An OpenLISEM Flooding Risk Assessment for the Tropical Volcanic Island of Réunion

Abstract

The island of Réunion is prone to natural hazards such as flooding and mass movements due to frequent tropical storms. The island is an overseas French territory in the Indian Ocean and one of the fastest growing regions in the EU with a population growth rate of 0.8%. Réunion holds various rainfall world records including most precipitation for 12, 24, 72 and 96 hour periods. These events and other tropical storms are responsible for considerable damage to property and people. In January 1980, Cyclone Hyacinthe, the wettest tropical cyclone worldwide struck claiming 25 lives and leaving 7,000 homeless. Cyclone Gaméde also broke rainfall records when it struck in February 2007 resulting in 2 fatalities and 90 severely wounded. If this growth trend persists and if more tropical storms are to be anticipated and produce record rainfall it is imperative to identify the degree of flooding hazard and subsequent risks. The objective of this study is to identify areas at risk of flooding and the potential water depth of flood prone regions. This study utilizes the hydrological erosion model OpenLISEM to simulate a 1 in 10 year storm event derived from 25 years of precipitation data. The most heavily impacted region is the floodplains of the Riviére du Mât where flood heights exceeding 4 meters were found to inundate 940 structures, placing them at extreme risk. Flood heights of 0.5 to 4 meters inundate the fourth most populated city of Le Tampon, home to 70,000 residents. When compared to a risk survey by the BRGM, this study differs in that it determined a much greater flooding risk on the eastern side of the island most likely to differences in methodology when creating the design storm and classification of risk based on flood heights.