Sediment transport dynamics and the evolution of subaqueous delta channel levees

Prograding river deltas construct new land along coastal regions through extension of subaqueous channels and accretion of levees and interdistributary bays. Predicting future coastal change requires an understanding of the controls on topographic evolution of these landforms. The evolution of fluvial channel levees has long been understood to be driven by laterally decreasing flow velocities. This allows sediment to settle, over time building levees on channel margins. However, flow in subaqueous channels on prograding river delta fronts is not constrained by channel margins. Previous laboratory experiments show similar patterns of sediment transport in subaqueous channels as in fluvial channels. We measured suspended sediment concentrations over subaqueous channel levees on the Wax Lake Delta in coastal Louisiana to test controls on levee construction in a field-scale system. We used in-situ laser diffraction sediment concentrations (from LISST-SL2) to calibrate backscatter from an acoustic Doppler current profiler to enable concentration, grain size, and flux determination across a channel-levee system. Contrasting with fluvial systems and previous experiments in subaqueous channel systems, we found suspended sediment concentrations increased from the channel center up the levee bank to a maximum along the channel margins, indicative of channel widening. Concentrations decreased laterally from the levee top toward the interdistributary bays indicating levees are being constructed by advection settling as flow decelerates. However, resuspension from the channel margin drives the delivery of sediment resulting in the accretion of these levees. These results indicate that the controls on the morphologic evolution of subaqueous delta channels does not conform to existing models. Further field work over a range of tidal and river input conditions will further elucidate the dynamic processes responsible for subaqueous channel levee evolution.