Updating knowledge of cyclonic wave hazard for Tahiti and Moorea Islands (French Polynesia) through a probabilistic approach

Abstract. This paper intends to reevaluate cyclonic wave scenarios used for hazard studies for two French Polynesian Islands (Tahiti and Moorea) with a combination of expertise and probabilistic analysis. The method entails (1) constituting a set of cyclones based on both historical data and synthetic events obtained by translating real historical tracks (2) computing the coastal wave characteristics all around the islands for each cyclone with numerical models (3) performing a statistical analysis on the model outputs and determining orders of magnitude of 10 and 100 yr wave characteristics (significant wave height: SWH, and peak period: T_p) for the different sectors of the islands.

The results show that if the western and southwestern coasts are more exposed to frequent cyclonic events (with 10 yr SWH up to 5 m and associated T_p up to 16 s), the northern coasts are the most vulnerable if a cyclone pass very close to the islands because they face the main direction of propagation of waves. For these coasts, the 100 yr SWH can reach 12 m with T_p around 12 s. On the contrary, southern coasts are the least exposed to extreme wave heights with 100 yr SWH below 7 m and associated T_p around 10 s. However, southern coasts can also be affected by very long southern waves all year long (100 yr SWH around 5 m and T_p up to 15 s) that can have similar effects than cyclonic waves at the coast.

Although these results are associated with high uncertainties related to the restricted sample of cyclones used, they provide realistic and rational orders of magnitude of cyclonic significant wave heights and associated periods for different return periods, taking into account the alongshore variability in wave exposure around the islands.