Water Resources Engineer IIHR-Hydroscience and Engineering, Iowa, United States
Abstract Submission: Charles City, Iowa has experienced flooding originating from a nearby small agricultural watershed for decades. A coalition of local, state and federal stakeholders led construction of 30 miles of terraces and water and sediment control basins (WASCOBs) during the 1970s and 80s at a cost of $500K ($4M today) to combat the flooding. While this investment constructed best management practices (BMPs) to capture 50% of the total drainage area and reduced the intensity of flooding, they are still experiencing flooding today. We developed a hydrologic/hydraulic model of the watershed that explicitly models all 250 conservation BMP’s storage, surface intake, and overflow using the rainfall-runoff model EPA-SWMM. The highly detailed model can simulate the nuanced behavior of each BMP structure during storm events. We used the model to explore hypothetical scenarios and their effect at the outlet of the watershed over a range of rainfall intensities. We ran simulations with all BMPs removed to estimate peak flow reductions provided by existing BMPs. The effect of back-to-back storm events was explored by simulating a 2-year event prior to each design storm to prematurely consume storage. The impact of increasing existing BMP storage by 50% to restore original design storage was explored. Additional detention storage for peak flow along larger waterways was explored by incorporating new pond projects with large primary spillways. These scenarios provided valuable insight into how broad implementation of terraces and WASCOBs can reduce peak flows but can be ineffective if storage is prematurely consumed.
