HomeMy WebLinkAboutCornelia_SummaryMemo_03172015_Final
Memorandum
To: Ross Bintner, City of Edina
From: Janna Kieffer
Subject: Lake Cornelia and Swimming Pool Pond Outlet Modification Review
Date: March 17, 2015
Project: 23270354.00 BCO 026
Per the City’s request, we have computed revised flood elevations for Lake Cornelia and adjacent water
bodies based on updated Atlas 14 precipitation estimates and re-evaluated historic water management
recommendations that affect flood levels in these waterbodies. Background information and a summary
of the results is provided below.
Existing Drainage Patterns
North Lake Cornelia
North Lake Cornelia has a large watershed encompassing approximately 860 acres, with several upstream
water bodies that receive stormwater runoff prior to reaching the lake (Figure 1). Land use within this
watershed is comprised of a large commercial area (including the Southdale Center), portions of T.H. 62
and T.H. 100, residential areas (high and low density), park, wetlands, and open water. Much of the runoff
from the highly impervious commercial area drains through the France Avenue and West 66th Street storm
sewer system and discharges to the Point of France pond (Subwatershed NC_4), located just northeast of
the West 66th Street and Valley View Road intersection. The Point of France pond drains to the Swimming
Pool Pond (Subwatershed NC_3) located west of Valley View Road and south of T.H. 62. Under normal
conditions, stormwater from the Swimming Pool Pond flows to North Lake Cornelia. During large storms
or times of high water levels, water from Swimming Pool Pond backs up into Garrison Pond (NC_30)
through the two 60-inch culverts located under T.H. 62. A secondary outlet is located on the north side of
Garrison Pond that conveys flows northward to Lake Pamela during high water conditions.
North Lake Cornelia spans approximately 29 acres. Based on storm sewer information provided by the
City, there are three systems that connect North Cornelia to South Cornelia beneath West 66th Street: two
15-inch corrugated metal pipe (CMP) systems and one 12-inch CMP system on the far east side of the
lake. The controlling invert elevations of the three CMP systems vary, but the lowest is elevation
857.9 MSL. The water level of North Lake Cornelia is generally controlled at 859.0 based on the weir outlet
structure at South Lake Cornelia. Water from North Cornelia overtops West 66th Street into South Lake
Date: March 17, 2015
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Cornelia at elevation 864.1 MSL, based on the Minnesota Department of Natural Resources (MnDNR)
2011 LiDAR elevation data.
South Lake Cornelia
The South Lake Cornelia watershed is a 112-acre residential watershed located south of the North Lake
Cornelia watershed. South Lake Cornelia spans approximately 32 acres. The normal elevation of the lake
is controlled by a weir structure at elevation 859 MSL. 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
large storm events, such as the 100-year frequency event, the 21-inch storm sewer system at Dunberry
Lane and Cornelia Drive restricts flow, resulting in flow northward through the 54-inch system and into
South Lake Cornelia.
Background
Several locations in the North and South Lake Cornelia watershed have experienced flooding problems in
the past. In particular, the 6600 France office building and a residential property near the outlet of South
Lake Cornelia have experienced damage. In the early 1990’s, the City asked Barr to evaluate drainage
within the watershed and recommend improvements to reduce the potential for flooding of these and
other vulnerable areas within the watershed. The outcome of this study was several recommended
improvements, some of which have been implemented including installation of an additional 66-inch
diameter pipe between the Point of France pond and Swimming Pool pond, redirection of the storm
sewer system at West 66th Street and Valley View Road directly to North Lake Cornelia (versus to
Swimming Pool pond), additional storm sewer capacity on Valley View Road between West 66th Street and
West 68th Street, and construction of a berm along the west edge of Valley View Road from West 68th
Street south to West 69th Street to prevent stormwater from running overland from Valley View to the
backyards of the residential lots in this area. The recommended improvements that haven’t been
implemented yet include:
1) Raising West 66th Street to eliminate overtopping of the roadway from North Cornelia to South
Cornelia in large storm events,
2) Replacing the existing 18-inch outlet from Swimming Pool Pond to North Lake Cornelia with a
42-inch equivalent reinforced concrete arch pipe (RCAP) and 20-foot control weir to increase
discharge from the Swimming Pool Pond to North Lake Cornelia, and
3) Lowering the surface overflow between Swimming Pool Pond and North Lake Cornelia to
elevation 863.5 MSL (current surface overflow is via the parking lot at elevation 865.5 MSL based
on MnDNR 2011 LiDAR elevation data).
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Historic project documentation indicates the following critical elevations within the Lake Cornelia
watershed:
• 6600 France Avenue office building (NC_113)- parking garage door sill elevation of 867.1 MSL
• 6900 Cornelia Drive (SC_1- South Lake Cornelia)- basement floor elevation of 861.3 MSL
• Garrison Pond (NC_30)- low home at elevation 866.3 MSL
A recent survey to verify these elevations or identify additional low structures adjacent to the water bodies
and low areas within the watershed was not completed as part of this scope of work, but should be
conducted prior to design and implementation of improvements.
In addition to the historic critical elevations identified above, flooding problems have historically occurred
at the Southdale Center, with the low areas of concern being the area on the west side of the shopping
center near Macy’s and the movie theater (subwatershed NC_101) and the area on the east side of the
shopping center near JCPenney’s (subwatershed NC_106). Historic project documentation indicates a
critical elevation of approximately 877 MSL. Stormwater from the west side (NC_101) is collected by the
France Avenue trunk storm sewer system, which flows northward to Point of France Pond (NC_4).
Stormwater from the east side (NC_106) is collected by the 66th Street trunk storm sewer, which also
discharges to the Point of France Pond. A detailed drainage model of the Southdale Center property has
not been developed, as this is considered private property. However, consideration should be given to
avoid unnecessary increases in the flood elevations of Point of France Pond (NC_4), as this may result in
increased flood elevations at the Southdale Center.
Modeling Methodology
The XP-SWMM model originally developed in 2002-2003 for the City’s Comprehensive Water Resources
Management Plan (CWRMP) was used as a base model for this analysis. The 100-year frequency, 24-hour
rainfall event of 7.5 inches was simulated, based on NOAA’s Atlas 14 precipitation estimates for the City of
Edina. The storage information (area/elevation relationships) for the water bodies and several low areas in
the Lake Cornelia watershed was updated to reflect MnDNR 2011 LiDAR elevation data. Surface overflows
between waterbodies and in the subwatersheds near Lake Cornelia were also reviewed and revised where
the overflow elevations from the MnDNR 2011 LiDAR data varied significantly from the City’s two-foot
topographic data (used in the original model development).
Modeled Improvement Scenarios
The three historic improvement recommendations identified above, and combinations thereof, were
evaluated to assess the change in flood levels of various water bodies or locations within the watershed.
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The following scenarios were evaluated:
A. Existing Conditions
B. Lower parking lot between Swimming Pool Pond and North Lake Cornelia to elevation
863.5 MSL.
C. Increase capacity of the Swimming Pool Pond outlet (20-foot weir and 42-inch equivalent RCAP)
D. Raise West 66th Street to eliminate road overtopping from North Lake Cornelia to South Lake
Cornelia
E. Lower the parking lot between Swimming Pool Pond and North Lake Cornelia AND increase
capacity of the Swimming Pool Pond outlet (Scenarios B and C)
F. Lower the parking lot between Swimming Pool Pond and North Lake Cornelia AND raise West
66th Street to eliminate road overtopping from North Lake Cornelia to South Lake Cornelia
(Scenarios B and D)
G. Increase capacity of the Swimming Pool Pond outlet AND raise West 66th Street to eliminate
roadway overtopping (Scenarios C and D)
H. Increase capacity of the Swimming Pool Pond outlet, raise 66th Street to eliminate roadway
overtopping, and lower the parking lot between Swimming Pool Pond and North Lake Cornelia
(Scenarios B, C, and D)
Results
Table 1 compares the Atlas 14 100-year frequency, 24-hour flood elevations for a number of waterbodies
and subwatersheds within the Lake Cornelia watershed, based on existing conditions and the
improvement scenarios described above. Under existing conditions, the Atlas 14 100-year flood elevations
exceed the historic critical elevations in subwatersheds SC_1 (South Lake Cornelia), NC_113 (6600 France
Avenue building) and NC_30 (Garrison Pond).
The Atlas 14 100-year, 24-hour flood elevation in South Lake Cornelia (864.3 feet MSL) exceeds the
historic critical low home elevation at 6900 Cornelia Drive (861.3 feet MSL) by three feet. Raising West
66th Street to prevent overtopping from North Lake Cornelia to South Cornelia results in a significant
decrease in flood levels in South Lake Cornelia, with the 100-year flood elevation in South Lake Cornelia
decreased to 862.1 MSL. While this flood elevation is still above the historic critical elevation, it is a
significant improvement from predicted flood levels under existing conditions. Currently, the surface
overflow between North and South Lake Cornelia is at an elevation of approximately 864.1 MSL based on
MnDNR 2011 LiDAR. The results in Table 1 assume that the roadway would be raised by approximately
two feet, to an elevation higher than the North Lake Cornelia 100-year flood elevation.
The 6600 France Avenue office building in the NC_113 subwatershed has historically had problems with
flooding. Under existing conditions and each of the modeled improvement scenarios, the Atlas 14
100-year, 24-hour flood elevation in subwatershed NC_113 is 868.7 MSL, which exceeds the low garage
entry elevation (867.1 MSL). To avoid flooding at the 6600 France Avenue office building, the storm sewer
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system that serves the 6600 France Avenue site should prevent water from backing up the drain in the
garage floor and into the building. Such an improvement was recommended following the stormwater
analysis in the late 1990’s and should be verified to ensure that it has been implemented.
Under existing conditions, the flood elevation in Garrison Pond (subwatershed NC_30) is 866.4 feet MSL,
which slightly exceeds the historic critical elevation (866.3 feet MSL). Lowering the parking lot between
the Swimming Pool Pond and North Lake Cornelia to provide a surface overflow elevation of 863.5 MSL
results in the most significant reduction in flood elevation in Garrison Pond (865.1 MSL). When lowering
the parking lot is combined with raising West 66th Street (Scenario F), the reduction to Garrison Pond
flood elevations is lessened, but the flood elevation (865.9 MSL) is still below the historic critical elevation.
The results presented in Table 1 assume the surface overflow between Swimming Pool Pond and North
Lake Cornelia is a trapezoidal channel with a 40-foot bottom width and 80:1 side slopes, based on the
MnDNR LiDAR elevation data. Under the lowered parking lot scenarios, the peak surface overflow from
Swimming Pool Pond to North Lake Cornelia is approximately 600 cubic feet per second (cfs).
Recommendations
As part of this analysis, several historic improvement recommendations were evaluated with a goal of
reducing flood elevations in several locations throughout the Lake Cornelia watershed, specifically South
Lake Cornelia, the 6600 France Avenue building (NC_113), Garrison Pond, and the Southdale Center
(controlled by Point of France Pond). Our recommendations are identified below:
• This analysis was based on critical low structure elevations identified in historic project
documentation. A recent survey to verify these elevations or identify additional low structures
around or near the water bodies and low areas within the watershed was not completed as part of
this scope of work, but should be conducted prior to design and implementation of
improvements.
• To lower the flood elevation of South Lake Cornelia, the portion of West 66th Street between
North and South Lake Cornelia should be raised to hold back additional water in North Lake
Cornelia and prevent overtopping of the roadway. The roadway should be raised by
approximately two feet at the current low points along West 66th Street to an elevation higher
than the North Lake Cornelia 100-year flood elevation.
• While raising West 66th Street between North and South Lake Cornelia lowers the flood elevation
of South Lake Cornelia, the 100-year flood elevation is still above the historic critical elevation at
6900 Cornelia Drive. Development of a flood-proofing plan for this property and other low
structures adjacent to the lake should be considered.
• To lower the flood elevation of Garrison Pond and Point of France Pond, the surface overflow
between the Swimming Pool Pond (NC_3) and North Lake Cornelia should be lowered. Currently,
the surface overflow is located in the parking lot adjacent to the play area at an approximate
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elevation of 865.5 MSL. Lowering the surface overflow to elevation 863.5 will allow floodwater to
flow to North Lake Cornelia at a lower elevation, resulting in decreased flood elevations in
upstream Garrison Pond (NC_30), Point of France Pond (NC_4) and the Southdale Center.
• The timing of implementing the recommendations to raise West 66th Street and lower the surface
overflow between Swimming Pool Pond (NC_3) and North Lake Cornelia (NC_62) should be
carefully considered. The raising of West 66th Street should be completed prior to lowering the
surface overflow in the parking lot between the Swimming Pool Pond and North Lake Cornelia to
avoid increases in the flood elevation of South Lake Cornelia. Model results indicate that if the
surface overflow between Swimming Pool Pond and North Lake Cornelia is lowered without
raising West 66th Street, the 100-year flood elevation in South Lake Cornelia will increase by
0.3 feet, as compared to the flood elevation under existing conditions.
• Historic improvement recommendations included increasing the flow capacity of the Swimming
Pool Pond outlet by replacing the existing outlet structure and pipe between Swimming Pool
Pond and North Lake Cornelia. Results of our analysis indicate that the incremental benefit of this
individual improvement is minimal when considering the 100-year frequency return period, with
flood elevations in Garrison Pond and Point of France Pond decreasing by only 0.1-0.2 feet.
Lowering the surface overflow by re-grading the parking lot between the Swimming Pool Pond
and North Lake Cornelia has a more significant benefit to flood elevations. Replacing the outlet
from the Swimming Pool Pond is not recommended at this time, unless lowering the surface
overflow in the parking lot between the Swimming Pool Pond and North Lake Cornelia is
determined to not be feasible. If the surface overflow is not lowered, increasing the flow capacity
of the Swimming Pool Pond outlet would lessen the frequency of flooding in the Edina Aquatic
Center parking lot adjacent to the Swimming Pool Pond.
• Results indicate continued potential for flooding at the 6600 France Avenue office building. To
reduce the potential of flooding at the 6600 France Avenue office building, the storm sewer
system that serves the 6600 France Avenue site should prevent water from backing up the drain
in the garage floor and into the building. Such an improvement was recommended following the
stormwater analysis in the late 1990’s and should be verified to ensure that it has been
implemented.
Southdale Center
North
Lake Cornelia
(NC_62)
NC_106
Garrison Pond
(NC_30)
Swimming
Pool Pond
(NC_3)Point of France
Pond
(NC_4)
NC_101
NC_2
NC_113
South
Lake Cornelia
(SC_1)
Source: Esri, DigitalGlobe, GeoEye, i-cubed, USDA, USGS, AEX,
Getmapping, Aerogrid, IGN, IGP, swisstopo, and the GIS User Community
!;NBarr Footer: Date: 2/17/2011 4:02:38 PM File: C:\DOCUME~1\abc\LOCALS~1\Temp\1.mxd User:400 0 400
Feet
Figure 1
Lake Cornelia and Swimming Pool
Pond Outlet Modification Review
City of Edina
Edina, MN
Legend
Storm Sewer
Subwatersheds
Table 1. Summary of Atlas 14 100-year, 24-hour Flood Elevations (feet MSL)NC_62 (North Lake Cornelia)NC_3 (Swim Pool Pond)NC_4 (Point of France Pond) NC_2Critical Target Elevation1AExisting conditions864.9 864.3(+3.0')866.2 867.1 868.7(+1.6')866.4 866.4(+0.1')BLower parking lot to 863.5 MSL865.0 864.6(+3.3')865.1 865.8 868.7(+1.6')866.1 865.1(-1.2')CIncrease capacity of Swimming Pool Pond outlet864.9 864.5(+3.2')866.1 866.9 868.7(+1.6')866.3 866.3 (0.0')DRaise West 66th Street to eliminate road overtopping from North Lake Cornelia to South Lake Cornelia865.9 862.1(+0.8')866.2 867.1 868.7(+1.6')866.4 866.4(+0.1')ELower the parking lot between Swimming Pool Pond and North Lake Cornelia AND increase capacity of the Swimming Pool Pond outlet (Scenarios B and C)865.0 864.6(+3.3')865.1 865.7 868.7(+1.6')866.1 865.1(-1.2')FLower the parking lot between Swimming Pool Pond and North Lake Cornelia AND raise West 66th Street to eliminate road overtopping from North Lake Cornelia to South Lake Cornelia (Scenarios B and D)865.9 862.2(+0.9')865.9 865.9 868.7(+1.6')866.1 865.9(-0.4')GIncrease capacity of the Swimming Pool Pond outlet AND raise West 66th Street to eliminate roadway overtopping (Scenarios C and D)865.9 862.1(+0.8')866.1 866.9 868.7(+1.6')866.3 866.3 (0.0')HIncrease capacity of the Swimming Pool Pond outlet, raise 66th Street to eliminate roadway overtopping, and lower the parking lot between Swimming Pool Pond and North Lake Cornelia (Scenarios B, C, and D)865.9 862.2(+0.9')865.9 865.9 868.7(+1.6')866.1 865.9(-0.4')1 Critical target elevations based on field surveys conducted as part of past drainage analyses (early-1990s)2 Values in parenthesis indicate the difference, in feet, between the critical target elevation and the modeled flood elevation .ScenarioSC_12 (South Lake Cornelia)NC_1132 (6600 France Building)NC_302 (Garrison Pond)861.3867.1 866.3P:\Mpls\23 MN\27\2327354\WorkFiles\2014\Cornelia Swim Pool Pond Analysis\Results_Summary_03172015.xlsx3/17/2015