Dune erosion near sea walls; XBeach validation

Abstract

During a storm dunes erode and provide sediment to the beach. The foreshore rises and the wave height decreases. Subsequently the wave-induced water level setup increases. Seawalls do not erode. The waves in front of the seawall remain high throughout the storm and the wave-induced water level setup hardly changes. Over a dune-dike connection a water level gradient drives a current which transports sediment from the dunes to the seabed in front of the seawall. The loss of sediment to the dike causes the foreshore of dunes near the connection to rise slower resulting in more erosion. The amount of additional erosion near structures depends on the angle of wave incidence w.r.t. the shore. Dunes situated downstream of the connection experience a significant increases in erosion. Near the upstream connection sediment is deposited on the seabed in front of the structure. Sediment will pile up against the structure resulting in less erosion near the connection. The influence of structures on dune erosion was investigated in a series of experiments and were aimed at 4 different connections between a dune and a structure. Two configurations of dunes and structures were investigated with 2 different wave periods. In XBeach the erosion in a dike breach is a function of the breach width and the wave period. The erosion increases for a smaller breach width and an increasing wave period. XBeach underestimates the erosion above the dune revetment. The erosion in the revetment breach is predicted well. For experiments V1 & V3 the predictive capabilities of XBeach in dune sections are very good. The performance of XBeach for experiments V2 & V4 is relatively good. The effect of the wave period for 2DH models with a large depth scale (nd = 60) is not properly simulated by XBeach.