Marine inundation hazards in French Polynesia: geomorphic impacts of Tropical Cyclone Oli in February 2010

Abstract

Abstract Marine inundation hazards in French Polynesia are various and unevenly distributed in the territory; they are strongly related to the physiography (topography, bathymetry, coral reef development) of these oceanic islands. Cyclones and tsunamis appear as predominant processes in the definition of coastal flooding risks for Polynesian people. This study examines the geomorphic impacts of Tropical Cyclone Oli, which struck the western part of French Polynesia in February 2010. Submarine reef erosion is quantified through coral colony degree of destruction and massive coral colony displacement. Sediment transport and beach retreat are quantified, and flow velocities at the coastline are estimated through boulder analysis. Erosion and resilience of a sandy bank (cay) at the reef margin is also considered on Tubuai Island through satellite image analysis and GPS field survey. Outer-reef slope angle appears as a major control factor for coral destruction, with vertical submarine cliffs relatively shielded compared to gentle slopes. Submarine boulder measurements provide valuable estimates of flow velocity profile with depth. Beachrock slab measurements also provide estimates of flow velocities at the reef–beach junction. Combining these different geomorphic markers might be a way to apprehend the flow velocity variation when the cyclone waves cross the coral reef.