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.
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.
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.
Reduce volume of stormwater runoff from new development and redevelopment and maintain existing water volume discharged from the subwatershed into Minnehaha Creek.
Limit new discharges from land locked basins and subbasins to prevent new impacts to downstream lakes.
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.
