Simulating the hydrologic impact of distributed flood mitigation practices and tile drainage in an agricultural catchment

Nicholas Thomas,Postdoctoral Scholar, University of Iowa

In 2008 flooding occurred over a majority of Iowa, damaging homes, displacing residents, and taking lives. In the wake of this event, the Iowa Flood Center was charged with the investigation of distributed flood mitigation strategies to reduce the frequency and magnitude of floods in Iowa. This work focused on the application of a numerical model to quantify the impact of flood mitigation strategies in an agricultural watershed. Variability in peak flow impact was a product of antecedent soil moisture, 24-hour design storm total depth, and initial structural storage capacity. The highest peak flow reductions occurred in scenarios with dry soil, empty project storage, and low rainfall depths. Peak flow reductions were estimated to dissipate approximately 2 km downstream of the small watershed outlet. Additional investigation into tile drainage illustrated the hydrologic impact of the commonly applied agricultural practice. A numerical tracer analysis identified the contribution of tile drainage to stream flow which varied through an annual cycle as a product of meteorological forcing. Beyond the analysis of individual agricultural features, this work assembled a framework to analyze the feature at the field scale for implementation at the watershed scale. It showed large scale simulations reproduce field scale results well. The product of this work was, a systematic hydrologic characterization of distributed flood mitigation structures, and pattern tile drainage systems facilitating the simulation of each practice in a physically-based coupled surface-subsurface model.