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Memorandum
To: Board of Managers, Nine Mile Creek Watershed District
From: Janna Kieffer
Subject: Lake Edina- Evaluation of Water Levels
Date: July 10, 2015
c: Kevin Bigalke, Nine Mile Creek Watershed District Administrator
Bob Obermeyer, Barr Engineering Co.
Background
Lake Edina is an approximately 23-acre lake located south of 70th Street and east of Trunk Highway 100
(T.H. 100) in the city of Edina. Lake Edina receives stormwater from the outflow from Lake Cornelia and
runoff from its direct watershed. Figure 1 shows the 1,365-acre drainage area to Lake Edina, including the
971-acre Lake Cornelia watershed and 394 acres of direct tributary area. The Lake Cornelia watershed is
primarily comprised of commercial (including the Southdale Shopping Center area), highway, and high
and low density residential land use. The Lake Edina direct watershed is primarily low density residential
land use. The normal elevation of Lake Edina is controlled by a weir structure at elevation 822 Mean Sea
Level (MSL). Discharge from Lake Edina flows through a 36-inch storm sewer system underneath T.H. 100
and into the North Fork of Nine Mile Creek.
In the past, the Southdale Shopping Center pumped groundwater through their heating and cooling
system. The continuous discharge from the system flowed to Lake Cornelia through the City's storm sewer
system, and ultimately to Lake Edina. Based on historic records, the Southdale cooling water totaled
approximately 30 to 40 million gallons of water per year, ranging from 1.0 million to 6.8 million gallons
per month. In 2011, the Minnesota Department of Natural Resources (MDNR) did not renew the
Southdale groundwater appropriation permit, which required Southdale to abandon the use of
groundwater in their heating and cooling system and eliminate the discharge of cooling water to Lake
Cornelia and downstream Lake Edina.
Spring 2015 Conditions
During the spring of 2015, water levels in Lake Edina were well below typical springtime lake levels. The
attached photos depict the low lake levels observed on April 17, 2015 by Barr staff. Continued low lake
levels throughout April and early-May prompted lakeshore residents to express concern to the City and
Nine Mile Creek Watershed District (NMCWD). In turn, Barr was asked to conduct a water balance analysis
for Lake Edina to evaluate the reason(s) for the low springtime water level conditions.
Barr Engineering Co. 4700 West 77th Street, Suite 200, Minneapolis, MN 55435 952.832.2600 www.barr.com
Date: July 10, 2015
Page: 2
Study Objectives
The objectives of the Lake Edina water balance were to gain a better understanding of the water level
fluctuations in the lake and determine if the low 2015 springtime water levels were a result of lower-than-
average spring snowmelt or other factors such as the elimination of the Southdale cooling water
discharge.
Methodology
A water balance spreadsheet model was developed that tracks daily inflows to and outflows from the lake.
Inflows of water to the lake included direct precipitation, watershed runoff from the area directly tributary
to Lake Edina, discharge from upstream Lake Cornelia, and Southdale cooling water (for years prior to
2011). Outflows from the lake included evaporation from the lake surface, discharge from the outlet
structure, and seepage losses. A P8 watershed loading model, originally developed as part of the City of
Edina Comprehensive Water Resources Management Plan and later adapted for the NMCWD Lake
Cornelia Use Attainability Analysis, was used to estimate watershed runoff and discharge from Lake
Cornelia. Hourly precipitation data from the Minneapolis/St. Paul International (MSP) Airport was used to
generate runoff. Discharge from the Lake Edina outlet structure was estimated using a rating curve
developed based on XP-SWMM modeling results that account for tailwater conditions from the North
Fork of Nine Mile Creek.
The water balance model was used to predict preliminary daily water levels based on estimated inflows
and outflows. Through comparison of preliminary water level predictions with observed lake levels, the
rate of seepage of water from the lake was estimated and the discharge rating curve was adjusted.
Figure 2 shows the predicted water levels in Lake Edina in comparison with the observed water levels for
the time period of April 2004 through June 2015. While the predicted water levels do not match up
exactly with observed water levels at times throughout the modeling period, the water balance model
provides a reasonable approximation of water levels throughout the past decade. Numerous factors can
result in differences between the predicted and observed water levels, including differences in
precipitation between Lake Edina and the MSP airport, timing of modeled versus actual snowmelt runoff,
variation in seepage rates as localized groundwater levels fluctuate, variation in discharge from upstream
Lake Cornelia, changes in the rate of Southdale Cooling water discharge, and impacts of Nine Mile Creek
water levels on surface discharge from Lake Edina.
Study Conclusions
Results of the Lake Edina water balance indicate that low water levels in the spring of 2015 were primarily
a result of the lower-than-average snowpack during the winter of 2014-2015, which yielded less snowmelt
runoff than typical years. Since mid-May, water levels in Lake Edina have rebounded, with a June 26, 2015
water level observation of elevation 821.9 MSL, just below the normal water level of the lake.
The impact of the discontinuation of the Southdale cooling water discharge on Lake Edina water levels
was also evaluated using the water balance model. Figure 3 shows the predicted water levels with
discontinuation of the Southdale cooling water discharge in 2011 in comparison with predicted water
levels if the Southdale discharge was continued through 2015. Model results indicate that continuation of
Date: July 10, 2015
Page: 3
Southdale cooling water discharge through 2015 results in periods where higher water levels are
maintained for longer durations (notably the fall of 2013 and 2014), but does not prevent the decline of
water levels that often occurs during the winter months.
Based on the lake level observations and water balance modeling, it appears that Lake Edina has a
seepage rate of approximately 10-12 inches per month. The North Fork of Nine Mile Creek is located
approximately 400 feet west of Lake Edina, with a creek bottom elevation of approximately 817 MSL near
the storm sewer outfall from Lake Edina. Given the proximity of the lake to the creek, the minimal
difference in water surface elevation, and the observed seepage, it is likely that localized groundwater
elevations and associated seepage rates from Lake Edina are highly influenced by the creek.
Study Recommendations
• Continue monthly monitoring of lake levels in Lake Edina
• Install a continuous lake level monitor in Lake Cornelia (South) prior to future water quality study
of Lake Edina to better track inflows from Lake Cornelia
• Conduct a field survey of the Lake Edina outlet to verify existing conditions and inspect for
potential debris that would modify the normal outlet elevation or restrict discharge
Feet
1,250
Figure 1
LAKE EDINA WATERSHED
Lake Edina Water Balance
Nine Mile Creek Watershed District
Edina, Minnesota
Legend
Major Watersheds
Tributary Drainage Area
to Lake Edina
Creek/Stream
Lake/Wetland
Trunk Storm Sewer
Surface Elevation
- High : 965.596
- Low : 812.027
; 4'4-L.4.17; ' P
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Source: Esri, DigitalGlobe, GeoEye—, ea:instal' Geographies, CNES/Airbus DS, USDA, USGS, AEX, Getrnapping, Aerogrid,
IGN, IGP, swisstopo, and the GIS User Community
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2.0
4.0
6.0
8.0
10.0
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14.0
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20.0
rp 111 11 T lrn yin„r7 r 1r11' r
Daily Precipitation
—Predicted Lake Levels
a Observed Lake Levels
— Precip (in)
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April-04 April-05 April-06 April-07 April-08 March-09 March-10 March-11 March-12 March-13 March-14 February-15
Figure 2. Comparison of Predicted
and Observed Water Levels
Lake Edina, Minnesota
La
ke
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(
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819
820
821
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—Predicted Lake Levels with Southdale Cooling Water Discharge Continued through 2015
i] Observed Lake Levels
—Predicted Lake Levels
April-04 April-05 April-06 April-07 April-08 March-09 March-10 March-11 March-12 March-13 March-14 February-15
Figure 3.
Comparison of Predicted Water Levels With and Without
Southdale Cooling Water Discharge Continued through 2015
Lake Edina, Minnesota
Photos
Lake Edina site visit- April 17, 2015 1
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Lake Edina site visit- April 17, 2015 2
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Lake Edina site visit- April 17, 2015 3
Lake Edina site visit- April 17, 2015 4