Erosive effects of the storm Helena (1963) on Basse Terre Island (Guadeloupe -- Lesser Antilles Arc)
Résumé
Erosive effects of the tropical storm Helena that hit the volcanic island of Basse Terre (Guadeloupe -- Lesser Antilles Arc) on 24 October 1963 has been measured using 80 aerial images acquired by the French geographic institute (IGN -- Institut Géographique National) at the approximate scale of 1/8000 less than three months after the storm Helena. On these images, 253 landslides triggered during the storm were identified and mapped. These landslides were located in the central region of the island where catchments exhibit the highest relief. Even though the average thickness of the landslides was only 1 m, i.e., less than the thickness of the weathered layer, the total volume of displaced sediments corresponded to an average denudation of 2800 t km− 2, i.e., 1.4 mm, on the watersheds affected by landsliding. To assess the erosional significance of this single climatic event, we compare the volume of sediment mobilized by the storm Helena to the long-term denudation rate. The latter, estimated from a calculation of the total volume of material eroded since the emplacement of lavas using a digital elevation model, is found to be 0.14 mm/y. Assuming that Helena is representative of the storms that hit Basse-Terre Island during the Quaternary, we find that a return period of about 10 to 15 years is enough to account for the long term denudation rate recorded for this Island. Such a period is comparable with the actual return period of the tropical cyclones of the order of 4 to 5 years, suggesting that the erosion of Basse-Terre Island is entirely controlled by tropical hurricanes.