Practical modelling of sand transport and beach profile evolution in the swash zone

Abstract

A proper prediction of the cross-shore profile evolution in the swash zone at time scales of days to years is important for evaluating beach management scenarios. However, this practical prediction is challenging due to a limited understanding of the complex physical processes in the swash zone. A quantitative evaluation of three existing practical swash-zone sand transport models, i.e. the Larson formula, the Van Rijn distribution model and the Karambas formula, has been conducted in this study. Measured net sand transport rates and beach profiles in seven large-scale flume tests, both low-energy accretive and high-energy erosive wave conditions, are used to assess these three models. Model performance is quantitatively evaluated with the Brier Skill Score ($BSS$), Root Mean Square Error ($RMSE$) and erosive/accretive volume. Overall, the Larson model shows the best performance. Nevertheless, the Larson model cannot capture the shoreline change, as it is assumed only valid for the higher part of the swash zone above the still water level (SWL). Additionally, it fails to predict the accretion in the upper swash zone during high-energy erosive conditions. Thus, two improvements are made for the Larson model by (1) extending the application of the Larson model from the still water level towards the run-down limit and by (2) developing shape functions for the equilibrium bed slope in the swash zone. The improved model is validated using six other large-scale wave flume tests. Results demonstrate that the improved Larson model works better than the original Larson model in predicting the profile evolution, shoreline change and total accretion/erosion volume in the swash zone. The improved model shows the potential to be coupled with wave-averaged morphological models for the nearshore zone to predict long-term evolutions of the entire beach profile.