Bioretention of Simulated Snowmelt: Cold Climate Performance and Design Criteria

Abstract

One of the primary tools used in decentralized urban stormwater management is routing runoff to bioretention systems integrated into the landscape (Oberts, 2003). To date, observation suggests that bioretention systems continue to infiltrate during the winter to varying degrees; however, little field research is currently available to quantify their snowmelt infiltration performance. A three-year study funded by the Water Environment Research Foundation (WERF) field-tested the cold climate hydrologic performance of four existing bioretention cells in the greater Twin Cities, MN, region. Sites were selected based on varying design applications including parking lot runoff, street runoff in a residential setting and a commercial application, in soils from sand and sandy loam to clay loam. The primary hydrologic test consisted of applying up to 6,000 gallons of water to a bioretention cell under various frost conditions. The synthetic snowmelt infiltration rates at the surface were measured as the pool receded while dynamic soil moisture readings tracked the subsurface water movement through the soil profile. Data on air, water and soil temperature, snow depth and frost penetration were collected on site. Measured responses reveal that these bioretention cells maintained hydrologie function in cold climates including many cases where rapid infiltration occurred. The primary study findings are that bioretention systems designed successfully for warm climate conditions will likely perform well in cold climate conditions and a well draining soil-type is the single most important design characteristic. The type of frost, rather than the presence or absence of frost, strongly influences bioretention performance, restricting infiltration under concrete frost conditions and facilitating rapid infiltration under granular frost conditions. Under-drains affect both the range of infiltration rates and the overall function.