Wave characteristics and morphological variations of pocket beaches in a coral reef-lagon setting, Mayotte Island, Indian Ocean
Résumé
Pocket beaches are commonworldwide but documentation on their hydrodynamics, sediment transport processes andmorphodynamics is sparse compared to open beaches. Studies of headland-bound pocket beaches in coral reef environments are evenmore sparse notwithstanding an increasing number of studies of coral reef shorelines. Mayotte Island, in the Indian Ocean, is characterised by a coral reef-lagoon complex and numerous pocket beaches nested between volcanic headlands. Field experimentswere conducted in order to: comparewave attenuation fromthe outer barrier reef to the inner reef flat fronting three pocket beaches, analyse attenuation patterns across an inner reef flat fronting one of the beaches, and document beach morphological changes.Wave attenuation exceeded 90%, and increased as wave heights increased, with maximum attenuation of moderately large waves (significant wave heights>1.8 m) generated by a category 1 cyclone (Jokwe). Further attenuation across the inner reef flat was neither related to reef width nor correlated with water depth, but the correlation was slightly better with relative wave height. Attenuation increased as relative wave height decreased. Patterns of beach morphological change driven by residual wave energy following reef attenuation were strongly affected by the degree of beach embayment. Mtsanga Gouela and Trevani beaches are characterised by a low bay indentation conducive to longshore sediment mobility, and provide rare examples of inferred rotation of reef-fronted beaches, similar to rotation of drift-aligned beaches in non-reef settings. In contrast, Dapani beach, nested in a strongly indented bay, was dominated by seasonal cross-shore sand exchange. In addition to reef-driven wave attenuation, an important factor differentiating pocket beaches in coral reef settings and non-reef settings is the inner reef flat. The historical stability of the beaches suggests that the outer limits of cross-shore seasonal or cyclone-induced sediment movements are set over these reef flats. Further studies of reef-fronted pocket beaches will require better elucidation of the effect of the fronting reef flats on sediment transport and storage, and of the role of heterogeneity in sediment grain size and density common in reef environments in volcanic settings.