Implementing a Thermal Urban Runoff Model (TURM)
An Erosion Control and Stormwater Management Ordinance was recently adopted in Dane County, Wisconsin, U.S.A. The thermal control section of this ordinance is one of the first in the nation designed to control heated runoff from impervious surfaces in order to protect cold water streams. This section of the ordinance applies to most new development projects draining to a cold water stream. Temperature reduction practices are required to reduce the potential thermal impact of such development projects.
In order for the thermal control section of this ordinance to be successfully implemented, developers, engineers and county officials must be able to assess the impact of proposed development projects on stream temperatures. A Thermal Urban Runoff Model (TURM) was developed for this purpose. TURM predicts runoff temperatures and flow rates from both pervious and impervious surfaces using weather and landscape factors. It also predicts the effectiveness of management practices for reducing thermal impact. Wilson et al. (2004) and Roa-Espinosa et al. (2003) validated TURM for predicting runoff temperatures in Dane County during four storms in the summer of 1999 and seven storms in the summer of 2000.
Assumed values of weather variables are required to use TURM to predict the thermal impact of a proposed development project. Weather statistics were collected from several stations and a sensitivity analysis of TURM to each of these variables was done in order to select storm conditions with significant thermal impact potential in Dane County. Selecting the design storm values was a key step in shifting TURM from being a validated model to actually being applied as a predictive tool. Although TURM is currently being used in the Dane County stormwater permitting process, further development of the model has the potential to make TURM a more general planning tool.
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