Flow and Sediment Flux Asymmetry in a Branching Channel Delta

Abstract

River deltas form complex branching patterns that distribute sediment to the coastal sea. The routing and storage of this sediment in deltas are poorly understood. We present results of a 1-month study of the sediment and water transport through two branches of the Wax Lake Delta on the coast of Louisiana. The two channels maintained near-equal total partitioning of flow and sediment discharge. East Pass was narrower and had higher tidally averaged velocities, lower tidal velocity fluctuations, more sediment flux, and less alluvial bed cover than Main Pass. We connected these differences to small differences in the geometry of the channels and feedbacks between these differences. East Pass was slightly shorter than Main Pass, leading to a steeper mean free surface profile, yielding higher subtidal currents and advective sediment fluxes. Main Pass received the tides from Atchafalaya Bay earlier than East Pass, allowing tides to reach their junction earlier from Main Pass. This led to Main Pass capturing tidal prism from East Pass and higher tidal velocity fluctuations in Main Pass. These fluctuations caused larger flow convergences in Main Pass, pumping sediment out of East Pass on tidal timescales. Since East Pass had higher mean velocities and lower tidal fluctuations, it spent less time in flow regimes that would allow the formation of alluvial bed cover. By explaining these asymmetries with physical arguments, we showed how geometric differences drive asymmetries in branching systems. Barrier island inlets, coastal lagoons, and other systems may display similar behavior.