Beach Cross-shore dynamics: Applicability of Equilibrium Shoreline models

Beaches are naturally dynamic environments and occur on all open coastlines in the world. The understanding of their behaviour is key to better management of these economically and socially important zones. The sediment in beaches is known to respond to physical forcing, such as incoming waves, tidal variation and sediment availability, having its variability associated with these factors. The use of equilibrium models to assess beach cross-shore movements have been increasing in the last few years, due to their relatively simple structure and lower computational and observational requirements when compared to process based numerical models. The existing models are often site specific and require local calibration. This work aimed to assess the performance of the equilibrium shoreline model developed by Yates et al. (2009c) when applied to 21 profiles from six long term beach records around the world. For all the data sets examined in this work, we found that the model performance was highly variable and never as good as the results presented in the original publication. Here we show that the model performed: moderately well for the same beach where it was developed to, Torrey Pines (California – USA); Narrabeen, (New South Wales – Australia) and Perranporth (Cornwall – UK). While having performed poorly for Duck, (North Carolina – USA), Sandown (Isle of Wight – UK) and Milford-on-Sea (Hampshire – UK). The reasons why the model is able or not to reproduce the shoreline movements are believed to be connected with sampling frequency and coverage, as well as the presence of clear seasonal variations and other physical factors that the model does not account for, such as tidal range and sediment gran size. A volumetric centroid analysis is proposed as a tool to estimate beach behaviours such as flattening, steepening, retreat and advance of the profiles. The expertise and knowledge on beach morphodynamics are evolving fields and the development of more universal models, which can capture the variations at different types of beach, are still a challenge to be overcome.