ABSTRACT
Flow regime, sediment supply and base level control geometry and evolution of alluvial channels and floodplains. Single thread rivers subject to constant forcing can reach equilibrium conditions in which the amount of sediment deposited on the floodplain through point bar deposition and overbank sedimentation is balanced by erosion of floodplain sediment through channel migration. At equilibrium, floodplain slope and sediment size distribution, reach-averaged channel geometry (width and depth) and channel migration rates do not change in time. In response to changes in sediment supply and floodplain width, channel geometry and migration rate, floodplain slope and size distribution are expected to evolve in space and time. Predicting this response remains an open problem for geoscientists and engineers. Here we use an equilibrium solution of a 1D morphodynamic framework of channel-floodplain evolution to investigate how equilibrium conditions change as a function of sediment supply and floodplain width. Sediment is modeled here as a mixture of two grain sizes, sand and mud. Channel migration rate and width are functions of near-bank flow properties and floodplain characteristics. We zero the model using input parameters based on the pre-1930 ~ reach of the Minnesota River from Mankato to Jordan, USA, where data is available for proper field scale model verification. We then use the validated model to quantify the long-term (equilibrium) response of the schematic reach to changes in sediment supply magnitude and size distribution, as well as to changes in floodplain width.
