When the suburban community of Maplewood, Minnesota, had to reduce pollution in its local lake, community planners focused on the nearby shopping mall.
As a tributary to the Mississippi River, Kohlman Lake is an especially important resource to protect from regional stormwater pollution. A 2007 report targeted Maplewood Mall as a key improvement site, especially for reducing pollution from phosphorous. Located just one mile east of Kohlman Lake, Maplewood Mall sits on a 70-acre lot, of which 35 acres was covered by the mall parking lot. The mall was chosen in part because 97 percent of the site’s surface is covered by impervious material, leading to an increase in unfiltered stormwater runoff into Kohlman Lake.
The goal of the Maplewood Mall Rainwater Runoff Retrofit Project was to infiltrate or filter one inch of runoff from the Mall’s parking lot per rainfall, reducing the phosphorus load from the Mall site between 60 and 80 percent, intending to contribute to a total 25 percent reduction of the phosphorus load from Kohlman Lake.
The Maplewood Mall Rainwater Runoff Retrofit Project has three specific features to target rainwater filtration and interception: public education, tree trenches, and rain gardens.
Public Education: The lead engineer, Barr Engineering, sought to make the relatively invisible water maintenance infrastructure more visible to the high volume of visitors to the mall. The goal was to contribute to the public understanding of water quality issues and the stormwater management process. Barr modified each of the Mall’s five entrances and fit them with artistic design elements that brought attention to the stormwater management infrastructure. This includes an interactive cistern that holds 5,700 gallons of roof runoff, marked with educational signage about the water filtration process and interactive water features.
Tree Trenches: The design of the tree trenches placed in the Maplewood Mall parking lot was adapted from a design created by tree specialists in Stockholm, Sweden, that produces a pervious, non-compacted growing environment in an impervious location. The tree trenches work when the stormwater is directed into sump catch basins–large canisters that store water under the surface–which are connected to porous pipes and drain tiles. When these canisters collect enough water, it is wicked up through the soil to water the trees and rain gardens. Barr Engineering added 375 tree trenches to the parking lot, which collect 11.2 million gallons of runoff a year.
Rain Gardens: The mall now features 55 rain gardens, strategically placed across the parking lot. Together they filter 9 million gallons of runoff per year. The rain gardens also serve as beautiful amenities to the entrance landscape. Planted with native plants with different blooming seasons, they provide an interesting array of color and texture. Trees were planted to eventually shade the seating around the rain gardens. The cistern at the main entrance of the Mall was constructed to distribute rainwater into the rain gardens through a decorative trench drain, which also absorbs groundwater through porous pavers and tree trenches. All water not used onsite by the gardens will eventually flow through Kohlman Lake into Lake Gervais before emptying into the Mississippi River, as shown on the map provided to mall visitors. Ninety percent of the stormwater collected in the parking lot drains into tree trenches and rain gardens onsite.
Taylor Griggs is an undergraduate student at DePaul University, completing a degree in Urban Sociology & Geography.