Analysis of the effect of stormwater runoff volume regulations on thermal loading to the Vermillion River

Abstract

Residential and commercial development dramatically alters the surface and groundwater hydrology of watersheds. Increasing areas of impervious surfaces can lead to higher peak stream flows and reduced baseflow, which both can lead to degradation of fish habitat. In addition, thermal pollution from surface runoff is increasing recognized as an additional mechanism for fish habitat degradation in coldwater stream systems. Stormwater best management practices (BMPs) such as infiltration ponds, rain gardens, and swales are often used to reduce stormwater surface runoff rates and volumes, and to increase infiltration and groundwater recharge. Wet detention ponds used to limit runoff flow rates and reduce sediment loads are not likely to reduce thermal loading, since there is no reliable mechanism for volume or temperature reduction. Infiltration practices that capture all of smaller storms and initial portion of larger storms are very likely to significantly reduce thermal loading, since the warmest surface runoff typically happens during low volume storms and the initial portions of larger storms. To predict changes in thermal loading to streams due to land use changes, a simulation model of runoff temperature is currently being developed as part of a research project that SAFL is conducting for the Minnesota Pollution Control Agency (“Development and Implementation of a Tool to Predict and Assess the Impact of Stormwater Runoff on Trout Streams”). The current version of the tool, MINUHET (MINnesota Urban Heat Export Tool), has components for simulating runoff volume and temperature for mixed land use sub-divisions, routing of runoff flow and heat through both conventional storm sewer systems and pervious channels, and for simulating wet ponds and infiltration ponds. Thus, MINUHET has the capability to quantify the reduction in thermal loading due to the addition of infiltration practices.