Effects of Wave Breaking Formulations and Wave Nonlinearity on XBeach Performance for Onshore and Offshore Sediment Transport Event

The performance of the sediment transport model XBeach for predicting onshore sediment transport was investigated using field observations at a dissipative beach in Japan. The field data were observed at the Hasaki Coast facing the Pacific Ocean. Based on the data from Jan. 1987 to Dec. 2003, 11 onshore dominant and 20 offshore dominant sediment transport events were selected using criteria, i.e. onshore sediment transport volume, longshore velocity, etc. Even with this reduction in data, it was still necessary to examine each profile to select a reasonable final number of events for comparison with XBeach. Finally, we considered the 4 events, i.e. one onshore dominant, two on-offshore dominant and one offshore dominant event. In this paper, by using the onshore event with a large fraction of sediment from the offshore bar area to the nearshore zone, the performance of XBeach for beach profile change is discussed. In particular, the analysis considered: (1) the effect of wave breaking on the spatial and temporal distributions of wave height, and (2) the sensitivity of wave nonlinearity parameters on bed profile change.

First, the effect of wave breaking formulations on wave height distributions were analyzed. The comparisons revealed that the wave breaking formulations affected the wave propagation and that the parameters of the default settings did not fit the observed wave heights in the nearshore. It was necessary to choose the wave breaking formulation and adjust the parameters to fit the spatial-temporal distribution of wave heights. The wave nonlinearity parameter, f_ua, for onshore dominant sediment transport event is calibrated using the observed beach profile data. Regardless of whether or not the model was tuned to predict the correct wave heights, the beach profile change could be adjusted using other parameter related to wave nonlinearity.