Using PCSWMM and HEC-RAS-1-D unsteady sediment transport models to assess the effectiveness of environmental site design

David Sample, Theresa Wynn-Thompson, Mohammad Alsmadi and Sami Towsif Khan


Environmental site design (ESD) seeks to minimize the impacts of urbanization on stream systems by conserving natural features, minimizing the use of impervious surfaces, and slowing runoff to increase lag time, infiltration, and evapotranspiration.  While there are numerous studies documenting improvements at the level of individual stormwater control measures (SCMs), few studies have evaluated benefits at the small watershed level.  The goal of this study is to compare the impacts of traditional stormwater management and ESD on watershed hydrology and channel stability. This analysis is based upon models developed within PCSWMM and HEC-RAS 6.0.  Essentially, we are asking whether our watershed management strategy, i.e., implementing SCMs to reduce runoff peak and volume, is having its intended effect. Our study watershed is in Clarksburg, an unincorporated portion of Montgomery County,  Maryland that is a headwater of Tributary 109 of Little Seneca Creek. This area was developed over the past decade with an extensive implementation of ESD-over 150 SCMs were installed and are modeled. A U.S. Geological Survey streamgage was installed before development, facilitating model calibration. We assess existing ESD and with traditional stormwater management practices to determine how extensive stream erosion would have been had ESD not been utilized. The analysis is then repeated with climate-altered rainfall and temperature files to assess the resiliency of ESD measures in protecting channel stability in the future. 

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