**1. Introduction**

Since the end of the 19th century, coastal areas have become increasingly sought-after worldwide, and are now largely urbanised [1]. Human populations have grown, with numerous urban centres and cities located along the shore. Infrastructures such as harbours, marinas, and seaside roads have been developed to support local development as well as global maritime trade and transport. This phenomenon is particularly widespread

**Citation:** Gairin, E.; Collin, A.; James, D.; Maueau, T.; Roncin, Y.; Lefort, L.; Dolique, F.; Jeanson, M.; Lecchini, D. Spatiotemporal Trends of Bora Bora's Shoreline Classification and Movement Using High-Resolution Imagery from 1955 to 2019. *Remote Sens.* **2021**, *13*, 4692. https://doi.org/ 10.3390/rs13224692

Academic Editors: Dar Roberts, Junshi Xia and Simona Niculescu

Received: 31 October 2021 Accepted: 17 November 2021 Published: 20 November 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

in Europe, Asia, Australia, and the USA, where up to 50% of coastlines are modified by engineering works [2]. However, the tropical Pacific Ocean, and the French Polynesian island of Bora Bora in particular, have not been spared. In French Polynesia, coastlines have been strongly modified for a range of reasons, mostly since the mid-1900s, from acquiring more land through infilling shallow reef areas to building walls to protect constructions from waves [3–6]. These modifications alter sediment transport processes. Along with the dredging of fringing reefs to open navigation channels, sea level rise, and background sediment transport, these modifications lead to changes in shoreline position [7].

These changes, beyond altering coastal landscapes, have impacts on local livelihoods as well as on economic activities. Many urban centres and residential areas are located along the coastline in French Polynesia, notably due to the lack of inland space, either due to steep topographies (e.g., for the volcanic Society islands such as Bora Bora) or simply to the limited size of the islands (e.g., in the case of the Tuamotu atolls). Furthermore, increased urbanisation and erosion pose a threat to beaches, which are a major resource for tourism activities. On Bora Bora, coastal erosion was already noted as a serious issue at the southern beach of Matira, a major tourism spot, in the 1990s [8]. Characterising coastal urbanisation and erosion in the coral reef islands of the tropical Pacific Ocean at a very high spatial resolution (VHR) can shed light on human impacts on sedimentary processes and provide key knowledge to better manage coastlines in the future, in the context of local urbanisation, global climate change-induced sea level rise, and more frequent and intense storm surges.

On Bora Bora, the availability of imagery from 1955 to 2019 enables long-term monitoring of changes through manual photointerpretation of VHR aerial and satellite imagery [9]. This article aims at addressing the issue of coastal modification and its impacts on the shoreline classification and position on Bora Bora from 1955 to 2019 through an original long-term and very high-resolution approach. This study adds to the growing awareness of the vulnerability of shorelines due to urbanisation and its effects in French Polynesia (e.g., [3]).
