Maplewood Mall Stormwater Infiltration Retrofit Project
The Kohlman Lake TMDL calls for the reduction of nutrients from watershed and in-lake loading. A major source of phosphorus loading is from the impervious areas in the District (roads, interstates, roofs, and parking lots). In the analysis of the Kohlman Lake watershed, one major land use feature stands out - Maplewood Mall. The District identified that retrofitting the Mall parking areas to infiltrate at least one inch of runoff would result in a large reduction in phosphorus to Kohlman Creek and the lake.
The District began discussions with the Mall in 2008 and implemented a Phase I project of rainwater gardens at each of the Mall entrances. This project was completed with District funds in 2010. Phase II of the project was submitted to the CWF and awarded in 2010. The project has been in the planning and design phase with construction slated for summer 2011. The project goal is to
infiltrate or filter the runoff from the northeast quadrant of the mall parking lot by using primarily tree trenches. Other practices will include rain garden and planter areas, porous pavement, and a cistern. The project will also include an educational and interpretive element which will include public art, signage, and exhibits.
The project is estimated to reduce phosphorus loading from 33 to 13 pounds per year and reduce sediment loads from 3.2 to 0.3 tons per year. The District is making plans to cooperatively address the remainder of the Mall lots within the next 3 years. The District has received a federal grant for the northwest quadrant of the mall parking lot. This construction will also take place in 2011.
(b) $2,800,000 the first year and $3,124,000 the second year are for grants to watershed districts and watershed management organizations for: (i) structural or vegetative management practices that reduce storm water runoff from developed or disturbed lands to reduce the movement of sediment, nutrients, and pollutants or to leverage federal funds for restoration, protection, or enhancement of water quality in lakes, rivers, and streams and to protect groundwater and drinking water; and (ii) the installation of proven and effective water retention practices including, but not limited to, rain gardens and other vegetated infiltration basins and sediment control basins in order to keep water on the land. The projects must be of long-lasting public benefit, include a local match, and be consistent with TMDL implementation plans or local water management plans. Watershed district and watershed management organization staff and administration may be used for local match. Priority may be given to school projects that can be used to demonstrate water retention practices. Up to five percent may be used for administering the grants (2010 - Runoff Reduction)
The project is estimated to reduce phosphorus loading from 33 to 13 pounds
per year and reduce sediment loads from 3.2 to 0.3 tons per year.
The project was completed in November 2011. The project was completed as designed and within budget. The Phase II project includes tree trenches , rain gardens, cistern porous pavement and educational features at the main entrance to the Mall. The stormwater features were tested and are performing as designed. A Maplewood Mall Stormwater Infiltration Retrofit Project Fact Sheet was also created as part of this project.