3.2 Water Quantity
- 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.
- 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.
- 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.
- Several landlocked subwatershed units
and individual subbasins are present in the subwatershed, primarily in Minnetonka and St.
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.
- Impoundments and grade control
structures along the creek cause sediment to be deposited and accumulate, limiting
- The HHPLS identified multiple locations
within the subwatershed that are predicted to overtop during the 100 year
event, including city streets, trails, and footbridges.
- 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.