Short-term river mouth bar development during extreme river discharge events: The role of the phase difference between the peak discharge and the tidal level

Abstract

This paper addresses the role of the phase difference between the peak discharge and the tidal level on the development of nearshore bars at river mouths during extreme river discharge events. We use a coupled hydrodynamic and morphodynamic numerical model (Delft3D) to reproduce extreme river discharge events on an idealized river mouth debouching on a nearshore shelf, characterized by a concave cross-shore profile. Different peak river discharges and lags between the peak discharge at the upstream boundary and the high tide at the offshore boundary are tested. The analysis of the water levels, currents, bed shear stresses, sediment transport rates and bed level changes shows that the lag between tidal conditions and river discharge triggers significant changes in (1) the instant in which the maximum currents at the outlet are reached, (2) the period for which the sediment is mobilized through the outlet, and (3) the maximum sediment transport rates during the events. These changes significantly modify the final characteristics of the river mouth bars, doubling their final extension and quadrupling the final bar volume for the same river discharge conditions and different phase lag, varying also their plan shape and the development of lateral subaqueous levees. These results emphasize the complex interplay between river and coastal hydrodynamics, being of major interest for researchers and managers dealing with projections of the consequences of coastal extreme events and plans to mitigate their effects.