5.8.5 Regional Volume and Load Reducation

Project

Minnehaha Creek Regional Volume and Load Reduction

Description

Implementation of opportunities to reduce stormwater volumes and nutrient loading, including but not limited to construction of infiltration basins and devices, wetland restoration, reforestation, revegetation, and stormwater detention or redirection.

Need

The Minnehaha Creek-Lake Hiawatha TMDL draft report (March 25, 2013) identifies the need to reduce phosphorus and bacterial (E. coli) loading to meet water quality targets for Lake Hiawatha and Minnehaha Creek. The TMDL draft report calls for a reduction of 1,907 lbs/year throughout the subwatershed in order for Lake Hiawatha to meet an in-lake nutrient concentration of 50 ug/L.  The TMDL draft report also identifies a need to reduce bacterial (E. coli) loading in order to meet the standard of 126 organisms/100 ml.  At this time with our current understanding, the best approaches for addressing excess bacteria loads appear to be source reduction or volume control practices.

 

In addition, the 2003 Minnehaha Creek Stream Assessment identified two major issues impacting water quality and biotic integrity in the Creek: flashy storm event flows that often result in streambank erosion; and low base flows, which reduce habitat and limit biotic integrity.  The high percent of impervious surface in this urbanized subwatershed has reduced the amount of stormwater that naturally infiltrates to surficial groundwater and which helps sustain base flow.  This stormwater is efficiently conveyed to the creek through stormsewers, which results in the flashy flows.

 

The Stream Assessment analyzed options for reducing peak discharges in Minnehaha Creek, and identified 11 key subwatersheds where reduction of peak discharge from the subwatersheds would collectively result in the greatest reduction in peak discharge in the Creek.  In these 11 subwatersheds, hypothetical ponds reducing peak discharge by 75 percent would result in a 27 percent reduction in Creek discharge at Cedar Avenue.  Regional infiltration or other abstraction or detention focused on those subwatershed units would have the most impact on reducing peak flows in Minnehaha Creek.

 

Specific project locations and methods will be identified and implemented to reduce nutrient and bacterial loading to Minnehaha Creek and thus to Lake Hiawatha; decrease peak discharge rates in Minnehaha Creek to reduce streambank erosion; and increase baseflow in the Creek to improve its biotic integrity.  These projects are intended to reduce annual volume and peak flows discharged to the Creek; increase infiltration to surficial groundwater; and reduce nutrient and bacterial export to the Creek.

 

Identifying specific implementation sites under this capital project element will be an ongoing process informed by refined technical knowledge of pollutant sources and geomorphological phenomena, available land and willing public or private partners.  Priorities are set foremost by diagnosing the spatial distribution of pollutant loading to Minnehaha Creek.

 

For example, on the basis of available data, subwatershed units between West 34th Street and Excelsior Boulevard are the source of greatest phosphorus loading to Minnehaha Creek, both in absolute amount and on a unit area basis.  Unit area loads for each subwatershed unit are determined by dividing the total phosphorus increase for each subwatershed unit by the area.

 

The list of specific implementation sites below reflects priorities determined based on available data to date.  However, over the term of this plan, the District: (a) may shift funds between projects included under 5.8.5 to reflect immediate priorities and implementation decisions; and (b) may identify and implement projects at other locations as well, on determining that the goals of 5.8.5 will be met and pursuant to the procedures described below.  There are three reasons for the District to reserve this role for its ongoing review:

 

(1) The District’s technical understanding of subwatershed hydrology and the hydraulic behavior of Minnehaha Creek will continue to develop and thereby refine the District’s capacity to determine where and what sorts of improvements will be most cost-effective. For instance, in 2011 the MCWD and the Mississippi Water Management Organization awarded a Joint Watershed Research Grant to the University of Minnesota to study stream base flow in Minnehaha Creek. The focus of the research is on investigating whether stormwater runoff can be infiltrated and stored in the shallow aquifer to contribute to stream base flow in Minnehaha Creek during periods of low flow and drought conditions. This study is ongoing and could help inform preferred locations for infiltration.

 

(2) Metropolitan area municipalities and the Minneapolis Park and Recreation Board (MPRB) are key District partners.  Municipalities are implementing their own comprehensive land use plans.  They are subject to legal obligations to reduce annual stormwater volumes and pollutant loadings, and to manage stormwater in ways that may be compatible with District objectives or which may be in conflict with them.

 

For example, the City of Minneapolis is obligated by the Metropolitan Council Environmental Services to reduce stormwater that enters the sanitary sewer system.   Actions such as disconnecting rooftop rain leaders from the sanitary system and completing the separation of combined sewers may increase the volume of annual stormwater runoff in the subwatershed, or may create or exacerbate local flooding issues.  Other LGUs in the subwatershed are required, under the Nondegradation requirement of their NPDES Phase II permits, to reduce annual stormwater volumes to at least 1988 volumes.

 

The District endeavors to work very closely with these public partners and is committed to investing funds where they will best serve the combined needs of involved public entities.  For this reason, the District needs to be prepared to be responsive to municipal needs and partnership interests as they evolve and arise.

 

(3) The Minnehaha Creek subwatershed is fully developed, land use is intensive and land prices are high.  For these reasons, the District must be opportunistic with respect to siting opportunities.  Opportunities may arise in conjunction with private redevelopment, public road reconstruction, MPRB capital work and other situations in which the opportunity occurs unexpectedly and is subject to the partner’s often-constrained timeline.    

 

Accordingly, the District intends to plan for and pursue implementation within the specific drainage areas enumerated below while also developing and pursuing other opportunities as they arise.  With respect to the latter, there will be several project development elements to ensure transparency and public accountability:

 

(a) The District will continue to work in partnership with the LGUs in the subwatershed to complete a more detailed study of the hydrologic and hydraulic conditions in the subwatershed to determine how District and LGU objectives can best be reconciled and advanced through partnership projects. For example, the District currently is working in coordination with the cities of Hopkins, St. Louis Park, Edina and Minneapolis and the MPRB to explore opportunities to partner on the regional treatment of stormwater runoff volumes and loads within priority subwatersheds. The City of Minneapolis has identified three specific priority projects for review and consideration for partnership with the District.  The cost share between project partners would be determined during the development of feasibility studies for each project.   These are:

 

  • Flood area 29-30, located in subwatershed unit MC-129.  This area drains to Lake Harriet, but has been considered in the past for redirection of some flow to the Creek.  If the volume and load reduction study identifies this as a feasible option, it may be possible to coordinate improvements to manage stormwater in that area with the stream restoration project already identified for Reach 12.  The estimated total cost of this project is $7.4 million.

 

  • Flood area 26 “C”, located in subwatershed unit MC-147.   Stormwater ponding and storm sewer upgrades are being considered for this third phase of projects in this flood area.  The estimated total cost of this project is $1.8 million.

 

  • Flood area 21-22, located in subwatershed unit MC-170.   This area is in the direct drainage area of Lake Hiawatha, and includes stormwater upgrades and combined sewer separation projects.  The estimated total cost of this project is $7.8 million.

 

For any significant implementation actions, whether the District pursues it in partnership or otherwise, a focused feasibility study will be prepared to examine feasibility and cost-effectiveness.  The study will be made publicly available and presented to the Board of Managers at a public meeting with the availability for public comment.

 

(b) As described earlier in this plan, each year, for budget and levy purposes, the District will review the status of its capital program in a public forum with opportunity for public input.  Proposed revisions to the District’s 10-year CIP will be provided to Hennepin and Carver Counties and all cities wholly or partly within the District.  At its budget and levy hearing, the District Board of Managers will make budgeting decisions that will set overall parameters for spending under this capital project 5.8.5 and set specific project implementation priorities for the upcoming fiscal year.

 

(c) Before expending levied funds on the design or construction of an implementation action, the Board will distribute design plans and provide public notice for a public hearing and project ordering in accordance with Minnesota Statutes §103B.251.  If the proposed work will exceed $300,000, the District will afford the additional procedures set forth in section 6.8 of the watershed plan (page 84).

 

Outcome

Minimized new pollutant loads conveyed by runoff and generated within Minnehaha Creek; minimized new volumes generated by new development; protection of stream base flows and wetland and surficial groundwater hydrology; wetland restorations; conservation of high-value native vegetation and habitat.

Estimated

Cost

Minnehaha Creek Subwatershed Volume and Load Reduction Study

$200,000

Project 1:  Improvements in MC-129.  Funding source is City of Minneapolis and District capital levy.

$2,299,800

Project 2:  Improvements in MC-147.  Funding source is City of Minneapolis and District capital levy.

$3,533,000

Project 3: Improvements in MC-170.  Funding source is City of Minneapolis and District capital levy.

$2,440,600

Project 4:  Improvements in MC-58.  Funding source is District capital levy.

$435,200

Project 5:  Improvements in MC-70-75.  Funding source is District capital levy.

$487,600

Project 6: Improvements in MC-95.  Funding source is District capital levy.

$2,221,500

Project 7: Improvements in MC-97.  Funding source is District capital levy.

$1,504,500

Project 8: Improvements in MC-134.  Funding source is District capital levy.

$485,400

Project 9: Improvements in MC-135.  Funding source is District capital levy.

$1,340,200

Project 10: Improvements in MC-139.  Funding source is District capital levy.

$511,000

Project 11: Improvements in MC-140.  Funding source is District capital levy.

$1,493,400

Project 12: Improvements in MC-146.  Funding source is District capital levy.

$564,300

Project 13: Improvements in MC-150.  Funding source is District capital levy.

$362,100

Project 14: Improvements in MC-151.  Funding source is District capital levy.

$524,700

Project 15: Improvements in MC-152.  Funding source is District capital levy.

 

Overall Implementation Estimate:

$678,500

 

 

$19,081,800

Schedule

2008:  Complete volume and load reduction study

2008:  Identify and construct improvements in MC-129

2009:  Identify and construct improvements in MC-147

2011:  Identify and construct improvements in MC-170

2012:  Identify and construct improvements in MC-134

2013:  Identify and construct improvements in MC-95

2014:  Identify and construct improvements in MC-97

2015:  Identify and construct improvements in MC-146

2015:  Identify and construct improvements in MC-70-75

2016:  Identify and construct improvements in MC-58

Unscheduled:  Identify and construct improvements in MC-152, MC-139, MC-150, MC-135, MC-140, MC-151; identify and construct improvements otherwise developed through described procedures.