Gwenaëlle Pennober

Effect of Tropical Cyclones on Short-Term Evolution of Carbonate Sandy Beaches on Reunion Island, Indian Ocean

ABSTRACT Mahabot, M.-M.; Pennober, G.; Suanez, S.; Troadec, R., and Delacourt, C., 2017. Effect of tropical cyclones on short-term evolution of carbonate sandy beaches on Reunion Island, Indian Ocean. Carbonate sandy beaches in Reunion Island show various forms of evidence of erosion. Extreme waves associated with tropical cyclones (TCs) play a major role in beach dynamics. The present study analyzes and quantifies back-reef beach response and recovery from forcing generated by TCs Dumile, Felleng, and Bejisa, which occurred in 2013 and 2014. The study focuses on carbonate beaches of Reunion Island from Cap Champagne to the Passe de Trois-Bassins. Morphological and volumetric changes on beaches were analyzed by comparing 19 beach profiles. The results show that TCs are able to cause significant morphosedimentary change on the back-reef beaches of Reunion Island. These changes affect beach topography and involve longshore and cross-shore sediment transport. An alongshore variation in beach response is observed, which varies according to tropical storm intensity and coastal morphology. The intensity of impact seems to be related to reef width. The most severe erosion occurred at Boucan Canot, where reef is absent with a loss of −24 ± 2 m3 after TC Dumile, −38.7 ± 1.2 m3 after Felleng, and −42.5 ± 1.6 m3 after Bejisa. Elsewhere, the volumetric changes is less than 5 m3 under TC Dumile and vary between 2 and 11 m3 under TC Felleng and between 2 and 23 m3 under TC Bejisa. No significant impact occurred at La Saline where the reef flat is large and provides good protection for the beach; however, relative coastline orientation and prestorm beach-profile morphology also play an essential role in storm impact. Wave height and water level are also determinant factors of storm erosion potential. After storms, the beaches show a relative capacity for recovering because of calm conditions; however, different behaviours are observed along the same beach compartment. This suggests local influence of coastal structure and/or reef geomorphology in sediment transport processes.