Effectiveness of Environmental Site Design In Protecting Stream Channel Stability

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, fewer studies have evaluated benefits at the watershed level. The goal of this project was to compare the impacts of different types of stormwater management on watershed hydrology and channel stability. Tributary 109 (Trib 109) to Little Seneca Creek near Clarksburg, MD was developed from 2006 to 2017 using ESD, with percent impervious cover increasing from 5% to 38%. A total of 64 stormwater management practices were represented in the model, including local, infiltration-enhancing measures, as well as end-of-pipe water detention systems. Monitoring of precipitation, stream discharge, repeated channel cross sections, and bed particle size started in 2004 and is ongoing. Because sediment load was not measured, incoming sediment load was estimated from measurements at five USGS gages with similar geology, climate, and land cover. The watershed was modeled using the Storm Water Management Model 5.3 (SWMM) and the Hydrologic Engineering Center River Analysis System 6.3 (HEC-RAS) to simulate changes in watershed hydrology and channel stability, respectively, due to different levels of stormwater management. Study results show that infiltration practices were more effective at reducing peak runoff from more frequent storm events than storage practices alone; however, storage practices are necessary to minimize increases in peak flows for larger, less frequent storm events. Ongoing HEC-RAS modeling will demonstrate the impact of these SCMs on channel stability.