Performance of Distributed Reservoir Storage in Mitigating Flood Impacts for a Small Rural Watershed

In 1988, stakeholders in the Soap Creek Watershed, a rural 660-km(2) basin located in southeast Iowa, USA, established a plan to construct 154 small reservoirs to reduce recurring flood damages to agricultural lands and roadway infrastructure. In a unique collaboration among local, state, and federal government entities, 132 of the planned reservoirs have been constructed since the plan was implemented. According to anecdotal information provided by basin residents, the reservoirs have significantly reduced the frequency and severity of flood damages. However, to date, there has not been an effort to quantify the cumulative benefit of the structures. As part of a broader mitigation project directed toward flood damage reduction in select Iowa watersheds, the Iowa Flood Center (IFC), a unit of the University of Iowa's IIHR-Hydroscience & Engineering, is conducting a hydrologic assessment of the Soap Creek Watershed. The goals of the assessment are to quantify the cumulative effects of the Soap Creek reservoirs on basin response to intense rainfall, to investigate potential reservoir enhancements, and to evaluate additive benefits of implementing additional management practices within the basin. A similar assessment is being performed for the adjacent Chequest Creek Watershed, which shares many physical characteristics but does not feature a system of flood storage reservoirs. The paired studies will not only inform continued flood management within the Soap and Chequest Creek Watersheds, but will also provide guidance for flood mitigation plans in other areas of the state. To perform hydrologic assessments, IFC is deploying instrumentation and developing basin-scale hydrologic models for the Soap and Chequest Creek Watersheds. No stream monitoring has been previously performed in either basin. IFC has installed six bridge-mounted water level sensors to track movement of water through the basins' stream networks. This information will be used to validate hydrologic models used to simulate the individual and cumulative effects of existing and proposed reservoirs or other flood mitigation practices. IFC is developing hydrologic models using the United States Army Corps of Engineers Hydrologic Engineering Center Hydrologic Modeling System (HEC-HMS). The models are being used to evaluate basin response and reservoir performance during historical and design storms. Models take advantage of several high-quality datasets, including comprehensive LiDAR topography and spatiotemporally-distributed radar rainfall estimates.