Deposition and Erosion Potential of Sediment in a Channelized Floodplain

High resolution topographic data reveal that meandering river floodplains are remarkably complex and often contain channel networks. Many questions remain as to the pervasiveness, function, and evolution of these channels. In this work, we quantify the potential for deposition and erosion of sediment in floodplain channels and focus our analysis along the East Fork White River near Seymour, Indiana, USA. We constructed a two-dimensional numerical model using 2-D HECRAS of the river-floodplain system from LiDAR data and from main-channel river bathymetry to elucidate the behavior of these floodplain channels across a range of flows. Simulated flow depths and velocities were used to estimate shear stresses and were combined with suspended sediment concentration and grain size data collected from the river channel and floodplain channels to assess deposition/erosion potential of various grain sizes at various flows. We find that for a given grain size, there are flows that will both deposit (lower flows) and erode (higher flows) that sediment. Furthermore, the locations at which a particular grain size switches from depositional to erosional varies spatially and this spatial pattern varies with increasing flow. The ultimate morphodynamic evolution of a particular floodplain then depends on the presence of certain sizes of sediment transported onto the floodplain and the frequency of flows acting to deposit or transport that caliber of sediment. Shifts in sediment supply or floodplain inundation frequency can therefore alter floodplain morphology. This has important implications for determining whether floodplain topography will be enhanced or annealed by river-floodplain processes.