Understanding Interactions between Shoreline Changes and Reef Outer Slope Morphometry on Takapoto Atoll (French Polynesia)

Low-lying atoll reef islands host populations who are highly exposed to increasing ocean-related pressures resulting from accelerated sea-level rise, ocean warming and acidification. Since 2010, changes in the position of atoll reef island shoreline were estimated on more than 400 Pacific islands using multi-date image analysis. These studies have emphasized island persistence and highlighted the major role of local controls, i.e., human activities and tropical cyclones, in shoreline change. However, they provide limited knowledge on the ecological-morphological linkages driving island change. In order to contribute filling this gap, will be studied the statistical relationships between multidecadal shoreline change and high-resolution outer slope morphometry of the eastern windward coast of Takapoto Atoll (Tuamotu Archipelago, French Polynesia). Shoreline change was estimated using 1969 airborne and 2013 spacebome imagery, while outer slope morphometry was mapped at 2 m spatial resolution using 2016 spacebome imagery. Nine classes of shoreline changes (N=1 402), ranging from >50 m accretion to >50 m erosion, were compared with averages of depth and slope metrics, derived from satellite-based bathymetry mapping. A strong positive correlation between shoreline erosion versus (i) depth and (ii) slope averages was obtained (R-2=0.93 and R-2=0.96, respectively): the deeper/steeper outer transects are, the more eroded shoreline transects are. Those findings suitably match the theory of wave propagation in shallow/gentle waters describing an increase in cross-shore sediment transfer with shallowness, thus explaining spatial variations in shoreline response. The implications of those original results for reef islands' future under climate change will be discussed.