Effect of Spatially Distributed Small Dams on Flood Frequency: Insights from the Soap Creek Watershed

Dams are ubiquitous in the United States, with more than 87,000 influencing streamflow across the nation. The significant majority of these dams are small and are often ignored in real-time flood forecasting operations and at-site and regional flood frequency estimations. Even though the impacts of individual small dams on floods is often limited, the combined flood attenuation effects of a system of such dams can be significant. In this study, the authors investigate how a system of spatially distributed small dams affect flood frequency across a range of drainage basin scales using the 660-km2 Soap Creek watershed in southeastern Iowa, which contains more than 144 small dams. Results from continuous simulation of the system of small dams indicate that peak discharges reduced between 20 and 70% with the effect decreasing as the drainage area increases. This means that neglecting the effects of the system of small dams may lead to an overestimation of flood risk, which has implications in both flood frequency estimation and real-time flood forecasting. Considering that more small dams are being built across watersheds in Iowa and elsewhere in the country, the results also highlight how the peak discharge attenuation effects of these dams is an additional factor that invalidates the stationarity assumption that is used in at-site and regional flood frequency analysis.