How does topography and river-floodplain connectivity influence flooding processes?

Recent high-resolution topographic data has revealed that meandering river floodplains can contain complex floodplain channel networks carved into their surface. How this topography influences flooding is unknown. In this study, we investigate how the connectivity among floodplain channels and between the floodplain channels and river channel influence flooding processes. To accomplish this, we conducted a series of numerical modeling experiments using a two-dimensional (2-D) model built in Hydrologic Engineering Center's River Analysis System (HEC-RAS). First, we simulated floods using 2-D HEC-RAS in the East Fork White River near Seymour, IN, USA, with the existing dense network of floodplain channels. Next, we simulated synthetic versions of this river system where we varied the connectivity among floodplain channels and between the floodplain channels and river channel. We found distinct differences in flooding patterns among model simulations. Numerical modeling experiments showed that increasing floodplain-channel connectivity caused increased variability of wet and dry surfaces across the floodplain, altered flow characteristics (e.g., water depth and velocity) in the river and floodplain, increased lateral exchange, and generally increased residence time on the floodplain. Our results suggest that floodplain channels should increase the ability of a floodplain to attenuate the peak discharge of a translating floodwave and also the ability of a floodplain to remove contaminates from floodwaters.