MORPHODYNAMIC INSIGHTS INTO A CHANNELIZED FLOODPLAIN ALONG THE MEANDERING EAST FORK WHITE RIVER, INDIANA, USA

High resolution topography reveals that meandering river floodplains in Indiana commonly have networks of channels. These floodplain channel networks are most prevalent in agricultural, low-gradient, wide floodplains. It appears that these networks are formed when floodplain channels connect oxbows to each other and the main river channel. Collectively, the channels in the floodplain create an interconnected network of pathways that convey water beginning at flows less than bankfull, and as stage increases, more of the floodplain becomes dissected by floodplain channels. In this work, we quantify the potential for erosion and deposition of sediment in floodplain channels along the East Fork White River near Seymour, Indiana, USA. We constructed a two-dimensional numerical model using HECRAS of the river-floodplain system from LiDAR data and from main-channel river bathymetry to elucidate the behaviour of these floodplain channels across a range of flows. Model calibration and verification data included stage from a USGS gage, high-water marks at a high and medium flow, and an aerial photograph of inundation in the floodplain channels. The simulated flow depths and velocities from the numerical model were combined with suspended-sediment concentration and particle-size data collected from the river channel and floodplain channels to assess incision/deposition potential (via shear stress). At flows just inundating the floodplain channels, fine sediment deposits. However, the continued existence of these channels suggests that erosion must occur during higher flows to evacuate and possibly further incise these floodplain channels. We suggest that these channels may form in low-gradient rivers as they transition to a new morphology, such as anabranching, or these floodplain channels may serve as preferential avulsion pathways.