Nearshore SWAN model sensitivities to measured and modelled offshore wave scenarios at an embayed beach compartment, NSW, Australia

Spectral wave modelling is a common dynamical approach to transform offshore wave climates to the nearshore zone for coastal hazard definition and engineering design. Knowledge of model limitations and sensitivities are thus of paramount importance to appropriate use for coastal engineering. This study reports the calibration and nearshore sensitivities of a SWAN model at Wamberal-Terrigal on the central New South Wales coast, when the model is forced with wave information from a regional WaveWatch III (WW-III) model, compared to model forcing from simultaneous offshore buoy observations. SWAN achieved good results for nearshore wave heights (R-2 = 0.86, RMSE = 0.2 m), but under-estimated mean wave period by approximately 1 s. Default SWAN physics were found to be largely appropriate. The inclusion of hindcast winds introduced a systematic over-estimation of high frequency (low period) wind-sea but improved the shape of the wave period distribution. Transformations of WW-III spectra through SWAN suggests that oblique swell is under-represented by WW-III at this location, with only wave directions between 80(circle) and 150(circle) accounted for. In modelling cases, the long shore transport component, typically driven by oblique long-period wave energy, would likely be under-estimated while shorter-period wind-waves that favour cross-shore sediment transport is preferenced.