Validation of wave transformation models for tropical coastal environments

Abstract

Very few studies have been performed to validate near-shore wave models for fringing reef conditions. Fringing reefs dissipate large amounts of energy suddenly through plunging wave breakers, providing a direct contrast to the slower and gentler energy dissipation through spilling mechanisms that most coastal wave models are based upon. This study examines the application of the spectral wave models, Steady-State Spectral Wave Model (STWAVE) and Simulating Waves Nearshore (SWAN) as well as the Boussinesq model COULWAVE to describe wave transformation for the fringing reef conditions of Mokuleia, Oahu and Ipan, Guam as well as Waimea Bay, Oahu during swell events. STWAVE and SWAN describe the transformation in time of recorded wave spectra from the offshore buoy to the shore. COULWAVE provides a more precise, wave-by-wave description of the transformation along the nearshore instrument transects during low and high wave conditions. The results are compared to nearshore measurements recorded at the three sites. The spectral models STWAVE and SWAN give very similar results and compare reasonably well with measurements prior to wave breaking. Inside the surf zone, the differences between STWAVE, SWAN, and COULWAVE are noticeable and none of the models can consistently describe the surf zone wave height at all three sites. This is due to the differences in the predefined breaking criteria in the models as well as the frictional dissipation mechanisms employed. There appears to be a dependence upon the relative roles of wave breaking and bottom friction that may influence the general predictive capabilities of the models.