Investigation of XBeach cross-shore capacity under fixed model parameters.

Abstract

An accurate, computationally efficient shoreline model is fundamental to understanding long term shoreline behaviour. Currently, one-line models can be used to perform this but are limited to either cross-shore or alongshore changes only. A simple, more robust model could be developed based on volumetric beach profile changes. However, this requires large empirical datasets to explore the relationships between wave and shoreline variables. In instances where such data is not available, process-based models such as XBeach are an attractive solution. Therefore, it is important to explore the ability of processes-based models such as XBeach to accurately predict shoreline changes under varying wave conditions. In this study two wave conditions associated with erosion (Hs =3m) and accretion ( Hs =1m) were used to evaluate the performance of XBeach to accurately predict shoreline changes. The east coast of South Africa was used as a case study site. The coastal areas of South Africa are known for their high energy wave climate resulting in a dynamic shoreline, with significant onshore and offshore sediment transportation. The effects of wave nonlinearities on sediment transport was estimated using XBeach’s built-in wave asymmetry and the skewness calibrating factors. The sensitivity of sediment transport to these parameters was investigated by running 180, 1D wave flume simulations. The 180 simulations were formulated by varying the wave asymmetry and skewness calibrating factors, sediment transport models, and approaching wave conditions. The results showed that increasing the magnitude of the calibrating factors increased onshore sediment transport. This was attributed to an increase in the advection velocity in the onshore direction. The investigations on the model capacity showed that the wave asymmetry and skewness related parameters control the cross-shore sediment movement direction. It was found that a single set of model input parameters were not able to produce both onshore sediment transport during low energy wave conditions and offshore sediment transport during high energy wave conditions. This study demonstrated that calibrating factors should depend on incoming wave energy. Currently, they are not implemented like this in XBeach.