Contribution of moderate climate events to atoll island building (Fakarava Atoll, French Polynesia)

Abstract

While the geomorphic impacts of extreme climate events, including in particular tropical cyclones and distant-source swells, on atoll islands are widely acknowledged, the influence of moderate climate events, such as tropical lows that do not reach the cyclone stage and low-magnitude distant-source swells, has to date been overlooked. Based on multi-date image analysis and field observations, the geomorphic impacts of a moderate climate event, tropical low 13F (February 2017), were assessed in the northern part of Fakarava Atoll, northwestern Tuamotu Archipelago, in French Polynesia. Results show that this event, which generated strong swells, predominantly caused shoreline retreat (noted along 54.14% of transects, with the minimum Net Shoreline Movement reaching −45.46 m) along the highly-exposed shoreline section, while the rest of the shoreline predominantly exhibited stability. The effects of this swell event were mainly constructional, as shown by the formation of two small storm ramparts and of extensive sediment sheets and tracts on the conglomerate platform. In low-lying areas, the waves crossed over the islands from ocean to lagoon, depositing sediment inland. The erosional impacts of the storm waves and overwash-induced sediment deposition were lower along shoreline exhibiting high vegetation density. One year after the event, intertidal sediment deposits had significantly migrated landward. This study, which reveals an important research gap in atoll geomorphic studies, calls for the more systematic assessment of the geomorphic impacts of moderate climate events on atoll islands, especially in atoll regions that are located outside the cyclone belt (e.g. Tuamotu, Maldives, etc.), where such events can be hypothesised to be more influential than extreme climate events.