Coupled evolution of rivers and floodplains

Abstract

River-floodplains are coupled systems in which the conveyance and cycling of sediment over centuries to millennia set the morphological and sedimentological boundary conditions that control fluvial processes over shorter time periods (e.g., years to decades). Although this is well known, understanding how such processes are coupled over short and long timescales remains a significant challenge. Moreover, fluvial geomorphology as a discipline has, arguably, focused more on the study of within-channel processes and dynamics, with less attention given to how river-floodplain interactions and feedbacks control the functioning of the wider river corridor (i.e., channel-belt). Growing concern over the potential impacts of 21st century environmental change (e.g., on river morphology, flood conveyance and floodplain ecosystems) highlights a pressing need to address this knowledge gap. This study applies a physically-based morphodynamic model to investigate the coupled evolution of channel-floodplain systems. The model solves the shallow water equations for in-channel and overbank flows, together with equations representing fine and coarse sediment transport, channel migration and floodplain development. Simulations are run over time periods sufficient for the floodplain to be reworked (and hence reconstructed) by the model. Simulation results provide new insights into: (i) the relative importance of river gradient, sediment flux and bank erodibility as controls on channel migration rates; (ii) the factors that determine the mechanisms and frequency of channel cutoffs; (iii) the degree to which floodplain dynamics (e.g., floodplain reworking and construction) control both floodplain topography and river pattern; (iv) the relative importance of bedload versus suspended load in determining overall fluvial system behaviour; and (v) the challenges involved in simulating these effects using numerical models.