Pacific Ocean

Rangiroa is one of the world’s largest atolls, located in the Tuamotu Archipelago of French Polynesia, northeast of Tahiti. It consists of a ring of low-lying coral islets surrounding a vast shallow lagoon. As the world’s second-largest atoll by lagoon surface, it extends 80 km in length and 32 km in width, with a lagoon of 1,446 km² and a maximum depth of just 35 m. The atoll is made up of around 240 motu separated by more than 100 narrow channels, with a total land area of 79 km² distributed across over 400 islets. Its lagoon, connected to the ocean through the Tiputa and Avatoru passes, can generate short waves and surges during periods of rough seas. Only two islands are permanently inhabited (fewer than 4,000 residents), while most remain natural or sparsely used. With elevations of only a few meters above sea level, Rangiroa is highly vulnerable to flooding and sea-level rise. Cyclones are uncommon, yet events such as Orama-Nisha (1983) produced waves up to 9 m and surges of nearly 2 m on the southern reef. The hydrodynamics of the atoll are complex: low tidal ranges combine with three main wave regimes—strong SW swells and S–SE waves during austral autumn–winter, and a mixed regime of weaker N–NW and S–SW waves more frequent in summer. Some of these wave regimes can generate long waves within the lagoon, affecting setup. In addition, trade winds and episodic winds such as the Mara’amu create short waves inside the lagoon, making the causes of flooding multiple and interconnected.