Extreme floods regionalisation in the tropical island of Tahiti, French Polynesia

Abstract

Flood hazards in the inactive volcanic island of Tahiti are very high. Indeed, the island is located in the South Pacific Ocean, in humid tropical climate and is exposed to cyclones. Even if the island is small (1,042 km2 ), rainfalls and runoffs are very different from one place to another. Precipitations are about 1,500 mm/year, and it rains twice as much on the East Coast than on the West Coast. Additionally, precipitations can reach 10 m/year in the summits. Above all, catchments are small and elongated. These characteristics induce flash-floods which may cause heavy damages. In this study, data coming from 10 water-level gages are analysed by using a distribution function. After water level transformation to streamflow, the maximum annual runoffs are extracted and fitted according to a Gumbel law. For the 10 stations, the two Gumbel parameters are selected and used to establish a model of extreme runoff distribution, at the catchment outlets, for different return periods. This model depends on drainage area, annual precipitations, shape and position of the watersheds. It highlights that the valleys the most affected by floods are the bigger ones in the Northern and Eastern parts of the island, what we noted, for example, in December 2015.