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The devastating Floods throughout Iowa in 2008 caused homes to be lost, people to be displaced, and cost billions in economic damages. This left State Officials pondering how to limit the damages of large magnitude floods in the future. From the legislative sessions following this tragedy came the Iowa Flood Center and funding through the Department of Housing and Urban Development (among others) to begin the Iowa Watersheds Project. The project was tasked with the planning, implementation and evaluation of watershed projects to lessen the severity and frequency of flooding in Iowa. One test watershed studied was the Middle Raccoon River watershed in West Central Iowa.
To study the impacts of basin-wide flood mitigation strategies on the Middle Raccoon River watershed, the hydrologic modeling software HEC-HMS was used in conjunction with the geographic analysis software, ArcGIS. A model was developed and calibrated to best represent the observed hydrologic response at USGS stream gages located at Bayard, IA and Panora, IA. Once complete, a series of flood mitigation techniques were applied to the watershed model, and run with the 10-, 25-, 50-, and 100-year SCS design storms. These techniques include increasing infiltration by modifying land use, and applying a distributed storage system (ponds). Both practices are shown to have the ability to reduce peak discharge, from 4 percent to 56 percent, depending on the location in the watershed, the severity of the design storm, and the extent of the flood mitigation technique.
Although research describing the effects of distributed storage and increased infiltration currently exist, this study details the process in which these effects can be modeled in a heavily agricultural Iowa watershed using a simplified lumped parameter model (HEC-HMS). With recent major flooding events in Iowa, the methods and tools in this report will be valuable in predicting the effectiveness of flood projects prior to project construction.