Extreme storm events and coastal inundation management in tropical island environment: a semi-empirical method for hazard assessment on Reunion Island coasts
Résumé
Located in the southwestern Indian Ocean, the overseas French department of Reunion Island is subject to extreme marine events, Northern cyclones and Southern storms, throughout the year and on all fronts. Moreover, given the volcanic mountains that occupy the central part of the island, urbanization and human activities are primarily located on a narrow coastal strip which is very vulnerable to attacks from the sea, especially in terms of coastal inundation. The risk prevention policy related to coastal flooding implemented by the French government passes first by the estimation and mapping of a 100-year reference hazard area in Coastal Risk Prevention Plans. For this purpose, various methods are possible, from naturalist field investigation to numerical modeling. Using a semi-empirical approach, which incorporates calculations (storm surge and wave-runup levels with parametric formulas) and calibration by expert analysis, is a compromise allowing the manager to have the most reliable results at limited cost and time. On the basis of available data (including regional numerical models and LIDAR DEM), the application on Reunion Island shows more accurate findings than an exclusively "hydrogeomorphic" field approach but also limitations due to the scope of application for empirical runup formulas (not for coral reefs coastlines for example) and the lack of past events observations to validate the outcomes. In these cases of limitations and in particular for high population density areas, the more time consuming and trustworthy approach of runup numerical modeling is required.