Evaluation of nearshore wave models in steep reef environments

Abstract

To provide coastal engineers and scientists with a quantitative evaluation of nearshore numerical wave models in reef environments, we review and compare three commonly used models with detailed laboratory observations. These models are the following: (1) SWASH (Simulating WAves till SHore) (Zijlema et al. 2011), a phase-resolving nonlinear shallow-water wave model with added nonhydrostatic terms; (2) SWAN (Simulating WAve Nearshore) (Booij et al. 1999), a phase-averaged spectral wave model; and (3) XBeach (Roelvink et al. 2009), a coupled phase-averaged spectral wave model (applied to modeling sea-swell waves) and a nonlinear shallow-water model (applied to modeling infragravity waves). A quantitative assessment was made of each model’s ability to predict sea-swell (SS) wave height, infragravity (IG) wave height, wave spectra, and wave setup ($\overline{\eta}$) at five locations across the laboratory fringing reef profile of Demirbilek et al. (2007). Simulations were performed with the “recommended” empirical coefficients as documented for each model, and then the key wave-breaking parameter for each model (α in SWASH and γ in both SWAN and XBeach) was optimized to most accurately reproduce the observations. SWASH, SWAN, and XBeach were found to be capable of predicting SS wave height variations across the steep fringing reef profile with reasonable accuracy using the default coefficients. Nevertheless, tuning of the key wave-breaking parameter improved the accuracy of each model’s predictions. SWASH and XBeach were also able to predict IG wave height and spectral transformation. Although SWAN was capable of modeling the SS wave height, in its current form, it was not capable of modeling the spectral transformation into lower frequencies, as evident in the underprediction of the low-frequency waves.