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 (IFC) was charged with the investigation of distributed flood mitigation strategies to reduce the frequency and magnitude of peak flows in Iowa. This dissertation is part of the several studies developed by the IFC and focused on the application of a coupled physics based modeling platform, to quantify the coupled benefits of distributed flood mitigation strategies on the reduction of peak flows in an agricultural watershed.
Additional investigation into tile drainage and terraces, illustrated the hydrologic impact of each commonly applied agricultural practice. The effect of each practice was represented in numerical simulations through a parameter adjustment. Systems were analyzed at the field scale, to estimate representative parameters, and applied at the watershed scale.
The impact of distributed flood mitigation wetlands reduced peak flows by 4 % to 17 % at the outlet of a 45 km2 watershed. Variability in reduction 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 beyond a total drainage area of 200 km2, approximately 2 km downstream of the small watershed outlet.
A numerical tracer analysis identified the contribution of tile drainage to stream flow (QT/Q) which varied between 6 % and 71 % through an annual cycle. QT/Q responded directly to meteorological forcing. Precipitation driven events produced a strong positive logarithmic correlation between QT/Q and drainage area. The addition of precipitation into the system saturated near surface soils, increased lateral soil water movement, and reduced the contribution of instream tile flow. A negative logarithmic trend in QT/Q to drainage area persisted in non-event durations.
Simulated gradient terraces reduced and delayed peak flows in subcatchments of less than 3 km2 of drainage area. The hydrographs were shifted responding to rainfall later than non-terraced scenarios, while retaining the total volumetric outflow over longer time periods. The effects of dense terrace systems quickly dissipated, and found to be inconsequential at a drainage area of 45 km2.
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, pattern tile drainage, and terrace systems facilitating the simulation of each practices in a physically-based coupled surface-subsurface model.