1. Stormwater Retrofit at a Challenging Site
Confined Site ● Steep Slopes ● Infiltration Restrictions ● High Traffic Loads Large Impervious Area ● Lack of Areas for BMPs
The Project Site
Snowmelt Area
Bioretention Cell and Snowmelt Area
Snowmelt Area and Bioretention Cell
The Wakota Ice Arena was retrofit with two biofiltration cells and three snowmelt treatment areas installed as part of a parking lot renovation project in These practices provide water quality treatment for runoff from 5.5 acres of impervious surfaces.
Bioretention Cell Design & Construction
A typical bioretention filtration design cross section was selected for this site. The clay liner limits infiltration into sandy steep slopes adjacent to the cells. Three feet of sand/compost media and 6” subdrains with rate control valves provide biofiltration treatment. Temporary pool depths are limited to 12 inches maximum.
Snowmelt Treatment Areas Low Maintenance Plantings
Winter
Summer
The grass turf areas adjacent to the cells are snow storage areas. The boulders prevent snowplow damages and snow storage within the cells.
Bioretention side slopes were designed at 5:1 and turfed for mowing. A dense planting of Minnesota native shrubs were planted on the bottom.
Retrofit Costs & Pollutant Reductions
Drainage Area
Volume Reductions
TSS Reductions
TP Reductions
ft3/yr %
lbs/yr
Wakota Bioretention Cells (5.5 acres)
149,000 35
1,727 63
4.05 56
30 Yr. Project Total
4,470,000 ft3
51,810 lbs
121.5 lbs
30 Yr. Cost / Unit
$637/acre-ft
$1.26/lb
$538/lb
Two bioretention cells totaling 2,075 square feet and two snow melt treatment areas totaling 2,075 square feet were installed for a total project cost of $60,
Clean Water Funds provided $33, or 55% of the project costs. The City of South Saint Paul Engineering Department provided engineering services for the project design.
Project Partners
Dakota County Soil and Water Conservation District
City of South St. Paul
Jim Davidson, CPSWQ, CPESC
Curt Coudron, CPESC
Mike Isensee, CPESC