Predicting nearshore sandbar migration using XBeach

Process-based modeling tools such as XBeach model have demonstrated a good performance in predicting dune erosion, overwash, and breaching processes (Van der Lugt et al., 2019). As the exchange of sediments between the foreshore and the inner-surf zone and more specifically the retention of eroded sediment in the inner surf zone is an important factor determining dune erosion, a more extensive evaluation has been performed of XBeach to concurrently simulate morphodynamic evolution in the surf zone and foreshore, particularly the onshore transport flux. The model was applied to simulate sandbar migration events observe during Duck94 field experiment. Using the default parameters, XBeach predicted almost no migration of the sand bar for both an energetic offshore bar migration event and a low-energy onshore event. Compared with measured hydrodynamic quantities, we found that while XBeach was able to predict the cross-shore distribution of wave height, the undertow current during the energetic offshore bar migration event was under-predicted near the bar by about a factor 3. A sensitivity study suggests that the parameters controlling the roller energy dissipation are most sensitive to the resulting undertow current intensity. By improving the roller dissipation parameterization, the model was able to predict the offshore migration event. With a better parameterization of roller dissipation, onshore bar migration can be captured by XBeach by slightly increasing the onshore transport parameter due to wave skewness/asymmetry. The increased onshore transport also improved the agreement with measured beach profile near the berm region. Our study suggests a more comprehensive formulation of roller dissipation across the entire surf zone and onshore transport parameterization are essential for extending XBeach’s capability to simulate the entire beach profile evolution. This preliminary finding is currently being tested with several large wave flume data on random waves over barred beach. The project has been supported by U.S. Office of Naval Research, Littoral Geosciences and Optics Program.