The Effect of Tides and Storm Surges on the Sediment Transport during Overwash Events

Storm events generally result in elevated water levels at the meso-tidal Wadden Sea coast, the Netherlands. This can lead to overwash and inundation of parts of the barrier islands. We hypothesize that the cross-shore sediment transport, caused by such events, can on the long term contribute to the vertical accretion of the barriers. Currently, large parts of the Dutch barriers are closed off by artificial dunes which prevent overwash, but the partial re-opening of dunes is considered by the Dutch management authorities. It is therefore important to identify the dominant cross-shore hydrodynamic and sediment transport processes during an overwash event and to study the potential long-term sediment transport. In addition, we focus on the role of the back-barrier basin on overwash dynamics. An XBeach model was set-up and validated against field data collected during overwash on East-Schiermonnikoog, the non-vegetated tip of the barrier. The simulated wave heights, periods, water levels and flow velocities agree well with the field data. Next, the model was run for a wide variety of storm and tidal characteristics. From the model simulations we conclude that: (1) The erosion and transport of sediment across the beach crest is mainly driven by the cross-shore currents. (2) Maximum onshore transport occurs during more gentle storms instead of big storms. (3) For mixed-energy, meso-tidal barrier systems like the Wadden Sea, the dynamics of the back-barrier have to be taken into account. Water level gradients across the barrier are strongly influenced by the tidal phase propagation and the difference in storm surge height between the back-barrier basin and adjacent sea. The model simulations substantiate our hypothesis that overwash events can on the long term contribute to the vertical accretion of the Wadden Islands. Physical explanations about these conclusions will be given during the presentation.