WATER AND SEDIMENT PARTITIONING THROUGH BACKWATER CHANNEL BIFURCATIONS

Water flow in river deltas has been shown to depend on backwater flow. No model of water and sediment partitioning in a channel bifurcation has explicitly taken backwater flow into account, although avulsions in river deltas are a key element of delta dynamics. Application of existing models is complicated by adverse channel-bed slopes that are commonly found in large deltas. Backwater flow contrasts from normal flow in that water surface height is forced by base level and is relatively independent of discharge in backwater reaches. We use this to simplify 1D shallow water equations, showing that flux partitioning primarily depends on the cumulative resistance of each downstream channel; a quantity proportional to channel length to its receiving basin and inversely proportional to depth squared and width. We conclude that in backwater bifurcations, the entirety of each downstream channel should be considered when predicting flux partitioning and not just local bathymetry. Further, we argue that suspended sediment flux entering an avulsion need not simply decant sediment laden water from above the channel lip. A channel can pull water and suspended sediment from below its lip height if its aforementioned dimensions force it to receive significant water flux. We use field data from the Atchafalaya/Wax Lake avulsion in Louisiana to confirm our model, presenting water velocity, sediment concentration, grain size, and bathymetry data. At this avulsion, 40% of water passes into the Wax Lake feeder channel over a channel lip that is 31% of the Atchafalaya River depth (5 to 16 m) and fine sand transported in suspension. We show that a standard decanting model under-predicts volumetric suspended sand concentrations (>60 μm) entering the Wax Lake feeder channel by a factor of three (1.5E-6 ± 6.8E-7 to 4.5E-6 ±1.9E-6).