3.2 Water Quantity

  1. Drainage is conveyed through the subwatershed through a network of storm sewers, ditches, wetlands and lakes that discharge to Minnehaha Creek.  The Minnehaha Creek Stream Assessment identified 35 locations on the creek with significant erosion or bank failure, and numerous locations with less severe erosion.   Channel bank stability is stressed by “flashy” storm discharges which produce high velocities and rapid increases and decreases in stage coupled with poor riparian zone management and numerous storm sewer outfalls.
  2. Development of the subwatershed has resulted in increased stormwater volumes and flow peaks and reduced infiltration and base flow in Minnehaha Creek.  Development, redevelopment, and reconstruction in the subwatershed may provide opportunities to achieve a net decrease in volume of stormwater runoff, nutrient and TSS loads conveyed to those water resources.
  3. Limitations on discharges from the Grays Bay dam, reduced infiltration and baseflow, multiple impoundments on the creek, and channel overwidening to accommodate high flows leads to extended periods when the flows and depths in the creek channel are insufficient for recreation and severely stress aquatic life.
  4. Several landlocked subwatershed units and individual subbasins are present in the subwatershed, primarily in Minnetonka and St. Louis Park.  As identified in the HHPLS, several of these subwatersheds or basins are being considered by the cities for outletting, altering local hydrology and potentially creating downstream volume or water quality impacts.  Within these landlocked basins, any future development or redevelopment should minimize creation of new stormwater volumes.  Outletting will generally be discouraged unless there is a demonstrated threat to property, structures or public safety.
  5. Impoundments and grade control structures along the creek cause sediment to be deposited and accumulate, limiting habitat values.
  6. The HHPLS identified multiple locations within the subwatershed that are predicted to overtop during the 100 year event, including city streets, trails, and footbridges.
  7. The HHPLS identified several locations within the subwatershed where, for both existing and future conditions, high velocities may result in erosion at outlets or culverts.   The Stream Assessment identified 178 storm sewer outfalls larger than eight inches in diameter discharging into the creek.  Erosion control or energy dissipation measures may be required in those locations.