4.5 Water Quantity

Goal 5 - Water Quantity

Maintain or reduce existing flows from drainage within the watershed to decrease the negative effects of stormwater runoff and bounce from existing and proposed development as well as provide low flow augmentation to surface waters.

Discussion

Development and the associated creation of new impervious surface increases the volume of stormwater runoff from the landscape, changes the rates and times to peak runoff flow, and decreases the amount of stormwater that naturally percolates into the soil to recharge groundwater.   The District’s long term goal in the Minnehaha Creek subwatershed is to achieve no increase in the volume of stormwater discharged from the subwatershed into Minnehaha Creek and thence into the Mississippi River.  Implementation strategies will include minimizing new runoff volumes from development and redevelopment, encouraging infiltration and groundwater recharge to maintain baseflow in the Creek and adequate hydrology to groundwater-fed wetlands, and limiting new volumes from existing landlocked subwatersheds.

A key strategy to achieve this goal is the adoption of a volume management standard for new development and redevelopment that requires the abstraction (removal from runoff through infiltration, capture and reuse, evapotranspiration, etc.) of one inch of rainfall.   Approximately 70 percent of annual runoff volume in Minnesota results from precipitation events of 1” or less (MPCA, 2000).  Requiring new development and redevelopment to abstract (retain on site through infiltration, evapotranspiration, or capture and reuse) runoff from that size event would significantly reduce new volumes of runoff flowing downstream and help reduce future erosion in streams and channels; minimize new pollutant loading that would have been conveyed by that stormwater; and help maintain groundwater levels, preserving wetlands.

Limiting discharges from subwatersheds and basins that are currently landlocked is necessary to prevent further degradation of downstream water quality as well as to limit new volumes discharged downstream to channels that are already experiencing erosion.

In the Minnehaha Creek subwatershed, another challenge will be balancing regulated flows from the GraysBay dam to manage levels in Lake Minnetonka with providing for adequate flow in the Creek to maintain aquatic life and recreational use while limiting volumes conveyed to the Creek in erosion-prone areas.

The phosphorus load reduction plan for Lake Hiawatha incorporates an expectation of a significant reduction of pollutant loading from existing development through abstraction and infiltration.   The additional new volume and the existing volume for improving downstream water quality could be mitigated through construction of regional infiltration basins, restoring drained wetlands, reforestation and revegetation, or other means.

Table 13.  Modeled annual volume of runoff in the Minnehaha Creek subwatershed, and estimated reductions resulting from application of a proposed 1” abstraction rule for new development and redevelopment (acre-feet).

2000 modeled annual subwatershed runoff volume

10,240

2020 modeled annual subwatershed runoff volume

10,470

Ultimate Development modeled annual subwatershed runoff volume

10,470

Increase between 2000 and Ultimate Development

230

Estimated volume abstracted by 1” rule

161

New volume to be abstracted through other means such as capital projects, wetland restorations, reforestation and revegetation, etc.

69

Total volume reduction needed to reduce pollutant loading to Minnehaha Creek as set forth in the Lake Hiawatha TMDL

1,298

Desired Outcomes:  Management of water volumes discharged from the subwatershed.

Metrics:  Acre-feet volume abstraction.

Goal 5.1 - Minnehaha Creek

Manage water volume to Minnehaha Creek in a way that balances desirable Lake Minnetonka and Minnehaha Creek levels and flows with recreation and ecological needs and channel impacts.

Actions

  1. Manage Lake Minnetonka discharges according to the Headwaters Control Structure Management Policy and Operating Procedures.
  2. Continue to partner with the US Army Corps of Engineers and the DNR to explore options for adjusting the GraysBay dam operating plan to balance maintenance of desirable Lake Minnetonka and Minnehaha Creek levels with recreation and ecological needs and impacts.
  3. Identify baseflow in Minnehaha Creek and continue to investigate the role of groundwater in gain or loss of baseflow.
  4. Continue to monitor flows in Minnehaha Creek.

Goal 5.2 - Minnehaha Creek

Reduce volume of stormwater runoff from new development and redevelopment and maintain existing water volume discharged from the subwatershed into Minnehaha Creek.

Actions

  1. Consider amending existing or establishing new District rules requiring abstraction of the first one inch of rainfall on new permitted development and redevelopment.
  2. Track volumes abstracted and new volumes created resulting from permitted development.
  3. Work cooperatively with the LGUs in the subwatershed to identify and construct abstraction and infiltration opportunities to reduce pollutant loading to Minnehaha Creek and thus Lake Hiawatha.
  4. Provide assistance to LGUs and developers to foster low impact development and redevelopment that minimizes new stormwater volumes.
    1. Develop and distribute model ordinances and design standards that incorporate low impact design principles.
    2. Sponsor educational opportunities for LGU staff, developers, elected and appointed officials and other interested parties to provide practical information and opportunities for sharing experiences.
    3. Provide education and training opportunities, technical and planning assistance for property owners and LGUs on methods to reduce runoff from and increase infiltration on their property by incorporating BMPs into landscaping, infrastructure maintenance, and reconstruction.
  5. Encourage the development and maintenance of depressional storage within the subwatershed.

Goal 5.3 - Minnehaha Creek

Limit new discharges from land locked basins and subbasins to prevent new impacts to downstream lakes.

Actions

  1. Require the LGUs to continue to manage basins without outlets as landlocked basins unless they can demonstrate that providing outlets would not negatively impact downstream water resources (see Figure 2 for landlocked basin locations).

Goal 5.4 - Minnehaha Creek

Require public stormwater conveyance and control structures in the watershed be sized and maintained properly to convey current and ultimate stormwater flows to minimize flooding and erosion potential.

Actions

  1. Require LGUs to provide to the District a copy of their annual NPDES report.