Dynamic Controls on the Asymmetry of Mouth Bars: Role of Alongshore Currents

Mouth bar formation is critical for channel avulsions and progradation of river deltas. The morphology of mouth bars results from different hydrodynamic forcings such as river jets, tides, and wind waves. Here we study the asymmetry of mouth bars due to alongshore currents. Adopting a numerical model, we study how alongshore propagating tides and net alongshore currents cause asymmetric mouth bar formation. The results indicate that alongshore propagating tides shift the depocenter of the mouth bar in the direction of the alongshore currents during peak ebb. Net alongshore currents shift the depocenter to its down-current side. The main channels are oriented in the opposite direction of peak flood flows (with tides) or in the direction of the net alongshore currents (without tides). Systems highly influenced by alongshore tidal flows tend to form more and wider distributary channels which are oriented toward the direction of the alongshore ebb flows. With increased river discharge and sediment influx, the number of bifurcations and channels increases while the mouth bar is less asymmetric. We developed a predictor showing that the mouth bar asymmetry is directly proportional to alongshore currents divided by river jet velocities and the width of the river mouth. Our findings provide insights into the evolution of river deltas and contribute to the management of mouth bars and channels.