Cumulative Effects of Multiple Stream Restoration Projects on Flood Attenuation and Nitrate Removal at the Watershed Scale
Erich Hester, PhD, PE
Department of Civil and Environmental Engineering
Virginia Tech
Blacksburg, VA
Authors: Erich T. Hester, Michael Calfe, Carly Federman, Luke Goodman, and Durelle T. Scott
Excess nutrients and resulting eutrophication of streams and downstream waterbodies are widespread, and stream restoration is increasingly popular in response, particularly in the Chesapeake Bay watershed in the Eastern USA. In addition, human land development and climate change have increased hydrologic flashiness, affecting channel erosion. Yet the cumulative effects of stream restoration projects on flood mitigation and watershed-scale nutrient loads are poorly understood. We developed a modeling approach linking the U. S. Army Corps of Engineers Hydrologic Engineering Center’s River Analysis System (HEC-RAS) to an auxiliary R script to simulate stream restoration projects that enhance hyporheic exchange and/or floodplain exchange, such as in-stream structures or pool-riffle sequences, floodplain creation or reconnection, and “Stage 0” or valley restoration (e.g., via legacy sediment removal and/or beaver dam analogs). We then simulated the cumulative effects of multiple restoration projects occurring in various patterns throughout a representative generic 4th-order watershed in the eastern USA on nitrate removal. Model results indicate that restoration of in-stream structures or pool-riffle sequences throughout all streams of a 4th-order watershed could reduce nitrate loading to downstream waterbodies by ~83%, but is highly dependent on sediment texture. Stage 0 techniques had greater effect than bankfull floodplain restoration approaches, including substantial flood attenuation effects even when implemented in relatively small portions of channel networks. The relationships between percent of the watershed channel network restored and both flood attenuation and percent nitrate load reduction at the watershed outlet were nonlinear, indicating that the location of restoration projects within a watershed is important in determining their benefits. Overall, our results indicate that stream restoration can significantly attenuate downstream flooding and reduce watershed nitrate loading to downstream waterbodies, but the required percentage of the watershed channel network that must be restored varies substantially among restoration goals and techniques. Watersheds must be viewed as a whole to understand the potential impacts of particular projects or groups of projects under consideration, and watershed level planning is necessary to maximize effectiveness and avoid wasting restoration funding.
About Erich Hester
Erich Hester is an Associate Professor in Civil and Environmental Engineering at Virginia Tech. He currently leads a research group focused on environmental hydraulics, which investigates how the flow of water through channels, floodplains, and groundwater affect flood attenuation, water quality, and ecosystems. This knowledge is applied to improve river management including river restoration and flood control, and has been presented at the Mid-Atlantic Stream Restoration Conference, Eco-Stream, and River Restoration Northwest. He also has nearly a decade of private sector experience in the Western United States also focused on river hydraulics and water quality management, and is a licensed professional engineer in Virginia and Washington State.