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Home My WebLink AboutEdina_2021_WRMP_Appendix_A_Final_12152022City of Edina Water Resources Management Plan Appendix A – Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas Prepared for City of Edina December 2022 Amendment 4300 MarketPointe Drive, Suite 200 Minneapolis, MN 55435 952.832.2600 www.barr.com City of Edina Water Resources Management Plan Appendix A – Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas Prepared for City of Edina December 2022 Amendment City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas i P:\Mpls\23 MN\27\23271799 Edina CWRMP Amendment\WorkFiles\Amended CWRMP\Edina_2021_WRMP_Appendix_Final_12152022.docx City of Edina Water Resources Management Plan Appendix A – Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas December 2022 Amendment Contents Executive Summary ...................................................................................................................................................................... A.0-1 A.1 Methodology for Modeling ....................................................................................................................................... A.1-1 A.1.1 Methodology for Hydrologic/Hydraulic Modeling .......................................................................... A.1-1 A.1.1.1 Hydrologic Modeling ................................................................................................................ A.1-1 A.1.1.1.1 Watershed Data ....................................................................................................................... A.1-1 A.1.1.1.2 Rainfall Data .............................................................................................................................. A.1-4 A.1.1.1.3 Infiltration Data ........................................................................................................................ A.1-4 A.1.1.1.4 Depression Storage Data ..................................................................................................... A.1-5 A.1.1.1.5 Overland Flow Roughness ................................................................................................... A.1-6 A.1.1.2 Hydraulic Modeling ................................................................................................................... A.1-6 A.1.1.2.1 Storm Sewer Network ........................................................................................................... A.1-6 A.1.1.2.2 Tailwater Effects ....................................................................................................................... A.1-7 A.1.1.2.3 Overland Flow Network ........................................................................................................ A.1-7 A.1.1.3 Stormwater System Analysis .................................................................................................. A.1-8 A.1.1.3.1 Problem Areas Selection Process .................................................................................. A.1-10 A.1.2 Methodology for Water Quality Modeling ...................................................................................... A.1-11 A.1.2.1 Watershed Characteristics ................................................................................................... A.1-11 A.1.2.1.1 Impervious Fraction ............................................................................................................. A.1-12 A.1.2.1.2 Pervious Curve Number .................................................................................................... A.1-12 A.1.2.1.3 Other P8 Watershed Input Parameters ....................................................................... A.1-13 A.1.2.2 Treatment Device Characteristics ...................................................................................... A.1-13 A.1.2.2.1 Dead Storage ......................................................................................................................... A.1-13 A.1.2.2.2 Live Storage ............................................................................................................................ A.1-13 A.1.2.2.3 Other P8 Treatment Device Input Characteristics .................................................. A.1-14 A.1.2.3 Precipitation and Temperature Data ............................................................................... A.1-14 A.1.2.4 Selection of Other P8 Model Parameters ...................................................................... A.1-15 A.1.2.4.1 Time Step, Snowmelt, and Runoff Parameters ........................................................ A.1-15 City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas ii A.1.2.4.2 Particle File Selection .......................................................................................................... A.1-15 A.1.2.4.3 Passes through the Storm File ........................................................................................ A.1-16 A.1.2.5 Stormwater System Analysis ............................................................................................... A.1-16 A.2 Nine Mile Creek—North ............................................................................................................................................. A.2-1 A.2.1 General Description of Drainage Area .................................................................................................. A.2-1 A.2.1.1 Drainage Patterns ....................................................................................................................... A.2-1 A.2.1.1.1 Mirror Lake ................................................................................................................................. A.2-1 A.2.1.1.2 Highlands Lake ......................................................................................................................... A.2-2 A.2.1.1.3 Hawkes Lake .............................................................................................................................. A.2-2 A.2.1.1.4 Mud Lake (Bredesen Park) ................................................................................................... A.2-2 A.2.1.1.5 Nine Mile North ....................................................................................................................... A.2-3 A.2.2 Stormwater System Results ....................................................................................................................... A.2-3 A.2.2.1 Hydrologic/Hydraulic Modeling Results ........................................................................... A.2-3 A.2.2.2 Water Quality Modeling Results ........................................................................................... A.2-3 A.2.3 Implementation Considerations .............................................................................................................. A.2-4 A.2.3.1 Potential Flood Risk Reduction Options ........................................................................... A.2-4 A.2.3.1.1 Maloney Avenue and Tyler Court (ML_35 and ML_19) ........................................... A.2-4 A.2.3.1.2 Between Leslee Lane and Kaymar Drive (MD_22) ..................................................... A.2-5 A.2.3.1.3 Parkwood Road and Schaefer Road (MD_28, MD_29, and MD_35) .................. A.2-5 A.2.3.1.4 Schaefer Road and View Lane (MD_38) ......................................................................... A.2-6 A.2.3.1.5 Nine Mile Village Townhomes (MD_49) ........................................................................ A.2-6 A.2.3.1.6 Hawkes Lake and Upstream Surrounding Area (HL_1, HL_11c, HL_11w, HL_49, and HL_12) ................................................................................................................................. A.2-7 A.2.3.2 Potential Construction/Upgrade of Water Quality Basins ............................................. 2-7 A.2.3.2.1 MD_15 .......................................................................................................................................... A.2-8 A.2.3.2.2 NMN_27 ...................................................................................................................................... A.2-8 A.2.3.2.3 NMN_24 ...................................................................................................................................... A.2-8 A.2.3.2.4 NMN_49 ...................................................................................................................................... A.2-8 A.2.3.2.5 MD_3 ............................................................................................................................................ A.2-9 A.3 Nine Mile Creek—Central ........................................................................................................................................... A.3-1 A.3.1 General Description of Drainage Area .................................................................................................. A.3-1 A.3.1.1 Drainage Patterns ....................................................................................................................... A.3-1 A.3.1.1.1 Colonial Ponds ......................................................................................................................... A.3-1 A.3.1.1.2 Indian Pond ............................................................................................................................... A.3-1 A.3.1.1.3 Nine Mile Central .................................................................................................................... A.3-2 A.3.2 Stormwater System Results ....................................................................................................................... A.3-2 City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas iii A.3.2.1 Hydrologic/Hydraulic Modeling Results ........................................................................... A.3-2 A.3.2.2 Water Quality Modeling Results ........................................................................................... A.3-2 A.3.3 Implementation Considerations .............................................................................................................. A.3-3 A.3.3.1 Potential Flood Risk Reduction Options ........................................................................... A.3-3 A.3.3.1.1 Antrim Road and Chapel Drive (NMC_41) .................................................................... A.3-3 A.3.3.1.2 Ridgeview Drive (NMC_106 and NMC_107) ................................................................ A.3-4 A.3.3.1.3 West 66th Street and Naomi Drive (NMC_71, NMC_74, and NMC_103) .......... A.3-4 A.3.3.2 Potential Construction/Upgrade of Water Quality Basins ......................................... A.3-5 A.4 Lake Cornelia/Lake Edina/Adam’s Hill ................................................................................................................... A.4-1 A.4.1 General Description of Drainage Area .................................................................................................. A.4-1 A.4.1.1 Drainage Patterns ....................................................................................................................... A.4-1 A.4.1.1.1 North Cornelia .......................................................................................................................... A.4-1 A.4.1.1.2 South Lake Cornelia ............................................................................................................... A.4-2 A.4.1.1.3 Lake Edina .................................................................................................................................. A.4-2 A.4.1.1.4 Adam’s Hill Pond ..................................................................................................................... A.4-2 A.4.2 Stormwater System Results ....................................................................................................................... A.4-3 A.4.2.1 Hydrologic/Hydraulic Modeling Results ........................................................................... A.4-3 A.4.2.2 Water Quality Modeling Results ........................................................................................... A.4-3 A.4.3 Implementation Considerations .............................................................................................................. A.4-3 A.4.3.1 Potential Flood Risk Reduction Options ........................................................................... A.4-4 A.4.3.1.1 Valley View and Southdale Road Neighborhood (LE_34, LE_36, and LE_43) – updated 2021 ............................................................................................................................ A.4-4 A.4.3.1.2 Southwest Corner of TH 62 and TH 100 (NC_7, NC_8, NC_13, NC_11, NC_12, NC_14, NC_15, NC_16, and NC_20) – updated 2021 ................................................ A.4-6 A.4.3.2 Potential Construction/Upgrade of Water Quality Basins ......................................... A.4-7 A.4.3.2.1 LE_38 ............................................................................................................................................. A.4-8 A.4.3.2.2 NC_88 ........................................................................................................................................... A.4-8 A.5 Nine Mile Creek South ................................................................................................................................................. A.5-1 A.5.1 General Description of Drainage Area .................................................................................................. A.5-1 A.5.1.1 Drainage Patterns ....................................................................................................................... A.5-1 A.5.1.1.1 Centennial Lakes ...................................................................................................................... A.5-1 A.5.1.1.2 South Pond (Border Basin) .................................................................................................. A.5-1 A.5.1.1.3 Nine Mile South ....................................................................................................................... A.5-2 A.5.2 Stormwater System Results ....................................................................................................................... A.5-2 A.5.2.1 Hydrologic/Hydraulic Modeling Results ........................................................................... A.5-2 A.5.2.2 Water Quality Modeling Results ........................................................................................... A.5-3 City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas iv A.5.3 Implementation Considerations .............................................................................................................. A.5-3 A.5.3.1 Potential Flood Risk Reduction Options ........................................................................... A.5-3 A.5.3.1.1 West 70th Street and West Shore Drive (NMS_38 and NMS_50) ........................ A.5-3 A.5.3.1.2 Centennial Lakes (CL_1) ........................................................................................................ A.5-4 A.5.3.2 Potential Construction/Upgrade of Water Quality Basins ......................................... A.5-5 A.5.3.2.1 West 77th Street and TH 100 .............................................................................................. A.5-5 A.5.3.2.2 NMS_76 ....................................................................................................................................... A.5-5 A.5.3.2.3 NMS_104 ..................................................................................................................................... A.5-6 A.5.3.2.4 NMS_72 and NMS_74 ............................................................................................................ A.5-6 A.5.3.2.5 SP_1 (South Pond/Border Basin) ....................................................................................... A.5-6 A.6 Nine Mile South Fork ................................................................................................................................................... A.6-1 A.6.1 General Description of Drainage Area .................................................................................................. A.6-1 A.6.1.1 Drainage Patterns ....................................................................................................................... A.6-1 A.6.1.1.1 Arrowhead Lake ....................................................................................................................... A.6-1 A.6.1.1.2 Indianhead Lake ....................................................................................................................... A.6-2 A.6.1.1.3 Pawnee Pond ............................................................................................................................ A.6-2 A.6.1.1.4 Eden Prairie ................................................................................................................................ A.6-2 A.6.1.1.5 Braemar Arena/Public Works ............................................................................................. A.6-2 A.6.1.1.6 Nine Mile South Fork ............................................................................................................. A.6-2 A.6.2 Stormwater System Results ....................................................................................................................... A.6-3 A.6.2.1 Hydrologic/Hydraulic Modeling Results ........................................................................... A.6-3 A.6.2.2 Water Quality Modeling Results ........................................................................................... A.6-3 A.6.3 Implementation Considerations .............................................................................................................. A.6-3 A.6.3.1 Potential Flood Risk Reduction Options ........................................................................... A.6-4 A.6.3.1.1 McCauley Trail West (AH_6)................................................................................................ A.6-4 A.6.3.1.2 Sally Lane and Valley View Road (NMSB_52, NMSB_69, and NMSB_77) ......... A.6-5 A.6.3.2 Potential Construction/Upgrade of Water Quality Basins ......................................... A.6-5 A.6.3.2.1 NMSB_3 and NMSB_2 ........................................................................................................... A.6-6 A.6.3.2.2 NMSB_12 .................................................................................................................................... A.6-6 A.6.3.2.3 NMSB_86 .................................................................................................................................... A.6-6 A.6.3.2.4 NMSB_7 ....................................................................................................................................... A.6-6 A.6.3.2.5 NMSB_85 .................................................................................................................................... A.6-7 A.7 Southwest Ponds (Dewey Hill Road Area) ........................................................................................................... A.7-1 A.7.1 General Description of Drainage Area .................................................................................................. A.7-1 A.7.1.1 Drainage Patterns ....................................................................................................................... A.7-1 A.7.1.1.1 Southwest Ponds ..................................................................................................................... A.7-1 City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas v A.7.1.1.2 Nine Mile I-494 ........................................................................................................................ A.7-1 A.7.2 Stormwater System Results ....................................................................................................................... A.7-2 A.7.2.1 Hydrologic/Hydraulic Modeling Results ........................................................................... A.7-2 A.7.2.2 Water Quality Modeling Results ........................................................................................... A.7-2 A.7.3 Implementation Considerations .............................................................................................................. A.7-2 A.7.3.1 Potential Flood Risk Reduction Options ........................................................................... A.7-3 A.7.3.1.1 Gleason Road and Bonnie Brae Drive (SWP_24) ........................................................ A.7-3 A.7.3.2 Potential Construction/Upgrade of Water Quality Basins ......................................... A.7-3 A.8 TH 169 North ................................................................................................................................................................... A.8-1 A.8.1 General Description of Drainage Area .................................................................................................. A.8-1 A.8.1.1 Drainage Patterns ....................................................................................................................... A.8-1 A.8.2 Stormwater System Results ....................................................................................................................... A.8-1 A.8.2.1 Hydrologic/Hydraulic Modeling Results ........................................................................... A.8-1 A.8.2.2 Water Quality Modeling Results ........................................................................................... A.8-2 A.8.3 Implementation Considerations .............................................................................................................. A.8-2 A.8.3.1 Potential Flood Risk Reduction Options ........................................................................... A.8-2 A.8.3.2 Potential Construction/Upgrade of Water Quality Basins ......................................... A.8-2 A.9 Northeast Minnehaha Creek ..................................................................................................................................... A.9-1 A.9.1 General Description of Drainage Area .................................................................................................. A.9-1 A.9.1.1 Drainage Patterns ....................................................................................................................... A.9-1 A.9.1.1.1 Morningside .............................................................................................................................. A.9-1 A.9.1.1.2 Minnehaha Creek North ....................................................................................................... A.9-1 A.9.1.1.3 Edina Country Club ................................................................................................................. A.9-2 A.9.2 Stormwater System Results ....................................................................................................................... A.9-2 A.9.2.1 Hydrologic/Hydraulic Modeling Results ........................................................................... A.9-2 A.9.2.2 Water Quality Modeling Results ........................................................................................... A.9-3 A.9.3 Implementation Considerations .............................................................................................................. A.9-3 A.9.3.1 Potential Flood Risk Reduction Options ........................................................................... A.9-3 A.9.3.1.1 Halifax Avenue South (MHN_84, MHN_3, MHN_56, MHN_89, MHN_55, MHN_61, MHN_62, MHN_63, MHN_87, MHN_88, MHN_90, and MHN_2) and Indianola Avenue South of West 50th Street (MHN_4, MHN_5, MHN_42 and MHN_72) – updated 2021 .................................................................................................... A.9-4 A.9.3.1.2 Morningside/Weber Park (MS_26, MS_25, MS_41, MS_32, MS_44, MS_24, MS_15, MS_52, MS_53, MS_2, MS_38, MS_40, MS_54, MS_31, MS_33, MS_39a, and MS_39b) – updated 2022.......................................................................................... A.9-21 A.9.3.1.3 Edinbrook Lane and Westbrook Lane (MHN_79) ................................................... A.9-22 City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas vi A.9.3.1.4 North of Morningside Road between Lynn Avenue and Crocker Avenue (MS_22) ..................................................................................................................................... A.9-23 A.9.3.1.5 Branson Street between West 44th Street and Morningside Road (MS_3, MS_48, and MS_7) ................................................................................................................................ A.9-23 A.9.3.2 Potential Construction/Upgrade of Water Quality Basins ...................................... A.9-24 A.10 Southeast Minnehaha Creek .................................................................................................................................. A.10-1 A.10.1 General Description of Drainage Area ............................................................................................... A.10-1 A.10.1.1 Drainage Patterns .................................................................................................................... A.10-1 A.10.1.1.1 Lake Pamela ............................................................................................................................ A.10-1 A.10.1.1.2 Minnehaha Creek South .................................................................................................... A.10-1 A.10.1.1.3 Melody Lake ........................................................................................................................... A.10-2 A.10.2 Stormwater System Results .................................................................................................................... A.10-2 A.10.2.1 Hydrologic/Hydraulic Modeling Results ........................................................................ A.10-2 A.10.2.2 Water Quality Modeling Results ........................................................................................ A.10-3 A.10.3 Implementation Considerations ........................................................................................................... A.10-3 A.10.3.1 Potential Flood Risk Reduction Options ........................................................................ A.10-3 A.10.3.1.1 East Golf Terrace Heights Neighborhood .................................................................. A.10-4 A.10.3.1.2 Concord and West 58th Street (MHS_59, MHS_26, MHS_58, MHS_42, MHS_53, and MHS_17) – updated 2021 ......................................................................................... A.10-4 A.10.3.2 Potential Construction/Upgrade of Water Quality Basins .................................... A.10-15 A.10.3.3 Potential Stream Improvement Projects ...................................................................... A.10-16 A.10.3.3.1 Minnehaha Creek Reach 14 Stream Restoration .................................................. A.10-16 A.11 Northwest Minnehaha Creek ................................................................................................................................. A.11-1 A.11.1 General Description of Drainage Area ............................................................................................... A.11-1 A.11.1.1 Drainage Patterns .................................................................................................................... A.11-1 A.11.1.1.1 TH 100 ....................................................................................................................................... A.11-1 A.11.1.1.2 Hopkins .................................................................................................................................... A.11-1 A.11.1.1.3 Interlachen .............................................................................................................................. A.11-1 A.11.2 Stormwater System Results .................................................................................................................... A.11-2 A.11.2.1 Hydrologic/Hydraulic Modeling Results ........................................................................ A.11-2 A.11.2.2 Water Quality Modeling Results ........................................................................................ A.11-2 A.11.3 Implementation Considerations ........................................................................................................... A.11-2 A.11.3.1 Potential Flood Risk Reduction Options ........................................................................ A.11-3 A.11.3.1.1 Blake Road South and Spruce Road (HO_4) ............................................................. A.11-3 A.11.3.2 Potential Construction/Upgrade of Water Quality Basins ...................................... A.11-4 A.12 References ..................................................................................................................................................................... A.12-1 City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas vii List of Tables Table A.0.1 Summary of Drainage Area Hydrologic and Hydraulic Analysis Updates since 2018 WRMP .................................................................................................................................................................................. A.0-2 Table A.1.1 Land Use Impervious Fraction Assumptions for Hydrologic Modeling ....................................... A.1-3 Table A.1.2 Modeled Design Storm Events..................................................................................................................... A.1-4 Table A.1.3 Horton Infiltration Parameters ..................................................................................................................... A.1-5 Table A.1.4 Roughness Coefficient Assumptions ......................................................................................................... A.1-6 Table A.1.5 Land Use Roughness Assumptions ............................................................................................................ A.1-8 Table A.1.6 Land Use Impervious Fraction Assumptions for Water Quality Modeling .............................. A.1-12 Table A.1.7 Infiltration Assumptions for Water Quality Modeling ..................................................................... A.1-14 Table A.2.1 Major Watersheds within the Nine Mile Creek—North Drainage Basin ..................................... A.2-1 Table A.3.1 Major Watersheds within the Nine Mile Creek—Central Drainage Basin ................................... A.3-1 Table A.4.1 Major Watersheds within the Lake Cornelia/Lake Edina/Adam’s Hill Drainage Basin ........... A.4-1 Table A.4.2 Residential Structure Impacts in the Valley View and Southdale Road area............................ A.4-5 Table A.4.3 Residential Structure Impacts in the Southwest Corner of TH 62 and TH 100 area .............. A.4-7 Table A.5.1 Major Watersheds within the Nine Mile Creek—South Drainage Basin ..................................... A.5-1 Table A.6.1 Major Watersheds within the Nine Mile South Fork Drainage Basin ........................................... A.6-1 Table A.7.1 Major Watersheds within the Southwest Ponds Drainage Basin ................................................... A.7-1 Table A.8.1 Major Watershed within the TH 169 North Drainage Area .............................................................. A.8-1 Table A.9.1 Major Watersheds within the Northeast Minnehaha Creek Drainage Basin ............................. A.9-1 Table A.9.2 Residential Structure Impacts in the Halifax area ................................................................................. A.9-6 Table A.9.3 Flood Elevations in Halifax Avenue South and Indianola Avenue Project Area for 1-percent-annual-chance event ..................................................................................................................................... A.9-17 Table A.9.4 Flood Elevations in the Halifax Avenue South and Indianola Avenue Project Area for 10-percent-annual-chance event.................................................................................................................... A.9-17 Table A.9.5 Residential structure impacts in the Halifax Avenue South and Indianola Avenue South areas ............................................................................................................................................................................... A.9-18 Table A.10.1 Major Watersheds within the Southeast Minnehaha Creek Drainage Basin .......................... A.10-1 Table A.10.2 Residential Structure Impacts in the Concord and West 58th Street Area ............................... A.10-6 Table A.10.3 Flood Elevations in Concord and West 58th Project Area for 1-percent-annual-chance event ............................................................................................................................................................................. A.10-11 Table A.10.4 Flood Elevations in Concord and West 58th Project Area for 10-percent-annual-chance event ............................................................................................................................................................................. A.10-12 Table A.10.5 Residential structure impacts in the Concord and West 58th Street area .............................. A.10-13 Table A.11.1 Major Watersheds within the Northwest Minnehaha Creek Drainage Basin ......................... A.11-1 City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas viii List of Figures Figure A.2.1 Nine Mile Creek—North Water Quality Modeling Results ............................................................ A.2-10 Figure A.3.1 Nine Mile Creek—Central Water Quality Modeling Results............................................................. A.3-6 Figure A.4.1 Lake Cornelia/Lake Edina/Adam's Hill Water Quality Modeling Results ..................................... A.4-9 Figure A.5.1 Nine Mile Creek—South Water Quality Modeling Results ............................................................... A.5-7 Figure A.6.1 Nine Mile South Fork Water Quality Modeling Results ..................................................................... A.6-8 Figure A.7.1 Southwest Ponds Water Quality Modeling Results ............................................................................. A.7-5 Figure A.8.1 TH 169 North Water Quality Modeling Results..................................................................................... A.8-4 Figure A.9.1 Existing and proposed street profile considered for reducing flood risk in the Halifax Avenue South area for Option 5 .................................................................................................................................. A.9-9 Figure A.9.2 Map showing components of Option 1 considered for reducing flood risk in the Halifax Avenue South and Indianola Avenue South areas ............................................................................ A.9-10 Figure A.9.3 Map showing components of Option 2 considered for reducing flood risk in the Halifax Avenue South and Indianola Avenue South areas ............................................................................ A.9-11 Figure A.9.4 Map showing components of Option 3 considered for reducing flood risk in the Halifax Avenue South and Indianola Avenue South areas ............................................................................ A.9-12 Figure A.9.5 Map showing components of Option 4 considered for reducing flood risk in the Halifax Avenue South and Indianola Avenue South areas ............................................................................ A.9-13 Figure A.9.6 Map showing components of Option 5 considered for reducing flood risk in the Halifax Avenue South and Indianola Avenue South areas ............................................................................ A.9-14 Figure A.9.7 Map showing components of Option 6 considered for reducing flood risk in the Halifax Avenue South and Indianola Avenue South areas ............................................................................ A.9-15 Figure A.9.8 Northeast Minnehaha Creek Water Quality Modeling Results .................................................... A.9-25 Figure A.10.1 Map showing components of the options considered for reducing flood risk in the Concord and West 58th Street area ............................................................................................................................ A.10-9 Figure A.10.2 Southeast Minnehaha Creek Water Quality Modeling Results .................................................. A.10-17 Figure A.11.1 Northwest Minnehaha Creek Water Quality Modeling Results ................................................... A.11-5 List of Attachments or Exhibits Attachment A City of Edina Imperviousness Assumptions for Stormwater Modeling Attachment B Summary of Nine Mile Creek and Minnehaha Creek Modeling Approach City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas x Certifications I hereby certify that this report was prepared by me or under my direct supervision and that I am a duly License Professional Engineer under the laws of the State of Minnesota. December 15, 2022 Cory Anderson PE #: 51305 Date City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas xi Acronyms Acronym Description ACE Annual Chance of Exceedance APWA American Public Works Association AWWA American Water Works Association BMPs Best Management Practices CAMP Citizen Assisted Monitoring Program CFS Cubic Feet per Second CIP Capital Improvement Program CWA Clean Water Act EOF Emergency Overflow EPA United States Environmental Protection Agency FAW Functional Assessment of Wetlands (MCWD) FEMA Federal Emergency Management Agency GIS Geographic Information Systems GPM Gallons per minute GSI Green Stormwater Infrastructure IAM Intelligent Asset Management IBI Index of Biotic Integrity IPWEA Institute of Public Works Engineering Australasia ISTS Individual Sewage Treatment Systems LGU Local Government Unit MCWD Minnehaha Creek Watershed District MnRAM Minnesota Routine Assessment Method MnDNR Minnesota Department of Natural Resources MnDOT Minnesota Department of Transportation MPCA Minnesota Pollution Control Agency MS4 Municipal Separate Storm Sewer System MUSA Metropolitan Urban Service Area NGVD29 National Geodetic Vertical Datum of 1929 NMCWD Nine Mile Creek Watershed District NOAA National Oceanic and Atmospheric Administration NPDES National Pollutant Discharge Elimination System NURP Nationwide Urban Runoff Program NWI National Wetlands Inventory City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas xii OHWL Ordinary High Water Level PWI Public Waters Inventory RCP Reinforced-Concrete Pipe SCS Soil Conservation Service SSURGO Soil Survey Geographic Database SWMM Stormwater Management Model SWPPP Stormwater Pollution Prevent Plan/Program TBD To Be Determined TH Trunk Highway TMDL Total Maximum Daily Load TSS Total Suspended Solids UAA Use Attainability Analysis USFWS United States Fish and Wildlife Service VIC Voluntary Investigation and Clean-up WCA Wetland Conservation Act WLA Wasteload allocation WMO Watershed Management Organization WRAPS Watershed Restoration and Protection Strategy WRMP Water Resources Management Plan City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.0-1 Executive Summary This appendix to the 2022 City of Edina Water Resources Management Plan (WRMP) includes a description of hydrologic, hydraulic, and water quality modeling methods and related analysis of ten major drainage areas within the city. Detailed modeling results, including tables and figures, were previously included within the main document of the 2018 City of Edina Water Resources Management Plan (2018 WRMP). Analyses in this appendix focuses on local flood areas versus regional flood areas – regional flood areas include creek corridors and regional basins; local flood areas include all other areas. The City of Edina is systematically updating its hydrologic, hydrologic, and water quality analysis. Thus, model results including water level elevations, inundation areas, and other data will be changing over time. The City’s interactive web map will be updated with the most recent data on an annual basis. Future analysis will be summarized in technical memoranda and included in this appendix as attachments. Generally, information provided in Section A.2 through Section A.11 of this document reflect analysis current at the time of development of the 2018 WRMP. Consult the City’s interactive web map for the most current information. Table A.0.1 identifies major updates made to modeling of specific areas within the city and will be used to track future modeling updates. Table A.0.1 is organized by drainage area; however, updates within a drainage area do not imply that modeling of the entire major drainage area was updated. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.0-2 Table A.0.1 Summary of Drainage Area Hydrologic and Hydraulic Analysis Updates since 2018 WRMP Appendix Drainage Area Date Updated Summary of Major Detailed Modeling Updates A.2 Nine Mile Creek North A.3 Nine Mile Creek Central A.4 Lake Cornelia/Lake Edina/Adam’s Hill 2021 - Updates to Valley View and Southdale Road Neighborhood (Section A.4.3.1.1) - Updates to Southwest corner of TH 62 and TH 100 (Section A.4.3.1.2) - Detailed modeling of the York Condos in support of drainage concerns from building owners A.5 Nine Mile Creek South A.6 Nine Mile South Fork A.7 Southwest Ponds (Dewey Hill Road Area) A.8 TH 169 North A.9 Northeast Minnehaha Creek 2021 - Updates to Halifax Avenue South and Indianola Avenue South (Section A.9.3.1.1) - Updates to Morningside/Weber Park area (Section A.9.3.1.2) A.10 Southeast Minnehaha Creek 2021 - Updates to Concord and West 58th Street (Section A.10.3.1.2) A.11 Northwest Minnehaha Creek City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.1-1 A.1 Methodology for Modeling A.1.1 Methodology for Hydrologic/Hydraulic Modeling The U.S. EPA’s Stormwater Management Model (SWMM), with a computerized graphical interface provided by XP Software/Innovyze (XPSWMM), was chosen as the computer modeling package for this study. XPSWMM uses rainfall and watershed characteristics to generate local runoff, which is routed through pipe and overland flow networks. The model can account for detention in ponding areas, backflow in pipes, surcharging of manholes, as well as tailwater conditions that may exist and affect upstream storage or pipe flows. The 5000 node version of XPSWMM 2014, was used to model the storm sewer, ponding and overland flow systems within the City of Edina. Since 2018, detailed study areas in the City utilized 2-dimensional (2D) modeling in XPSWMM to add additional detail and more accurately reflect hydrologic and hydraulic conditions. These detailed study areas are maintained in separate models and the results are integrated into the citywide inundation mapping. The City storm sewer system was split into two XPSWMM models: one model that reflects the parts of Edina that drain to Minnehaha Creek and one that reflects the parts of Edina that drain to Nine Mile Creek, with overflow connections between the two where necessary. A.1.1.1 Hydrologic Modeling Three major types of information are required by XPSWMM for hydrologic modeling: (1) watershed data, (2) rainfall data, and (3) infiltration data. This data is used by XPSWMM to generate inflow hydrographs at various points into the storm sewer, ponding, and overland flow networks. The following sections describe each of these data. A.1.1.1.1 Watershed Data The amount of runoff from a watershed depends on numerous factors, including the total watershed area, the soil types within the watershed, the percent of impervious area, the runoff path through the watershed, and the slope of the land within the watershed. Esri’s ArcMap geographic information systems (GIS) software was used extensively in assessing the above mentioned characteristics of each watershed within the City. The software also allowed mapping of the drainage network for the area. Watershed Area The watershed delineation was based on watersheds delineated for the 2003 WRMP that were then modified using newer data including the MnDNR LiDAR elevation data collected in 2011, the updated City storm sewer GIS data (provided by the City in 2016), and newer aerial imagery. In certain cases, the watershed divides were field verified. Since 2018, approximately 1,650 separate watersheds have been delineated citywide (an increase of about 180 since 2018), with approximately 420 watershed divides in the integrated 1D/2D detailed study areas. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.1-2 Land Use Data The percent of impervious area within each watershed was estimated using land use data provided by the City of Edina. An electronic land use coverage was provided by the City for the 2003/2011 WRMPs. For areas outside of the Edina City limits that were included in the model, such as small portions of Eden Prairie, Bloomington, Hopkins, St. Louis Park, Minneapolis, and Richfield, land use data based on the 2010 Metropolitan Council aerial photographs was used. Land use within the study area was divided into the designations shown in Table A.1.1. The land use information provided by the City categorized residential land use as single-family residential or multiple-family residential. For modeling purposes, these residential categories were further broken down based on the density of housing units within the area. The City’s electronic land use coverage and aerial imagery was used in ArcMap to determine the density of the residential areas. The single-family residential areas were further categorized as very low density residential (<1 unit/acre) or low density residential (1-4 units/acre). The areas categorized as multiple-family residential by the City were broken down into the following categories: low density residential (1-4 units/acre), medium density residential (4-8 units/acre), and high density residential (>8 units/acre). The land use type “Other” is new to the analysis for this WRMP update. The “Other” land use type essentially reflects railroad corridors and is about 20 percent impervious. The land use categories were used to estimate the total and “directly-connected” impervious fractions for each subwatershed within the study area. The total impervious fraction of a watershed represents the portion of the watershed that is covered by an impervious surface. The “directly-connected” impervious fraction represents the impervious surfaces that are hydraulically connected to a stormwater conveyance system. For example, if a rooftop drains onto an adjacent pervious area such as a yard, it is not a “directly- connected” impervious area. However, if a rooftop drains onto a driveway, which drains to the street and then to a stormwater catch basin, the rooftop would be a “directly-connected” impervious area. A separate study estimating the total impervious fraction and the “directly-connected” impervious fraction for each land use type was completed in October 2016 (City of Edina Imperviousness Assumptions for Stormwater Modeling, Barr Engineering, 2016; also attached as Attachment A) using the land use layer from the City, and a 2011 gridded impervious layer of the City from the University of Minnesota. Resulting imperviousness for each land use type throughout the City are listed in Table A.1.1. The “directly- connected” imperviousness for each watershed was calculated as an area-weighted average of the different land use types existing within the watershed. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.1-3 Table A.1.1 Land Use Impervious Fraction Assumptions for Hydrologic Modeling Land Use Designation Total Impervious % Directly-Connected Impervious % Commercial 85% 80% Developed Park 30% 20% Golf Course 5% 2% High Density Residential 65% 50% Highway 65% 65% Industrial/Office 75% 75% Institutional 60% 30% Institutional - High Imperviousness 80% 70% Low Density Residential 40% 25% Medium Density Residential 50% 40% Natural/Park/Open 2% 0% Open Water 100% 100% Other 20% 20% Very Low Density Residential 25% 15% Wetland 100% 100% Watershed Width and Slope The SWMM Runoff Non-linear Reservoir Method was used as the hydrograph generation technique for this project. This method computes outflow as the product of velocity, depth and a watershed width factor. The watershed “width” in XPSWMM is defined as twice the length of the main drainage channel, with adjustments made for watersheds that are skewed (i.e., the areas on both sides of the main drainage channel are not equal). This factor is a key parameter in determining the shape of the hydrograph for each watershed and is often used as a calibration parameter. To determine the width parameter, the main drainage channel for each watershed was digitized in ArcMap and a customized ArcMap script was used to calculate the width based on the skew of the drainage path within the subwatershed. For this WRMP update, the widths of unchanged or slightly modified watersheds were left the same. Only in watersheds where the boundary was significantly modified or the watershed was subdivided were the widths recalculated using the method described above. The average slope (feet/feet) for each watershed was calculated in ArcMap using the electronic gridded topographic LiDAR elevation data. The slope was then determined for each grid cell by calculating the steepest slope in any direction. The average of all of the gridded slope values within each watershed was then used as the representative slope of the watershed. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.1-4 A.1.1.1.2 Rainfall Data Storm events for several return periods were analyzed in the citywide 1D XPSWMM model: the 1-percent- annual-chance 24-hour storm, the 10-percent-annual-chance 24-hour storm, and the 1-percent-annual- chance 10-day snowmelt event. For the 1-percent-annual-chance return period, a centrally nested distribution was applied to a total rainfall depth of 7.47 inches, based on the National Oceanic and Atmospheric Administration’s Atlas 14 Volume 8 precipitation frequency curves. A nested distribution was selected to be consistent with the XPSWMM models of Nine Mile Creek and Minnehaha Creek. For the snowmelt event, a longer 10-day distribution that has historically been used was applied to a total of 7.2 inches with no losses (generating 7.2 inches of runoff). A precipitation depth of 4.29 inches was used for the 10-percent-annual-chance 24-hour storm. For the integrated 1D/2D XPSWMM models in the detailed study areas (Concord and 58th, Halifax, TH62/100, Valley View, and Morningside), five return periods were analyzed, as summarized in Table A.1.2. The 24-hour precipitation depths and peak rainfall intensity for each design storm are listed in the table as well. The MSE3 temporal distribution was used for these events because that is the distribution recommended for the entire state of MN by the Natural Resource Conservation Service (NRCS). In developing these models, a comparison was made between the centrally- nested distribution and the MSE3 distribution, finding there is very little difference between the two. Table A.1.2 Modeled Design Storm Events Design Storm Event Annual Chance of Exceedance (ACE) 24-hour Depth Peak Intensity 5-year 20% 3.59 inches 5.9 in/hr 10-year 10% 4.29 inches 7.1 in/hr 25-year 4% 5.41 inches 8.9 in/hr 50-year 2% 6.39 inches 10.5 in/hr 100-year 1% 7.49 inches 12.3 in/hr A.1.1.1.3 Infiltration Data Soils Soils data for the City of Edina was obtained through two sources: the 2012 Natural Resources Conservation Service Soil Survey Geographic (SSURGO) Database and the soils layer used for the modeling effort associated with the 2003 WRMP. The hydrologic soil group (HSG) designation classifies soils into groups (A, B, C, and D) based on the infiltration capacity of the soil (well drained, sandy soils are classified as “A” soils; poorly drained, clayey soils are classified as “D” soils). When a HSG designation was not included in the new SSURGO soils database, the 2003 WRMP HSGs were used. For the 2003 modeling effort, when a HSG designation was not included in the soils database, the soil description was used to estimate the HSG. If a soil description was unavailable, the most dominant soil group in the vicinity was assumed. Although all soil types are represented in the City, the predominant soil type in the City is Type B (sandy loam). City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.1-5 Horton Infiltration Infiltration was simulated in the XPSWMM models using the Horton Infiltration equation. This equation is used to represent the exponential decay of infiltration capacity of the soil that occurs during heavy storm events. The soil infiltration capacity is a function of the following variables: Fc (minimum or ultimate value of infiltration capacity), Fo (maximum or initial value of infiltration capacity), k (decay coefficient), and time. The actual values of Fc, Fo, and k are dependent upon soil, vegetation, and initial moisture conditions prior to a rainfall event. Because it was not feasible to obtain this detailed information for each subwatershed through field samples, it was necessary to make assumptions based on the various soil types throughout the City. Table A.1.3 summarizes the Horton infiltration values used for each Hydrologic Soil Group to calculate composite infiltration parameters for each subwatershed. The values shown in the table are based on suggested values in the Stormwater Management Model, Version 4: User’s Manual, U.S. EPA, 1988. Composite Fc and Fo values were calculated for each subwatershed based on the fraction of each soil type within the subwatershed. Global databases containing the infiltration parameters for each subwatershed were developed and imported into the XPSWMM models. Table A.1.3 Horton Infiltration Parameters Hydrologic Soil Group Fo (in/hr) Fc (in/hr) k (1/sec) A 5 0.38 0.00115 B 3 0.23 0.00115 C 2 0.1 0.00115 D 1 0.03 0.00115 A.1.1.1.4 Depression Storage Data Depression storage represents the volume (in inches) that must be filled with rainfall prior to the occurrence of runoff in XPSWMM. It characterizes the loss or "initial abstraction" caused by such phenomena as surface ponding, surface wetting, interception and evaporation. Separate depression storage input values are required in XPSWMM for pervious and impervious areas. The depression storage assumptions used for the models were based on the values used in the XPSWMM model developed for the Nine Mile Creek Watershed District Bloomington Use Attainability Analysis, Barr Engineering, 2001. For this model, the depression storage was estimated by plotting total precipitation for several measured rainfall events at a Bloomington continuous recording precipitation gage versus runoff from several Bloomington monitoring sites. A regression of the data yielded a y-intercept that was assumed to be the depression storage (in inches). Based on this analysis, the assumed impervious depression storage was 0.06 inches and the pervious depression storage was 0.17 inches. XPSWMM also uses a “Zero Detention Storage” parameter to account for areas that generate immediate runoff (i.e., water surface areas). This parameter was estimated for each subwatershed by dividing the water surface area by the directly connected impervious surface area. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.1-6 A.1.1.1.5 Overland Flow Roughness The SWMM Runoff Non-linear Reservoir Method requires an estimate of the roughness of both pervious and impervious areas in the watershed. These values were calibrated in the Nine Mile Creek Watershed District Bloomington Use Attainability Analysis (Barr Engineering, 2001) and were used in previous WRMPs as well as this WRMP. The values for impervious area and for pervious area are 0.200 and 0.355, respectively. A.1.1.2 Hydraulic Modeling A.1.1.2.1 Storm Sewer Network Data detailing the storm sewer network within the City of Edina was provided by the City; updated data is provided by the City annually. An electronic GIS feature dataset provided detailed information on the storm sewer system, including the type of pipe (material of construction), invert elevations, pipe sizes, pipe lengths, and manhole rim elevations. Where this data was incomplete, additional information was obtained from other sources such as construction plans, field surveys, or previous modeling efforts. All elevations entered into the model reflect the National Geodetic Vertical Datum of 1929 (NGVD29). Assumptions A variety of pipe types are used throughout the City. The assumptions used for the roughness coefficient (Manning’s “n”) for each pipe type are listed in Table A.1.4. Table A.1.4 Roughness Coefficient Assumptions Pipe Type Abbreviation Assumed Roughness Coefficient Corrugated Metal Pipe CMP 0.024 Clay - 0.015 Steel - 0.015 Ductile Iron Pipe DIP 0.014 Reinforced-Concrete Pipe RCP 0.013 Poly Vinyl Chloride PVC 0.010 High Density Polyethylene HDPE 0.008 Outlets from ponding areas that may be inlet controlled were modeled in XPSWMM assuming a groove end projecting concrete pipe inlet condition. This allowed XPSWMM to determine the controlling flow condition in the outlet pipe (i.e., is the flow in the pipe controlled by the inlet size, barrel capacity, or tailwater conditions) and accurately estimate the pond’s water surface elevation. Surface inundation, or flooding, occurs when the runoff from the watershed exceeds the capacity of the storm sewer system to convey the stormwater downstream. Capacity of the storm sewer system can be limited by either the capacity of the storm sewer pipes or the inlet capacity (i.e., flow through catch basins City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.1-7 and other minor roadway inlets into storm sewer pipes). For most of the city, the XPSWMM model does not include inlet capacity, due to the high-level of detail required. Therefore, model results reflect the capacity of the storm sewer system and may not fully characterize the existing flood risk where inlet capacity is restrictive. Inlet capacity was modeled for certain areas of the city where additional detailed analysis was performed. These results are among data included in the City’s interactive web map and/or technical memoranda attached to this appendix. A.1.1.2.2 Tailwater Effects For the portion of the City that drains to Nine Mile Creek, the City XPSWMM model incorporated the relevant portions of the Nine Mile Creek XPSWMM model to more accurately account for tailwater impacts from the creek. For the portion of the City that drains to Minnehaha Creek, the City XPSWMM model uses the tailwater elevations (stage hydrographs) along the creek system as boundary conditions for each corresponding storm event (i.e., the Minnehaha Creek model provided by Wenck Associates/Stantec is not integrated into the City model, but the creek model is used to establish boundary conditions). A more detailed description of the Nine Mile Creek and Minnehaha Creek model development, as well as a description of how the tailwater conditions were incorporated into the City of Edina models is provided in Attachment B. A.1.1.2.3 Overland Flow Network Overland flow networks were entered into the XPSWMM models because preliminary modeling results indicated that water was being routed out of the systems and lost (i.e., manholes and ponding areas would surcharge and the model assumed the water disappeared once it exceeded the respective spill crest elevation). An iterative process was used by adding storage and overland flow network data until all of the stormwater had been accounted for by XPSWMM. Data for the overland flow network were primarily based on MnDNR 2011 LiDAR elevation data (ground surface), and in some cases, site visits or storm sewer data from the City. Storage was also added to XPSWMM nodes based on the MnDNR 2011 LiDAR elevation. The storage curves were defined as depth-area tables, at depth increments of 0.2 feet. Initially, storage was added only to the XPSWMM nodes representing ponds or backyard depression areas. After the initial model runs, storage was added to subwatersheds where ponding or surface inundation occurs (e.g., parking lots). Overland flow paths were added with the following characteristics: Overland flow along streets • Trapezoidal channels • Bottom width = 16 feet (approx. ½ street width), or 32 feet if the entire street is flowing • Side slopes = 1H:1V • Manning’s “n” for the surface flow channels was set equal to 0.014 for flow down paved streets • Channel depth = 1 foot City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.1-8 Natural overland flow paths • Trapezoidal channels • Bottom width = variable based on topographic information. Typically, an estimate of 10 feet was used. • Side slopes = variable based on topographic information. Typically, 5H:1V was used. • Manning’s “n” = 0.035 where overland flow was clearly over vegetated areas or onto boulevards. • Channel depth = 1 foot or deeper where necessary. For the detailed study areas modeled in 2D, overland flow and surface storage is accounted for using a digital terrain model (DTM) generated from LiDAR elevation data. In these areas, links representing overland flow from the original 1D model were removed and storage data in the storm sewer system nodes was removed. Nodes representing manholes and catch basins are linked to the DTM at the rim so that when surcharging occurs, water is transferred from the node to the surface and allowed to flow onto the surface following topographical flow paths represented by the DTM or is stored on the surface in natural depressions represented in the DTM. The hydraulics of the overland flow on the 2D model surface is represented by the DTM data (i.e., elevation change, slope) and Manning’s roughness coefficients assigned by land use classification shown in Table A.1.5. Table A.1.5 Land Use Roughness Assumptions Land Use Classification Manning’s Roughness Value Deciduous Tree Canopy 0.035 Roads/Paved Surfaces 0.014 Grass/Shrub 0.035 Buildings 1.000 Coniferous Tree Canopy 0.035 Bare Soil 0.020 Emergent Wetland 0.050 Lakes/Ponds 0.050 A.1.1.3 Stormwater System Analysis Ten- and 1-percent-annual-chance frequency flood analyses were performed for all of the major drainage basins. The 10-percent-annual-chance analysis was based on a 24-hour storm with 4.29 inches of rain. The 1-percent-annual-chance analysis was based on a 24-hour storm event with 7.47 inches of rain. In the detailed study areas where 1D/2D modeling was utilized, the twenty-, ten-, four-, two- and 1-percent- annual-chance frequency flood analyses were performed, as described in Section A.1.1.1.2. Watershed information and the results for the 1-percent-annual-chance frequency hydrologic analyses in the citywide City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.1-9 1D model are available in City’s interactive web map and/or technical memoranda attached to this appendix. Available data include: • Subwatershed • Outlet Type (from subwatersheds) • Outlet Elevation (from waterbodies or storage areas) • Flood Elevation—The maximum water elevation reached in the 1-percent-annual-chance storm event • Inundation Area—The area estimated to be covered with water at the 1-percent-annual-chance flood elevation • Stormsewer Network—Including inlets, outlets, and conduits The results of the 1-percent-annual-chance frequency hydraulic analyses are available from the City’s interactive web map. The City’s interactive web map includes the drainage basin boundary, subwatershed boundaries, the modeled storm sewer network, and the flood-prone areas identified in the modeling In past modeling efforts and WRMPs, one objective of the hydraulic analyses was to evaluate the level of service provided by the current storm sewer system. The level of service was examined by determining the surcharge conditions of the manholes and catch basins within the storm sewer system during the 10- and 1-percent-annual-chance frequency storm events. More recent modeling shows clearly that many portions of the city are not currently equipped to completely handle the 1-percent-annual-chance event of Atlas 14 without some surcharging and surface inundation. Therefore, these detailed hydraulic analysis to identify particular restrictions in the storm sewer system are now carried out on a case-by-case basis as smaller focused areas of the city are investigated for potential stormwater management issues and/or improvements. Another objective of the hydraulic analyses was to evaluate the level of protection offered by the current stormwater system. Level of protection is defined as the pipe and overland overflow capacity provided by a municipal drainage system to prevent damage to principal structures damage and assure a reasonable degree of public safety following a rainstorm. A 1-percent-annual-chance event has historically been and often been recommended as a standard for the design of stormwater management basins. To evaluate the level of protection of the stormwater system within each major drainage area, the 1-percent-annual- chance frequency flood elevations for the ponding basins and depressed areas were compared to the approximate low elevations of structures surrounding each basin. The approximate low elevations were determined using the 2011 MnDNR LiDAR elevation data in ArcMap where actual survey data was not available. The LiDAR elevation may not be representative of the actual lowest elevation; therefore, a field survey would need to be conducted to determine potential impacts. The inundation areas predicted to potentially impact structures during the 1-percent-annual-chance storm event are shown in the City’s interactive web map. The detailed study areas modeled using 1D/2D modeling were chosen to further quantify the level of protection in these areas and provide more detailed results. Additionally, proposed improvements to the stormwater system to increase the level of protection in these areas were evaluated. Discussion and recommended improvement considerations for these areas are included in Sections A.2 through A.11. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.1-10 A.1.1.3.1 Problem Areas Selection Process The Atlas 14 XPSWMM modeling analysis performed in developing the City’s 2018 WRMP identified many locations throughout the city where the 1-percent-annual-chance flood elevations appear to impact roadways and/or structures. As part of this plan, some of these flood-prone areas were evaluated further (at varying levels of detail) to better understand the problem(s) and identify potential improvement options. The discussion below summarizes the criteria and approach taken to identify and categorize the most severe flood-prone areas throughout the city. The inundation areas were given a score based on meeting some or all of the following criteria that are based on 1-percent-annual-chance flood event modeling results (except where otherwise noted): • 1 point for each potentially impacted residential home • 1 point for each potentially impacted apartment building • 1 point for each potentially impacted commercial building, as defined by a GIS structure layer provided by the City • 1 point for road flooding for roads identified as an evacuation route, as defined by an evacuation route GIS layer provided by the City • 1 point for each area where the depth of flooding over a road is greater than or equal to 2 feet, and the road is not an evacuation route, with evacuation routes defined by a GIS layer provided by the City • 1 point for each area where a public building (school, church, fire station, daycare) is potentially impacted with public buildings defined by a GIS layer provided by the City • 1 point for each area where the flooding problem is a regional problem (versus flooding in a backyard depression area with no outlet) • 2 points for each area where the flooding from the 10-percent-annual-chance precipitation event is similar to the 1-percent-annual-chance precipitation event, indicating higher probability of impacts • 2 to 14 points based on the duration of flooding during the 1-percent-annual-chance precipitation event. Areas where the lowest structure is potentially impacted for a short time (less than 1 hour) received 2 points, whereas areas where the lowest structure is potentially impacted for a long time (greater than 72 hours) received 14 points. • 3 points for any area that impacts a water facility (treatment, wells, towers, etc.), as identified by a GIS layer provided by the City The top 80 flood-prone areas based on the 2018 CWRMP modeling were identified (“registered”) as most severe, based on the criteria presented above. In general, inundation areas that potentially impacted less than five residential homes with no other criterion did not register within the top approximately 80 identified flood-prone areas. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.1-11 The registered flood-prone areas were reviewed further; five areas were selected to be studied in detail and another 20 areas were selected for a screening-level review. The identification process was objective, data-driven, and was collaborative between City staff and Barr Engineering staff. The resulting 25 flood- prone areas that were identified for further study during the 2018 WRMP update are discussed in Sections A.2 through A.11. Detailed study of the five areas consisted of evaluating the nature of the problem based on 1D modeling results and identifying and modeling potential improvement options to reduce flood risk, including combinations of the proposed improvement options. Since adoption of the 2018 CWRMP, 1D/2D models were developed for the five areas to refine the results and further evaluate the nature of the problems and identify and model additional improvement options to reduce flood risk (see modeling updates in the Executive Summary). Results of these analyses have been incorporated into Sections A.4, A.9, and A.10). For the 20 flood-prone areas selected for a screening level analysis, a desktop review was conducted to assess the nature of the problem and identify potential improvement options (without additional modeling analysis). The remaining registered flood-prone areas that were not evaluated in detail or at a screening level are included for future evaluation. Inundation areas that did not register in the top 80 areas are not described further in the narrative of this plan; however, they are shown in the City’s interactive web map. A.1.2 Methodology for Water Quality Modeling Water quality modeling was not updated for the 2018 WRMP or for this iteration of the WRMP. The following sections are based on the work completed for the 2003 WRMP. As described in this 2022 WRMP the City will be developing a Clean Water strategy to target and prioritize pollutant reduction. P8 (Program for Predicting Polluting Particle Passage through Pits, Puddles and Ponds, IEP, Inc., 1990) is a computer model used for predicting the generation and transport of stormwater runoff pollutants in urban watersheds. The P8 model was used in this study to simulate the hydrology and phosphorus loads introduced from the watershed of each pond and the transport of phosphorus throughout the stormwater system. P8 is a useful diagnostic tool for evaluating and designing watershed improvements and BMPs. The model requires user input on watershed characteristics, pond attributes, local precipitation and temperature, and other parameters relating to water quality and pond removal performances. A.1.2.1 Watershed Characteristics Examination of the watershed characteristics for each pond being modeled involved assessment of soil type, land use and residential density, and the impervious fraction of the land in the watershed. ArcMap software was used extensively in assessing the watershed characteristics. The software also allowed mapping of the drainage network for the area. In P8, pervious and impervious areas are modeled separately. Runoff volumes from pervious areas are computed using the Soil Conservation Service (SCS) Curve Number method. Runoff from impervious areas begins once the cumulative storm rainfall exceeds the specified depression storage, with the runoff rate equal to the rainfall intensity. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.1-12 A.1.2.1.1 Impervious Fraction Because P8 calculates runoff separately from pervious and impervious areas, it was necessary to determine the impervious fraction of each watershed. For the P8 model, the impervious fraction included only the directly-connected impervious area, the impervious surfaces that are “connected” directly to a stormwater conveyance system, where stormwater does not cross over pervious areas. The directly- connected impervious fraction was calculated for each watershed based on the land use(s) within the watershed and impervious fraction assumptions for each land use. The assumptions made for the total impervious fraction and directly-connected impervious fraction for each land use for the water quality modeling are listed in Table A.1.6. Table A.1.6 Land Use Impervious Fraction Assumptions for Water Quality Modeling Land Use Designation Total Impervious % Directly-Connected Impervious % Commercial 90% 80% Golf Course 5% 2% Highway 50% 50% Industrial/Office 90% 80% Institutional 40% 20% Institutional- High Imperviousness 70% 50% Natural/Park/Open 2% 0% Open Water 100% 0% Residential- Very Low Density 12% 8% Residential- Low Density 40% 20% Residential- Medium Density 55% 30% Residential- High Density 70% 40% Wetlands 0% 0% A.1.2.1.2 Pervious Curve Number Watershed runoff volumes from pervious areas are computed in P8 using the SCS Curve Number method. Thus, it was necessary to determine a pervious curve number for each watershed. The soil type(s) within each watershed were determined and a pervious curve number was selected for the watershed based on the soil type, land use, and hydrologic conditions (e.g., if watershed soils are Type B and pervious areas comprise grassed areas with >75 percent cover, then a curve number of 61 would be selected). The City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.1-13 pervious curve number was then weighted with the indirect (i.e., unconnected) impervious area in each subwatershed as follows: ()()[]Area) (PerviousArea Impervious Indirect Number)] Curve (Pervious* Area) [(Pervious +(98)] * Area Impervious Indirect[ = CNwt + A.1.2.1.3 Other P8 Watershed Input Parameters Outflow Device Number: The Device Number of the device receiving runoff from the watersheds was selected to match the pond or manhole node ID used for the hydrologic/hydraulic modeling. Swept/Not Swept: An “Unswept” assumption was made for the entire impervious watershed area. A Sweeping Frequency of 0 was selected. Selected parameters were placed in the “Unswept” column since a sweeping frequency of 0 was selected. Depression Storage = 0.03 (P8 default value) Impervious Runoff Coefficient = 0.94 (P8 default value) A.1.2.2 Treatment Device Characteristics The treatment devices in P8 provide collection, storage, and/or treatment of watershed discharges. A variety of treatment devices can be modeled in P8, including detention ponds (wet or dry), infiltration basins, swales and buffers, aquifers, and pipe/manholes. For this study, nearly all ponds were modeled as detention basins. The user-defined characteristics of these ponds are described in the following sections. A.1.2.2.1 Dead Storage Detailed information pertaining to the permanent pool storage volume (dead storage) was only available for a small number of the ponds that were modeled. Pond depth data for the ponds in the Mirror Lake watershed was available as a result of pond surveys being performed for the Draft Mirror Lake Use Attainability Analysis (Barr Engineering, 2004). Pond depth information for Indianhead Lake was available from the MnDNR. Where detailed information on pond depths was not available, it was necessary to make assumptions. The surface area of each pond was determined from the 2-foot topographic information provided by the City. Where detailed information was not available, pond depths were estimated based on the type of wetland, which was determined in the wetland inventory process. An average depth of 4 feet was typically assumed for Type 5 wetlands; 2 feet for Type 3 and Type 4 wetlands; 0.5 feet for Type 1, 2, 6, and 7 wetlands. A.1.2.2.2 Live Storage The flood pool storage volume (live storage) for each pond was calculated in ArcMap using the electronic topographic data provided by the City. The live storage represents the storage volume between the City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.1-14 normal water elevation and the flood elevation. The overflow elevation from each pond was determined from the 2-foot topographic data. The live storage volume was then calculated in ArcMap based on the slope of the flood pool. A.1.2.2.3 Other P8 Treatment Device Input Characteristics Infiltration Rate (in/hr): An infiltration rate was entered only for land-locked detention ponds. The rates applied were dependent upon the type of soil surrounding each pond. The infiltration rates used for each soil type are listed in Table A.1.7. Table A.1.7 Infiltration Assumptions for Water Quality Modeling Hydrologic Soil Group Infiltration Rate Assumption for Dead Storage Areas [in/hr] Infiltration Rate Assumption for Live Storage Areas [in/hr] A 0.02 0.06 B 0.015 0.05 C 0.015 0.02 D 0.005 0.01 • Orifice Diameter and Weir Length: The orifice diameter or weir length of the pond outlet was determined from storm sewer system data provided by the City of Edina. For landlocked basins, the overflow was represented as a weir, with the weir length estimated using ArcMap and available topographic information. • Particle Removal Scale Factor: 0.3 for ponds less than 2 feet deep and 1.0 for all ponds 3 feet deep or greater. For ponds with normal water depths between 2 and 3 feet, a particle removal factor of 0.6 was selected. These factors were selected based on development of a similar P8 model for the Round Lake Use Attainability Analysis (Barr Engineering, 1999). • Pipe/Manhole— Time of Concentration: The time of concentration for each pipe/manhole device was determined and entered here. Time of concentration was determined in accordance with Kirpich’s method (Schwab et al., 1993). A.1.2.3 Precipitation and Temperature Data The P8 model requires hourly precipitation and daily temperature data; long-term data can be used so that watersheds and BMPs can be evaluated for varying hydrologic conditions. Hourly precipitation data was obtained from the Minneapolis-St. Paul International Airport for October 1994 through September 1995 (1995 water year, which represents average yearly precipitation). Average daily temperature data was obtained from the National Weather Service site at the Minneapolis-St. Paul International Airport. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.1-15 A.1.2.4 Selection of Other P8 Model Parameters A.1.2.4.1 Time Step, Snowmelt, and Runoff Parameters • Time Steps per Hour (Integer) = varied. This parameter varied between each P8 model. Selection was based upon the number of time steps required to eliminate continuity errors greater than 2 percent. • Minimum Inter-Event Time (Hours) = 10. The selection of this parameter was based upon an evaluation of storm hydrographs from the summer of 1999 to determine which storms should be combined and which storms should be separated to accurately depict runoff from the lake’s watershed. Precipitation data from 1999 was used for the analysis due to the high frequency of storms during the summer, particularly during July. • Snowmelt Factors—Melt Coef (Inches/Day-Deg-F) = 0.03. The P8 model predicts snowmelt runoff beginning and ending earlier than observed snowmelt. The lowest coefficient of the recommended range was selected to minimize the disparity between observed and predicted snowmelt (i.e., the coefficient minimizes the number of inches of snow melted per day and maximizes the number of snowmelt runoff days). • Snowmelt Factors— Scale Factor for Max Abstraction = 1. This factor controls the quantity of snowmelt runoff (i.e., controls losses due to infiltration). Selection of this factor was based upon other calibrated P8 models developed for lakes within the metropolitan area (Reference Glen Lake, Smetana Lake). • Growing Season AMC-II = .05 and AMC-III = 100. Selection of this factor was based upon calibration efforts for the P8 model developed for the Glen Lake Use Attainability Analysis (Barr Engineering, 1999). In development of this calibrated model, it was observed that the model accurately predicted runoff water volumes from monitored watersheds when the Antecedent Moisture Condition II was selected (i.e., curve numbers selected by the model are based upon antecedent moisture conditions). Modeled water volumes were less than observed volumes when Antecedent Moisture Condition I was selected, and modeled water volumes exceeded observed volumes when Antecedent Moisture Condition III was selected. The selected parameters direct the model to only use Antecedent Moisture Condition I when less than 0.05 inches of rainfall occur during the 5 days prior to a rainfall event and to only use Antecedent Moisture Condition III if more than 100 inches of rainfall occur within 5 days prior to a rainfall event, thus causing the model to simulate Antecedent Moisture Condition II throughout the majority of the simulation period. A.1.2.4.2 Particle File Selection The NURP50.PAR file was selected for the P8 models. The NURP 50 particle file represents typical concentrations and the distribution of particle settling velocities for a number of stormwater pollutants. The component concentrations in the NURP 50 file were calibrated to the 50th percentile (median) values compiled in the U.S. EPA’s Nationwide Urban Runoff Program (NURP). City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.1-16 A.1.2.4.3 Passes through the Storm File The number of passes through the storm file was determined after the model had been set up and a preliminary run completed. The selection of the number of passes through the storm file was based upon the number required to achieve model stability. Multiple passes through the storm file were required because the model assumes that dead storage waters contain no pollutants. Consequently, the first pass through the storm file results in lower pollutant loading than occurs with subsequent passes. Stability occurs when subsequent passes do not result in a change in pollutant concentration in the pond waters. To determine the number of passes to select, the model was run with five passes and 10 passes. A comparison of pollutant predictions for all devices was evaluated to determine whether changes occurred between the two scenarios. If there is no difference between five and 10 passes, five passes are sufficient to achieve model stability. This parameter was determined for all of the P8 model areas and no differences were noted between five and 10 passes. Therefore, it was determined that five passes through the storm file resulted in model stability for these models. A.1.2.5 Stormwater System Analysis The effectiveness of the stormwater system in removing stormwater pollutants such as phosphorus was analyzed using the P8 water quality model. The P8 model simulates the hydrology and phosphorus loads introduced from each pond’s watershed and the transport of phosphorus throughout the stormwater system. Since site-specific data on pollutant wash-off rates and sediment characteristics were not available, it was necessary to make assumptions based on national average values. Because of these assumptions and lack of in-lake water quality data for model calibration, the analysis of modeling results was based on the percent of phosphorus removed and not on actual phosphorus concentrations. Figures included in this Appendix illustrate the results of the water quality modeling for each of the major drainage basins. The results present the fraction of total phosphorus removal for each water body as well as the cumulative total phosphorus removal in the watershed. The individual water bodies are colored various shades of blue, indicating the percent of the total annual mass of phosphorus entering the water body that is removed (through settling). It is important to note that the percent of phosphorus removed is based on total phosphorus, including phosphorus in the soluble form. Therefore, the removal rates in downstream ponds will likely decrease due to large soluble fractions of incoming phosphorus that could not settle in upstream ponds. The watersheds are depicted in various shades of gray, indicating the cumulative total phosphorus removal achieved. The cumulative percent removal represents the percent of the total annual mass of phosphorus entering the watershed that is removed by the pond and all upstream ponds. Ponds that had an average annual total phosphorus removal rate of 60 percent or greater under average climatic conditions were considered to be performing well. For ponds with total phosphorus removal below 60 percent, the permanent pool storage volume was analyzed to determine whether additional capacity is necessary. Based on recommendations from the MPCA publication Protecting Water Quality in Urban Areas (March 2000), the permanent pool for detention ponds should be equal to or greater than the runoff from a 2.0-inch rainfall, plus sediment storage for at least 25 years of accumulation. This City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.1-17 recommended permanent pool storage volume was calculated for each pond with less than 60 percent total phosphorus removal and compared to the existing storage volume. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.2-1 A.2 Nine Mile Creek—North A.2.1 General Description of Drainage Area The City’s interactive web map depicts the Nine Mile Creek—North drainage area. The Nine Mile Creek— North drainage area is located in the northwest portion of Edina. The drainage area encompasses approximately 2,050 acres that ultimately drain to the stretch of Nine Mile Creek between the intersection of the North fork with TH 169 and TH 62. A.2.1.1 Drainage Patterns The stormwater system within this drainage area comprises storm sewers, ponding basins, wetlands, drainage ditches, and overland flow paths. The Nine Mile Creek—North drainage area has been divided into several major watersheds based on the drainage patterns. These major watersheds are depicted in the City’s interactive web map as “Major Watersheds” Each major watershed has been further delineated into numerous subwatersheds. The naming convention for each subwatershed is based on the major watershed where it is located. Table A.2.1 lists each major watershed and the associated subwatershed naming convention. Table A.2.1 Major Watersheds within the Nine Mile Creek—North Drainage Basin Major Watershed Subwatershed Naming Convention Mirror Lake ML_## Highlands Lake HI_## Hawkes Lake HL_## Mud Lake MD_## Nine Mile North NMN_## / EdCrk## A.2.1.1.1 Mirror Lake The Mirror Lake major watershed is located in the northwest portion of Edina. The 288-acre watershed contains six ponding basins of varying sizes that drain to Mirror Lake via storm sewer. There are also three landlocked ponds within the watershed. The Mirror Lake watershed is almost entirely residential area, with the exception of a few subwatersheds that are a part of the Interlachen Country Club golf course. Mirror Lake spans approximately 23 acres. The water level of the lake is controlled at an elevation of approximately 908,5 feet by a pumped outlet (4 cubic feet per second, or cfs) on the southwest side of the lake. The pumped outlet flows in a southwest direction and eventually connects to the storm sewer system along Blake Road. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.2-2 A.2.1.1.2 Highlands Lake The Highlands Lake watershed is located east of Mirror Lake and north of Vernon Avenue. This 276-acre watershed is bordered on the north by Interlachen Boulevard and portions of the Interlachen Country Club golf course. The Highlands Lake watershed land use is characterized by residential areas, part of the Interlachen golf course, several ponding basins that ultimately drain to Highlands Lake, a wetland area directly east of the lake referred to as East Basin, Highlands Park directly south of the lake, and a portion of the drainage from Highlands Elementary School. Highlands Lake spans approximately 11 acres. The water level of Highlands Lake is controlled at elevation of 888.4 feet by a pumped outlet (approximately 1 cfs). The lift station was installed in 1994 and is located in the southwest corner of Highlands Park, near the intersection of Ayrshire Boulevard and Glengarry Parkway. Water from the pumped outlet flows south, connecting with the storm sewer system along Vernon Avenue, which discharges into Hawkes Lake. A.2.1.1.3 Hawkes Lake The Hawkes Lake watershed is located south of the Highlands Lake watershed. The approximately 300-acre watershed includes residential (low and high density), institutional, open space/parks, and a small commercial area. Portions of Highland Elementary and Countryside Elementary drain to Hawkes Lake. Garden Park is also located within this watershed. There are several ponding basins within the watershed that drain to Hawkes Lake via drainage ditches and storm sewer systems. Hawkes Lake spans approximately 6 acres. A pumped outlet at an elevation of 888 feet limits flooding conditions in Hawkes Lake. However, the average water level for this lake between the period of 1963 to 2001 is 885.5 feet (MnDNR Lake Finder webpage http://www.dnr.state.mn.us/lakefind/index.html). The pumped outlet, located on the western side of the lake near the intersection of Wycliffe Road and Merold Drive, discharges southwest into the Mud Lake watershed. A.2.1.1.4 Mud Lake (Bredesen Park) The Mud Lake watershed spans an area of approximately 432 acres and is predominantly residential (ranging from very low to medium density). The watershed has a complex drainage system characterized by numerous ponding basins connected by storm sewer and a unique diversion structure that routes stormwater from the northwest portion of the watershed to the North Fork of Nine Mile Creek or Mud Lake, depending on the amount of precipitation and the water level of the pond near the structure. The diversion structure is located within a manhole on Blake Road, between Saxony Road and Jeffrey Lane, and receives water from the upstream Blake Road storm sewer system and Mirror Lake outlet. The structure controls the amount of incoming water that flows southward toward Mud Lake or into the pond directly west of Blake Road and encircled by Knoll Drive (Knoll Pond). A small orifice in the structure directs flows from small precipitation events southward to Mud Lake. A weir structure within the manhole directs larger stormwater flows into the adjacent Knoll Pond. During large rain events, stormwater will continue to flow into Knoll Pond until the water level within the pond has equalized with the weir elevation; at that point, the remainder of the water will drain south towards Mud Lake. In the event that inflow to Knoll Pond from upstream watersheds surpasses the capacity of the normal outlet to the North Fork of Nine Mile Creek, the diversion structure will act as a second overflow from the pond, directing water to Mud Lake. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.2-3 Mud Lake is a meandered lake located just east of the North Fork of Nine Mile Creek, between TH 62 and Vernon Avenue. The lake and surrounding wetlands are part of Bredesen Park. The park spans an area of approximately 126 acres and is a mix of open water, wet marsh, and floating bog. Mud Lake outlets to the North Fork of Nine Mile Creek just north of TH 62 through a culvert at elevation 849 feet. A.2.1.1.5 Nine Mile North The Nine Mile North watershed encompasses the area in northwest Edina that drains directly to the floodplain of the North Fork of Nine Mile Creek. The floodplain within this area is relatively flat and is characterized by wetland conditions. The Nine Mile North watershed is bounded by TH 169 on the west and Schaeffer Road on the east, extending north to Malibu Drive and south to TH 62. The 718-acre watershed drains to the creek through a series of storm sewer systems and stormwater detention ponds. There are a wide range of land uses within the watershed, including residential (very low density, low density, and medium density), industrial, commercial, highway, wetlands, and natural/park area. A.2.2 Stormwater System Results A.2.2.1 Hydrologic/Hydraulic Modeling Results The 10- and 1-percent-annual-chance flood analyses were performed for the Nine Mile Creek—North drainage basin. The 10-percent-annual-chance analysis was based on a 24-hour storm with 4.29 inches of rain. The 1-percent-annual-chance analysis was based on a 24-hour storm event with 7.47 inches of rain and on a 10-day snowmelt event with 7.2 inches of runoff; the higher resulting flood level of the two events was chosen for the 1-percent-annual-chance analysis. Data available from the City’s interactive web map presents the watershed information and results for the 1-percent-annual-chance frequency hydrologic analyses for the Nine Mile Creek—North basin. A more detailed description of the stormwater system analysis is provided in Section A.1.1.3. The City’s interactive web map depicts the Nine Mile Creek—North drainage basin boundary, subwatershed boundaries, the modeled storm sewer network, and the 1-percent-annual-chance flood inundation areas identified in the modeling analyses. To evaluate the level of protection of the stormwater system within the Nine Mile Creek—North drainage area, the 1-percent-annual-chance flood elevations for the ponding basins and depressed areas were compared to the elevations of structures surrounding each basin. The areas predicted to be inundated during the 1-percent-annual-chance storm event are shown on the City’s interactive web map. Discussion and recommended improvement considerations for these areas are included in Section A.2.3. A.2.2.2 Water Quality Modeling Results The effectiveness of the stormwater system in removing stormwater pollutants such as phosphorus was analyzed using the P8 water quality model. The P8 model simulates the hydrology and phosphorus loads introduced from each pond’s watershed and the transport of phosphorus throughout the stormwater system. A more detailed description of the stormwater system analysis is provided in Section A.1.1.3. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.2-4 Figure A.2.1 depicts the results of the water quality modeling for the Nine Mile Creek—North drainage basin. The figure shows the fraction of total phosphorus removal for each water body as well as the cumulative total phosphorus removal in the watershed. A.2.3 Implementation Considerations The XPSWMM hydrologic and hydraulic modeling analyses in support of the 2018 WRMP and the 2003 P8 water quality analysis helped identify locations throughout the watershed where improvements to the City’s stormwater management system may be warranted. The following sections discuss potential improvement options identified as part of the modeling analyses. As opportunities to address identified flooding issues and improve water quality arise (e.g., street reconstruction projects or public facilities improvements), the City will use a comprehensive approach to stormwater management. This approach will include consideration of infiltration or volume retention practices to address flooding and/or improve water quality, reduction of impervious surfaces, increased storm sewer capacity where necessary and feasible to alleviate flooding, construction and/or expansion of water quality basins, and implementation of other stormwater BMPs to reduce pollutant loading to downstream waterbodies. A.2.3.1 Potential Flood Risk Reduction Options The 2018 WRMP identified several locations within the Nine Mile Creek—North drainage basin where the 1-percent-annual-chance flood elevations may impact structures or public safety. Several of these problem areas, and potential corrective measures, were evaluated as part of 2018 WRMP development and are discussed in more detail in this section. There may be additional flood-prone areas within the drainage basin that warrant further analysis— see the City’s interactive web map for the 1-percent- annual-chance flood inundation areas. The 2003 hydrologic and hydraulic modeling analyses also identified several locations within the Nine Mile Creek—North drainage basin where the 1-percent-annual-chance level of protection was not provided by the stormwater system, based on TP-40 precipitation frequency estimates. The discussions related to those areas are included in Appendix B of the 2022 WRMP, along with a short summary of what has been done in those areas since 2003. A.2.3.1.1 Maloney Avenue and Tyler Court (ML_35 and ML_19) On either side of Tyler Court, south of Maloney Avenue, there are local depressions with inlets and outlets draining to Mirror Lake. Stormwater runoff from a drainage area of approximately 29 acres discharges into this area. These depressions are drained by a 24-inch pipe. However, downstream, at the intersection of Arthur Street and Waterman Avenue, the pipe flowing east toward Mirror Lake is reduced to 18 inches. Modeling results indicate that the 1-percent-annual-chance flood elevation in this area (940.0 feet west of Tyler Court and 938.1 feet east of Tyler Court) exceeds the low house elevations at 505, 509, and 513 Tyler Court (surveyed at 932.9, 933.1, and 934.07 feet, respectively in 2003) and the apparent low elevations at 500 and 508 Tyler Court. Additionally, the 1-percent-annual-chance flood elevation is more than 2 feet higher than the low elevation on Tyler Court, with the duration of the impacts more than 60 hours, based City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.2-5 on a 24-hour duration storm event. This area was also identified as an area of concern in the 2003 and 2009 WRMPs. The flooding problem in this area is primarily related to the capacity of the storm sewer system between this area and Mirror Lake. To alleviate a portion of the flooding problem, it is recommended that the 18-inch pipe flowing east from the Arthur Street and Waterman Avenue intersection be upgraded to a larger pipe. There are currently no potentially impacted principal structures around Mirror Lake during the 1-percent-annual-chance 24-hour storm event, which is more critical than the 10-day snowmelt event. Therefore, the additional conveyance of water to Mirror Lake does not appear to create problems downstream. Further analysis is needed to balance reducing impacts in this area without creating impacts around Mirror Lake. A.2.3.1.2 Between Leslee Lane and Kaymar Drive (MD_22) A backyard depression area exists between the properties on the south side of Leslee Lane and north side of Kaymar Drive. The depression area collects stormwater from a drainage area of approximately 10 acres with 6.4 acres directly contributing. A 15-inch pipe extends southward into the backyard depression area from Leslee Lane, collecting stormwater from the low area, and continues west toward Jeffrey Lane. This system eventually connects with the Blake Road South system at the intersection of Blake Road South and Kaymar Drive. During the 1-percent-annual-chance 24-hour storm event, flow is restricted in the 15-inch pipe and water pools in the backyard depression area to a peak flood level of 916.8 feet. Eventually, the water overflows to the west between 6104 Jeffrey Lane and 6017 Leslee Lane. Based on the LiDAR data and the approximate building footprints, it appears that seven principal structures may be impacted (6016–6028 Kaymar Drive and 6001–6009 Leslee Lane). This area was also identified as an area of concern in the 2003 and 2009 WRMPs. To reduce the backyard flooding, it is recommended that the size of both the 15-inch pipe draining the backyard depression and the downstream 18-inch pipe be increased to increase discharge capacity. It is also recommended that the surface overflow from this area to Jeffrey Lane be lowered if existing or future road grades allow for it. At a minimum, the existing surface overflow should be maintained. A.2.3.1.3 Parkwood Road and Schaefer Road (MD_28, MD_29, and MD_35) A small, 0.5-acre stormwater detention pond is located just northwest of the intersection of Schaefer Road and Parkwood Road. The outlet from the pond is a 12-inch pipe that drains south to Parkwood Road, east toward Blake Road, and then south toward the wet pond along Knoll Drive. During the peak of the 1- percent-annual-chance 24-hour storm event, inundation occurs along Parkwood Road due to downstream pipe capacity restrictions and overflows to the south between 6213 and 6217 Parkwood Road (approximately 936.5 feet based on LiDAR data). As the water level rises along Parkwood Road, flow from the detention pond west of Schaefer is reduced due to tailwater effects. The detention pond then fills up, overtopping Schaefer Road to the east, at an elevation of approximately 937.6 feet, to another stormwater detention basin in MD_29. Inundation from the 1-percent-annual-chance event appears to impact four principal structures within this area: 6216 and 6217 Parkwood Road, 6316 Westwood Court, and 5316 Schaefer Road. This area was also identified as an area of concern in the 2003 and 2009 WRMPs. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.2-6 Due to the presence of downstream flood-prone structures, increasing downstream pipe capacity is not recommended. Review of LiDAR data and approximate building footprints indicates there may be impacts to the principal structures identified above. A survey of the low entry elevations of these structures is recommended. It is also recommended that the surface overflows across Schaefer Road, between 6213 and 6217 Parkwood Road, and between 6212 and 6216 Parkwood Road be maintained or slightly lowered during any future road improvement projects in this area. Underground storage may be an option to reduce flood risk in this area. In particular, Parkwood Road has a significant depression area. If Parkwood Road is reconstructed it could be regraded to have positive drainage to the east with storage under Parkwood Road. Soil maps indicate that the soils here are “B” soils, with moderate infiltration capacity. The effectiveness of underground storage could be optimized by allowing infiltration from the bottom of the underground system. A.2.3.1.4 Schaefer Road and View Lane (MD_38) Adjacent to Bredesen Park on the west side, where View Lane, Schaefer Road, and Killarney Lane South meet, there is a backyard depression with a private lift station. Surface overflow to the east, between 6008 and 6012 Schaefer Road, occurs at an elevation of approximately 867.5 feet (based on LiDAR data), which is higher than the 1-percent-annual-chance flood level. This particular area has a history of flooding and of private efforts to reduce flooding. Modeling results indicate that during the 1-percent-annual-chance 24-hour storm event, runoff from the MD_38 watershed would drain to the backyard depression, resulting in a peak flood level of 865.0 feet, and may impact 6021 View Lane and 6016–6032 Schaefer Road. It is recommended that a gravity outlet be installed from the backyard depression area to Bredesen Park to lower the flood elevation in the depression area. This proposed gravity outlet would supplement or replace the private lift station. Due to the larger area of Bredesen Park, and the relatively small volume of water that would be conveyed from the backyard depression, the impact to flood elevations in Bredesen Park would likely be negligible. Further analysis is warranted to verify. A.2.3.1.5 Nine Mile Village Townhomes (MD_49) The Nine Mile Village Townhomes are located east of Bredesen Park, across Villa Lane. Flood water overtops Villa Lane (surface overflow at approximately 857.3 feet) during the 1-percent-annual-chance 24-hour storm event, with water flowing down Sandpiper Court towards several townhomes. These townhomes may be impacted at the 1-percent-annual-chance flood level (857.7 feet). Water can then overflow from MD_49, traveling southeast to a pond around Colonial Church (CO_2). Modeling results indicate that the peak flood level of the waterbodies within Bredesen Park during this event is 858.2 feet. Construction of a berm on the west side of Villa Lane is recommended to prevent surface overflows from waterbodies within Bredesen Park to the Nine Mile Village Townhomes. Potential surface overflows from Bredesen Park should be directed towards the low area between Sandpiper Court and Red Fox Lane, which eventually flows to the ponds around Colonial Church. If the opportunity arises, additional surface grading within the Nine Mile Village Townhomes complex, particularly in the southeast corner where MD_49 overflows into CO_2, should be considered to promote drainage of the local contributing watershed through a surface overflow. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.2-7 A.2.3.1.6 Hawkes Lake and Upstream Surrounding Area (HL_1, HL_11c, HL_11w, HL_49, and HL_12) Hawkes Lake is in the Nine Mile Creek watershed, south of Vernon Avenue South and west of Tracy Avenue. The lake has many inlet points from all directions and one pumped outlet. The pumped outlet drains through a gravity system under Merold Drive to the south, crosses Amy Drive, and eventually discharges to a wetland area (subwatershed MD_3) within Bredesen Park. The 1-percent-annual-chance flood level of Hawkes Lake (HL_1) and the immediately surrounding area is determined by the 10-day snowmelt event (894.1 feet). The 1-percent-annual-chance flood level of HL_12 to the east of Hawkes Lake is determined by the 24-hour precipitation event (903.1 feet). Based on modeling results, LiDAR data, and approximate building footprints, there are nine principle structures around Hawkes Lake (5621, 5625, 5629, and 5701 Wycliffe Road, 5708 and 5705 Warden Avenue, 5717 Hawkes Drive, and 5712 and 5716 Tracy Avenue), six principle structures immediately north of Vernon Avenue South (5533, 5537, and 5604 Dundee Road, 5532, 5536, and 5541 Mirror Lakes Drive), and one principle structure near Warden Avenue (5537 Warden Avenue) that are potentially impacted at the 1-percent-annual-chance flood level. It is recommended that capacity of the pumped outlet and the downstream gravity storm sewer system be increased to reduce flood risk around Hawkes Lake; however, this may require that the capacity be increased all the way downstream to Bredesen Park, where the downstream impacts are expected to be minimal. The capacity of the storm sewer systems draining to Hawkes Lake, particularly from north of Vernon Avenue South and from Warden Avenue, should also be increased to alleviate flooding in the areas upstream of Hawkes Lake. Providing additional flood storage within the city-owned parcel in subwatershed MD_15 could be considered to mitigate the additional flows to Bredesen Park, if needed, assuming flood elevations in MD_15 are not increased and the existing surface overflow elevation is maintained or lowered. A cursory analysis of this option was completed by the city in 2015 in preparation for the city’s 2016 street reconstruction project (Countryside H). A.2.3.2 Potential Construction/Upgrade of Water Quality Basins The 2003 P8 modeling analysis indicated that under average conditions the annual removal of total phosphorus from several ponds in the Nine Mile Creek—North drainage area was predicted to be below the desired 60 percent removal rate. For those ponds with total phosphorus removal below 60 percent, the permanent pool storage volume was analyzed to determine whether additional capacity is necessary. The ponds with deficiencies in total phosphorus removal and permanent pool volume are listed below (and are also summarized in Appendix C of the 2022 WRMP), with recommended pond upgrades. Construction of new or expansion of existing water quality basins is one way to increase pollutant removal prior to stormwater reaching downstream waterbodies. Many additional techniques are available to reduce pollutant loading, including impervious surface reduction or disconnection, implementation of infiltration or volume-retention BMPs, installation of underground stormwater treatment structures and sump manholes, and other good housekeeping practices such as street sweeping. As opportunities arise, the City will consider all of these options to reduce the volume and improve the quality of stormwater runoff. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.2-8 A.2.3.2.1 MD_15 Pond MD_15 is located just north of the 5904, 5908, and 5912 Sun Road properties, south of Amy Drive. The pond receives runoff from an area of approximately 25 acres. Pond MD_15 outlets to the storm sewer system along Sun Road via an 18-inch RCP pipe. The pond is a Type 5 wetland and was assumed to have an average depth of 4 feet. Based on this depth assumption and the 2-foot topographic information for the pond, the current permanent pool storage is 1.1 acre-feet. This is less than the MPCA-recommended storage volume for detention basins. It is recommended that an additional 0.3 acre-feet of dead storage volume be provided to meet the MPCA design criteria for detention basins. A.2.3.2.2 NMN_27 Pond NMN_27 is located northeast of the TH 62 and TH 169 intersection. The pond is south of Langford Court, directly east of Lincoln Road, and northwest of Waterford Court. The pond receives runoff from an area of approximately 37 acres, including drainage from TH 169 and Lincoln Drive. The pond is a Type 5 wetland and was assumed to have an average depth of 4 feet. Based on this depth assumption and the 2-foot topographic information for the pond, the current permanent pool storage volume is 1.7 acre-feet. This is less than the MPCA-recommended storage volume for Pond NMN_27. It is recommended that an additional 1.4 acre-feet of dead storage volume be provided to meet the MPCA design criteria for detention basins. A.2.3.2.3 NMN_24 Pond NMN_24 is located between Waterford Court and Habitat Court, downstream and to the southeast of Pond NMN_27. The pond receives runoff from a 5-acre watershed. The pond is a Type 4 wetland and was assumed to have an average depth of 2 feet. Based on this depth assumption and the 2-foot topographic information for the pond, the current permanent pool storage volume is 1.7 acre-feet. This is greater than the MPCA-recommended storage volume for detention basins; however, because water quality modeling results indicate that the total phosphorus removal in Pond NMN_24 is below desired levels, it is recommended that the depth of the pond be increased to 4 feet to improve removal efficiency. A.2.3.2.4 NMN_49 Pond NMN_49 is a sedimentation basin located directly west of the 5521 Malibu Drive property. The sedimentation basin receives runoff from a watershed of approximately 6 acres, in addition to incoming flows from the upstream sedimentation basin (NMN_48). Pond NMN_49 discharges to the North Fork of Nine Mile Creek. Based on storm sewer information from the City, a 2-foot average depth was assumed. Considering this depth assumption and the 2-foot topographic data for the pond area, the current permanent pool storage volume is 0.14 acre-feet. Compared to the calculated MPCA-recommended storage volume for Pond NMN_49, this is not an adequate amount of permanent pool storage for this basin. It is recommended that 0.2 acre-feet of dead storage volume be added to meet the MPCA design criteria for detention basins. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.2-9 A.2.3.2.5 MD_3 Pond MD_3 is located in Bredesen Park, directly east of the parking area off Olinger Boulevard. Pond MD_3 receives runoff from a 48-acre watershed, in addition to discharges from ponds MD_15 and MD_13. The pond is a Type 5 wetland and was assumed to have an average depth of 4 feet. Based on this depth assumption and 2-foot topographic information for the pond area, the current permanent pool storage volume is 4.7 acre-feet—greater than the MPCA-recommended storage volume for detention ponds. However, because water quality modeling results indicate that the total phosphorus removal in Pond MD_3 is below desired levels, it is recommended that the pond be excavated to remove accumulated sediment its depth increased to improve removal efficiency. Nine M i l e Creek H o p k i n sHopkins M i n n e t o n k aMinnetonka E d e n P r a i r i eEden P r ai r i e S t . L o u i s P a r kSt. L o ui s P a r k ML_1 MD_50 MD_1 HI_1 HL_1 MD_4 ML_16 MD_25 HI_13 MD_1 MD_50 ML_28 HL_44 ML_15 NMN_50 MD_2 ML_2 HI_18 HL_28 MD_21 NMN_75 ML_32 NMN_76 HI_5 MD_13 HI_20 ML_40 MD_2 MD_4 ML_26 MD_3MD_2 NMN_77 NMN_62 MD_11 HL_9 HL_50 HL_39 NMN_24 MD_7 HL_40 ML_38 MD_15 NMN_55 HL_24 ML_19 MD_28 HL_13 NMN_27 MD_2 NMN_84 MD_29 HI_17 NMN_20 NMN_63 HL_25 NMN_73 MD_39 ML_6 HI_21 HI_22 NMN_74 ML_3 ML_27 NMN_49 NMN_48 Barr Footer: ArcGIS 10.4.1, 2017-09-21 07:57 File: \\barr.com\gis\Client\Edina\Projects\CRWMP_Update_2017\Maps\Reports\Figures_CityReviewDraft\Fig_5_4_NMC_North_Water_Quality.mxd User: rcs2NINE MILE CREEK NORTHWATER QUALITY MODELING RESULTSWater ResourceManagement PlanCity of Edina, Minnesota FIGURE A.2.1 1,200 0 1,200Feet !;N 400 0 400Meters Percent TP Removal in Water Body*This number represents the percent of the total annual massof phosphorus entering the water body that is removed. Cumulative TP Removal in Watershed*This number represents the percent of the total annual massof phosphorus entering the watershed and upstream watershedsthat is removed in the pond and all upstream ponds. *Data based on results of 2003 P8 modeling. 0 - 25% (Poor/No Treatment) 25 - 40% (Moderate Removal) 40 - 60% (Good Removal) 60 - 100% (Excellent Removal) 0 - 25% (Poor/No Treatment) 25 - 40% (Moderate Removal) 40 - 60% (Good Removal) 60 - 100% (Excellent Removal) Area Draining Directly to the NorthFork of Nine Mile Creek Flow Direction Imagery Source: USDA 2016 NAIP via MnGeo City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.3-1 A.3 Nine Mile Creek—Central A.3.1 General Description of Drainage Area The City’s interactive web map depicts the Nine Mile Creek—Central drainage area, labeled as “Drainage Areas”. The Nine Mile Creek—Central drainage area is located in the central portion of Edina and encompasses 1,236 acres that ultimately drain to the stretch of the North Fork of Nine Mile Creek between TH 62 and West 70th Street. A.3.1.1 Drainage Patterns The stormwater system within this drainage area comprises storm sewers, ponding basins, drainage ditches, and overland flow paths. The Nine Mile Creek—Central drainage basin has been divided into three “Major Watersheds” based on the drainage patterns. These major watersheds are depicted in the City’s interactive web map. Each major watershed has been further delineated into many subwatersheds. The naming convention for each subwatershed is based on the major watershed where it is located Table A.3.1 lists each major watershed and the associated subwatershed naming convention. Table A.3.1 Major Watersheds within the Nine Mile Creek—Central Drainage Basin Major Watershed Subwatershed Naming Convention Colonial Ponds CO_## Indian Pond IP_## Nine Mile Central NMC_## A.3.1.1.1 Colonial Ponds The Colonial Ponds major watershed is located in central Edina and encompasses approximately 115 acres. The watershed is bordered by TH 62 to the south, Villa Lane on the west, extends northward to Benton Avenue, and slightly eastward past Westridge Boulevard. Six stormwater detention ponds are located within the watershed. The most downstream detention basin is located just south of the Colonial Church (subwatershed CO_1) and outlets to the North Fork of Nine Mile Creek via a 48-inch culvert under TH 62. The land use within the watershed is primarily residential, with the exception of the Colonial Church property and adjacent Countryside Park. A.3.1.1.2 Indian Pond The Indian Pond major watershed is located in central Edina, southwest of Creek Valley Elementary School. The 24-acre watershed is characterized by a single storm sewer system that drains to Indian Pond. Indian Pond is a land-locked basin. In the unlikely event of overflow from this pond, which would occur at an approximate elevation of 897 feet, the overflow would discharge to the intersection of Indian Hills Pass and Cherokee Trail. It would then be picked up by the Gleason Road storm sewer system and eventually City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.3-2 discharge to the North Fork of Nine Mile Creek, just northwest of the Edina High School complex. The land use within the Indian Pond watershed is low-density residential. A.3.1.1.3 Nine Mile Central The Nine Mile Central major watershed is also located in central Edina and spans approximately 1,097 acres. Stormwater within the watershed drains to the North Fork of Nine Mile Creek between TH 62 and West 70th Street via a network of ponding basins and storm sewer. The watershed extends north to the intersection of Hansen Road and West 56th Street and includes the area north of TH 62 that drains to the storm sewer system along the Soo Line railroad. The Soo Line storm sewer system flows beneath TH 62 and eventually discharges to the creek near the intersection of Valley Lane and Limerick Lane. The watershed is bordered by West 70th Street on the south, Gleason Road on the west, and TH 100 on the east. There are five stormwater detention basins within the Nine Mile Central watershed. The land use is characterized by residential areas, the Edina High School complex, freeway, several parks, the Soo Line Railroad, several ponding basins, and the floodplain of the North Fork of Nine Mile Creek. A.3.2 Stormwater System Results A.3.2.1 Hydrologic/Hydraulic Modeling Results The 10- and 1-percent-annual-chance flood analyses were performed for the Nine Mile Creek—Central drainage basin. The 10-percent-annual-chance analysis was based on a 24-hour storm with 4.29 inches of rain.. The 1-percent-annual-chance analysis was based on a 24-hour storm event with 7.47 inches of rain and on a 10-day snowmelt event with 7.2 inches of runoff; the higher resulting flood level of the two events was chosen for the 1-percent-annual-chance analysis. Data available from the City’s interactive web map presents the watershed information and the results for the 1-percent-annual-chance hydrologic analyses for the Nine Mile Creek—Central basin. A more detailed description of the stormwater system analysis is provided in Section A.1.1.3. City’s interactive web map depicts the Nine Mile Creek—Central drainage basin boundary, subwatershed boundaries, the modeled storm sewer network, and the flood- prone areas identified in the modeling analyses. To evaluate the level of protection of the stormwater system within the Nine Mile Creek—Central drainage area, the 1-percent-annual-chance frequency flood elevations for the ponding basins and depressed areas were compared to the low elevations of structures surrounding each basin. The areas predicted to be inundated during the 1-percent-annual-chance storm event are shown on the City’s interactive web map. Discussion and recommended improvement considerations for these areas are included in Section A.3.3. A.3.2.2 Water Quality Modeling Results The effectiveness of the stormwater system in removing stormwater pollutants such as phosphorus was analyzed using the P8 water quality model. The P8 model simulates the hydrology and phosphorus loads City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.3-3 introduced from each pond’s watershed and the transport of phosphorus throughout the stormwater system. A more detailed description of the stormwater system analysis is provided in Section A.1.1.3. Figure A.3.1 depicts the results of the water quality modeling for the Nine Mile Creek—Central drainage basin. The figure shows the fraction of total phosphorus removal for each water body as well as the cumulative total phosphorus removal in the watershed. A.3.3 Implementation Considerations The XPSWMM hydrologic and hydraulic modeling analyses in support of the 2018 WRMP and 2003 P8 water quality analysis helped identify locations throughout the watershed where improvements to the City’s stormwater management system may be warranted. The following sections discuss potential improvement options identified as part of the modeling analyses. As opportunities to address identified flooding issues and improve water quality arise (e.g., street reconstruction projects or public facilities improvements), the City will use a comprehensive approach to stormwater management. This approach will include consideration of infiltration or volume retention practices to address flooding and/or improve water quality, reduction of impervious surfaces, increased storm sewer capacity where necessary to alleviate flooding, construction and/or expansion of water quality basins, and implementation of other stormwater BMPs to reduce pollutant loading to downstream waterbodies. A.3.3.1 Potential Flood Risk Reduction Options The 2018 WRMP identified several locations within the Nine Mile Creek—Central drainage basin where the 1-percent-annual-chance level of protection is not provided by the current stormwater system. As part of the 2018 WRMP modeling analyses, potential corrective measures were identified for problem areas. These preliminary corrective measures are also discussed below. As the City evaluates flooding issues and potential system modifications in these areas, other potential modifications, including (but not limited to) volume-retention practices, increases in conveyance capacity, and/or stormwater infiltration (where soils are conducive) will be given consideration. The 2003 hydrologic and hydraulic modeling analyses also identified several locations within the Nine Mile Creek—Central drainage basin where the 1-percent-annual-chance level of protection was not provided by the stormwater system, based on TP-40 precipitation frequency estimates. The discussions related to those areas are included in Appendix B of the 2022 WRMP, along with a short summary of what has been done in those areas since 2003. A.3.3.1.1 Antrim Road and Chapel Drive (NMC_41) A depression exists in the backyards of the homes between Antrim Road and Erin Terrace, south of Chapel Drive. The backyard depression has a 12-inch CMP outlet that connects to the existing storm sewer system on Chapel Drive. Modeling results indicate that the 1-percent-annual-chance flood elevation (943.3 feet) in this area is determined by the 24-hour precipitation event and may exceed up to five low principal structure- elevations, based on LiDAR data and approximate building footprint information. The City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.3-4 problem is caused by the limited capacity of the 12-inch pipe and the capacity of the surface overflow between homes (approximate elevation of 942.5 feet based on LiDAR). It is recommended that a survey of low house elevations be conducted at 5901 and 5905 Chapel Drive, and 6812–6820 Antrim Road to determine potential flood impacts. Flood-proofing measures on the individual properties could be considered for those that are demonstrated to be impacted. Additionally, the natural surface overflow north to Chapel Drive between 5901 and 5905 Chapel Drive should be maintained, and potentially lowered if feasible. Increasing the capacity of the outlet pipe is not recommended due to the long length of pipe (approximately 5,000 feet) between NMC_41 and the ultimate discharge, Nine Mile Creek. Increasing the capacity at any point may require increasing the capacity in all downstream pipes from that point. A.3.3.1.2 Ridgeview Drive (NMC_106 and NMC_107) A depression exists in the backyards of the homes along Ridgeview Road, adjacent to the Soo Line railroad tracks south of West 66th Street. The backyard depression is not connected to a storm sewer pipe but does have a private lift station in NMC_107. Modeling results indicate that the flood elevation from the 1-percent-annual-chance 24-hour storm event (846.0 in NMC_106 and 844.5 in NMC_107) may exceed the low house elevations of 10 principal structures, based on LiDAR data and approximate building footprints. The results of the 24-hour precipitation event and the 10-day snowmelt event are nearly identical. It is recommended that a survey of low house elevations be conducted at 6700–6716 Ridgeview Road and 6808–6824 Ridgeview Road to determine potential for flood impacts. This area was also identified as an area of concern in the 2003 and 2009 WRMPs. The problem is due to the land-locked nature of these subwatersheds and the limited capacity of the private lift station. It is recommended that a gravity storm sewer system with a backflow preventer be installed that conveys stormwater from the backyard depression areas to Nine Mile Creek (on the other side of the railroad tracks). Finally, a storm sewer pipe could be added and connected to the existing storm sewer system at the intersection of Tifton Drive and Ridgeview Drive (upstream invert of existing system is approximately 840.7 feet). A.3.3.1.3 West 66th Street and Naomi Drive (NMC_71, NMC_74, and NMC_103) During intense rainstorms, flooding problems have historically occurred at the low-lying intersection of West 66th Street and Naomi Drive. The backyard depression area in the rear of the homes on the east and west sides of Naomi Drive and Normandale Park have also been subject to flooding. Stormwater overflow from the West 66th Street and Naomi Drive intersection flows into the adjacent Normandale Park storage area (ball field). The intersection and ball field are eventually drained by a 33-inch trunk storm sewer system that flows northwest to the low area along Warren Avenue and, eventually, west to the North Fork of Nine Mile Creek. Modeling results indicate that the 1-percent-annual-chance flood elevation (863.9 feet in NMC_103 and 863.8 feet in NMC_71 and NMC_74) exceeds the low house elevation of up to nine principal structures (6600 and 6604 Naomi Drive, 6605–6617 Naomi Drive, 6608 and 6612 Kenney Place, and 5177 West 66th Street). City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.3-5 The backyard depression area behind the Naomi Drive homes is drained by a 15-inch culvert that connects to the 15-inch pipe heading north from Circle Drive Pond. During periods of intense rainfall, the water in this system backs up and flows south into Circle Drive Pond. A flap gate has been installed on the culvert draining the backyard depression area to prevent backflow from inundating the area. However, with the flap gate closed, there is no outlet from this area and the backyard storage volume is not sufficient to prevent flooding of the structures along Naomi Drive. This flooding problem has been analyzed in the past and recommendations to alleviate the flooding were made, in which case some were implemented. The recommendations to add additional outlet capacity to the backyard depression area via a pumped outlet to the Normandale Park storage area or a separate gravity system flowing west to the North Fork of Nine Mile Creek, have not been implemented but should be considered further. If the recommendation to install a gravity outlet under the railroad tracks from NMC_106 and NMC_107 is implemented, additional storm sewer could be installed to connect this area to NMC_106. If a pumped outlet is installed to drain the backyard area, it will be necessary to add additional storage capacity in Normandale Park. A.3.3.2 Potential Construction/Upgrade of Water Quality Basins The 2003 P8 modeling analysis indicated that under average conditions the annual removal of total phosphorus from several ponds in the Nine Mile Creek—Central drainage area was predicted to be below the desired rate of 60 percent. For those ponds with total phosphorus removal below 60 percent, the permanent pool storage volume was analyzed to determine whether additional capacity was necessary. Based on the MPCA-recommended permanent pool storage volume for detention basins, all of the basins were found to have sufficient dead storage volume. As a result, no specific recommendations for water quality basin upgrades in the Nine Mile Creek—Central drainage basin have been made based on the 2003 analysis. Construction of new or expansion of existing water quality basins is one way to increase pollutant removal prior to stormwater reaching downstream waterbodies. Many additional techniques are available to reduce pollutant loading, including impervious surface reduction or disconnection, implementation of infiltration or volume-retention BMPs, installation of underground stormwater treatment structures and sump manholes, and other good housekeeping practices such as street sweeping. As opportunities arise, the City will consider all of these options to reduce the volume and improve the quality of stormwater runoff. NMC_112 NMC_112 NMC_114 CO_7 CO_2 CO_5 IP_1 CO_3 CO_4 CO_1 NMC_70 NMC_77 NMC_44 N in e M ile Creek Barr Footer: ArcGIS 10.4.1, 2017-09-21 07:57 File: \\barr.com\gis\Client\Edina\Projects\CRWMP_Update_2017\Maps\Reports\Figures_CityReviewDraft\Fig_5_4_NMC_North_Water_Quality.mxd User: rcs2NINE MILE CREEK CENTRALWATER QUALITY MODELING RESULTSWater ResourceManagement PlanCity of Edina, Minnesota FIGURE A.3.1 1,200 0 1,200Feet !;N 400 0 400Meters Percent TP Removal in Water Body*This number represents the percent of the total annual massof phosphorus entering the water body that is removed. Cumulative TP Removal in Watershed*This number represents the percent of the total annual massof phosphorus entering the watershed and upstream watershedsthat is removed in the pond and all upstream ponds. *Data based on results of 2003 P8 modeling. 0 - 25% (Poor/No Treatment) 25 - 40% (Moderate Removal) 40 - 60% (Good Removal) 60 - 100% (Excellent Removal) 0 - 25% (Poor/No Treatment) 25 - 40% (Moderate Removal) 40 - 60% (Good Removal) 60 - 100% (Excellent Removal) Imagery Source: USDA 2016 NAIP via MnGeo Area Draining Directly to the NorthFork of Nine Mile Creek Flow Direction City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.4-1 A.4 Lake Cornelia/Lake Edina/Adam’s Hill A.4.1 General Description of Drainage Area The City’s interactive web map depicts the Lake Cornelia/Lake Edina/Adam’s Hill “Drainage Area”. This drainage area is located in the southeast portion of Edina and encompasses 1,482 acres. A.4.1.1 Drainage Patterns This chapter discusses four “Major Watersheds” within the drainage area: North Lake Cornelia, South Lake Cornelia, Lake Edina, and the Adam’s Hill Pond drainage area. These major watersheds are depicted in the City’s interactive web map. North and South Lake Cornelia ultimately drain to Lake Edina, which outlets into the North Fork of Nine Mile Creek. The Adam’s Hill drainage area includes those watersheds within the City of Edina that drain to the Adam’s Hill stormwater detention basin in Richfield. This drainage area was analyzed in conjunction with the North Cornelia watershed because the storm sewer systems draining to North Lake Cornelia and Adam’s Hill Pond are adjoined at the intersection of 69th Street and York Avenue. Adam’s Hill Pond in Richfield ultimately is pumped out to Centennial Lakes which is part of the Nine Mile Creek – South Drainage Area. Each of the four major watersheds have been further delineated into numerous subwatersheds. The naming convention for each subwatershed is based on the major watershed where it is located. Table A.4.1 lists each major watershed and the associated subwatershed naming convention. The stormwater system within these drainage areas includes storm sewers, ponding basins, drainage ditches, and overland flow paths. Table A.4.1 Major Watersheds within the Lake Cornelia/Lake Edina/Adam’s Hill Drainage Basin Major Watershed Subwatershed Naming Convention Lake Cornelia—North NC_## Lake Cornelia—South SC_## Lake Edina LE_## Adam's Hill (Richfield) AHR_## A.4.1.1.1 North Cornelia North Lake Cornelia has a large watershed, encompassing 867 acres. The North Cornelia watershed is characterized by several ponding basins within the watershed. Land use within this watershed comprises a large commercial area (including the Southdale Shopping Center), portions of TH 62 and TH 100, residential areas (high and low density), parks, wetlands, and open water. The majority of the runoff from the highly impervious commercial area drains through the France Avenue and West 66th Street storm sewer system and discharges into the Point of France pond, located just northeast of the West 66th Street and Valley View Road intersection. The Point of France pond drains to the Swimming Pool Pond west of City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.4-2 Valley View Road, which typically drains to North Lake Cornelia. During large storms, such as the 1- percent-annual-chance event, when North Lake Cornelia nears its capacity, the Swimming Pool Pond will flow northward through two 60-inch culverts located under TH 62 that connect the Swimming Pool Pond with the Brookview Pond, just north of TH 62. An outlet control structure on the north side of this pond allows flows to the north into Lake Pamela when the water elevation reaches 863.3 feet. North Lake Cornelia covers approximately 31 acres and serves as a recreation area for the City of Edina. The lake outlets to South Lake Cornelia through a 12-inch culvert beneath West 66th Street. A.4.1.1.2 South Lake Cornelia The South Lake Cornelia watershed is located south of the North Lake Cornelia watershed. The 112-acre watershed includes two stormwater detention areas in addition to Lake Cornelia. The land use within the watershed is low-density residential and open water. South Lake Cornelia spans approximately 32 acres. The normal elevation of the lake is controlled by a weir structure at 859 feet. Discharge from South Cornelia flows southward through a 54-inch system for approximately 1,000 feet, where it connects with a 21-inch system at the intersection of Dunberry Lane and Cornelia Drive. This system ultimately drains to Lake Edina. During extreme storm events, such as the 1-percent-annual-chance event, the 21-inch pipe at Dunberry Lane and Cornelia Drive restricts flow, resulting in flow northward through the 54-inch system and into South Lake Cornelia. A.4.1.1.3 Lake Edina The Lake Edina watershed is located south of the Lake Cornelia drainage basins. The watershed encompasses approximately 394 acres. Land use within the watershed is mainly low-density residential, with smaller portions of high density residential, commercial, institutional (Cornelia Elementary School), park, wetland, and open water. A wetland along the west side of Lake Edina, directly east of TH 100, receives runoff from an area of approximately 40 acres. Flow from this wetland discharges into Lake Edina via a weir structure and pipe system. Lake Edina spans an area of approximately 23 acres. The normal elevation of the lake is controlled by a weir structure at 822 feet. Discharge from Lake Edina flows through a 36-inch system underneath TH 100 and into the North Fork of Nine Mile Creek. A.4.1.1.4 Adam’s Hill Pond The Adam’s Hill drainage area discussed in this analysis includes the 109-acre area within the City of Edina that drains to the Adam’s Hill Pond in Richfield. The outlet from Adam’s Hill Pond is a pumped outlet that discharges 10 cfs to Centennial Lakes. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.4-3 A.4.2 Stormwater System Results A.4.2.1 Hydrologic/Hydraulic Modeling Results The 10- and 1-percent-annual-chance flood analyses were performed for the Lake Cornelia/Lake Edina/Adam’s Hill drainage basins. The 10-percent-annual-chance analysis was based on a 24-hour storm with 4.29 inches of rain. The 1-percent-annual-chance analysis was based on a 24-hour storm with 7.47 inches of rain and on a 10-day snowmelt event with 7.2 inches of runoff; the higher resulting flood level of the two events was chosen for the 1-percent-annual-chance analysis. Data available from the City’s interactive web map presents the watershed information and the results for the 1-percent-annual- chance hydrologic analyses. A more detailed description of the stormwater system analysis is provided in Section A.1.1.3. The City’s interactive web map depicts the boundaries of the drainage areas, subwatershed boundaries, the modeled storm sewer network, and the flood-prone areas identified in the modeling analyses. To evaluate the level of protection of the stormwater system within the Lake Cornelia/Lake Edina/Adam’s Hill drainage areas, the 1-percent-annual-chance flood elevations for the ponding basins and depressed areas were compared to the low elevations of structures surrounding each basin. The areas predicted to be inundated during the 1-percent-annual-chance storm event are shown on the City’s interactive web map. Discussion and recommended improvement considerations for these areas are included in Section A.4.3. A.4.2.2 Water Quality Modeling Results The effectiveness of the stormwater system in removing stormwater pollutants such as phosphorus was analyzed using the P8 water quality model. The P8 model simulates the hydrology and phosphorus loads introduced from each pond’s watershed and the transport of phosphorus throughout the stormwater system. A more detailed description of the stormwater system analysis is provided in Section A.1.1.3. Figure A.4.1 depicts the results of the water quality modeling for the Lake Cornelia/Lake Edina/Adam’s Hill drainage areas. The figure shows the fraction of total phosphorus removal for each water body as well as the cumulative total phosphorus removal in the watershed. A.4.3 Implementation Considerations The XPSWMM hydrologic and hydraulic modeling analyses in support of the 2018 WRMP and 2003 P8 water quality analysis helped identify locations throughout the watershed where improvements to the City’s stormwater management system may be warranted. The following sections discuss potential improvement options identified as part of the modeling analyses. As opportunities to address identified flooding issues and improve water quality arise (e.g., street reconstruction projects or public facilities improvements), the City will use a comprehensive approach to stormwater management. This approach will include consideration of infiltration or volume retention practices to address flooding and/or improve water quality, reduction of impervious surfaces, increased storm sewer capacity where necessary to City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.4-4 alleviate flooding, construction and/or expansion of water quality basins, and implementation of other stormwater BMPs to reduce pollutant loading to downstream waterbodies. A.4.3.1 Potential Flood Risk Reduction Options The 2018 WRMP identified several locations within the Lake Cornelia, Lake Edina, and Adam’s Hill drainage area where the 1-percent-annual-chance level of protection is not provided by the current stormwater system. As part of the 2018 WRMP modeling analyses, potential corrective measures were identified for the problem areas. These preliminary corrective measures are also discussed below. As the City evaluates flooding issues and potential system modifications in these areas, other potential modifications, including (but not limited to) implementation of volume-retention practices, increases in conveyance capacity, and/or stormwater infiltration (where soils are conducive) will be given consideration. The 2003 hydrologic and hydraulic modeling analyses identified several locations within the Lake Cornelia, Lake Edina, and Adam’s Hill drainage basin where the 1-percent-annual-chance level of protection was not provided by the stormwater system, based on TP-40 precipitation frequency estimates. The discussions related to those areas are included in Appendix B of the 2022 WRMP, along with a short summary of what has been done in those areas since 2003. A.4.3.1.1 Valley View and Southdale Road Neighborhood (LE_34, LE_36, and LE_43) – updated 2021 A significant local depression exists along Southdale Road south of West 68th Street, extending west to Dawson Lane and east to Valley View Road. Under existing conditions, the low areas at Southdale Road and Valley View Road are drained by a 36-inch pipe that conveys runoff to the storm sewer system along Cornelia Drive. In this area, a 100-year storm (a storm that has a 1-percent-annual-chance of occurring) would inundate Dawson Lane, Southdale Road, and Valley View Road, with portions under more than 2 feet of water. This storm may also impact up to 40 principal structures (not counting sheds or other smaller outbuildings). In addition to the fact that the low point of Southdale Road is only a few feet higher than the normal level of South Cornelia Lake, the flooding problem in this area is primarily related to the capacity of the existing storm sewer system. Additionally, surface overflows from Valley View Road occur at West 68th Street, contributing additional runoff to the low area along Southdale Road. Potential flood risk reduction options were proposed in the previous WRMP. With updated and more detailed modeling, those options have remained. Raise West 68th Street at its Intersection with Valley View Road: This option would reduce or eliminate surface overflow from Valley View Road. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.4-5 Add Pipe Capacity to South Cornelia Lake: Double the capacity of the storm sewer system under Cornelia Drive between South Cornelia and Dunberry Lane (both the pipe and weir) and increase the pipe size under Dunberry Lane and Southdale Road to 54 inches. Underground Storage at Cornelia School Park and Additional Pipe Capacity: Add storm sewer capacity at the Southdale Road area and pipe water to an underground storage vault at Cornelia School Park rather than through the existing pipes along Cornelia Drive. This way, the Southdale Road area is connected directly to additional underground storage, with no other areas competing for that storage. The proposed underground storage would be at the southern end of the park under the smaller baseball field and existing hockey rink. It would be pumped out after storm events to the existing gravity storm sewer along W 72nd Street and to Oaklawn Avenue. This storage would also have the added benefit of preventing impacts to downstream areas such as Lake Edina. As presented in Table A.4.2, Options 2 and 3 have very comparable benefits, with most focused on the Southdale Road portion rather than the other roads in that neighborhood. Beyond these options, the strategy of requiring minimum elevations for newly constructed or rebuilt homes may be the most successful for the remaining low-lying homes in this neighborhood. Table A.4.2 Residential Structure Impacts in the Valley View and Southdale Road area Flood Risk Reduction Option Number of At-Risk Principal Structures1 (Reduction from Existing Conditions) 20-percent-annual-chance event 10-percent-annual-chance event 4-percent-annual-chance event 2-percent-annual-chance event 1-percent-annual-chance event Existing Conditions2 8 13 20 29 40 Option 1: Raise Intersection at Valley View and W 68th St. 8 (0) 13 (0) 19 (-1) 28 (-1) 36 (-4) Option 2: Additional Pipe Capacity to South Cornelia Lake 5 (-3) 9 (-4) 15 (-5) 23 (-6) 29 (-11) Option 3: Underground Storage at Cornelia School Park and Additional Pipe Capacity 5 (-3) 8 (-5) 17 (-3) 24 (-5) 29 (-11) (1) The homes to the south of the study area along Heatherton Trail are not included in the counts above because they are not in the Southdale Road area; (2) In the existing condition, if the city elevation data properly captures the elevation at which flood levels would impact a home, the number of structures would be 7 for the 5-year, 12 for the 10-year, 16 for the 25-year, 25 for the 50-year, and 35 for the 100-year events. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.4-6 A.4.3.1.2 Southwest Corner of TH 62 and TH 100 (NC_7, NC_8, NC_13, NC_11, NC_12, NC_14, NC_15, NC_16, and NC_20) – updated 2021 A portion of the Normandale Park neighborhood southwest of the TH 62 and TH 100 interchange and north of West 66th Street is subject to flooding. This area drains to North Lake Cornelia via a storm sewer pipe under TH 100; several depressions within the area include portions of Warren, Mildred, Rolf, Tingdale, Wilryan, and Josephine Avenues. Based on LiDAR elevation data and approximate building footprints in this area, peak water surface elevations from the 1-percent-annual-chance (100-year), 24- hour storm event may impact up to 39 principal structures, not counting sheds or other smaller outbuildings. Earlier modeling efforts that did not account for the inlet capacity of the storm sewer system suggested that the flooding problem was primarily related to the capacity of the storm sewer system that conveys stormwater from this area to North Lake Cornelia—particularly one 30-inch pipe segment running from west to east under Parnell Avenue along West 65th Street. There, friction losses are significant under full pipe-flow conditions. However, more recent, detailed 1D/2D modeling that accounts for inlet capacity has provided more insight to the flooding problem in this area. 1D/2D modeling suggests that the number and capacity of the catch basin inlets to this system are the limiting factor. Even during large storm events, the pipes on the west side of TH 100 are not flowing to their capacity. One example is the two inlets at the dead-end of West 65th Street, just west of TH 100. At this location, there are only two catch basins for the low area where all surface flow will collect without an emergency overflow (EOF). Images in Google Street View (as of 2021) show that these catch basins were entirely plugged, making the inlets completely ineffective. When the inlets are insufficient, water pools in the depression areas, impacting structures below the surface-overflow elevations. There is no planned street reconstruction in this area in the next few years; therefore, there are no near- term opportunities to install significant infrastructure projects. As such, the City evaluated two main flood- risk-reduction options: 1. Adding inlets in strategic areas to fully use the pipe capacity without causing surcharging elsewhere in the neighborhood 2. Adding under-road storage in the upstream part of the neighborhood to offset the additional flow in the pipes For the first option, additional catch basin inlets were modeled along Mildred, Rolf, and Tingdale Avenues and in some backyard depressions between these roads. Inlets were also modeled along Josephine and West 65th Street. For the second option, under-road storage was included along Mildred, Rolf, and Tingdale Avenues. Small pumps would be required (a few hundred gallons per minute) to dewater these storage vaults after storm or snowmelt events. While these alternatives do not remove all of the structures from flood risk (up to the 100-year event), they substantially reduce the flood risk in this neighborhood (Table A.4.3). With the second option, the City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.4-7 number of structures at risk of surface-level flooding was reduced by more than half for each of the five modeled storm events. Based on the City’s available data, only one home has been rebuilt in this neighborhood in the last 20 years (in 2008). The elevation data available suggests that this home may still be impacted by the 100- year flood level of 937.4 feet. Beyond the options described above, the City’s strategy of requiring minimum elevations when homes are initially constructed or rebuilt may be the most successful for the remaining low-lying homes in this neighborhood. Table A.4.3 Residential Structure Impacts in the Southwest Corner of TH 62 and TH 100 area Flood Risk Reduction Option Number of At-Risk Principal Structures (Reduction from Existing Conditions) 20-percent-annual-chance event 10-percent-annual-chance event 4-percent-annual-chance event 2-percent-annual-chance event 1-percent-annual-chance event Existing conditions 12 19 30 39 51 Option 1: Add inlets at strategic locations 8 (-4) 11 (-8) 16 (-14) 26 (-13) 32 (-19) Option 2: Add under-road storage and associated inlets, pipes, and pumps 5 (-7) 7 (-12) 12 (-18) 17 (-22) 24 (-27) A.4.3.2 Potential Construction/Upgrade of Water Quality Basins The 2003 P8 modeling analysis indicated that under average conditions the annual removal of total phosphorus from several ponds in the Lake Cornelia/Lake Edina drainage area was predicted to be below the desired 60 percent removal rate. For ponds with total phosphorus removal below 60 percent, the permanent pool storage volume was analyzed to determine whether additional capacity was necessary. The ponds with deficiencies in total phosphorus removal and permanent pool volume are listed below (and are also summarized in Appendix C of the 2022 WRMP), with recommended pond upgrades. Construction of new or expansion of existing water quality basins is one way to increase pollutant removal prior to stormwater reaching downstream waterbodies. Many additional techniques are available to reduce pollutant loading, including impervious surface reduction or disconnection, implementation of infiltration or volume-retention BMPs, installation of underground stormwater treatment structures and sump manholes, and other good housekeeping practices such as street sweeping. The City recently installed a filter BMP in Rosland Park (subwatershed NC_62) to reduce nutrient loading to downstream lakes. As further opportunities arise the City will consider available options to reduce the volume and improve the quality of stormwater runoff. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.4-8 A.4.3.2.1 LE_38 Pond LE_38 is located along the west side of Lake Edina, directly east of TH 100 (primarily within the Minnesota Department of Transportation [MnDOT] right-of-way). The pond receives runoff from an area of approximately 36 acres. Flow from this pond is discharged into Lake Edina via a weir structure and pipe system. Based on the recommended storage volume discussed above, Pond LE_38 is deficient in permanent pool storage volume. It is recommended that an additional 1.4 acre-feet of dead storage volume be provided to meet the MPCA design criteria for detention basins. A.4.3.2.2 NC_88 Pond NC_88 is located southeast of the intersection of York Avenue and West 64th Street. This basin has two pumped outlets, with discharge eventually entering both the Point of France Pond and the Swimming Pool Pond. Based on the MPCA-recommended storage volume for detention basins, there is not an adequate amount of permanent pool storage in this basin. However, since the predicted total phosphorus removal rate from this pond is approximately 50 percent and the pumped stormwater leaving this basin will receive additional water quality treatment through several subsequent ponding basins, recommendations for additional dead-storage volume are not being made at this time. NC_5 NC_6 NC_2 NC_30 NC_62NC_78 NC_72 NC_88 NC_4 SC_3 LE_38 LE_1 LE_44 NC_135 SC_2 LE_54LE_51 NC_130 SC_1 SC_1 NC_3 NC_3 R i c h f i e l dRichfield M i n n e a p o l i sMinneapolis NineMileCreek Barr Footer: ArcGIS 10.4.1, 2017-09-21 08:24 File: \\barr.com\gis\Client\Edina\Projects\CRWMP_Update_2017\Maps\Reports\Figures_CityReviewDraft\Fig_7_4_Lk_Cornelia_Water_Quality.mxd User: rcs2LAKE CORNELIA/LAKE EDINAWATER QUALITY MODELING RESULTSWater ResourceManagement PlanCity of Edina, Minnesota FIGURE A.4.1 1,200 0 1,200Feet !;N 400 0 400Meters Percent TP Removal in Water Body*This number represents the percent of the total annual massof phosphorus entering the water body that is removed. Cumulative TP Removal in Watershed*This number represents the percent of the total annual massof phosphorus entering the watershed and upstream watershedsthat is removed in the pond and all upstream ponds. *Data based on results of 2003 P8 modeling. 0 - 25% (Poor/No Treatment) 25 - 40% (Moderate Removal) 40 - 60% (Good Removal) 60 - 100% (Excellent Removal) 0 - 25% (Poor/No Treatment) 25 - 40% (Moderate Removal) 40 - 60% (Good Removal) 60 - 100% (Excellent Removal) Imagery Source: USDA 2016 NAIP via MnGeo Flow Direction City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.5-1 A.5 Nine Mile Creek South A.5.1 General Description of Drainage Area The City’s interactive web map depicts the Nine Mile Creek—South “Drainage Area”, located in the southeast portion of Edina. The drainage area encompasses approximately 1,176 acres that ultimately drain to the North Fork of Nine Mile Creek between West 70th Street and the south Edina City limits. A.5.1.1 Drainage Patterns The stormwater system within this drainage area comprises storm sewers, ditches, overland flow paths, wetlands, and ponding basins. The Nine Mile Creek—South drainage area has been divided into several “Major Watersheds” based on the drainage patterns. These major watersheds are depicted on the City’s interactive web map. Each major watershed has been further delineated into many subwatersheds. The naming convention for each subwatershed is based on the major watershed where it is located Table A.5.1 lists each major watershed and the associated subwatershed naming convention. Table A.5.1 Major Watersheds within the Nine Mile Creek—South Drainage Basin Major Watershed Subwatershed Naming Convention Centennial Lakes CL_## South Pond SP_## Nine Mile South NMS_## / EdCrk## A.5.1.1.1 Centennial Lakes The 207-acre Centennial Lakes watershed is located in southeast Edina and drains to Centennial Lakes. The watershed is bordered by West 69th Street on the north, West 78th Street on the south, France Avenue on the west, and York Avenue on the east. Runoff from France Avenue between West 69th Street and just south of Gallagher Drive drains to Centennial Lakes. France Avenue drainage south of Gallagher Drive flows to the South Pond. The watershed is characterized by mainly commercial and high-density residential land use. Centennial Lakes spans approximately 9.5 acres, stretching south from Gallagher Drive to Minnesota Drive, and receives runoff from the direct watershed as well as flow from Adam’s Hill Pond (10 cfs). The normal elevation of Centennial Lakes is 838 feet, controlled by a weir structure that discharges to the South Pond. A.5.1.1.2 South Pond (Border Basin) The South Pond is located on the border between Edina and Bloomington, just west of the intersection of Minnesota Drive and West 77th Street. The watershed draining to the South Pond encompasses 224 acres. The land use within the watershed is entirely commercial and industrial, thus highly impervious. In City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.5-2 addition to the runoff from the direct watershed, the South Pond receives flow from Centennial Lakes. The South Pond was categorized as a Type 4 wetland in the wetland inventory, a shallow (0.5 to 3 foot), marshy wetland with vegetation such as grasses, cattails, and bulrushes. The normal elevation of the South Pond is controlled at 814.7 feet by a weir structure. Discharge from the South Pond flows west through the storm sewer system along Viking Drive and eventually discharges to the North Fork of Nine Mile Creek. A.5.1.1.3 Nine Mile South The Nine Mile South watershed encompasses the area that drains to the North Fork of Nine Mile Creek between West 70th Street and the southern border of Edina. The 745-acre watershed extends to Cahill Road to the west, France Avenue to the east, West 66th Street to the north, and West 78th Street to the south. The watershed is characterized by multiple land uses, including residential, commercial, industrial, highway, and golf course. The portion of the watershed west of the North Fork of Nine Mile Creek is almost entirely commercial and industrial, thus highly impervious. The northern portion is low-density residential. The southeast portion consists mainly of high-density residential buildings, a large commercial and industrial area, and the Fred Richards Golf Course. The golf course is characterized by a series of ponding basins that receive runoff from an area of approximately 180 acres. Discharge from the golf course ponds flows southward through a storm sewer system located between the 4700 and 4660 West 77th Street properties. This system connects to the trunk system that flows westward from the South Pond to the North Fork of Nine Mile Creek. A.5.2 Stormwater System Results A.5.2.1 Hydrologic/Hydraulic Modeling Results The 10- and 1-percent-annual-chance flood analyses were performed for the Nine Mile Creek- South drainage basin. The 10-percent-annual-chance analysis was based on a 24-hour storm with 4.29 inches of rain. The 1-percent-annual-chance analysis was based on a 24-hour storm event with 7.47 inches of rain and on a 10-day snowmelt event with 7.2 inches of runoff; the higher resulting flood level of the two events was chosen for the 1-percent-annual-chance analysis. Data available from the City’s interactive web map presents the watershed information and the results for the 1-percent-annual-chance hydrologic analyses. A more detailed description of the stormwater system analysis is provided in Section A.1.1.3. City’s interactive web map depicts the boundaries of the drainage areas, subwatershed boundaries, the modeled storm sewer network, and the flood-prone areas identified in the modeling analyses. To evaluate the level of protection of the stormwater system within the Nine Mile Creek—South drainage basin, the 1-percent-annual-chance flood elevations for the ponding basins and depressed areas were compared to the low elevations of structures surrounding each basin.. The areas predicted to be inundated during the 1-percent-annual-chance storm event are shown on the City’s interactive web map. Discussion and recommended improvement considerations for these areas are included in Section A.5.3. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.5-3 A.5.2.2 Water Quality Modeling Results The effectiveness of the stormwater system in removing stormwater pollutants such as phosphorus was analyzed using the P8 water quality model. The P8 model simulates the hydrology and phosphorus loads introduced from each pond’s watershed and the transport of phosphorus throughout the stormwater system. A more detailed description of the stormwater system analysis is provided in Section A.1.1.3. Figure A.5.1 depicts the results of the water quality modeling for the Nine Mile Creek—South drainage basin. The figure shows the fraction of total phosphorus removal for each water body as well as the cumulative total phosphorus removal in the watershed. Implementation Considerations A.5.3 Implementation Considerations The XPSWMM hydrologic and hydraulic modeling analyses in support of the 2018 WRMP and 2003 P8 water quality analysis helped identify locations throughout the watershed where improvements to the City’s stormwater management system may be warranted. The following sections discuss potential improvement options identified as part of the modeling analyses. As opportunities to address identified flooding issues and improve water quality arise (e.g., street reconstruction projects or public facilities improvements), the City will use a comprehensive approach to stormwater management. This approach will include consideration of infiltration or volume retention practices to address flooding and/or improve water quality, reduction of impervious surfaces, increased storm sewer capacity where necessary to alleviate flooding, construction and/or expansion of water quality basins, and implementation of other stormwater BMPs to reduce pollutant loading to downstream waterbodies. A.5.3.1 Potential Flood Risk Reduction Options The 2018 WRMP identified several locations within the Nine Mile Creek—South drainage basin where the 1-percent-annual-chance level of protection is not provided by the current stormwater system. As part of the 2018 WRMP modeling analyses, potential corrective measures were identified for problem areas. These preliminary corrective measures are also discussed below. As the City evaluates flooding issues and potential system modifications in these areas, other potential modifications, including (but not limited to) implementation of volume-retention practices, increases in conveyance capacity, and/or stormwater infiltration (where soils are conducive) will be given consideration. The 2003 hydrologic and hydraulic modeling analyses identified several locations within the Nine Mile Creek –South drainage basin where the 1-percent-annual-chance level of protection was not provided by the stormwater system, based on TP-40 precipitation frequency estimates. The discussions related to those areas are included in Appendix B of the 2022 WRMP, along with a short summary of what has been done in those areas since 2003. A.5.3.1.1 West 70th Street and West Shore Drive (NMS_38 and NMS_50) Approximately 42 acres drain to the local depression at the intersection of West 70th Street and West Shore Drive. A 36-inch storm sewer drains this intersection to the west to approximately TH 100, where City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.5-4 the storm sewer size increases to 42-inch and 48-inch pipes and eventually drains to Nine Mile Creek. Water pools in this intersection until it reaches the surface overflow elevation of approximately 869.4 feet (according to LiDAR data) and flows west. The 1-percent-annual-chance flood level is determined by the 24-hour precipitation event and is 869.4 feet. Four principal structures (4701 West 70th Street, and 6905 through 6913 West Shore Drive) are potentially impacted at this intersection and the depth of flooding is approximately 2.0 feet. Additional flow capacity is needed in the West 70th Street system. Additionally, in conjunction with future road reconstruction, a second option would be to lower the high elevation along West 70th Street to the west of the intersection. This would allow for the pooled water to overflow at a lower elevation and limit the peak flooding elevation and extents. Another potential option to reduce flood risk at this intersection is to divert high flows within the existing storm sewer system along West 70th Street southward to the low area in Arnesen Acres Park (LE_54). Further analysis is warranted. A.5.3.1.2 Centennial Lakes (CL_1) Centennial Lake is located in the southeast portion of Edina, east of France Avenue and north of Interstate 494 (I-494) within Centennial Lakes Park. Approximately 207 acres drain to Centennial Lake. Water levels in the lake are controlled by a 25-foot long weir structure at elevation 838.04 feet, with a 60- inch pipe that conveys runoff to the Border Basin (SP_1) and eventually to the North Fork of Nine Mile Creek. Modeling results indicate that during the 1-percent-annual-chance 24-hour storm event the peak flood level is 842.3 feet. The storage options around this area are limited and the overall capacity of the outlet could be enlarged to pass more water through the system. However, current flood levels downstream are already elevated and increasing drainage capacity from Centennial Lakes Park may cause further impacts downstream. It is recommended that a survey be conducted to determine low entry elevations for structures adjacent to Centennial Lake. Given the existing flood issues downstream of Centennial Lakes and limited potential for upstream storage, flood-proofing may be the most appropriate strategy to address flood risk in this area. It appears that the structures along the north part of Centennial Lakes have a minimal number of entry locations directly adjacent to the lake; flood proofing may not be necessary in this area. Toward the south end of Centennial Lakes, there are several structures with low entries facing the lake. A 3-foot floodwall around the lake would provide approximately 1 foot of freeboard above the 1-percent-annual-chance flood level. Additional storage could also be created in Centennial Lakes by drawing the lake down during dry periods in anticipation of storm events. This could be accomplished in a number of ways. The current weir structure could be modified to include a small orifice below the existing weir crest. The orifice could be fitted with a gate valve for periodic use if preferred. Alternatively, the valve could be fitted with automatic controls that are based on weather prediction. Another approach could be periodically pumping the lake level down using a small lift station near the outlet, which could be controlled manually or automatically based on weather prediction. This approach is referred to as Adaptive Level Control Systems within Edina City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.5-5 and is beginning to be evaluated in 2022 with potential partners in Nine Mile Creek Watershed District, City of Bloomington, and City of Richfield. A.5.3.2 Potential Construction/Upgrade of Water Quality Basins The 2003 P8 modeling analysis indicated that under average conditions the annual removal of total phosphorus from several ponds in the Nine Mile Creek—South drainage area was below the desired 60 percent removal rate. For ponds with total phosphorus removal below 60 percent, the permanent pool storage volume was analyzed to determine whether additional capacity was necessary. The ponds with deficiencies in total phosphorus removal and permanent pool volume are listed below (and are also summarized in Appendix C of the 2022 WRMP), with recommended pond upgrades. Construction of new or expansion of existing water quality basins is one way to increase pollutant removal prior to stormwater reaching downstream waterbodies. Many additional techniques are available to reduce pollutant loading, including impervious surface reduction or disconnection, implementation of infiltration or volume-retention BMPs, installation of underground stormwater treatment structures and sump manholes, and other good housekeeping practices such as street sweeping. As opportunities arise, the City will consider all of these options to reduce the volume and improve the quality of stormwater runoff. A large portion of stormwater runoff from the Nine Mile Creek—South drainage basin drains through the storm sewer system directly to the North Fork of Nine Mile Creek without any water quality treatment prior to entering the creek. The large area draining directly to the Creek (approximately 500 acres) is depicted on Figure A.5.1. To remove pollutants and improve the quality of the discharge to Nine Mile Creek, it is recommended that the City consider installation of a water quality treatment basin upstream of the discharge location at West 77th Street and TH 100 (discussed in additional detail below), as well as other water quality treatment techniques as opportunities arise. A.5.3.2.1 West 77th Street and TH 100 The southwest portion of the Nine Mile Creek—South drainage basin is an industrial, highly impervious area. Stormwater from this area is collected via storm sewer and discharged to the North Fork of Nine Mile Creek without any water quality treatment. Construction of a water quality basin in the southwest quadrant of the intersection of TH 100 and West 77th Street is being considered to provide some pollutant removal prior to discharge. The basin will receive runoff from an area of approximately 50 acres along Industrial Boulevard. Based on the MPCA-recommended design criteria for permanent pool storage in detention basins, the total required dead storage volume for this basin is 4.4 acre-feet. A.5.3.2.2 NMS_76 Pond NMS_76 is located on the east side of the Fred Richards Golf Course, just northwest of the intersection of West 76th Street and Parklawn Avenue. The pond receives runoff from an area of approximately 120 acres. The pond outlets to Pond NMS_104 via a 108-inch “round equivalent” arch pipe. The pond is a Type 5 wetland and was assumed to have an average depth of 4 feet. Based on this assumed depth and the 2-foot topographic data, the permanent pool storage volume was estimated to City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.5-6 be 4.4 acre-feet in 2004. According to NURP pond design standards, this storage volume is inadequate. In 2008, the City removed approximately 0.9 acre-feet of sediment from Pond NMS_76. To upgrade the pond to meet the NURP standards, an additional 1.6 acre-feet of dead storage volume is recommended. A.5.3.2.3 NMS_104 Pond NMS_104 is located along the southeast border of the Fred Richards Golf Course, just north of the parking lot for the Pentagon Park office complex. In addition to runoff from adjacent parking lots, this detention basin receives discharge from Pond NMS_76. Based on the wetland inventory, the pond is a Type 5 wetland and was assumed to have an average depth of 4 feet. Pond NMS_104 is connected to the downstream pond NMS_72 by two 30-inch equalizer pipes. According to MPCA recommendations, there is not an adequate amount of permanent pool storage in this basin. It is recommended that an additional 0.2 acre-feet of dead storage volume be provided. A.5.3.2.4 NMS_72 and NMS_74 Ponds NMS_72 and NMS_74 are located within the Fred Richards Golf Course, connected by a 36-inch equalizer pipe. Pond NMS_72 is upstream of NMS_74 and receives discharge from Pond NMS_79 and NMS_104, as well as runoff from the 7-acre direct watershed. Pond NMS_74 receives discharge from NMS_72 in addition to runoff from the 6.5-acre direct watershed. Based on the wetland inventory, both ponds are Type 5 wetlands and were assumed to have an average depth of 4 feet. Considering this depth and the 2-foot topographic information, the permanent pool storage volume of each pond is greater than the MPCA-recommended volume for detention ponds. However, because water quality modeling results indicate that the total phosphorus removal in Ponds NMS_72 and NMS_74 is below desired levels, it is recommended that the depth of the ponds be increased to improve removal efficiency. A.5.3.2.5 SP_1 (South Pond/Border Basin) Pond SP_1 is located on the border between Edina and Bloomington, just west of the intersection of Minnesota Drive and West 77th Street. In addition to stormwater runoff from a large, highly impervious 215-acre watershed, Pond SP_1 receives discharge from Centennial Lakes. The water level in Pond SP_1 is controlled by a weir structure. Discharge from the pond flows to the North Fork of Nine Mile Creek on the west side of TH 100, just south of the West 77th Street crossing. The pond is a Type 4 wetland and was assumed to have an average depth of 2 feet. Based on this depth and the pond area from the 2-foot topographic data, the existing dead storage volume was calculated to be 6.8 acre-feet, which is less than the MPCA-recommended permanent pool storage volume for this basin. It is recommended that an additional 19.6 acre-feet of dead storage volume be provided to meet the MPCA design criteria for detention basins and improve the efficiency of total phosphorus removal. NMS_76NMS_79 NMS_88 NMS_72NMS_103 NMS_104 SP_1 NMS_23 NMS_40 NMS_28 NMS_84 NMS_74 NMS_74 CL_1 CL_1 AH_1 B l oo m i n g t o nBloomington Ri c h f i e l dRichfield NineMi l eCreek Barr Footer: ArcGIS 10.4.1, 2017-09-21 08:35 File: \\barr.com\gis\Client\Edina\Projects\CRWMP_Update_2017\Maps\Reports\Figures_CityReviewDraft\Fig_8_4_NMC_South_Water_Quality.mxd User: rcs2NINE MILE CREEK SOUTHWATER QUALITY MODELING RESULTSWater ResourceManagement PlanCity of Edina, Minnesota FIGURE A.5.1 1,200 0 1,200Feet !;N 400 0 400Meters Percent TP Removal in Water Body*This number represents the percent of the total annual massof phosphorus entering the water body that is removed. Cumulative TP Removal in Watershed*This number represents the percent of the total annual massof phosphorus entering the watershed and upstream watershedsthat is removed in the pond and all upstream ponds. *Data based on results of 2003 P8 modeling. 0 - 25% (Poor/No Treatment) 25 - 40% (Moderate Removal) 40 - 60% (Good Removal) 60 - 100% (Excellent Removal) 0 - 25% (Poor/No Treatment) 25 - 40% (Moderate Removal) 40 - 60% (Good Removal) 60 - 100% (Excellent Removal) Imagery Source: USDA 2016 NAIP via MnGeo Area Draining Directly to the NorthFork of Nine Mile Creek Flow Direction City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.6-1 A.6 Nine Mile South Fork A.6.1 General Description of Drainage Area The City’s interactive web map depicts the 1,385-acre “Drainage Area” to the South Fork of Nine Mile Creek and the individual subwatersheds within this area. The Nine Mile South Fork drainage area is located in the southwest corner of Edina and includes a small portion of Eden Prairie. Several land-locked lakes are located within this drainage area, including Arrowhead Lake and Indianhead Lake. These areas would become tributary to the South Fork of Nine Mile Creek only under extreme flooding circumstances (storms greater than the 1-percent-annual-chance storm event). A.6.1.1 Drainage Patterns The stormwater system within this drainage area comprises storm sewers, ditches, overland flow paths, wetlands, and ponding basins. The Nine Mile South Fork drainage area has been divided into several “Major Watersheds” based on the drainage patterns. These major watersheds are depicted on the City’s interactive web map. Each major watershed has been further delineated into many subwatersheds. The naming convention for each subwatershed is based on the major watershed where it is located Table A.6.1 lists each major watershed and the associated subwatershed naming convention. Table A.6.1 Major Watersheds within the Nine Mile South Fork Drainage Basin Major Watershed Subwatershed Naming Convention Arrowhead Lake AH_## Indianhead IH_## Pawnee Pond PA_## Eden Prairie EP_## Braemar Arena/Public Works BA_## Nine Mile South Fork NMSB_## / BRCrk## A.6.1.1.1 Arrowhead Lake The Arrowhead Lake watershed extends north of TH 62 and is bordered on the west side by TH 169 and generally bordered on the east and south side by Indian Hills Road/Pass. Land use in the 178-acre watershed is mainly single family residential; however, portions of TH 62 and the TH 62/TH 169 intersection are tributary to the lake. Within the watershed there are three stormwater detention basins that ultimately drain to Arrowhead Lake. Arrowhead Lake is a land-locked basin covering approximately 22 acres. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.6-2 A.6.1.1.2 Indianhead Lake The Indianhead Lake watershed is located southeast of Arrowhead Lake. Within the 108-acre watershed, there are two stormwater detention basins in addition to the lake. The residential watershed ultimately drains to Indianhead Lake via storm sewer networks and overland flow channels. Indianhead Lake is a land-locked basin covering approximately 14 acres. A.6.1.1.3 Pawnee Pond The Pawnee Pond watershed is approximately 39 acres. The watershed is a residential area consisting of two stormwater detention basins, Pawnee Pond, and a smaller basin east of the intersection of Apache Road and Sally Lane. The Pawnee Pond is located directly north of Apache Road, bordered by Indian Way West on the west, Pawnee Road on the east, and Indian Hills Road on the north. The normal elevation of Pawnee Pond is controlled at an elevation of 864 feet by a pumped outlet. The outflow from Pawnee Pond flows westerly through a cross-culvert beneath TH 169, then south on the west side of TH 169, and ultimately into the Braemar Branch of Nine Mile Creek. A.6.1.1.4 Eden Prairie The Eden Prairie watershed consists of approximately 217 acres of land west of TH 169 that drains to the South Fork of Nine Mile Creek via the Braemar Branch of Nine Mile Creek. Stormwater runoff from this area flows through a succession of storm sewer systems and ponding basins, eventually outletting to the drainage way that flows south along the west side of TH 169. The Eden Prairie watershed boundaries were based on the watershed divides from the Nine Mile Creek Watershed District Water Management Plan (May 1996). Land use within this area consists mainly of industrial and office property. A.6.1.1.5 Braemar Arena/Public Works The Braemar Arena/Public Works watershed includes drainage from the south parking lot of the Braemar Arena, Braemar Boulevard, and the Public Works and Public Safety Training Site. The remaining portion of the Braemar Sports Complex parking lot that does not drain to the south drains westerly to the TH 169 drainage system. The 27-acre Braemar Arena/Public Works watershed drains south through a storm sewer system to a 0.24-acre stormwater detention pond. The water level of this detention pond is controlled at an elevation of 846 feet by a 24-inch-diameter outlet pipe that discharges south to the floodplain of the South Fork of Nine Mile Creek. A.6.1.1.6 Nine Mile South Fork The Nine Mile South Fork watershed drains through the Braemar Golf Course, ultimately discharging to the South Fork of Nine Mile Creek. The 816-acre watershed has a wide range of land uses, including residential, industrial, wetlands, open area/park, and the golf course. The stormwater system throughout this area is characterized by storm sewer, ditches, ponds, and overland flow networks. The extent of the Nine Mile South Fork watershed spans west of TH 169, where drainage from the Washington Avenue storm sewer system combines with flows from the Eden Prairie and Pawnee Pond watersheds. This stormwater flows easterly under TH 169 through a large culvert, midway between Hamilton Road and West 69th Street, and discharges to the Braemar Branch of Nine Mile Creek. The Braemar Branch drains City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.6-3 southward through Braemar Park towards the Braemar Golf Course. The Braemar Branch flows through several ponding basins on the west side of the golf course before discharging to the South Fork. Stormwater from the remaining portion of the Nine Mile South Fork watershed flows through a series of storm sewer pipes, wetlands, and ponds on the east side of the Braemar Golf Course before reaching the South Fork of Nine Mile Creek. A.6.2 Stormwater System Results A.6.2.1 Hydrologic/Hydraulic Modeling Results The 10- and 1-percent-annual-chance flood analyses were performed for the Nine Mile South Fork Watershed. The 10-percent-annual-chance analysis was based on a 24-hour storm with 4.29 inches of rain. The 1-percent-annual-chance analysis was based on a 24-hour storm event with 7.47 inches of rain and on a 10-day snowmelt event with 7.2 inches of runoff; the higher resulting flood level of the two events was chosen for the 1-percent-annual-chance analysis. Data available from the City’s interactive web map presents the watershed information and the results for the 1-percent-annual-chance hydrologic analyses. A more detailed description of the stormwater system analysis is provided in Section A.1.1.3. The City’s interactive web map depicts the Nine Mile South Fork drainage area boundary, subwatershed boundaries, the modeled storm sewer network, and the flood-prone areas identified in the modeling analyses. To evaluate the level of protection of the stormwater system within the Nine Mile South Fork drainage area, the 1-percent-annual-chance flood elevations for the ponding basins and depressed areas were compared to the low elevations of structures surrounding each basin.. The areas predicted to be inundated during the 1-percent-annual-chance storm event are shown on the City’s interactive web map. Discussion and recommended improvement considerations for these areas are included in Section A.6.3. A.6.2.2 Water Quality Modeling Results The effectiveness of the stormwater system in removing stormwater pollutants such as phosphorus was analyzed using the P8 water quality model. The P8 model simulates the hydrology and phosphorus loads introduced from each pond’s watershed and the transport of phosphorus throughout the stormwater system. A more detailed description of the stormwater system analysis is provided in Section A.1.1.3. Figure A.6.1 depicts the results of the water quality modeling for the Nine Mile South Fork drainage basin. The figure shows the fraction of total phosphorus removal for each water body as well as the cumulative total phosphorus removal in the watershed. A.6.3 Implementation Considerations The XPSWMM hydrologic and hydraulic modeling analyses in support of the 2018 WRMP and 2003 P8 water quality analysis helped identify locations throughout the watershed where improvements to the City’s stormwater management system may be warranted. The following sections discuss potential City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.6-4 improvement options identified as part of the modeling analyses. As opportunities to address identified flooding issues and improve water quality arise (e.g., street reconstruction projects or public facilities improvements), the City will use a comprehensive approach to stormwater management. This approach will include consideration of infiltration or volume retention practices to address flooding and/or improve water quality, reduction of impervious surfaces, increased storm sewer capacity where necessary to alleviate flooding, construction and/or expansion of water quality basins, and implementation of other stormwater BMPs to reduce pollutant loading to downstream waterbodies. A.6.3.1 Potential Flood Risk Reduction Options The 2018 WRMP identified several locations within the Nine Mile South Fork drainage basin where the 1- percent-annual-chance level of protection is not provided by the current stormwater system. As part of the 2018 WRMP modeling analyses, potential corrective measures were identified for the problem areas. These preliminary corrective measures are also discussed below. As the City evaluates flooding issues and potential system modifications in these areas, other potential modifications, including (but not limited to) implementation of volume-retention practices, increases in conveyance capacity, and/or stormwater infiltration (where soils are conducive) will be given consideration. The 2003 hydrologic and hydraulic modeling analyses also identified several locations within the Nine Mile South Fork drainage basin where the 1-percent-annual-chance level of protection was not provided by the stormwater system, based on TP-40 precipitation frequency estimates. The discussions related to those areas are included in Appendix B of the 2022 WRMP, along with a short summary of what has been done since 2003. A.6.3.1.1 McCauley Trail West (AH_6) A stormwater pond located behind 6533–6545 McCauley Trail West and 6301 Timber Trail receives runoff from approximately 39.3 acres with approximately 13 acres being direct drainage, mostly north of TH 62. The pond has a 21-inch gravity drain outlet that discharges to land-locked Arrowhead Lake. There is a pumped outlet (Lift Station 8) that appears to be in place to draw the pond down below the outlet elevation, but the pump capacity appears to be very minor compared to the gravity drain capacity. Without definitive information on the pump capacity, and due to its relatively small size, this pump was not included in the modeling. Modeling results indicate that the 1-percent-annual-chance flood elevation (885.0 feet) may impact up to nine principal structures for a duration of approximately 6 hours. Several options to alleviate flooding in this area should be considered, including providing additional outlet capacity and storing additional water upstream. An increase in discharge capacity from the pond could likely reduce the number of principle structures potentially impacted in AH_6 without creating impacts to principal structures around Arrowhead Lake. The 1-percent-chance annual flood elevation in downstream Arrowhead Lake is well below the low houses adjacent to the lake. Additionally, the critical 1- percent-annual-chance event for Arrowhead Lake is the 1-percent-annual-chance 10-day snowmelt—not the 24-hour storm event. A restrictive outlet upstream of TH 62 could also be implemented to store more water in AH_3 during a precipitation event and limit the amount of water contributing to AH_6. A City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.6-5 restrictive outlet, without changing the invert, would allow for AH_3 to drain dry between events. Finally, there are some options for creating additional stormwater storage. In subwatershed AH_9, there is room for additional regrading work to create additional storage. A.6.3.1.2 Sally Lane and Valley View Road (NMSB_52, NMSB_69, and NMSB_77) The ravine located in the backyards of the homes between Sally Lane and McCauley Trail South receives stormwater flows from a 436-acre tributary area that includes portions of Eden Prairie and Edina. This ravine, which is the headwaters of the Braemar Branch of Nine Mile Creek, crosses under Valley View Road through a 6-foot by 11.7-foot box culvert. Modeling results indicate that the 1-percent-annual-chance flood elevations along this ravine may impact up to 19 principal structures (6713–6721 Sioux Trail, 6800– 7008 Sally Lane, 7016, 7020, and 7028 Sally Lane). The storm sewer system at the Paiute Pass and Sally Lane intersection collects stormwater from a total drainage area of approximately 27 acres. The system discharges into the Braemar Branch, west of Sally Lane, via two 24-inch pipes. Flooding issues at the intersection of Sally Lane and Paiute Pass were analyzed in 2013 (Project STS-406). However, that work was focused on the intersection and not the backyards of the principal structures along Sally Lane that are affected by high water levels within the ravine. Some of the recommendations from the 2013 analysis to reduce flood risk at this intersection were implemented as part of the 2016 street reconstruction project. There are several options that should be considered to reduce flood risk in the backyard ravine area, including increasing capacity of the Braemar Branch culvert under Valley View Road and providing additional flood storage in the tributary drainage area. It is recommended that the City work with the NMCWD to evaluate potential impacts of increasing the capacity under Valley View Road. It is also recommended that the City work with the City of Eden Prairie and Nine Mile Creek Watershed District to identify opportunities for increased flood storage upstream of the Braemar Branch. A.6.3.2 Potential Construction/Upgrade of Water Quality Basins The 2003 P8 modeling analysis predicted that under average conditions the annual removal of total phosphorus from several ponds in the Nine Mile South Fork drainage area would be below the desired 60 percent removal rate. For those ponds with total phosphorus removal below 60 percent, the permanent pool storage volume was analyzed to determine whether additional capacity was necessary. The ponds with deficiencies in total phosphorus removal and permanent pool volume are listed below (and are also summarized in Appendix C of the 2022 WRMP), with recommended pond upgrades. Construction of new or expansion of existing water quality basins is one method to increase the pollutant removal achieved prior to stormwater reaching downstream waterbodies. Many additional techniques are available to reduce pollutant loading, including impervious surface reduction or disconnection, implementation of infiltration or volume-retention BMPs, installation of underground stormwater treatment structures and sump manholes, and other good housekeeping practices such as street sweeping. As opportunities arise, the City will consider all of these options to reduce the volume and improve the quality of stormwater runoff. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.6-6 A.6.3.2.1 NMSB_3 and NMSB_2 Pond NMSB_3 is located on the Braemar Golf Course, southeast of the intersection of Valley View Road and Braemar Boulevard. This pond receives stormwater from an immediate watershed of approximately 21 acres, as well as discharge from the Braemar Branch. The Braemar Branch drains an area of approximately 259 acres and flow from Pond NMSB_33 to the east. The pond is a Type 5 wetland and was assumed to have an average depth of 4 feet. Pond NMSB_2 is downstream of Pond NMSB_3 and connected by a 30-inch equalizer pipe. Pond NMSB_2 receives stormwater from an immediate watershed of approximately 5 acres, as well as the flow from NMSB_3. This pond is also a Type 5 wetland and was assumed to have an average depth of 4 feet. Based on modeling results, the annual removal of total phosphorus from these two ponds was predicted to be below 60 percent. Consequently, the MPCA-recommended permanent pool storage volume for these ponds was calculated and compared to the existing dead storage volume. For the permanent pool volume analysis, the two ponds were considered as one. Based on this assumption, the ponds are deficient in dead storage volume. We recommend that an additional 1.2 acre-feet of dead storage volume be provided to these two ponds to meet the MPCA design criteria for detention basins. A.6.3.2.2 NMSB_12 Pond NMSB_12 is located on the Braemar Golf Course, approximately 700 feet southwest of the clubhouse. The pond receives stormwater runoff from the direct watershed of approximately 12 acres, in addition to flow from the upstream wetland NMSB_56. According to the wetland inventory, the pond is a Type 5 wetland and assumed to have an average depth of 4 feet. Based on this depth and the 2-foot topographic information, the current permanent pool storage volume is greater than the MPCA- recommended volume for detention ponds. It is recommended that the basin be maintained on a regular basis to ensure the removal efficiency is maintained. A.6.3.2.3 NMSB_86 Pond NMSB_86 is located on the Braemar Golf Course, directly south of the clubhouse parking lot. Pond NMSB_86 is a small pond that receives stormwater runoff from a direct watershed of approximately 21 acres, as well as discharge from the upstream pond (NMSB_57). The pond discharges directly to the South Fork of Nine Mile Creek via a 30-inch pipe. Based on modeling results, the annual removal of total phosphorus from Pond NMSB_86 was predicted to be well below 60 percent. According to the MPCA-recommended storage volume for detention basins, there is not an adequate amount of permanent pool storage in this basin. It is recommended that 0.15 acre-feet of dead storage volume be added to meet the MPCA design criteria for detention basins. A.6.3.2.4 NMSB_7 Pond NMSB_7 is located on the Braemar Golf Course on the north side of Hilary Lane. Pond NMSB_7 is a small detention pond that receives stormwater runoff from a 2.4-acre watershed in addition to discharge from an upstream wetland (NMSB_90). Pond NMSB_7 discharges to Pond NMSB_85 via a 24-inch pipe. Based on Braemar Golf Course design plans, Pond NMSB_7 was assumed to be shallow, with an average City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.6-7 depth of 2.3 feet. Using this depth assumption and the pond area from the 2-foot topographic information, the current permanent pool storage volume was calculated to be 0.6 acre-feet. This volume is greater than the MPCA-recommended storage volume for detention ponds. However, because the water quality modeling results indicate that the total phosphorus removal in Pond NMSB_7 is below 60 percent, it is recommended that the depth of the pond be increased to 4 feet to improve removal efficiency. A.6.3.2.5 NMSB_85 Pond NMSB_85 is located on the Braemar Golf Course, downstream of Pond NMSB_7, on the north side of Braemar Boulevard. Pond NMSB_85 receives stormwater runoff from a 67.5-acre watershed, as well as discharge from Pond NMSB_7 and discharge from a backyard depression area northeast of the intersection of Gleason Road and Dewey Hill Road (NMSB_15). According to the wetlands inventory, the pond is Type 5 and was assumed to have an average depth of 4 feet. Based on this depth assumption and the pond area from the 2-foot topographic data, the current permanent pool storage volume is 1.3 acre- feet. This storage volume is less than the MPCA-recommended storage volume for detention basins. It is recommended that an additional 1.2 acre-feet of dead storage volume be provided to meet the MPCA design criteria for detention basins. PA_1 PA_6 NMSB_2 NMSB_6 NMSB_7 NMSB_5 NMSB_62 NMSB_86 NMSB_34 NMSB_59 NMSB_15 NMSB_33NMSB_3 NMSB_3 NMSB_85 NMSB_85 NMSB_12 NMSB_12 NMSB_57 NMSB_57 EP_2A NMSB_90 NMSB_8 EP_2B EP_1 PA_9 IH_1 IH_14 AH_4 AH_6 AH_1 AH_32 BA_6 Sou th B ranchNin e M i leCree k N or t h B r a nch NineMil e Creek Blo o m i n g t o nBloomington E de n P r a i rieEden P ra i ri e Min n e t o n k aMinnetonka Barr Footer: ArcGIS 10.4.1, 2018-03-26 11:28 File: I:\Client\Edina\Projects\CRWMP_Update_2017\Maps\Reports\Figures_CityReviewDraft\Fig_9_4_NMC_South_Fork_Water_Quality.mxd User: EMANINE MILE CREEK SOUTH FORKWATER QUALITY MODELING RESULTSWater ResourceManagement PlanCity of Edina, Minnesota FIGURE A.6.1 1,200 0 1,200Feet !;N 400 0 400Meters Percent TP Removal in Water Body*This number represents the percent of the total annual massof phosphorus entering the water body that is removed. Cumulative TP Removal in Watershed*This number represents the percent of the total annual massof phosphorus entering the watershed and upstream watershedsthat is removed in the pond and all upstream ponds. *Data based on results of 2003 P8 modeling. 0 - 25% (Poor/No Treatment) 25 - 40% (Moderate Removal) 40 - 60% (Good Removal) 60 - 100% (Excellent Removal) 0 - 25% (Poor/No Treatment) 25 - 40% (Moderate Removal) 40 - 60% (Good Removal) 60 - 100% (Excellent Removal) Imagery Source: USDA 2016 NAIP via MnGeo Area Draining Directly to the SouthFork of Nine Mile Creek Flow Direction City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.7-1 A.7 Southwest Ponds (Dewey Hill Road Area) A.7.1 General Description of Drainage Area The City’s interactive web map depicts the “Drainage Area” to the Southwest Ponds drainage area and the individual subwatersheds within this area. The Southwest Ponds watershed is located in southwest Edina, bordered by West 70th Street to the north, West 78th Street to the south, Gleason Road on the west, and the Soo Line railroad on the east. The drainage area encompasses approximately 461 acres that ultimately drain to the South Fork of Nine Mile Creek south of West 78th Street. A.7.1.1 Drainage Patterns The stormwater system within this drainage area comprises storm sewers, ditches, overland flow paths, wetlands, and ponding basins. The Southwest Ponds drainage area has been divided into two “Major Watersheds” based on the drainage patterns. These major watersheds are depicted on the City’s interactive web map. Each major watershed has been further delineated into numerous subwatersheds. The naming convention for each subwatershed is based on the major watershed where it is located. Table A.7.1 lists each major watershed and the associated subwatershed naming convention. Table A.7.1 Major Watersheds within the Southwest Ponds Drainage Basin Major Watershed Subwatershed Naming Convention Southwest Ponds SWP_## Nine Mile—I-494 NM494_## A.7.1.1.1 Southwest Ponds The Southwest Ponds watershed encompasses approximately 411 acres. The land use within the watershed is mainly low- and medium-density residential, in addition to the commercial and industrial area along Cahill Road and Lewis Park (the eastern portion of the watershed). The watershed is characterized by a series of ponding basins that outlet to the South Fork of Nine Mile Creek via a storm sewer system that travels south from the intersection of West 78th Street and Delaney Boulevard and discharges to a detention pond north of I-494. Discharge from this detention pond flows beneath I-494 and enters the South Fork of Nine Mile Creek. A.7.1.1.2 Nine Mile I-494 The Nine Mile I-494 watershed encompasses approximately 50 acres. The land use within the watershed is mainly low- and medium-density residential. There is one stormwater detention basin within the watershed. The watershed ultimately drains to the South Fork of Nine Mile Creek through a storm sewer system that discharges to the creek southeast of the intersection of Marth Court and West 78th Street, on the north side of I-494 City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.7-2 A.7.2 Stormwater System Results A.7.2.1 Hydrologic/Hydraulic Modeling Results The 10- and 1-percent-annual-chance flood analyses were performed for the Southwest Ponds drainage basin. The 10-percent-annual-chance event was based on a 24-hour storm with 4.29 inches of rain. The 1-percent-annual-chance analysis was based on a 24-hour storm event with 7.47 inches of rain and on a 10-day snowmelt event with 7.2 inches of runoff; the higher resulting flood level of the two events was chosen for the 1-percent-annual-chance analysis. Data available from the City’s interactive web map presents the watershed information and the results for the 1-percent-annual-chance hydrologic analyses for the Southwest Ponds basin. A more detailed description of the stormwater system analysis is provided in Section A.1.1.3. The City’s interactive web map depicts the Southwest Ponds drainage basin boundary, subwatershed boundaries, the modeled storm sewer network, and the flood-prone areas identified in the modeling analyses. To evaluate the level of protection of the stormwater system within the Southwest Ponds drainage area, the 1-percent-annual-chance flood elevations for the ponding basins and depressed areas were compared to the low elevations of structures surrounding each basin. The areas predicted to be inundated during the 1-percent-annual-chance storm event are shown on the City’s interactive web map. Discussion and recommended improvement considerations for these areas are included in Section A.7.3. A.7.2.2 Water Quality Modeling Results The effectiveness of the stormwater system in removing stormwater pollutants such as phosphorus was analyzed using the P8 water quality model. The P8 model simulates the hydrology and phosphorus loads introduced from each pond’s watershed and the transport of phosphorus throughout the stormwater system. A more detailed description of the stormwater system analysis is provided in Section A.1.1.3. Figure A.7.1 depicts the results of the water quality modeling for the Southwest Ponds drainage basin. The figure shows the fraction of total phosphorus removal for each water body as well as the cumulative total phosphorus removal in the watershed. A.7.3 Implementation Considerations The XPSWMM hydrologic and hydraulic modeling analyses in support of the 2018 WRMP and 2003 P8 water quality analysis helped identify locations throughout the watershed where improvements to the City’s stormwater management system may be warranted. The following sections discuss potential improvement options identified as part of the modeling analyses. As opportunities to address identified flooding issues and improve water quality arise (e.g., street reconstruction projects or public facilities improvements), the City will use a comprehensive approach to stormwater management. This approach will include consideration of infiltration or volume retention practices to address flooding and/or improve water quality, reduction of impervious surfaces, increased storm sewer capacity where necessary to City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.7-3 alleviate flooding, construction and/or expansion of water quality basins, and implementation of other stormwater BMPs to reduce pollutant loading to downstream waterbodies. A.7.3.1 Potential Flood Risk Reduction Options The 2018 WRMP identified several locations within the Southwest Ponds drainage basin where the 1- percent-annual-chance level of protection is not provided by the current stormwater system. As part of the 2018 WRMP modeling analyses, potential corrective measures were identified for problem areas. These preliminary corrective measures are also discussed. As the City evaluates flooding issues and potential system modifications in these areas, other potential modifications, including (but not limited to) implementation of volume-retention practices, increases in conveyance capacity, and/or stormwater infiltration (where soils are conducive) will be given consideration. The 2003 hydrologic and hydraulic modeling analyses also identified several locations within the Southwest Ponds drainage basin where the 1-percent-annual-chance level of protection was not provided by the stormwater system, based on TP-40 precipitation frequency estimates. The discussions related to those areas are included in Appendix B of the 2022 WRMP, along with a short summary of what has been done since 2003. A.7.3.1.1 Gleason Road and Bonnie Brae Drive (SWP_24) A local depression exists in the backyards of the homes along Gleason Road, Bonnie Brae Drive, and Hyde Park Drive, with a single 18-inch outlet positioned east. The outlet size increases to 30 inches before the pipes discharge to the existing wet pond to the east. This pipe size increase was part of a 2015 street reconstruction project. Modeling results indicate that flooding would occur in the backyard depression during the 1-percent-annual-chance 24-hour storm event, with a peak flood level of 842.3 feet. Based on LiDAR data and building footprints, two principal structures are potentially impacted by this flooding (7501 and 7505 Gleason Road), and two principal structures are nearly potentially impacted (7436 and 7500 Hyde Park Drive). The problem is mostly caused by the limited capacity of the remaining 18-inch storm sewer pipe. The first recommendation is to survey the potentially impacted principal structures to verify the impact. If the impacts exist, flood-proofing these eight principal structures may be the most cost-effective solution. Alternatively, additional flood storage could be created in the City-owned parcels south of Bonnie Brae Drive, and storm sewer could be added to connect the backyard depression to the new storage. Finally, the remaining portion of the existing storm sewer pipe that has not be upsized could be modified to add more outlet capacity. A.7.3.2 Potential Construction/Upgrade of Water Quality Basins The 2003 P8 modeling analysis indicated that under average conditions the annual removal of total phosphorus from several ponds in the Southwest Ponds drainage area was below the desired 60 percent rate. For those ponds with total phosphorus removal below 60 percent, the permanent pool storage volume was analyzed to determine whether additional capacity is necessary. Based on the MPCA- City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.7-4 recommended permanent pool storage volume for detention basins, all of the basins were found to have sufficient dead storage volume. As a result, no specific recommendations for water quality basin upgrades in the Southwest Ponds drainage basin have been made based on the 2003 analysis. Construction of new or expansion of existing water quality basins is one method to increase the pollutant removal achieved prior to stormwater reaching downstream waterbodies. Many additional techniques are available to reduce pollutant loading, including impervious surface reduction or disconnection, implementation of infiltration or volume-retention BMPs, installation of underground stormwater treatment structures and sump manholes, and other good housekeeping practices such as street sweeping. As opportunities arise, the City will consider all of these options to reduce the volume and improve the quality of stormwater runoff. NM494_4 SWP_40 SWP_47 SWP_37 SWP_34 SWP_35 SWP_3 SWP_3 SWP_14 SWP_5 SWP_59SWP_4 SWP_2 SWP_10 SWP_31 SWP_1 SWP_33 SWP_58 SWP_57 SWP_9 Nine M ile Cre ek N i n e MileCreek B l oo m in g t o nBloomington Barr Footer: ArcGIS 10.4.1, 2018-03-26 11:28 File: I:\Client\Edina\Projects\CRWMP_Update_2017\Maps\Reports\Figures_CityReviewDraft\Fig_10_4_SW_Ponds_Water_Quality_Rev052014.mxd User: EMASOUTHWEST PONDSWATER QUALITY MODELING RESULTSWater ResourceManagement PlanCity of Edina, Minnesota FIGURE A.7.1 600 0 600Feet !;N 200 0 200Meters Percent TP Removal in Water Body*This number represents the percent of the total annual massof phosphorus entering the water body that is removed. Cumulative TP Removal in Watershed*This number represents the percent of the total annual massof phosphorus entering the watershed and upstream watershedsthat is removed in the pond and all upstream ponds. *Data based on results of 2003 P8 modeling. 0 - 25% (Poor/No Treatment) 25 - 40% (Moderate Removal) 40 - 60% (Good Removal) 60 - 100% (Excellent Removal) 0 - 25% (Poor/No Treatment) 25 - 40% (Moderate Removal) 40 - 60% (Good Removal) 60 - 100% (Excellent Removal) Imagery Source: USDA 2016 NAIP via MnGeo Area Draining Directly to the SouthFork of Nine Mile Creek Flow Direction City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.8-1 A.8 TH 169 North A.8.1 General Description of Drainage Area The City’s interactive web map depicts the TH 169 North drainage area and the individual subwatersheds within this area. The TH 169 North drainage area is located in the northwest corner of Edina. The drainage area encompasses approximately 141 acres that ultimately drain to the TH 169 drainage system. A.8.1.1 Drainage Patterns The stormwater system within this drainage area comprises storm sewers, ditches, overland flow paths, and ponding basins. Stormwater from this drainage area ultimately flows to the TH 169 storm sewer system at several locations along TH 169 between the intersection of the highway with Malibu Drive and the Edina City limits. The TH 169 North drainage area has only one major watershed, also referred to as TH 169 North . The drainage area has been delineated into several subwatersheds. Table A.8.1 describes the naming convention for subwatersheds within the drainage area. Land use within the drainage area includes low-density residential, open area, Van Valkenburg Park, and a small commercial area. Table A.8.1 Major Watershed within the TH 169 North Drainage Area Major Watershed Subwatershed Naming Convention TH 169 North 169N_## A.8.2 Stormwater System Results A.8.2.1 Hydrologic/Hydraulic Modeling Results The 10-percent-annual-chance and 1-percent-annual-chance flood analyses were performed for the TH 169 North drainage area. The 10-percent-annual-chance analysis was based on a 24-hour storm with 4.29 inches of rain. The 1-percent-annual-chance analysis was based on a 24-hour storm event with 7.47 inches of rain and on a 10-day snowmelt event with 7.2 inches of runoff; the higher resulting flood level of the two events was chosen for the 1-percent-annual-chance analysis. Data available from the City’s interactive web map presents the watershed information and the results for the 1-percent-annual- chance hydrologic analyses for the TH 169 North drainage area. A more detailed description of the stormwater system analysis is provided in Section A.1.1.3. The City’s interactive web map depicts the TH 169 North drainage area boundary, subwatershed boundaries, the modeled storm sewer network, and the flood-prone areas identified in the modeling analyses. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.8-2 To evaluate the level of protection of the stormwater system within the TH 169 North drainage area, the 1-percent-annual-chance flood elevations for the ponding basins and depressed areas were compared to the low elevations of structures surrounding each basin. At this time, none of the areas in the TH 169 North drainage area have been evaluated for flood risk management improvements. A.8.2.2 Water Quality Modeling Results The effectiveness of the stormwater system in removing stormwater pollutants such as phosphorus was analyzed using the P8 water quality model. The P8 model simulates the hydrology and phosphorus loads introduced from each pond’s watershed and the transport of phosphorus throughout the stormwater system. A more detailed description of the stormwater system analysis is provided in Section A.1.1.3. Figure A.8.1 depicts the results of the water quality modeling for the TH 169 North drainage area. The figure shows the fraction of total phosphorus removal for each water body as well as the cumulative total phosphorus removal in the watershed. A.8.3 Implementation Considerations The XPSWMM hydrologic and hydraulic modeling analyses in support of the 2018 WRMP and 2003 P8 water quality analysis helped identify locations throughout the watershed where improvements to the City’s stormwater management system may be warranted. The following sections discuss potential improvement options identified as part of the modeling analyses. As opportunities to address identified flooding issues and improve water quality arise (e.g., street reconstruction projects or public facilities improvements), the City will use a comprehensive approach to stormwater management. This approach will include consideration of infiltration or volume retention practices to address flooding and/or improve water quality, reduction of impervious surfaces, increased storm sewer capacity where necessary to alleviate flooding, construction and/or expansion of water quality basins, and implementation of other stormwater BMPs to reduce pollutant loading to downstream waterbodies. A.8.3.1 Potential Flood Risk Reduction Options The 2018 WRMP identified several locations within the TH 169 North drainage basin where the 1-percent- annual-chance level of protection is not provided by the current stormwater system. However, at this time, none of those areas have been evaluated further to identify possible improvement options. It is recommended that those areas be evaluated in the future. A.8.3.2 Potential Construction/Upgrade of Water Quality Basins The 2003 P8 modeling analysis indicated that under average conditions the predicted annual removal of total phosphorus from Pond 169N_16 in the TH 169 North drainage area was below the desired 60 percent removal rate. The permanent pool storage volume was analyzed to determine whether additional capacity was necessary. Based on the MPCA-recommended volume of permanent pool storage for removal of particulate phosphorus, the basin was found to have sufficient dead storage volume. As a result, no specific recommendations for water quality basin upgrades in the TH 169 North drainage area have been made based on the 2003 analysis. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.8-3 Construction of new or expansion of existing water quality basins is one method to increase the pollutant removal achieved prior to stormwater reaching downstream waterbodies. Many additional techniques are available to reduce pollutant loading, including impervious surface reduction or disconnection, implementation of infiltration or volume-retention BMPs, installation of underground stormwater treatment structures and sump manholes, and other good housekeeping practices such as street sweeping. As opportunities arise, the City will consider all of these options to reduce the volume and improve the quality of stormwater runoff. 169N_16 169N_15 H o pk i n sHopkins Barr Footer: ArcGIS 10.4.1, 2017-09-22 12:14 File: I:\Client\Edina\Projects\CRWMP_Update_2017\Maps\Reports\Figures_CityReviewDraft\Fig_11_4_TH169_Ponds_Water_Quality.mxd User: jrvTH 169 NORTHWATER QUALITY MODELING RESULTSWater ResourceManagement PlanCity of Edina, Minnesota FIGURE A.8.1 400 0 400Feet !;N 150 0 150Meters Percent TP Removal in Water Body*This number represents the percent of the total annual massof phosphorus entering the water body that is removed. Cumulative TP Removal in Watershed*This number represents the percent of the total annual massof phosphorus entering the watershed and upstream watershedsthat is removed in the pond and all upstream ponds. *Data based on results of 2003 P8 modeling. 0 - 25% (Poor/No Treatment) 25 - 40% (Moderate Removal) 40 - 60% (Good Removal) 60 - 100% (Excellent Removal) 0 - 25% (Poor/No Treatment) 25 - 40% (Moderate Removal) 40 - 60% (Good Removal) 60 - 100% (Excellent Removal) Imagery Source: USDA 2016 NAIP via MnGeo Flow Direction City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.9-1 A.9 Northeast Minnehaha Creek A.9.1 General Description of Drainage Area The City’s interactive web map depict the Northeast Minnehaha Creek drainage area and the individual subwatersheds within this area. The Northeast Minnehaha Creek drainage area is located in the northeast corner of Edina. This watershed contains a limited number of ponds and no lakes. A.9.1.1 Drainage Patterns The stormwater system within this drainage area comprises storm sewers, ponding basins, wetlands, drainage ditches, and overland flow paths. The Northeast Minnehaha Creek basin has been divided into several major watersheds, based on the drainage patterns. These major watersheds are depicted on the City’s interactive web map. Each major watershed has been further delineated into numerous subwatersheds. The naming convention for each subwatershed is based on the major watershed where it is located. Table A.9.1 lists each major watershed and the associated subwatershed naming convention. Table A.9.1 Major Watersheds within the Northeast Minnehaha Creek Drainage Basin Major Watershed Subwatershed Naming Convention Morningside MS_## Minnehaha Creek North MHN_## / MHC_## Edina Country Club ECC_## A.9.1.1.1 Morningside The 232-acre Morningside watershed is located in the northeast corner of Edina, primarily north of West 44th Street. Land use in this watershed, which includes Weber Park, is primarily single-family residential. The Edina trunk storm sewer system through this area connects to the incoming St. Louis Park system just southeast of Susan Lindgren Elementary School (Natchez Avenue and 41st Street). From this junction the system runs east to the east side of Weber Park and an inlet/outlet to the Weber Park pond. The inlet/outlet allows stormwater to flow into the basin until the head differential between the basin and trunk sewer system results in a discharge from the basin. From the Weber Park Pond, the system drains north to St. Louis Park and then east to connect with the Minneapolis system, eventually draining to Lake Calhoun. A.9.1.1.2 Minnehaha Creek North The Minnehaha Creek North watershed lies primarily east of Minnehaha Creek, west of France Avenue, north of West 54th Street and south of West 44th Street. There are only two wetlands within this 564-acre watershed and no ponds; all other areas discharge directly to Minnehaha Creek. Land use is primarily City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.9-2 single-family residential; however, there is some commercial land adjacent to France Avenue. There is very little open space in this watershed except for areas directly adjacent to Minnehaha Creek. A.9.1.1.3 Edina Country Club The Edina Country Club watershed is a small 116-acre watershed that encompasses the Edina Country Club golf course and areas east of the Country Club to Minnehaha Creek. The watershed area outside of the golf course is low-density residential and contains no ponds or wetlands; all areas discharge directly to Minnehaha Creek. There are no known pipes connecting the ponds of the Edina Country Club to the adjacent storm sewer network along Wooddale Avenue. A.9.2 Stormwater System Results A.9.2.1 Hydrologic/Hydraulic Modeling Results The 10-, and 1-percent-annual-chance flood analyses were performed for the Northeast Minnehaha Creek drainage basin. For the Minnehaha Creek North and the Edina Country Club drainage areas, the storm sewers were evaluated using 10- and 1-percent-annual-chance storm events. The 10-percent-annual- chance analysis was based on a 24-hour storm with 4.29 inches of rain. The 1-percent-annual-chance analysis was based on a 24-hour storm event with 7.47 inches of rain and on a 10-day snowmelt event with 7.2 inches of runoff; the higher resulting flood level of the two events was chosen for the 1-percent- annual-chance analysis. Data available from the City’s interactive web map presents the watershed information and the results for the 10-, and 1-percent-annual-chance hydrologic analyses for the Northeast Minnehaha Creek basin. A more detailed description of the stormwater system analysis is provided in Section A.1.1.3. The results of the 1-percent-annual-chance hydraulic analyses for the Northeast Minnehaha Creek drainage basin are available from the City’s interactive web map. Modeling results for a range of flood risk reduction options are presented in Table A.9.4 and Table A.9.3 for the 10-, and 1-percent-annual-chance hydraulic analyses. The City’s interactive web map depicts the Northeast Minnehaha Creek drainage basin boundary, subwatershed boundaries, the modeled storm sewer network, and the flood-prone areas identified in the modeling analyses. To evaluate the level of protection of the stormwater system within the Northeast Minnehaha Creek drainage area, the 1-percent-annual-chance flood elevations for the ponding basins and depressed areas were compared to the low elevations of structures surrounding each basin. The areas predicted to be inundated during the 1-percent-annual-chance storm event are shown on the City’s interactive web map. Additional modeling results evaluating multiple flood risk reduction options are presented in this section. Discussion and recommended improvement considerations for these areas are included in Section A.9.3. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.9-3 A.9.2.2 Water Quality Modeling Results The effectiveness of the stormwater system in removing stormwater pollutants such as phosphorus was analyzed using the P8 water quality model. The P8 model simulates the hydrology and phosphorus loads introduced from each pond’s watershed and the transport of phosphorus throughout the stormwater system. A more detailed description of the stormwater system analysis is provided in Section A.1.1.3. Figure A.9.8 depicts the results of the water quality modeling for the Northeast Minnehaha Creek drainage basin. The figure shows the fraction of total phosphorus removal for each water body as well as the cumulative total phosphorus removal in the watershed. A.9.3 Implementation Considerations The XPSWMM hydrologic and hydraulic modeling analyses in support of the 2018 WRMP and 2003 P8 water quality analysis helped identify locations throughout the watershed where improvements to the City’s stormwater management system may be warranted. The following sections discuss potential improvement options identified as part of the modeling analyses. As opportunities to address identified flooding issues and improve water quality arise (e.g., street reconstruction projects or public facilities improvements), the City will use a comprehensive approach to stormwater management. This approach will include consideration of infiltration or volume retention practices to address flooding and/or improve water quality, reduction of impervious surfaces, increased storm sewer capacity where necessary to alleviate flooding, construction and/or expansion of water quality basins, and implementation of other stormwater BMPs to reduce pollutant loading to downstream waterbodies. A.9.3.1 Potential Flood Risk Reduction Options The 2018 WRMP identified several locations within the Northeast Minnehaha Creek drainage basin where the 1-percent-annual-chance level of protection is not provided by the current stormwater system. As part of the 2018 WRMP modeling analyses, potential corrective measures were identified for problem areas. These preliminary corrective measures are also discussed below. As the City evaluates the flooding issues and potential system modifications in these areas, other potential modifications, including (but not limited to) implementation of volume-retention practices, increases in conveyance capacity, and/or stormwater infiltration (where soils are conducive) will be given consideration. The 2003 hydrologic and hydraulic modeling analyses also identified several locations within the Northeast Minnehaha Creek drainage basin where the 1-percent-annual-chance level of protection was not provided by the stormwater system, based on TP-40 precipitation frequency estimates. The discussions related to those areas are included in Appendix B of the 2022 WRMP, along with a short summary of what has been done since 2003. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.9-4 A.9.3.1.1 Halifax Avenue South (MHN_84, MHN_3, MHN_56, MHN_89, MHN_55, MHN_61, MHN_62, MHN_63, MHN_87, MHN_88, MHN_90, and MHN_2) and Indianola Avenue South of West 50th Street (MHN_4, MHN_5, MHN_42 and MHN_72) – updated 2021 Halifax Avenue South and Indianola Avenue south of 50th Street West, including the surrounding areas, have historically experienced flooding. These two flood-prone areas (collectively referred to as the study area) are within the Northeast Minnehaha Creek drainage basin, which is tributary to Minnehaha Creek near 52nd Street West. The first flood-prone area is located east of Minnehaha Creek, west of France Avenue, and north of 54th Street West. Within this study area, flooding occurs primarily at localized depressions along Halifax Avenue South and adjacent yards, and at the intersection of Halifax Avenue South and 52nd Street West, as shown on the City’s interactive web map. Halifax Avenue South, from 54th Street West up to and including 52nd Street West, is undulating; two low areas occur along the roadway and extend onto the adjacent properties. The two low-lying areas along Halifax Avenue South are drained by an existing 18- inch RCP storm sewer system that conveys stormwater to the intersection of Halifax Avenue South and 52nd Street West and, ultimately, to Minnehaha Creek. However, when stormwater flows exceed the capacity of the existing system, water will flow north only after rising above the downstream crests in the roadway. The controlling crest elevations for the southern and northern low-lying depressions along Halifax Avenue South are 878.9 feet and 879.9, respectively. Another significant depression exists at the intersection of Halifax Avenue South and 52nd Street West. At this intersection, the trunk storm sewer from Halifax Avenue South increases to 48-inches in size and conveys stormwater west from the intersection along 52nd Street West to Minnehaha Creek. Parallel to the 48-inch trunk storm sewer, a 24-inch trunk storm sewer accepts stormwater runoff from the Halifax Avenue South and 52nd Street West intersection and runs along 52nd Street West to Minnehaha Creek. An equalization pipe connects the 24-inch and 48-inch pipes. However, when stormwater flows exceed the capacity of the existing system at the intersection, water pools in the depression at the intersection and adjacent yards. Pooled water will ultimately flow west over the downstream crest along 52nd Street West if it rises above the controlling roadway crest elevation of 882.9 feet near 4201 52nd Street West. The Indianola Avenue flood-prone area is east of Minnehaha Creek, north of 52nd Street West, and south of 50th Street West. As with Halifax Avenue South, the flooding problems in this area are primarily related to the limited conveyance capacity of the storm sewer system. Additionally, when the capacity of the downstream system is exceeded (to the northwest), surface flows from 50th Street West can overflow to Indianola, around the church and further fill this low-lying area. During large events such as the 1- and 2- percent-annual-chance storms, this area fills enough to overflow to the south, and contribute flow to Juanita and Indianola Avenues. On Juanita Avenue, this is less of an issue because that road continually slopes south and then to the creek. On the other hand, Indianola slopes to a localized depression in the road, which fills and allows water from the street to flow west between homes into the backyard depression. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.9-5 There are other localized potential flood risk areas associated with water flowing from streets or alleys, between homes ultimately to the creek. The emergency overflow between 5028 Bruce Avenue and 5030 Bruce Avenue is an example of one that should be maintained and not filled in with landscaping, for instance. Previous hydrologic and hydraulic modeling efforts have indicated significant flood risk potential at the intersection of 52nd Street West and Halifax Avenue South, along with portions of Halifax Avenue South from 54th Street West to 52nd Street West. The detail of the modeling has been increased to a combined 1D/2D model and current modeling results indicate that the 1-percent-annual-chance flood elevations may impact up to 35 principal structures in the area (based on LiDAR and approximate building footprint data). For larger events (25-year event and greater), inundated water in low points along Halifax Avenue South cannot flow into the catch basins, largely because the tailwater elevation in the storm sewer system is so high. For smaller events (less than the 25-year event), inlet capacity along Halifax Avenue South is not an issue that would cause impacts to structures. There is road flooding, but it is contained within the low points along the road. Street reconstruction is not planned in this area by the city in the next few years and therefore does not offer a near-term opportunity to install significant infrastructure projects. Options to address the inlet and pipe capacity issues in this area would be to add pipe capacity in various places and/or add storage in upstream areas. We do not expect that there is a near-term opportunity to do a large pipe capacity project and so those types of options were not included here. Two main flood risk reduction options were evaluated in detail: (1) underground storage under the church parking lot in the southeast of the neighborhood, (2) localized regrading of Indianola Avenue to force overflow water west along Juanita Avenue (an opportunity to partner with the church to redo the parking lot), and (3) a small inlet and pipe capacity increase project on Indianola Avenue. These two options are isolated enough that they were combined as one option. The storage vault would require small pumps (a few hundred gallons per minute) to dewater them after storm or snowmelt events. While these alternatives do not remove all of the structures from flood risk (up to the 1-percent-annual- chance event), they do reduce the flood risk in this neighborhood (see Table A.9.2). The storage at the church parking lot provides a significant benefit for the homes on the east side of Halifax, particularly for the smaller storm (4-percent-annual-chance event and more frequent). Because of the limited volume of that storage, it is not as effective for the same structures in the larger storms. For the larger storms, the simple road grading and additional pipe provide a benefit for the homes along Indianola Avenue. With regard to moving the stormwater downstream, keeping the water in a street that continually drains (Juanita Avenue) is more beneficial than letting that water pool in a low point of a street (Indianola Avenue). City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.9-6 Table A.9.2 Residential Structure Impacts in the Halifax area Flood Risk Reduction Option Number of At-Risk Principal Structures (Reduction from Existing Conditions) 20-percent-annual-chance event 10-percent-annual-chance event 4-percent-annual-chance event 2-percent-annual-chance event 1-percent-annual-chance event Existing Conditions 10 13 20 32 35 Option 1: UG Storage at the Church, Road Grading, Increase Inlet and Pipe Capacity 9 (-1) 10 (-3) 15 (-5) 22 (-10) 27 (-8) Based on the City’s available data, five of the potentially impacted homes have been rebuilt since the LiDAR was flown in 2011. The following list describes the flood condition at those four homes that appear to be impacted based on 2011 MnDNR LiDAR elevation data available, but additional elevation data from the City suggests they would likely not be impacted. • 5112 Indianola Avenue; water flows around the house during the 1-percent-annual-chance storm at about 880.4 feet; the first floor level is 882.0 feet. • 5124 Indianola Avenue; water flows around the house during the 1-percent-annual-chance storm at about 880.4 feet; the first floor level is 881.8 feet. • 5128 Indianola Avenue; water flows around the house during the 1-percent-annual-chance storm at about 880.4 feet; the first floor level is 882.4 feet. • 5101 Indianola Avenue; the 1-percent-annual-chance existing flood level is 880.6 feet; the first floor level is 883.4 feet. Beyond the options described herein, the strategies of requiring minimum elevations for newly built or rebuilt homes and maintaining emergency overflows may be the most successful strategies for the remaining low-lying homes in this neighborhood. Proposed Infrastructure Options to Reduce Flood Risk Six potential flood risk reduction options were evaluated using the 1D/2D XPSWMM model for the Halifax Avenue South and Indianola Avenue south of 50th Street West study area. The evaluated options address both areas of increased flood risk in the study area: the Halifax Avenue South area, and the Indianola Avenue south of 50th Street West area. Five of the six options consist of an individual approach (i.e., extra pipe capacity, storage, etc.) to address each of the flood-prone areas, although the approach may differ between the two areas. The sixth option represents a combination of approaches to address the Indianola Avenue area. Five 24-hour storm events were simulated (the 20-, 10-, 4-, 2-, and the 1-percent-annual-chance events) for each flood risk reduction City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.9-7 option to evaluate the benefit of each flood risk reduction option. The number of potentially impacted residential principal structures was evaluated for each option and each modeled storm event (see Table A.9.5). The six flood risk reduction options are described below, and additional details are also shown on Figure A.9.2 through Figure A.9.7. Option 1 – Increased storm sewer capacity at Halifax flood-prone area and added surface storage at Indianola flood-prone area • Halifax Avenue South Alternative: Increase the capacity of the main trunk storm sewer along 52nd Street West from Halifax Avenue South to the outfall at Minnehaha Creek. The proposed option includes increasing the existing 48-inch storm sewer to a 66-inch storm sewer. This option includes increasing the existing 48-inch junction storm sewer (approximately 13 feet in length) upstream of the main trunk storm sewer running west to Minnehaha Creek to 66-inch storm sewer. This option also includes upsizing the junction storm sewer at the intersection accepting flow from Halifax Avenue South (approximately 9 feet in length) from 18-inch storm sewer to 36-inch storm sewer. • Indianola Avenue Alternative: Provide additional flood storage by excavating (i.e., lowering) the open area of the residential lot located at 5017 Indianola Avenue (approximate volume of 0.9 acre-ft; the approximate footprint is shown on Figure A.9.2). The basin will connect to the storm sewer along Indianola Avenue via a 12-inch outlet. Option 2 – Increased storm sewer capacity at Halifax flood-prone area and added underground storage at Indianola flood-prone area • Halifax Avenue South Alternative: Increase the capacity of the main trunk storm sewer along 52nd Street West from Halifax Avenue South to the outfall at Minnehaha Creek. This option includes increasing the existing 24-inch storm sewer to a 48-inch storm sewer pipe. • Indianola Avenue Alternative: Provide additional flood storage by installing underground chambers (approximate volume of 1.7 acre-ft) under the Mercy Commons Covenant Church parking lot and connect the outlet to the storm sewer conveying stormwater to the 50th Street West storm sewer system. A pumped outlet would likely be necessary to drain the underground storage following storm events. Option 3 – Added underground storage at Halifax flood-prone area and increased storm sewer capacity at Indianola flood-prone area • Halifax Avenue South Alternative: Provide additional flood storage by installing underground chambers (approximate volume of 1.6 acre-ft) under the Calvary Christian Reformed Church parking lot and connect outlets to the existing storm sewer conveying stormwater to the Halifax Avenue South system. A pumped outlet would likely be necessary to drain the underground storage following storm events. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.9-8 • Indianola Avenue Alternative: Increase the capacity of the storm sewer from the low area at Indianola Avenue south of 50th Street West to Jay Place. This option would increase the existing 12- and 18-inch storm sewer to 36-inch storm sewer. Option 4 – Increased storm sewer capacity and conveyance at Halifax and Indianola flood- prone areas • Halifax Avenue South Alternative: Installation of a piped outlet at the backyard depression near 5308 Halifax Avenue South (subwatershed MHN_87), in combination with installation of new 42-inch storm sewer running south along Halifax Avenue South, and west along 54th Street West to a new discharge at Minnehaha Creek. • Indianola Avenue Alternative: Increase capacity of storm sewer along Indianola Avenue from the low depression south of 50th Street West to the intersection of Juanita Avenue and 52nd Street West. This option includes installation of 24-inch storm sewer from Indianola Avenue to Juanita Avenue and increasing the capacity of the existing Juanita Avenue storm sewer from 12-inch to 24-inch storm sewer. Option 5 – Street regrading at Halifax flood-prone area and added underground storage at Indianola flood-prone area • Halifax Avenue South Alternative: Regrade portions of Halifax Avenue South and 52nd Street West to lower the controlling road crest elevations that result in the inundation of the roadway and principal structures, as shown in Figure A.9.1. This option may not be desirable for the foreseeable future because the roadway was recently reconstructed. The reduction in the flood elevation would depend on the final controlling elevations after regrading. • Indianola Avenue Alternative: Provide additional flood storage by installing underground chambers (modeled chamber as 200 feet long by 28 feet wide by 7 feet deep, totaling 0.9 acre-ft) below the street at the low depression along Indianola Avenue south of 50th Street West. The underground storage would discharge to the existing system, which conveys stormwater from the Indianola Avenue low area to 50th Street West via Jay Place. A pumped outlet would likely be necessary to drain the underground storage following storm events. Option 6 – Added underground storage at Halifax flood-prone area, and added surface storage and increased storm sewer conveyance at Indianola flood-prone area • Halifax Avenue South Alternative: Provide additional flood storage by installing underground chambers (approximate volume of 1.1 acre-ft) under the roadway at the intersection of Halifax Avenue South and 52nd Street West. • Indianola Avenue Alternative: A combination of additional flood storage with underground chambers (as in Option 2) under the Mercy Commons Covenant parking lot and increasing the storm sewer capacity along Indianola Avenue from the low area depression south of 50th Street West to the intersection of Juanita Avenue and 52nd Street West (as in Option 4). This option includes installation of 24-inch storm sewer from Indianola Avenue to Juanita Avenue City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.9-9 and increasing the capacity of the existing Juanita Avenue storm sewer from 12-inch to 24- inch storm sewer. Figure A.9.1 Existing and proposed street profile considered for reducing flood risk in the Halifax Avenue South area for Option 5 Some of the options discussed above send water to Minnehaha Creek more quickly. Because the local peak discharges occur in these subwatersheds much earlier than peak flows in Minnehaha Creek following a large event, timing may allow for additional water to be added to the creek and passed downstream without negatively impacting peak flood level or the number of potentially impacted structures along the creek. Arden Park France Ave S52nd St W Maple RdTownesRdEdina CtW o o d d a l eLn Arden Ave52nd St W 49 1/2 St W Indianola AveBruce Ave54th St W Fuller St 53rd St W 52nd St W 51st St W 50th St W 49th St W Halifax Ave51st St W51st St WCasco Ave53rd St W C o u n tr y C l u b R d Jay PlKellogg AveOaklawn AveBrookview AveHalifax LnBruce Pl Park Pl Gorgas AveJuanita AveMinnehaha BlvdBarr Footer: ArcGIS 10.7.1, 2021-02-19 09:47 File: I:\Client\Edina\Projects\CWRMP_Amendment_2D_Modeling\Maps\Basemaps\Halifax Results\Alternative 1.mxd User: EMA FLOOD RISK REDUCTIONOPTION 1 - HALIFAX/INDIANOLAWater Resources Management PlanCity of Edina FIGURE A.9.2 !;N 0 150 300 Feet Proposed Indianola Ave Alternative: 0.85 acre-feet surfacestorage at vacant residential lot, 5017 Indianola Ave. Proposed 52nd St W & Halifax Ave Alternative: Increase existing48-inch diameter pipe to 66-inch diameter pipe (southern of dual pipes conveying stormwater from east to west) Surface Storageat Vacant Lot Increase 52nd St WStorm Sewer Capacity Arden Park France Ave S52nd St W Maple RdTownesRdEdina CtW o o d d a l eLn Arden Ave52nd St W 49 1/2 St W Indianola AveBruce Ave54th St W Fuller St 53rd St W 52nd St W 51st St W 50th St W 49th St W Halifax Ave51st St W51st St WCasco Ave53rd St W C o u n tr y C l u b R d Jay PlKellogg AveOaklawn AveBrookview AveHalifax LnBruce Pl Park Pl Gorgas AveJuanita AveMinnehaha BlvdBarr Footer: ArcGIS 10.7.1, 2021-02-19 10:14 File: I:\Client\Edina\Projects\CWRMP_Amendment_2D_Modeling\Maps\Basemaps\Halifax Results\Alternative 2.mxd User: EMA FLOOD RISK REDUCTIONOPTION 2 - HALIFAX/INDIANOLAWater Resources Management PlanCity of EdinaFIGURE A.9.3 !;N 0 150 300 Feet Proposed Indianola Ave Alternative: 1.7 acre-feet underground storage at Mercy Commons Covenant Church Proposed 52nd St W & Halifax Ave Alternative: Increase existing24-inch diameter pipe to 48-inch diameter pipe (northern of dual pipes conveying stormwater from east to west) Mercy Commons Covenant ChurchParking Lot Underground Storage Increase 52nd St WStorm Sewer Capacity Calvary Christian Reformed Church Underground Storage Increase Storm Sewer Capacityfrom Indianola Ave to Jay Place Arden Park France Ave S52nd St W Maple RdTownesRdEdina CtW o o d d a l eLn Arden Ave52nd St W 49 1/2 St W Indianola AveBruce Ave54th St W Fuller St 53rd St W 52nd St W 51st St W 50th St W 49th St W Halifax Ave51st St W51st St WCasco Ave53rd St W C o u n tr y C l u b R d Jay PlKellogg AveOaklawn AveBrookview AveHalifax LnBruce Pl Park Pl Gorgas AveJuanita AveMinnehaha BlvdBarr Footer: ArcGIS 10.7.1, 2021-02-25 16:08 File: I:\Client\Edina\Projects\CWRMP_Amendment_2D_Modeling\Maps\Basemaps\Halifax Results\Alternative 3.mxd User: EMA FLOOD RISK REDUCTIONOPTION 3 - HALIFAX/INDIANOLAWater Resources Management PlanCity of Edina FIGURE A.9.4 !;N 0 150 300 Feet Proposed Indianola Ave Alternative: Increase existing 12-inch and 18-inch diameter pipes to 36-inch diameter pipes from Indianola Ave to Jay Place Proposed 52nd St W & Halifax Ave Alternative: 1.6 acre-feetunderground storage at Calvary Christian Reformed Churchparking lot Install Piped Outlet at BackyardDepression & Storm Sewer toMinnehaha Creek Addition of Storm Sewer from Indianola Depressionto Juanita Ave System Increase Storm SewerCapacity from Juanita Aveto 52nd St W Arden Park France Ave S52nd St W Maple RdTownesRdEdina CtW o o d d a l eLn Arden Ave52nd St W 49 1/2 St W Indianola AveBruce Ave54th St W Fuller St 53rd St W 52nd St W 51st St W 50th St W 49th St W Halifax Ave51st St W51st St WCasco Ave53rd St W C o u n tr y C l u b R d Jay PlKellogg AveOaklawn AveBrookview AveHalifax LnBruce Pl Park Pl Gorgas AveJuanitaAveMinnehaha BlvdBarr Footer: ArcGIS 10.7.1, 2021-02-25 16:18 File: I:\Client\Edina\Projects\CWRMP_Amendment_2D_Modeling\Maps\Basemaps\Halifax Results\Alternative 4.mxd User: EMA FLOOD RISK REDUCTIONOPTION 4 - HALIFAX/INDIANOLAWater Resources Management PlanCity of Edina FIGURE A.9.5 !;N 0 150 300 Feet Proposed Indianola Ave Alternative: Install 24-inch storm sewerfrom Indianola Ave to Juanita Ave. Increase capacity of Juanita Ave storm sewer from 12-inch storm sewer to 24-inch storm sewer Proposed 52nd St W & Halifax Ave Alternative: Install piped outletat backyard depression. Install 42-inch storm sewer running southalong Halifax Ave to 54th St W, then west to Minnehaha Creek RegradeHalifax Ave S & 52nd St W Underground Storage BelowIndianola Ave Arden Park France Ave S52nd St W Maple RdTownesRdEdina CtW o o d d a l eLn Arden Ave52nd St W 49 1/2 St W Indianola AveBruce Ave54th St W Fuller St 53rd St W 52nd St W 51st St W 50th St W 49th St W Halifax Ave51st St W51st St WCasco Ave53rd St W C o u n tr y C l u b R d Jay PlKellogg AveOaklawn AveBrookview AveHalifax LnBruce Pl Park Pl Gorgas AveJuanita AveMinnehaha BlvdBarr Footer: ArcGIS 10.7.1, 2021-02-25 16:01 File: I:\Client\Edina\Projects\CWRMP_Amendment_2D_Modeling\Maps\Basemaps\Halifax Results\Alternative 5.mxd User: EMA FLOOD RISK REDUCTIONOPTION 5 - HALIFAX/INDIANOLAWater Resources Management PlanCity of EdinaFIGURE A.9.6 !;N 0 150 300 Feet Proposed 52nd St W & Halifax Ave Alternative: Regrade portions ofHalifax Ave S and 52nd St W to remove controlling road crest elevations that result in inundation of roadway and principal structures Proposed Indianola Ave Alternative: 0.9 acre-feet of undergroundstorage below Indianola Ave Underground Storageat 52nd St W andHalifax Ave SIntersection Mercy Commons Covenant ChurchParking Lot Underground Storage Increase Storm SewerCapacity to 52nd St W Arden Park France Ave S52nd St W Maple RdTownesRdEdina CtW o o d d a l eLn Arden Ave52nd St W 49 1/2 St W Indianola AveBruce Ave54th St W Fuller St 53rd St W 52nd St W 51st St W 50th St W 49th St W Halifax Ave51st St W51st St WCasco Ave53rd St W C o u n tr y C l u b R d Jay PlKellogg AveOaklawn AveBrookview AveHalifax LnBruce Pl Park Pl Gorgas AveJuanitaAveMinnehaha BlvdBarr Footer: ArcGIS 10.7.1, 2021-02-22 14:18 File: I:\Client\Edina\Projects\CWRMP_Amendment_2D_Modeling\Maps\Basemaps\Halifax Results\Alternative 6.mxd User: EMA FLOOD RISK REDUCTIONOPTION 6 - HALIFAX/INDIANOLAWater Resources Management PlanCity of Edina FIGURE A.9.7 !;N 0 150 300 Feet Proposed Indianola Ave Alternative: 1.7 acre-feet of undergroundstorage at Mercy Commons Covenant Church. Proposed Indianola Ave Alternative: Install 24-inch storm sewerfrom Indianola Ave to Juanita Ave. Increase capacity of Juanita Avestorm sewer from 12-inch storm sewer to 24-inch storm sewer. Proposed 52nd St W & Halifax Ave Alternative: 1.1 acre-feet ofunderground storage at intersection of 52nd St W and Halifax Ave S. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.9-16 Modeling Results and Risk Reduction of Proposed Options Modeling of the proposed options yielded varying degrees of flood risk reduction relative to existing conditions. Modeled flood elevations in key locations of the Halifax Avenue South and Indianola Avenue study area are presented in Table A.9.3 and Table A.9.4 for the 1-percent-annual-chance and 10-percent- annual-chance events, respectively. Of the six options, Options 4, 5, and 6 resulted in the greatest benefit (i.e., greatest number of principal structures removed from risk), while the Options 1, 2 and 3 provided less benefit. The number of residential structures at risk in the existing condition and with each proposed option is presented in Table A.9.3. Principal structures that are referred to as “removed from risk” in Table A.9.3 are structures that are no longer at risk of damage for the storm based on modeling results. However, no structure is ever removed from all flood risk. Note that some flood risk reduction options resulted in flood risk being transferred to principal structures previously not at risk under existing conditions, as described in the following section. In Option 4, backyard and street flooding along Halifax Avenue South is reduced due to increased inlet capacity, capture of backyard depression and road flooding, and diversion of stormwater to Minnehaha Creek. Several structures along the west side of Halifax Avenue South near the backyard depression (subwatershed MHN_87) are removed from risk of flooding during the 1-percent-annual-chance event, and the risk is not transferred to other structures (i.e., no structures became at risk which were not at risk under existing conditions). These structures are most likely removed from risk due to the decrease in stormwater pooling at the southern inundation area along Halifax Avenue South. Addition of storm sewer to a new discharge point along the creek from 54th Avenue West alleviates capacity issues in the Halifax Avenue storm sewer system. Within the Indianola Avenue flood-prone area, reduced flood elevations at the Indianola Avenue roadway depression south of 50th Street West removes one structure from flood risk. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.9-17 Table A.9.3 Flood Elevations in Halifax Avenue South and Indianola Avenue Project Area for 1-percent-annual-chance event Number of At-Risk Structures 52nd Street West and Halifax Avenue South Backyard Depression (MHN_87) Backyard Depression (MHN_62) Halifax Avenue South (Southern Low Area) Halifax Avenue South (Northern Low Area) Indianola Avenue south of 50th Street West Existing Conditions 36 876.6 878.1 877.2 877.4 877.2 885.2 Option 1 36 876.8 878.1 876.9 877.3 876.9 885.2 Option 2 36 877.0 878.1 877.1 877.3 877.1 885.2 Option 3 36 876.7 878.1 876.7 877.4 876.7 885.6 Option 4 32 876.6 877.5 877.2 877.5 877.2 885.1 Option 5 33 875.4 878.1 876.8 875.9 875.8 884.8 Option 6 34 877.0 878.1 877.2 877.4 877.2 885.1 Table A.9.4 Flood Elevations in the Halifax Avenue South and Indianola Avenue Project Area for 10-percent-annual-chance event 52nd Street West and Halifax Avenue South Backyard Depression (MHN_87) Backyard Depression (MHN_62) Halifax Avenue South (Southern Low Area) Halifax Avenue South (Northern Low Area) Indianola Avenue south of 50th Street West Existing Conditions 876.0 877.0 876.0 876.1 875.6 885.2 Option 1 875.4 877.0 876.0 876.0 875.6 885.2 Option 2 875.6 877.0 876.0 876.1 875.6 885.2 Option 3 876.1 876.9 874.6 876.1 875.6 885.6 Option 4 876.0 876.0 876.0 876.0 875.6 885.0 Option 5 874.9 876.9 876.0 875.7 875.4 884.7 Option 6 875.9 876.9 876.0 876.1 875.6 885.1 City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.9-18 Table A.9.5 Residential structure impacts in the Halifax Avenue South and Indianola Avenue South areas Flood Risk Reduction Option Number of At-Risk Structures (Reduction from Existing Conditions) 20-percent- annual- chance event 10-percent- annual- chance event 4-percent- annual- chance event 2-percent- annual- chance event 1-percent- annual- chance event Existing Conditions 19 21 24 33 36 Option 1 19 (0) 20 (-1) 23 (-1) 33 (0) 36 (0) Option 2 19 (0) 20 (-1) 23 (-1) 33 (0) 36 (0) Option 3 19 (0) 21 (0) 23 (-1) 34 (+1) 36 (0) Option 4 18 (-1) 20 (-1) 22 (-2) 28 (-5) 32 (-4) Option 5 19 (0) 20 (-1) 22 (-2) 29 (-4) 33 (-3) Option 6 18 (-1) 21 (0) 24 (0) 33 (0) 34 (-2) Discussion of Proposed Options The proposed options discussed in Section A.9.3.1.1 utilize flood storage, increased conveyance, addition of storm sewer, and street regrading to remove residential principal structures from flood risk for storm events between 20-percent-annual-chance event and 1-percent-annual-chance event. The performance of the evaluated options, potential implementation considerations, and prioritization based on the flood risk reduction benefit is discussed in this section. Increased Conveyance Options The options that increased conveyance of stormwater and included street regrading (Options 4 and 5, respectively) were among the most impactful for removing structures from flood risk within the Halifax Avenue South flood-prone area. The options that increased conveyance of stormwater primarily through addition of storm sewer and increased capacity of the existing storm sewer (Option 4), and a combination of underground storage and increased conveyance, were among the most impactful for removing structures from flood risk within the Indianola Avenue flood-prone area. Options 4 and 6 were the most impactful for removing structures from risk, while not transferring risk to principal structures previously not at risk under existing conditions. Flood risk reduction Options 1, 2, 3 and 5 resulted in flood risk being transferred to principal structures previously not at risk under existing conditions. Implementation of portions of these options would likely coincide with street reconstruction projects to minimize disturbance along these residential roads. Implementation of the Halifax Avenue South portion of Option 4 may include removal and replacement of storm sewer along 54th Street West, and installation of storm sewer along Halifax Avenue South. This planning level evaluation has not accounted for feasibility due to potential utility conflicts. Additionally, storm sewer depths along 54th Street West would be approximately 10 to 15 feet below existing grade. This may require significant excavation/trenching. The Halifax Avenue South flood-prone area of Option 4 lowers the 1-percent-annual-chance peak flood City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.9-19 elevation in the backyards west of Halifax Avenue South (subwatershed MHN_87) by 0.6 feet and removes 3 adjacent principal structures from flood risk. Implementation of the Indianola Avenue portion of Option 4 may include removal, replacement, and addition of storm sewer along Indianola Avenue and Juanita Avenue. This planning level evaluation has not accounted for feasibility due to potential utility conflicts. This improvement option lowers the 1- percent-annual-chance peak flood elevation of the Indianola Avenue depression south of 50th Street West by 0.1 feet and removes 1 downstream structure from flood risk, while not transferring risk to other structures in the study area. The lowest two principal structure elevations in the Indianola Avenue flood- prone area are at approximately 883.0 feet and 882.2 feet (based on LiDAR and approximate building footprint data). Further lowering flood levels by approximately 2.2 feet and 3.0 feet, respectively, below the local depression 1-percent-annual-chance flood level of 885.2 feet is necessary to remove the structures from flood risk during the 1-percent-annual-chance event. This is particularly difficult because the overflow elevation along Indianola Avenue to the south is at elevation 884.5 feet. The only storm sewer draining this area is to the west along 50th Street West, and this system backflows into the Indianola Avenue flood-prone area under existing conditions. Effectively, this area is a local depression with no outlet during the 1-percent-annual-chance event. The four lowest structure elevations adjacent to the backyard depression on the east side of Halifax Avenue South (MHN_62) are: • 874.3 feet located at 5241 Halifax Avenue South • 876.6 feet located at 5301 Halifax Avenue South • 875.4 feet located at 5305 Halifax Avenue South • 875.1 feet located at 5309 Halifax Avenue South Protection of these structures during the 1-percent-annual-chance event would require lowering flood elevations by a further 0.6-2.9 feet below the 1-percent-annual-chance flood elevation at the local depression (877.2 feet). This is particularly difficult because when stormwater flows exceed the capacity of the existing system, water pools in the low-lying area along the roadway and flows into the backyard depression (MHN_62), which also receives overflow from the Calvary Christian parking area during the 1- percent-annual-chance event. Both flood-prone areas evaluated in Option 4 send water to Minnehaha Creek more quickly. Because the peak discharge in these subwatersheds occurs much earlier than the regional flood peak in Minnehaha Creek, timing may allow for additional water to be added to the creek and passed downstream before the regional flood peak, avoiding impacts to the peak flood level or the number of potentially impacted structures along the creek. Per MCWD stormwater management rules, discharge from a proposed project may not exceed discharge under existing conditions. Discussions with MCWD staff will be needed to determine the application of the rules to these proposed options. Storage Options Adding storage in key locations through the Halifax Avenue South and Indianola Avenue study area reduced at-risk principal structures. The benefits of the underground storage options evaluated within the City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.9-20 Halifax Avenue South flood-prone area (Options 3 and 6) are similar to those of the increased conveyance options evaluated as part of Option 4 (addition of storm sewer to Minnehaha Creek via 54th Street West). The most flood risk reduction benefit seen from adding storage was associated with Option 3, where storage was added at Calvary Christian Reformed Church to reduce overland flow into backyard depressions and prevent storm sewer backups from surcharging. The underground storage option evaluated under the Calvary Christian Reformed Church parking lot (Option 3) lowers the 1-percent- annual-chance peak flood elevation of the local depression near 5241 Halifax Avenue by 0.5 feet, and removes 1 adjacent principal structure from flood risk, while not transferring risk to other principal structures in the study area. The underground storage options evaluated within the Indianola Avenue flood-prone area (Options 2, 5 and 6) did not result in significant benefit as the storage was quickly filled by local inflow and backflow from the existing storm sewer system. A combination of storage and increased conveyance (Option 6) showed similar results to Option 4: one structure removed from flood risk in the 1-percent-annual-chance event within the Indianola Avenue flood-prone area. Options 4 and 6 did not transfer risk to residents previously not at risk under existing conditions. In all of the options that utilized storage (either above or below ground), street corridors, parking lots, and open space were prioritized for placement. However, further evaluation of the street corridors is needed to identify potential utility conflicts. Implementation of underground storage at Calvary Christian Reformed Church would need to be coordinated with the church and may provide an opportunity for resurfacing the parking lot. Surface storage evaluated within the Indianola Avenue flood-prone area may not be a viable option, as a significant portion of the open space would be converted to flood storage and would have temporary standing water during most storm events (for greater than 24 hours). All storage options would require routine maintenance to ensure that the BMPs are performing as designed. Hydrodynamic separators or an equivalent BMP would be needed upstream of any storage option to prevent sediment accumulation (i.e., loss of flood storage volume). Street Regrading Option The street regrading option (existing and proposed profiles shown on Figure A.9.1) evaluated within the Halifax Avenue South flood-prone area (Options 5) removed more structures from risk than the increased conveyance option (Option 4). However, by regrading portions of Halifax Avenue South and 52nd Street West to alter the controlling road crest elevations and reduce local inundation, the risk may be transferred to a principal structure near Minnehaha Creek and 52nd Street West. The Halifax Avenue South street regrading option lowers the 1-percent-annual-chance peak flood elevation of the two local depression along the roadway in existing conditions by approximately 1.5 feet at the depressions. The Halifax Avenue South street regrading option lowers the 1-percent-annual-chance peak flood elevation of the Halifax Avenue South and 52nd Street West intersection by 1.2 feet, removing 3 principal structures around the intersection from flood risk. Discussion of Flood Risk City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.9-21 Flood risk reduction Options 1, 2, 3 and 5 resulted in flood risk being transferred to principal structures previously not at risk under existing conditions. The Halifax Avenue South portion of Options 1 and 2 provided little flood reduction benefit with the evaluated increase in storm sewer capacity along 52nd Street West to Minnehaha Creek. With Option 1, two structures are removed from risk and with Option 2, three structures are removed from risk. However, 2 and 3 structures, respectively, were added to the risk count (not at risk under existing conditions) effectively creating no benefit and transferring risk. The increased conveyance of stormwater from the intersection to the creek resulted in a higher 1-percent- annual-chance flood elevation at the intersection. In existing conditions, the 1-percent-annual-chance flood elevation at the intersection of Halifax Avenue South and 52nd Street West is 876.6 feet. In Options 1 and 2, the 1-percent-annual-chance flood elevation at the intersection is 876.8 and 877.0 feet, respectively, likely due to potential increased tailwater conditions of the creek as a result of the increased trunk storm sewer sizes. With Option 3, one principal structure is removed from flood risk in the Indianola Avenue flood-prone area, but two structures are added to the risk count. Increasing the pipe sizes from 12- and 18-inch storm sewer to 36-inch storm sewer from the local depression to Jay Place increases backflow from the existing storm sewer system (5.7 cubic feet per second in existing conditions to 73.2 cubic feet per second in proposed conditions). Increased backflow from the existing system results in a higher peak elevation in the local depression, effectively transferring risk to downstream residents. There are several options that could be considered to reduce flood risk in both the Halifax Avenue South and Indianola Avenue flood-prone areas, including installing underground flood storage, increasing discharge capacity and conveyance, and street regrading. Underground storage (with sufficient inlet capacity) would reduce the flood elevations in these areas by storing flows that exceed the capacity of the existing storm sewer systems and infiltrating a portion of the runoff. Planning-level soil maps indicate that the soils in this area are “A” soils with a high potential for infiltration. Animations of the overland flow during the 1-percent-annual-chance and 10-percent-annual-chance storm events are available and have been shared with the City. If desired, additional views, storm events, and detailed animations can be made to assist with fully understanding the flood problem in this area. A.9.3.1.2 Morningside/Weber Park (MS_26, MS_25, MS_41, MS_32, MS_44, MS_24, MS_15, MS_52, MS_53, MS_2, MS_38, MS_40, MS_54, MS_31, MS_33, MS_39a, and MS_39b) – updated 2022 The Morningside neighborhood is in the far northeastern corner of Edina, bordering St. Louis Park to the north and Minneapolis to the east. The neighborhood is fully developed with primarily single-family homes built between 1910 and 1960, with some infill happening later and redevelopment currently replacing some structures. The area is characterized by numerous backyard depressions and several large low-lying areas, including Weber Park. There are two large stormwater detention basins in the area (Weber Pond and a temporary inundation area between Lynn Avenue and Kipling Avenue, north of West 42nd Street). The area is drained by a piped outlet that conveys stormwater to Bde Maka Ska in Minneapolis. The storm sewer and detention basins in this area were originally designed for the 2- percent-annual-chance (i.e., 50-year) storm event using now outdated rainfall estimates. Portions of this City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.9-22 watershed have experienced flooding. Flood risk in the Morningside area has been studied in detail by Barr Engineering Co. (Barr) and others. Street reconstruction in the Morningside neighborhood planned for 2022 and 2023 presented an opportunity to explore infrastructure-based solutions to help manage flood risk. Therefore, flood-related studies have been completed between 2018 and 2022 in advance of the expected street reconstruction. These studies continued to refine flood risk reduction options with consideration for the economics and benefits of a flood risk reduction project. The studies considered community values identified through direct engagement with the Flood Risk Reduction Task Force (July 2019 through February 2020) and through a specific engagement plan as part of work completed in 2020 and early 2021. Given the space available in the neighborhood, the values of the community, and the funding available, refined options to reduce flood risk were summarized in Barr’s February 2021 technical memo (Infrastructure Options to Reduce Flood Risk in the Morningside Neighborhood). To provide a significant flood risk reduction benefit in the Morningside neighborhood, large infrastructure changes were required. With some of those large infrastructure changes come similarly large changes to the landscape. Because the neighborhood is mostly developed and open space is limited, the remaining open space is highly valued by the community. However, additional stormwater storage is a necessary component to reduce flood risk. Thus, the City considered many social, aesthetic, and recreational tradeoffs and opportunities as it evaluated the feasibility of the refined flood risk reduction options. Ultimately, the recommended conceptual design was based on combinations of flood risk reduction alternatives evaluated through previous engineering studies, community engagement, and cost/benefit analyses. The final project design (fully described in the 100% Basis of Design Report, Barr, May, 2022) increases storm sewer pipe capacity, improves flood conveyance through streets and a constructed swale within Weber Park, and expands the flood storage capacity of Weber Pond and the Lynn/Kipling Inundation Area. The City started construction on the Morningside Flood Infrastructure Project (MFIP) in June 2022 with construction anticipated to be complete in spring of 2023. A.9.3.1.3 Edinbrook Lane and Westbrook Lane (MHN_79) There is a backyard depression west of TH 100 and southeast of the intersection of Edinbrook Lane and Westbrook Lane that receives runoff from subwatershed MHN_79, the watershed to the west (MHN_78), and TH 100. The depression has a 21-inch piped outlet to the TH 100 storm sewer system and a surface overflow north at approximately 893.6 feet, adjacent to 5005 Edinbrook Lane. Modeling results indicate that the 1-percent-annual-chance flood elevation (892.5 feet) may impact three principal structures (5013 Edinbrook Lane and 4801–4805 Westbrook Lane) and may very nearly impact three other principal structures (5009 and 5017 Edinbrook Lane, and 4811 Westbrook Lane), based on LiDAR and approximate building footprint data. The modeling results suggest the flooding problem is not driven by the water level in Minnehaha Creek. Instead, it is primarily related to the quantity of water coming into the depression from west of MHN_79 (MHN_78) and from backflow from the TH 100 storm sewer system. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.9-23 There are several options that should be considered to reduce flood risk in this area, including diverting flows to reduce the volume of water to the low area and/or improving outlet conditions. The runoff contributing from MHN_78, which has a peak flow rate of over 50 cfs, could be rerouted through new storm sewer pipes running north under Westbrook Lane to Minnehaha Creek. Alternatively, the surface overflow could be made more efficient (e.g., a concrete flume with vegetation cleared) and the elevation could be lowered. However, this second option may require purchasing 5005 Edinbrook Lane. The City will also consider working with MnDOT to evaluate options to reduce or eliminate flow into this area from the TH 100 storm sewer system, with a backflow preventer or by increasing the conveyance capacity in the TH 100 system, for example. Finally, all of the potentially impacted principal structures in this area could be acquired so that the area could be used and possibly improved as a dry basin for storage and, potentially, a water quality treatment basin. Soil maps show that the soils in this area are “B” soils with moderate potential for infiltration. A.9.3.1.4 North of Morningside Road between Lynn Avenue and Crocker Avenue (MS_22) There is a backyard depression north of Morningside Road between Lynn Avenue and Crocker Avenue. The depression is land-locked, with a surface overflow to the east (adjacent to 4226 Crocker Avenue) at approximately 875.8 feet, which is about 5 feet above the approximate low principal structure elevations in this area. The 1-percent-annual-chance flood elevation (872.4 feet) is determined by the 10-day snowmelt event and may impact up to six principal structures (4226–4236 Crocker Avenue), based on LiDAR data and approximate building footprints. It is recommended that a survey be conducted to determine low entry elevations for these seven principal structures. If the survey indicates that these principal structures are potentially impacted by the 1-percent- annual-chance flood elevation, it is recommended that a storm sewer outlet be installed in this backyard depression. A new storm sewer could connect to either the existing system on Crocker Avenue or Lynn Avenue, which both drain to the trunk storm sewer on West 42nd Street and ultimately to Weber Park Pond. Additional pipe capacity downstream to Weber Park Pond and, potentially, additional storage in Weber Park may also be required with this option due to other flood concerns within this general area. Access to the backyard depression for installation of a new pipe may be challenging due to the developed nature of the neighborhood and private property ownership. A.9.3.1.5 Branson Street between West 44th Street and Morningside Road (MS_3, MS_48, and MS_7) There are local depression areas along Branson Street and in the backyards north and south of Branson Street just west of Grimes Avenue. Branson Street (MS_48) and the backyard area to the north (MS_3) have catch basins and are drained by a 15-inch pipe originating at Branson Street and connecting north to the storm sewer system on Morningside Road. The backyard depression to the south (MS_7) is land- locked and does not have a piped outlet. It is hydraulically connected via surface overflows between 4303, 4301, and 4215 Branson Street at approximately 901.5 feet (according to LiDAR data). In addition, the storm sewer system on West 44th Street surcharges during the 1-percent-annual-chance 24-hour storm event; as a result, stormwater flows from West 44th Street into the backyard depression area (MS_7). At the City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.9-24 1-percent-annual-chance flood level, there are 12 principal structures that are potentially impacted and Branson Street will have approximately 2 feet of water in its lowest spot. This area was also identified as an area of concern in the 2003 and 2009 WRMPs. It is recommended that one or more inlets be installed in the backyard depression areas, pipes be added, and pipe sizes increased north to the existing storm sewer system under Morningside Road. The addition of an inlet to MS_7 with a connection to the pipe system on West 44th Street was evaluated, but this alternative would require the entire pipe system along West 44th Street and Morningside Avenue to be upgraded. Additional flow capacity in the storm sewer system to Weber Park Pond and, potentially, additional storage in Weber Park may be required with this option due to other flood concerns within this general area. Other considerations include adding additional storage capacity in this area. Storage could be added in MS_7 by lowering the backyard depression. Alternatively, in the event of street reconstruction, the low area of Branson Street could be raised and underground storage could be added under Branson Street and Oakdale Avenue (up to approximately 700 linear feet). Underground storage under Oakdale Avenue could intercept runoff from the upper parts of the MS_48 watershed. A.9.3.2 Potential Construction/Upgrade of Water Quality Basins The 2003 P8 modeling analysis indicated that under average conditions the predicted annual removal of total phosphorus from the ponds and wetlands in the Northeast Minnehaha Creek drainage area is greater than the desired 60 percent removal rate. As a result, no specific recommendations are given for the construction or upgrade of water quality basins in this watershed. Many techniques are available to reduce pollutant loading from stormwater runoff, including impervious surface reduction or disconnection, implementation of infiltration or volume-retention BMPs, installation of underground stormwater treatment structures and sump manholes, and other good housekeeping practices such as street sweeping. As opportunities arise, the City will consider all of these options to reduce the volume and further improve the quality of stormwater runoff from this drainage area. JP_40JP_26 MHN_1 MHN_66 MHN_11 ECC_6 ECC_5 ECC_9 ECC_4 ECC_3 ECC_2 JP_10 S t . Lo u i sSt. Lo u i s P a r kPark M i n n e a p o l isMinneapolis MinnehahaCree kBarr Footer: ArcGIS 10.4.1, 2018-03-26 11:35 File: I:\Client\Edina\Projects\CRWMP_Update_2017\Maps\Reports\Figures_CityReviewDraft\Fig_12_4_MC_Northeast_Water_Quality.mxd User: EMANORTHEAST MINNEHAHA CREEKWATER QUALITY MODELING RESULTSWater ResourceManagement PlanCity of Edina, Minnesota FIGURE A.9.8 1,000 0 1,000Feet !;N 300 0 300Meters Percent TP Removal in Water Body*This number represents the percent of the total annual massof phosphorus entering the water body that is removed. Cumulative TP Removal in Watershed*This number represents the percent of the total annual massof phosphorus entering the watershed and upstream watershedsthat is removed in the pond and all upstream ponds. *Data based on results of 2003 P8 modeling. 0 - 25% (Poor/No Treatment) 25 - 40% (Moderate Removal) 40 - 60% (Good Removal) 60 - 100% (Excellent Removal) 0 - 25% (Poor/No Treatment) 25 - 40% (Moderate Removal) 40 - 60% (Good Removal) 60 - 100% (Excellent Removal) Imagery Source: USDA 2016 NAIP via MnGeo Area Draining Directly to Minnehaha Creek Flow Direction City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.10-1 A.10 Southeast Minnehaha Creek A.10.1 General Description of Drainage Area The City’s interactive web map depicts the Southeast Minnehaha Creek drainage area and the individual subwatersheds within this area. The Southeast Minnehaha Creek drainage area is located in east-central Edina and contains several ponds, Lake Harvey, Lake Pamela, and Melody Lake. A.10.1.1 Drainage Patterns The stormwater system within this drainage area comprises storm sewers, ponding basins, wetlands, drainage ditches, and overland flow paths. The Southeast Minnehaha Creek area has been divided into several major watersheds based on the drainage patterns. These major watersheds are depicted on the City’s interactive web map. Each major watershed has been further delineated into numerous subwatersheds. The naming convention for each subwatershed is based on the major watershed where it is located. Table A.10.1 lists each major watershed and the associated subwatershed naming convention. Table A.10.1 Major Watersheds within the Southeast Minnehaha Creek Drainage Basin Major Watershed Subwatershed Naming Convention Lake Pamela LP_## Minnehaha Creek South MHS_## / MHC_## Melody Lake ML_## A.10.1.1.1 Lake Pamela The Lake Pamela watershed is located in the east central portion of Edina. The entire 279-acre watershed drains to Lake Pamela and then north to Minnehaha Creek. The land use in this watershed is primarily low-density residential, with Pamela Park surrounding Lake Pamela. Four stormwater management basins in this watershed, two on the south end of Lake Pamela and two on the north end, have recently been constructed to treat stormwater. The two ponds on the north end of the lake also receive runoff from about half of the Minnehaha Creek South watershed (described below). This runoff is routed through the ponds, over a weir, to the north bay of Lake Pamela, and finally, to Minnehaha Creek. These ponds were designed to treat runoff from the Minnehaha Creek South watershed before it is discharged to Minnehaha Creek. Lake Pamela has been excavated to increase the dead storage volume within the lake for water quality treatment. A.10.1.1.2 Minnehaha Creek South The Minnehaha Creek South watershed extends from areas just south of the Edina Country Club at Lake Harvey, west to TH 100 and south to West 54th Street. The land use in this 589-acre watershed is City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.10-2 predominantly low-density residential with some scattered areas of institutional land use. There are no ponds east of Minnehaha Creek and only a few wet and dry detention ponds in the western half of this watershed. However, stormwater from most of the western half of the watershed is routed through ponds and through the northern bay of Lake Pamela before discharging to Minnehaha Creek. Areas directly east and west of Minnehaha Creek are drained by short storm sewer systems or directly by overland flow. There is a stormwater control weir in a manhole just east of the intersection of West 58th Street and Concord Avenue. A pipe leading from the backyard area is part of this system and connects to the downstream end of the weir-manhole; a flap gate stops water from backing into the pipe. The weir, located at node 1849, forces water to back into a pipe that discharges to a ball field along Concord Avenue. This entire system was designed to store water in the ballpark and slowly release it back into the storm sewer system to reduce flood elevations for the nearby principal structures during large storm events. A.10.1.1.3 Melody Lake Land use in this 173-acre watershed is low-density residential, institutional, and TH 100. The outlet from Melody Lake is a pumped outlet to the TH 100 drainage system. This system flows north and ultimately discharges to Minnehaha Creek. The TH 100 storm sewer system was not modeled as part of this study. A.10.2 Stormwater System Results A.10.2.1 Hydrologic/Hydraulic Modeling Results The 10-percent-annual-chance and 1-percent-annual-chance flood analyses were performed for the Southeast Minnehaha Creek drainage basin. The 10-percent-annual-chance analysis was based on a 24- hour storm with 4.29 inches of rain. The 1-percent-annual-chance analysis was based on a 24-hour storm event with 7.47 inches of rain and on a 10-day snowmelt event with 7.2 inches of runoff; the higher resulting flood level of the two events was chosen for the 1-percent-annual-chance analysis. Data available from the City’s interactive web map presents the watershed information and the results for the 1-percent-annual-chance hydrologic analyses for the Southeast Minnehaha Creek basin. A more detailed description of the stormwater system analysis is provided in Section A.1.1.3. The results of the 1-percent-annual-chance hydraulic analyses for the Southeast Minnehaha Creek drainage basin are available from the City’s interactive web map. Modeling results for a range of flood risk reduction options are presented in Table A.10.4 and Table A.10.3 for the 10-, and 1-percent-annual- chance hydraulic analyses. The web map depicts the Southeast Minnehaha Creek drainage basin boundary, subwatershed boundaries, the modeled storm sewer network, and the flood-prone areas identified in the modeling analyses. To evaluate the level of protection of the stormwater system within the Southeast Minnehaha Creek drainage area, the 1-percent-annual-chance flood elevations for the ponding basins and depressed areas were compared to the low elevations of structures surrounding each basin. The areas predicted to be City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.10-3 inundated during the 1-percent-annual-chance storm event are shown on the City’s interactive web map. Additional modeling results evaluating multiple flood risk reduction options are presented in this section. Discussion and recommended improvement considerations for these areas are included in Section A.10.3. A.10.2.2 Water Quality Modeling Results The effectiveness of the stormwater system in removing stormwater pollutants such as phosphorus was analyzed using the P8 water quality model. The P8 model simulates the hydrology and phosphorus loads introduced from each pond’s watershed and the transport of phosphorus throughout the stormwater system. A more detailed description of the stormwater system analysis is provided in Section A.1.1.3. Figure A.10.2 depicts the results of the water quality modeling for the Southeast Minnehaha Creek drainage basin. The figure shows the fraction of total phosphorus removal for each water body as well as the cumulative total phosphorus removal in the watershed. A.10.3 Implementation Considerations The XPSWMM hydrologic and hydraulic modeling analyses in support of the 2018 WRMP and 2003 P8 water quality analysis helped identify locations throughout the watershed where improvements to the City’s stormwater management system may be warranted. The following sections discuss potential improvement options identified as part of the modeling analyses. As opportunities to address identified flooding issues and improve water quality arise (e.g., street reconstruction projects or public facilities improvements), the City will use a comprehensive approach to stormwater management. This approach will include consideration of infiltration or volume retention practices to address flooding and/or improve water quality, reduction of impervious surfaces, increased storm sewer capacity where necessary to alleviate flooding, construction and/or expansion of water quality basins, and implementation of other stormwater BMPs to reduce pollutant loading to downstream waterbodies. A.10.3.1 Potential Flood Risk Reduction Options The 2018 WRMP identified several locations within the Southeast Minnehaha Creek drainage basin where the 1-percent-annual-chance level of protection is not provided by the current stormwater system. As part of the 2018 WRMP modeling analyses, potential corrective measures were identified for problem areas. These preliminary corrective measures are also discussed below. As the City evaluates flooding issues and potential system modifications, other potential modifications, including (but not limited to) implementation of volume-retention practices, increases in conveyance capacity, and/or stormwater infiltration (where soils are conducive) will be given consideration. The 2003 hydrologic and hydraulic modeling analyses also identified several locations within the Southeast Minnehaha Creek drainage basin where the 1-percent-annual-chance level of protection was not provided by the stormwater system, based on TP-40 precipitation frequency estimates. The discussions related to those areas are included in Appendix B of the 2022 WRMP, along with a short summary of what has been done since 2003. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.10-4 A.10.3.1.1 East Golf Terrace Heights Neighborhood There are several low depression areas within the Golf Terrace Heights neighborhood, including (1) Tower Street between Fairfax Avenue and Wooddale Avenue and the backyard depressions of the homes along Tower Street (MHS_75, MHS_76, and MHS_86), (2) the backyard depression north of West 58th Street between Fairfax Avenue and Wooddale Avenue (MHS_16), and (3) the parking lot of Wooddale Church and the backyards of homes along West 56th Street (MHS_83). Along Tower Street there are 8 catch basin inlets with 24-inch pipe, and in the parking lot of Wooddale Church there are 3 catch basin inlets with 15- inch pipe—both leading to the 24-inch pipe under Wooddale Avenue. In the backyard depression of MHS_16, there are no inlets and, according to LiDAR data, the surface overflow is between 5712 and 5716 Wooddale Avenue at approximately 880.0 feet. Modeling results indicate that the 1-percent-annual-chance flood levels are 886.3 feet (MHS_83), 882.4 feet (MHS_75), 880.6 feet (MHS_86), 880.6 feet (MHS_76), and 880.0 feet (MHS_16). For all of these subwatersheds, except for MHS_16, the 24-hour precipitation event determines the 1-percent-annual- chance flood level. Based on LiDAR data and approximate building footprints, the 1-percent-annual- chance flood elevation in subwatershed MHS_83 may impact up to three principal structures (4516, 4520, and 4524 West 56th Street) in addition to inundating the Wooddale Church parking lot. The 1-percent- annual-chance flood elevation may impact up to nine principal structures in subwatersheds MHS_75, MHS_86, and MHS_76 (4519 West 56th Street, 4536–4516 Tower Street, and 4517 and 4513 Tower Street). Up to six principal structures in subwatershed MHS_16 may be impacted by the 1-percent-annual-chance flood elevation (5712–5720 Wooddale Avenue and 5709–5717 Fairfax Avenue). The flooding problem is primarily related to the capacity of the downstream storm sewer system that conveys stormwater east to Minnehaha Creek. The high flows passing through the single 48-inch pipe east to Minnehaha Creek result in high velocities, high friction head losses, and a higher surface water profile in this area. Several system modifications were evaluated using the detailed 1D/2D XPSWMM model. The options and results are discussed in Section A.10.3.1.2. A.10.3.1.2 Concord and West 58th Street (MHS_59, MHS_26, MHS_58, MHS_42, MHS_53, and MHS_17) – updated 2021 Location and Existing Flooding Description The intersection of Concord Avenue and West 58th Street and the surrounding area has long experienced flooding problems. A storm sewer system drains this low-lying area; however, when stormwater flows exceed the system’s capacity (3.5 inches over 24 hours), water pools along the roadway and in adjacent yards until it rises enough to flow eastward along West 58th Street toward Wooddale Avenue. This type of storm has a 20 percent chance of occurring in a given year (also known as a 5-year storm or 20-percent- annual-chance storm). The flooding problem in this area results from the limited conveyance capacity of the downstream storm sewer system. The 48-inch pipe between Wooddale Avenue and Minnehaha Creek limits the outflow of stormwater, which results in the backup of water at West 58th Street and Concord Avenue. Results from hydrologic and hydraulic modeling using Atlas 14 precipitation inputs (the most recent precipitation data) City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.10-5 indicate that the intersection of Concord Avenue and West 58th Street would be inundated during a 24- hour storm that has a 1 percent chance of occurring in a given year (a 100-year storm). Portions of West 58th Street east of Concord Avenue and parts of Ashcroft Avenue and Fairfax Avenue would also be inundated. A storm of this magnitude may produce flooding with the potential to impact 29 principal structures between Concord Avenue and St. Johns Avenue and around the intersection of West 58th Street and Fairfax Avenue. Ponding of water in backyards and along roads also impacts several principal residential structures in the East Golf Terrace neighborhood north of West 58th Street and west of Wooddale Avenue. A 100-year storm has the potential to impact 33 principal structures along Golf Terrace, Lakeview Drive, Oak Drive, West 56th Street, Tower Street, and Woodland Road W. The flooding problem in this area is primarily related to the limited inlet and local conveyance capacity of the existing storm sewer system along these roads, which connect to the main storm trunk line along Wooddale Avenue. Stormwater modeling shows that flooding can occur relatively frequently—even during a 24-hour storm that has a 50 percent chance of occurring in a given year (a 2-year storm). Proposed Infrastructure Options to Reduce Flood Risk Nine potential flood risk reduction options were evaluated using a model developed for the Concord and West 58th Street study area. Seven of the nine options consisted of individual approaches (i.e., extra pipe capacity, storage, etc.), and two represented a combination of approaches. Flood risk reduction was accomplished by three primary measures: increased conveyance (increasing trunk storm sewer sizes or adding storm sewer), increased storage in key locations, and grading. Modeling Results Five 24-hour storm events were simulated (20-, 10-, 4-, 2-, and 1-percent-annual-chance) to evaluate the benefit of each flood-risk-reduction option. The number of potentially impacted principal residential structures was evaluated for each option and each storm event. Though no home is ever removed from all flood risk, of nine options evaluated, the following were determined to be most effective and were therefore studied in additional detail (see Table A.10.2). • Increase trunkline capacity along West 58th Street—Increase the capacity of the main trunk storm sewer along West 58th Street by 300 percent—from the low area of South View School (the ballfield) to the Minnehaha Creek outfall. • Add surface storage at South View School property—Provide additional flood storage by excavating (i.e., lowering) the open area at South View School (a dry pond). • Add underground street storage and increase inlet capacity in the East Golf Terrace neighborhood and add underground storage in Wooddale Church parking lot—Provide additional flood storage by installing underground storage chambers in the East Golf Terrace neighborhood under Lakeview Drive, Oak Drive, Lexington Street, West 56th Street, Tower Street, Woodland Road, and in the Wooddale Church parking lot. This flood storage option provided the greatest benefit. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.10-6 • Raise overflow through private property on Woodland Road West—Increase the curb height and residential front yards approximately 2 to 3 feet near the principal residential structure at 18 West Woodland Road to redirect flows north along Woodland Road. This option has a benefit similar to some of the conveyance and storage options but would cost significantly less. Flood inundation mapping for this area is available through the City’s interactive web map. Table A.10.2 Residential Structure Impacts in the Concord and West 58th Street Area Flood Risk Reduction Option Number of At-Risk Principal Structures (Reduction from Existing Conditions) 20-percent-annual-chance event 10-percent-annual-chance event 4-percent-annual-chance event 2-percent-annual-chance event 1-percent-annual-chance event Existing conditions 22 33 52 69 86 Increase trunkline capacity along W 58th Street 22 (0) 31 (-2) 48 (-4) 60 (-9) 74 (-12) Add above-ground storage in Southview Park 22 (0) 31 (-2) 48 (-4) 62 (-7) 78 (-8) Add underground street storage and inlet capacity in neighborhood northeast of Concord and W 58th 18 (-4) 26 (-7) 41 (-11) 57 (-12) 73 (-13) Raise overflow through private property on Woodland Road W 19 (-3) 30 (-3) 48 (-4) 65 (-4) 82 (-4) 1The total number of principal structures considered at risk includes structures within the model domain but outside of the extents of the proposed options (i.e., near Dalrymple Avenue, east of Wooddale Avenue along West 55th Street). Therefore, the localized benefit of some of the alternatives may be greater than when compared to the entire model area results (i.e., six out of 12 local structures removed versus 6 out of 86 structures removed). City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.10-7 Proposed Infrastructure Options to Reduce Flood Risk Nine potential flood risk reduction options were evaluated using the detailed 1D/2D XPSWMM model for the Concord and West 58th study area. Seven of the nine options consist of individual approaches (i.e., extra pipe capacity, storage, etc.) and two of the nine options represent a combination of approaches. Five 24-hour storm events were simulated (the 20-, 10-, 4-, 2-, and 1-percent-annual-chance events) for each flood risk reduction option to help understand the amount of improvement each option can achieve depending on the severity of the rainfall event. Barr conducted an evaluation of potentially impacted structures for each potential flood risk reduction option to assess which options provided the greatest level of flood risk reduction in terms of a reduction in the number of potentially impacted structures. The nine options are described below, and additional details are also shown on Figure A.10.1. Option 1: Divert Ashcroft Avenue Flows to Pamela Park: Add a 36-inch pipe diversion at the intersection of West 58th Street and Ashcroft Avenue. The diversion trunk runs south along Ashcroft Avenue, turns east on West 59th Street and outlets to the Pamela Park pond. Option 2: Increase Trunkline Capacity Along West 58th Street: Increase the capacity (triple capacity) of the main trunk storm sewer along West 58th Street from South View Park to the outfall to Minnehaha Creek. Option 3: Add Underground Storage in South View Park: Provide additional flood storage by installing underground chambers (modeled as a chamber 450 feet long by 100 feet wide by 10 feet deep, totaling 10.3 acre-feet) in the open area of South View Park that outlets to the main trunk storm sewer along West 58th Street. Option 4: Add Surface Storage in South View Park: Provide additional flood storage by excavating (i.e. lowering) the open area of South View Park (modeled as a dry pond 340 feet long by 200 feet wide and ranging from 8 to 10 feet deep, totaling 19 acre-ft; the footprint is shown on Figure A.10.1) with a basin bottom at the invert of the park outlet structure connecting to the main trunk storm sewer along West 58th Street. Option 5: Increase Trunkline Capacity Along West 58th Street and Add Underground Storage in South View Park: A combination of tripling the storm sewer capacity along West 58th Street (Option 2) and underground flood storage installation in South View Park (Option 3). Option 6: Add Underground Storage in Wooddale Church Parking Lot: Provide additional flood storage by installing underground storage chambers (modeled as a chamber 200 feet long by 100 feet wide by 8 feet deep, totaling 3.6 acre-ft) under the Wooddale Church parking lot; add inlets in the parking lot and at the low point of Lexington Street to increase conveyance to the added storage. Option 7: Add Underground Street Storage and Increase Inlet Capacity in the East Golf Terrace Neighborhood and Add Underground Storage in Wooddale Church Parking Lot: Provide additional flood storage by installing underground storage chambers (modeled as chambers the length of each road, as shown on Figure A.10.1, totaling 14 acre-feet) in the East Golf Terrace City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.10-8 neighborhood under Lakeview Drive, Oak Drive, Lexington Street, West 56th Street, Tower Street, Woodland Road, and in the Wooddale Church parking lot (as in Option 6). The breakdown of storage is 10.4 acre-feet of street storage and 3.6 acre-feet of storage under the Wooddale Church parking lot. Add inlets along streets and at intersections to increase conveyance to the added storage. Option 8: Raise Overflow through Private Property on Woodland Road West: Raise natural overflow by increasing curb height and residential front yards approximately 2 to 3 feet near 18 Woodland Road to redirect flows north along Woodland Road rather than through backyards. Option 9: Lower West 58th Street and Woodland Road High Points: Lower the crest along West 58th Street from 882.0 to 880.6 and the crest along Woodland Road from 882.3 to 881.0 to allow for water to spill east out of the West 58th Street and Concord Avenue intersection and West 58th Street and Fairfax Avenue intersection. Some of the potential improvement alternatives discussed above rely on sending water to Minnehaha Creek more quickly and using available storage in Pamela Park. Because the local storm event in these subwatersheds peaks much earlier than the regional flood event in Minnehaha Creek, timing may allow for additional water to be added to the creek and passed downstream before the regional peak comes through without creating additional impacts to the peak flood level or the number of potentially impacted structures along the creek. Wooddale AveLakeview Dr Southview La Woodland Rd Lexington St W 54th St Oaklawn AveKellog AveSt Johns AveConcord AveGolf Ter Woodland Rd Woodcrest DrW 56th St Woodcrest Dr Brookview AveConcord AvePark PlW 55th St Kellogg Pl W 53rd St W 60th StLakeview DrW 58th St W 59th St Woodland RdSchool Rd Oak Dr Lakeview DrFairfax AveConcord AveWoodale AveAshcroft AveKellogg AveSt Johns AveVi rginia AveOaklawn AveTower St Lexington St Concord Ter Brookview AveWoodland Rd Dever DrSt Andrews AveMinnehaha BlvdWo o dcrest Dr Philbrook La Barr Footer: ArcGIS 10.7.1, 2021-03-01 10:43 File: I:\Client\Edina\Projects\CWRMP_Amendment_2D_Modeling\Users\TAO2\Edina_Concord_Alternatives.mxd User: tao2 FLOOD RISK REDUCTIONFEATURES - CONCORD & W 58THWater Resources Management PlanCity of EdinaFIGURE A.10.10500 Feet !;N Options 6/7: Street storage in East Golf Terrace +Wooddale Church parking lot storageTotal Volume = 14 ac-ft Options 3/4/5: Southview Park storageSurface Volume = 19 ac-ftORUnderground Volume = 10 ac-ft ParcelRaise EOF and redirect flow down WoodlandRedirect Ashcroft flows to Pamela ParkIncrease W 58th Storm Sewer CapacityParking lot storage at Wooddale Church Street storage in northern neighborhoodSurface storage at Southview ParkUnderground storage at Southview Park Option 1: 36-inch diversion Option 2/5: Triple capacity of current system Option 8: Raise curb and private property grading 2-3 ft to redirect flows north on Woodland Ave Option 9: Lower West 58th Street Lower Woodland Road Lower road City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.10-10 Modeling Results and Risk Reduction of Proposed Options The proposed options resulted in varying levels of flood risk reduction. Flood elevations in key areas of the Concord and West 58th project area are presented in Table A.10.3 and Table A.10.4 for the 1-percent- annual-chance event and the 10-percent-annual-chance event, respectively. Of the nine options, Options 2, 5, and 7 resulted in the greatest benefit, while Options 6, 8 and 9 provided little benefit. However, these options were created to provide localized benefit and therefore do not show similar results as the more regional alternatives. Additionally, some structures in the entire model domain are not helped by any of the alternatives and are therefore still considered at-risk. In Option 7, backyard and street flooding in the East Golf Terrace neighborhood is reduced due to increased storage and capture of road flooding. Additionally, several structures along the West 58th Street corridor are removed from risk of flooding during the 1-percent-annual-chance event. These structures are most likely removed due to the decrease of overflow south along Wooddale Ave and through the trunk storm sewer under Wooddale Ave. Because the East Golf Terrace and West 58th Street systems are connected north of Philbrook Lane (before the storm sewer turns east towards Minnehaha Creek), adding storage in one system allows for the other to utilize more of the system’s capacity. This can be seen in the results of the other evaluated options where storage or system capacity is added along West 58th Street and structures are removed from risk in the East Golf Terrace neighborhood. The number of residential structures impacted in the existing condition and with each proposed option is presented in Table A.10.5. Homes that are referred to as “removed from risk” in the summary below are homes that are no longer at risk of damage for the storm based on modeling results. However, no home is ever removed from all flood risk. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.10-11 Table A.10.3 Flood Elevations in Concord and West 58th Project Area for 1-percent-annual-chance event South View Park/Concord Avenue and West 58th Street West 58th Street and Fairfax Avenue West 58th Street and Wooddale Avenue West 56th Street Backyards along Woodland Road EOF Existing Conditions 883.0 882.4 883.7 885.6 863.2 Divert Ashcroft Ave Flows to Pamela Park 882.6 882.2 883.7 885.6 863.2 Increase Trunkline Capacity Along W 58th Street 881.8 882.2 883.7 885.6 863.2 Add Underground Storage in Southview Park 882.5 882.2 883.7 885.6 863.2 Add Above Ground Storage in Southview Park 881.9 882.2 883.7 885.6 863.2 Increase Trunkline Capacity Along W 58th Street and Add Underground Southview Park Storage 881.8 882.2 883.7 885.6 863.2 Add Underground Storage in Wooddale Church Parking Lot 883.0 882.3 883.7 885.5 863.2 Add Underground Street Storage and Inlet Capacity in Neighborhood Northeast of Concord and W 58th 882.9 882.2 883.7 885.4 863.2 Raise Overflow through Private Property on Woodland Road W 883.0 882.4 883.7 885.6 863.1 Lower West 58th Street and Woodland Road 882.6 882.4 883.7 885.6 863.2 City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.10-12 Table A.10.4 Flood Elevations in Concord and West 58th Project Area for 10-percent-annual-chance event South View Park/Concord Avenue and West 58th Street West 58th Street and Fairfax Avenue West 58th Street and Wooddale Avenue West 56th Street Backyards along Woodland Road EOF Existing Conditions 880.5 881.1 883.5 885.3 863.2 Divert Ashcroft Ave Flows to Pamela Park 880.2 881.1 883.5 885.2 863.2 Increase Trunkline Capacity Along W 58th Street 879.2 881.1 883.5 885.2 863.2 Add Underground Storage in Southview Park 879.1 881.1 883.5 885.2 863.2 Add Above Ground Storage in Southview Park 877.2 881.1 883.5 885.2 863.2 Increase Trunkline Capacity Along W 58th Street and Add Underground Southview Park Storage 879.1 881.1 883.5 885.2 863.2 Add Underground Storage in Wooddale Church Parking Lot 880.4 881.1 883.5 885.2 863.2 Add Underground Street Storage and Inlet Capacity in Neighborhood Northeast of Concord and W 58th 880.2 881.1 883.5 885.0 863.2 Raise Overflow through Private Property on Woodland Road W 880.5 881.1 883.5 885.3 863.1 Lower West 58th Street and Woodland Road 880.5 880.8 883.5 885.3 863.2 City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.10-13 Table A.10.5 Residential structure impacts in the Concord and West 58th Street area Flood Risk Reduction Alternative Number of At-Risk Structures (Reduction from Existing Conditions) 20-percent-annual-chance event 10-percent-annual-chance event 4-percent-annual-chance event 2-percent-annual-chance event 1-percent-annual-chance event Existing Conditions 22 33 52 69 86 Option 1: Divert Ashcroft Ave Flows to Pamela Park 22 (0) 31 (-2) 49 (-3) 64 (-5) 80 (-6) Option 2: Increase Trunkline Capacity Along W 58th Street 22 (0) 31 (-2) 48 (-4) 60 (-9) 74 (-12) Option 3: Add Underground Storage in Southview Park 22 (0) 31 (-2) 48 (-4) 62 (-7) 80 (-6) Option 4: Add Above Ground Storage in Southview Park 22 (0) 31 (-2) 48 (-4) 62 (-7) 78 (-8) Option 5: Increase Trunkline Capacity Along W 58th Street and Add Underground Southview Park Storage 22 (0) 31 (-2) 48 (-4) 60 (-9) 74 (-12) Option 6: Add Underground Storage in Wooddale Church Parking Lot 22 (0) 32 (-1) 52 (0) 66 (-3) 83 (-3) Option 7: Add Underground Street Storage and Inlet Capacity in Neighborhood Northeast of Concord and W 58th 18 (-4) 26 (-7) 41 (-11) 57 (-12) 73 (-13) Option 8: Raise Overflow through Private Property on Woodland Road W 19 (-3) 30 (-3) 48 (-4) 65 (-4) 82 (-4) Option 9: Lower West 58th Street and Woodland Road 22 (0) 33 (0) 50 (-2) 68 (-1) 83 (-3) 1The total number of structures considered at-risk includes structures that are within the model domain but may be far from any of the proposed alternatives (i.e., near Dalrymple Avenue; east of Wooddale Avenue along West 55th Street). Therefore, the localized benefit of some of the alternatives may be greater than when compared to the entire model area results (i.e., 6 out of 12 local structures removed versus 6 out of 86 structures removed). City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.10-14 Discussion of Proposed Options The proposed alternatives discussed above utilize flood storage, increased conveyance, and minor grading to remove residential structures from flood risk for storm events between the 20-percent-annual- chance event and the 1-percent-annual-chance event. The performance of the evaluated options is discussed below, along with potential implementation considerations and prioritization based on the flood risk reduction benefit. The options that evaluated increased conveyance of stormwater, primarily through increasing trunk storm sewer sizes or adding storm sewer, were among the most impactful for removing structures from flood risk. However, no structures were removed from flood risk for the 20-percent-annual-chance storm event. Several residential structures in the East Golf Terrace neighborhood proved difficult to remove from flood risk for any of the analyzed storm events due to the minimal drainage in low backyards in addition to street flooding that overflows into the backyards. Significant underground storage would be needed (even beyond the storage volume provided in Option 7) to remove all structures from flood risk. Implementation of these alternatives would most likely coincide with street reconstruction projects to minimize disturbance along these residential roads. Implementation of these alternatives would include removal and replacement of storm sewer along West 58th Street, installation of storm sewer along Ashcroft Avenue and West 59th Street, or some other combination. It is unknown if adding storm sewer under West 58th Street, Ashcroft Avenue, or West 59th Street is feasible due to potential utility conflicts. Additionally, storm sewer depths along West 59th Street would be approximately 10 to 30 feet below existing grade. This would require significant excavation/trenching (high cost), especially around a high point on West 59th Street at its intersection with Kellogg Avenue. These options rely on sending water to Minnehaha Creek more quickly and using available storage in Pamela Park. Because the local storm event in these subwatersheds peaks much earlier than the regional flood event in Minnehaha Creek, timing may allow for additional water to be added to the creek and passed downstream before the regional peak comes through without creating additional impacts to the peak flood level or the number of potentially impacted structures along the creek. Adding storage in key locations through the Concord Avenue and West 58th Street project area reduced at-risk residential structures. The storage options evaluated (Options 3, 4, and 6) did not reduce the number of at-risk structures more than the increased conveyance options (Options 1 and 2). A combination of storage and increased conveyance (Option 5) showed similar results to the option that only had increased conveyance (Option 2). The most flood risk reduction benefit seen from adding storage was for Option 7, where storage was added in the East Golf Terrace neighborhood to reduce overland flow into backyard depressions and prevent storm sewer backups from surcharging. In all of the options that utilized storage (either above or below ground), both street corridors and open space were prioritized for placement. However, further evaluation of the street corridors would be needed to identify potential utility conflicts. The storage options that were evaluated in the Wooddale Church parking lot and the South View park open space would be ideal candidates, as there are not likely to be many utility or land use conflicts. Implementation of underground storage in the South View park would need to be coordinated with the school to ensure that the space can still be usable. Surface storage may not be a viable option, as a significant portion of the open space would be converted to flood storage and would City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.10-15 have temporary standing water during most storm events. All storage options would require routine maintenance to ensure that the BMPs are performing correctly. Hydrodynamic separators or an equivalent BMP would be needed upstream of any storage option to prevent sediment accumulation (i.e., loss of flood storage volume). One less extensive alternative that was evaluated was Option 8, where the curb and front yard area along Woodland Road (near 18 Woodland Road) was raised to prevent road overflow from following the existing flow path down a low point on the private property. The existing flow path cuts through several backyards and causes four homes along the flow path to be at-risk of flooding for the 1-, 2-, and 4- percent-annual-chance storm events and three homes to at-risk for the 10- and 20-percent-annual- chance storm events. By raising the flow path, runoff is redirected north down Woodland Road and does not impact the previously at-risk structures for any of the events. This less extensive alternative has a similar flood risk reduction benefit to the conveyance and storage options but would cost significantly less. In other words, this option may provide a large flood risk reduction benefit for comparatively small effort. Animations of the overland flow during the 10- and 1-percent-annual-chance existing conditions storm events are available and have been shared with the City. If desired, additional views, storm events, and detailed animations can be developed to assist with fully understanding the flood problem in this area. A.10.3.2 Potential Construction/Upgrade of Water Quality Basins When considered individually, the ponds MHS_13, LP_5, and LP_13, and the two bays of Lake Pamela (LP_14 and LP_26) are removing less that 60 percent of the total phosphorus in stormwater inflows. Because water from a watershed greater than 500 acres in size is routed through the ponds and the two bays of Lake Pamela before being discharged to Minnehaha Creek, the cumulative phosphorus removal by the ponds should be considered. In addition, ponds MHS_13, LP 5, and LP_13 were recently constructed and designed to function as a treatment train and not individually. On a cumulative basis, the ponds and Lake Pamela are removing 63 percent of the total phosphorus load from this entire watershed; as such, it is not necessary to upgrade these ponds. Because over 60 percent of the total phosphorus in stormwater runoff is being removed by all other ponds and wetlands in the Southeast Minnehaha Creek watershed, no recommendations are given for the construction or upgrade of water quality basins in this watershed. Many techniques are available to reduce pollutant loading from stormwater runoff, including impervious surface reduction or disconnection, implementation of infiltration or volume-retention BMPs, installation of underground stormwater treatment structures and sump manholes, and other good housekeeping practices such as street sweeping. As opportunities arise, the City will consider all of these options to reduce the volume and further improve the quality of stormwater runoff from this drainage area. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.10-16 A.10.3.3 Potential Stream Improvement Projects A.10.3.3.1 Minnehaha Creek Reach 14 Stream Restoration The MCWD’s 2007 Comprehensive Water Resources Management Plan identified a potential capital improvement project in Edina to implement a stream restoration project on Reach 14 of Minnehaha Creek. This reach extends from France Avenue to West 54th Street. This project, which incorporated streambank stabilization, in-stream habitat enhancement, and buffer enhancement has been completed. MHS_35 MHS_22 MHS_19 MHS_41 ML_8 LP_13LP_5 MHS_13 MHS_13 LP_14 LP_26 Ri c h f i e l dRichfield M i n n e a p o l isMinneapolis Minnehaha C r ee kBarr Footer: ArcGIS 10.4.1, 2018-03-26 11:36 File: I:\Client\Edina\Projects\CRWMP_Update_2017\Maps\Reports\Figures_CityReviewDraft\Fig_13_4_MC_Southeast_Water_Quality.mxd User: EMASOUTHEAST MINNEHAHA CREEKWATER QUALITY MODELING RESULTSWater ResourceManagement PlanCity of Edina, Minnesota FIGURE A.10.2 1,000 0 1,000Feet !;N 300 0 300Meters Percent TP Removal in Water Body*This number represents the percent of the total annual massof phosphorus entering the water body that is removed. Cumulative TP Removal in Watershed*This number represents the percent of the total annual massof phosphorus entering the watershed and upstream watershedsthat is removed in the pond and all upstream ponds. *Data based on results of 2003 P8 modeling. 0 - 25% (Poor/No Treatment) 25 - 40% (Moderate Removal) 40 - 60% (Good Removal) 60 - 100% (Excellent Removal) 0 - 25% (Poor/No Treatment) 25 - 40% (Moderate Removal) 40 - 60% (Good Removal) 60 - 100% (Excellent Removal) Imagery Source: USDA 2016 NAIP via MnGeo Area Draining Directly to Minnehaha Creek Flow Direction City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.11-1 A.11 Northwest Minnehaha Creek A.11.1 General Description of Drainage Area The City’s interactive web map depicts the Northwest Minnehaha Creek drainage area and the individual subwatersheds within this area. The Northwest Minnehaha Creek drainage area is located in the far northwest corner of Edina, east of the TH 169 North drainage area. This watershed is the smallest of the Minnehaha Creek watersheds and extends from TH 100 to areas west of the Interlachen Golf Course. A.11.1.1 Drainage Patterns The stormwater system within this drainage area comprises storm sewers, ponding basins, wetlands, drainage ditches, and overland flow paths. The Northwest Minnehaha Creek area has been divided into several major watersheds based on the drainage patterns. These major watersheds are depicted on the City’s interactive web map. Each major watershed has been further delineated into numerous subwatersheds. The naming convention for each subwatershed is based on the major watershed where it is located. Table A.11.1 lists each major watershed and the associated subwatershed naming convention. Table A.11.1 Major Watersheds within the Northwest Minnehaha Creek Drainage Basin Major Watershed Subwatershed Naming Convention TH 100 H100_## Hopkins HO_## Interlachen EI_## A.11.1.1.1 TH 100 The land use in this 150-acre watershed is primarily commercial and most of the runoff drains to a trunk storm sewer system along TH 100 that flows north to Minnehaha Creek. The pipe system and ponds within the Grandview Square development were also incorporated in the hydraulic and water quality models. A.11.1.1.2 Hopkins The Hopkins watershed is located just south of the City of Hopkins and west of the Interlachen Country Club. This is a small watershed and consists of 94 acres of total land area. All of the water in this watershed is routed north by a lift station to a Hopkins storm sewer system. The land use is primarily low- density residential with several ponds and wetlands. A.11.1.1.3 Interlachen The 360-acre Interlachen watershed consists of the Interlachen Country Club and residential areas adjacent to the golf course. There are several ponds and wetlands that provide storage and treatment for City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.11-2 runoff in this watershed, but few storm sewer pipes. Water conveyed by the storm sewer system drains north to Minnehaha Creek. Modeling of the culverts and storm sewer within the Interlachen Country Club is based on historical reference maps showing proposed pipes connecting the ponds. A survey of actual field conditions may be warranted in the future to improve the accuracy of model results within this watershed. A.11.2 Stormwater System Results A.11.2.1 Hydrologic/Hydraulic Modeling Results The 10- and 1-percent-annual-chance flood analyses were performed for the Northwest Minnehaha Creek drainage basin. For the Northwest Minnehaha Creek watersheds, the storm sewers were evaluated using 10- and 1-percent-annual-chance storm events. The 10-percent-annual-chance analysis was based on a 24-hour storm with 4.29 inches of rain. The 1-percent-annual-chance analysis was based on a 24-hour storm event with 7.47 inches of rain and on a 10-day snowmelt event with 7.2 inches of runoff; the higher resulting flood level of the two events was chosen for the 1-percent-annual-chance analysis. The storm sewers in Data available from the City’s interactive web map present the watershed information and the results for the 1-percent-annual-chance hydrologic analyses for the Northwest Minnehaha Creek basin. A more detailed description of the stormwater system analysis is provided in Section A.1.1.3. The City’s interactive web map depicts the Northwest Minnehaha Creek drainage basin boundary, subwatershed boundaries, the modeled storm sewer network, and the flood-prone areas identified in the modeling analyses. To evaluate the level of protection of the stormwater system within the Northwest Minnehaha Creek drainage area, the 1-percent-annual-chance flood elevations for the ponding basins and depressed areas were compared to the low elevations of structures surrounding each basin. The areas predicted to be inundated during the 1-percent-annual-chance storm event are shown on the City’s interactive web map. Discussion and recommended improvement considerations for these areas are included in Section A.11.3. A.11.2.2 Water Quality Modeling Results The effectiveness of the stormwater system in removing stormwater pollutants such as phosphorus was analyzed using the P8 water quality model. The P8 model simulates the hydrology and phosphorus loads introduced from each pond’s watershed and the transport of phosphorus throughout the stormwater system. A more detailed description of the stormwater system analysis is provided in Section A.1.1.3. Figure A.11.1 depicts the results of the water quality modeling for the Northwest Minnehaha Creek drainage basin. The figure shows the fraction of total phosphorus removal for each water body as well as the cumulative total phosphorus removal in the watershed. A.11.3 Implementation Considerations The XPSWMM hydrologic and hydraulic modeling analyses in support of the 2018 WRMP and 2003 P8 water quality analysis helped identify locations throughout the watershed where improvements to the City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.11-3 City’s stormwater management system may be warranted. The following sections discuss potential improvement options identified as part of the modeling analyses. As opportunities to address identified flooding issues and improve water quality arise (e.g., street reconstruction projects or public facilities improvements), the City will use a comprehensive approach to stormwater management. This approach will include consideration of infiltration or volume retention practices to address flooding and/or improve water quality, reduction of impervious surfaces, increased storm sewer capacity where necessary to alleviate flooding, construction and/or expansion of water quality basins, and implementation of other stormwater BMPs to reduce pollutant loading to downstream waterbodies. A.11.3.1 Potential Flood Risk Reduction Options The 2018 WRMP identified several locations within the Northwest Minnehaha Creek drainage basin where the 1-percent-annual-chance level of protection is not provided by the current stormwater system. As part of the 2018 WRMP modeling analyses, potential corrective measures were identified for problem areas. These preliminary corrective measures are also discussed below. As the City evaluates flooding issues and potential system modifications in these areas, other potential modifications, including (but not limited to) implementation of volume-retention practices, increases in conveyance capacity, and/or stormwater infiltration (where soils are conducive) will be given consideration. The 2003 hydrologic and hydraulic modeling analyses also identified several locations within the Northwest Minnehaha Creek drainage basin where the 1-percent-annual-chance level of protection was not provided by the stormwater system, based on TP-40 precipitation frequency estimates. The discussions related to those areas are included in Appendix B of the 2022 WRMP, along with a short summary of what has been done since 2003. A.11.3.1.1 Blake Road South and Spruce Road (HO_4) At the southeast corner of Blake Road South and Spruce Road, there is a local depression in the backyard area with one inlet draining to a 21-inch pipe and a surface overflow at approximately 922.5 feet. The 21-inch pipe draining this area connects to the storm sewer on Spruce Road that eventually discharges to a wet pond south of Belmore Lane (HO_8) and just west of the Interlachen golf course. The 1-percent- annual-chance flood elevation of HO_4 (921.9 feet) may impact five principal structures (300–308 John Street and 301 and 309 Blake Road South), based on LiDAR and approximate building footprint information. Further downstream, an 18-inch pipe drains HO_8 to another pond (HO_19) which significantly restricts the outflow from HO_8 and raises the water level in HO_8. This creates a tailwater effect on the pipes between HO_4 and HO_8 and limits the outflow from HO_4. The most likely option to reduce flooding impacts in the backyard depression area is to increase the downstream pipe capacity. This may also require upsizing the existing lift station in HO_19 (currently about 500 gpm capacity). There are multiple options to achieve this goal: (1) increase the size of the pipe connecting the two wet ponds (HO_8 and HO_19), (2) add an additional outlet pipe from the wet pond to City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.11-4 the Interlachen Golf Course where storage is available and no structures can be impacted, or (3) install a new pipe connecting the existing storm sewer under Spruce Road to the Interlachen Golf Course. Additional storage could also be created around several small depressions within the golf course. The feasibility of conveying additional water to the golf course and/or adding storage volume in the golf course may be low because it is private property. A.11.3.2 Potential Construction/Upgrade of Water Quality Basins Several ponds in this watershed are removing less than 60 percent of the average annual phosphorus load from stormwater inflows. The light blue ponds on Figure A.11.1 are achieving less than 60 percent total phosphorus removal; however, the cumulative phosphorus removal was greater than 60 percent for all the subwatersheds. Additional analysis was performed to identify which ponds were functioning properly and which were functioning poorly. In the P8 model, phosphorus particles are grouped into several fractions. The unsettleable (dissolved) fraction is called P0. From the mass balance output of the P8 model, the percent of total phosphorus removal for the other settleable phosphorus fractions was evaluated for the apparently “non-performing” ponds to determine if the ponds were removing greater than 60 percent of the settleable phosphorus fractions. From this analysis it was determined that these ponds were removing greater than 60 percent of the settleable phosphorus and performing adequately. As a result, no ponds in this watershed require upgrades. Many techniques are available to reduce pollutant loading from stormwater runoff, including impervious surface reduction or disconnection, implementation of infiltration or volume-retention BMPs, installation of underground stormwater treatment structures and sump manholes, and other good housekeeping practices such as street sweeping. As opportunities arise, the City will consider all of these options to reduce the volume and further improve the quality of stormwater runoff from this drainage area. HO_16 HO_8 HO_19 EI_32 EI_29 EI_31 EI_30 EI_27 EI_26 EI_25 EI_25 EI_1 EI_13 EI_19 EI_24 EI_21 EI_10 EI_12 EI_6 EI_20 H100_4 H100_14 Ho p k i n sHopkins S t . L o uisSt. L o u is P a r kPark Min n ehahaCreek Barr Footer: ArcGIS 10.4.1, 2018-03-26 11:36 File: I:\Client\Edina\Projects\CRWMP_Update_2017\Maps\Reports\Figures_CityReviewDraft\Fig_14_4_MC_Northwest_Water_Quality.mxd User: EMANORTHWEST MINNEHAHA CREEKWATER QUALITY MODELING RESULTSWater ResourceManagement PlanCity of Edina, Minnesota FIGURE A.11.1 1,000 0 1,000Feet !;N 300 0 300Meters Percent TP Removal in Water Body*This number represents the percent of the total annual massof phosphorus entering the water body that is removed. Cumulative TP Removal in Watershed*This number represents the percent of the total annual massof phosphorus entering the watershed and upstream watershedsthat is removed in the pond and all upstream ponds. *Data based on results of 2003 P8 modeling. 0 - 25% (Poor/No Treatment) 25 - 40% (Moderate Removal) 40 - 60% (Good Removal) 60 - 100% (Excellent Removal) 0 - 25% (Poor/No Treatment) 25 - 40% (Moderate Removal) 40 - 60% (Good Removal) 60 - 100% (Excellent Removal) Imagery Source: USDA 2016 NAIP via MnGeo Area Draining Directly to Minnehaha Creek Flow Direction City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas A.12-1 A.12 References Barr Engineering Company, 1999a. Glen Lake Use Attainability Analysis. Barr Engineering Company, 1999b. Round Lake Use Attainability Analysis. Barr Engineering Company, 2001. Bloomington Use Attainability Analysis. Prepared for Nine Mile Creek Watershed District. Barr Engineering Company, 2004. Draft Mirror Lake Use Attainability Analysis. Barr Engineering Company, 2006. Draft Lake Cornelia Use Attainability Analysis. Barr Engineering Company, 2016. City of Edina Imperviousness Assumptions for Stormwater Modeling. Prepared for City of Edina. Barr Engineering Company, February 2021. Infrastructure Options to Reduce Flood Risk in the Morningside Neighborhood. Prepared for City of Edina. Barr Engineering Company, May 2022. 100% Basis of Design Report – Morningside Flood Infrastructure Project. Prepared for City of Edina. City of Edina. 2022. City of Edina Interactive Web Map: https://www.edinamn.gov/894/Maps Minnesota Department of Natural Resources. 2022. LakeFinder web page: http://www.dnr.state.mn.us/lakefind/index.html Minnesota Pollution Control Agency (MPCA), 2000. Protecting Water Quality in Urban Areas, March 2000. Minnesota Pollution Control Agency (MPCA), 2015. South Metro Mississippi River Total Suspended Solids Total Maximum Daily Load Study. Prepared by MPCA and LimnoTech. National Oceanic and Atmospheric Administration, 2013. Atlas 14 Volume 8 - Precipitation-Frequency Atlas of the United States, Midwestern States. Natural Resource Conservation Service (NRCS). 2012 (accessed). Web Soil Survey. https://websoilsurvey.nrcs.usda.gov/app/WebSoilSurvey.aspx Schwab, G.O., D. Fangmeier, W. Elliot, and R. Frevert, 1993. Soil and Water Conservation Engineering. U.S. Environmental Protection Agency, 1988. Stormwater Management Model, Version 4: User’s Manual. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas Attachments Attachment A – City of Edina Impervious Assumptions for Stormwater Modeling Attachment B – Summary of Nine Mile Creek Minnehaha Creek Modeling Approach City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas Attachment A City of Edina Imperviousness Assumptions for Stormwater Modeling Barr Engineering Co. 4300 MarketPointe Drive, Suite 200, Minneapolis, MN 55435 952.832.2600 www.barr.com Technical Memorandum To: Jessica Wilson and Ross Bintner From: Cory Anderson, Sarah Stratton, and Janna Kieffer Subject: City of Edina Imperviousness Assumptions for Stormwater Modeling Date: October 25, 2016 Project: 23/27-0354.00 BCO 160 1.0 Introduction Redevelopment throughout the City of Edina (City), particularly the rebuilding of older homes with newer, larger homes, has raised questions about the imperviousness assumptions used for stormwater modeling. Therefore, as directed by the City, Barr evaluated the most recent imperviousness data throughout different neighborhoods of the city to help determine if the assumptions that were previously used for stormwater modeling are representative of current conditions. This memo documents the findings of this imperviousness assessment, referred to herein as the “2016 analysis”. There are two forms of imperviousness: (1) “Total Impervious” which represents the total area of impervious surfaces such as pavement, roof tops, etc., and (2) “Directly Connected Impervious” which represents the area of impervious surface from which water flows directly into storm sewer or water bodies. The Directly Connected Impervious area is the area that is most important for hydrologic modeling. The majority of this memo discusses the Total Impervious, and Section 5.0 discusses methods for converting from Total Impervious area to Directly Connected Impervious area. Table 1 provides a summary of the imperviousness assumptions used for modeling associated with both the 2003 and 2011 CWRMPs (2003/2011 CWRMPs). Table 1 Imperviousness assumptions from the 2003/2011 CWRMPs Land Use Type Total Impervious % Directly Connected Impervious % Ratio of Directly Connected to Total Commercial 90% 80% 0.889 Developed Park Not previously used Not previously used N/A Golf Course 5% 2% 0.400 High Density Residential 70% 40% 0.571 Highway 50% 50% 1.000 Industrial/Office 90% 80% 0.889 Institutional 40% 20% 0.500 Institutional - High Imperviousness 70% 50% 0.714 Low Density Residential 40% 20% 0.500 Medium Density Residential 55% 30% 0.545 Natural/Park/Open 2% 0% 0.000 Open Water 100% 100% 1.000 To: Jessica Wilson and Ross Bintner From: Cory Anderson, Sarah Stratton, and Janna Kieffer Subject: City of Edina Imperviousness Assumptions for Stormwater Modeling Date: October 25, 2016 Page: 2 P:\Mpls\23 MN\27\23271514 2017 CWRMP SW Modeling Updates\WorkFiles\Imperviousness Analysis\Imperviousness Analysis Summary.docx Land Use Type Total Impervious % Directly Connected Impervious % Ratio of Directly Connected to Total Other Not previously used Not previously used N/A Very Low Density Residential 12% 8% 0.667 Wetland 100% 100% 1.000 2.0 Data Sources The main data source for this 2016 analysis is the 2011 Twin Cities impervious surface area dataset developed by the University of Minnesota (reference [1]). This geographic information system (GIS) dataset is a 30-meter resolution raster (grid) of impervious surface classification for the seven-county Twin Cities Metropolitan Area. The values in this GIS layer represents total imperviousness, not directly connected imperviousness. The impervious surface classification was created using a combination of multi-temporal Landsat (satellite) data and Light Detection and Ranging (LiDAR) data. This raster data set is shown in Figure 1. Barr analyzed the imperviousness data by land use type and neighborhood. This approach allowed us to review the range of results by neighborhood for imperviousness of each land use type. A neighborhood analysis was performed (as opposed to a parcel analysis) due to the larger grid size of the imperviousness raster dataset (i.e., the U of M’s imperviousness data is too coarse for a parcel-level analysis). The City provided the neighborhood GIS layer containing 45 neighborhoods throughout the city (Figure 2 (reference [2]). The land use data utilized for this analysis was the same land use data provided by the City for the 2003/2011 CWRMPs (reference [3]). Using the same land use data allowed us to analyze results with the understanding that changes were strictly based on the changing imperviousness within the city. The land use data is shown in Figure 3. 3.0 Analysis Methods The neighborhood and land use type polygon GIS layers were intersected to define smaller polygons of land use type within each neighborhood. Zonal statistics were then used to calculate the average raster cell value for each land use type within each neighborhood (Table 2). Additionally, the area of each land use type within each neighborhood was calculated to understand which land use types are more prevalent in each neighborhood (Table 3). The data from Table 2 and Table 3 were then used to create a histogram of imperviousness and a cumulative area function to understand the range of imperviousness for each land use type. Figure 4 also shows the average and range of the resulting imperviousness values of all neighborhoods by land use type. These results are presented and discussed in Section 4.0. CahillBraemar Hills Countryside Parkwood Knolls Concord Southdale Bredesen Park Lake Cornelia Indian Hills Highlands Dewey Hill Creek Valley Todd Park Birchcrest Grandview Presidents The Heights Prospect Knolls Parklawn South Cornelia Melody Lake Arden Park Normandale Park Pentagon Park Morningside Pamela Park Lake Edina Fox Meadow Golf Terrace Heights Country ClubInterlachen Park Chowen Park Indian Trails Hilldale Promenade Rolling Green Brookview Heights Edinborough Minnehaha Woods Strachauer Park White Oaks Centennial Lakes Sunny Slope Creek Knoll 50th and France Source: Esri, DigitalGlobe, GeoEye, i-cubed, USDA, USGS, AEX,Getmapping, Aerogrid, IGN, IGP, and the GIS User Community PERCENT IMPERVIOUSNESS2011 U OF M DATASETImperviousness AnalysisCity of Edina FIGURE 1 Barr Footer: ArcGIS 10.4, 2016-09-16 16:52 File: I:\Client\Edina\Projects\CRWMP_Update_2017\Maps\Meetings\Percent Imperviousness.mxd User: cda1 0 1 Miles !;N Percent ImperviousnessHigh : 100 Low : 0 Edina Neighborhoods Note: Raster grid cells with0% imperviousness aretransparent and the backgroundimagery is visible. §¨¦494§¨¦494 £¤212 £¤169 £¤169 100 62 7 10062 7 456728 456717 456753 456734 456731 45673 4567158 456720 456732 456761 CahillBraemar Hills Countryside Parkwood Knolls Concord Southdale Bredesen Park Lake Cornelia Indian Hills Highlands Dewey Hill Creek Valley Todd Park Birchcrest Grandview Presidents The Heights Prospect Knolls Parklawn South Cornelia Melody Lake Arden Park Pentagon Park Morningside Pamela Park Lake Edina Fox Meadow Golf Terrace Heights Country ClubInterlachen Park Chowen Park Normandale Park Indian Trails Hilldale Promenade Rolling Green Brookview Heights Edinborough Minnehaha Woods Strachauer Park White Oaks Centennial Lakes Sunny Slope Creek Knoll 50th and France Source: Esri, DigitalGlobe, GeoEye, i-cubed, USDA, USGS, AEX,Getmapping, Aerogrid, IGN, IGP, and the GIS User Community EDINA NEIGHBORHOODSImperviousness AnalysisCity of Edina FIGURE 2 Barr Footer: ArcGIS 10.4, 2016-09-16 16:24 File: I:\Client\Edina\Projects\CRWMP_Update_2017\Maps\Meetings\Edina Neighborhoods.mxd User: cda1 0 1 Miles !;N Edina Neighborhoods Streets and Highways Interstate Highway US Highway State Trunk Highway County State-Aid Highway Source: Esri, DigitalGlobe, GeoEye, i-cubed, USDA, USGS, AEX,Getmapping, Aerogrid, IGN, IGP, and the GIS User Community LAND USE TYPEImperviousness AnalysisCity of Edina FIGURE 3 Barr Footer: ArcGIS 10.4, 2016-09-16 16:44 File: I:\Client\Edina\Projects\CRWMP_Update_2017\Maps\Meetings\Land Use Map.mxd User: cda1 0 1 Miles !;N Edina Neighborhoods Land Use Natural/Park/Open Developed Parkland Golf Course Very Low Density Residential Low Density Residential Medium Density Residential High Density Residential Institutional Institutional - High Imperviousness Highway Commercial Industrial/Office Other Open Water Wetland Table 2 - Mean total imperviousness by land use type within each neighborhood Commercial Developed Park Golf Course High Density Residential Highway Industrial/ Office Institutional Institutional - High Imperviousness Low Density Residential Medium Density Residential Natural/Park/ Open Open Water Other Very Low Density Residential Wetland Average % Impervious Legend 50th and France 87.3 72.6 52.0 61.8 86.5 Arden Park 64.6 0.0 34.3 63.4 39.4 32.6 65.6 7.3 100.0 36.6 100.0 Birchcrest 48.9 68.1 32.8 18.4 100.0 21.0 36.9 90.0 Braemar Hills 66.7 3.5 63.3 69.3 54.8 27.9 14.2 100.0 12.5 100.0 29.7 80.0 Bredesen Park 72.2 4.6 40.9 61.8 33.7 42.1 5.6 100.0 100.0 46.4 70.0 Brookview Heights 71.8 59.7 30.9 12.6 100.0 21.3 100.0 37.9 60.0 Cahill 72.4 49.9 74.1 60.4 63.5 24.3 100.0 41.9 100.0 70.5 50.0 Centennial Lakes 88.0 41.3 60.0 83.0 100.0 76.9 40.0 Chowen Park 42.2 38.9 7.7 100.0 38.6 30.0 Concord 53.2 38.6 53.6 60.6 35.3 19.4 100.0 100.0 45.5 20.0 Country Club 33.8 65.6 38.0 100.0 41.1 10.0 Countryside 22.3 37.7 32.7 28.6 49.1 25.7 100.0 18.8 100.0 32.0 0.0 Creek Knoll 62.2 34.2 76.7 14.9 100.0 36.9 Creek Valley 12.8 39.6 30.8 27.7 2.0 100.0 100.0 36.9 Dewey Hill 6.5 65.5 32.0 41.2 11.8 100.0 100.0 39.3 Edinborough 64.5 63.1 57.9 64.5 76.3 34.0 47.9 57.0 Fox Meadow 19.6 28.9 51.4 6.9 100.0 21.6 37.4 Golf Terrace Heights 65.6 27.5 7.2 68.3 61.3 35.1 100.0 28.7 Grandview 80.0 42.0 46.8 59.0 46.5 66.6 37.7 54.0 100.0 44.7 50.4 Highlands 12.5 26.0 35.3 28.4 10.7 100.0 27.8 100.0 31.8 Hilldale 0.0 21.5 100.0 100.0 39.7 Indian Hills 62.8 30.5 56.3 27.6 100.0 18.1 100.0 38.0 Indian Trails 65.8 56.0 71.7 28.6 4.6 13.3 100.0 33.5 Interlachen Park 57.5 6.6 25.8 100.0 100.0 17.6 Lake Cornelia 60.2 34.1 60.6 48.1 33.5 11.3 100.0 100.0 45.2 Lake Edina 90.7 9.2 0.0 62.4 34.6 15.2 100.0 100.0 43.9 Melody Lake 53.4 44.5 30.3 3.1 100.0 23.7 33.7 Minnehaha Woods 56.2 4.4 16.7 34.6 71.5 34.7 100.0 100.0 35.4 Morningside 68.2 15.2 45.1 32.1 2.7 15.4 100.0 100.0 32.3 Normandale Park 10.0 53.5 43.0 31.6 10.0 100.0 24.5 100.0 34.0 Pamela Park 72.0 8.4 59.0 59.2 37.1 28.0 100.0 100.0 35.9 Parklawn 77.2 6.7 61.9 72.8 26.7 100.0 51.6 Parkwood Knolls 66.4 19.7 47.5 59.0 29.5 51.7 3.6 100.0 22.1 100.0 31.7 Pentagon Park 78.0 40.4 60.5 71.3 0.0 100.0 75.2 Presidents 13.8 63.1 56.5 29.5 24.8 100.0 29.1 Promenade 80.2 52.8 63.6 73.8 71.9 Prospect Knolls 57.4 17.1 34.3 52.2 27.4 45.7 0.4 100.0 100.0 29.7 Rolling Green 17.4 21.4 100.0 100.0 27.1 South Cornelia 76.9 58.2 34.3 39.0 30.7 41.0 Southdale 81.2 67.7 59.0 60.7 84.5 49.9 100.0 76.1 Strachauer Park 7.1 55.5 39.7 39.1 Sunny Slope 39.8 68.7 75.0 29.4 100.0 41.0 The Heights 64.0 15.2 74.3 39.2 30.9 45.1 8.6 16.0 100.0 33.3 Todd Park 37.4 12.5 60.9 31.0 39.0 22.8 100.0 100.0 37.2 White Oaks 40.6 44.5 30.3 47.8 100.0 100.0 36.0 Commercial Developed Park Golf Course High Density Residential Highway Industrial/ Office Institutional Institutional - High Imperviousness Low Density Residential Medium Density Residential Natural/Park/ Open Open Water Other Very Low Density Residential Wetland Maximum 90.7 67.7 40.4 72.6 71.8 83.0 75.0 84.5 60.4 76.7 34.7 100.0 44.7 27.8 100.0 Minimum 37.4 0.0 0.0 38.6 37.7 52.2 16.7 56.3 21.4 2.7 0.0 100.0 16.0 12.5 100.0 Average 77.6 18.7 5.4 58.7 53.8 71.7 41.7 71.6 31.7 42.6 10.5 100.0 31.3 20.1 100.0 Table 3 - Area (acres) of each land use type within each neighborhood Commercial Developed Park Golf Course High Density Residential Highway Industrial/ Office Institutional Institutional - High Imperviousness Low Density Residential Medium Density Residential Natural/Park/ Open Open Water Other Very Low Density Residential Wetland Total Acres of Neighborhood 50th and France 18.91 0.97 0.004 0.06 20 Arden Park 8.87 0.003 0.60 4.40 2.48 114.16 1.98 12.47 6.20 151 Birchcrest 25.23 3.95 150.04 2.73 4.76 3.91 191 Braemar Hills 31.69 263.86 28.79 32.89 23.17 134.46 91.91 23.33 4.42 43.16 678 Bredesen Park 2.72 12.57 44.07 40.70 125.31 52.99 104.13 17.25 97.77 497 Brookview Heights 13.80 5.28 144.99 2.56 3.01 2.51 5.88 178 Cahill 64.78 26.48 255.13 0.26 0.03 7.41 4.52 14.58 5.43 379 Centennial Lakes 38.64 13.35 17.48 18.10 10.05 98 Chowen Park 1.26 176.30 4.25 1.33 183 Concord 1.87 3.97 28.21 48.29 192.44 1.15 17.91 1.38 295 Country Club 5.74 1.49 164.24 8.68 180 Countryside 35.12 14.79 42.68 355.49 5.11 1.98 17.09 4.60 4.05 481 Creek Knoll 2.83 33.47 1.05 13.15 4.27 55 Creek Valley 18.36 21.55 97.42 73.60 18.11 0.95 35.28 265 Dewey Hill 16.17 12.15 111.44 60.86 16.00 20.62 1.48 239 Edinborough 8.36 0.39 43.10 16.01 6.79 10.32 12.70 98 Fox Meadow 0.25 132.88 5.58 10.21 27.29 20.89 197 Golf Terrace Heights 5.92 5.57 127.51 18.80 7.81 130.09 10.04 306 Grandview 25.54 0.13 28.21 9.59 23.51 20.87 77.02 1.32 0.40 3.17 190 Highlands 13.72 0.30 12.26 226.84 19.85 16.89 10.81 4.34 305 Hilldale 0.74 59.42 5.42 12.99 79 Indian Hills 28.33 3.83 6.20 166.68 42.64 88.49 0.98 337 Indian Trails 5.69 13.63 14.02 88.52 4.76 22.82 0.18 150 Interlachen Park 1.96 153.62 53.14 13.46 0.88 223 Lake Cornelia 0.12 30.50 0.15 29.27 289.09 14.18 66.43 8.32 438 Lake Edina 2.06 14.78 0.07 11.31 112.77 7.03 25.43 0.58 174 Melody Lake 6.72 0.31 157.97 4.35 8.51 3.00 181 Minnehaha Woods 0.02 1.06 3.69 132.39 1.14 0.67 1.06 1.58 142 Morningside 7.90 12.08 7.79 192.01 10.28 6.02 3.15 0.82 240 Normandale Park 14.07 31.98 0.05 155.17 6.51 0.79 4.75 3.71 217 Pamela Park 4.98 51.10 0.01 3.89 153.82 0.08 4.08 10.94 229 Parklawn 28.42 38.14 58.45 7.95 0.77 4.91 139 Parkwood Knolls 11.34 20.33 4.76 3.68 369.33 18.96 42.00 30.65 118.87 4.30 624 Pentagon Park 86.52 0.26 6.88 49.05 0.18 2.53 145 Presidents 5.11 1.24 2.89 135.05 35.08 0.77 180 Promenade 59.92 8.69 42.49 9.46 121 Prospect Knolls 0.17 19.23 0.56 0.67 174.03 36.74 10.25 4.13 0.51 246 Rolling Green 0.26 126.78 4.65 5.31 137 South Cornelia 8.75 11.01 22.24 167.28 2.71 212 Southdale 248.23 0.15 61.71 12.39 13.86 8.14 3.24 348 Strachauer Park 5.89 7.85 101.19 115 Sunny Slope 0.35 6.11 0.07 55.82 8.01 70 The Heights 0.03 7.83 0.05 4.06 171.32 2.15 6.00 1.35 10.07 203 Todd Park 8.58 15.52 6.41 129.88 14.22 0.33 0.05 16.02 191 White Oaks 0.19 0.05 61.87 1.33 0.23 4.95 69 Commercial Developed Park Golf Course High Density Residential Highway Industrial/ Office Institutional Institutional - High Imperviousness Low Density Residential Medium Density Residential Natural/Park/ Open Open Water Other Very Low Density Residential Wetland Maximum 248 51 264 62 44 255 97 21 369 61 104 66 15 119 98 Minimum 0.02 0.003 0.07 0.01 1.24 0.05 0.004 3.95 0.26 0.03 0.08 0.05 1.35 4.42 0.18 Total Acres in Edina 683 315 602 272 404 456 312 52 5416 227 446 396 38 266 309 To: Jessica Wilson and Ross Bintner From: Cory Anderson, Sarah Stratton, and Janna Kieffer Subject: City of Edina Imperviousness Assumptions for Stormwater Modeling Date: October 25, 2016 Page: 8 P:\Mpls\23 MN\27\23271514 2017 CWRMP SW Modeling Updates\WorkFiles\Imperviousness Analysis\Imperviousness Analysis Summary.docx 4.0 Results The average imperviousness for each land use type and the range of imperviousness among neighborhoods is shown in Figure 4. The imperviousness values assumed for the 2003/2011 CWRMPs are also shown in Figure 4. For some land use types such as Golf Course, Highway, Institutional, and Institutional – High Imperviousness, the 2016 analysis average value matches very closely with the 2003/2011 CWRMPs assumed value. For others, such as Commercial, High Density Residential, and Industrial/Office, the 2003/2011 CWRMPs assumed value is substantially higher when compared to the results of this 2016 analysis. For a few other land use types, such as Natural/Park/Open and Very Low Density Residential, the 2003/2011 CWRMPs assumptions appear to be low compared to the results of the 2016 analysis. Low and Medium Density Residential land use types both have wide ranges of imperviousness based on the 2016 analysis, and the 2003/2011 CWRMPs assumptions are on the high end of these new results. Open Water and Wetland land use types are 100% in both the 2003/2011 CWRMPs and this 2016 analysis; those will not change. Land use types Developed Park and Other were not used previously. The following figures (Figure 5 through Figure 17) show the resulting histograms of each of the land use types. Figure 4 - Average and range of imperviousness within all neighborhoods by land use type 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 Imperviousness (%)Maximum Minimum Average 2003/2011 CWRMPs To: Jessica Wilson and Ross Bintner From: Cory Anderson, Sarah Stratton, and Janna Kieffer Subject: City of Edina Imperviousness Assumptions for Stormwater Modeling Date: October 25, 2016 Page: 10 P:\Mpls\23 MN\27\23271514 2017 CWRMP SW Modeling Updates\WorkFiles\Imperviousness Analysis\Imperviousness Analysis Summary.docx Figure 5 Percent impervious histogram of the Commercial land use type Figure 6 Percent impervious histogram of the Developed Park land use type 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0 1 2 3 4 5 6 0% - 5%5% - 10%10% - 15%15% - 20%20% - 25%25% - 30%30% - 35%35% - 40%40% - 45%45% - 50%50% - 55%55% - 60%60% - 65%65% - 70%70% - 75%75% - 80%80% - 85%85% - 90%90% - 95%95% - 100%Fraction of Total Land Use AreaNumber of NeighborhoodsPercent Impervious (U of M 2011 data) Commercial; N = 27 Neighborhoods Total Commercial area in Edina = 683 acres 2003/2011 CWRMP Total Imp% = 90% 2003/2011 CWRMP Directly Connected Imp% = 80% 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0 1 2 3 4 5 6 0% - 5%5% - 10%10% - 15%15% - 20%20% - 25%25% - 30%30% - 35%35% - 40%40% - 45%45% - 50%50% - 55%55% - 60%60% - 65%65% - 70%70% - 75%75% - 80%80% - 85%85% - 90%90% - 95%95% - 100%Fraction of Total Land Use AreaNumber of NeighborhoodsPercent Impervious (U of M 2011 data) Developed Park; N = 27 Neighborhoods Total Developed Park area in Edina = 315 acres 2003/2011 CWRMP Total Imp% = N/A 2003/2011 CWRMP Directly Connected Imp% = N/A To: Jessica Wilson and Ross Bintner From: Cory Anderson, Sarah Stratton, and Janna Kieffer Subject: City of Edina Imperviousness Assumptions for Stormwater Modeling Date: October 25, 2016 Page: 11 P:\Mpls\23 MN\27\23271514 2017 CWRMP SW Modeling Updates\WorkFiles\Imperviousness Analysis\Imperviousness Analysis Summary.docx Figure 7 Percent impervious histogram of the Golf Course land use type Figure 8 Percent impervious histogram of the High Density Residential land use type 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 0% - 5%5% - 10%10% - 15%15% - 20%20% - 25%25% - 30%30% - 35%35% - 40%40% - 45%45% - 50%50% - 55%55% - 60%60% - 65%65% - 70%70% - 75%75% - 80%80% - 85%85% - 90%90% - 95%95% - 100%Fraction of Total Land Use AreaNumber of NeighborhoodsPercent Impervious (U of M 2011 data) Golf Course; N = 13 Neighborhoods Total Golf Course area in Edina = 602 acres 2003/2011 CWRMP Total Imp% = 5% 2003/2011 CWRMP Directly Connected Imp% = 2% 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0 1 2 3 4 5 6 0% - 5%5% - 10%10% - 15%15% - 20%20% - 25%25% - 30%30% - 35%35% - 40%40% - 45%45% - 50%50% - 55%55% - 60%60% - 65%65% - 70%70% - 75%75% - 80%80% - 85%85% - 90%90% - 95%95% - 100%Fraction of Total Land Use AreaNumber of NeighborhoodsPercent Impervious (U of M 2011 data) High Density Residential; N = 12 Neighborhoods Total High Density Residential area in Edina = 272 acres 2003/2011 CWRMP Total Imp% = 70% 2003/2011 CWRMP Directly Connected Imp% = 40% To: Jessica Wilson and Ross Bintner From: Cory Anderson, Sarah Stratton, and Janna Kieffer Subject: City of Edina Imperviousness Assumptions for Stormwater Modeling Date: October 25, 2016 Page: 12 P:\Mpls\23 MN\27\23271514 2017 CWRMP SW Modeling Updates\WorkFiles\Imperviousness Analysis\Imperviousness Analysis Summary.docx Figure 9 Percent impervious histogram of the Highway land use type Figure 10 Percent impervious histogram of the Industrial/Office land use type 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0 1 2 3 4 5 6 7 8 0% - 5%5% - 10%10% - 15%15% - 20%20% - 25%25% - 30%30% - 35%35% - 40%40% - 45%45% - 50%50% - 55%55% - 60%60% - 65%65% - 70%70% - 75%75% - 80%80% - 85%85% - 90%90% - 95%95% - 100%Fraction of Total Land Use AreaNumber of NeighborhoodsPercent Impervious (U of M 2011 data) Highway; N = 25 Neighborhoods Total Highway area in Edina = 404 acres 2003/2011 CWRMP Total Imp% = 50% 2003/2011 CWRMP Directly Connected Imp% = 50% 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0 1 2 3 4 5 6 0% - 5%5% - 10%10% - 15%15% - 20%20% - 25%25% - 30%30% - 35%35% - 40%40% - 45%45% - 50%50% - 55%55% - 60%60% - 65%65% - 70%70% - 75%75% - 80%80% - 85%85% - 90%90% - 95%95% - 100%Fraction of Total Land Use AreaNumber of NeighborhoodsPercent Impervious (U of M 2011 data) Industrial/Office; N = 13 Neighborhoods Total Industrial/Office area in Edina = 456 acres 2003/2011 CWRMP Total Imp% = 90% 2003/2011 CWRMP Directly Connected Imp% = 80% To: Jessica Wilson and Ross Bintner From: Cory Anderson, Sarah Stratton, and Janna Kieffer Subject: City of Edina Imperviousness Assumptions for Stormwater Modeling Date: October 25, 2016 Page: 13 P:\Mpls\23 MN\27\23271514 2017 CWRMP SW Modeling Updates\WorkFiles\Imperviousness Analysis\Imperviousness Analysis Summary.docx Figure 11 Percent impervious histogram of the Institutional land use type Figure 12 Percent impervious histogram of the Institutional – High Imperviousness land use type 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 0% - 5%5% - 10%10% - 15%15% - 20%20% - 25%25% - 30%30% - 35%35% - 40%40% - 45%45% - 50%50% - 55%55% - 60%60% - 65%65% - 70%70% - 75%75% - 80%80% - 85%85% - 90%90% - 95%95% - 100%Fraction of Total Land Use AreaNumber of NeighborhoodsPercent Impervious (U of M 2011 data) Institutional; N = 20 Neighborhoods Total Institutional area in Edina = 312 acres 2003/2011 CWRMP Total Imp% = 40% 2003/2011 CWRMP Directly Connected Imp% = 20% 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0 0.5 1 1.5 2 2.5 0% - 5%5% - 10%10% - 15%15% - 20%20% - 25%25% - 30%30% - 35%35% - 40%40% - 45%45% - 50%50% - 55%55% - 60%60% - 65%65% - 70%70% - 75%75% - 80%80% - 85%85% - 90%90% - 95%95% - 100%Fraction of Total Land Use AreaNumber of NeighborhoodsPercent Impervious (U of M 2011 data) Institutional -High Imperviousness; N = 5 Neighborhoods Total Institutional -High Imperviousness area in Edina = 52 acres 2003/2011 CWRMP Total Imp% = 70% 2003/2011 CWRMP Directly Connected Imp% = 50% To: Jessica Wilson and Ross Bintner From: Cory Anderson, Sarah Stratton, and Janna Kieffer Subject: City of Edina Imperviousness Assumptions for Stormwater Modeling Date: October 25, 2016 Page: 14 P:\Mpls\23 MN\27\23271514 2017 CWRMP SW Modeling Updates\WorkFiles\Imperviousness Analysis\Imperviousness Analysis Summary.docx Figure 13 Percent impervious histogram of the Low Density Residential land use type Figure 14 Percent impervious histogram of the Medium Density Residential land use type 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0 2 4 6 8 10 12 14 16 18 0% - 5%5% - 10%10% - 15%15% - 20%20% - 25%25% - 30%30% - 35%35% - 40%40% - 45%45% - 50%50% - 55%55% - 60%60% - 65%65% - 70%70% - 75%75% - 80%80% - 85%85% - 90%90% - 95%95% - 100%Fraction of Total Land Use AreaNumber of NeighborhoodsPercent Impervious (U of M 2011 data) Low Density Residential; N = 41 Neighborhoods Total Low Density Residential area in Edina = 5416 acres 2003/2011 CWRMP Total Imp% = 40% 2003/2011 CWRMP Directly Connected Imp% = 20% 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0 1 2 3 4 5 6 0% - 5%5% - 10%10% - 15%15% - 20%20% - 25%25% - 30%30% - 35%35% - 40%40% - 45%45% - 50%50% - 55%55% - 60%60% - 65%65% - 70%70% - 75%75% - 80%80% - 85%85% - 90%90% - 95%95% - 100%Fraction of Total Land Use AreaNumber of NeighborhoodsPercent Impervious (U of M 2011 data) Medium Density Residential; N = 17 Neighborhoods Total Medium Density Residential area in Edina = 227 acres 2003/2011 CWRMP Total Imp% = 55% 2003/2011 CWRMP Directly Connected Imp% = 30% To: Jessica Wilson and Ross Bintner From: Cory Anderson, Sarah Stratton, and Janna Kieffer Subject: City of Edina Imperviousness Assumptions for Stormwater Modeling Date: October 25, 2016 Page: 15 P:\Mpls\23 MN\27\23271514 2017 CWRMP SW Modeling Updates\WorkFiles\Imperviousness Analysis\Imperviousness Analysis Summary.docx Figure 15 Percent impervious histogram of the Natural/Park/Open land use type Figure 16 Percent impervious histogram of the Other land use type 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0 1 2 3 4 5 6 7 0% - 5%5% - 10%10% - 15%15% - 20%20% - 25%25% - 30%30% - 35%35% - 40%40% - 45%45% - 50%50% - 55%55% - 60%60% - 65%65% - 70%70% - 75%75% - 80%80% - 85%85% - 90%90% - 95%95% - 100%Fraction of Total Land Use AreaNumber of NeighborhoodsPercent Impervious (U of M 2011 data) Natural/Park/Open; N = 29 Neighborhoods Total Natural/Park/Open area in Edina = 446 acres 2003/2011 CWRMP Total Imp% = 2% 2003/2011 CWRMP Directly Connected Imp% = 0% 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 0% - 5%5% - 10%10% - 15%15% - 20%20% - 25%25% - 30%30% - 35%35% - 40%40% - 45%45% - 50%50% - 55%55% - 60%60% - 65%65% - 70%70% - 75%75% - 80%80% - 85%85% - 90%90% - 95%95% - 100%Fraction of Total Land Use AreaNumber of NeighborhoodsPercent Impervious (U of M 2011 data) Other; N = 8 Neighborhoods Total Other area in Edina = 38 acres 2003/2011 CWRMP Total Imp% = N/A 2003/2011 CWRMP Directly Connected Imp% = N/A To: Jessica Wilson and Ross Bintner From: Cory Anderson, Sarah Stratton, and Janna Kieffer Subject: City of Edina Imperviousness Assumptions for Stormwater Modeling Date: October 25, 2016 Page: 16 P:\Mpls\23 MN\27\23271514 2017 CWRMP SW Modeling Updates\WorkFiles\Imperviousness Analysis\Imperviousness Analysis Summary.docx Figure 17 Percent impervious histogram of the Very Low Density Residential land use type Table 4 shows the fraction of the area throughout the city in which the imperviousness from this 2016 analysis is below the assumptions used for the 2003/2011 CWRMPs. In other words, high numbers in Table 4 suggest that the previously used assumptions are conservative with respect to runoff volume because they may be overestimating the imperviousness of the land use type in some areas within Edina. Percentages in Table 4 around 40% to 50% suggest that imperviousness is underestimated for about half the area, and therefore, overestimated for the other half of the area. Low percentages in Table 4 (e.g., Very Low Density Residential) suggest that the previous assumptions in the 2003/2011 CWRMPs for associated land use types may be too low, and consideration should be given for increasing those imperviousness values. Table 4 Percent of total area of Edina where new average imperviousness value is below 2003/2011 CWRMP values Land Use Type Percent of Area below 2003/2011 CWRMP Imperviousness value Commercial ~100% Developed Park Not previously used Golf Course ~44% High Density Residential ~100% Highway ~41% Industrial/Office ~100% Institutional ~60% 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0 0.5 1 1.5 2 2.5 0% - 5%5% - 10%10% - 15%15% - 20%20% - 25%25% - 30%30% - 35%35% - 40%40% - 45%45% - 50%50% - 55%55% - 60%60% - 65%65% - 70%70% - 75%75% - 80%80% - 85%85% - 90%90% - 95%95% - 100%Fraction of Total Land Use AreaNumber of NeighborhoodsPercent Impervious (U of M 2011 data) Very Low Density Residential; N = 6 Neighborhoods Total Very Low Density Residential area in Edina = 266 acres 2003/2011 CWRMP Total Imp% = 12% 2003/2011 CWRMP Directly Connected Imp% = 8% To: Jessica Wilson and Ross Bintner From: Cory Anderson, Sarah Stratton, and Janna Kieffer Subject: City of Edina Imperviousness Assumptions for Stormwater Modeling Date: October 25, 2016 Page: 17 P:\Mpls\23 MN\27\23271514 2017 CWRMP SW Modeling Updates\WorkFiles\Imperviousness Analysis\Imperviousness Analysis Summary.docx Land Use Type Percent of Area below 2003/2011 CWRMP Imperviousness value Institutional - High Imperviousness ~60% Low Density Residential ~100% Medium Density Residential ~98% Natural/Park/Open < 18% Open Water ~100% Other Not previously used Very Low Density Residential < 10% Wetland ~100% A discussion of the results for four different land use types is presented here to provide guidance for interpreting the results. • Open Water: This land use type, by definition is 100% impervious. Therefore, the imperviousness values of this 2016 analysis match the 2003/2011 CWRMPs and do not need to be adjusted. • Commercial: There are 27 neighborhoods that contain the Commercial land use type. The total area of Commercial land use is about 680 acres, with nearly 250 acres of Commercial land use falling within the Southdale neighborhood. There are five neighborhoods with imperviousness less than 60%, and there is one neighborhood with imperviousness greater than 90%. However, those extremes comprise only about 13 acres of the 680 total acres of Commercial land use. Close to 50% of the area of Commercial land use is less than 80% impervious, and about 90% of the Commercial land use area is below 85% impervious. Finally, essentially all of the Commercial land use area is less than 90% impervious. Therefore, the assumption of 90% impervious used in the 2003/2011 CWRMPs for Commercial land use may be overestimated. Alternatively, 90% impervious can be thought of as a conservative assumption with respect to runoff volume. • Institutional: There are 20 neighborhoods that contain the Institutional land use type. The total area of Institutional land use is about 310 acres, with nearly 190 acres of Institutional land use within the Concord, Countryside, and Creek Valley neighborhoods. There is one neighborhood with imperviousness less than 20%, and there are two neighborhoods with imperviousness greater than 70%. However, those extremes comprise only about 13 acres of the 310 total acres of Institutional land use. Roughly 60% of the area of Institutional land use is less than 40% impervious. Therefore, the assumption of 40% impervious used in the 2003/2011 CWRMPs for Institutional land use is right in the middle of the imperviousness results of the 2016 analysis. • Very Low Density Residential: There are six neighborhoods that contain the Very Low Density Residential land use type. The total area of Very Low Density Residential land use is almost 270 acres, with about 230 acres of Very Low Density Residential land use within the Indian Hills, Indian Trails, and Parkwood Knolls neighborhoods. The three neighborhoods between 15% and 25% impervious make up about 85% of the Very Low Density Residential area. Close to 50% of the total area of Very Low Density Residential land use is less than about 20% impervious, and about To: Jessica Wilson and Ross Bintner From: Cory Anderson, Sarah Stratton, and Janna Kieffer Subject: City of Edina Imperviousness Assumptions for Stormwater Modeling Date: October 25, 2016 Page: 18 P:\Mpls\23 MN\27\23271514 2017 CWRMP SW Modeling Updates\WorkFiles\Imperviousness Analysis\Imperviousness Analysis Summary.docx 95% of the Very Low Density Residential land use area is below 25% impervious. There are no neighborhoods with imperviousness less than 12%. Therefore, the assumption of 12% impervious used in the 2003/2011 CWRMPs for Very Low Density Residential land use may be underestimated which is consistent with the assumption that increasing development has impacted imperviousness. However, the increase in imperviousness does not appear to be significant enough to make the imperviousness values for this land use type consistent with the imperviousness values for the Low Density Residential land use type. There is still a difference in the imperviousness values of these two land use types. 5.0 Conversion from Total Imperviousness to Directly Connected Imperviousness Sections 1.0 – 4.0 of this memo have discussed total imperviousness for each land use type. However, what is important for hydrologic modeling is the directly connected imperviousness which is similar to effective impervious area. A July 2015 report on effective impervious area suggests that these terms are slightly different (reference [4]). The report describes how the effective impervious area is usually less, about 80% to 90% of the directly connected impervious area. Two possible approaches for converting from total to directly connected imperviousness are listed below. First, the simplest approach for converting the total imperviousness described in Section 4.0 to directly connected imperviousness is to simply use the same conversion ratios (ratio of directly connected to total) used in the 2003/2011CWRMPs as shown in Table 1 and then apply some engineering judgment to the results. For example, if the total imperviousness of Commercial land use was changed from 90% to 80%, and the same ratio was then used to convert total imperviousness to directly connected imperviousness (0.889), the result for Commercial land use would be 71%, or potentially rounded to 70% directly connected imperviousness. Second, an alternative method is proposed in a report by John Gulliver and others at the University of Minnesota (reference [4]). The proposed method of determining the directly connected impervious area fraction in ungauged urban watersheds is summarized in the following steps: • Extract total imperviousness from land use and the hydrologic soil groups from the SSURGO data set and calculate the weighted average saturated hydraulic conductivity of the soil. • Estimate the actual curve number of the watershed as a function of total imperviousness and the saturated hydraulic conductivity. • Determine the fraction of effective impervious area as a function of the actual curve number. • Assume that the effective impervious area is roughly 85% of the directly connected impervious area, and scale up the values to account for this difference with a factor of 1.176 (or 0.85-1). To: Jessica Wilson and Ross Bintner From: Cory Anderson, Sarah Stratton, and Janna Kieffer Subject: City of Edina Imperviousness Assumptions for Stormwater Modeling Date: October 25, 2016 Page: 19 P:\Mpls\23 MN\27\23271514 2017 CWRMP SW Modeling Updates\WorkFiles\Imperviousness Analysis\Imperviousness Analysis Summary.docx The approach suggested in the paper by Gulliver could be followed to determine the directly connected impervious area for the purposes of the 2017 XP-SWMM modeling. However, there are some concerns about the applicability of the paper to this modeling. First, much of the method relies on regression equations that do not account for the spread in the data and the error bars, which appear to be relatively significant. Second, the suggested approach is likely more useful for simpler hydrologic modeling methods, such as the rational method. In XP-SWMM, hydrologic factors such as depression storage and infiltration parameters based on soil type are treated as independent inputs. In the method described in the paper, it appears that these other hydrologic factors are implicitly included in the estimated value of effective impervious area. Therefore, we do not recommend using this approach to estimate imperviousness for the 2017 XP-SWMM modeling. 6.0 Consequences and Risks Understanding the consequences and risks of over- or under-estimating the imperviousness can help determine an appropriate value for each land use type in the city of Edina. Figure 18 is a simple diagram to help illustrate this decision making process. Currently, there is a range of imperviousness throughout the city, and it varies by land use type (residential versus commercial versus park space, etc.). Accounting for the trend that the city is becoming more impervious, it is reasonable to expect that in the near future, the imperviousness will be higher than what it is today. However, with policies and regulations being put in place to limit the increase in imperviousness and to offset any additional imperviousness being created (e.g., using stormwater BMPs), the long term outlook is much more uncertain. If the current imperviousness is used in the modeling for the 2017 CWRMP, then the risk is that it will likely be outdated and too low in the near future. The consequence is that flooding of structures may increase, stormwater infrastructure may be undersized, and the level of service provided by the City will decrease creating frustration within the community. If the current trend of increasing imperviousness is extended into the future, the risk is that the imperviousness will be overestimated. The consequence is that more locations may be identified as flood risk locations and may require expensive updates to infrastructure. The flooding of structures may decrease because the stormwater infrastructure will generally be oversized. The level of service will increase, but it will come at a significant and potentially unnecessary cost to the community. Finally, choosing an imperviousness value that is higher than the current average, but one that captures the current trend of increasing imperviousness without extending it too far into the future may be the best selection. Risk of over- or under-estimating the imperviousness still exists, but the consequences may be less because the error in the selected value will likely be less. Therefore, for each land use type, selecting a value that is higher than 80% to 90% of the total area of that land use type is expected to be a reasonably protective, yet still accurate value. To: Jessica Wilson and Ross Bintner From: Cory Anderson, Sarah Stratton, and Janna Kieffer Subject: City of Edina Imperviousness Assumptions for Stormwater Modeling Date: October 25, 2016 Page: 20 P:\Mpls\23 MN\27\23271514 2017 CWRMP SW Modeling Updates\WorkFiles\Imperviousness Analysis\Imperviousness Analysis Summary.docx Figure 18 Total imperviousness estimation; consequences and risks diagram current status likely future ??? current trend low total imperviousness high Consequences and risks •Flood risk •Infrastructure size •Level of service current range of imperviousness likely near future range of imperviousness To: Jessica Wilson and Ross Bintner From: Cory Anderson, Sarah Stratton, and Janna Kieffer Subject: City of Edina Imperviousness Assumptions for Stormwater Modeling Date: October 25, 2016 Page: 21 P:\Mpls\23 MN\27\23271514 2017 CWRMP SW Modeling Updates\WorkFiles\Imperviousness Analysis\Imperviousness Analysis Summary.docx 7.0 Conclusions An analysis of the imperviousness throughout the city of Edina for multiple land use types was completed using the most recent available imperviousness data set. For some of the land use types, the imperviousness has historically been over- or under-estimated, and for others, the current value has been estimated very well. The values for total imperviousness were updated based on the 2016 imperviousness analysis and consideration of the risks and consequences presented in the previous section. Recommended total imperviousness values for stormwater modeling associated with the 2017 CWRMP are listed in Table 5. Additionally, after discussion with City staff concerning the trends in residential development throughout the city, recommendations for updates to the directly connected imperviousness are also presented in Table 5. For most of the land use types, the recommended total imperviousness for the 2017 CWRMP is at or above the average imperviousness of the 2016 analysis. The two exceptions to this are the “Natural/Park/Open” and “Other” (essentially a railroad corridor) land use types. In both cases, these land use polygons tend to be small and narrow and the analysis was highly affected by the adjacent land use polygons which were often Industrial/Office or Commercial and were raising the average imperviousness. A closer look at the aerial imagery within the small and narrow land use polygons representing Natural/Park/Open and Other justifies using lower numbers for the total imperviousness. Table 5 Summary of imperviousness values and recommendation for impervious assumptions for the 2017 CWRMP update Land Use Type Total Area (acres) Imperviousness Value Assumptions (%) 2003/2011 CWRMPs 2016 Imperviousness Analysis Recommended for 2017 CWRMP Total Directly Connected Total (Range) Total (Average) Total Directly Connected Commercial 683 90% 80% 37% - 91% 78% 85% 80% Developed Park 315 not previously used 0% - 68% 19% 30% 20% Golf Course 602 5% 2% 0% - 40% 5% 5% 2% High Density Residential 272 70% 40% 39% - 73% 59% 65% 50% Highway 404 50% 50% 38% - 72% 54% 65% 65% Industrial/Office 456 90% 80% 52% - 83% 72% 75% 75% Institutional 312 40% 20% 17% - 75% 42% 60% 30% Institutional - High Imperviousness 52 70% 50% 56% - 85% 72% 80% 70% Low Density Residential 5,416 40% 20% 21% - 60% 32% 40% 25% Medium Density Residential 227 55% 30% 3% - 77% 43% 50% 40% Natural/Park/Open 446 2% 0% 0% - 35% 11% 2% 0% Open Water 396 100% 100% N/A 100% 100% 100% Other 38 not previously used 16% - 45% 32% 20% 20% Very Low Density Residential 266 12% 8% 13% - 29% 20% 25% 15% Wetland 309 100% 100% N/A 100% 100% 100% To: Jessica Wilson and Ross Bintner From: Cory Anderson, Sarah Stratton, and Janna Kieffer Subject: City of Edina Imperviousness Assumptions for Stormwater Modeling Date: October 25, 2016 Page: 22 P:\Mpls\23 MN\27\23271514 2017 CWRMP SW Modeling Updates\WorkFiles\Imperviousness Analysis\Imperviousness Analysis Summary.docx 8.0 References [1] Remote Sensing and Geospatial Analysis Laboratory, University of Minnesota, Marvin Bauer, "Twin Cities Metropolitan Area Land Cover Classification and Impervious Surface Area by Landsat Remote Sensing: 2011 Update," St. Paul, MN, 2011. [2] City of Edina, "Neighborhood Layer," Edina, MN, 2016. [3] City of Edina, "Land Use Data," 2000. [4] J. S. Gulliver, A. Ebrahimian and B. N. Wilson, "Determination of Effective Impervious Area in Urban Watersheds," Minnesota Department of Transportation, St. Paul, Minnesota, July, 2015. City of Edina Water Resources Management Plan – Appendix A Modeling Methods and Hydrologic and Hydraulic Analysis of Drainage Areas Attachment B Summary of Nine Mile Creek and Minnehaha Creek Modeling Approach Barr Engineering Co. 4300 MarketPointe Drive, Suite 200, Minneapolis, MN 55435 952.832.2600 www.barr.com Technical Memorandum To: Jessica Vanderwerff Wilson and Ross Bintner From: Cory Anderson and Sarah Stratton Subject: History of Nine Mile Creek and Minnehaha Creek Model Development and Incorporation of Tailwater Conditions into the Edina XP-SWMM models Date: September 22, 2017 Project: 23/27-1514.00 c: Janna Kieffer The City of Edina updated its Comprehensive Water Resources Management Plan (CWRMP) in 2017. As part of this update, 10 previously developed XP-SWMM models of the city were updated. The storm sewer data (City’s GIS layer provided in June 2016), soils information (2012 Natural Resources Conservation Service Soil Survey Geographic (SSURGO) data), imperviousness (reflecting 2011 University of Minnesota imperviousness raster data), and watershed divides and detention storage (2011 Minnesota DNR LiDAR) were all updated in the XP-SWMM models to reflect newer data. In addition to updating the data in the 10 XP-SWMM models that cover the city, the models were merged together so that there is now one model covering the areas of Edina draining to Nine Mile Creek, and one model covering the areas of Edina draining to Minnehaha Creek. The details of the model construction are described in the 2017 CWRMP. This memo provides some historical context for how Nine Mile Creek and Minnehaha Creek were previously modeled and also describes the methodology used to account for each creek within the City of Edina XP-SWMM modeling updated for the 2017 CWRMP. 1.0 Nine Mile Creek Methodology An XP-SWMM model of Nine Mile Creek was developed by Barr in 2005 for the Nine Mile Creek Watershed District to determine the 1%-annual-chance flood (100-yr flood) elevations along the creek. Due to the large size of the Nine Mile Creek watershed (~50 square miles) and the detail of the modeling to be done (e.g. 3065 subwatersheds), the study area was split into 15 different “city” models and one creek model. Watershed drainage divides (subwatersheds) were delineated using 2-foot topography data provided by each of the cities that the creek flows through, with the exception of the South Fork of Nine Mile Creek that passes through the City of Eden Prairie. The City of Eden Prairie did not have 2-foot topography data at the time the model was developed, therefore watersheds delineated for that portion of the model are less detailed. For the entire stretch of the creek itself, surveyed cross-sections were used to estimate the To: Jessica Vanderwerff Wilson and Ross Bintner From: Cory Anderson and Sarah Stratton Subject: History of Nine Mile Creek and Minnehaha Creek Model Development and Incorporation of Tailwater Conditions into the Edina XP-SWMM models Date: September 22, 2017 Page: 2 backwater effects of the bridges and culverts and to reflect variations in the stream valley topography. All of the cross-sections within the different reaches of Nine Mile Creek were surveyed within the channel banks and extended on the floodplain using 2-foot topography to define the changes in slopes for the large overbank areas. In the City of Eden Prairie where 2-foot topography data was unavailable, full cross- sections (stream channel and overbanks) for that stretch of the creek were surveyed. Road crossing information was obtained primarily through survey; plan sheets were reviewed as available. Loss coefficients for bridges, other channel obstructions, channel roughness, and overbank roughness (Manning’s “n”) were estimated by field inspection and photographs. Lakes and large wetland areas that have a consistent water surface elevation along Nine Mile Creek were modeled as storage areas in the XP- SWMM model. The peak flood flow that was predicted by the XP-SWMM model was compared to the expected peak flow using the National Flood Frequency Program (NFF) regional regression analysis on Nine Mile Creek as a QA/QC measure. Flows were compared on the north fork of Nine Mile Creek at Highway 169, Highway 62, and Interstate 494. The peak flows calculated for the three different locations using the regional regression were all within 11% of the peak hydraulically routed flow calculated using XP-SWMM. Since the standard error for the 1%-annual-chance recurrence interval using NFF is 54%, the flows calculated by XP-SWMM are reasonable. Two other precipitation events were used to calibrate and validate the model. The calibration event was in April 2004, and the amount of precipitation varied throughout the watershed. Therefore, average precipitation amounts were calculated by major watershed and were used in the appropriate models. Flows for the calibration event were compared to monitored flows at four different locations throughout the watershed—one location on the North Fork of the Creek, one location on the South Fork of the Creek, and two locations on the Lower Valley portion of the Creek (below the confluence of the North Fork and South Fork). The validation event was in June 2003 and the amount of precipitation was also varied throughout the watershed. Therefore, average precipitation amounts were calculated by major watershed and were used in the appropriate models. The water surface elevation for the validation event was also compared to a monitored water surface elevation at Normandale Lake (the confluence of the North Fork and South Fork of Nine Mile Creek). Simulating storm events using 15 different city models and one creek model required an iterative process to account for overflows and tailwater conditions between models. Each city model had numerous points of outflow into the creek. XP-SWMM creates a hydrograph at each outflow point and each outflow point from the city model was assigned an inflow point in the creek model. Since some of the inflow points had several outflow points attached to it (not a 1:1 ratio) it was necessary to sum the hydrographs created To: Jessica Vanderwerff Wilson and Ross Bintner From: Cory Anderson and Sarah Stratton Subject: History of Nine Mile Creek and Minnehaha Creek Model Development and Incorporation of Tailwater Conditions into the Edina XP-SWMM models Date: September 22, 2017 Page: 3 by XP-SWMM. An Excel macro was used to sum hydrographs so there would be one hydrograph for each inflow point into the creek. The hydrographs were input into the creek model as text files. In some of the city models, there was overflow out of the model into a different city model. These points were treated similar to the outflow into the creek. A hydrograph was created by XP-SWMM and then imported into the downstream city model. In cases where more than one overflow went into a single downstream system, the hydrographs were summed. For larger events (100-yr and 500-yr) it was necessary to consider the tailwater elevation in the creek. When the creek model was run XP-SWMM saved the elevation versus time data for each node that had inflow from a city model. These elevations were imported back into the city models at locations where there was outflow to the creek. This replaced the original downstream condition (free outfall) with the actual downstream elevation in the creek at that outlet. The city models then needed to be re-run to determine new hydrographs. The hydrographs were again summed where necessary and imported into the creek model. The creek model was re-run, and a new tail-water file was created. The process was repeated until the elevation in the creek stabilized. The model of Nine Mile Creek (including all of the associated city models) was updated by Barr in 2014 to reflect Atlas 14 precipitation depths. Faster computing times and newer versions of XP-SWMM have now made it more feasible to combine the city models and creek model into one model. Prior to combining the models (for this 2017 CWRMP update), the modeling connection between Nine Mile Creek and the XP-SWMM models of the city was time-consuming and required iterations between model runs to properly determine boundary conditions. Therefore, to remove the iteration process that was necessary, the nodes and links of the Nine Mile Creek model were merged directly into the city model. This created one complete model of the areas of Edina draining to Nine Mile Creek including the creek itself, extending from Hopkins northwest of Edina down through Bloomington southeast of Edina. However, the input data associated with the creek nodes and links was not updated with any new data. Because the creek nodes and links were included in the overall city model contributing to Nine Mile Creek, the modeled storm events could be routed through all of the runoff nodes in the model to determine the runoff hydrographs, and through the hydraulic layer nodes and links to route the storm events through both the creek and the city. The iteration process is no longer necessary to determine flood levels in the creek or in the nodes adjacent to the city that might be affected by creek tailwater levels. To: Jessica Vanderwerff Wilson and Ross Bintner From: Cory Anderson and Sarah Stratton Subject: History of Nine Mile Creek and Minnehaha Creek Model Development and Incorporation of Tailwater Conditions into the Edina XP-SWMM models Date: September 22, 2017 Page: 4 2.0 Minnehaha Creek Methodology The previous XP-SWMM models developed for the city of Edina did not account for tailwater conditions from Minnehaha Creek. Previously, any areas of Edina that drained to Minnehaha Creek through storm sewer assumed that those areas could freely drain to the creek. This assumption may reflect actual conditions for some areas along the creek and/or for some shorter duration storm events, but may not be true for other lower-lying areas adjacent to the creek or for longer duration storm events. The XP-SWMM models of Minnehaha Creek and its upstream contributing area were developed by EOR for the Minnehaha Creek Watershed District (MCWD) in 2003. The methodology used for developing these models is provided on MCWD’s website (Hydrologic, Hydraulic, and Pollutant Loading Study, 2003). For this 2017 CWRMP modeling effort, the Minnehaha Creek XP-SWMM models were provided by Wenck (MCWD’s engineer) in August 2016. Wenck provided two XP-SWMM models – one for the Upper Watershed of Minnehaha Creek (the upper watershed down to Lake Minnetonka at Gray’s Bay Dam) and one for the stretch of Minnehaha Creek below Lake Minnetonka at Gray’s Bay Dam). The Upper Watershed model determines the outflow from Lake Minnetonka to Minnehaha Creek. The models provided by Wenck used TP-40 1%-annual-chance precipitation event inputs. Barr then updated the two Minnehaha Creek models with the Atlas 14 1%-annual-chance precipitation event to be consistent with the Atlas 14 1%-annual-chance precipitation event depths and nested distribution used for the other Edina XP-SWMM models. The two Minnehaha Creek models provided by Wenck were also re-run for other events (Atlas 14 10%-annual-chance precipitation and 1%-annual-chance snowmelt). No other model changes were made. The separation of the models has been maintained (i.e., the Minnehaha Creek model provided by Wenck is not combined with the City of Edina model). Therefore, to account for Minnehaha Creek tailwater conditions, user-defined stage hydrographs were extracted from the Minnehaha Creek model for each modeled event and were included as downstream boundary conditions at all model node locations where Edina subwatersheds drain to Minnehaha Creek.