Numerical modeling of the influence of tides on beach morphodynamics

Beach morphodynamics is strongly interlinked with the wave and tidal climate. While a lot of effort has been put in the past decades to study and model the effects of waves on the morphological evolution of beaches, studies on the influence of tides are relatively scarce.

In this study, we use the numerical model XBeach to identify the influence of tides on sediment transport in the cross-shore direction. The tidal wave is prescribed in the model as an alongshore Kelvin wave.

The effect of tide is twofold: on one hand, tidal fluxes increase the combined wave-current bed shear stress, thus increasing suspended concentration. On the other hand, the water level variation modulates the orbital wave velocity and resuspension, both of which increase for smaller depths. We show how the average water depth over which the tide is propagating changes the character of the tidal wave: for higher offshore depths, the tidal current is in phase with the water level. As the tidal wave enters frictionally-dominated shallow waters, the tidal current signal displays a phase lag with respect to the water level signal. This shift changes the relative importance of tidal fluxes and water level modulation.

We conclude showing that tide favors on-shore sediment transport and it is an important driver for cross shore beach recovery during fair-weather conditions. The larger the tidal amplitude, the larger is its contribution on beach recovery.