ROLE OF URBAN STORMWATER BEST MANAGEMENT PRACTICES IN TEMPERATURE TMDLS

Temperature TMDLs are being developed to protect habitats in coldwater streams throughout the nation, particularly in the Pacific Northwest. Concurrently, urban stormwater Best Management Practices (BMPs) are increasingly adopted as a requirement in local land development ordinances and under Phase II Stormwater Regulations to target pollutant loading and flow moderation. Sporadic pieces of information in the literature, however, have suggested that thermal enrichment of coldwater streams by urban stormwater runoff can be exacerbated by traditionally designed BMPs. We report in this paper the results of a five-year thermal enrichment study in a recently constructed regional stormwater treatment system discharging to a designated trout stream in Portage, Michigan. The Consolidated Drain stormwater treatment system is a three-stage treatment design with a sediment forebay, a wet detention pond, and an enhanced existing wetland. Results showed that the design of the stormwater treatment facility was an effective means to reduce stormwater thermal loading (45.2% during August-September of 2002). Mitigation of higher stormwater temperatures, however, was not accomplished in the more traditional two-stage treatment process (i.e., forebay and wet detention). The enhanced natural wetland accounted for most of the thermal load and temperature reductions with its dense canopy coverage and higher infiltration rate. The outflow temperature of the wetland was always near ambient air. Event-based thermal loading analysis indicated that flow reduction via infiltration was the primary mechanism for thermal load reduction by the wetland, while temperature mitigation was achieved mainly through vegetative shading and the utilization of natural groundwater input to the Consolidated Drain channel. Using the concept of temperature equivalent (TE), the flow-weighted average temperature of a flow component, we established the locations and polluting potency of thermal enrichment source areas as well as the mitigating sources in the study watershed. It was concluded that stormwater BMPs that promote infiltration and provide sufficient vegetative shading to detained runoff should be used in urban areas to protect the thermal integrity of coldwater streams.