Setup in the surfzone

Abstract

Surfzone wave height transformation and wave-breaking-driven increases in the mean sea level (setup) are examined on alongshore-uniform beaches with alongshore homogeneous and inhomogeneous wave forcing. While previously derived models predict wave heights adequately (root-mean-square errors typically less than 20%), the models can be improved by tuning a free parameter or by using a new parameterization based on the deep-water wave height. Based on a sensitivity analysis of the cross-shore momentum balance used to predict setup, a one-dimensional (1-D) model is developed that includes wave rollers and bottom stress owing to the mean offshore-directed flow. The model predicts setup accurately at three alongshore homogeneous field sites, as well as at a site where the incident wave field is alongshore non-uniform, suggesting that setup is driven primarily by the cross-shore (1-D) forcing. Furthermore, alongshore gradients of setup can be important to driving alongshore flows in the surfzone, and the 1-D setup model predicts these gradients accurately enough to simulate the observed flows.