Inundation flow velocities generated by tropical cyclones across atoll islands, derived from two centuries of megaclast deposits in French Polynesia

Document Type

Article

Source of Publication

Natural Hazards

Publication Date

1-26-2026

Abstract

Understanding potential flow velocities during high-energy marine inundation events is crucial for coastal risk assessment. However, modelling struggles to simulate wave energy dissipation across atoll island coastlines. Here, we examined coral reefblocks transported by past tropical cyclones to calculate the minimum flow velocities (MFVs) responsible. Fieldwork on 6 atolls in the Tuamotu archipelago (South Pacific) examined 196 reefblocks, some megaclasts exceeding 300 cubic metres in size. These blocks are scattered between the oceanside reef edge and the atoll lagoon over several hundred metres and suggest flow velocities much higher than those modelled in an assumed ‘extreme reference scenario’ (HS = 12 m). Through U/Th dating and by studying archives and historical aerial photos, the cyclones that moved these reefblocks were identified. Inundation flows generated by two recent cyclones (March and April 1983), two historical cyclones (1903 AD and 1906 AD) and one prehistorical cyclone (54–80 AD) were calculated (storms with swells 10–18.5 m in height). Calculations reveal that previous modelling underestimates flow velocities across atoll islands (inhabited areas) for two reasons: the underestimation of extreme swell heights and the unaccounted-for degradation of shoreline rubble ramparts. During a supercyclone (HS > 15 m), flows can exceed 3 m/s at 350 m from the reef edge and are capable of transporting 20-ton coral blocks. Findings have a wider significance to tropical coral reef coastlines beyond these atolls studied, where the presence of reefblocks can allow hindcasting of the characteristics of prehistorical cyclone inundations.

ISSN

0921-030X

Publisher

Springer Science and Business Media LLC

Volume

122

Issue

2

Disciplines

Earth Sciences

Keywords

Coral rampart, Flow velocities, French Polynesia, Megaclasts, Reefblocks, Tropical cyclones, Tuamotus

Scopus ID

105028278843

Indexed in Scopus

yes

Open Access

no

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