MINNEHAHA CREEK

MCWD H/H and Pollutant Loading Study – 2003 L-1 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Minnehaha Creek Table of Contents

L. Minnehaha Creek...... 3 L.1. General Description...... 3 L.2. Physical Features...... 6 L.2.a. Land Cover/Land Use...... 6 L.2.b. Geology ...... 9 L.2.c. Soils ...... 10 L.2.d. Groundwater...... 10 L.3. Water Quantity ...... 17 L.3.a. Watershed Hydrology...... 17 L.3.b. Watershed Hydraulics ...... 28 L.3.c. Water Quantity Findings and Discussion ...... 30 L.3.d. Watershed Recommendations ...... 56 L.3.e. Watershed References ...... 56 L.4. Scour and Erosion-Prone Areas...... 59 L.4.a. Streams ...... 59 L.4.b. Lakeshore ...... 67 L.5. Water Quality ...... 68 L.5.a. Watershed Pollutant Load Analysis...... 68 L.5.b. Lake Modeling and Associated Goals...... 72 L.5.c. MPCA Impaired Waters and Point Source Permits...... 80 L.6. Recommendations ...... 84

MCWD H/H and Pollutant Loading Study – 2003 L-2 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

L. Minnehaha Creek

L.1. General Description

This report segment addresses the portion of the Minnehaha Creek watershed that is located downstream of Grays Bay dam (Figure IV.L.1-1). This area includes Minnehaha Creek (about 22 miles to the Mississippi). It is also referred to as the “lower watershed.” Also discussed in this report section are several subwatersheds that are located within the political boundaries of the MCWD, yet are non-contributing to Minnehaha Creek itself. These subwatersheds are indicated on Figure IV.L.1-1, and the following table lists the all of the subwatersheds that are discussed in this section, along with their acreages: Table IV.L.1-1 List of “Lower Watershed” Subwatersheds and Acreages Watershed Subwatersheds Acres Contributing: Minnehaha Creek MC-1 through MC-184 30,290 Non-Contributing: Wood Lake (Grass Lake) WL-1 through WL-3 1135 Powderhorn Lake MR-3 332 Mississippi River Direct MR-1 through MR-2 950

Figure IV.L.1-2 shows the subwatersheds and their drainage configuration.

MCWD H/H and Pollutant Loading Study – 2003 L-3 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Plymouth g%55 .35 Wayzata RT 3 ,- RT 1 RT 2 Golden Valley 394 ,.- ,.-394 ,.-94 Saint Cedar Lake Lake Louis of the Isles Gray's Park Bay Outlet Lake ,-.35 Calhoun g%55 Minneapolis

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N Figure IV.L.1-1 Minnehaha Creek Political Boundaries H&H Report/Basins/Projects/flow_030331_kl

MCWD H/H and Pollutant Loading Study – 2003 L-4 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion 1 < 3

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N Figure IV.L.1-2 Minnehaha Creek Flow Direction H&H Report/Basins/Projects/flow_030417_kl

MCWD H/H and Pollutant Loading Study – 2003 L-5 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion L.2. Physical Features

The following sections detail the MLCCS, geology, soils, and groundwater of the Minnehaha Creek watershed.

L.2.a. Land Cover/Land Use

For comparison purposes, the various MLCCS land cover classifications have been combined into five impervious surface area categories and six vegetative cover type categories (Figures IV.L.2-1 and IV.L.2-2). Although not shown here, each of the impervious surface area categories was further broken down with respect to type of land use and vegetative cover found on non-impervious surface areas. A more detailed map showing MLCCS cover types to Level 3 for the entire MCWD is included in Appendix 3 (Figure IV.Appendix.3-1). A description of all MLCCS cover types is also included in Appendix 3 and is incorporated into the District's interactive GIS tool.

Land use is dominated by single family residential. Blocks of parks and recreational areas are scattered throughout the watershed, in addition to areas with a high concentration of commercial and industrial land uses. Percent imperviousness is, on the average, higher in the eastern portion of the watershed.

Currently in the Minnehaha Creek watershed, the “26% to 50% impervious cover” category of land use dominates the landscape (Figure IV.L.2-2), making up 50% of the landscape (Figure IV.L.2-2). Under 2020 land use conditions, “26% to 50% impervious cover” remains the most dominant category, with few changes predicted in the majority of the categories. The biggest percent increase (Table IV.L.2-1) occurs in the “0% to 10% impervious cover” category. The biggest percent decrease was found in the forests and woodlands category.

MCWD H/H and Pollutant Loading Study – 2003 L-6 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion MCWD H/H and Pollutant Loading Study – 2003 L-7 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Figure IV.L.2-2 Minnehaha Creek Land Cover

16000 50% Existing Conditions 14000 2020 Conditions

12000 40%

10000 30% 8000

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er er er s r d s s s s s v v v u ve n d d d ea d o o o io o a n n an r n c c c v c L la la l A la s s r l d s et l et us u u e s ra o as a o o o p ou u o r W r W i i i m i lt r u v v v i v u W G e at r er er r c t pe p p % pe i & a N 5 gr s W d im im im 7 im st e to A re en in % % % % o p ta 0 5 0 % 0 F n 1 2 5 1 0 O i o o o 5 1 a t t t o & t s M % % % ke 0 11 26 % a 76 L Land Cover Category

MCWD H/H and Pollutant Loading Study – 2003 L-8 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

Table IV.L.2-1 Minnehaha Creek Watershed Land Cover Percent Change Percent Change Land Cover Category (from existing to 2020 conditions) 0% to 10% impervious cover 44% 11% to 25% impervious cover 0% 26% to 50% impervious cover 0% 51% to 75% impervious cover 0% 76% to 100% impervious cover 0% Agricultural Land 0% Forests & Woodlands -15% Grasslands 0% Lakes & Open Water Wetlands 0% Maintained Natural Areas 0% Wetlands 0%

L.2.b. Geology

In contrast to other subwatersheds in the west part of MCWD, the Minnehaha Creek watershed has the full stratigraphic sequence of bedrock units found in the Twin Cities Basin. The Platteville-Glenwood Limestone is the prominent uppermost bedrock unit beneath the Minnehaha Creek watershed. The Platteville-Glenwood Limestone is the youngest bedrock unit substantially represented within the entire watershed district. There is a bedrock valley trending north-south beneath the region of the district known as the chain of lakes, which includes Cedar, Calhoun, Harriet, and Lake of the Isles. The bedrock valley is cut through the St. Peter Sandstone to the Prairie du Chien Limestone.

The Quaternary deposits are associated primarily with Des Moines Lobe glaciation but also till from the Superior Lobe glaciation. Superior Lobe deposits are sandy to silty loam and only exist in east central Minnetonka. Des Moines Lobe Deposits are composed primarily of outwash plain

MCWD H/H and Pollutant Loading Study – 2003 L-9 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion from Lake Minnetonka eastward. Outwash plain deposits were deposited as Minnehaha Creek drained the glacial lake occupying the area of present-day Lake Minnetonka. The creek changed its course through time and fanned outwash deposits over a wide area. Outwash plain deposits are sand and gravelly sand. In the east, a substantial portion of the land is river terrace deposits from the glacial rivers. River terrace deposits in this region are composed of sand, gravelly sand, and loamy sand.

(See Volume II: Framework and Methodology, D. Groundwater for a description of methodology for sections L.2.b through L.2.d.)

L.2.c. Soils Soil Hydrologic Groups are shown on Figure IV.L.2-3. The predominant hydrologic group is B type (moderate infiltration rate when wet). Group B soils have moderately fine to moderately coarse texture. Much of the area is classified only as “urban” in the Hennepin County Soil Survey, meaning that urban development had disturbed the soils and prevented field identification or verification. For purposes of this investigation, Group B type soils were assigned to soils in the urban areas. Criteria other than soil hydrologic group were used when determining infiltration potential and runoff characteristics in urban areas. The soils figure should be used as a guide for further soil examination at the site of inquiry.

L.2.d. Groundwater Water table elevation contours are shown on Figure IV.L.2-4. Shallow groundwater flow is away from Lake Minnetonka toward the Mississippi River. Groundwater flow roughly follows the southeast flow of Minnehaha Creek.

Typically, groundwater elevation contours show a characteristic “V” pattern pointing upstream in the area of creeks and rivers where groundwater is discharging. This pattern is not observed along Minnehaha Creek, and in fact the “V” points downstream in many areas. This leads to the conclusion that Minnehaha Creek is losing, not gaining water to the shallow water table aquifer in many areas. In some areas this conclusion can be readily explained:

MCWD H/H and Pollutant Loading Study – 2003 L-10 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion • In Edina, creek levels have been artificially raised above the natural water table elevation by the Browndale Dam and the spillway in Arden Park. • Close to Minnehaha Falls, groundwater table levels decrease rapidly as groundwater approaches the Mississippi River Valley and the many springs in the area. Several observers have noted that during periods of low water, parts of Minnehaha Creek will continue to flow (although at very low volume) while other parts may be completely dry. This is consistent with the idea that the Creek looses or gains water along different reaches.

The Minneapolis chain of lakes (Brownie, Cedar, Isles, Calhoun, and Harriet) lie along a bedrock valley filled with relatively coarse, unconsolidated glacial sediment. A study conducted from 1971 to 1973 concluded that all of the chain of lakes except Harriet had an ongoing net loss of water to groundwater (Shapiro, J. and H. Phannkuch, 1973. The Minneapolis Chain of Lakes, A Study of Urban Drainage and Its Effects, 1971-1973. Interim Report No. 9, Limnological Research Center, University of Minnesota). Some of the lost water entered other lakes or Minnehaha Creek. No information was available about whether this condition continued beyond the study period.

Water levels in excavations along the shore of Lake Nokomis are regularly lower than the elevation of the lake (HDR Engineering, 2002. Personal communication). This suggests that parts of Lake Nokomis are not well connected (hydraulically) to groundwater.

North-south trending bedrock valleys filled with very coarse unconsolidated material are found in the western part of the Minneapolis-St. Paul International Airport. MCWD and others expressed concern that large-scale construction dewatering in these areas could affect groundwater and surface water levels in the area of Nokomis, Taft, and Mother Lake. A network of monitoring wells and staff gauges was established to record water levels. From July 2001 to April 2003, groundwater levels fluctuated in areas close to the dewatering, but surface water levels remained at or above normal elevations. This was at least partly due to the unusually high precipitation during that time period. Further information about the construction, dewatering, and monitoring is available from the Metropolitan Airport Commission at www.mspairport.com/MSP/Airport_Expansion/Tunnel_Construction/Monitoring/.

MCWD H/H and Pollutant Loading Study – 2003 L-11 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

In the extreme southeast part of the watershed is Coldwater Spring. The Spring has cultural significance to Native Americans and historic significance to the State of Minnesota. Concerns were raised that road construction associated with the Highway 55/62 interchange would alter local groundwater flow patterns and affect flow toward the Spring. Shallow groundwater flow was found to be directed by joints and cracks in the Platteville Limestone bedrock. Geophysical testing, dye trace testing, and monitoring wells were used to determine the rate and direction of groundwater flow throughout the area impacted by construction. Contact MCWD for more information about the testing and results.

Figure IV.L.2-5 shows depth to the water table. The water table is shallow beneath much of the land surrounding Lake Minnetonka due to the presence of the lake and adjacent low areas where the topography doesn’t change much. The water table is shallow along Minnehaha Creek and its low marshy areas, as well as the southeast parts of Minneapolis where there is not much change in topography. The water table is deeper near the Mississippi River where springs discharge along the bluffs and lower the water table.

The combination of type B soils (moderately good infiltration potential) along with glacial outwash and river terrace deposits in the urban areas of Minnehaha Creek watershed create a high infiltration potential as seen in Figure IV.L.2-6. There are a number of variable infiltration potential zones near wet and low areas due to organic material being present.

Further geology and groundwater discussion can be found in Volume V: Watershed Issues Integration.

MCWD H/H and Pollutant Loading Study – 2003 L-12 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion MCWD H/H and Pollutant Loading Study – 2003 L-13 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion MCWD H/H and Pollutant Loading Study – 2003 L-14 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion MCWD H/H and Pollutant Loading Study – 2003 L-15 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion MCWD H/H and Pollutant Loading Study – 2003 L-16 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion L.3. Water Quantity

L.3.a. Watershed Hydrology

A description of the watershed morphology, drainage, land use, land cover, and soils exists under Minnehaha Creek and Non-Contributing Area watershed sections L.1. and L.2. For labeling of modeled features and XP-SWMM diagram, refer to the Minnehaha Creek and Non-Contributing Watershed Figures IV.Appendix.1-L1 through L4 in the Appendix.

Input Parameters:

The hydrology of the Minnehaha Creek watershed is influenced primarily by urban residential and commercial development. The majority of the area the creek passes through is completely developed and many wetlands that may have existed have long since been drained and filled. Nearly all discharge in the watershed is carried to Minnehaha Creek through city stormsewer. Exceptions to typical urban stormsewer exist in the upper portion of Minnehaha Creek mostly in the City of Minnetonka. The City of Minnetonka is characterized by large wetland complexes and multiple smaller landlocked wetlands.

Hydrologic input parameters include: area, slope, width, percent impervious, depression storage, hydraulic conductivity, capillary suction, and initial soil moisture deficit. The methodology used to generate these parameters is described under the model methodology section (II.F.1). The input parameter values for Minnehaha Creek watershed are shown on Table IV.Appendix.1-L1.

Subwatershed Boundaries:

The Minnehaha Creek watershed was subdivided into a total of 184 subwatersheds with an average subwatershed size of approximately 166 acres. The non contributing areas were divided into 5 subwatersheds for which the average subwatershed size is approximately 458 acres. The largest subwatershed is MC-170 with a total area of about 893 acres. This subwatershed is entirely residential and is drained by an extensive stormsewer system that ultimately discharges

MCWD H/H and Pollutant Loading Study – 2003 K-17 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

into Lake Hiawatha through two 5 x 6 foot box culverts. Larger subwatersheds which contain the creek itself generally represent the lower gradient portions such as the large wetland (DNR ID 27-761W), located in subwatershed MC-10, at the headwaters or sections of Minnehaha Creek running through lake. Lakes and wetlands that Minnehaha Creek run through include: • Unnamed Wetland 27-761W (MC-10 and MC-16) • Meadowbrook Lake (MC-75) • Mill Pond (MC-81) • Lake Hiawatha (MC-172)

Other significant subwatersheds that are tributary to Minnehaha Creek include subwatersheds containing metro lakes: • Bass Lake (MC-115) in St. Louis Park • Chain of Lakes:

- Brownie (MC-108)

- Cedar Lake (MC-109)

- Lake of the Isles (MC-121)

- Lake Calhoun (MC-126)

- Lake Harriet (MC-130) • Diamond Lake (MC-144) • Mother Lake (MC-159) • Taft Lake (MC-160) • Legion Lake (MC-158) • Lake Nokomis (MC-167)

The total area draining to Minnehaha Creek downstream of Lake Minnetonka is approximately 30,472 acres (47.6 sq. miles). The total area of non-contributing subwatersheds located in the MCWD’s jurisdiction is about 2,291 acres (3.6 sq. miles).

The subwatershed boundaries drawn were aided by the use of 2 foot topography and stormsewer from the Cities of: Minnetonka, Hopkins, St. Louis Park, Edina, Minneapolis and Richfield (stormsewer only). New MCWD topography (10 foot resolution, generated with 5 foot

MCWD H/H and Pollutant Loading Study – 2003 K-18 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

interpolated intervals) and USGS 10 foot topography were also referenced. Where applicable and reasonable, subwatershed boundaries were matched with those already in use by others (i.e. city subwatershed boundaries). The greatest boundary challenges emerged in areas with incomplete or conflicting stormsewer data. This problem frequently arose near jurisdictional boundaries between cities and parks where data was not continuous.

Boundary considerations: • MC-64 and 65: Stormsewer information available was incomplete and difficult to decipher at the border of Hopkins and St. Louis Park due to the lack of continuity in data across jurisdictional boundaries. Additional stormsewer data was collected from both Hopkins and St. Louis Park engineering offices to clarify the drainage near Powell Street. • MC-75 (Interlachen Golf Course): Stormsewer from the golf course was unavailable. Partial stormsewer data from Hopkins and Edina were used to determine drainage boundaries. It was assumed that all golf course ponds were ultimately drained to Meadowbrook Lake. • MC-79: A small portion of this subwatershed (west of T.H. 100) may drain under T.H. 100 to Lake Harvey. Lake Harvey is located in the upper reaches of subwatershed MC- 89 and not modeled explicitly. Additional investigation suggested only if particular interest in Lake Harvey arises. • MC-90: Pamela Lake drainage area. An earlier EOR survey performed on August 7, 2002 surveyed water surface elevations, structure elevations and gathered structure details for the pipes and control structures connecting the ponds located generally between Pamela Lake, Lake Cornelia of the Nine Mile Creek Watershed District and the Southdale Mall commercial area. The area is bisected by the Crosstown T.H. 62. Ponds surveyed included: Point of France Pond, Swimming Pool Pond (SPP), Lake Cornelia, Garrison Pond S., Garrison Pond N., and Miller Pond. Based on the survey, it appears that the normal operation of the system was such that stormwater received by the Point of France Pond, SPP, and the Garrison Pond (North and South) were all equalized and typically flowed north into Pamela Lake. Flow into Pamela Lake from Garrison Pond is controlled by a two stage weir located near the Garrison Lane cul-de-sac. The weir crest

MCWD H/H and Pollutant Loading Study – 2003 K-19 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

of the lowest notch of this structure is about 0.9 feet lower than the weir crest of the structure controlling flows between the SPP and Lake Cornelia.

From the information gathered thus far, it can be concluded that all areas draining to the equalized system of ponds (Point of France Pond, SPP, and Garrison Pond) drain first into Pamela Lake and Minnehaha Creek. Only under high water conditions (water levels at or above 863.3 feet) or large storm events (exceeding a 10-year event) would discharge from the equalized system flow both into Pamela Lake and Lake Cornelia.

An issue to investigate in the future is to determine how much area drains to the equalized system of ponds (Point of France Pond, SSP, and Garrison Pond). A partial review of Edina’s local stormsewer system appears to indicate that Southdale Mall, a portion of Highway 62, and some of the commercial area surrounding the Southdale Mall drain to the equalized pond system. Based on this information, it appears that areas currently outside of the MCWD jurisdictional boundary drain, at least partially, into Minnehaha Creek.

During a recent field inspection (June 2003) it was found that the two 60 inch CMP’s which equalized the Point of France Pond and the SSP (both on the south side of Hwy 62) with the Garrison Pond (on north side of Hwy 62) were not operational due to the Crosstown (Hwy 62) construction. It was unknown at that time whether that was a temporary block for construction purposes or if it was a designed (permanent) flow pattern change. If the equalizer pipes are permanently removed, that will change the current flow pattern from Southdale to Minnehaha Creek. It is recommended that Mn/DOT be contacted regarding the final design of the equalizer culverts.

• MC-147: Stormsewer information available indicates a possible split near Columbus Ave S and E 41st Street which potentially transfers the uppermost portion of subwatershed to the adjacent subwatershed MC-170. The boundary was defined to be consistent with the existing Minneapolis subwatershed boundaries. It was assumed that

MCWD H/H and Pollutant Loading Study – 2003 K-20 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

the existing Minneapolis subwatershed boundaries reflected the most current and up-to- date stormsewer data available.

Subwatershed growth:

The subwatershed area and percent impervious for the Minnehaha Creek and Non-contributing watersheds for existing and 2020 conditions are listed in Table IV.L.3-1.

Highlighted by the last column of Table IV.L.3-1, the most significant increase of impervious surfaces predicted to occur by the year 2020 are in subwatersheds tributary to the upper stretches of Minnehaha Creek. Although significant growth is expected in localized areas, the overall increase of impervious surfaces predicted for the entire Minnehaha Creek watershed (computed by weighted average) is only about 1.0%.

As Minnehaha Creek and Non-Contributing areas developed or are re-developed, care should be taken to ensure development does not negatively impact Minnehaha Creek or the areas lakes and wetlands. Most new development in the Minnehaha Creek watershed will occur in the upper portion of the Creek. Re-development that occurs in the lower watershed will present the District with an opportunity to provide protection to resources. Specific recommendations concerning development in the Minnehaha Creek and Non-Contributing watersheds are located in section L.6: Recommendations.

MCWD H/H and Pollutant Loading Study – 2003 K-21 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

Table IV.L.3-1 Minnehaha Creek & Non Contributing Area Growth by Subwatershed

Percent 2020 Percent Change in Percent Area Impervious** Impervious** Impervious Subwatershed I.D. (acres) (%) (%) (%) MC-1 79 57 64 7 MC-2 381 56 62 6 MC-3 334 51 60 9 MC-4 195 44 46 2 MC-5 142 28 29 1 MC-6 42 31 34 3 MC-7 110 47 48 1 MC-8 105 45 60 15 MC-9 410 24 25 1 MC-10 804 57 59 2 MC-11 119 17 18 1 MC-12 115 22 24 2 MC-13 152 35 36 1 MC-14 90 25 27 2 MC-15 207 23 24 1 MC-16 210 46 48 2 MC-17 197 25 26 1 MC-18 59 34 39 5 MC-19 97 40 42 2 MC-20 336 40 42 2 MC-21 454 55 56 1 MC-22 186 41 42 0 MC-23 220 41 41 0 MC-24 61 41 41 1 MC-25 85 30 31 1 MC-26 185 36 37 1 MC-27 81 24 28 5 MC-28 140 34 35 1 MC-29 72 38 42 4

MCWD H/H and Pollutant Loading Study – 2003 K-22 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

Table IV.L.3-1 Minnehaha Creek & Non Contributing Area Growth by Subwatershed

Percent 2020 Percent Change in Percent Area Impervious** Impervious** Impervious Subwatershed I.D. (acres) (%) (%) (%) MC-30 184 39 39 0 MC-31 30 58 60 2 MC-32 26 66 67 1 MC-33 103 27 28 0 MC-34 136 58 58 0 MC-35 311 47 48 1 MC-36 80 33 34 1 MC-37 72 43 43 0 MC-38 88 40 41 0 MC-39 155 64 65 1 MC-40 61 21 21 0 MC-41 237 63 64 2 MC-42 161 18 18 1 MC-43 153 44 46 2 MC-44 61 54 54 0 MC-45 143 42 42 0 MC-46 176 41 44 2 MC-47 62 43 43 0 MC-48 193 50 52 2 MC-49 175 52 56 5 MC-50 24 44 45 1 MC-51 155 44 45 0 MC-52 47 41 41 0 MC-53 121 41 43 2 MC-54 31 33 38 5 MC-55 140 40 40 0 MC-56 46 67 73 6 MC-57 54 91 91 0 MC-58 154 55 57 2 MC-59 229 54 55 1 MC-60 123 70 72 2

MCWD H/H and Pollutant Loading Study – 2003 K-23 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

Table IV.L.3-1 Minnehaha Creek & Non Contributing Area Growth by Subwatershed

Percent 2020 Percent Change in Percent Area Impervious** Impervious** Impervious Subwatershed I.D. (acres) (%) (%) (%) MC-61 36 86 86 0 MC-62 89 43 44 1 MC-63 52 44 44 0 MC-64 234 54 54 1 MC-65 36 79 83 4 MC-66 284 41 42 1 MC-67 176 65 69 4 MC-68 188 68 72 5 MC-69 125 66 69 3 MC-70 71 35 36 1 MC-71 73 53 53 1 MC-72 63 38 43 4 MC-73 49 37 39 2 MC-74 69 41 41 0 MC-75 439 36 36 0 MC-76 95 43 44 1 MC-77 70 49 49 0 MC-78 153 43 45 1 MC-79 167 62 63 1 MC-80 41 38 38 0 MC-81 137 47 47 0 MC-82 135 9 9 0 MC-83 108 41 42 1 MC-84 72 40 40 0 MC-85 33 80 80 0 MC-86 88 40 42 1 MC-87 41 35 35 0 MC-88 38 48 48 0 MC-89 265 40 40 0 MC-90 153 40 41 0 MC-91 115 34 34 0

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Table IV.L.3-1 Minnehaha Creek & Non Contributing Area Growth by Subwatershed

Percent 2020 Percent Change in Percent Area Impervious** Impervious** Impervious Subwatershed I.D. (acres) (%) (%) (%) MC-92 101 39 39 0 MC-93 226 39 39 0 MC-94 45 37 37 0 MC-95 357 38 38 0 MC-96 25 32 32 0 MC-97 234 39 39 0 MC-98 67 35 35 0 MC-99 67 37 37 0 MC-100 520 36 38 2 MC-101 582 50 52 2 MC-102 170 44 44 0 MC-103 301 51 51 0 MC-104 142 47 48 1 MC-105 232 39 39 0 MC-106 51 41 41 0 MC-107 322 56 57 1 MC-108 72 36 36 0 MC-109 293 75 75 0 MC-110 464 47 47 0 MC-111 68 90 90 0 MC-112 134 55 55 0 MC-113 75 63 66 3 MC-114 179 60 60 1 MC-115 365 74 76 3 MC-116 179 25 34 9 MC-117 178 67 68 1 MC-118 55 56 57 1 MC-119 108 26 26 0 MC-120 246 59 59 0 MC-121 592 50 50 0 MC-122 90 63 63 0

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Table IV.L.3-1 Minnehaha Creek & Non Contributing Area Growth by Subwatershed

Percent 2020 Percent Change in Percent Area Impervious** Impervious** Impervious Subwatershed I.D. (acres) (%) (%) (%) MC-123 241 56 56 0 MC-124 463 36 37 1 MC-125 417 40 40 0 MC-126 649 75 76 0 MC-127 206 17 17 0 MC-128 143 41 41 0 MC-129 313 43 43 0 MC-130 833 65 65 0 MC-131 105 40 40 0 MC-132 126 39 39 0 MC-133 34 41 41 0 MC-134 53 61 61 0 MC-135 174 45 45 0 MC-136 108 32 32 0 MC-137 46 35 35 0 MC-138 19 23 23 0 MC-139 92 37 37 0 MC-140 326 43 43 0 MC-141 41 35 35 0 MC-142 181 68 69 2 MC-143 114 52 52 0 MC-144 449 48 48 0 MC-145 44 17 17 0 MC-146 88 38 38 0 MC-147 313 37 37 0 MC-148 20 30 30 0 MC-149 46 30 30 0 MC-150 54 38 38 0 MC-151 88 33 33 0 MC-152 116 33 33 0 MC-153 20 49 49 0

MCWD H/H and Pollutant Loading Study – 2003 K-26 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

Table IV.L.3-1 Minnehaha Creek & Non Contributing Area Growth by Subwatershed

Percent 2020 Percent Change in Percent Area Impervious** Impervious** Impervious Subwatershed I.D. (acres) (%) (%) (%) MC-154 36 26 26 0 MC-155 435 40 40 0 MC-156 148 46 46 0 MC-157 65 38 38 0 MC-158 548 50 50 0 MC-159 484 57 57 0 MC-160 159 45 45 0 MC-161 85 38 38 0 MC-162 203 37 37 0 MC-163 113 37 37 0 MC-164 165 39 39 0 MC-165 24 34 34 0 MC-166 55 38 38 0 MC-167 348 67 67 0 MC-168 13 15 15 0 MC-169 58 32 32 0 MC-170 893 41 41 0 MC-171 95 38 38 0 MC-172 207 30 34 5 MC-173 9 28 30 2 MC-174 22 34 34 0 MC-175 7 32 33 0 MC-176 228 41 41 0 MC-177 49 38 38 0 MC-178 31 31 31 0 MC-179 109 38 38 0 MC-180 70 33 34 0 MC-181 44 35 36 0 MC-182 25 42 43 0 MC-183 115 44 44 0

MCWD H/H and Pollutant Loading Study – 2003 K-27 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

Table IV.L.3-1 Minnehaha Creek & Non Contributing Area Growth by Subwatershed

Percent 2020 Percent Change in Percent Area Impervious** Impervious** Impervious Subwatershed I.D. (acres) (%) (%) (%) MC-184 75 31 31 0 Average ** 166 46.2 47.2 1.0 Total Area 30472 WL-1 440 46 47 0 WL-3 570 59 59 0 MR-1 322 41 41 0 MR-2 628 44 44 0 MR-3 332 38 38 0 Average ** 458 47.1 47.1 0.0 Total Area 2291 * Includes open water and saturated wetlands. ** Percent impervious average is weighted on area.

L.3.b. Watershed Hydraulics

Input Parameters:

Table IV.Appendix.1-L2 in the Appendix shows a summary of the hydraulic input parameters for the modeled links. More specific input information like; cross section slopes, friction coefficients, or entrance/exit conduit losses (minor losses) can be found under the hydraulic mode of the XP-SWMM model.

Stage/storage information for Minnehaha Creek and Non-Contributing subwatersheds is available in the hydraulic mode of the XP-SWMM model.

Drainage Routing:

MCWD H/H and Pollutant Loading Study – 2003 K-28 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

The majority of all surface flows in the Minnehaha Creek and Non-Contributing watersheds are routed by stormsewer directly into the nearest lake or wetland, or into Minnehaha Creek itself. Exceptions to the typical stormsewer are found mostly in the upper half (western half) of the Minnehaha Creek watershed where several landlocked lakes, and areas characterized by many smaller landlocked depressions and wetlands exist. A more detailed description of general flow patterns in the Minnehaha Creek and Non-Contributing watersheds is available under section L.1. General Description, and visible in Figure IV.L.1-2.

Areas drained to Minnehaha Creek through a stormsewer system with an outfall equal to or greater than 30 inches diameter were routed through pipes in the XP-SWMM hydraulic mode, in order to assess potential impacts on velocity and discharge within the channel. Areas drained by stormsewer with an outfall less than 30 inches in diameter were modeled as an area allowed to discharge directly into the downstream waterbody (creek, wetland or lake) but the pipe system was not specifically modeled. Flows from these areas were regulated by the subwatersheds time of concentration, which is defined by the width and slope model parameters (discussed in the methodology section II.F.1). In general, stormsewer systems that were modeled represent the main system’s trunks before its outfall. Most stormsewer laterals are not included in the model but, could easily be added to provide additional detail or address local issues.

The main landlocked basins in the Minnehaha Creek watershed (shaded in Figure IV.L.1-2) include: • Spring Lake (MC-12) • Minnetonka Mills wetland (MC-27) • Cedar Manor Lake (MC-47) • Hannan Lake (MC-48) • Wolfe Lake (MC-113) • Unnamed wetlands located in MC-26, 33, 46, 73, and 83.

Areas containing several landlocked and semi-landlocked pockets (small wetlands and depressions) that do not typically contribute to Minnehaha Creek watershed include: • Southern half of MC-9

MCWD H/H and Pollutant Loading Study – 2003 K-29 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

• Southern half of MC-15 • South-eastern portion of MC-17

The landlocked basins are shaded in Figure IV.L.1-2. The unnamed wetland in subwatershed MC-44 and Windsor Lake in MC-45 have outlets. However, they are denoted as landlocked since they flow into a landlocked basin and do not typically contribute to Minnehaha Creek. Cedar Manor Lake in subwatershed MC-47 is equalized with Hannon Lake above about 894 feet (NGVD 1929). As a combined basin, both are landlocked.

Several lakes exist in the watershed that were once landlocked but now have pumped discharge to Minnehaha Creek. These lakes include: • Lake Victoria (MC-49) • Westling Pond (MC-50) • Cobble Crest Lake (MC-52) • Unnamed wetland 27-780W (MC-63) • South Oak Pond (MC-67) • Melody Lake (MC-78) • Lamplighter Pond (MC-100)

Powderhorn Lake (MR-3) in the Non-Contributing watershed is pumped to the Mississippi River.

L.3.c. Water Quantity Findings and Discussion

A summary of subwatershed findings and notes resulting from the modeling effort are compiled in Table IV.L.3-2. Additional modeling specific comments are in Table IV.Appendix.1-L5.

Results Summary:

The normal water level (NWL), high water level (HWL), peak discharge, and peak velocities predicted for the 100-year events are listed in Table IV.Appendix.1-L3 and Table

MCWD H/H and Pollutant Loading Study – 2003 K-30 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

IV.Appendix.1-L4 of this volume. Hydrographs and time dependent stages and velocities for continuous simulations and other event runs can be found in the provided XP-SWMM models. Figures IV.Appendix.1-L1 through L4 in the Appendix shows the XP-SWMM model diagram depicting the names of the links and nodes representing the hydraulics (water routing) of the watershed.

MCWD H/H and Pollutant Loading Study – 2003 K-31 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

Table IV.L.3-2 Summary of Water Quantity Findings for Minnehaha Creek and Non-Contributing Watersheds Issues Flooding Issues Appendix.1-L5) Water Quantity Boundary Issues DNR jurisdiction Landlocked Issues Named waterbody Subwatershed I.D. Notes (see Table IV. Infrastructure notes Backwater conditions Improvement Priority 100-year Critical Event Flow Velocities/Erosion Significant 2020 Impacts Impacts Significant 2020 Additional H/H Modeling

MC-1 Rainfall medium MC-2 Snowmelt medium MC-3 Snowmelt Unnamed Wetland 27-740W medium MC-4 Snowmelt Unnamed Wetland 27-741W medium MC-5 Snowmelt Unnamed Wetland 27-753W medium MC-6 Snowmelt Unnamed Wetland 27-750W medium MC-7 Snowmelt Unnamed Wetland 27-749W medium MC-8 Snowmelt Unnamed Wetland 27-748W medium MC-9 Snowmelt high MC-10 Rainfall Unnamed Wetland 27-761W medium MC-11 Snowmelt medium MC-12 Snowmelt Spring Lake 27-771W high MC-13 Rainfall Unnamed Wetland 27-773W medium MC-14 Rainfall medium MC-15 Rainfall high MC-16 Rainfall Unnamed Wetland 27-761W medium MC-17 Rainfall Minnehaha Creek high

MCWD H/H and Pollutant Loading Study – 2003 K-32 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

Table IV.L.3-2 Summary of Water Quantity Findings for Minnehaha Creek and Non-Contributing Watersheds Issues Flooding Issues Appendix.1-L5) Water Quantity Boundary Issues DNR jurisdiction Landlocked Issues Named waterbody Subwatershed I.D. Notes (see Table IV. Infrastructure notes Backwater conditions Improvement Priority 100-year Critical Event Flow Velocities/Erosion Significant 2020 Impacts Impacts Significant 2020 Additional H/H Modeling

MC-18 Snowmelt medium MC-19 Rainfall double wooden weir outlet medium MC-20 Snowmelt Unnamed Wetland 27-755W medium MC-21 Rainfall Unnamed Wetland 27-759W medium MC-22 Snowmelt Unnamed Wetland 27-758W medium MC-23 Snowmelt Unnamed Wetland 27-756P medium MC-24 Snowmelt Unnamed Wetland 27-763W medium MC-25 Snowmelt medium MC-26 Rainfall high MC-27 Snowmelt Minnetonka Mills high MC-28 Rainfall Minnehaha Creek 27-721P high MC-48 Snowmelt Hannan Lake 27-52P high MC-29 Rainfall Minnehaha Creek high MC-49 Snowmelt pumped outlet Lake Victoria 27-51P medium MC-30 Rainfall Unnamed Wetland 27-720W medium MC-31 Rainfall new bridge at Cedar Crossings Minnehaha Creek 27-719P medium MC-32 Rainfall Minnehaha Creek 27-719P medium

MCWD H/H and Pollutant Loading Study – 2003 K-33 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

Table IV.L.3-2 Summary of Water Quantity Findings for Minnehaha Creek and Non-Contributing Watersheds Issues Flooding Issues Appendix.1-L5) Water Quantity Boundary Issues DNR jurisdiction Landlocked Issues Named waterbody Subwatershed I.D. Notes (see Table IV. Infrastructure notes Backwater conditions Improvement Priority 100-year Critical Event Flow Velocities/Erosion Significant 2020 Impacts Impacts Significant 2020 Additional H/H Modeling

MC-33 Snowmelt Unnamed Wetland 27-83W high MC-34 Snowmelt Unnamed Wetland 27-722W medium MC-35 Rainfall Unnamed Wetland 27-718P medium MC-36 Rainfall medium MC-37 Rainfall Henn. Co. Bridge No. 27672 Minnehaha Creek 27-719P high MC-38 Rainfall medium no outlet found - suspect flow high MC-39 Snowmelt through RR berm Unnamed Wetland 27-84P MC-40 Rainfall medium MC-41 Rainfall Minnehaha Creek 27-84P high MC-42 Rainfall Unnamed Wetland 27-716W medium MC-43 Rainfall Minnehaha Creek 27-716W high MC-44 Snowmelt Unnamed Wetland 27-726W high MC-45 Snowmelt Windsor Lake 27-82P high MC-46 Snowmelt Unnamed Wetland 27-712W high MC-47 Snowmelt Cedar Manor Lake 27-713W high MC-50 Rainfall pumped outlet Westling Pond 27-714W medium

MCWD H/H and Pollutant Loading Study – 2003 K-34 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

Table IV.L.3-2 Summary of Water Quantity Findings for Minnehaha Creek and Non-Contributing Watersheds Issues Flooding Issues Appendix.1-L5) Water Quantity Boundary Issues DNR jurisdiction Landlocked Issues Named waterbody Subwatershed I.D. Notes (see Table IV. Infrastructure notes Backwater conditions Improvement Priority 100-year Critical Event Flow Velocities/Erosion Significant 2020 Impacts Impacts Significant 2020 Additional H/H Modeling

MC-51 Rainfall medium MC-52 Snowmelt pumped outlet Cobble Crest Lake 27-53P medium MC-53 Rainfall Minnehaha Creek 27-715W high MC-54 Rainfall Minnehaha Creek high MC-55 Rainfall medium MC-56 Rainfall Minnehaha Creek high MC-57 Rainfall Minnehaha Creek high MC-58 Rainfall Minnehaha Creek high MC-59 Rainfall Auburn Pond medium MC-60 Rainfall Minnehaha Creek 27-779W high MC-61 Rainfall Lake St. bridge rebuilt in 2000 Minnehaha Creek high MC-62 Rainfall Minnehaha Creek high MC-63 Snowmelt pumped outlet Unnamed Wetland 27-780W medium MC-64 Snowmelt medium MC-65 Rainfall Minnehaha Creek high MC-66 Rainfall Oak Pond 27-660P medium MC-67 Snowmelt pumped outlet South Oak Pond 27-661W medium

MCWD H/H and Pollutant Loading Study – 2003 K-35 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

Table IV.L.3-2 Summary of Water Quantity Findings for Minnehaha Creek and Non-Contributing Watersheds Issues Flooding Issues Appendix.1-L5) Water Quantity Boundary Issues DNR jurisdiction Landlocked Issues Named waterbody Subwatershed I.D. Notes (see Table IV. Infrastructure notes Backwater conditions Improvement Priority 100-year Critical Event Flow Velocities/Erosion Significant 2020 Impacts Impacts Significant 2020 Additional H/H Modeling

MC-68 Rainfall Minnehaha Creek 27-663W high MC-69 Rainfall Minnehaha Creek 27-662W high MC-70 Rainfall Minnehaha Creek high MC-71 Snowmelt medium MC-72 Rainfall Unnamed Wetland 27-666W medium MC-73 Snowmelt high MC-74 Rainfall medium Minnehaha Creek/Meadowbroo high MC-75 Rainfall k Lake 27-54P MC-76 Snowmelt Minnehaha Creek high MC-77 Rainfall Minnehaha Creek high MC-78 Snowmelt pumped outlet Melody Lake 27-669W medium MC-79 Rainfall medium MC-80 Rainfall medium Minnehaha high MC-81 Rainfall location of Browndale dam Creek/Mill Pond 27-41P MC-82 Rainfall Minnehaha Creek high

MCWD H/H and Pollutant Loading Study – 2003 K-36 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

Table IV.L.3-2 Summary of Water Quantity Findings for Minnehaha Creek and Non-Contributing Watersheds Issues Flooding Issues Appendix.1-L5) Water Quantity Boundary Issues DNR jurisdiction Landlocked Issues Named waterbody Subwatershed I.D. Notes (see Table IV. Infrastructure notes Backwater conditions Improvement Priority 100-year Critical Event Flow Velocities/Erosion Significant 2020 Impacts Impacts Significant 2020 Additional H/H Modeling

MC-83 Snowmelt high MC-84 Rainfall medium MC-85 Rainfall medium MC-86 Rainfall Minnehaha Creek high MC-87 Rainfall small concrete dam Minnehaha Creek high MC-88 Rainfall Minnehaha Creek high MC-89 Rainfall medium MC-90 Rainfall MCWD Improvement Project Pamela Lake 27-675P high Unnamed Wetland (south Pamela Lake medium MC-91 Rainfall MCWD Improvement Project wetlands) 27-675P MC-92 Rainfall Minnehaha Creek high MC-93 Rainfall Minnehaha Creek high MC-94 Rainfall Minnehaha Creek high MC-95 Rainfall pumped storm drain medium MC-96 Rainfall Minnehaha Creek high MC-97 Rainfall pumped storm drain medium

MCWD H/H and Pollutant Loading Study – 2003 K-37 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

Table IV.L.3-2 Summary of Water Quantity Findings for Minnehaha Creek and Non-Contributing Watersheds Issues Flooding Issues Appendix.1-L5) Water Quantity Boundary Issues DNR jurisdiction Landlocked Issues Named waterbody Subwatershed I.D. Notes (see Table IV. Infrastructure notes Backwater conditions Improvement Priority 100-year Critical Event Flow Velocities/Erosion Significant 2020 Impacts Impacts Significant 2020 Additional H/H Modeling

MC-98 Rainfall Minnehaha Creek high MC-99 Rainfall Minnehaha Creek high MC-100 Snowmelt pumped outlet Lamp Lighter Pond 27-710P medium 27-1087W, medium MC-101 Snowmelt Unnamed Wetland 27-658W MC-102 Rainfall Unnamed Wetland 27-659W medium MC-103 Rainfall medium MC-104 Snowmelt MCWD Improvement Project Twin Lake 27-657P medium MC-105 Rainfall MCWD Improvement Project Cedar Meadows medium MC-106 Snowmelt medium MC-107 Rainfall medium MC-108 Snowmelt Brownie Lake 27-38P medium MC-109 Snowmelt Cedar Lake 27-39P medium MC-110 Rainfall medium MC-111 Rainfall medium MC-112 Snowmelt medium MC-113 Snowmelt Wolfe Park Lake 27-664P high

MCWD H/H and Pollutant Loading Study – 2003 K-38 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

Table IV.L.3-2 Summary of Water Quantity Findings for Minnehaha Creek and Non-Contributing Watersheds Issues Flooding Issues Appendix.1-L5) Water Quantity Boundary Issues DNR jurisdiction Landlocked Issues Named waterbody Subwatershed I.D. Notes (see Table IV. Infrastructure notes Backwater conditions Improvement Priority 100-year Critical Event Flow Velocities/Erosion Significant 2020 Impacts Impacts Significant 2020 Additional H/H Modeling

MC-114 Rainfall medium MC-115 Snowmelt Bass Lake 27-15P medium MC-116 Snowmelt medium MC-117 Rainfall medium MC-118 Rainfall medium MC-119 Rainfall medium MC-120 Rainfall medium MC-121 Snowmelt Lake of the Isles 27-40P medium MC-122 Snowmelt medium MC-123 Snowmelt medium overflow by passes Calhoun medium MC-124 Rainfall Wetland MC-125 Rainfall MCWD Improvement Project Calhoun Wetlands medium MC-126 Snowmelt new outlet to Lake Harriet Lake Calhoun 27-31P medium aging infrastructure-poor Lakewood Cemetery medium MC-127 Snowmelt condition Pond 27-17P MC-128 Rainfall medium

MCWD H/H and Pollutant Loading Study – 2003 K-39 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

Table IV.L.3-2 Summary of Water Quantity Findings for Minnehaha Creek and Non-Contributing Watersheds Issues Flooding Issues Appendix.1-L5) Water Quantity Boundary Issues DNR jurisdiction Landlocked Issues Named waterbody Subwatershed I.D. Notes (see Table IV. Infrastructure notes Backwater conditions Improvement Priority 100-year Critical Event Flow Velocities/Erosion Significant 2020 Impacts Impacts Significant 2020 Additional H/H Modeling

MC-129 Rainfall medium MC-130 Snowmelt Lake Harriet 27-16P medium MC-131 Rainfall medium MC-132 Rainfall Minnehaha Creek high MC-133 Rainfall Minnehaha Creek high MC-134 Rainfall medium MC-135 Rainfall medium MC-136 Rainfall Minnehaha Creek high MC-137 Rainfall Minnehaha Creek high MC-138 Rainfall Minnehaha Creek high MC-139 Rainfall medium MC-140 Rainfall medium MC-141 Rainfall Minnehaha Creek high MC-142 Rainfall medium MC-143 Snowmelt medium MC-144 Snowmelt Diamond Lake 27-22P high

MCWD H/H and Pollutant Loading Study – 2003 K-40 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

Table IV.L.3-2 Summary of Water Quantity Findings for Minnehaha Creek and Non-Contributing Watersheds Issues Flooding Issues Appendix.1-L5) Water Quantity Boundary Issues DNR jurisdiction Landlocked Issues Named waterbody Subwatershed I.D. Notes (see Table IV. Infrastructure notes Backwater conditions Improvement Priority 100-year Critical Event Flow Velocities/Erosion Significant 2020 Impacts Impacts Significant 2020 Additional H/H Modeling

flat pipe (Diamond Lake medium MC-145 Rainfall drainage pipe) MC-146 Rainfall medium MC-147 Rainfall medium MC-148 Rainfall Minnehaha Creek high MC-149 Rainfall Minnehaha Creek high MC-150 Rainfall medium MC-151 Rainfall Minnehaha Creek high MC-152 Rainfall medium MC-153 Rainfall medium MC-154 Rainfall Minnehaha Creek high MC-155 Rainfall Norby's Pond 27-685W medium MC-156 Rainfall Milner's Pond 27-684W medium MC-157 Rainfall Christian Park Pond medium MC-158 Snowmelt Legion Lake 27-24P high MC-159 Snowmelt Mother Lake 27-23P medium MC-160 Snowmelt Taft Lake 27-683P medium

MCWD H/H and Pollutant Loading Study – 2003 K-41 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

Table IV.L.3-2 Summary of Water Quantity Findings for Minnehaha Creek and Non-Contributing Watersheds Issues Flooding Issues Appendix.1-L5) Water Quantity Boundary Issues DNR jurisdiction Landlocked Issues Named waterbody Subwatershed I.D. Notes (see Table IV. Infrastructure notes Backwater conditions Improvement Priority 100-year Critical Event Flow Velocities/Erosion Significant 2020 Impacts Impacts Significant 2020 Additional H/H Modeling

MC-161 Rainfall outlet pipe sediment impacted Unnamed Wetland 27-682W medium MC-162 Rainfall MCWD Improvement Project Amelia Wetland medium MC-163 Rainfall MCWD Improvement Project Gateway Wetland medium MC-164 Rainfall medium MC-165 Rainfall medium Nokomis Knoll medium MC-166 Rainfall MCWD Improvement Project Wetland MC-167 Rainfall inflatable weir Lake Nokomis 27-19P medium control dam downstream of high MC-168 Rainfall Nokomis tributary Minnehaha Creek MC-169 Rainfall Minnehaha Creek high MC-170 Rainfall medium MC-171 Rainfall medium Minnehaha Creek/Lake high MC-172 Rainfall control weir Hiawatha 27-18P MC-173 Rainfall Minnehaha Creek high MC-174 Snowmelt Minnehaha Creek high

MCWD H/H and Pollutant Loading Study – 2003 K-42 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

Table IV.L.3-2 Summary of Water Quantity Findings for Minnehaha Creek and Non-Contributing Watersheds Issues Flooding Issues Appendix.1-L5) Water Quantity Boundary Issues DNR jurisdiction Landlocked Issues Named waterbody Subwatershed I.D. Notes (see Table IV. Infrastructure notes Backwater conditions Improvement Priority 100-year Critical Event Flow Velocities/Erosion Significant 2020 Impacts Impacts Significant 2020 Additional H/H Modeling

MC-175 Snowmelt Minnehaha Creek high MC-176 Rainfall medium MC-177 Snowmelt medium WOMP station at 32nd Ave S high MC-178 Snowmelt footbridge Minnehaha Creek MC-179 Snowmelt Minnehaha Creek high MC-180 Snowmelt control weir removed. Minnehaha Creek high control weir lowered, new Minnehaha Parkway and high MC-181 Snowmelt Hiawatha bridges (2000-2001) Minnehaha Creek MC-182 Snowmelt Minnehaha Creek Falls Minnehaha Creek high MC-183 Rainfall medium Discharge into Mississippi high MC-184 Snowmelt River. Minnehaha Creek MR-1 Rainfall medium MR-2 Rainfall medium MR-3 Snowmelt pumped outlet Powderhorn Lake 27-14P low WL-1 Rainfall Grass Lake 27-681W low

MCWD H/H and Pollutant Loading Study – 2003 K-43 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

Table IV.L.3-2 Summary of Water Quantity Findings for Minnehaha Creek and Non-Contributing Watersheds Issues Flooding Issues Appendix.1-L5) Water Quantity Boundary Issues DNR jurisdiction Landlocked Issues Named waterbody Subwatershed I.D. Notes (see Table IV. Infrastructure notes Backwater conditions Improvement Priority 100-year Critical Event Flow Velocities/Erosion Significant 2020 Impacts Impacts Significant 2020 Additional H/H Modeling

WL-3 Snowmelt Richfield Lake 27-21P low

MCWD H/H and Pollutant Loading Study – 2003 K-44 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

Subwatershed Critical Event:

The modeled 100-year event that produced the “critical” or greater HWL in the watershed basins varied. Table IV.L.3-2 and Table IV.Appendix.1-L3 should be referenced for each subwatershed’s “critical” event and HWL.

In general, the 100-year 24-hour rainfall event (6.0 inches) produced the greater HWL and peak discharge in the Minnehaha Creek channel. These findings are related to assumptions made regarding operation of the Grays Bay dam. It was assumed that a discharge of 250 cfs (close to the highest operational discharge of the dam) was allowed through the dam at the time the 100- year 24-hour storm event occurred. This conservative but realistic approach representing the scenario of a wet season preceding the event (such as occurred in 2002 when multiple smaller storm events added to significant volumes producing high water levels in Lake Minnetonka) or perhaps the occurrence of a significant event some few days prior to the 100-year event simulated.

2020 Impacts:

Overall, high water levels are not predicted to be significantly impacted as a result of 2020 landuse changes in Minnehaha Creek or Non-Contributing watersheds. The vast majority of the area has already been developed. However, opportunities to influence some new development in the upper portion of the watershed along with redevelopment projects throughout the watershed will continue to arise. Stormwater improvements which minimize discharge rates to Minnehaha Creek and implement volume control will help mitigate channel erosion. From Figure IV.L.2-6 Infiltration Potential it can be seen that many areas of the lower watershed may be very well suited for the implementation of infiltration practices.

The operation of the Grays Bay dam has a significant impact on Minnehaha Creek flows. Sustained base-flows observed in Minnehaha Creek are, for the most part, dictated by flows allowed through the dam. The sharp peaks and flashy high water levels observed in the channel are strongly influenced by the direct drainage of the Minnehaha Creek watershed. Reduction of

MCWD H/H and Pollutant Loading Study – 2003 L-45 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion channel flashiness will require addressing stormsewer flow delivery to the Minnehaha Creek channel, regardless of potential alteration of dam operation.

For further discussion of impacts related to water quality, refer to section L.5.

Special Subwatershed Issues:

Initial Conditions:

The simulation of a single event occurring in the Minnehaha Creek watershed required the formulation of multiple assumptions that described the watershed conditions at the time the event occurs. The following assumptions were applied • Grays Bay dam operates as designed (no debris clogging simulated). • No lake evaporation simulated over the duration of the storm event. • 100-year, 24-hour rainfall events (existing and 2020 conditions):

o 250 cfs constant discharge over Grays Bay dam. This flow represents a significant discharge representative of a very wet season preceding the design storm or the occurrence of a significant event prior to the simulated design event. It does not represent the highest flows allowed (or observed) through the dam. • 100-year, 10-day snowmelt runoff event:

o 20 cfs constant discharge over Grays Bay dam. This discharge produces a base flow consistent with the current operation plan. It is the intent of the operation plan to retain volumes in Lake Minnetonka until the melt season has passed before releasing flows to Minnehaha Creek. Although it is more likely that snow from the lower watershed (Minnehaha Creek) will melt before snow accumulated in the upper watershed (Lake Minnetonka), it is feasible and conservative to assume the District could be in the process of slowly drawing down Lake Minnetonka at the time a melt event occurs.

o All ground 100% impervious (no infiltration) • 1.5-year, 24-hour rainfall event (existing and 2020 conditions):

MCWD H/H and Pollutant Loading Study – 2003 L-46 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion o 20 cfs constant discharge over Grays Bay dam. This represents a realistic scenario of maintenance discharge maintained through dam operation.

The results presented represent the scenario of conditions as outlined above. However, any number of storm events, initial conditions, and alternative dam configurations can be simulated using the XP-SWMM model provided. Other common design events and situations that could be modeled include (but are not limited to): • Back-to-back events • 10-day Rainfall event • Shorter duration events (e.g. 6 hour probable maximum event ) • 5, 10, 25, 50, and 500 year return events • Historical rainfall events (real events) • “What-if” scenarios:

o Alternative starting water elevations (higher and lower)

o Grays Bay Dam clogged or closed

o Alternative Grays Bay dam operation

o No infiltration.

Continuous simulations were performed for the calibration of Minnehaha Creek. Knowledge of actual operation and discharge of flow through Grays Bay dam was very important. Additional discussion and calibration results are included and discussed under the model methodology section (Volume II, Section F.1.b.: Model Calibration). Future model simulation of conditions other than a single or design events should take actual dam discharge into consideration.

Travel time and attenuation:

To determine the time it takes for water to navigate the length of Minnehaha Creek from the Grays Bay dam to the Minnehaha Creek Falls during base flow conditions, a two-stage pulse of water was entered at the upstream end of Minnehaha Creek. The first stage of the pulse consisted of a long steady flow rate. This was necessary to fill any channel depressions and bring the creek to equilibrium at base-flow. The second stage consisted of a dramatic and short

MCWD H/H and Pollutant Loading Study – 2003 L-47 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion lived pulse of water followed by a return to the steady stage base-flow condition. In addition to observation of creek travel time, simulation of the pulse allowed measurement of peak attenuation due to routing as well as quantifying the volume of storage available in the channel. The results of the model simulation are visible in Figure IV.L.3-1.

Results of the simulation predict that it will take a drop of water approximately 2-3 days to travel from the Grays Bay dam to “Minnehaha Falls” near its outlet to the Mississippi and that during that travel time, approximately 65% of the original peak was attenuated. Approximately 80 ac-ft of in-channel storage was available (up to 20 cfs base-flow water levels) and it took approximately 7 days for the channel to fill and reach steady-state on the leading edge of the pulse.

Figure IV.L.3-1 Minnehaha Creek Travel Time and Attenuation 220 ~ 2.3 days Minnehaha Falls 200 (leading edge to leading edge) Pulse @ Grays Bay dam 180 Under base flow conditions of the creek, it takes about 2-3 days for 160 water at Grays Bay dam to reach Minnehaha Falls. 140 About 65% of peak attenuated

120 ~ 3 days (center to center) 100

Discharge (cfs) 80 Peak Flow ~ 83 cfs

60

40 channel filling to steady-state 20

0 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 1

Time (days)

MCWD H/H and Pollutant Loading Study – 2003 L-48 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

Nokomis Weir:

Discharge from Lake Nokomis (MC-167) into Minnehaha Creek is controlled by an inflatable weir. The minimum weir elevation is 815.1 feet. When creek elevations are below 815.1, this elevation defines the NWL of Lake Nokomis. When Minnehaha Creek elevations are between 815.1 and 817.5 feet, backwater flows into Lake Nokomis are limited by the weir (high water levels trigger the weir to raise in elevation). Flows can only be limited to the maximum height of the weir, which is at 817.5 feet.

Creek flows simulated, assuming a base-flow of 20 cfs from the Gray’s Bay dam, during storm events up to and including the 5-year, 24-hour rainfall event (3.6 inches) were not able to flow into Lake Nokomis due to the inflatable weir. Simulation of the 10-year, 24 hour rainfall event (4.2 inches) on a 20 cfs base-flow showed that flows from Minnehaha Creek initially backflow into Lake Nokomis. Following the peak attenuation into Lake Nokomis, flows reverse and discharge moves in the direction of Lake Nokomis to Minnehaha Creek. This pattern was also observed during simulation of the 100-year rainfall and 100-year snowmelt events and would indicate that the inflatable weir does not prevent flows exceeding the 5-year recurrence from entering Lake Nokomis.

During the 100-year, 24-hour simulation, peak flow of the creek is attenuated (reduced) by approximately 450 cfs due to discharge into Lake Nokomis.

Chain of Lakes:

The Chain-of-Lakes in the City of Minneapolis refers to Brownie, Cedar, Lake of the Isles, Calhoun, and Lake Harriet. The upper four lakes of the chain (Brownie, Cedar, Lake of the Isles, and Calhoun) equalize and operate as one basin. Discharge from Lake Calhoun flows into Lake Harriet, which is about 5 feet lower in elevation. Ultimately discharge from the chain passes through Lake Harriet and flows through a short channel which enters Minnehaha Creek just downstream of Humboldt Avenue South.

MCWD H/H and Pollutant Loading Study – 2003 L-49 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

The combined upper basin has an extended draw down time which exceeds one month to return from the HWL to the NWL. The lakes were modeled starting at their NWL (851.9 feet NGVD 1929), however, because of the slow draw down, it is highly probable that initial water levels will be elevated prior to storm event.

To develop more conservative HWLs, the upper chain could be treated differently from other lakes in the MCWD model. Simulating the 100-year return events on slightly elevated initial water surface conditions is an option given the lengthy draw down time. In any case, the HWLs for the upper Chain of Lakes determined in this report would only be increased by 0.2 – 0.3 feet. Other modeling alternatives could include simulation of back-to-back 100-year storm events.

Bank flooding:

Channel flooding that overtops the defined banks of Minnehaha Creek (typically spilling into parkland) occurs in many locations along the length of the creek. In most cases, the flooding may occur without causing obvious structural impacts. Impacts on structures as a result of bank flooding are listed in Tables IV.L.3-2 and IV.L.3-4. Locations of structure impacts from flooding are indicated in Figure IV.L.3-2.

All channel cross-sections and simulation HWLs may be viewed in the XP-SWMM using the model’s dynamic view function. Areas of interest should be reviewed for bank flooding directly from the XP-SWMM model.

Landlocked Basins:

Several landlocked basins and many smaller landlocked pocket wetlands exist in the upper reaches of the Minnehaha Creek drainage area including large areas within the City of Minnetonka and portions of Hopkins, Edina and St. Louis Park. Landlocked basins and the areas characterized by multiple smaller landlocked wetlands or depressions are noted above under

MCWD H/H and Pollutant Loading Study – 2003 L-50 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion section L.3.b Drainage Routing. Landlocked areas are also noted in the Summary Table (Table IV.L.3-2).

Proposed culverts (new and/or upgrades to existing) shown in the various city surface water and water resources management plans indicate that many of these basins and smaller wetlands areas are proposed to be connected and drained. While providing outlets to landlocked areas (as suggested in several city water resource plans) could address potential flooding issues, the additional drainage area released would result in increased channel erosion and potentially increased pollutant loading to the receiving water body. For discussion of water quality issues see section L.5.

To avoid new flooding problems or exacerbation of existing issues, it is recommended that all existing landlocked basins remain unconnected, that low impact development techniques be employed as these areas develop or are re-developed, and that the ability of landlocked basins and pocket wetlands to retain runoff be maintained. Particular attention to volume control and green space planning (where feasible) will greatly ease that burden.

Additionally, landlocked basins and depressions are particularly sensitive to additional stormwater volumes. For this reason, strong volume control standards are recommended in all areas draining to landlocked areas.

Backwater:

Several lakes and wetlands in the Minnehaha Creek Watershed are influenced by backwater conditions during times of higher water levels in Minnehaha Creek. Basins impacted by backwater conditions are indicated in Table IV.L.3-2.

Flow Velocities/Erosion Issues:

The following Table IV.L.3-3 highlights pipes showing unusually high velocities for the modeled events. It is recommended that inlet and outlet erosion control measures or energy

MCWD H/H and Pollutant Loading Study – 2003 L-51 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion dissipation designs are implemented at these outfall locations. Section L.4 provides additional information on erosion prone areas in the Minnehaha Creek channel.

Table IV.L.3-3 Significant Conduit Velocities in Minnehaha Creek and Non-Contributing Watersheds Peak Velocity (ft/s) Existing 2020 Snow-melt Event Link or Multi 1.5-year, 100-year, 100-year, 100-year, Description Link Name 24-hour 24-hour 24-hour 10-day Excelsior stormsewer MC-70EBdC3 8.4 11.0 11.0 9.0 Excelsior stormsewer (outfall to Meadowbrook Lake) MC-70EBdC4 10.3 13.4 13.4 11.0 Minnehaha Creek under 50th Street Bridge MC-81 50 5.3 11.9 11.9 10.4 stormsewer (enters creek just north of Pamela Park) MC-89 SS1 7.1 10.1 10.1 6.6 stormsewer (outfall into ditch leading to Minnehaha Creek just north of Pamela Park) MC-89 SS2 7.4 10.5 10.5 7.4 stormsewer (outfall into Lake of the Isles) MC-120 SS1 5.8 12.2 12.2 6.0 stormsewer (outfall into Lake Calhoun) MC-123 SS1 3.7 10.3 10.3 3.2 stormsewer (outfall into Lake Harriet) MC-128 SS2 4.7 10.4 10.4 4.7 stormsewer (outfall into Lake Harriet) MC-128 SS2 4.7 10.4 10.4 4.7 stormsewer (outfall into Minnehaha Creek downstream of Lyndale Ave. S. bridge) MC-134 SS1 12.4 29.1 29.1 6.5 stormsewer (drains to MC-135 SS2) MC-135 SS1 13.6 13.3 13.3 13.2 stormsewer (outfall into Minnehaha Creek downstream of Pleasant Ave. S. bridge) MC-135 SS2 10.6 10.9 10.9 8.7 stormsewer (outfall into Diamond Lake) MC-142 SS1 9.5 21.2 21.3 5.1 stormsewer (outfall into Diamond Lake) MC-143 54C 4.3 10.4 10.4 1.9

MCWD H/H and Pollutant Loading Study – 2003 L-52 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Table IV.L.3-3 Significant Conduit Velocities in Minnehaha Creek and Non-Contributing Watersheds Peak Velocity (ft/s) Existing 2020 Snow-melt Event Link or Multi 1.5-year, 100-year, 100-year, 100-year, Description Link Name 24-hour 24-hour 24-hour 10-day residential stormsewer (outfall into Minnehaha Creek downstream of Minnehaha Parkway bridge near Oakland Ave. S.) MC-147 SS1 6.3 13.1 13.1 5.1 stormsewer outletting Norby’s Pond MC-155 SS1 4.0 13.0 13.2 7.2 stormsewer (outfall into Legion Lake) MC-156 SS3 7.9 10.3 10.4 6.9 stormsewer tunnel (outfall into Mississippi River) MR-1 SS 10.2 14.4 14.4 10.1 stormsewer tunnel (outfall into Mississippi River) MR-2 SS 10.9 15.6 15.6 11.8

Where: Velocity Range 10 to 11.9 12 to 14.9 15+

Flooding Issues:

Modeling of the Minnehaha Creek and Non-Contributing Watersheds predicted that several roads will overtop during large storm events. Areas predicted to flood during the modeled events are shown in Figure IV.L.3-2. The structure and modeled event(s) resulting in flooding are listed in Table IV.L.3-4.

There may be a number of structure, roads and trails adjacent to Minnehaha Creek or other modeled basins that are under or within the freeboard (2 feet) required by the district. The HWLs corresponding to the modeled events can be obtained from Table IV.Appendix.1-L3.

MCWD H/H and Pollutant Loading Study – 2003 L-53 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Stanton 35 Wayzata Drive Plymouth g%55 ,-. RT 3 RT 1 RT 2 Golden Valley 394 . 394 Cedar,- Lake ,.- Road ,.-94 Cedar Saint Lake Lake g%55 E Minnehaha Louis of the Fremont Parkway Park Isles Ave S Grays Footbridge Cedar Ave S Bay Outlet Lake ,-.35 Park/Longfellow Calhoun Ave S Footbridge Trail Golf Course Minneapolis Bridge Bridges (4) 3rd Footbridge DS of falls Hopkins Lake Lake g%7 Minnetonka Harriet Hiawatha

Berm just Boulevard #

east of # 494 169 Lake Haven Road -. /(Kresse # , Circle Natural Nokomis Berm 14th Ave S 6th Footbridge Diamond Footbridge DS of falls Minnetonka RT 2 RT 1 Lake Park Grass Utility Bridge Lake

g%62 12th Flooding Area Newton 35 ,-. Ave S Subwatershed Boundaries W. 58th Ave S. Tarrymore/ Major Watershed Boundary Edina g%100 Street Footbridge Clinton Ave S Non-contributing Areas Footbridge Major Roads City Boundaries Stevens Richfield g%77 Streams 0.500.5Miles Ave S Lakes

N Figure IV.L.3-2 Minnehaha Creek Flooding H&H Report/Basins/Projects/flow_030331_kl

MCWD H/H and Pollutant Loading Study – 2003 L-54 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Table IV.L.3-4 Flood Magnitude for Minnehaha Creek and Non Contributing Watersheds Flood Event Existing 2020 Snow- Link or melt Description Multi Link 1.5- 100- 100- Event Name year, year, year, 100- 24- 24- 24- year, hour hour hour 10- day Berm just east of Haven Road MC-12nat X Stanton Drive road top MC-24 SDr X Minnetonka Boulevard road top MC-26MDrT X X X X Cedar Lake Road road top MC-30 CRd X X X Kresse Circle MC-63 KCiT X X X berm flooding MC-72 Out berm flooding MC-73 natT X Bank flooding around Park Utility bridge MC-86 PBr X X W. 58th Street MC-91 58St X X overflow pipe delivers flow upstream of 54th Street Bridge MC-92 eof X X bank flooding around Newton Avenue footbridge MC-98 NFt X X X overflow pipe bypassing Calhoun Wetland MC-124 eof X X X X overflow pipe from Calhoun Wetland to Lake Calhoun MC-125eofP X X X X trail over channel between Harriet and Minnehaha Creek MC-131 Tr X X Freemont Ave S footbridge MC-132 FFt X X X X Stevens Ave S MC-137 SAv X X Tarrymoore/Clinton Ave S footbridge MC-138 TFt X X X E Minnehaha Parkway bridge near 50th Street MC-141 MPk X X 12th Avenue S bridge MC-149 12 X X X 14th Ave S footbridge MC-151 14 X X X X Cedar Avenue S bridge MC-154 CAv X X Park/Longfellow Ave S footbridge MC-168 PFt X X X 1st Golf Course footbridge (downstream of Minnehaha Parkway) MC-169 Ft1 X X X X 2nd Golf Course footbridge (downstream of Minnehaha Parkway) MC-169 Ft2 X X X 3rd Golf Course footbridge (downstream of Minnehaha Parkway) MC-169 Ft3 X X X X 4th Golf Course footbridge (downstream of Minnehaha Parkway) MC-169 Ft4 X X X X 3rd footbridge downstream of Falls MC-184 Ft3 X X X 6th footbridge downstream of Falls (last bridge) MC-184 Ft6 X X

MCWD H/H and Pollutant Loading Study – 2003 L-55 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

L.3.d. Watershed Recommendations

Recommendations specific to water quantity issues can be found in L.6: Recommendations, along with recommendations relevant to other aspects of Minnehaha Creek and Non- Contributing water resources.

L.3.e. Watershed References

Pertinent information available to aid model construction and to compare and contrast XP- SWMM model results included: ∗ MCWD hydrodata (including flow monitoring at Grays Bay Dam, continuous flow monitoring at Mill Pond, and various grab sample locations along Minnehaha Creek) ∗ MCWD Water Resource Plan TR-20 Model results ∗ MCWD Operation and Maintenance Manual Grays Bay Operation Plan Pamela Park Water Quality Improvement Project Twin Lakes Subwatershed Improvement Project (Cedar Meadows plans included) Lake Nokomis Water Quality Improvement Project: Water Quality Wetlands ∗ MCWD data: Mother Lake Outlet Structure ∗ MNDNR Minnehaha Creek HEC-2 models ∗ MNDNR historical water elevations: Cedar Manor Lake DNR ID 27-713W (MC-47) note: one data entry Twin Lake DNR ID 27-656P (MC-104) note: one data entry Brownie Lake DNR ID 27-38P (MC-108) Lake of the Isles DNR ID 27-40P (MC-121) Lake Calhoun DNR ID 27-31P (MC-126) Cemetery Pond DNR ID 27-17P (MC-127)

MCWD H/H and Pollutant Loading Study – 2003 L-56 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Lake Harriet DNR ID 27-16P (MC-130) Lake Nokomis DNR ID 27-19P (MC-167) Lake Hiawatha DNR ID 27-18P (MC-172) ∗ MNDNR Hydrographic Survey Reports Unnamed Wetland 27-84P (6/18/1992) Cedar Manor Lake 27-713W (7/12/1990) Mother Lake 27-23P (11/13 and 11/14/ 1996) ∗ Surface Water Management Plans including stormsewer/structure detail: Minnetonka Hopkins St. Louis Park Edina Minneapolis Richfield ∗ Bridge plans and surveys (all cities) ∗ Minneapolis Data (among others): Stormdrain index maps Minneapolis Chain of Lakes Clean Water Partnership Project (1991) Minneapolis Chain of Lakes Phase I-Diagnostic Report 1991 Clean Water Partnership Project Cities of Minneapolis Storm Water NPDES Part II Monitoring and Modeling Results Lake Calhoun Outlet – Hydrologic Model, June 15, 1997 (study by BARR Engineering completed for Minneapolis Park and Recreation Board) Lake Calhoun Outlet – Phase II (8/24/1999) contract drawings 1980 Creek Survey 1995 Creek Survey 1997 MPRB Creek Survey Penn Avenue HEC-RAS model ∗ Mn/DOT / Hennepin County Road Plans T.H. 35W: State Project No. 2782-9612

MCWD H/H and Pollutant Loading Study – 2003 L-57 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion T.H. 7: State Project No. 2706-83 (Henn. Co. Proj. No. 6706) T.H. 100: State Project No. 2734 Cedar Lake Parkway: Min. Proj. No. Brnh 007-2(080), Bridge No. 27A70 T.H. 62: State Project No. 2775-707 T.H 55: State Project No. 2724-27192 (E. Minnehaha Parkway Bridge No. 27192) T.H 55: State Project No. 2724-27X03 (Hiawatha Bridge No.27X03) ∗ 15 minute precipitation from the City of Minneapolis ∗ USGS Quadrangle Maps

This information was used to aid model construction and also for model validation where applicable. That engineering and modeling judgment was used to assess and resolve conflicting information. Additional field information was gathered when necessary to fill in gaps, update and/or resolve conflicting information.

The XP-SWMM model results were calibrated against measured data and compared to other models for general results validation.

MCWD H/H and Pollutant Loading Study – 2003 L-58 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion L.4. Scour and Erosion-Prone Areas

L.4.a. Streams

A scour analysis was performed on Minnehaha Creek for the 1.5-yr (2.6 inch) event based on creek flow velocity and soil composition in and around the channel.

Based on the soil reports from the Soil Survey Geographic (SSURGO) database for Hennepin County, Minnesota, the soil composition for Minnehaha Creek is mostly made up of sandy loam and fine sandy loam. Soils along the banks of the creek have been greatly altered and compacted from development in the area. Because of such alterations, a permissible creek velocity of 2.0 fps was assigned to the entire channel for the purpose of this analysis. Nearly 120 modeled creek sections ranging from 5 to 1000 feet in length were found to have high erosion potential based on the ranking criteria displayed in Table IV.F.4-2 (Volume II: Framework and Methodology). About 60 modeled creek sections ranging from 6 to 940 feet in length were found to have medium to very low erosion potential for the 1.5-yr rainfall event. Also, about 60 sections of Minnehaha Creek were found to have velocities greater than 1.5 fps but less than the soil’s permissible velocity. Such velocities may result in scour occurring where lenses of very fine and/or non-cohesive grains are present. Modeled links found to have erosion potential are detailed in Table IV.E.4-1. Locations for those links are found on figures IV.Appendix.1-L1 through L4 in appendix and can be identified by the link name in Table IV.E.4-1.

MCWD H/H and Pollutant Loading Study – 2003 L-59 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Table IV.E.4-1 Minnehaha Creek channel links with velocities greater than 1.5 fps for the 1.5-yr event Link 1.5-yr Velocity Erosive potential Link Name Length (ft) (fps) ranking MC-17sec12 78 3.19 High MC-28 RR 15 3.21 High MC-31 CCr 60 2.53 High MC-31 PD 16 3.39 High MC-31xsec1 5 3.05 High MC-41xsec6 275 2.68 High MC-41xsec7 360 2.63 High MC-42 ORd 80 3.55 High MC-42 ORd 80 3.55 High MC-53 34St 40 3.08 High MC-58CSt 54 2.93 High MC-61LSt 53 3.50 High MC-61xsec3 29 2.80 High MC-62 BT 10 2.80 High MC-62 RR 25 3.48 High MC-62sec10 46 3.49 High MC-62sec11 30 4.24 High MC-62sec13 8 2.77 High MC-62xsec8 28 2.63 High MC-65MRd 59 3.54 High MC-65MRd 59 3.91 High MC-65xsec1 10 3.26 High MC-65xsec2 34 3.56 High MC-65xsec4 290 3.15 High MC-65xsec5 43 3.50 High MC-68 LAv 39.7 3.28 High MC-69 EBd 62 3.65 High MC-81 50 80 5.47 High MC-81 BAv 11.5 6.27 High MC-81xsec1 20 5.80 High MC-81xsec2 30 3.68 High MC-81xsec3 10 5.64 High MC-82 WAv 41 2.93 High

MCWD H/H and Pollutant Loading Study – 2003 L-60 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Link 1.5-yr Velocity Erosive potential Link Name Length (ft) (fps) ranking MC-82xsec4 31 3.85 High MC-86 PBr 45 2.86 High MC-87 54St 33 2.69 High MC-87xsec4 32 3.47 High MC-88xsec1 402 3.23 High MC-88xsec2 491 2.62 High MC-93 XAv 54 2.57 High MC-93xsec9 437 2.73 High MC-94 UAv 48 2.79 High MC-98 MFt 7 5.43 High MC-98 NFt 6 2.88 High MC-98sec10 320 2.88 High MC-98sec11 140 3.13 High MC-98xsec3 33 2.77 High MC-98xsec4 55 4.96 High MC-98xsec5 33 2.91 High MC-98xsec6 33 2.51 High MC-98xsec8 33 2.68 High MC-98xsec9 33 3.75 High MC-99xsec1 33 3.01 High MC-99xsec3 33 3.59 High MC-132 DFt 6 3.75 High MC-132sec3 53 2.51 High MC-132sec4 587 2.71 High MC-133 LAv 50 3.22 High MC-133sec1 294 2.62 High MC-133sec4 65 3.43 High MC-136 GFt 7 2.80 High MC-136 NFt 7 3.85 High MC-136 PAv 64 3.97 High MC-136MPk1 48 2.61 High MC136sec10 672 2.55 High MC136sec12 522 2.56 High MC136sec13 223 3.07 High

MCWD H/H and Pollutant Loading Study – 2003 L-61 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Link 1.5-yr Velocity Erosive potential Link Name Length (ft) (fps) ranking MC-136sec3 208 3.17 High MC-136sec4 100 2.99 High MC-136sec6 43 2.92 High MC-137 I35 205 3.32 High MC-137sec2 150 3.41 High MC-138 TFt 9 2.59 High MC-138sec1 792 2.75 High MC-138sec2 258 3.23 High MC-138sec3 562 2.95 High MC-141 MPk 48 2.74 High MC-141 PAv 64 2.96 High MC-141sec2 67 3.84 High MC-141sec3 472 3.37 High MC-148 MPk 67 3.64 High MC-148sec1 239 3.21 High MC-148sec2 589 3.61 High MC-148sec4 214 3.30 High MC-149sec1 184 2.91 High MC-151 BAv 73 3.60 High MC-151sec2 947 2.67 High MC-154 17 7 2.74 High MC-154sec2 343 2.59 High MC-168 MPk 49 3.01 High MC-168sec1 1065 2.63 High MC-168sec2 213 2.96 High MC-169 Ft2 24 2.96 High MC-169 Ft4 6 2.84 High MC-169sec2 749 3.27 High MC-169sec5 484 2.66 High MC-174 28 50 3.37 High MC-175 NAv 54 2.95 High MC-175sec1 129 3.05 High MC-175sec2 377 2.55 High MC-178 Ft1 7 3.09 High

MCWD H/H and Pollutant Loading Study – 2003 L-62 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Link 1.5-yr Velocity Erosive potential Link Name Length (ft) (fps) ranking MC-178 Ft2 9 3.79 High MC-180 Ft 52 3.52 High MC-180sec2 624 3.34 High MC-180sec4 85 2.81 High MC-181 MPk 51.54 2.56 High MC-181sec4 18 5.09 High MC-182 Ft 18 4.88 High MC-182 MAv 77 7.39 High MC-182 RR 19 5.78 High MC-182sec2 52 4.19 High MC-182sec3 337 3.44 High MC-182sec4 42 4.83 High MC-184 Ft1 13 6.95 High MC-184 Ft2 10 5.80 High MC-184 Ft3 9 3.86 High MC-184sec1 449 5.48 High MC-184sec2 610 2.60 High MC-184sec3 586 4.32 High MC-17 MRd 66.3 2.24 Medium MC-17sec11 360 2.44 Medium MC-56 36St 67 2.22 Medium MC-56xsec7 26 2.38 Medium MC-61xsec1 37 2.27 Medium MC-62sec12 95 2.34 Medium MC-62sec14 10 2.33 Medium MC-62xsec6 145 2.34 Medium MC-62xsec7 36 2.41 Medium MC-62xsec9 132 2.37 Medium MC-82xsec1 480 2.38 Medium MC-86xsec1 535 2.24 Medium MC-93xsec4 57 2.40 Medium MC-94xsec2 434 2.25 Medium MC-94xsec3 76 2.30 Medium MC-96xsec1 86 2.26 Medium

MCWD H/H and Pollutant Loading Study – 2003 L-63 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Link 1.5-yr Velocity Erosive potential Link Name Length (ft) (fps) ranking MC-96xsec5 375 2.22 Medium MC-132sec2 360 2.25 Medium MC-132sec6 275 2.35 Medium MC-133sec3 325 2.32 Medium MC-136 NAv 22 2.31 Medium MC-136MPk2 35 2.41 Medium MC136sec11 473 2.37 Medium MC-136sec5 272 2.26 Medium MC-136sec8 260 2.36 Medium MC-137 SAv 45 2.40 Medium MC-137sec1 683 2.42 Medium MC-148sec3 463 2.44 Medium MC-149sec3 350 2.38 Medium MC-151 14 8 2.25 Medium MC-154 CAv 65 2.31 Medium MC-154sec3 500 2.39 Medium MC-168sec4 20 2.39 Medium MC-168sec5 159 2.38 Medium MC-169 Ft1 6 2.37 Medium MC-169sec1 340 2.29 Medium MC-169sec4 274 2.48 Medium MC-178sec2 938 2.43 Medium MC-179 34 78 2.30 Medium MC-181 HAv 131.23 2.49 Medium MC-28xsec1 28 2.13 Low MC-29 AMi 37 2.20 Low MC-37xsec2 193 2.00 Low MC-60BRd 112 2.19 Low MC-61xsec2 180 2.09 Low MC-82xsec2 435 2.08 Low MC-88xsec3 354 2.17 Low MC-88xsec4 596 2.00 Low MC-96xsec3 566 2.03 Low MC-98 LAv 68 2.00 Low

MCWD H/H and Pollutant Loading Study – 2003 L-64 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Link 1.5-yr Velocity Erosive potential Link Name Length (ft) (fps) ranking MC-132 FFt 8 2.16 Low MC-132sec5 532 2.12 Low MC-136sec1 90 2.02 Low MC-136sec2 445 2.10 Low MC-136sec7 477 2.19 Low MC-169 Ft3 6 2.18 Low MC-169sec3 251 2.12 Low MC-178sec1 321 2.18 Low MC-181sec1 116 2.17 Low MC-10PDBr 8 1.95 Very Low MC-17sec10 430 1.62 Very Low MC-28sec12 380 1.60 Very Low MC-28sec14 72 1.63 Very Low MC-28xsec5 180 1.72 Very Low MC-31xsec3 70 1.55 Very Low MC-32 RR 34 1.82 Very Low MC-32xsec1 5 1.85 Very Low MC-32xsec2 97 1.94 Very Low MC-32xsec5 40 1.66 Very Low MC-37xsec1 70 1.53 Very Low MC-41xsec5 295 1.85 Very Low MC-56xsec6 10 1.62 Very Low MC-57 7 108 1.56 Very Low MC-62xsec2 541 1.50 Very Low MC-62xsec3 265 1.90 Very Low MC-62xsec4 343 1.53 Very Low MC-62xsec5 538 1.86 Very Low MC-68 LAv 39.7 1.52 Very Low MC-68 LAv 39.7 1.54 Very Low MC-76 44St 41 1.92 Very Low MC-76xsec1 49 1.67 Very Low MC-77 100 133 1.94 Very Low MC-77xsec2 446 1.56 Very Low MC-82xsec3 23 1.90 Very Low

MCWD H/H and Pollutant Loading Study – 2003 L-65 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Link 1.5-yr Velocity Erosive potential Link Name Length (ft) (fps) ranking MC-86xsec2 977 1.87 Very Low MC-88 56St 40 1.56 Very Low MC-92 FAv 137 1.83 Very Low MC-92xsec2 518 1.70 Very Low MC-93xsec5 31 1.90 Very Low MC-93xsec6 319 1.55 Very Low MC-94xsec1 561 1.97 Very Low MC-96 PAv 62 1.68 Very Low MC-96xsec2 608 1.50 Very Low MC-96xsec4 187 1.95 Very Low MC-98 LAv 68 1.51 Very Low MC-98 LAv 68 1.51 Very Low MC-98xsec1 65 1.67 Very Low MC-98xsec2 1000 1.96 Very Low MC-98xsec7 33 1.69 Very Low MC-99xsec2 100 1.73 Very Low MC-99xsec4 170 1.80 Very Low MC-99xsec5 70 1.72 Very Low MC-133sec2 700 1.93 Very Low MC-141sec1 433 1.93 Very Low MC-148 CAv 66 1.81 Very Low MC-149 12 48 1.97 Very Low MC-149sec2 300 1.72 Very Low MC-149sec4 522 1.62 Very Low MC-149sec5 208 1.60 Very Low MC-154sec1 602 1.56 Very Low MC-168 PFt 8 1.93 Very Low MC-168sec3 79 1.92 Very Low MC-173 Ft 9 1.65 Very Low MC-173sec1 16 1.59 Very Low MC-173sec2 382 1.96 Very Low MC-174sec1 711 1.62 Very Low MC-179sec1 641 1.62 Very Low MC-180sec1 1010 1.92 Very Low

MCWD H/H and Pollutant Loading Study – 2003 L-66 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Link 1.5-yr Velocity Erosive potential Link Name Length (ft) (fps) ranking MC-180sec3 598 1.66 Very Low MC-181sec2 462 1.54 Very Low MC-182sec1 103 1.79 Very Low

L.4.b. Lakeshore

The identification of lakeshore erosion areas was conducted primarily at the Regional Team meetings, when participants were asked to locate any known erosion areas on a map of the area represented. The RT 1 and 2 meetings did not identify any locations. This, however, does not necessarily mean that none exist; rather, it indicates that the members have not seen specific problems. The District should remain vigilant in locating lakeshore erosion because of the direct threat that these problems present through sediment delivery into lakes.

MCWD H/H and Pollutant Loading Study – 2003 L-67 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion L.5. Water Quality

L.5.a. Watershed Pollutant Load Analysis

The pollutant loads (lbs/ac-yr) for the Minnehaha Creek watershed are illustrated in Figures IV.L.5-1 through -3. The remaining water quality model results, including runoff volume and pollutant loads (lbs/yr), are listed in Appendix 2 of this volume.

As the Minnehaha Creek watershed develops, small increases are expected in the pollutant loads generated from the changing land uses, as summarized in Table IV.L.5-1. Impervious cover is expected to increase only slightly (Figure IV.L.2-2). In order to maintain current pollutant loading rates, about 800 lbs. per year of phosphorus will need to be removed in the watershed. Similar relative increases in total nitrogen and total suspended solids will also have to be eliminated (Table IV.L.5-1). These load reduction targets should be spread out over the entire watershed to avoid the detrimental cumulative effects of development.

Table IV.L.5-1 Minnehaha Creek Watershed Pollutant Load Summary TP load (lbs/yr) TN load (lbs/yr) TSS load (lbs/yr) % % % Existing 2020 Increase Existing 2020 Increase Existing 2020 Increase Increase Increase Increase 9,812 10,623 811 8% 40,717 43,328 2611 6% 2,928,661 3,236,981 308,320 11%

Under current conditions, TP loads (per unit area) in the Minnehaha Creek (lower) watershed are lower in the western portion, with several areas of high pollutant loads throughout the rest of the watershed (Figure IV.L.5-1). TP loads are not predicted to increase substantially by the year 2020.

The TN and TSS loads follow a similar pattern (Figures IV.L.5-2 and IV.L.5-3), with lower loads in the western portion of the watershed. Management implications for these modeling results are discussed in Section L.6: Recommendations.

MCWD H/H and Pollutant Loading Study – 2003 L-68 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Existing Conditions 2020 Conditions

Watershed Boundary

Non-contributing Areas

Lakes

Minnehaha Creek

Watershed TP Loads (lbs/ac-yr): 0.0 - 0.13 1 0 1 Miles 0.13 - 0.2 0.2 - 0.28 Figure IV.L.5-1 N 0.28 - 0.35 Minnehaha Creek Watershed 0.35- 1.0 Total Phosphorus Loads

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MCWD H/H and Pollutant Loading Study – 2003 L-69 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Existing Conditions 2020 Conditions

Watershed Boundary

Non-contributing Areas

Lakes

Minnehaha Creek

Watershed TN Loads (lbs/ac-yr): 0.0 - 0.9 1 0 1 Miles 0.9 - 1.2 1.2 - 1.5 Figure IV.L.5-2 N 1.5 - 2.0 Minnehaha Creek Watershed 2.0 -7.8 Total Nitrogen Loads

H&H Report/Basins/Projects/pload_030417_kl

MCWD H/H and Pollutant Loading Study – 2003 L-70 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Existing Conditions 2020 Conditions

Watershed Boundary

Non-contributing Areas

Lakes

Minnehaha Creek

Watershed TSS Loads (lbs/ac-yr): 0 - 40 101Miles 40 - 60 60 - 80 Figure IV.L.5-3 N 80 - 110 Minnehaha Creek Watershed 110 - 445 Total Suspended Solids Loads

H&H Report/Basins/Projects/pload_030417_kl

MCWD H/H and Pollutant Loading Study – 2003 L-71 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion L.5.b. Lake Modeling and Associated Goals

This section summarizes the lake modeling results for the lakes within the lower watershed that were modeled. WiLMS input parameters (see Volume II: Framework and Methodology) are presented in Table IV.L.5-2, and the lake modeling results are presented in Table IV.L.5-3.

For these lakes, the current load to each lake is derived from the observed in-lake concentration, along with lake and watershed characteristics (see Volume II. Framework and Methodology, Section F for a complete explanation). The mean annual runoff used for the lake modeling was 4.81 inches for existing conditions and 4.91 inches for 2020 conditions, the same values used in the watershed pollutant loading model. The runoff volume increases as development fills in currently undeveloped parts of the watershed.

Table IV.L.5-2 WiLMS Input Parameters Mean Watershed TP Load Lake Area Volume Drainage Lake Depth to Lake (lbs./year) (acres) (ac-ft) Area (ac) (ft) Existing 2020 Brownie 18 396 22 373 40 41 Cedar 170 3400 20 2513 107 114 Isles* 103 927 9 3525 233 259 Calhoun 421 14,651 35 6765 346 390 Harriet 353 10,061 29 8328 407 462 Nokomis* 204 2876 14 2633 537 569 Hiawatha* 504 788 15 111,477 13,256 --** Diamond* 54 162 3 689 225 249 Powderhorn* 11 44 4 320 119 138 * On MPCA 303d list of “impaired waters” for excess nutrients; see also Table IV.L.5-6. **A 2020 estimate of the TP load to Lake Hiawatha was not possible, due to the fact that Minnehaha Creek was not modeled, and therefore a 2020 estimate of the load coming from the creek was not available.

MCWD H/H and Pollutant Loading Study – 2003 L-72 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

Table IV.L.5-3 Lake Modeling Results Observed (µg/L) Predicted 2020 TP (µg/L)

Observed Based on modeled Lake “Adjusted 2020” TP (June- Years averaged watershed estimate* Sept mean) loadings

Brownie 38 1997 - 2001 41 Cedar 22 1997 - 2001 22 Isles 38 1997 - 2001 40 Calhoun 21 1997 - 2001 26 Harriet 23 1997 - 2001 24 Nokomis 64 1997 - 2001 65 Hiawatha 70 1997 - 2001 --** Diamond 141 1997 - 2000 152 Powderhorn 172 1997 - 1999, 2001 195 *See methodology, Volume II, section F.2: Modeling, water quality. **A 2020 estimate for Lake Hiawatha in-lake TP was not possible, due to the fact that Minnehaha Creek was not modeled, and therefore a 2020 estimate of the load coming from the creek was not available.

Table IV.L.5-4 reviews the lake goals recommended by Regional Team 1 & 2 and identifies the total phosphorus loads that correspond to the RT recommendations. Additionally, the percent load reduction necessary to achieve the desired goal is presented.

The priorities for water quality goal establishment set by RT 1 & 2 were:

• The goals should reflect the water quality relationships between Grays Bay, Minnehaha Creek, Lake Hiawatha and the Mississippi River.

• Where the current lake water quality is better than the District’s water quality goals, then the goal should be revised to current conditions, or better.

• Where local water quality studies exist, then the MCWD should set water quality goals that are consistent with the goals of the local study.

MCWD H/H and Pollutant Loading Study – 2003 L-73 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion • If Minnehaha Creek backflows into a lake or wetland, then the water quality of the Creek should not be degraded by the quality of the lake, nor should the water quality of the Creek negatively affect the quality of the lake.

• Water quality goals should be established for Minnehaha Creek.

• Water quality goals for lakes without recent studies should remain unchanged until a reduced goal is justified based on detailed analyses.

More information regarding the RT 1 & 2 goal recommendations can be found in Volume III: Public Involvement, B. Regional Team 1 & 2.

Table IV.L.5-4 Minnehaha Creek Watershed Goals and Target Loads TP Goal (µg/L) TP Load to Lake (lbs/yr) Required % Current Load Load Proposed Reduction in Lake MCWD (calculated from either Goal Regional Load 1997 PLOAD estimate or (calculated Team 1 and (current vs. goal) Goal observed in-lake from RT 2 TP Goal concentration) goal) Brownie 50 35 40 35 13 Cedar 50 25 107 133 0* Isles 50 40 233 249 0* Calhoun 30 25 346 468 0* Harriet 30 20 407 325 20 Libbs none 30 --** -- -- Nokomis 50 50 537 372 32 Hiawatha 50 50 13,256 8795*** 34 Diamond none 90 225 185 18 Powderhorn 90 120 119 79 34 *For those lakes where the load goal > the current load, the required % reduction was noted as 0. **Libbs Lake was not modeled as part of this study. ***Assumes no short-circuiting in lake; see text for further explanation.

Minneapolis Chain of Lakes The Minneapolis Chain of Lakes Clean Water Partnership Project implemented a series of water quality improvements, including construction of two stormwater basins, increased frequency of

MCWD H/H and Pollutant Loading Study – 2003 L-74 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion street sweeping, extensive public education, and in-lake alum treatment. The current water quality of these lakes has improved to conditions that are better than the current MCWD goals. For these lakes (Harriet, Calhoun, Isles, Cedar and Brownie) the Regional Team has recommended that the MCWD formally adopt the goals that were established at the start of this successful partnership. The recommended goals are listed in Table IV.L.5-4.

Libbs Lake Based on a classification system used by the Minnesota Pollution Control Agency, Libbs Lake is safe for all activities, including swimming. However, this lake has the potential for degradation based on a high percentage of urbanized area within its watershed and minimal best management practices being utilized in its watershed. Therefore it is recommended that a water quality goal be established at the observed in-lake phosphorus concentration of 30 µg/l, and that measures be taken to ensure that future degradation does not occur.

Lake Nokomis The observed phosphorus concentration for Lake Nokomis is an average of the years 1997 through 2001 In 2001 the MCWD completed construction of the three stormwater wetlands recommended by the Blue Water Commission. It is recommended that the in-lake phosphorus concentration goal of 50 µg/l be reassessed in 2005 to reflect the actual improvements achieved in this lake. A second recommendation by RT 1 & 2 is to reassess the need for in-lake alum treatment.

Lake Hiawatha Over 90% of the phosphorus load to Lake Hiawatha is directly contributed by Minnehaha Creek. The 1999 mean in-lake TP was 48 µg/l, thus meeting the current goal, while the 2000 mean reached 108 µg/l, over twice the concentration as the year before. During the year 2000, flow in Minnehaha Creek was low, with no flow discharging from Grays Bay dam. This results in variability in both the annual load and the reductions necessary to achieve the in-lake TP goal – ranging from 0% for the 1999 data to approximately 80% for the 2000 data. This load analysis is preliminary due to the lack of data regarding the amount of inflow short-circuiting that occurs as the creek flows through Lake Hiawatha. If 27% short-circuiting is assumed (as reported in the

MCWD H/H and Pollutant Loading Study – 2003 L-75 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Lakes Nokomis and Hiawatha Diagnostic-Feasibility Study, February 1998), then the three year average (1999-2001) load reduction needed to achieve the lake water quality goal is only 9%, while if no short-circuiting is assumed, the average load reduction necessary increases to 33%. Load reductions to the lake are more crucial during the low flow years, when a greater percent of the creek water consists of stormwater runoff, with little dilution from Grays Bay outflow. Thus, management practices to achieve a long-term in-lake TP goal of 50 µg/l need to be focused on stormwater runoff throughout the entire RT 1 & 2 drainage area; if stormwater runoff water quality is improved, the water quality of both Minnehaha Creek and Lake Hiawatha should also be improved. However, these improvements should be more apparent during low flow years, due to the lower than average dilution effect from Lake Minnetonka outflow.

This load analysis underscores the importance of examining the water quality data on a year-to- year basis. Patterns emerge that would not have been evident when looking at either long-term averages or only a single year of monitoring data.

Diamond Lake There are is no existing in-lake phosphorus goal for Diamond Lake. RT 1 & 2 recommends that a goal be established that ensures that the water from Diamond Lake does not increase the phosphorus concentrations within Minnehaha Creek. The predicted in-lake TP concentration based on land use (WiLMS model) is 157 µg/L, and the predicted concentration based on ecoregion (MNLEAP model) is 74 µg/L, compared to the observed concentration of 141 µg/L. This case is similar to that of Powderhorn Lake in that they are both shallow lakes with high phosphorus loads due to the highly residential land use in their watersheds. A combination of residential BMPs and shallow lake management strategies are recommended to improve the water quality of Diamond Lake.

Powderhorn Lake The predicted in-lake TP concentration for Powderhorn Lake based on current land use (WiLMS model) is 201 µg/L, and the predicted concentration based on ecoregion (MNLEAP model) is 82 µg/L, compared to the observed concentration of 172 µg/L. This suggests that the water quality of the lake is approximately what we would expect to see based on current land use, but worse

MCWD H/H and Pollutant Loading Study – 2003 L-76 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion than average ecoregion values for a lake of Powderhorn’s size and watershed area. In 1999 the Powderhorn Lake Neighborhood along with the Minneapolis Park and Recreation Board studied the quality of Powderhorn Lake and concluded that a goal of 120 µg/l was obtainable. BMPs were implemented, including the construction of grit chambers in each storm drain that discharges to the lake. The current MCWD water quality goal of 90 µg/l for Powderhorn Lake is a general recommendation based on what could be accomplished for a lake with a mean depth of four feet. Therefore, Regional Team 1 & 2 recommends that the MCWD adjust its goal to be consistent with the local goal (120 µg/L) developed through the more detailed study of the lake.

Minnehaha Creek The total phosphorus analysis for Minnehaha Creek was performed with monitoring data from the Chicago Avenue monitoring station. This station was chosen based on the fact that it is located towards the lower portion of the watershed, and therefore the monitoring data reflect the majority of the watershed, yet it is located before Lake Hiawatha where in-lake phosphorus settling likely occurs. Table IV.L.5-5 presents the observed total phosphorus concentration along with the recommended goal.

Table IV.L.5-5 Minnehaha Creek Total Phosphorus Concentration – Observed and Goals Observed TP Proposed (µg/L, at Chicago MCWD 1997 Regional Team 1 Years averaged Ave. monitoring Goal and 2 TP Goal site) (µg/L) 107 1997 - 2001 none 80

The recommended goal for Minnehaha Creek is based on several considerations:

1) What concentration in the creek is necessary for Lake Hiawatha to attain its goal of 50 µg/L? If no short-circuiting of the creek as it passes through Hiawatha is assumed, an in- stream TP average of 61 µg/L (calculated with data from 1999-2001) at the Chicago Ave. monitoring site would allow Lake Hiawatha to achieve its goal. If 27% short-circuiting is assumed, then an in-stream TP average of 86 µg/L would achieve the same goal.

MCWD H/H and Pollutant Loading Study – 2003 L-77 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion 2) What is the EPA recommended in-stream total phosphorus concentration for streams? The EPA recommended goal for any stream that discharges into a lake is 50 µg/L, and the recommendation for any stream that does not discharge into a lake is 100 µg/L.

3) What is the best, realistic scenario that we can hope for in the creek, taking into account BMP limitations? This analysis was based on the concept of “irreducible concentrations,” or the background level of stormwater pollutants that represents the best that can be achieved through current technology. The values 100 µg/L and 200 µg/L were used as two estimates of the irreducible total phosphorus concentration (values from Technical Note #75, Watershed Protection Techniques; 2(2):369-372) of stormwater runoff originating in the watershed, between the monitoring sites at I-494 and Chicago Avenue. In-stream monitored loads originating above I-494 were added into the calculation. This analysis indicated that if the TP concentration in stormwater runoff that reaches the creek were 100 µg/L, the concentration in the creek would be 62 µg/L (a five year average using monitored volumes and loads from each year separately). If the stormwater runoff were 200 µg/L, the concentration in the creek would be 101 µg/L. Keep in mind that the “irreducible concentration” is an average across the watershed. Since this geographical area includes the Chain of Lakes watershed, which has a low TP concentration in its outflow (average of 23 µg/L), it allows for other areas of the watershed to have TP concentrations higher than the stated irreducible concentration. According to this analysis, in order to reach an in-stream concentration of approximately 80 µg/L (to achieve the goal for Lake Hiawatha), the average TP concentration of runoff (including the outflow from the Chain of Lakes) would need to be 150 µg/L.

4) What is the average TP concentration in runoff between I-494 and Chicago (monitored and modeled)? The irreducible concentrations were then compared to the average total phosphorus concentration in runoff in the same geographic area, calculated from in-stream monitoring data. The five year average (1997-2001) of the monitoring data was 176 µg/L. This concentration is “post-treatment,” in that it represents the concentration of stormwater that reaches the creek, as opposed to the concentration of stormwater runoff originating directly from the watershed before it passes through a

MCWD H/H and Pollutant Loading Study – 2003 L-78 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion water body or treatment system. Additionally, this concentration takes into account in- stream processes, such as deposition, erosion, and internal loading. Therefore, the concentration of runoff from the watershed is likely higher than 176 µg/L. This figure can be compared to the average TP concentration in runoff predicted by the PLOAD model developed for the watershed. This flow-weighted event mean concentration is 266 µg/L. Unlike the previous estimate, this estimate is “pre-treatment,” and the concentration of runoff that actually reaches Minnehaha Creek is likely to be lower than the 266 µg/L. These figures all indicate that the average TP concentration in runoff that eventually reaches the creek is likely close to 176 µg/L. Using monitored creek volumes, runoff of this quality would lead to an in-stream concentration of 90 µg/L, slightly lower than the five year average of 107 µg/L. Therefore, if we use a middle value of 150 µg/L for the ideal “irreducible concentration” of stormwater runoff, the same calculations suggest that in-stream concentrations would be reduced to approximately 81 µg/L. This would mean that the stormwater runoff reaching the creek would have to be reduced from the current average of 176 µg/L to 150 µg/L, or by 15%.

Even though Minnehaha Creek flows into Lake Hiawatha, the RT felt, over the short-term, that a goal of 50 µg/L (as suggested by EPA recommendations) was unrealistic, and that the middle ground of 80 µg/L would be more appropriate. The creek’s annual TP concentration average is usually below this threshold of 80 µg/L, except during years with low amounts of runoff. Therefore, management strategies to achieve this goal will need to be focused on stormwater runoff, which will more dramatically benefit the creek during the low-flow conditions (see above Lake Hiawatha discussion). Additionally, as the Lake Hiawatha load analysis indicates (question #1 from above), this in-stream concentration of 80 µg/L should allow Hiawatha to achieve its water quality goal of 50 µg/L. The exact inflow concentration necessary for the lake to achieve this goal is dependent on the amount of short-circuiting that the creek experiences as it flows through the lake. This load analysis will therefore be able to be refined when more information regarding creek-lake mixing becomes available.

In addition to the lack of information on the creek-lake mixing, there is limited information regarding the in-stream processes of Minnehaha Creek. The morphology of the creek’s several

MCWD H/H and Pollutant Loading Study – 2003 L-79 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion ponded reaches suggests that internal loading might be a factor in the annual phosphorus budget. This internal loading could be exacerbated during low flow years due to stagnant water in the ponds leading to greater amounts of algal growth, and thus higher oxygen demand above the sediments. Another potential factor in the creek’s phosphorus budget is in-stream erosion. The amount and variability of the phosphorus load originating from erosion is unknown. A Minnehaha Creek Diagnostic Study would clarify the contribution of each of these phosphorus sources, in addition to providing guidance on management strategies to improve the water quality of both the creek and Lake Hiawatha.

L.5.c. MPCA Impaired Waters and Point Source Permits

Within the Minnehaha Creek watershed, several water bodies are on the MPCA’s 303(d) list of impaired waters. Table IV.L.5-6 outlines these listed water bodies.

Table IV.L.5-6 Impaired Water Bodies in the Lower Watershed Impairment TMDL Development Target PCB FCA Lake Mercury/FCA Excess nutrients Start/Completion Dates for (Aquatic (Aquatic life)* (swimming) Nutrient-Impaired Waters life)* Brownie Cedar Isles 2004/2008 Calhoun Harriet Nokomis 2004/2008 Hiawatha 2005/2009 Diamond 2006/2011 Powderhorn 2006/2011 *Target start/completion dates for Mercury/FCA and PCB/FCA listed waters are all 2002/2015.

MCWD H/H and Pollutant Loading Study – 2003 L-80 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Today, very few point source discharges of any treated material occur in the entire Minnehaha Creek Watershed. Of the six discharges that currently exist, four are located in the lower watershed, as illustrated in Figure IV.L.5-4. The discharges are described in Table IV.L.5-7. All of the discharges comply with MPCA discharge requirements.

MCWD H/H and Pollutant Loading Study – 2003 L-81 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

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MCWD H/H and Pollutant Loading Study – 2003 L-82 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

Table IV.L.5-7 MPCA Permitted Point Source Discharges in the Lower Watershed Permit Holder Discharge Type and City Pollutants Discharged (permit number) Receiving Water Ceridian Corp. Bloomington Contaminated groundwater Permitted for: 1,1,1-Trichloroethane; 1,1,2,2- (MN0051942) pump-out; Minnehaha Creek Tetrachloroethane; 1,1-Dichloroethane; 1,1-Dichloroethylene; 1,2-Dichloroethane; 1,2-Dichloroethylene; pH; Flow; Tetrachloroethylene; and Vinyl Chloride

Former TPI Petroleum Minneapolis Contaminated groundwater Benzene; Ethylbenzene; Organics (diesel); Organics (gasoline); Facility (MNG790034) pump-out; Minnehaha Creek pH; Flow; Toluene; and Xylenes (total) Northland Aluminum St. Louis Park Non-contact, chlorine cooling Chlorine; pH; Flow; and Temperature Products, Inc. water; Bass Lake to Lake (MNG255027) Calhoun St. Louis Park St. Louis Park Contaminated groundwater Anthracene; Fluoranthene; Iron; Manganese; pH; Flow; Wastewater Treatment pump-out and industrial Phenanthrene; Phenols; Carcinogenic PAHs; and Non- (MN0045489) process wastewater; South carcinogenic PAHs Oak Lake and storm sewer to Minnehaha Creek

MCWD H/H and Pollutant Loading Study – 2003 L-83 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

L.6. Recommendations

The Lower Watershed of Minnehaha Creek encompasses some 25 main lakes and 22 miles of Minnehaha Creek. The Lower Watershed includes the largest area of densely developed urban land in the MCWD and for much of its length, Minnehaha Creek is the center piece of park systems, particularly for the City of Minneapolis. Because of the heavy public use in this area, all of the lakes as well as the Minnehaha Creek is given a high priority.

To address the load reduction needs identified in Table IV.L.5-4 and to incorporate the management alternatives in Table III.B-3 (Volume III: Public Involvement, B. Regional Team 1 & 2), the management scheme outlined in Table IV.L.6-1 is proposed for the Minnehaha Creek (“Lower”) watershed. Details of the recommendations follow the table. Recommendations applicable to the entire MCWD are discussed in Volume V: Watershed Issues Integration.

Table IV.L.6-1 Minnehaha Creek Watershed (“Lower Watershed”) Recommended Actions Category

Receiving Recommended Action Water Body Priority Maintenance Responsible Party* Capital Improvement Information /Education Permitting/Enforcement Monitoring/Investigation Monitoring/Investigation 1) Create shoreline stabilization Minnehaha High X X X A, B, E program Creek 2) Retrofit new stormwater A, B, practices into redeveloping urban All High X X D, E areas All listed under 3) Preserve and manage L.3. b landlocked basins (maximize High X X A, B Drainage infiltration, bounce, and retention) Routing

MCWD H/H and Pollutant Loading Study – 2003 L-84 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Category

Receiving Recommended Action Water Body Priority Maintenance Responsible Party* Capital Improvement Information /Education Permitting/Enforcement Monitoring/Investigation Monitoring/Investigation Upper Minnehaha Creek (mostly 4) Preservation of smaller Minnetonka, High X X A, B landlocked pockets some St. Louis Park, Hopkins and Edina areas) Minnehaha Creek Mississippi 5) Install energy dissipation and River erosion control at outfalls with Lake of the Isles High X X B, D high pipe velocities Lake Calhoun Lake Harriet Diamond Lake Legion Lake Pamela Lake 6) Investigate watershed boundary High X X A near Southdale Mall Minnehaha Creek Isles 7) Develop TMDL allocations and Nokomis management strategies for lakes Hiawatha High X X X X X A. B with “excess nutrient” impairments Diamond Powderhorn Minnehaha Ck. Brownie Cedar 8) Revise water quality goals for Isles A, B, Minnehaha Creek and lakes of the High X X E, F lower watershed Calhoun Harriet Diamond Powderhorn

MCWD H/H and Pollutant Loading Study – 2003 L-85 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Category

Receiving Recommended Action Water Body Priority Maintenance Responsible Party* Capital Improvement Information /Education Permitting/Enforcement Monitoring/Investigation Monitoring/Investigation 9) Develop Implementation Plan Minnehaha A, B, to Improve Minnehaha Creek and Creek High X X X X X E, F Lake Hiawatha Lake Hiawatha 10) Implement flood mitigation policies for flooding along Minnehaha A, B, Medium Minnehaha Creek and within Creek E, F neighborhoods Brownie Lake 11) Simulate back-to-back 100- Cedar Lake year storm events on Chain of Lake of the Isles Medium X X X X X A, B Lakes Calhoun Harriet Spring Lake Windsor Lake Wetland 27- 12) Implement volume control 712W standards in all subwatersheds Cedar Manor Medium X X A, B draining to or containing Lake landlocked depressions Hannan Lake Wolfe Park Pond

13) Stormwater rate reduction Minnehaha Medium X X A, B, D downstream of Browndale Dam Creek 14) Coordinate with Cities’ A, B, “Roadway Reconstruction All Medium X X D, F /Infrastructure Upgrade” program Lake Nokomis 15) Conduct a Nokomis weir Medium X A operation investigation Minnehaha Creek 16) Conduct a Legion Legion Lake Medium X X A, B Lake/Infiltration Capacity Study Lake Nokomis

MCWD H/H and Pollutant Loading Study – 2003 L-86 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Category

Receiving Recommended Action Water Body Priority Maintenance Responsible Party* Capital Improvement Information /Education Permitting/Enforcement Monitoring/Investigation Monitoring/Investigation 17) Knollwood Plaza Stormwater Minnehaha Medium X X A, B Improvements Creek 18) Create incentives and/or matching grants for property owners (commercial and residential) that are willing to All Medium X X X A, B, E create innovative or infiltration practices to benefit runoff water quality 19) Further strengthen water quality and construction site BMP All Medium X X A, D, G requirements for future highway projects *Responsible party: A – MCWD, B – City, C – Three Rivers Park District, D – Mn/DOT, E – Private Landowners, F- DNR, G- County

1) Create shoreline stabilization program. One important source of water quality degradation is the continual erosion of soils into the creek and lakes. It is recommended that a comprehensive approach to shoreline management be created that establishes incentives for proper shoreline buffers, investment by public agencies in the most severe problem areas, and a balanced regulatory approach. Key to these recommendations is investments in matching grants and demonstration projects. Key components of this program include: a. Investment in shoreline stabilization in areas of most severe erosion b. Creation of matching grant program for private property owners willing to invest in creation of shoreline buffers (commercial and/or residential properties). c. Adoption of realistic shoreline buffer requirements that respect the space constraints of fully developed communities.

MCWD H/H and Pollutant Loading Study – 2003 L-87 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion 2) Retrofit new stormwater practices into redeveloping urban areas. As redevelopment opportunities arise, low impact development approaches should be incorporated into the stormwater management design. Pollutant load reduction goals for the lakes in the lower watershed and Minnehaha Creek could be partly met through this approach.

3) Preserve and manage landlocked basins (maximize infiltration, bounce, and retention). Numerous natural wetlands and depressions characterize large portions of the upper Minnehaha Creek Watershed in Minnetonka, the west half of St. Louis Park, parts of Hopkins, and the area south of Meadowbrook Lake in the City of Edina. Some of these depressions contain wetlands or vernal pools, while others quickly lose water to infiltration or evapotranspiration. Landlocked subwatersheds are identified in Figure IV.L.1-2 and in Table IV.L.3-2. Maintaining existing hydrology and functions of these depressions will minimize potential downstream flooding and pollutant loading to receiving waters. This can be achieved through a combination of infiltration and volume control practices in the watershed as development occurs, along with specific management practices of the landlocked depressions. Management strategies recommended for landlocked basins include: a. Design of 2-stage, drop outlet facilities that mimic natural conditions by maximizing bounce, retention, and infiltration in the basin. Where sensitive wetlands are present (wetlands designated as “preserve” in the Functional Assessment of Wetlands), stormwater pollutant loading and increases in bounce should not exceed MN Stormwater Advisory Group guidelines. These outlet structures would also include controlled emergency overflow and draw down maintenance gates. b. Incorporate low maintenance infiltration enhancement techniques in the basin (i.e. infiltration gravel trenches, perforated tubes, subsoil unconnected drain tiles etc.) to ensure long-term performance. c. Vegetation management to promote deep-rooted natural species and capillary suction and evapotranspiration at all hydrologic regimes in the basin.

MCWD H/H and Pollutant Loading Study – 2003 L-88 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion 4) Preservation of smaller landlocked pockets. Areas containing smaller landlocked depressions in the watershed not explicitly modeled help reduce watershed impacts by minimizing downstream discharge rates, volumes, and transfer of sediment loads. Their preservation is important to minimize development impacts to downstream water bodies. To the extent possible, it is recommended that the smaller landlocked pockets in the watershed be retained, or their function be retained as the area develops.

5) Install energy dissipation and erosion control at outfalls with high pipe velocities High pipe velocities predicted are listed in Table IV.L.3-3. Appropriate structural improvements will vary by site. Alternatives for energy dissipation may include: a. Upstream ponding. The addition of upstream storage capacity in a stormsewer system allows for temporary extension of storage time and slower release of flows. This option also helps reduce peak rates. b. In-pipe energy dissipation. Several devises have been used to reduce in-pipe velocities. Examples include various configurations of baffles and orifice rings within an expanded section of pipe. c. Outlet energy dissipation. Apron configuration, submergence or partial submergence of outlet, plunge pools and baffles are all examples of methods used for energy dissipation at an outlet.

6) Investigate watershed boundary near Southdale Mall As part of this study, modeling results appear to indicate that Southdale Mall, a portion of Highway 62, and some of the commercial area surrounding the Southdale Mall drain to an equalized pond system (Point of France Pond, Swimming Pool Pond, and Garrison Pond). Based on this information, it appears that areas currently outside of the MCWD jurisdictional boundary drain, at least partially, into Minnehaha Creek. It is recommended that a thorough investigation be conducted to clarify drainage boundary issues and assess the accuracy of the current jurisdictional boundary.

MCWD H/H and Pollutant Loading Study – 2003 L-89 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion 7) Develop TMDL allocations and management strategies for lakes with “excess nutrient” impairments. This report identifies load allocation requirements to meet phosphorus load reduction goals. The MCWD should work with local units of government to implement specific load reduction strategies to meet water quality goals for these lakes. Strategies include shoreline buffers, residential BMPs to achieve water quality improvements, use of rain gardens, infiltration and incorporation of new or additional stormwater practices into redeveloping areas.

8) Revise water quality goals for Minnehaha Creek and lakes of the lower watershed. As outlined in Table IV.L.5-4, water quality goals for Minnehaha Creek and many of the lakes in the lower watershed should be changed to reflect current water quality conditions. The District should consider revision of these recommended water quality

goals as part of its 509 Plan update.

9) Develop Implementation Plan to Improve Minnehaha Creek and Lake Hiawatha. Minnehaha Creek is often viewed as a storm drain outlet for discharge of runoff from the tributary communities. The HHPLS analysis views the creek as a resource which must be protected and as the primary hydrologic and pollutant input to Lake Hiawatha. Recommendations for management of the creek should be based on usage goals (recreation, flood protection, and protection of unique resources), rather than restoration of pre-development conditions. It is recommended, therefore, that the water quality management and flood management be inter-related and administered as combined rather than separate projects. This can be accomplished by thorough study of the creek and creation of an Implementation Plan that would define those capital improvement projects that will improve the water quality of Minnehaha Creek and/or create flood retention in areas defined with additional flood storage capacity. The Implementation Plan should

include the following key elements: a. Water quality monitoring of in-stream pond-like reaches to determine which ponds release phosphorus to creek flows vs. which ponds remove phosphorus (and sediment) from the creek.

MCWD H/H and Pollutant Loading Study – 2003 L-90 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion b. Mixing study of Lake Hiawatha to determine the percentage of Minnehaha Creek flows which mix into the lake and the percentage of flows that bypass the lake. (Previous studies had made assumptions based on theory and not on field data). c. Identification of subwatersheds which are major contributors of phosphorus to Minnehaha Creek and Lake Hiawatha. Investigate the feasibility of water quality improvement projects in these subwatersheds. d. Identification of areas within Minnehaha Creek and adjacent floodplains which have unused flood storage capacity. Identify areas within the floodplain of Minnehaha Creek that have been hydraulically disconnected. Investigate the feasibility of reconnection for purposes of both flood mitigation and water quality improvement. Define specific projects. e. Identification of areas within the floodplain of Minnehaha Creek with inadequate vegetative buffers and/or excessive sedimentation. Define specific projects. f. Identification of wetlands tributary to Minnehaha Creek that are critical to the water quality of the creek. Define projects and regulatory protections to ensure long-term health of these wetlands. g. Compilation and prioritization of all identified projects into a comprehensive Minnehaha Creek Implementation Plan with a menu of projects and preliminary cost estimates for the MCWD to include in future CIPs. h. Identification of potential partners for each proposed project. i. Identification of water quality goals for Minnehaha Creek.

10) Implement flood mitigation policies for flooding along Minnehaha Creek and within neighborhoods. Hopkins, St. Louis Park, and Minneapolis have identified areas within their municipalities where regular, and sometimes severe flooding occurs after intense rainstorms. The cause of this flooding often is related to the hydraulic relationships between their municipal storm drainage system and Minnehaha Creek. The problems range in severity from the less severe regular flooding of intersections to the most problematic backflow of sewage into basements. Typically the solutions to this sort of flooding involve increased capacities of the storm drains and/or retention in new ponds.

MCWD H/H and Pollutant Loading Study – 2003 L-91 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Neither solution increases the volume of stormwater being discharged to Minnehaha Creek; however, each solution does impact the rate and timing of stormwater discharge to the creek. To compound this problem, the rainstorm that causes neighborhood flooding may also cause creek flooding.

It is recommended that the MCWD manage flooding using a holistic approach, whether it is along the creek or within a neighborhood. Flooding would be accepted as a natural occurrence that should be tolerated whenever the flooding does create structural problems. Flooding that results in water or sewage in buildings must be corrected. Participants of the lower watershed Regional Teams favor flood management strategies founded on a range of flooding acceptability. For example, flooding that simply overtops the creek bank without creating structural and/or health and safety problems should be allowable. When flood mitigation is determined to be necessary, then the project should also include features that ensure that downstream problems do not occur. MCWD should adopt flood mitigation policies that look beyond the boundaries of the creek and balance the hydraulic inter-relationship between the creek and the municipal drainage systems. Key components of recommended MCWD flood mitigation policies include: a. Flooding that overtops the creek banks without creating structural problems to bridges, buildings or other structures should be tolerated. b. Flooding that creates structural and/or health and safety problems must be mitigated. c. Flood mitigation projects should include measures that ensure downstream problems are not created where no current problem exists. d. MCWD should adopt a separate flood mitigation permitting process rather than fit flood mitigation projects into permitting processes more suited to development reviews. e. MCWD should include mitigation of health and safety problems as criteria when evaluating flood mitigation projects. f. MCWD should accept additional rate of stormwater discharge to Minnehaha Creek in segments of the Creek where the hydraulic capacity exists and does not create downstream problems.

MCWD H/H and Pollutant Loading Study – 2003 L-92 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion g. MCWD should require flood capacity compensation when accepting increased rates of stormwater discharge to Minnehaha Creek for all new stormwater discharges to Minnehaha Creek, regardless of whether the new discharges are related to increased development or neighborhood flood mitigation.

11) Simulate back-to back 100-year storm events on Chain of Lakes. The combined upper basin has an extended draw down time which exceeds one month to return to the NWL. The lakes were modeled starting at their NWL (851.9 feet NGVD 1929); however, because of the slow draw down, it is highly probable that initial water levels will be elevated prior to storm events. In order to define more conservative HWLs, the upper chain can be modeled differently from other lakes in the MCWD. To develop more conservative HWLs, the upper chain could be treated differently from other lakes in the MCWD model. Simulating the 100-year return events on slightly elevated initial water surface conditions is an option given the lengthy draw down time. In any case, the HWLs for the upper Chain of Lakes determined in this report would only be increased by 0.2 – 0.3 feet. Other modeling alternatives could include simulation of back-to-back 100- year storm events.

12) Implement volume control standards in all subwatersheds draining to or containing landlocked depressions. Landlocked basins are particularly sensitive to additional stormwater volumes. As development occurs, special emphasis should be given to volume control regulation within all subwatersheds containing or draining to landlocked basins and/or pocket wetlands. Simple runoff volume management techniques like rain gardens, infiltration swales, or dry ponding are strongly recommended in those areas to mimic natural watershed hydrology and control the runoff volumes discharged into landlocked basins. Local soils and groundwater issues (see Figure IV.L.2-6 Minor Watersheds Infiltration Potential) should be considered at the design and review (permitting) phase to assess the suitability, placement, and sizing of these runoff volume reduction techniques.

MCWD H/H and Pollutant Loading Study – 2003 L-93 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion 13) Stormwater rate reduction downstream of Browndale Dam. A significant increase in Minnehaha Creek flow and velocity can be observed between the Browndale Avenue dam and the WOMP station (at 32nd Avenue S) monitoring locations. The erosion and scour analysis also indicates that this portion of the creek contains many areas of high erosion potential.

14) Coordinate with Cities’ “Roadway Reconstruction/Infrastructure Upgrade” programs. This plan should incorporate stormwater management improvements and low impact designs as road and stormsewer infrastructure are maintained and upgraded. The MCWD should coordinate with the Cities’ CIP and Maintenance Plans to look for opportunities to collaborate and reduce downstream impacts. Emphasis should be place in areas draining to highly erosive segments of the creek (see Table IV.E.4-1) or stormsewer systems with high velocities (Table IV.L.3-3). Re-grading of streets to include rain-gardens (“Gutters to Gardens”) in areas of high infiltration potential (Figure IV.L.2-6) will help mitigate high discharge and velocity rates in Minnehaha Creek and reduce runoff volumes and sediment transport from streets.

15) Conduct a Nokomis weir operation investigation. Simulation of the 10-year, 24 hour rainfall event (4.2 inches) on a 20 cfs base-flow showed that flows from Minnehaha Creek initially backflow into Lake Nokomis. Following the peak attenuation into Lake Nokomis, flows reverse and discharge moves in the direction of Lake Nokomis to Minnehaha Creek. This pattern was also observed during simulation of the 100-year rainfall and 100-year snowmelt events and would indicate that the inflatable weir does not prevent flows exceeding the 5-year recurrence from entering Lake Nokomis. Base on the new and more accurate model, it is recommended that the effectiveness of the Nokomis weir as currently designed be reassessed.

16) Conduct a Legion Lake/Infiltration Capacity Study. Legion Lake is a unique area with naturally high infiltration/groundwater recharge capacity. Potential exists to take advantage of the basin’s natural infiltration capacity to

MCWD H/H and Pollutant Loading Study – 2003 L-94 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion further reduce the runoff volume discharged into Lake Nokomis. The District, Minneapolis, and Richfield could co-sponsor an assessment/feasibility study to better determine Legion Lake’s current infiltration capacity and hydrologic regime, and determine any potential infiltration enhancement practices (i.e. trenching, vegetation management, etc.). The study could also look at potential stormwater re-routing options.

17) Knollwood Plaza Stormwater Improvements Knollwood Plaza encompasses one of the larger contiguous tracts of impervious surfaces in the lower watershed, yet generally lacks stormwater treatment facilities. The MCWD should work with the City of St. Louis Park to investigate options for retrofitting existing stormwater infrastructure and incorporating new practices wherever practical. In particular, the feasibility of installing large infiltration ponding facilities, should be investigated, since the area around Knollwood Plaza has a high infiltration potential. Additionally, vegetative buffers should be established along either side of Minnehaha Creek as it flows though this area.

18) Create incentives and/or matching grants for property owners (commercial and residential) that are willing to create innovative or infiltration practices to benefit runoff

water quality. Individual opportunities to improve the quality of runoff discharged should not be overlooked. Through the creation of matching grants and demonstration projects, the MCWD could lead the effort to rebalance the volumes of stormwater that infiltrates to the groundwater vs. the volume that is discharged to lakes and the creek. This would be coupled with the adoption of volume control requirements for new developments. Key

components include: a. Work with Lake Associations and Neighborhood Organizations to create one residential infiltration demonstration project in each minor watershed. b. Create matching grant program to financially support creation of infiltration practices on private property (residential and commercial). Funding priority could be given to those infiltration projects that remove runoff tributary to a flood mitigation project.

MCWD H/H and Pollutant Loading Study – 2003 L-95 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion c. Adopt volume control standards for new developments that require no net increase in volume discharged from site.

19) Further strengthen water quality and construction site management requirements for

future highway projects. Local highway departments appear to give little attention to water resources issues that are related to either the construction of new highways and/or maintenance of existing highways. It is recommended that highway departments implement stronger measures to control sedimentation during construction, such as controlling access in and out of construction zones and daily sweeping in areas where no other sediment control is possible. Further, these road authorities should implement structural and non-structural Best Management Practices for highways not programmed for future reconstruction. Key

components include: a. Strengthen sediment control permitting requirements for highway construction projects. b. Require post-project dredging for those projects where sediment control has failed and resulted in downstream sedimentation of water resource. c. Include long-term sweeping and other non-structural BMPs in all highway construction permits.

A complete presentation of the recommendations made for this watershed by Regional Team 1&2 can be found in Volume III: Public Involvement, B. Regional Team 1&2. This includes information regarding the priority of each issue, who would be responsible for undertaking each suggested management approach, and a recommendation of when the approach should be undertaken.

MCWD H/H and Pollutant Loading Study – 2003 L-96 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion