Hiawatha Golf Course Area Water Management Alternatives ...

4300 MarketPointe Drive, Suite 200 Minneapolis, MN 55435 952.832.2600 www.barr.com

Hiawatha Golf Course Area Water Management Alternatives Assessment

Water Management Alternatives

Prepared for Minneapolis Park and Recreation Board, City of Minneapolis, & Minnehaha Creek Watershed District

6/21/2017

Barr Engineering Co. 4300 MarketPointe Drive, Suite 200, Minneapolis, MN 55435 952.832.2600 www.barr.com

Technical Memorandum MPRB Hiawatha Golf Course Alternatives Assessment Project Water Management Alternatives To: Michael Schroeder, MPRB & Katrina Kessler, City of Minneapolis From: Jennifer Koehler, PE & Kurt Leuthold, PE Subject: Hiawatha Golf Course Area – Water Management Alternatives Date: 6/21/2017 Project: 23/27-1466.03 c: Della Schall Young, Paul Hudalla, Tyler Pederson, Rachel Crabb, Deb Pilger, James

Wisker

Table of Contents 1.0 Introduction ............................................................................................................................................................................. 2

2.0 Water Management Alternatives .................................................................................................................................... 2

3.0 Groundwater Pumping and Impact Assessment ................................................................................................... 12

4.0 Surface Water Evaluation and Impact Assessment ............................................................................................... 19

5.0 Water Quality Impacts ...................................................................................................................................................... 38

6.0 Summary of Water Management Alternatives ....................................................................................................... 43

To: Michael Schroeder, MPRB & Katrina Kessler, City of Minneapolis From: Jennifer Koehler, PE & Kurt Leuthold, PE Subject: Hiawatha Golf Course Area – Water Management Alternatives Date: 6/21/2017 Page: 2

P:\Mpls\23 MN\27\23271466 Hiawatha Golf Course Analyses\WorkFiles\2017Scope\AlternativeAssessment\ImpactAssessment\Memos\WaterManagement\Final\HiawathaGC_WaterManagementAlternatives_Final_06212017.docx

1.0 Introduction Barr Engineering Company (Barr) has been assisting the Minneapolis Park and Recreation Board (MPRB) and the City of Minneapolis (City) on the evaluation of the surface, storm, and groundwater management issues related to the Hiawatha Golf Course area. Minnehaha Creek Watershed District (MCWD) staff have also been involved with the current phase of this project as well. This memorandum summarizes additional water management analyses for the Hiawatha Golf Course area, building off work completed and summarized in a memo dated 2/28/2017.

As part of this phase of the project, Barr will be performing a detailed alternatives assessment of two selected alternatives for the Hiawatha Golf Course area, including one alternative that maintains the existing pumping rates and a second alternative that considers reduced pumping and modifications to water management in the golf course area.

Because the issues in the Hiawatha Golf Course area are primarily related to groundwater and surface water management, the first step in defining the two alternatives to be considered is identifying the preferred water management approach for the area. This memorandum summarizes the estimated impacts of the various water management alternatives considered. These analyses helped MPRB, City, and MCWD staff to identify the preferred water management alternatives to proceed with through the complete alternatives assessment process (which will be summarized in future memorandums and deliverables).

All elevations included in this memo are in NGVD 1929 vertical datum.

2.0 Water Management Alternatives This section outlines the main water management objectives and a summary of each of the various water management alternatives.

2.1 Objectives Public input was collected during the development MPRB Nokomis-Hiawatha Regional Park Master Plan (the Plan, 2015), which included public preference related to water management. Although the Plan did not specifically include the Hiawatha Golf Course area, the public input indicated that water quality of the resources (Lake Hiawatha, Lake Nokomis, and Minnehaha Creek) are of high concern for the regional park users and included the reduction of trash in Lake Hiawatha. The Plan also indicated preference for the development of stormwater treatment areas/wetlands, utilizing a more naturalized style of landscaping (replacing approximately half the turf area in strategic locations with native plantings and wildlife habitat), and continuing to look for shoreline restoration opportunities.

Based on initial discussions with the MPRB and City staff, the following objectives were identified as the highest priority for the water management alternatives:



1. Reduce pumping while still protecting adjacent basements 2. Continue to provide water quality treatment for the watershed to the west of the golf course 3. No increases in surface flooding in the adjacent neighborhoods, or potential improvement to

local flood areas 4. Capturing trash from the larger Hiawatha watershed

Further discussions with MCWD staff also identified trying to maximize the opportunity for water quality treatment to help achieve the phosphorus load reduction to Lake Hiawatha (to help achieve the Total Maximum Daily Load (TMDL)) as an important consideration for each of the various water management alternatives.

Additionally, at a public meeting held on 4/20/2017, the public attendees were divided into nine (9) working groups and were tasked with identifying potential recreation concepts for the potential upland and wetland/open water areas expected under a reduced-pumping water management alternative. The groups were tasked with identifying potential recreation concepts for the potential upland and wetland/open water areas expected under a reduced-pumping alternative. Common water management related-themes from this exercise included selection of intermittently-inundated wetlands (7 of 9 groups) and prairie/wet meadow restoration (8 of 9 groups). Several groups identified habitat development and wildlife related uses for the area as well, including development of pollinator gardens/areas (8 of 9 groups), amphibian education trail (6 of 9 groups), bird sanctuary (3 of 9 groups), and wildlife blinds (5 of 9 groups), and shoreline habitat restoration (1 of 9 groups). The MPRB, City, and MCWD staff also considered this public input when selecting the preferred reduced-pumping water management concept for the Hiawatha golf course area.

2.2 Management Alternatives Existing conditions along with five additional water management alternatives were evaluated to understand the impacts due to modifications to the current water management in the golf course area. To evaluate each alternative, we utilized the analysis tools (e.g. models) and data collected in earlier phases of the project to estimate impacts (see memo dated 2/28/2017 for additional details).

• Existing Conditions: No modifications to existing pumping rates • Water Management Alternative 1: No pumping • Water Management Alternative 2: No modifications to existing pumping rates, daylight storm

sewer at corner of 19th Avenue S and E 43rd Street into an open channel that drains around the existing golf course ponds to Lake Hiawatha

• Water Management Alternative 3a: Reduced pumping to protect homes (Longfellow drain, well at E 43rd Street/17th Avenue S), open channel connection through berm to Lake Hiawatha (no stormwater pumping), daylight storm sewer at corner of 19th Avenue S and E 43rd Street into an open channel that drains through the existing golf course ponds.



• Water Management Alternative 3b: Reduced pumping to protect homes (Longfellow drain, well at E 43rd Street/17th Avenue S), open channel connection through berm to Lake Hiawatha (no stormwater pumping), daylight storm sewer at corner of 19th Avenue S and E 43rd Street into an open channel that drains through the existing golf course ponds, and realignment of Minnehaha Creek

• Water Management Alternative 4: Reduced pumping to protect homes (Longfellow drain, well at E 43rd Street/17th Avenue S), stormwater pumping above elevation 812.3 ft Mean Sea Level (MSL), daylight storm sewer at corner of 19th Avenue S and E 43rd Street into an open channel that drains around the existing golf course ponds to Lake Hiawatha, 3x36” pipe connection with backflow prevention through berm to Lake Hiawatha

• Water Management Alternative 5: Reduced pumping to protect homes (Longfellow drain, well at E 43rd Street/17th Avenue S), stormwater pumping above elevation 812.3 ft MSL, daylight storm sewer at corner of 19th Avenue S and E 43rd Street into an open channel that drains through the existing golf course ponds, 9x36” pipe connections with backflow prevention through berm to Lake Hiawatha

Each of these water management alternatives are summarized in Figures 1 through 7.



Figure 1: Existing Conditions



Figure 2: Water Management Alternative 1



Figure 4: Water Management Alternative 3a



Figure 5: Water Management Alternative 3b



3.0 Groundwater Pumping and Impact Assessment This section summarizes the estimated groundwater and pumping related impacts based on the use of the calibrated groundwater modeling and the additional basement data collected since the completion of the memo dated 2/28/2017.

3.1 Revised Groundwater Inflow Table The groundwater model (ModFLOW), recalibrated in January 2017 based on additional groundwater and flow data collected from 2016 through 2017, was used to estimate the regional groundwater inflows into the golf course area based on a variety of Lake Hiawatha tailwater conditions and golf course pond elevations. The estimated groundwater inflows are incorporated into the surface/storm water modeling (XP-SWMM, See discussion in Section 4.0 below) to account for the regional groundwater and lake inflows (though the berm) into the golf course area when estimating the surface water implications of different water management alternatives.

Table 1 summarizes the final groundwater inflow values based on the recalibrated groundwater model.

Table 1 – Lake Hiawatha Stage/Pond Groundwater Inflow Relationship (for use in the XP-SWMM Model)

Lake Hiawatha Stage (ft NGVD29)

Golf Course Ponds

Stage (ft NGVD29)

Steady-State Groundwater Inflow Rate

(gpm)

812.8 808.0 490 812.8 808.4 460 812.8 809.0 409 812.8 810.0 325 812.8 811.0 239 812.8 812.0 129 812.8 812.8 0 810.3 808.4 348 811 808.4 382

811.8 808.4 419 812.8 808.4 460 813.5 808.4 496 814.1 808.4 521 815 808.4 552

816.4 808.4 564



3.2 Revised Pumping Analysis - Additional Basement Surveys/Basement Impacts

MPRB and City staff were able to collect basement survey data on several additional low homes in the area immediately adjacent to and west of the Hiawatha Golf Course. To date, 28 homes have been surveyed with estimates of the basement floor elevations. These homes along with the surveyed basement floor elevation are summarized in Figure 8.

The original groundwater pumping analysis was completed using the calibrated 2017 groundwater model to evaluate steady-state future pumping scenarios, assuming the three varying tailwater conditions at Lake Hiawatha. The tailwater conditions used in this evaluation were 811.8 ft MSL (low flow conditions on Minnehaha Creek), 812.8 ft MSL (average water level & MnDNR Ordinary High Water Level (OHWL)), and 814.1 ft MSL (higher flow conditions on Minnehaha Creek).

To incorporate the additional basement survey information, we revised the original groundwater pumping analysis completed earlier in 2017 (summarized in memo dated 2/28/2017) to determine the pumping required for the protection of nearby basements. However, based on the additional survey data collected since the original pumping analysis was done, we discovered the pumping rate defined in the original groundwater pumping analysis only using the proposed drain system along Longfellow Avenue would not sufficiently protect several lower basements several blocks west of the golf course (south of the intersection of E 43rd Street and 17th Avenue S).

To provide the targeted level of protection for these homes (basement elevation minus freeboard (additional vertical distance between water surface and basement floor) of 0.5 feet (6.0 inches)), the updated pumping analysis considered two additional pumping scenarios to provide protection of the low basements south of the intersection of E 43rd Street and 17th Avenue S to determine the best collection and pumping method for the protection of all the surveyed low basements (resulting in the lowest pumping rate):

1) Evaluation of the pumping required by utilizing only the proposed Longfellow Drain along the west edge of the golf course to protect all surveyed basements, including those several blocks to the west (for both keeping the berm in-place as well as having a direct gravity connection to Lake Hiawatha).

2) Evaluation of the pumping required by utilizing the proposed Longfellow Drain to protect the surveyed basements adjacent to the golf course (not including the home with the sport court basement) and a well at the intersection of E 43rd Street and 17th Avenue S to protect the low basements in this area. Again, the pumping was evaluated for both the berm in-place as well as the direct gravity connection to Lake Hiawatha).



For the first pumping scenario which uses only the proposed Longfellow drain to protect all the basements in the Hiawatha area, including the low homes several blocks to the west of the golf course, required significantly more pumping than the original reduced pumping analysis suggested. Using this method under normal conditions (Lake Hiawatha at 812.8 ft MSL), the estimated annual groundwater pumping required to protect the low homes/basements is 131 million gallons per year to 228 million gallons per year. This is 6-45% reduction from the existing groundwater pumping rate (242 million gallons per year), depending on if the berm between the golf course and Lake Hiawatha remains in-place or not.

For the second pumping scenario, we used both the proposed Longfellow drain system along with a well system at the intersection of E 43rd Street and 19th Avenue S to protect the basements in the Hiawatha area. Using this method under normal conditions (Lake Hiawatha at 812.8 ft MSL), the estimated average annual groundwater pumping required to protect the low homes/basements is 54 million gallons per year to 94 million gallons per year. This is a 61-78% reduction from the existing groundwater pumping rate (242 million gallons per year). Because this approach resulted in a lower pumping rate, we will assume this approach for the remainder of our alternatives assessment.

The updated results of the pumping analyses are summarized in Table 2, including the two new scenarios above, along with estimated impacts to nearby basements and regional resources.

3.3 Impacts to Stormwater Infrastructure We anticipate that the discharge from the Longfellow Drain would discharge to either Minnehaha Creek or Lake Hiawatha. However, for the well located at the intersection of E 43rd Street and 19th Avenue S, we would expect this system to discharge to the existing storm sewer along E 43rd Street. The estimated discharge rate for the well is 20 – 24 GPM (0.04 -0.06 cfs). Per the XP-SWMM model for the watershed, the storm sewer along E 43rd Street is modeled as a semi-eclipse pipe (~78 inch (6.5 ft) diameter equivalent round pipe) with a full-flow capacity of ~175-180 cfs. Therefore the well discharge will take up less than 0.06% the conveyance capacity of the storm sewer, and should not have a significant impact on the existing storm sewer capacity.

The water levels in Lake Hiawatha have a more significant impact on the conveyance capacity of the storm sewer along E 43rd Street. There is approximately one (1) foot of standing water in the storm sewer system when the lake is at 811.8 ft MSL (~10% of pipe capacity), two (2) feet when the lake is at 812.8 ft MSL (~26% of pipe capacity), and 3.3 feet when the lake is at 814.1 ft MSL (~50% of pipe capacity).

3.4 Impacts to Regional Resources Table 2 also includes a summary of the estimated impact of modifications to the various pumping regimes on the regional water resources. Modifications to pumping at the Hiawatha Golf Course are not expected to have a significant impact on water levels of most regional resources. However, based on the



groundwater model results, we anticipate a slight increase in the long-term water levels in Powderhorn Lake. Powderhorn Lake is a landlocked lake (with no surface discharge) and the outlet includes pumps to manage runoff and maintain water levels. During wet years, the pumps run frequently; however, during drier years, the pumps may only run intermittently.

Assuming that all groundwater pumping stopped at the golf course, the modeling estimated an increase in the Powderhorn Lake water levels of 0.7-0.8 feet. To offset this increase, we would anticipate an increase in pumping at Powderhorn Lake of 12-19 gpm (0.03-0.04 cfs, 6.3-10.0 MGY).

Under the reduced pumping scenarios, the groundwater modeling estimates an increase in the Powderhorn Lake water levels of 0.3-0.4 feet. To offset this increase, we would anticipate an increase in pumping at Powderhorn Lake of 7-10 gpm (0.015-0.02 cfs, 3.7-5.3 MGY).

Table 2: Hiawatha Golf Course Area Pumping Scenario Summary

521 460 419 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 00 0 0 0 0 0 0 0 0 0 0 0 0 0 0 140 96 63 142 100 64 288 249 219 95 57 370 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 46 46 43521 460 419 0 0 0 0 0 0 0 0 0 0 0 0 140 96 63 142 100 64 288 249 219 141 103 800 0 0 0 0 0 0 0 0 0 0 0 19 15 12 0 0 0 0 0 0 0 0 0 0 0 0

‐‐ ‐‐ ‐‐ ‐‐ ‐‐ ‐‐ ‐‐ ‐‐ ‐‐ ‐‐ ‐‐ ‐‐ ‐‐ ‐‐ ‐‐ 811.8 812.1 812.2 811.9 812.0 812.2 809.7 809.9 810.0 812.1 812.3 812.3

274 242 220 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 00 0 0 0 0 0 0 0 0 0 0 0 0 0 0 74 51 33 75 52 34 151 131 115 50 30 190 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 24 24 2366 66 66 66 66 66 0 0 0 0 0 0 66 66 66 66 66 66 66 66 66 66 66 66 66 66 66340 308 286 66 66 66 0 0 0 0 0 0 66 66 66 140 117 99 141 118 100 217 197 181 140 120 108

814.1 812.8 811.8 814.1 812.8 811.8 814.1 812.8 811.8 814.1 812.8 811.8 814.1 812.8 811.8 814.1 812.8 811.8 814.1 812.8 811.8 814.1 812.8 811.8 814.1 812.8 811.8808.4 808.4 808.4 813.1 812.7 812.4 813.6 813.2 812.9 813.7 813.3 813.0 813.1 812.7 812.4 812.5 812.3 812.1 812.5 812.3 812.1 811.8 811.4 811.2 812.6 812.3 812.0815.7 815.8 815.8 815.6 815.8 815.8 815.6 815.8 815.8 815.6 815.8 815.8 815.6 815.8 815.8 815.7 815.8 815.8 815.7 815.8 815.8 815.7 815.8 815.8 815.6 815.8 815.8819.5 819.5 819.5 820.3 820.2 820.1 820.3 820.2 820.2 820.3 820.3 820.2 819.5 819.5 819.5 820.0 820.0 819.9 820.0 820.0 819.9 819.7 819.7 819.7 819.8 819.9 819.8822.8 822.8 822.8 822.8 822.8 822.8 822.8 822.8 822.8 822.8 822.8 822.8 822.8 822.8 822.8 822.8 822.8 822.8 822.8 822.8 822.8 822.8 822.8 822.8 822.8 822.8 822.8814.2 814.3 814.3 814.2 814.3 814.3 814.2 814.3 814.3 814.2 814.3 814.3 814.2 814.3 814.3 814.3 814.3 814.3 814.3 814.3 814.3 814.2 814.3 814.3 814.2 814.3 814.3810.9 811.0 811.0 810.9 811.0 811.0 810.9 811.0 811.0 810.9 811.0 811.0 810.9 811.0 811.0 810.9 811.0 811.0 810.9 811.0 811.0 810.9 811.0 811.0 810.9 811.0 811.0

Address

Estimated Basement Elevation (ft NGVD29)

Estimated Basement Elev. Minus Freeboard (ft NGVD29)

4343 Longfellow Ave S 817.91 817.4 810.8 810.7 810.7 813.9 813.6 813.4 814.3 814.0 813.7 814.4 814.1 813.9 813.9 813.6 813.3 812.5 812.6 812.6 812.5 812.6 812.6 810.8 810.9 810.9 812.5 812.6 812.54354 Longfellow Ave S 814.57 814.1 810.9 810.9 810.8 814.0 813.7 813.4 814.3 814.0 813.8 814.4 814.1 813.9 814.0 813.7 813.4 812.3 812.5 812.6 812.3 812.5 812.6 810.5 810.6 810.7 812.4 812.5 812.54400 Longfellow Ave S 813.70 813.2 810.8 810.8 810.8 814.0 813.7 813.4 814.3 814.0 813.7 814.4 814.1 813.9 813.9 813.6 813.3 812.2 812.4 812.5 812.2 812.4 812.5 810.3 810.4 810.5 812.4 812.5 812.44406 Longfellow Ave S 814.03 813.5 810.8 810.8 810.8 813.9 813.6 813.3 814.2 814.0 813.7 814.3 814.1 813.8 813.9 813.6 813.3 812.1 812.3 812.4 812.1 812.3 812.4 810.2 810.3 810.4 812.3 812.4 812.44414 Longfellow Ave S 812.93 812.4 810.8 810.8 810.7 813.9 813.6 813.3 814.2 813.9 813.7 814.3 814.1 813.8 813.9 813.6 813.3 812.1 812.3 812.4 812.1 812.3 812.4 810.2 810.3 810.4 812.3 812.4 812.44432 Longfellow Ave S 810.83 810.3 810.7 810.7 810.7 813.8 813.5 813.2 814.1 813.9 813.6 814.2 814.0 813.7 813.8 813.5 813.2 812.0 812.2 812.4 812.1 812.2 812.4 810.1 810.2 810.3 812.3 812.4 812.44434 Longfellow Ave S 816.13 815.6 810.7 810.7 810.7 813.8 813.5 813.2 814.1 813.8 813.6 814.2 814.0 813.7 813.8 813.5 813.2 812.1 812.3 812.4 812.1 812.3 812.4 810.2 810.3 810.4 812.3 812.4 812.44438 Longfellow Ave S 815.75 815.3 810.7 810.7 810.7 813.8 813.5 813.2 814.1 813.8 813.6 814.2 814.0 813.7 813.8 813.5 813.2 812.1 812.3 812.4 812.1 812.3 812.4 810.2 810.3 810.4 812.3 812.4 812.44446 Longfellow Ave S 817.91 817.4 810.7 810.8 810.7 813.7 813.5 813.2 814.1 813.8 813.6 814.2 813.9 813.7 813.7 813.4 813.2 812.1 812.3 812.4 812.1 812.3 812.4 810.3 810.4 810.5 812.3 812.4 812.44454 Longfellow Ave S 819.03 818.5 810.8 810.8 810.8 813.7 813.4 813.2 814.0 813.8 813.5 814.2 813.9 813.7 813.7 813.4 813.2 812.2 812.4 812.4 812.2 812.4 812.4 810.5 810.6 810.7 812.4 812.5 812.4

1617 E 43rd Street 813.76 813.3 813.1 813.0 813.0 815.2 815.0 814.8 815.5 815.3 815.1 815.6 815.4 815.2 815.2 815.0 814.8 814.3 814.3 814.4 814.3 814.3 814.4 813.3 813.3 813.2 813.3 813.3 813.34304 17th Avenue S 814.02 813.5 812.9 812.9 812.9 815.1 814.9 814.7 815.4 815.2 815.0 815.5 815.3 815.1 815.1 814.8 814.6 814.2 814.2 814.2 814.2 814.2 814.2 813.1 813.1 813.1 813.4 813.4 813.44312 17th Avenue S 813.86 813.4 813.0 812.9 812.9 815.2 815.0 814.7 815.4 815.2 815.0 815.5 815.3 815.1 815.1 814.9 814.7 814.2 814.3 814.3 814.2 814.3 814.3 813.1 813.1 813.1 813.2 813.2 813.24441 18th Avenue S 817.06 816.6 811.8 811.9 811.8 814.3 814.1 813.9 814.6 814.4 814.2 814.7 814.5 814.3 814.3 814.1 813.8 813.1 813.2 813.3 813.2 813.2 813.3 811.8 811.9 811.9 813.1 813.1 813.14733 18th Avenue S 818.51 818.0 812.5 812.6 812.6 813.4 813.4 813.3 813.5 813.5 813.5 813.6 813.6 813.5 813.4 813.4 813.3 813.1 813.2 813.2 813.1 813.2 813.2 812.7 812.8 812.8 813.1 813.2 813.14255 19th Avenue S 823.10 822.6 811.0 811.0 810.9 814.0 813.7 813.4 814.3 814.0 813.8 814.4 814.2 813.9 814.0 813.7 813.4 813.0 813.0 812.9 813.0 813.0 813.0 811.9 811.8 811.7 812.9 812.9 812.74300 19th Avenue S 832.70 832.2 811.0 811.0 810.9 814.0 813.7 813.4 814.3 814.1 813.8 814.4 814.2 813.9 814.0 813.7 813.4 813.0 813.0 812.9 813.0 813.0 812.9 811.7 811.7 811.6 812.9 812.8 812.74310 19th Avenue S 820.30 819.8 810.8 810.8 810.7 813.9 813.6 813.3 814.3 814.0 813.7 814.4 814.1 813.8 813.9 813.6 813.3 812.8 812.8 812.8 812.8 812.8 812.8 811.6 811.5 811.4 812.8 812.7 812.64316 19th Avenue S 819.74 819.2 810.7 810.7 810.7 813.9 813.6 813.3 814.2 813.9 813.7 814.3 814.1 813.8 813.9 813.6 813.3 812.8 812.8 812.8 812.8 812.8 812.8 811.5 811.5 811.4 812.8 812.7 812.64320 19th Avenue S 814.49 814.0 810.7 810.6 810.6 813.9 813.6 813.3 814.2 813.9 813.7 814.3 814.0 813.8 813.9 813.5 813.2 812.8 812.8 812.7 812.8 812.8 812.7 811.4 811.4 811.3 812.7 812.7 812.64324 19th Avenue S 813.22 812.7 810.6 810.6 810.5 813.9 813.5 813.2 814.2 813.9 813.6 814.3 814.0 813.7 813.8 813.5 813.2 812.7 812.7 812.7 812.7 812.7 812.7 811.4 811.3 811.3 812.7 812.7 812.54326 19th Avenue S 813.88 813.4 810.5 810.5 810.4 813.8 813.5 813.2 814.1 813.9 813.6 814.3 814.0 813.7 813.8 813.5 813.2 812.6 812.6 812.6 812.6 812.6 812.6 811.3 811.2 811.2 812.6 812.6 812.54340 19th Avenue S 814.15 813.7 810.2 810.2 810.1 813.7 813.4 813.1 814.0 813.7 813.5 814.1 813.9 813.6 813.7 813.3 813.0 812.4 812.4 812.5 812.4 812.4 812.5 810.9 810.9 810.8 812.5 812.5 812.44346 19th Avenue S 813.35 812.9 810.1 810.1 810.1 813.7 813.3 813.0 814.0 813.7 813.4 814.1 813.8 813.6 813.6 813.3 813.0 812.3 812.4 812.4 812.3 812.4 812.4 810.7 810.7 810.7 812.4 812.5 812.44354 19th Avenue S 812.95 812.4 810.0 810.0 810.0 813.6 813.3 813.0 814.0 813.7 813.4 814.1 813.8 813.5 813.6 813.3 813.0 812.1 812.2 812.3 812.1 812.2 812.3 810.2 810.3 810.3 812.3 812.4 812.34255 20th Avenue S 818.26 817.8 810.7 810.6 810.5 813.8 813.4 813.1 814.1 813.8 813.5 814.2 813.9 813.6 813.7 813.4 813.1 812.9 812.8 812.7 812.9 812.8 812.7 811.9 811.7 811.6 812.8 812.7 812.54250 21st Avenue S 820.59 820.1 810.6 810.5 810.4 813.6 813.3 812.9 813.9 813.6 813.3 814.0 813.7 813.4 813.6 813.2 812.9 812.9 812.7 812.6 812.9 812.7 812.6 812.0 811.7 811.6 812.8 812.6 812.44254 21st Avenue S 819.38 818.9 810.6 810.4 810.3 813.6 813.2 812.9 813.9 813.6 813.3 814.0 813.7 813.4 813.6 813.2 812.9 812.9 812.7 812.5 812.8 812.7 812.5 811.9 811.7 811.5 812.8 812.6 812.4

1 1 1 12 11 9 14 12 11 14 14 11 12 11 9 4 4 4 4 4 4 0 0 0 1 1 1

Berm between Lake Hiawatha and Golf Course intact, Ponds A+B inplace, Longfellow drain

operating

Berm between Lake Hiawatha and Golf Course intact, Ponds A+B filled, Longfellow drain

operating

Berm between Lake Hiawatha and Golf Course intact, Ponds A+B inplace, Longfellow drain operating to protect ALL low

basements

Denotes potentially imapcted basement

Existing ConditionsNo groundwater pumping, Stormwater pumping only

No pumping (groundwater or stormwater), 30 in. annual

rainfall

No pumping (groundwater or stormwater), 40 in. annual

rainfall

No groundwater pumping, Additional pumping to maintain

Powderhorn Lake level

Annual Pumping Volumes (MG)

Groundwater Pumping Rates (gpm)Golf Course Pumping Rate

Longfellow Drain Pumping Rate43rd Street Well Pumping Rate

Berm between Lake Hiawatha and Golf Course intact, Ponds A+B inplace, Longfellow drain operating, 43rd Street Well operating to protect low

basements to west of Cedar Ave.

Pumping Scenarios

Total Groundwater Pumping RatePowderhorn Lake Pumping Rate

Drain Elevation (ft NGVD29)Drain Elevation

Golf Course Groundwater PumpingLongfellow Drain Groundwater Outflow

43rd Street Well PumpingStormwater Pumping

Total Pumping

Taft LakeMother Lake

Estimated Groundwater Elevations (ft NGVD29)

Potentially Impacted basements

Lake Water Levels (ft NGVD29)Lake HiawathaCourse PondsLake Nokomis

Powderhorn LakeDiamond Lake

Table 2: Hiawatha Golf Course Area Pumping Scenario Summary

Address

Estimated Basement Elevation (ft NGVD29)

Estimated Basement Elev. Minus Freeboard (ft NGVD29)

4343 Longfellow Ave S 817.91 817.44354 Longfellow Ave S 814.57 814.14400 Longfellow Ave S 813.70 813.24406 Longfellow Ave S 814.03 813.54414 Longfellow Ave S 812.93 812.44432 Longfellow Ave S 810.83 810.34434 Longfellow Ave S 816.13 815.64438 Longfellow Ave S 815.75 815.34446 Longfellow Ave S 817.91 817.44454 Longfellow Ave S 819.03 818.5

1617 E 43rd Street 813.76 813.34304 17th Avenue S 814.02 813.54312 17th Avenue S 813.86 813.44441 18th Avenue S 817.06 816.64733 18th Avenue S 818.51 818.04255 19th Avenue S 823.10 822.64300 19th Avenue S 832.70 832.24310 19th Avenue S 820.30 819.84316 19th Avenue S 819.74 819.24320 19th Avenue S 814.49 814.04324 19th Avenue S 813.22 812.74326 19th Avenue S 813.88 813.44340 19th Avenue S 814.15 813.74346 19th Avenue S 813.35 812.94354 19th Avenue S 812.95 812.44255 20th Avenue S 818.26 817.84250 21st Avenue S 820.59 820.14254 21st Avenue S 819.38 818.9

Denotes potentially imapcted basement

Golf Course Pumping RateLongfellow Drain Pumping Rate43rd Street Well Pumping Rate

Total Groundwater Pumping RatePowderhorn Lake Pumping Rate

Drain Elevation

Golf Course Groundwater PumpingLongfellow Drain Groundwater Outflow

43rd Street Well PumpingStormwater Pumping

Total Pumping

Taft LakeMother Lake

Potentially Impacted basements

Lake HiawathaCourse PondsLake Nokomis

Powderhorn LakeDiamond Lake

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 096 57 37 427 184 41 431 183 42 633 433 282 382 134 21 382 134 2046 46 43 0 0 0 0 0 0 0 0 0 38 45 41 38 45 41141 104 80 427 184 41 431 183 42 633 433 282 419 179 62 420 179 629 8 7 0 0 0 0 0 0 0 0 0 0 0 0 10 9 7

812.1 812.3 812.3 810.5 811.7 812.2 810.6 811.7 812.2 808.5 809.2 809.8 810.8 811.9 812.3 810.8 811.9 812.3

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 050 30 19 225 97 22 227 96 22 333 228 148 201 71 11 201 70 1124 24 23 0 0 0 0 0 0 0 0 0 20 24 22 20 24 2266 66 66 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0140 120 108 225 97 22 227 96 22 333 228 148 220 94 33 220 94 32

814.1 812.8 811.8 814.1 812.8 811.8 814.1 812.8 811.8 814.1 812.8 811.8 814.1 812.8 811.8 814.1 812.8 811.8812.6 812.3 812.1 814.1 812.8 811.8 814.1 812.8 811.8 814.1 812.8 811.8 814.1 812.8 811.8 814.1 812.8 811.8815.6 815.8 815.8 815.6 815.8 815.8 815.6 815.8 815.8 815.6 815.8 815.8 815.6 815.8 815.8 815.6 815.8 815.8819.5 819.5 819.5 820.0 820.0 820.0 820.0 820.0 820.0 819.8 819.7 819.6 820.0 819.9 819.8 819.5 819.5 819.5822.8 822.8 822.8 822.8 822.8 822.8 822.8 822.8 822.8 822.8 822.8 822.8 822.8 822.8 822.8 822.8 822.8 822.8814.2 814.3 814.3 814.2 814.3 814.3 814.2 814.3 814.3 814.2 814.3 814.3 814.2 814.3 814.3 814.2 814.3 814.3810.9 811.0 811.0 810.9 811.0 811.0 810.9 811.0 811.0 810.9 811.0 811.0 810.9 811.0 811.0 810.9 811.0 811.0

812.5 812.6 812.5 811.9 812.4 812.6 811.9 812.4 812.6 810.5 810.7 810.9 811.9 812.5 812.5 811.9 812.5 812.5812.4 812.5 812.5 811.4 812.2 812.6 811.5 812.3 812.6 809.8 810.2 810.6 811.6 812.3 812.5 811.5 812.3 812.5812.3 812.5 812.4 811.2 812.1 812.5 811.2 812.1 812.5 809.4 809.9 810.4 811.3 812.2 812.4 811.3 812.2 812.4812.3 812.4 812.4 811.0 812.0 812.4 811.1 812.0 812.4 809.2 809.8 810.3 811.2 812.2 812.4 811.2 812.2 812.4812.3 812.4 812.4 811.0 811.9 812.4 811.1 812.0 812.4 809.1 809.8 810.2 811.2 812.2 812.4 811.2 812.2 812.4812.3 812.4 812.4 810.9 811.9 812.4 811.0 812.0 812.4 809.1 809.7 810.2 811.1 812.1 812.4 811.1 812.1 812.4812.3 812.4 812.4 811.0 811.9 812.4 811.1 812.0 812.4 809.2 809.8 810.2 811.2 812.1 812.4 811.2 812.1 812.4812.3 812.4 812.4 811.0 811.9 812.4 811.1 812.0 812.4 809.2 809.8 810.2 811.2 812.2 812.4 811.2 812.2 812.4812.3 812.4 812.4 811.1 812.0 812.4 811.2 812.0 812.4 809.4 809.9 810.3 811.3 812.2 812.4 811.3 812.2 812.4812.4 812.5 812.4 811.5 812.1 812.4 811.6 812.2 812.4 810.0 810.3 810.6 811.7 812.3 812.4 811.7 812.3 812.4813.2 813.2 813.2 814.1 814.3 814.3 814.1 814.3 814.3 813.3 813.3 813.3 813.3 813.3 813.3 813.2 813.2 813.2813.4 813.4 813.4 813.9 814.1 814.2 814.0 814.2 814.2 813.0 813.1 813.1 813.3 813.4 813.4 813.3 813.4 813.4813.2 813.2 813.2 814.0 814.2 814.2 814.0 814.2 814.2 813.1 813.1 813.1 813.2 813.2 813.2 813.2 813.2 813.2813.0 813.1 813.1 812.7 813.1 813.2 812.8 813.1 813.3 811.6 811.8 811.9 812.7 813.0 813.0 812.7 813.0 813.0813.1 813.2 813.1 813.3 813.2 813.2 813.3 813.2 813.2 813.0 813.0 812.9 813.2 813.2 813.1 813.2 813.2 813.1812.9 812.9 812.7 813.2 813.0 812.8 813.2 813.1 812.9 812.4 812.1 811.8 813.1 812.9 812.6 813.1 812.9 812.6812.9 812.8 812.7 813.1 813.0 812.8 813.0 813.0 812.8 812.2 811.9 811.7 813.0 812.9 812.6 813.0 812.9 812.6812.8 812.7 812.6 812.9 812.8 812.7 812.9 812.9 812.7 812.0 811.7 811.6 812.9 812.8 812.5 812.9 812.8 812.5812.8 812.7 812.6 812.9 812.8 812.7 812.8 812.8 812.7 811.9 811.7 811.5 812.8 812.8 812.5 812.8 812.8 812.5812.7 812.7 812.6 812.8 812.8 812.6 812.8 812.8 812.7 811.8 811.6 811.4 812.8 812.7 812.5 812.8 812.7 812.5812.7 812.7 812.5 812.7 812.7 812.6 812.7 812.7 812.6 811.7 811.5 811.4 812.7 812.7 812.4 812.7 812.7 812.4812.6 812.6 812.5 812.6 812.6 812.5 812.6 812.6 812.6 811.6 811.4 811.3 812.6 812.6 812.4 812.6 812.6 812.4812.5 812.5 812.4 812.2 812.4 812.4 812.2 812.4 812.4 811.0 810.9 810.9 812.3 812.5 812.3 812.3 812.5 812.3812.4 812.5 812.4 812.0 812.3 812.3 812.0 812.3 812.3 810.7 810.7 810.8 812.1 812.4 812.2 812.1 812.4 812.2812.3 812.4 812.3 811.2 811.9 812.2 811.2 812.0 812.2 809.5 809.9 810.2 811.4 812.2 812.2 811.4 812.1 812.2812.8 812.7 812.5 813.5 812.9 812.6 813.4 812.9 812.6 812.9 812.2 811.8 813.4 812.9 812.4 813.4 812.9 812.4812.8 812.6 812.4 813.6 812.9 812.4 813.6 812.9 812.4 813.2 812.3 811.8 813.5 812.8 812.3 813.5 812.8 812.3812.8 812.6 812.4 813.6 812.9 812.4 813.6 812.9 812.4 813.2 812.3 811.8 813.6 812.8 812.3 813.6 812.8 812.31 1 1 4 4 4 4 4 4 0 0 0 1 1 1 1 1 1

Berm between Lake Hiawatha and Golf Course removed

(gravity connection), Ponds A+B inplace, Longfellow drain operating, 43rd Street Well operating to protect low

basements to west of Cedar Ave., Additional pumping to

Berm between Lake Hiawatha and Golf Course intact, Ponds A+B inplace, Longfellow drain operating, 43rd Street Well operating to protect low

basements to west of Cedar Ave., Additional pumping to

maintain Powderhorn Lake level


(gravity connection), Ponds A+B inplace, Longfellow drain

operating


(gravity connection), Ponds A+B filled, Longfellow drain

operating


(gravity connection), Ponds A+B inplace, Longfellow drain

operating to protect ALL low basements


(gravity connection), Ponds A+B inplace, Longfellow drain operating, 43rd Street Well operating to protect low

basements to west of Cedar Ave.

L a k e H i a w a t h a

Minn

ehah

a Cree

k

Surveyed EL= 817.91

Surveyed EL= 814.02

Surveyed EL= 814.49

Surveyed EL= 815.75Surveyed EL= 815.75

Surveyed EL= 812.95

Surveyed EL= 813.76

Surveyed EL= 817.91

Surveyed EL= 812.93

Surveyed EL= 814.02

Surveyed EL= 817.06

Surveyed EL= 810.83

Surveyed EL= 814.57

Surveyed EL= 813.7

Surveyed EL= 823.1

Surveyed EL= 814.49

Surveyed EL= 818.51

Surveyed EL= 819.38

Surveyed EL= 819.03

Surveyed EL= 817.91

Surveyed EL= 813.86

Surveyed EL= 832.7

Surveyed EL= 816.13Surveyed EL= 815.75

Surveyed EL= 814.15

Surveyed EL= 813.22

Surveyed EL= 820.3

Surveyed EL= 818.26

Surveyed EL= 813.35

Surveyed EL= 814.03

Surveyed EL= 819.74

Surveyed EL= 813.88

Surveyed EL= 820.59

Surveyed EL= 815.75

4567152

456746

456742

E 43rd St

E 42nd St

28th

Ave S16

th Av

e S

17th

Ave S

18th

Ave S

Ceda

r Ave

S

Long

fellow

Ave S

E 44th St

E Minnehaha Pkwy

E 47th St

E 46th St

E 45th St

27th

Ave S

19th

Ave S

21st

Ave S

26th

Ave S

24th

Ave S

25th

Ave S

20th

Ave S

23rd

Ave S

22nd

Ave S

Stand

ish Av

e S

E 48th St

E 47th St

27th

Ave S

I:\Projects\23\27\1466\Users\JAK2\Building_2017Survey_callout_032317.mxd

Building Basement Survey SummaryHiawatha Golf Course Area

Minneapolis Park & Recreation Board

FIGURE 8

BuildingSurveyed1

Building with Est Basement Elevation Below 814.1

Min Building Ground Elevation (LiDAR)GrndElevNGVD29

815.0 - 820.0

820.0 - 830.0

830.0 - 840.0

840.0 - 850.0

850.0 - 860.0

860.0 - 870.0

I0 240 480120

Feet

1 -Buildings Surveyed by MPRB & City Staff through 3/10/3017.

DRAFT



4.0 Surface Water Evaluation and Impact Assessment This section summarizes the surface water and stormwater analyses and related impacts based on the XP-SWMM modeling and other data collected as part of and since the completion of the memo dated 2/28/2017.

4.1 Lowering of Lake Hiawatha Lowering of Lake Hiawatha was evaluated as a method to reduce the amount of pumping to maintain the golf course and/or protect homes as well as reduce flooding in the watershed to the north of the Hiawatha Golf Course area. A GPS survey was conducted by Barr along Minnehaha Creek downstream of Lake Hiawatha in January and February 2017 to identify high points in the channel that may control the water level in Lake Hiawatha and to estimate how much the normal water level in the lake could be lowered.

Based on this survey, we have estimated that the control elevation of Lake Hiawatha in the channel downstream could be reduced by 1.0 -1.5 feet. We’ve assumed this reduction would result in an average water surface elevation approximately one (1) foot lower than that the OHWL/average water elevation, lowering the average lake level from 812.8 ft NGVD29 to 811.8 ft NGVD29. These are the elevations we have been using for our groundwater and stormwater evaluations (see memo dated 2/28/2017 for more details related to the channel survey).

4.1.1 Summary of Utility Conflicts In follow-up to the Minnehaha Creek survey conducted in January/February 2017 from Lake Hiawatha downstream to the pedestrian bridge downstream of Nokomis Avenue, a Gopher State One call was placed for this section of the creek to determine what public and private utility conflicts might exist.

Based on the utility information received to date:

• There is a City of Minneapolis water main crossing upstream of 28th Ave S Bridge. The depth of the water main is unknown.

• There are several City of Minneapolis gravity sanitary sewer crossings, including: downstream of the 28th Ave S bridge, at 29th Ave S, and at 30th Ave S. These all tie into the Metropolitan Council Environmental Services (MCES) sanitary system (next bullet).

• There is a MCES gravity sanitary sewer (11’-0” span reinforced concrete horseshoe) that generally runs through the Minnehaha Creek corridor and crosses under the creek at two locations between the outlet of Lake Hiawatha and Nokomis Avenue (between 28th and 29th Ave S & between 30th and Nokomis Ave S).

• There are no CenturyLink crossings under the creek from Lake Hiawatha to Nokomis Avenue. • There is an abandoned CenterPoint gas main along 28th Ave S and crosses on the downstream

side of the 28th Ave S Bridge. It is unclear if the abandoned main has been removed, but there



are no other active lines crossing under Minnehaha Creek between Lake Hiawatha and Nokomis Avenue.

• There is an overhead electric Xcel crossing at 28th Ave S and at 31st Ave S (just downstream of Nokomis Ave), not affecting the creek.

Figure 9 shows the results of the Minnehaha Creek survey, the location of the City of Minneapolis and MCES sanitary sewer crossings (as provided in GIS by the City of Minneapolis), and the estimated elevations of the top of the MCES and City sanitary sewer pipes in comparison to the bottom of the creek channel. Also shown on this figure is the proposed elevation needed to lower the Lake Hiawatha water level.

In order to lower the average water level in Lake Hiawatha, we anticipate that the following activities would be needed:

• Removal of the existing concrete weir structure at the outlet of Lake Hiawatha. • Redesign/lowering of ~2,000 feet of the Minnehaha Creek stream channel downstream of Lake

Hiawatha to downstream of Nokomis Avenue to invert elevation 809.0 ft MSL. • Redesign/modification to or reconstruction of several creek crossings including the 28th Avenue S

Bridge, the pedestrian bridge at 30th Avenue S, and the Nokomis Avenue S Bridge. • Lowering of the City of Minneapolis water main located upstream of 28th Avenue S Bridge to

provide sufficient cover. • Providing sufficient cover/protection of the existing City of Minneapolis and MCES sanitary sewer

crossings. • Removal of the abandoned CenterPoint gas main located downstream of 28th Avenue S Bridge. • All associated review and permitting activities related to the modifications to the average water

level in Lake Hiawatha, the stream channel redesign and construction, the modifications/reconstruction of existing bridge/pedestrian crossings and utility work.

4.1.2 Impact on FEMA Flood Elevations The Hiawatha Golf Course is located within a FEMA-mapped floodplain for Minnehaha Creek with a 1% chance (100-year) flood elevation of 817.0 ft MSL.

Earlier in this study, the Minnesota Department of Natural Resources (MnDNR) provided the XP-SWMM model used for the floodplain mapping along Minnehaha Creek (Note: This is different than the XP-SWMM model developed by the City of Minneapolis for evaluation of the local watersheds to Lake Hiawatha). We utilized the Minnehaha Creek XP-SWMM model to evaluate the potential lowering of Minnehaha Creek. To reflect the alternatives, we modeled a scenario where the starting elevation of Lake Hiawatha was lowered by a foot below the existing conditions and the inverts of the structures and



channel bottom from Lake Hiawatha to approximately 2000 feet downstream of Lake Hiawatha were lowered to elevation 809.0 ft MSL.

The expected reduction in the 1% chance (100-year) flood elevation for Lake Hiawatha is estimated to be approximately 0.6 feet.



4.2 Lake Level Analyses Lake levels on Lake Hiawatha are significantly influenced by the flows on Minnehaha Creek. However, the flows on Minnehaha Creek are influenced by precipitation/runoff events as well as the operations at Gray’s Bay dam on Lake Minnetonka. The discharge from Gray’s Bay dam can vary throughout the year; however, high flows are typically discharged from the dam during wet climatic periods when levels on Lake Minnetonka are high or in the fall when Lake Minnetonka is being drawn down to develop flood storage for the spring snowmelt.

To understand the typical magnitude of the lake level bounce and duration in Lake Hiawatha, we utilized the USGS gage flow data at Hiawatha Avenue (USGS station 5289800) and the regression developed between the USGS gage flows and Lake Hiawatha water elevations (see memo dated 2/28/2017) to develop a continuous lake level record for Lake Hiawatha for the available flow data. The USGS gage flow data is typically 15-minute data, with occasional gaps in the data record, especially during winter months when the gage is impacted by ice and does not record flow. We utilized the entire flow data record available for the USGS gage from 11/5/2005 to 3/24/2017.

The maximum flow rates used to develop the regression equation were approximately 530 cfs. However, there were periods during the USGS flow record (in June 2014) where the recorded flow exceeded the maximum flows rate used to develop the regression. For flow rates where the regression equation estimated lake levels greater than 817.0 ft MSL (the effective FEMA floodplain elevation), we assumed the lake level to be 817.0 ft MSL.

We performed an “event” analysis of the estimated lake level data, defining each “event” as the period of time in the continuous record where the lake levels first exceeded a defined threshold elevation to when it returned to the threshold elevation. We utilized the following thresholds to evaluate the events: 811.8 ft MSL (Lake Hiawatha elevation at low flow conditions in the creek) and 812.8 ft MSL (Lake Hiawatha average elevations). For each event, we summarized the average and maximum bounce from the threshold elevation as well as the estimated duration of the event. Table 3 summarizes the results of the event bounce (time-weighted average) and duration analysis. We also fitted probability distributions to the “event” duration data for the defined events to understand the probability of the different event durations which is summarized in Table 4. We also looked at the relationship between the duration of the event and the average bounce in lake level. These data are shown in Figure 10, along with the time-weighted average “event” bounce and average “event” duration (in gray).

MCWD also provided the long-term Gray’s Bay Dam discharge data to help highlight those events where the dam was discharging high flows. The Gray’s Bay Dam discharge data was aligned with the defined events based on the Lake Hiawatha estimated water level threshold analysis. The data points circled in orange in Figure 10 are events where the maximum discharge from Gray’s Bay Dam was greater than



150 cfs. This suggests that many of the events with a longer duration or higher average elevation are associated with periods of higher discharge from the Gray’s Bay Dam.

Table 3: Lake Hiawatha Lake Level “Event” Bounce & Duration Analysis Summary

Average Lake Elevation1

(ft MSL) 812.5

Standard Deviation (ft) 1.0 Event Threshold

Elevation (ft MSL) 811.80 812.80

Duration of Lake Levels At/Below

Threshold

(hr) 28396 53435 (days) 1183 2226

(% of Total Time in Flow Data

Record)

38 72

Events (#) 239 215 Time-Weighted Average Bounce Above Threshold

(ft) 1.2 1.0

Time-Weighted Average Elevation

(ft MSL) 813.0 813.8

Average Duration above Threshold

(hr) 193.8 96.9 (days) 8.1 4.0

Maximum Duration above Threshold

(hr) 5632 3461

(days) 235 144 Maximum Bounce above Threshold

(ft) 5.2 4.2

Maximum Elevation (ft MSL) 817.0 817.0 1 - Based on estimated Lake Hiawatha lake levels using USGS flow record at Hiawatha Avenue from late 2005 through early 2017 and the previously developed regression developed for the USGS flow vs Lake Hiawatha Elevation (memo dated 2/28/2017)



Table 4: Lake Hiawatha Lake Level “Event” Duration Exceedance Probability Summary – Threshold 811.8 ft MSL

“Event” Percent Chance of Exceedance (%)

“Event” Duration

64 1 hour

54 2.5 hours

50 6.2 hours

25 3.3 days

15 8.7 days

10 15.8 days

5 36.7 days

3 62.3 days / 2.0 months



0.5 280.3 days / 9.2 months



Figure 10 Lake Hiawatha Lake Level “Event” Analysis Summary (Threshold Elevation 811.8 ft MSL)

A flow frequency analysis was performed based on the USGS gage data from Minnehaha Creek at Hiawatha Avenue, using the observed annual peak flow discharge data. The 11-year data set available from the USGS (2005-2016) was used to calculate the statistical information such as mean value, standard deviation, skewness and recurrence intervals. The statistical data are then used to construct frequency distributions that help define the likelihood of various discharges as a function of recurrence interval or exceedance probability. Specifically, the Log-Pearson Type III Distribution was used for this study, which is the standard technique for fitting frequency distribution data to predict the design flood for a river used by Federal Agencies in the United States.

The Log-Pearson Type III distribution is calculated using the general equation:

log 𝑥𝑥 = 𝑙𝑙𝑙𝑙𝑙𝑙𝑥𝑥�� + 𝐾𝐾𝜎𝜎𝑙𝑙𝑙𝑙𝑙𝑙𝑥𝑥

Where 𝑥𝑥 is the flood discharge value of the specified probability, log 𝑥𝑥�� is the average of the log 𝑥𝑥 discharge values, 𝐾𝐾 is a frequency factor, and 𝜎𝜎 is the standard deviation of the log 𝑥𝑥 values. The frequency factor 𝐾𝐾 is a function of the skewness coefficient and the return period can be found using the frequency factor table for Log-Pearson Type III Distributions by Haan, 1977.



The flow rates at different recurrence intervals were translated to Lake Hiawatha elevations using the regression relationship between the USGS gage flows at Hiawatha Avenue and the Lake Hiawatha elevation data. The results of this analysis are summarized in Table 5. Also included in this table are the estimated Lake Hiawatha peak elevations and flow rates at the USGS station based on the Minnehaha Creek XP-SWMM model provided by the MnDNR for the 10% chance (10-year) and 1% chance (100-year) events.

To interpret this table, it is important to keep in mind how the return period on a given flow rate (and corresponding lake elevation) translates to the likelihood of this flow happening in any given year. For example, a flow rate (or corresponding elevation) with a 10-year return period has a 10 percent chance of occurring in any given year (which on average translates to once every 10 years).

Table 5: USGS Gage at Hiawatha Avenue – Flow-Duration-Frequency Relationship

Flood Frequency Calculations using log-Pearson Analysis III

Using Average Daily Maximums (period of record 2005-2016)

Lake Hiawatha

Elevation (per Regression w/ USGS

gage)1

Lake Hiawatha Elevation (per

MnDNR Minnehaha

Creek Model/ Effective FEMA

maps)

Peak Flows near Hiawatha Ave

(USGS gage location) (per

MnDNR Minnehaha

Creek XP-SWMM Model)

Return Period

Percent Chance of

Occurrence in Any

Given Year

Discharge in Minnehaha

Creek at Hiawatha Avenue

(years) (%) Q (cfs) (ft MSL) (ft MSL) (cfs) 1.0 100 71 812.7 -- -- 1.5 67 198 813.9 -- -- 2.0 50 244 814.2 -- -- 5.0 20 390 815.1 -- --

10.0 10 501 816.3 815.8 482 25.0 4 664 -- -- -- 50.0 2 797 -- -- --

100.0 1 942 -- 817.00 668 200.0 0.5 1100 -- -- --

1 - Lake level regression developed with creek flows up to 520 cfs and not applied to flows greater than 520 cfs



4.3 XP-SWMM Modeling of the Golf Course Area We utilized the existing City of Minneapolis XP-SWMM model of the local watershed to the Hiawatha Golf Course (Hiawatha west) and the northern watershed contributing to the storm sewer along 43rd Street S that discharges to Lake Hiawatha on the north side of the lake (Hiawatha north).

This XP-SWMM modeling effort is focused on the local watersheds and does not include the larger Minnehaha Creek watershed or creek conveyance. However, much of the Hiawatha golf course is located within the FEMA-mapped floodplain (1% chance (100-yr) event) for Minnehaha Creek (elevation 817.0 ft MSL). Additionally, the Minnehaha Creek XP-SWMM model provided by the MnDNR suggests that the golf course area could also be impacted by the 10 percent chance (10-yr) event on the creek with water levels reaching the top of the berm separating the golf course area from Lake Hiawatha (approx. elevation 815.7 ft MSL).

For this analysis, we evaluated the Atlas 14 10-year and 100-year, 24 hour design storm events on the local watersheds for three different tailwater conditions for Lake Hiawatha, including the following:

• 811.8 ft MSL (Elevations at low flows on the creek or a future average lake level if Lake Hiawatha lowered),

• 812.8 ft MSL (Average lake level of Lake Hiawatha), and • 814.1 ft MSL (Lake elevation at high flows on the creek (~250-300 cfs when Grays Bay Dam is

discharging))

The XP-SWMM modeling is being used to estimate impacts of the various water management alternatives on flooding in the Hiawatha north and west watersheds and estimate peak elevations, bounce, and duration of inundation in the Hiawatha golf course area.

Additionally, the results of the XP-SWMM modeling was used to identify potential impacts in the watershed related to surface flooding, impacts to homes, and stormwater infrastructure. Critical elevations in the Hiawatha north watershed around Columbus Pond, Sibley Pond, and Bloomington Pond are based on the critical elevations established in the Lake Hiawatha Stormwater Management Feasibility Study - Task 7 (Houston, 2014) completed for the City of Minneapolis. Critical elevations for low areas along E 43rd Street in the Hiawatha north watershed and in the Hiawatha west watershed were based on the surveys completed by the MPRB/City of Minneapolis (using the estimated elevation of the window sash as the low opening) and/or the minimum building elevation (estimated from the MnDNR Lidar data) for those structures that have not had a survey completed. We also considered the minimum building elevation in cases where the window sash was associated with an egress window to confirm if water would actually reach the window well associated with the egress windows. Potentially impacted structures are those where the modeled peak elevation exceeds either the surveyed window sash opening (assumed to



be low opening) or the minimum building elevation (for non-surveyed structures). This analysis does not take into account typical freeboard requirements.

Tables 6a and 6b summarize the peak flood elevations at select locations in the Hiawatha north and west watersheds for existing conditions along with the various water management alternatives to help understand the impact of the various water management alternatives on surface flooding/potential surface flooding. The locations summarized in Tables 6a and 6b are shown in Figure 11. Figure 12 shows the structures potentially impacted by surface flooding during the 10% chance (10-year) and 1% chance (100-year) design storm events in the Hiawatha west watershed and along E 43rd Street in the Hiawatha north watershed. However, these structures should be surveyed to determine their actual flood risk.

In the Hiawatha west watershed, there are no primary structures that are identified as being at-risk of flooding during either the 10 percent chance (10-year) and 1percent chance (100-year) design storm events in the local watershed. Although under the various water management alternatives, the XP-SWMM modeling suggests there will be increases in the peak elevations during the 10 percent chance event, these increases appear to be fully-contained within the existing storm sewer infrastructure and do not result in new surface flooding or an increase in the number of primary structures potentially at-risk in the Hiawatha west watershed. During the 1 percent chance event under the various water management alternatives, there are no anticipated increases in flood elevations in the Hiawatha west watershed except at the corner of 19th Avenue S and E 44th Street. This increase in peak elevations does not increase the number of at-risk structures and can be mitigated with modifications to the curb in the area to direct any water leaving the storm sewer system to the golf course area.

Because the XP-SWMM modeling suggests that the proposed water management alternatives for the golf course area have no impact or a slight reduction in flood elevations in the Hiawatha north watershed for both the 10 percent chance and 1 percent chance events, the analysis for identifying potentially impacted structures is focused on the low areas along E. 43rd Street, immediately adjacent to the Hiawatha west watershed and along the north edge of the Hiawatha Golf Course. Under 1 percent chance event for existing conditions, there are an estimated 24 primary structures that could be at-risk of flooding in the area along E. 43rd Street. In all water management scenarios that include the open channel concept, we have estimated that the number of at-risk primary structures could be reduced to 18 primary structures.

Table 6a: Summary of 10‐Year Peak Elevations at Select Areas in Hiawatha Watershed

Peak Elev (ft MSL 29)

Depth of Flooding

(ft)Change (ft)


Depth of Flooding

(ft)Change (ft)


Depth of Flooding

(ft)Change (ft)


Depth of Flooding

(ft)Change (ft)

Sibley Pond A 1939F 826.0 819.5 N/A 826.8 7.2 0.8 826.6 7.1 ‐0.2 826.6 7.1 ‐0.2 826.6 7.1 ‐0.1 826.6 7.1 ‐0.1Low Spot on the 19th Ave S, south of the crossing of E 40th St

B 1941B 827.6 827.1 N/A 827.8 0.8 0.2 827.8 0.8 0.0 827.8 0.8 0.0 827.8 0.8 0.0 827.8 0.8 0.0

Bloomington Pond C node559 820.0 813.4 N/A 817.9 4.5 ‐2.1 817.5 4.1 ‐0.4 817.5 4.1 ‐0.4 817.5 4.1 ‐0.4 817.5 4.1 ‐0.4Low Spot in the Alley in the southwest of the Bloomington Ave S and E 42nd St. Crossing

D 1542H N/A 818.6 N/A 818.6 0.1 N/A 818.4 0.0 ‐0.2 818.4 0.0 ‐0.2 818.4 0.0 ‐0.2 818.4 0.0 ‐0.2

Columbus Pond E Basin East 845.3 841.1 N/A 846.0 4.9 0.7 846.0 4.9 0.0 846.0 4.9 0.0 846.0 4.9 0.0 846.0 4.9 0.0Low Spot on the 17th Ave S, south of the crossing of E 43rd St

F 1743c.1 819.3 818.1 819.4 818.8 0.7 ‐0.5 818.7 0.6 ‐0.1 818.7 0.6 ‐0.1 818.7 0.6 ‐0.1 818.7 0.6 ‐0.1

Low spot at the corner of 19th Ave S & E 44th St G MH6 N/A 816.2 818.6 812.2 0.0 N/A 812.1 0.0 ‐0.1 813.9 0.0 1.7 813.8 0.0 1.6 814.0 0.0 1.8

Low spot at the corner of 19th Ave S & E 44th St H MH‐9 N/A 815.7 818.6 812.9 0.0 N/A 812.9 0.0 0.0 814.7 0.0 1.8 814.6 0.0 1.7 814.7 0.0 1.8

Low spot along E 44th St.between 16th Ave S and 17th Ave S

I 1644B N/A 820.8 N/A 820.6 0.0 N/A 820.6 0.0 0.0 820.6 0.0 0.1 820.6 0.0 0.1 820.6 0.0 0.1

Low spot in alley between 18th Ave S and Cedar Ave S just north of E 44th St.

J 1844B N/A 821.1 N/A 817.0 0.0 N/A 817.0 0.0 0.0 817.3 0.0 0.2 817.3 0.0 0.2 817.3 0.0 0.2

Low spot in alley between Longfellow Ave S and 19th Ave S just north of E 44rd St.

K LO44D N/A 816.6 818.6 816.2 0.0 N/A 816.2 0.0 0.0 816.5 0.0 0.3 816.5 0.0 0.3 816.5 0.0 0.3

:ow Spot in alley between 16th Ave S and 17th Ave S just south of E 44th St

L MH7 N/A 818.0 N/A 816.6 0.0 N/A 816.6 0.0 0.0 816.8 0.0 0.2 816.8 0.0 0.2 816.8 0.0 0.2

:ow Spot in alley between 18th Ave S and Cedar Ave S just south of E 44th St

M CB8 N/A 819.9 822.7 816.9 0.0 N/A 816.9 0.0 0.0 817.1 0.0 0.2 817.1 0.0 0.2 817.1 0.0 0.3

Southwest Corner of the 19th Ave S & E 43rd St Crossing

N 1943 N/A 828.5 820.8 817.1 0.0 N/A 814.6 0.0 ‐2.6 814.5 0.0 ‐2.6 814.6 0.0 ‐2.6 814.5 0.0 ‐2.6

Northeast Corner of the 20th Ave S & E 43rd St Crossing

O Node620 N/A 819.0 821.0 817.6 0.0 N/A 816.2 0.0 ‐1.4 816.2 0.0 ‐1.4 816.2 0.0 ‐1.4 816.2 0.0 ‐1.4

Northeast Corner of the 21st Ave S & E 43rd St Crossing

P Node622 N/A 820.2 820.6 817.6 0.0 N/A 817.3 0.0 ‐0.3 817.2 0.0 ‐0.3 817.2 0.0 ‐0.3 817.2 0.0 ‐0.3

Low spot in the southeast corner of Longfellow Ave S & E 44th St crossing

Q MH22 N/A 816.4 817.9 813.8 0.0 N/A 813.8 0.0 0.0 815.5 0.0 1.7 815.4 0.0 1.7 815.6 0.0 1.8

Low spot in the northeast corner of Longfellow Ave S & E 45th St Crossing

R LO45A N/A 820.8 821.4 820.5 0.0 N/A 820.5 0.0 0.0 820.5 0.0 0.0 820.5 0.0 0.0 820.5 0.0 0.0

Hiawatha Golf Course Area T Pond C N/A 809.9 N/A 812.2 2.3 N/A 811.6 1.7 ‐0.6 812.3 2.5 0.2 812.5 2.7 0.4 812.7 2.9 0.5Hiawatha Golf Course Area S Pond E N/A 808.3 N/A 812.2 3.9 N/A 811.6 3.3 ‐0.6 811.9 3.6 ‐0.3 812.5 4.2 0.4 812.7 4.4 0.5Hiawatha Golf Course Area (Drain Time) Pond E N/A 808.3 N/A 240hr 192hr 9hr 20hr 15hr

Golf Course: 30ft wide 2:1 Open Channel from 1043C to Lake Hiawatha (2000ft,

810.11 to 808.11)

Future Recreation: Ponds @ Lake elevation, 30ft wide slope 2:1 Open Channel from 1043C to Pond B (500ft, 810.11 to 809.61), No Stormwater Pumping, 60ft wide slope 2:1 Open

Channel cut on Berm invert from 809.81 to 809.78

Flood areaXP‐SWMM Node

NameCritical

Elevation1Ground

Elevation2Surveyed Elevation3

Water Management Alternative 4

Exsiting Condition: Golf Course


Future Recreation: Ponds @ elev per pumping to protect Adj homes (~812.3), 30ft wide 2:1 Open Channel from 1043C to Lake Hiawatha (2000ft, 810.11 to

808.11), Stormwater Pumps (2) starting at 812.301/813.3,stop at 812.3, 3 x 36" 30ft 1% Pipe w/ Backflow Preventer at 811.0

Lake Hiawatha = 811.8

Depth of Flooding

(ft)

Depth Above Critical Elevation

(ft)

Future Recreation: Ponds @ elev per pumping to protect Adj homes (~812.3), 30ft wide 2:1 Open Channel from 1043C

to Pond B (500ft, 810.11 to 809.61),Stormwater Pumps (2) starting at 812.301/813.3,stop at 812.3,9x30ft 1%

36" Pipe w/ Backflow Preventer at 809.81

Map Node (See Figure 11)

Proposed Condition






Depth of Flooding

(ft)Change (ft)


Depth of Flooding

(ft)Change (ft)


Depth of Flooding

(ft)Change (ft)


Depth of Flooding

(ft)Change (ft)


810.11 to 808.11)




NameCritical

Elevation1Ground







Depth of Flooding

(ft)


(ft)





Proposed Condition





B 1941B 827.6 827.1 N/A 827.8 0.8 0.2 827.8 0.8 0.0 827.8 0.8 0.0 827.8 0.8 0.0 827.8 0.8 0.0


D 1542H N/A 818.6 N/A 818.7 0.1 N/A 818.4 0.0 ‐0.3 818.4 0.0 ‐0.3 818.4 0.0 ‐0.2 818.4 0.0 ‐0.3


F 1743c.1 819.3 818.1 819.4 818.8 0.7 ‐0.5 818.7 0.6 ‐0.1 818.7 0.6 ‐0.1 818.7 0.6 ‐0.1 818.7 0.6 ‐0.1




I 1644B N/A 820.8 N/A 820.6 0.0 N/A 820.6 0.0 0.0 820.6 0.0 0.1 820.6 0.0 0.1 820.6 0.0 0.1


J 1844B N/A 821.1 N/A 817.0 0.0 N/A 817.0 0.0 0.0 817.4 0.0 0.3 817.3 0.0 0.3 817.3 0.0 0.3


K LO44D N/A 816.6 818.6 816.2 0.0 N/A 816.2 0.0 0.0 816.5 0.0 0.3 816.5 0.0 0.3 816.5 0.0 0.3


L MH7 N/A 818.0 N/A 816.6 0.0 N/A 816.6 0.0 0.0 816.9 0.0 0.3 816.8 0.0 0.3 816.9 0.0 0.3


M CB8 N/A 819.9 822.7 816.9 0.0 N/A 816.9 0.0 0.0 817.2 0.0 0.3 817.2 0.0 0.3 817.2 0.0 0.3


N 1943 N/A 828.5 820.8 817.2 0.0 N/A 814.7 0.0 ‐2.5 814.6 0.0 ‐2.6 814.7 0.0 ‐2.4 814.6 0.0 ‐2.6


O Node620 N/A 819.0 821.0 817.6 0.0 N/A 816.3 0.0 ‐1.3 816.3 0.0 ‐1.4 816.3 0.0 ‐1.4 816.3 0.0 ‐1.4


P Node622 N/A 820.2 820.6 817.6 0.0 N/A 817.3 0.0 ‐0.3 817.3 0.0 ‐0.3 817.3 0.0 ‐0.3 817.3 0.0 ‐0.3


Q MH22 N/A 816.4 817.9 813.8 0.0 N/A 813.8 0.0 0.0 816.1 0.0 2.3 815.8 0.0 2.0 816.0 0.0 2.3


R LO45A N/A 820.8 821.4 820.5 0.0 N/A 820.5 0.0 0.0 820.5 0.0 0.1 820.5 0.0 0.0 820.5 0.0 0.0

Hiawatha Golf Course Area T Pond C N/A 809.9 822.4 812.3 0.0 N/A 811.6 0.0 ‐0.7 812.9 0.0 0.6 812.9 0.0 0.6 813.3 0.0 1.0Hiawatha Golf Course Area S Pond E N/A 808.3 N/A 812.3 0.0 N/A 811.6 0.0 ‐0.7 812.8 0.0 0.5 812.9 0.0 0.7 813.3 0.0 1.0Hiawatha Golf Course Area (Drain Time) Pond E N/A 808.3 N/A 240hr 204hr 9hr* 129hr 93hr




Depth of Flooding

(ft)Change (ft)


Depth of Flooding

(ft)Change (ft)


Depth of Flooding

(ft)Change (ft)


Depth of Flooding

(ft)Change (ft)


810.11 to 808.11)




NameCritical

Elevation1Ground







Depth of Flooding

(ft)


(ft)





Proposed Condition





B 1941B 827.6 827.1 N/A 827.8 0.8 0.2 827.8 0.8 0.0 827.8 0.8 0.0 827.8 0.8 0.0 827.8 0.8 0.0


D 1542H N/A 818.6 N/A 818.7 0.1 N/A 818.4 0.0 ‐0.3 818.4 0.0 ‐0.3 818.4 0.0 ‐0.3 818.4 0.0 ‐0.3


F 1743c.1 819.3 818.1 819.4 818.8 0.7 ‐0.5 818.7 0.6 ‐0.1 818.7 0.6 ‐0.1 818.7 0.6 ‐0.1 818.7 0.6 ‐0.1


Low spot at the corner of 19th Ave S & E 44th St H MH‐9 N/A 815.7 818.6 812.9 0.0 N/A 812.9 0.0 ‐0.1 816.0 0.3 3.1 815.1 0.0 2.2 815.4 0.0 2.4


I 1644B N/A 820.8 N/A 820.6 0.0 N/A 820.6 0.0 0.0 820.7 0.0 0.1 820.6 0.0 0.1 820.6 0.0 0.1


J 1844B N/A 821.1 N/A 817.0 0.0 N/A 817.0 0.0 0.0 817.8 0.0 0.7 817.3 0.0 0.3 817.4 0.0 0.3


K LO44D N/A 816.6 818.6 816.2 0.0 N/A 816.2 0.0 0.0 816.6 0.0 0.4 816.5 0.0 0.3 816.5 0.0 0.3


L MH7 N/A 818.0 N/A 816.6 0.0 N/A 816.6 0.0 0.0 817.1 0.0 0.5 816.9 0.0 0.3 816.9 0.0 0.3


M CB8 N/A 819.9 822.7 816.9 0.0 N/A 816.9 0.0 0.0 817.4 0.0 0.5 817.2 0.0 0.3 817.2 0.0 0.4


N 1943 N/A 828.5 820.8 817.2 0.0 N/A 815.1 0.0 ‐2.1 815.0 0.0 ‐2.2 815.1 0.0 ‐2.1 814.6 0.0 ‐2.6


O Node620 N/A 819.0 821.0 817.7 0.0 N/A 816.6 0.0 ‐1.1 816.6 0.0 ‐1.1 816.6 0.0 ‐1.1 816.6 0.0 ‐1.1


P Node622 N/A 820.2 820.6 817.6 0.0 N/A 817.4 0.0 ‐0.2 817.4 0.0 ‐0.2 817.4 0.0 ‐0.2 817.4 0.0 ‐0.2


Q MH22 N/A 816.4 817.9 813.8 0.0 N/A 813.8 0.0 ‐0.1 816.6 0.2 2.8 815.9 0.0 2.1 816.2 0.0 2.3


R LO45A N/A 820.8 821.4 820.5 0.0 N/A 820.5 0.0 0.0 820.5 0.0 0.1 820.5 0.0 0.0 820.5 0.0 0.1

Hiawatha Golf Course Area T Pond C N/A 809.9 822.4 812.8 2.9 N/A 811.6 1.7 ‐1.2 814.1 4.2 1.3 813.3 3.4 0.5 814.3 0.0 1.5Hiawatha Golf Course Area S Pond E N/A 808.3 N/A 812.6 4.3 N/A 811.6 3.3 ‐1.0 814.1 5.8 1.5 813.3 5.0 0.7 814.3 0.0 1.7Hiawatha Golf Course Area (Drain Time) Pond E N/A 808.3 N/A >>240hr 228hr 9hr ~230hr >>240hr1 ‐ Critical Elevation is referencing Houston Report with exception of the Golf Course, which is referencing the FEMA Flood Elevation2 ‐ Ground elevation based on node location and 2011 MnLiDAR data, used to estimate depth of surface flooding3 ‐ Surveyed elevation from Mpls Basement Survey, approx elevation of window sash, only reflects structures surveyed to date*‐ Due to the tailwater condition in Lake Hiawatha, the final elevation will not drain to the starting elevation


Table 6b: Summary of 100‐Year Peak Elevations at Select Areas in Hiawatha Watershed


Depth of Flooding

(ft)Change (ft)


Depth of Flooding

(ft)Change (ft)


Depth of Flooding

(ft)Change (ft)


Depth of Flooding

(ft)Change (ft)


B 1941B 827.6 827.1 N/A 828.5 1.4 0.9 828.3 1.3 ‐0.1 828.3 1.3 ‐0.1 828.3 1.3 ‐0.1 828.3 1.3 ‐0.1

Bloomington Pond C node559 820.0 813.4 N/A 821.4 8.0 1.4 820.8 7.4 ‐0.6 820.8 7.4 ‐0.6 820.8 7.4 ‐0.6 820.8 7.4 ‐0.6Low Spot in the Alley in the southwest of the Bloomington Ave S and E 42nd St. Crossing

D 1542H N/A 818.6 N/A 820.9 2.3 N/A 820.7 2.1 ‐0.2 820.7 2.1 ‐0.2 820.7 2.1 ‐0.2 820.7 2.1 ‐0.2


F 1743c.1 819.3 818.1 819.4 820.1 2.0 0.8 820.1 2.0 ‐0.1 820.1 2.0 0.0 820.1 2.0 0.0 820.1 2.0 0.0




I 1644B N/A 820.8 N/A 821.0 0.2 N/A 821.0 0.2 0.0 821.0 0.2 0.0 821.0 0.2 0.0 821.0 0.2 0.0


J 1844B N/A 821.1 N/A 820.7 0.0 N/A 820.7 0.0 0.0 820.7 0.0 0.0 820.7 0.0 0.0 820.7 0.0 0.0


K LO44D N/A 816.6 818.6 816.7 0.1 N/A 816.7 0.1 0.0 816.7 0.1 0.0 816.7 0.1 0.0 816.7 0.1 0.0


L MH7 N/A 818.0 N/A 817.9 0.0 N/A 817.9 0.0 0.0 817.9 0.0 0.0 817.9 0.0 0.0 817.9 0.0 0.0


M CB8 N/A 819.9 822.7 818.1 0.0 N/A 818.0 0.0 0.0 818.1 0.0 0.0 818.1 0.0 0.0 818.1 0.0 0.0


N 1943 N/A 828.5 820.8 818.1 0.0 N/A 815.6 0.0 ‐2.4 815.6 0.0 ‐2.5 815.6 0.0 ‐2.4 815.6 0.0 ‐2.5


O Node620 N/A 819.0 821.0 818.4 0.0 N/A 816.9 0.0 ‐1.5 816.8 0.0 ‐1.5 816.8 0.0 ‐1.5 816.8 0.0 ‐1.5


P Node622 N/A 820.2 820.6 818.1 0.0 N/A 817.5 0.0 ‐0.6 817.5 0.0 ‐0.6 817.5 0.0 ‐0.6 817.5 0.0 ‐0.6


Q MH22 N/A 816.4 817.9 817.2 0.8 N/A 817.2 0.8 0.0 817.2 0.8 0.0 817.2 0.8 0.0 817.2 0.8 0.0


R LO45A N/A 820.8 821.4 820.8 0.0 N/A 820.8 0.0 0.0 820.8 0.0 0.0 820.8 0.0 0.0 820.8 0.0 0.0

Hiawatha Golf Course Area T Pond C N/A 809.9 822.4 814.3 4.5 N/A 813.1 3.2 ‐1.2 812.7 2.8 ‐1.6 813.3 3.5 ‐1.0 813.5 3.6 ‐0.8Hiawatha Golf Course Area S Pond E N/A 808.3 N/A 814.3 6.0 N/A 813.1 4.8 ‐1.2 812.0 3.7 ‐2.3 813.3 5.0 ‐1.0 813.5 5.2 ‐0.8Hiawatha Golf Course Area (Drain Time) Pond E N/A 808.3 N/A >>240hr >>240hr 16hr 27hr 18hr


XP‐SWMM Node Name



810.11 to 808.11)





(ft)



to Pond B (500ft, 810.11 to 809.61),Stormwater Pumps (2) starting at 812.301/813.3,stop at 812.3, 9x30ft 1% 36" Pipe w/ Backflow Preventer at 809.81

Flood areaCritical


Ground Elevation2

Proposed Condition







Depth of Flooding

(ft)



Depth of Flooding

(ft)Change (ft)


Depth of Flooding

(ft)Change (ft)


Depth of Flooding

(ft)Change (ft)


Depth of Flooding

(ft)Change (ft)


XP‐SWMM Node Name



810.11 to 808.11)




(ft)




Flood areaCritical


Ground Elevation2

Proposed Condition







Depth of Flooding

(ft)


B 1941B 827.6 827.1 N/A 828.5 1.4 0.9 828.3 1.3 ‐0.1 828.3 1.3 ‐0.1 828.3 1.3 ‐0.1 828.3 1.3 ‐0.1


D 1542H N/A 818.6 N/A 821.3 2.7 N/A 820.7 2.1 ‐0.6 820.7 2.1 ‐0.6 820.7 2.1 ‐0.6 820.7 2.1 ‐0.6


F 1743c.1 819.3 818.1 819.4 820.1 2.0 0.8 820.1 2.0 0.0 820.1 2.0 0.0 820.1 2.0 0.0 820.1 2.0 0.0




I 1644B N/A 820.8 N/A 821.0 0.2 N/A 821.0 0.2 0.0 821.0 0.2 0.0 821.0 0.2 0.0 821.0 0.2 0.0


J 1844B N/A 821.1 N/A 820.7 0.0 N/A 820.7 0.0 0.0 820.7 0.0 0.0 820.7 0.0 0.0 820.7 0.0 0.0


K LO44D N/A 816.6 818.6 816.7 0.1 N/A 816.7 0.1 0.0 816.7 0.1 0.0 816.7 0.1 0.0 816.7 0.1 0.0


L MH7 N/A 818.0 N/A 817.9 0.0 N/A 817.9 0.0 0.0 817.9 0.0 0.0 817.9 0.0 0.0 818.0 0.0 0.1


M CB8 N/A 819.9 822.7 818.1 0.0 N/A 818.0 0.0 0.0 818.1 0.0 0.0 818.1 0.0 0.0 818.1 0.0 0.0


N 1943 N/A 828.5 820.8 818.1 0.0 N/A 815.7 0.0 ‐2.4 815.6 0.0 ‐2.5 815.7 0.0 ‐2.4 815.7 0.0 ‐2.4


O Node620 N/A 819.0 821.0 818.4 0.0 N/A 817.1 0.0 ‐1.2 817.0 0.0 ‐1.3 817.0 0.0 ‐1.3 817.0 0.0 ‐1.3


P Node622 N/A 820.2 820.6 818.2 0.0 N/A 817.6 0.0 ‐0.6 817.6 0.0 ‐0.6 817.6 0.0 ‐0.6 817.6 0.0 ‐0.6


Q MH22 N/A 816.4 817.9 817.2 0.0 N/A 817.2 0.0 0.0 817.2 0.0 0.0 817.2 0.0 0.0 817.2 0.0 0.0


R LO45A N/A 820.8 821.4 820.8 0.0 N/A 820.8 0.0 0.0 820.8 0.0 0.0 820.8 0.0 0.0 820.8 0.0 0.0

Hiawatha Golf Course Area T Pond C N/A 809.9 822.4 814.5 0.0 N/A 813.1 0.0 ‐1.3 813.0 0.0 ‐1.5 813.6 0.0 ‐0.9 814.0 0.0 ‐0.5Hiawatha Golf Course Area S Pond E N/A 808.3 N/A 814.5 0.0 N/A 813.1 0.0 ‐1.3 812.9 0.0 ‐1.6 813.6 0.0 ‐0.9 814.0 0.0 ‐0.5Hiawatha Golf Course Area (Drain Time) Pond E N/A 808.3 N/A >240hr >>240hr 17hr 83hr 95hr




Depth of Flooding

(ft)Change (ft)


Depth of Flooding

(ft)Change (ft)


Depth of Flooding

(ft)Change (ft)


Depth of Flooding

(ft)Change (ft)


XP‐SWMM Node Name



810.11 to 808.11)




(ft)




Flood areaCritical


Ground Elevation2

Proposed Condition







Depth of Flooding

(ft)


B 1941B 827.6 827.1 N/A 828.5 1.4 0.9 828.4 1.3 ‐0.1 828.4 1.3 ‐0.1 828.4 1.3 ‐0.1 828.3 1.3 ‐0.1


D 1542H N/A 818.6 N/A 821.3 2.8 N/A 820.7 2.1 ‐0.7 820.7 2.1 ‐0.7 820.7 2.1 ‐0.7 820.7 2.1 ‐0.7


F 1743c.1 819.3 818.1 819.4 820.1 2.0 0.9 820.1 2.0 ‐0.1 820.1 2.0 0.0 820.1 2.0 0.0 820.1 2.0 0.0




I 1644B N/A 820.8 N/A 821.0 0.2 N/A 821.0 0.2 0.0 821.0 0.2 0.0 821.0 0.2 0.0 821.0 0.2 0.0


J 1844B N/A 821.1 N/A 820.7 0.0 N/A 820.7 0.0 0.0 820.7 0.0 0.0 820.7 0.0 0.0 820.7 0.0 0.0


K LO44D N/A 816.6 818.6 816.7 0.1 N/A 816.7 0.1 0.0 816.7 0.1 0.0 816.7 0.1 0.0 816.7 0.1 0.0


L MH7 N/A 818.0 N/A 817.9 0.0 N/A 817.9 0.0 0.0 818.0 0.0 0.1 818.0 0.0 0.0 818.0 0.0 0.1


M CB8 N/A 819.9 822.7 818.1 0.0 N/A 818.0 0.0 0.0 818.1 0.0 0.1 818.1 0.0 0.0 818.1 0.0 0.0


N 1943 N/A 828.5 820.8 818.1 0.0 N/A 815.9 0.0 ‐2.2 815.8 0.0 ‐2.3 815.9 0.0 ‐2.2 815.7 0.0 ‐2.4


O Node620 N/A 819.0 821.0 818.4 0.0 N/A 817.5 0.0 ‐0.9 817.5 0.0 ‐0.8 817.5 0.0 ‐0.8 817.5 0.0 ‐0.8


P Node622 N/A 820.2 820.6 818.2 0.0 N/A 817.7 0.0 ‐0.6 817.7 0.0 ‐0.5 817.7 0.0 ‐0.5 817.7 0.0 ‐0.5


Q MH22 N/A 816.4 817.9 817.2 0.8 N/A 817.2 0.8 0.0 817.2 0.8 0.0 817.2 0.8 0.0 817.2 0.0 0.0


R LO45A N/A 820.8 821.4 820.8 0.0 N/A 820.8 0.0 0.0 820.8 0.0 0.0 820.8 0.0 0.0 820.8 0.0 0.0

Hiawatha Golf Course Area T Pond C N/A 809.9 822.4 814.8 4.9 N/A 813.1 3.2 ‐1.7 814.1 4.3 ‐0.6 814.0 4.2 ‐0.7 814.8 0.0 0.0Hiawatha Golf Course Area S Pond E N/A 808.3 N/A 814.8 6.5 N/A 813.1 4.8 ‐1.7 814.1 5.8 ‐0.7 814.0 5.7 ‐0.7 814.8 0.0 0.0Hiawatha Golf Course Area (Drain Time) Pond E N/A 808.3 N/A <>>240hr >>240hr 19hr >>240hr >>240hr1 ‐ Critical Elevation from Houston Report, 2014 ( the Golf Course from the FEMA Flood Elevation)2 ‐ Ground elevation based on node location and 2011 MnLiDAR data, used to estimate depth of surface flooding3 ‐ Surveyed elevation from Mpls Basement Survey, approx elevation of window sash, only reflects structures surveyed to date*‐ Due to the tailwater condition in Lake Hiawatha, the final elevation will not drain to the starting elevation


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16th

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17th

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Steve

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Blaisd

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E 40th St

E 29th St

W 31st St

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W Lake St

W 35th St

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W 40th St

W 44th St

W 45th St

W 43rd St

W 47th St

E 39th St

E 41st St

W 32nd St

W 42nd St

E 37th St

E 33rd St

Clinto

n Ave

S

E 36th 1/2 St

W 34th St

26th Ave S

W 33rd St

Elliot

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Crosby Pl

W 48th St

Elroy St

Van N

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23rd

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E 45th St

E 41st St

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29th

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22nd

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E 43rd St

35th

Ave S

26th

Ave S

E 47th St

E 37th St

14th

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E 39th St

E 45th St

Clinto

n Ave

S

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W 33rd St

E 41st St

E 34th St E 34th St

35th

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Blaisd

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Pleas

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Steve

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E 31st St

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E 36th St

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E 39th St

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E 44th St

E 29th St

E 33rd St

E 48th St

10th

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E 37th St

Hiawatha Ave

30th

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n Ave

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E 33rd St

E 32nd St

E 33rd St

19th

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10th

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11th

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28th

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36th

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E 40th St

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P:\Mpls\23 MN\27\23271466 Hiawatha Golf Course Analyses\WorkFiles\2017Scope\WetlandDelineation\Draft\Hiawatha_Wetland_2017.pdf

XP-SWMMLocation of Interest

Hiawatha Golf Course Surface Water Management Analysis

Minneapolis Park & Recreation Board

FIGURE 11

0 600 1,200

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!( Location of Interest

Modeled XP-SWMM Link

XP-SWMM Model Boundary

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Sibley Pond

Bloomington Pond

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I:\Projects\23\27\1466\Users\XF2\Building _2017_analysis.mx d

Potentially Impacted Structures – Ex isting Conditions

Hiawatha Golf Course AreaCity of Minneapolis

FIGURE 12

1% Chance (100Yr) Potentially Impacted Primary Structures1

10% Chance (10Yr) Potentially Impacted Primary Structures2

GrndElevNGVD29815.0 - 820.0

820.0 - 830.0

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! XP-SWMM Node

XP-SWMM Link

I0 240 480 720120

Feet

1 24 Primary Structures/ 29 Garages2 2 Primary Structures / 8 Garages

DRAFT



5.0 Water Quality Impacts Water quality of Lake Hiawatha was identified by City, MPRB, and MCWD staff and the general public as an important consideration. The following section summarizes water quality implications for the various water management alternatives.

5.1 Golf Course Pond Water Quality (Phosphorus) Barr staff originally collected water quality grab samples from the Hiawatha Golf Course Ponds on January 20, 2016. These grab samples were tested for a variety of parameters (see memo dated 2/28/2017). Based on these samples, the total phosphorus concentration in Golf Course Pond E was 0.076 mg/L and in Lake Hiawatha, the total phosphorus concentration was 0.028 mg/L.

More recently, MPRB staff collected additional water quality grab samples from Lake Hiawatha and the Hiawatha Golf Course Ponds (collected at Pond E, in pump vault, or at pump discharge pipe, depending on accessibility). These samples were collected on 1/31/2017, 4/12/2017 (Lake Hiawatha), 4/13/2017 (Hiawatha Golf Course Ponds), 4/25/2017, 5/11/2017, and 5/26/2017. These samples were tested for total phosphorus and soluble reactive phosphorus (dissolved phosphorus). The 2017 total phosphorus data is summarized in Figure 13.

The total phosphorus concentrations in the Hiawatha Golf Course ponds are 2-3 times the concentrations observed in Lake Hiawatha, similar to the samples collected in January 2016. The pond total phosphorus concentrations ranged from 0.099 mg/L to 0.163 mg/L, which is typical of concentrations seen in stormwater ponds.

The total phosphorus concentrations observed in Lake Hiawatha in the surface sample (0-2 m) is similar to the water quality observed in the deeper lake sample (4 m). The surface concentrations ranged from 0.041 mg/L to 0.091 mg/L while the deeper samples ranged from 0.031 mg/L to 0.080 mg/L.

5.2 Comparison with the Lake Hiawatha TMDL Lake Hiawatha is listed as impaired for excess nutrients by the Minnesota Pollution Control Agency (MPCA). A Total Maximum Daily Load (TMDL) study (Tetra Tech, 2013) was completed and approved on February 24, 2014. Site-specific standards were established for Lake Hiawatha including the following water quality goals for the growing season (June-September):

• Total Phosphorus < 50ug/L • Chlorophyll a < 14 ug/L • Secchi Depth > 1.4 m

Through the TMDL process, the seasonal (June – September) flow volumes, phosphorus loads, and lake residence time were summarized from 2001 through 2011 (11 years). The average seasonal inflow to the



lake is 1,053 million cubic feet with 77% of the inflow from Minnehaha Creek and 23% coming from stormflow. The seasonal total phosphorus load to Lake Hiawatha is 6,463 pounds. A 30-percent reduction in the total phosphorus load would be needed to achieve the TMDL. For the City of Minneapolis, a load reduction of 400.4 pounds total phosphorus per growing season is required.

The estimated average residence time for Lake Hiawatha is 4.4 days (ranging from 1.8 to 47.4 days). The TMDL indicates that internal loading may be a source of phosphorus to the lake during low flow conditions/years when the residence time is longer and the lake may become stratified. However, under normal and high flow conditions, stratification typically does not occur and residence times are short, minimizing the impact of internal loading on lake water quality.

Based the estimated seasonal volume pumped from the golf course to Lake Hiawatha using the monitored pumping rate, the estimated volume pumped from the golf course to the lake could range from 0.3% (wet conditions) to 7.6% (dry conditions) of the total seasonal inflow volume. Using the water quality data collected in the golf course ponds and the seasonal volume to the lake, the estimated seasonal phosphorus load from the golf course represents approximately 1.0 percent of the total seasonal phosphorus load to Lake Hiawatha.

Additionally, based on the estimates using the Minnesota Pollution Control Agency (MPCA) Minimal Impact Design Standards (MIDS) calculator analysis (see Section 5.2 below), the annual total phosphorus load from the local Lake Hiawatha watersheds (Hiawatha west and Hiawatha north), is 660 pounds per year. To translate this to a comparable seasonal load (June through September) as summarized in the TMDL study, we utilized the average annual monthly precipitation for the Twin Cities area to estimate that 50 percent of the annual precipitation/runoff occurs from June through September. Applying this percentage to the MIDS annual total phosphorus watershed loads, we estimated that the local watershed contributes approximately 330 pounds of total phosphorus to the lake from June through September. The estimated loading from the local watersheds represents approximately five (5) percent of the seasonal phosphorus load to Lake Hiawatha (6,463 pounds), as estimated in the TMDL study.

5.3 Estimated Phosphorus Removals Utilizing the existing topography and pond bathymetry for the golf course area, the existing and/or proposed normal water levels in the golf course area, and the contributing watersheds, we were able to estimate how the water quality treatment volume to be provided by the water management alternatives compares to the volume needed to meet the National Urban Runoff Program (NURP) standards. A pond designed to meet NURP standards typically achieves 60 percent of the phosphorus associated with particulates. Stormwater ponds/open water typically do not remove the dissolved phosphorus fraction in runoff.



We utilized the MPCA MIDS calculator to estimate the annual phosphorus loads (particulate and dissolved) from the Hiawatha north, Hiawatha west, and direct golf course watersheds. Based on the fraction of the required NURP volume the proposed volume provides, we assumed that the wetland will remove 60 percent of the particulate phosphorus load from the equivalent fraction of the proposed watershed. If the volume provided was greater than the volume required to meet NURP standards, 60 percent removal was applied to the entire particulate phosphorus load from the contributing watershed.

Based on this analysis, Table 7 summarizes the estimated annual phosphorus removal for each of the water management alternatives, including existing conditions. We have also utilized the estimated fraction of precipitation that occurs from June through September (discussed in Section 5.1 above) to estimate the seasonal total phosphorus removals.

As previously mentioned, this water quality analysis was performed applying the existing topography for the golf course area and ponds. Additional water quality treatment may be developed by creating deeper ponds/more dead pool volume and should be considered in future phases of this project. The realignment of Minnehaha Creek, such as in Alternative 3b, could provide treatment of a portion of the creek flows as well. For example, if the treatment volume could be provided to remove 1% of the creek’s seasonal phosphorus load, this could result in the removal of an additional 60-65 pounds of total phosphorus seasonally (approximately 120-130 pounds annually).



Table 7: Estimated Annual and Seasonal (June-September) Total Phosphorus Removals

Alternative Estimated Annual Total Phosphorus Removal (lbs/yr)

Estimated Seasonal Total Phosphorus Removal

(lbs/yr)

Existing Conditions 46 23

Alternative 1 46 23

Alternative 2 46 23

Alternative 3a / 3b 229 114

Alternative 4 46 23

Alternative 5 180 90

5.4 Other Water Quality Considerations A site visit to Hiawatha Golf Course on 3/28/2017 confirmed that the turf areas of the golf course, especially those areas closest to the existing ponds, are heavily used by geese and that the area has significant quantities of geese droppings. These droppings can have a impact on the pond water quality, especially nutrients and bacteria, in the golf course as well as Lake Hiawatha. During future phases of the project, the MPRB should consider way of managing the existing golf course and/or the future alternative area to discourage the congregation of geese.

Additionally, residents from the larger Hiawatha community have expressed concerns about trash accumulating in the lake, and University of Minnesota (U of MN) civil engineering students are currently doing a capstone project on the trash problem in the area. The U of MN students are expected to have a report summarizing recommendations for the management of trash in the larger Hiawatha watershed as well as in Lake Hiawatha which the City and MPRB will consider.

The various alternatives which could include the daylighting of an existing storm sewer and design of open channel conveyance and potential reconfiguration of the part of the golf course or changes in land use will provide the opportunity to incorporate trash mitigation/collection measures.



Figure 13 – Summary of 2017 Lake Hiawatha and Golf Course Pond Water Quality (Total Phosphorus)



6.0 Summary of Water Management Alternatives The following section summarizes the expected impacts of the evaluation of the water management alternatives for the Hiawatha golf course area. Table 8 attached summarizes the technical results and expected water resource related impacts for the various water management alternatives based on the modeling and technical analyses completed for this area. Table 9 and the discussion below highlights some of the key takeaways from the data summarized in Table 8. In Table 9, those items highlighted in green indicate those changes that could be perceived as a potential benefit while those in red, could be considered a potential drawback.

6.1 Impact of Lowering Lake Hiawatha • Reduces the amount of groundwater pumping needed (reduces amount of inflow through the

berm from the Lake) • Reduces the expected normal water level in the golf course area (resulting in a larger upland park

area) • Potentially reduces the amount of water quality treatment (reduction in dead pool volume)

provided • Slight reduction in the amount of flooding at Bloomington Pond Area/Sibley Pond (Hiawatha

north watershed) (~0.1 ft reduction) • Minimal/no impact on flooding in Hiawatha west watershed • Reduces the FEMA flood elevation for Lake Hiawatha by ~0.6 feet

However, to achieve a lower control elevation of Lake Hiawatha, it will require the following:

• Removal of the existing concrete weir structure at the outlet of Lake Hiawatha. • Redesign/lowering of ~2,000 feet of the Minnehaha Creek stream channel downstream of Lake

Hiawatha to downstream of Nokomis Avenue to invert elevation 809.0 ft MSL (+/-). • Redesign/modification to or reconstruction of several creek crossings including the 28th Avenue S

Bridge, the pedestrian bridge at 30th Avenue S, and the Nokomis Avenue S Bridge. • Lowering of the City of Minneapolis water main located upstream of 28th Avenue S Bridge to

provide sufficient cover. • Providing sufficient cover/protection of the existing City of Minneapolis and MCES sanitary sewer

crossings. • Removal of the abandoned CenterPoint gas main located downstream of 28th Avenue S Bridge. • All associated permitting activities related to the modifications to the average water level in Lake

Hiawatha, the stream channel redesign and construction, the modifications/reconstruction of existing bridge/pedestrian crossings and utility work.

MPRB, City, and MCWD staff discussed the lowering of Lake Hiawatha as part of the pumping mitigation strategy; however, because the lowering of the lake would require significant design and permitting



efforts for only slight improvements, they decided that pursuing the lowering of Lake Hiawatha would not be part of the long-term water management strategies.

6.2 Existing Conditions • Overflows from the existing storm sewer along 43rd to the golf course during larger events • No improvement on surface flooding in the Hiawatha north watershed – there are 2 potentially

impacted primary structures during the 10% chance event and 24 potentially impacted primary structures during the 1% chance event.

• No impact on flooding in Hiawatha west watershed – there are no potentially impacted primary structures during the 10% chance event and no potentially impacted primary structures during the 1% chance event.

• Long event drawdown times on Hiawatha golf course area due to reliance entirely on pumping rate

• No reduction in groundwater or stormwater pumping (~308 MGY) • No impact on regional resources • No change in water quality treatment – treatment of Hiawatha west watershed only (~46 lbs total

phosphorus (TP) removal/year)

6.3 Water Management Alternative #1 (No Pumping) • Higher normal water level in golf course area (if berm in place) • Impacts to 11-14 basements (per basements surveyed to date) • No improvement on surface flooding in the Hiawatha north watershed – there are 2 potentially

impacted primary structures during the 10% chance event and 24 potentially impacted primary structures during the 1% chance event.

• Likely increases in peak elevations in the Hiawatha west watershed during 10% chance event (due to increased tailwater conditions on storm sewer system); however, no expected impact on surface flooding in Hiawatha west watershed – there are no potentially impacted primary structures during the 10% chance event and no potentially impacted primary structures during the 1% chance event.

• Increase in water quality treatment – treatment of Hiawatha west watershed (~46 lbs TP removal/year)

• Increases water levels in Powderhorn Lake by 0.7-0.8 feet (requires additional ~12-19 gpm of pumping at Powderhorn Lake)

6.4 Water Management Alternative #2 (Open Channel around Golf Course Area)

• Open channel will require berm/separation from golf course area



• Reduces flood elevations in the golf course by eliminating the overflow from the existing storm sewer along 43rd to the golf course during larger events

• Reduces flooding at Bloomington Pond Area/Sibley Pond (Hiawatha north watershed) (~0.1-0.4 ft) in the Hiawatha north watershed – there are 2 potentially impacted primary structures during the 10% chance event and 18 potentially impacted primary structures during the 1% chance event (6 homes removed).

• No impact on flooding in Hiawatha west watershed – there are no potentially impacted primary structures during the 10% chance event and no potentially impacted primary structures during the 1% chance event.

• Long event drawdown times on Hiawatha golf course area due to reliance entirely on pumping rate

• No reduction in groundwater or stormwater pumping • No impact on regional resources • No change in water quality treatment – treatment of Hiawatha west watershed only (~46 lbs TP

removal/year) • Will require reconfiguring the golf course to maintain 18 holes

6.5 Water Management Alternative #3a & #3b (Open Channel through Golf Course Area, Open Channel Connection to Lake, Realignment of Minnehaha Creek (#3b))

• Open channel can pass through golf course area • Reduces flood elevations in the golf course area by eliminating the overflow from the existing

storm sewer along 43rd to the golf course during larger events • Reduces flooding at Bloomington Pond Area/Sibley Pond (Hiawatha north watershed) (~0.2-0.6 ft

reduction at Bloomington Pond area, ~0.2 ft reduction at Sibley Pond) – there are 2 potentially impacted primary structures during the 10% chance event and 18 potentially impacted primary structures during the 1% chance event (6 homes removed).

• Increases in peak elevations in the Hiawatha west watershed during 10% chance event (due to increased tailwater conditions on storm sewer system); however, there are no potentially impacted primary structures during the 10% chance event and no potentially impacted primary structures during the 1% chance event.

• Reduction in event drawdown times on Hiawatha golf course area – no stormwater pumping required

• Reduction in groundwater pumping – amount of pumping highly dependent on Lake Hiawatha water levels (~69% reduction in total pumping)




• Significant increase in water quality treatment – treatment of Hiawatha west & north watersheds (~229 lbs TP removal/yr)

• Significant water level fluctuations throughout the year directly tied to Lake Hiawatha water levels (resulting in less upland park area as much of the park area could potentially be under water for longer durations when Minnehaha Creek flows are high) – expected range ~40+ acres (69 acres – 111 acres)

• Alternative #3b would allow realignment of Minnehaha Creek through the golf course area and could potentially provide more water quality treatment (e.g if 1% of phosphorus load from Minnehaha Creek could be treated, additional ~120 lbs TP removal/yr)

6.6 Water Management Alternative #4 (Open Channel around Golf Course Area, Pipe Connection w/ Backflow Prevention and Stormwater Pumping above NWL)

• Open channel will require berm/separation from golf course area • Reduces flood elevations in the golf course area by eliminating the overflow from the existing

storm sewer along 43rd to the golf course during larger events • Reduces flooding at Bloomington Pond Area/Sibley Pond (Hiawatha north watershed) (~0.2-0.6 ft

reduction at Bloomington Pond area, ~0.2 ft reduction at Sibley Pond) – there are 2 potentially impacted primary structures during the 10% chance event and 18 potentially impacted primary structures during the 1% chance event (6 homes removed).


• Reduction in event drawdown times on Hiawatha golf course area due to combined gravity (3x36” pipes w/ backflow prevention)/pumping system

• Reduction in groundwater & stormwater pumping – less dependent on Lake Hiawatha water levels (than direct connection to lake) (~76% reduction in total pumping)


• Increase in water quality treatment – treatment of Hiawatha west watershed only (~46 lbs TP removal/year)

• More controlled water level fluctuations in the golf course area (resulting in a more constant upland park area as less dependent on the water level fluctuations) – expected range within ~10 acres (99 – 109 acres)

• Could allow realignment of Minnehaha Creek; however, creek would still need to be separated from the golf course area by a berm.



6.7 Water Management Alternative #5 (Open Channel through Golf Course Area, Pipe Connection w/ Backflow Prevention and Stormwater Pumping above NWL)

• Reduces flood elevations in the golf course area by eliminating the overflow from the existing storm sewer along 43rd to the golf course during larger events

• Reduces flooding at Bloomington Pond Area/Sibley Pond (Hiawatha north watershed) (~0.2-0.6 ft reduction at Bloomington Pond area, ~0.2 ft reduction at Sibley Pond) – there are 2 potentially impacted primary structures during the 10% chance event and 18 potentially impacted primary structures during the 1% chance event (6 homes removed).


• Reduction in event drawdown times on Hiawatha golf course area due to combined gravity (9x36” pipes w/ backflow prevention)/pumping system

• Reduction in groundwater & stormwater pumping – less dependent on Lake Hiawatha water levels (than direct connection to lake) (~81% reduction in total pumping)


• Significant increase in water quality treatment – treatment of Hiawatha west & north watershed (~180 lbs TP removal/year)

• More controlled water level fluctuations in the golf course area (resulting in a more constant upland park area as less dependent on the water level fluctuations) – expected range within ~10 acres (99 – 109 acres)

• Could allow realignment of Minnehaha Creek; however, creek would still need to be separated from the golf course area by a berm.

6.8 Preferred Water Management Alternatives The various water management alternatives were initially presented to and discussed with MPRB, City, and MCWD staff at a meeting held on 4/11/2017. Several follow-up items were further investigated and summarized (in memo above) for staff based on questions asked at that meeting including:

• Evaluating the storm sewer capacity along E. 43rd Street to accommodate the pumping required at 43rd and 17th,

• Investigation into the pumping rates needed to maintain existing water levels at Powderhorn Lake during the reduced pumping scenarios,

• Utilizing Gray’s Bay discharge data from MCWD to correlate long durations of elevated water levels on Lake Hiawatha to high discharges from the Gray’s Bay dam,



• Further investigations into surface flooding in the Hiawatha North and West watersheds, and • Additional summary of TMDL and water quality implications.

Additionally, the public input from meeting held on 4/20/2017 identified water management related-themes including: intermittently-inundated wetlands, prairie/wet meadow restoration, and shoreline habitat restoration as well as the restoration of habitat such as natives and pollinator gardens/areas, amphibian education trail, bird sanctuaries, and wildlife blinds. The MPRB, City, and MCWD staff considered this public input and opportunities to incorporate these water-related concepts when selecting the preferred reduced-pumping water management concept for the Hiawatha golf course area.

Considering all the information and analysis presented for each alternative and the water management objectives (listed below), the preferred water management approach for a reduced-pumping alternative was selected by the MPRB, City, and MCWD staff at a meeting held on 4/21/2017.

1. Reduce pumping while still protecting adjacent basements 2. At a minimum, continue to provide water quality treatment for the watershed to the west of the

golf course and if possible, increase water quality treatment as much as possible to help achieve the phosphorus load reduction to Lake Hiawatha (as required by the TMDL)

3. No increases in surface flooding in the adjacent neighborhoods, or potential improvement to local flood areas

4. Capturing trash from the larger Hiawatha watershed

Through the water management evaluation and discussion process, the MPRB, City, and MCWD staff initially eliminated the consideration of the lowering the normal/average water level of Lake Hiawatha in any water management alternatives due to the significant design and permitting efforts that would be needed for limited benefit (see Section 6.1 above). For example, although the lowering of the lake could reduce the amount of pumping slightly, it is expected to have minimal impact on reducing flooding in the watershed to the north of the golf course area and could actually reduce the amount of water quality treatment that could be developed due to the creation of smaller water quality treatment volumes.

For the reduced pumping scenario, MPRB, City, and MCWD staff selected Alternative #3b as it:

• eliminates the need to pump stormwater and reduces the average annual total pumping rates by 60-70% while still protecting the basements of the adjacent homes,

• reduces flooding in the Hiawatha north watershed and does not impact flooding in the Hiawatha west watershed,

• provides the opportunity to restore ecological function including restoration of wetlands and shoreland habitat, creation new stream channels/floodplain, restoration of uplands with pollinator/native plantings, and creation of wildlife habitat (many of which were identified by public input on the reduced pumping scenario), and



• maximizes the potential for water quality treatment of runoff from the Hiawatha North and Hiawatha West watersheds as well as for a portion of the Minnehaha Creek flows with the potential to help the City of Minneapolis achieve approximately 25-40% of the load reduction required by the Lake Hiawatha TMDL.

The two alternatives selected for full-evaluation in alternatives assessment included:

• Alternative #2 (Alternative A) – is similar to existing conditions, with no changes to the existing pumping rates in order to maintain the operation of an 18-hole golf course. However, the alternative includes the incorporation of an open channel around the golf course area to help alleviate flooding in the watershed to the north. This channel will primarily serve a conveyance function and will be separated from the golf course area by a berm. This alternative will be called Alternative A for the remainder of the alternatives assessment process.

• Alternative #3b (Alternative B) – is the preferred reduced-pumping alternative that provides a direct, gravity connection between the golf course area and Lake Hiawatha and reduces the total annual pumping by 60-70%. This alternative includes the development of the open channel through the golf course area helping alleviate flooding in the watershed to the north of the golf course area and also allows for the realignment of Minnehaha Creek. This alternative will be called Alternative B for the remainder of the alternatives assessment process.

Table 8: Summary of Water Management Alternative Impacts

811.8 812.8 814.1 811.8 812.8 814.1 811.8 812.8 814.1 811.8 812.8 814.1 811.8 812.8 814.1 811.8 812.8 814.1220 242 274 0 0 0 220 242 274 33 94 220 42 54 74 42 54 7466 66 66 0 0 0 66 66 66 0 0 0 20 20 20 4 4 4286 308 340 0 0 0 286 308 340 33 94 220 62 74 94 46 58 780 0 0 100 100 100 0 0 0 85 61 19 81 78 73 81 78 730 0 0 100 100 100 0 0 0 100 100 100 70 70 70 94 94 940 0 0 100 100 100 0 0 0 89 69 35 78 76 72 84 81 771 1 1 11 12 14 1 1 1 1 1 1 1 1 1 1 1 1

809.0 809.0 809.0 812.9 813.2 813.6 809.0 809.0 809.0 811.8 812.8 814.1 812.0 812.3 812.6 812.0 812.3 812.6Golf Course Area Peak Elevation (ft MSL) 812.2 812.3 812.8 ‐‐ ‐‐ ‐‐ 811.6 811.6 811.6 812.3 812.9 814.1 812.5 812.9 813.3 812.7 813.1 814.3

Event Bounce (ft) 3.2 3.3 3.8 ‐‐ ‐‐ ‐‐ 2.6 2.6 2.6 0.6 0.1 0.0 0.5 0.6 0.7 0.7 0.7 1.7Drawdown Time (hours)4 240 240 240+ ‐‐ ‐‐ ‐‐ 192 204 228 9 9 9 20 129 230 15 93 240+

Golf Course Area Peak Elevation (ft MSL) 814.3 814.5 814.8 ‐‐ ‐‐ ‐‐ 813.1 813.1 813.1 812.7 813.0 814.1 813.3 813.6 814.0 813.5 814.0 814.8Event Bounce (ft) 5.3 5.5 5.8 ‐‐ ‐‐ ‐‐ 4.1 4.1 4.1 0.9 0.2 0.1 1.3 1.3 1.5 1.5 1.7 2.2

Drawdown Time (hours)4 240+ 240+ 240+ ‐‐ ‐‐ ‐‐ 240+ 240+ 240+ 16 17 19 27 83 240+ 18 95 240+

10‐yr: No Impacted Structures100‐yr: No Impacted Structures



‐‐ ‐‐ ‐‐

10‐yr: Red 0.1‐2.6 ft, No Surface Flooding/Impacted Structures100‐yr: Red 0.3‐2.4 ft, No Surface Flooding/Impacted Structures



10‐yr: Incr 0.1‐1.7 ft ‐ No Surface Flooding/Impacted Structures100‐yr: No change except 19th/44th (incr 0.2 ft)/No Impacted Structures




10‐yr: Incr 0.1‐2.6 ft ‐ No Surface Flooding/Impacted Structures100‐yr: No change except19th/44th (incr 0.3 ft)/No Impacted Structures

10‐yr: Incr 0.1‐2.1ft ‐ No Surface Flooding/Impacted Structures100‐yr: No change except 19th/44th (incr 0.2 ft)/No Impacted Structures




10‐yr: 2 Impacted Structures100‐yr: 24 Impacted Structures



‐‐ ‐‐ ‐‐

10‐yr: 0.4 ft red in BP, 0.2 ft red S of BP ‐2 impacted structures100‐yr: 0.6 ft red in BP, 0.2 ft red S of BP ‐18 impacted structures (6 removed)












‐‐ ‐‐ ‐‐ ‐‐ ‐‐ ‐‐

10‐yr: 0.1‐0.2 ft red in SP100‐yr: 0.1‐0.2 ft red in SP












‐‐ ‐‐ ‐‐ ‐‐ ‐‐ ‐‐ No Change No Change No Change No Change No Change No Change No Change No Change No Change No Change No Change No Change

815.8 815.8 815.8 815.8 815.8 815.8 815.8 815.8 815.8 815.8 815.8 815.8 815.8 815.8 815.8 815.8 815.8 815.8

817.0 817.0 817.0 817.0 817.0 817.0 817.0 817.0 817.0 817.0 817.0 817.0 817.0 817.0 817.0 817.0 817.0 817.0

0.6 ft N/A N/A 0.6 ft N/A N/A 0.6 ft N/A N/A 0.6 ft N/A N/A 0.6 ft N/A N/A 0.6 ft N/A N/A

No ImpactIncrease at Powderhorn by 0.7‐0.8 ftNo Impact

Estimated Reduction in Total Pumping (%)

Lake Hiawatha Tailwater Elevation (ft MSL)

Estimated Reduction in Groundwater Pumping (%)Estimated Reduction in Stormwater Pumping (%)

Alternative 5 ‐ Open Channel Through Golf Course Area, Berm in Place,

Reduced GW Pumping, Gravity Pipe w/ Backflow Prevention (9x36"),

Stormwater Pumping above 812.3

Increase at Powderhorn by 0.3‐0.4 ftIncrease at Powderhorn by 0.3‐0.5 ft

Alternative 3a/3b ‐ Open Channel Through Golf Course Area, Open Channel Connection Through Berm to Lake (No

Stormwater Pumping)

Alternative 4 ‐ Open Channel Around Golf Course Area, Berm in Place,



FEMA/Lake Hiawatha & Minnehaha Creek

Summary

Estimated 10% Chance (10‐yr) Flood Elevation (ft MSL)5

Impact on Peak Elevations (ft) ‐ Hiawatha West Watershed

Increase at Powderhorn by 0.3‐0.4 ft

Potential Reduction on FEMA 1% Chance (100‐year) Flood Elevation (ft)

Effective FEMA 1% Chance (100‐yr) Flood Elevation (ft MSL)5

Golf Course Pond Normal Elevation (ft MSL)

Impact on Peak Flood Elevations (ft) ‐ Bloomington Pond Area/E 43rd Street

Impact on Peak Flood Elevations (ft) ‐ Sibley Pond Area

Impact on Peak Flood Elevations (ft) ‐ Columbus Pond Area

10% Chance (10‐yr)3

1% Chance (100‐yr)3

Impacts to Regional Resources

Alternative 1 ‐ No Groundwater Pumping, No Stormwater Pumping

Existing Conditions: Golf CourseAlternative 2 ‐ Golf Course with Open

Channel

Annual Groundwater Pumping (MGY)1

Annual Stormwater Pumping (MGY)Annual Total Pumping (MGY)

Estimated # of Impacted Basements2

Groundwater Pumping Summary

Design Storm Event Summary


811.8 812.8 814.1 811.8 812.8 814.1 811.8 812.8 814.1 811.8 812.8 814.1 811.8 812.8 814.1 811.8 812.8 814.1Lake Hiawatha Tailwater Elevation (ft MSL)

Alternative 5 ‐ Open Channel Through Golf Course Area, Berm in Place,



Alternative 3a/3b ‐ Open Channel Through Golf Course Area, Open Channel Connection Through Berm to Lake (No

Stormwater Pumping)

Alternative 4 ‐ Open Channel Around Golf Course Area, Berm in Place,



Alternative 1 ‐ No Groundwater Pumping, No Stormwater Pumping

Existing Conditions: Golf CourseAlternative 2 ‐ Golf Course with Open

Channel

Hiawatha West/Golf Course









Hiawatha West/Golf

Course/Hiawatha North

Hiawatha West/Golf


Hiawatha West/Golf





Hiawatha West/Golf


Hiawatha West/Golf


Hiawatha West/Golf


193 193 193 193 193 193 193 193 193 1128 1128 1128 193 193 193 1128 1128 112816.6 16.6 16.6 78.2 89.5 106.6 16.6 16.6 16.6 46.1 74.6 134.8 50.6 58.7 67.9 50.6 58.7 67.913.8 13.8 13.8 13.8 13.8 13.8 13.8 13.8 13.8 129.6 129.6 129.6 13.8 13.8 13.8 129.6 129.6 129.6120 120 120 565 647 771 120 120 120 36 58 104 366 425 491 39 45 5246 46 46 46 46 46 46 46 46 141 229 397 46 46 46 155 180 208

23 23 23 23 23 23 23 23 23 71 114 198 23 23 23 78 90 104

146.0 146.0 146.0 146.0 146.0 146.0 146.0 146.0 146.0 146.0 146.0 146.0 146.0 146.0 146.0 146.0 146.0 146.05.6 5.6 5.6 30.1 33.5 38.5 5.6 5.6 5.6 19.1 34.7 65.3 24.9 29.0 32.4 24.9 29.0 32.43.2 3.2 3.2 12.5 15.4 26.8 3.2 3.2 3.2 15.6 18.2 11.5 12.3 12.2 13.9 12.3 12.2 13.9137.2 137.2 137.2 103.4 97.1 80.7 137.2 137.2 137.2 111.3 93.1 69.2 108.8 104.8 99.7 108.8 104.8 99.7

1 ‐ Proposed conditions scenarios assume Longfellow Drain pumping to protect homes adjacent to the Golf Course area and a well at 43rd St to protect low homes to the west2 ‐ Based on basement floor elevation + 0.5 feet of freeboard for structures surveyed by MPRB/City of Minneapolis to date.3 ‐ Peak elevations for Golf Course Area are based on the design storm events applied to the local contributing watersheds. It does not reflect the critical flood events for the entire Minnehaha Creek watershed.4 ‐ Estimated drawdown times assume the golf course lift station is operating at the same rates as monitored in 2015. There may be opportunities to improve the existing pumping performance to increase the pumping capacity of the lift station.5 ‐ Flood elevations for 10% (10‐yr) based on MnDNR Minnehaha Creek XP‐SWMM model results for Lake Haiwatha. FEMA 1% Chance (100‐yr) elevation based on elevation listed on the effective Flood Insurance Rate Map for Lake Hiawatha.6 ‐ Based on existing topography in the golf course and estimated normal water level for each scenario. Additional water quality treatment could be developed during final design.7 ‐ Based on MIDS particulate phosphorus load, assuming Level 1 treatment (60%) of the fraction of the watershed being treated (based on the % of NURP volume provided); Seasonal estimate based on that 50% of annual precipitation/runoff occurs from June ‐ September8 ‐ Based on existing topography in the golf course area. "Saturated" area assumed to be the additional area within 1.0 ft of the Pond Normal Water Level.9 ‐ Based on estimated Lake Hiawatha lake levels based on USGS flow record (11/2005 ‐ 3/2017) and the regression developed for the USGS flow vs lake level

812.5

Data Record Below Threshold Elevation (%)

Standard Deviation (ft)Threshold Evaluation (ft MSL)

1.0811.838.3

1.2

8.15.2

234.6

Time‐Weighted Average Bounce Elevation (ft MSL) 813.0

Maximum Bounce Elevation (ft MSL) 817.0

Upland Park Area (ac)

Watershed Area Treated

Water Quality (Dead Pool) Volume (Ac‐ft)6

% of NURP volumeRequired NURP Volume (Ac‐ft)

Watershed Area Treated (ac)

Estimated Annual Total Phosphorus Removal (lbs/yr)7

Water Quality Impacts

Estimated Seasonal Total Phosphorus Removal (lbs/season (June‐Sept))7

Hiawatha "Event" Analysis

Average Lake Elevation (ft MSL)9

Time‐Weighted Average Bounce for Events Above Threshold (ft)

Average Duration for Events Above Threshold (days)Maximum Bounce for Events Above Threshold (ft)

Maximum Duration for Events Above Threshold (days)

Total Park/Golf Course Area (ac)Pond Water Surface Area at Normal Water Level (ac)8Park Area Summary

"Saturated" Area (ac)8


Parameter

Existing Conditions ‐ Golf CourseWater Management Alternative 1 ‐

No Groundwater PumpingWater Management Alternative 2 ‐ Golf Course with Open Channel

Water Management Alternative 3 ‐ Open Channel Through Golf Course Area, Open Channel Connection

Through Berm to Lake

Water Management Alternative 4 ‐ Open Channel Around Golf Course Area, Berm in Place, Reduced GW Pumping, Gravity Pipe w/ Backflow Prevention (3x36"), Stormwater

Pumping above 812.3

Water Management Alternative 5 ‐ Open Channel Through Golf Course Area, Berm in Place, Reduced GW Pumping, Gravity Pipe w/ Backflow Prevention (9x36"), Stormwater

Pumping above 812.3

Reduce Groundwater Pumping (to around 40 MGY): Longfellow Drain and Well at 43rd/17th

No Reduction in GW Pumping (242 MGY)

No GW PumpingNo Reduction in GW Pumping (242

MGY)20‐85% reduction in GW pumping

(dependent on Lake Level) (94 MGY)73‐81% reduction in GW pumping (54

MGY)73‐81% reduction in GW pumping (54

MGY)

Reduce/Eliminate Stormwater PumpingNo Reduction in SW Pumping (66

MGY)No SW Pumping

No Reduction in SW Pumping (66 MGY)

No SW Pumping70% reduction in SW Pumping (20

MGY)94% reduction in SW Pumping (4

MGY)Protect basements from Groundwater Flooding 1 impacted basement (sport court) 11‐14 impacted basements 1 impacted basement (sport court) 1 impacted basement (sport court) 1 impacted basement (sport court) 1 impacted basement (sport court)

No Impact to Other Regional Resources No impactsIncrease in Powderhorn 0.7‐0.8 ft (~12‐19 gpm pumping required to

maintain water levels)No impacts

Increase in Powderhorn 0.3‐0.5 ft (~7‐10 gpm required to maintain water

levels)


levels)


levels)

Avoid Increases to Peak Water Surface Elevations in the Neighborhood (Hiawatha West)

10‐yr: No impacted structures100‐yr: No impacted structures

Not evaluated in XP‐SWMM

10 yr: No change or slight reduction in peak elevations ‐ no impacted

structures100 yr: No change or slight reduction

in peak elevations ‐ no impacted structures

10 yr: Increases in peak elevations in Hiawatha west due to higher

elevation in golf course ponds (no surface flooding at these locations) ‐

no impacted structures100 yr: No impact in most locations during 100‐yr; Slight increase at the corner of 19th Ave and 44th (still

lower than FEMA and modifications to curb can help reduce surface

flooding) ‐ no impacted sturctures

10 yr: Increases in flood elevations in Hiawatha west due to higher




flooding) ‐ no impacted structures

10 yr: Increases in flood elevations in Hiawatha west due to higher




flooding) ‐ no impacted structures

Reduce Peak Water Surface Elevations in the Neighborhood (Hiawatha North/Open Channel)

10‐yr: 2 impacted primary structures100‐yr: 24 impacted primary

structures Not evaluated in XP‐SWMM

10 yr: Slight decrease in flood elevations around Bloomington

Pond, Sibley Pond, and around 43rd & 19th, 20th, 21st ‐ 2 impacted

primary structures100 yr: Slight decrease in flood elevations around Bloomington

Pond, Sibley Pond, and around 43rd & 19th, 20th, 21st ‐ 18 impacted primary structures (6 removed)













Impacts to Flood Elevations in Golf Course Area No impacts Not evaluated in XP‐SWMM10 yr Peak Change: ‐0.7 ft100 yr Peak Change: ‐1.4 ft

10 yr Peak Change: +0.6 ft100 yr Peak Change: ‐1.5 ft



Minimize bounce of water levels/draw down time in the golf couse area

10 yr Bounce/Drawdown: 3.3 ft/240+ hours


Not evaluated in XP‐SWMM



10 yr Bounce/Drawdown: 0.1 ft/9 hours






Development of additional stream/riparian channel No increase No increase ~2000 ft of open channel to Lake Hiawatha ‐ separated from golf

course by berm

~500 ft of open channel to golf course/wetland

~2000 ft of open channel to Lake Hiawatha ‐ separated from golf

course/wetland by berm

~500 ft of open channel to golf course/wetland

Increase open water area No increase ‐ ~8.8 acres Increase to ~48.9 acres No increase ‐ ~8.8 acres Increase to ~52.9 acres Increase to ~41.2 acres Increase to ~41.2 acres

Maximize Upland Park Land Area ~137.2 acres~97.1 acres (range due to lake level:

80.7 acres ‐ 103.4 acres)~137.2 acres

~93.1 acres (range due to lake level: 69.2 acres ‐ 111.3 acres)



Provide Water Quality Treatment for Hiawatha West/Hiawatha North

Yes ‐ Treats only Hiawatha West watershed; Estimated 46 lb TP

removed/yr

Yes ‐ Treats only Hiawatha West watershed; Increases treatment to

46 lb TP removed/yr

Yes ‐ Treats only Hiawatha West watershed; Estimated 46 lb TP

removed/yr

Yes ‐ Treats both Hiawtha west/north watersheds and more water quality treatment volume provided with

increased pond elevations; Increases treatment to 229 lb TP removed/yr

Yes ‐ Treats only Hiawatha West watershed; Increases treatment to

46 lb TP removed/yr

Yes ‐ Treats both Hiawtha west/north watersheds and more water quality treatment volume provided with

increased pond elevations; Increases treatment to 180 lb TP removed/yr

Capture Trash from Large Hiawatha WatershedOpportunity in open channel or along

existing storm sewer on 43rdOpportunity to address in watershed

or in future designOpportunity in open channel or along

existing storm sewer on 43rd

Opportunity in open channel, along existing storm sewer on 43rd, or with

future design


future design


future designLower FEMA Flood Elevation on Lake Hiawtha/Lowering Average Lake Level

Reduces 1% Chance Flood Elevation by 0.6 ft






Potential to realign Minnehaha Creek No potential to realign creek

Potential to realign creek ‐ separated by berm from golf/course wetland (not enough pipe capacity to convey

creek flows)

No potential to realign creekPotential to realign creek through

golf course/wetland


creek flows)


creek flows)

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