Extreme wave transformation and runup on a beach fronted by a very steep forereef profile

Coral reefs offer natural coastal protection through the efficient dissipation of incident wave energy. Various studies have focused on wave transformation over coral reefs considering an idealised reef profile. However, less effort has been dedicated to studying wave transformation and runup across realistic bathymetric profiles on beaches protected by reefs with steep slopes (${\beta }_f>1/5)$ and subject to extreme wave conditions (Tr > 100 years). In this study, laboratory experiments were conducted using a physical model, considering the bathymetry of the Teahupo’o coral reef, located in Tahiti, French Polynesia. This reef is known for violent incident wave breaking due to the sudden change in water depth induced by a very steep forereef slope (1/2.29). Laboratory measurements were used to investigate the ability of the SWASH model to simulate wave transformation and runup under extreme wave conditions. The numerical model SWASH predicts the variation in significant wave height, low-frequency wave transformation, and runup satisfactorily for return periods of <100 years. However, the numerical model underpredicts the low-frequency swash and runup for the more extreme wave conditions. An existing runup parameterisation is compared with the runup measurements, but overestimates the observed values due to the omission of steeper forereef slopes and very large waves in its derivation. As a result, the parameterisation is recalibrated using additional SWASH simulations, which show better agreement with laboratory measurements. However, limitations remain in predicting runup under to the most energetic wave conditions owing to the SWASH model’s inability to simulate plunging breakers.