Impact of Stream Restoration on Flood Attenuation and Channel-Floodplain Exchange During Small Recurrence Interval Storms

Extreme flooding and excess nutrient pollution have been detrimental to river health under increased environmental stress from human activities (e.g., agriculture, urbanization). Riverine flooding can be detrimental to human life and infrastructure yet provides important habitat and ecosystem services. Traditional flood control approaches (e.g., levees, dams) negatively impact habitat and ecosystem services, and cause flooding elsewhere along the river. Prior studies have shown that stream restoration can enhance flood attenuation, and increased exchange of water between the channel and floodplain can improve water quality. However, the effects of floodplain restoration during small and sub annual recurrence interval storms have not been thoroughly studied, nor have cumulative impacts of floodplain restoration on water quality at watershed scales. We used HEC-RAS to perform 1D unsteady simulations on a 2nd-order generic stream from the Chesapeake Bay Watershed to study flood attenuation under small and sub-annual recurrence interval storms (i.e., 2-year, 1-year, 0.5-year, and monthly). In HEC-RAS we varied percent of channel restored, location of restoration, bank height of restoration, floodplain width, and floodplain Manning's n. Overall, stream restoration reduced peak flow (up to 37%) and decreased time to peak (up to 93%). We found the timing of tributary inflows could obscure the attenuation achieved, and even reverse the trends with certain parameters in the sensitivity analysis. The greatest exchange with the floodplains (greater volume and exchange under more recurrence interval storms) was observed from Stage 0 restoration, which reduces bank height more than other approaches. We also conducted a quantitative literature synthesis of nitrate removal rates from stream restoration projects. We focused on how removal rates varied with properties relevant at watershed scales, such as effects of stream order. The resulting database will aid in determining which stream restoration parameters better reduce nutrient loads and in simulating the effects of stream restoration on water quality at watershed scales. Floodplain restoration practices, and particularly Stage 0 approaches, enhance flood attenuation which can help to counteract urban hydrologic effects.