Wave dynamics within a reef-fringed pocket beach

Upper dune erosion occurs in Gnarabup Beach during extreme events; a popular reef-fringed pocket beach in the Margaret River region of south Western Australia. The site is exposed to considerable year-round wave energy from the Southern Ocean with offshore significant wave heights during winter often exceeding 4 m. Even though pocket beaches with fronted fringing-reefs are common setting all over the world, the characteristics of the waves and their impacts on beaches have been the subject of little research. There is a lack of scientific knowledge on how waves transform within these nearshore reef systems and interact with the coastal morphology. Moreover, the interaction between reefs and headlands of pocket beaches will presumably create different hydrodynamic characteristics to traditional coastal reefs. To understand these hydrodynamic processes, a fifteen-month measuring campaign was carried out at Gnarabup Beach to monitor the waves and water levels including the installation of a directional wave gauge offshore of the reef and pressure sensors inside the lagoon in cross-shore and alongshore configuration. The results show offshore wave height ranged between less than 1 m to up to 8 m. Inside the reef lagoon, wave height dissipated to around 20% of the offshore wave height but with considerable infragravity energy. Both sea-swell and infragravity wave heights within the lagoon were correlated with the offshore wave height but were also weakly correlated with regional sub-tidal water level variations. While the reefs at the site only occupy approximately one-third of the total alongshore extent of the beach the reef effectively regulate wave energy along the beach in a similar manner that would be expected onshore of a continuous fringing reef.