The Effect of Urban Stormwater BMPs on Runoff Temperature in Trout Sensitive Waters

Urbanization can increase the temperature of coldwater stream environments by transferring heat from solar radiation, captured by pavement materials, to receiving water bodies through stormwater runoff. A number of aquatic organisms, including trout, are sensitive to elevated stream temperatures and temperature increases associated with urbanization have been shown to have a negative impact on the aquatic ecosystem. Research was conducted in regions of trout sensitive waters in Western North Carolina to determine, the effect that urban stormwater BMPs have on the thermal load of stormwater runoff at 4 bioretention areas, 1 stormwater wetland, and 1 wet pond. Water temperature and associated flows were remotely logged every 5 minutes at all BMP inlets and outlets, with additional temperature sensors located at specified depths and receiving waters. Analysis focused on flow and temperature reductions between inlets and outlets, as well as temperature changes within the BMPs due to diurnal fluctuations and storm events. Variations in temperature throughout the soil and water columns suggest the opportunity for modified outlet structures to reduce the thermal load discharged from BMPs. Results have also indicated the possibility of lowering runoff temperatures through conveyance in buried pipes. A comparison of the effect shading within the BMPs has on thermal load is also presented. An understanding of the temperature reduction mechanisms involved in urban stormwater BMPs should provide engineers with design criteria tos effectively mitigate the effect of thermal loads from stormwater runoff.