Human- and climate-driven shoreline changes on a remote mountainous tropical Pacific Island: Tubuai, French Polynesia

Few studies have focused on shoreline change on high mountainous tropical islands, whereas their low-lying coastal areas generally host most population centres and human assets. This paper contributes to filling this gap by assessing shoreline change on a remote Pacific island, Tubuai, French Polynesia. The study considers different shoreline proxies and time periods based on the 32-year (1982–2014) aerial imagery available. Over the multi-decadal timescale, the base of the beach predominantly exhibited retreat, observed along 57% of the shoreline. The stability line, which consists of the vegetation line or the base of coastal defences, depending on shoreline sections, was more resistant to coastal erosion, as 61% of the shoreline remained stable while 32% experienced retreat. At shorter timescales (i.e. for the four sub-periods considered in this study), results show high spatial-temporal variability. Clarifying the respective contributions of human activities and intense tropical cyclones to shoreline change, results show that the former had a major influence despite low population density, whereas the latter had a moderate influence. In fact, human disturbances have extensively destabilized sediment cells, which will likely exacerbate the negative impacts of future cyclones on tropical shorelines and their inhabitants. Findings provide insights for developing strategies for risk reduction in remote islands with low population densities and available space.