**2. Materials and Methods**

*2.1. Study Site*

#### 2.1.1. Physical Setting

Bora Bora (16◦29 S, 151◦44 W, highest summit: 727 m) is a ~20 km<sup>2</sup> tropical volcanic island circled by a 70 km2 barrier reef, in the Society archipelago of French Polynesia in the South Pacific (Figure 1). It has an approximately 40 km complex coastline forming numerous bays and peninsulas that are bordered by 50 to 150 m-wide fringing reefs [8]. There are multiple motu (sandy islands) on the reef margin around the island which hinder water circulation in and out of the lagoon as well as constrain sediment transport from the barrier reef crest and from lagoonal sand accumulations to the island [10]. There are a few shallow channels (hoa) through which oceanic water enters the lagoon. A 48 m deep pass connects the lagoon to the ocean to the west of the island [10]. The general water circulation in the lagoon is south to north [8] but depends on tides (spring tidal range: 0.4 m [11]), wind conditions, and temperature-related water column stratification [12]. The only available wave height data for Bora Bora is based on a record from a sensor located outside the lagoon near Bora Bora's only pass on the western side of the island [13]. The main island is sheltered from the waves by the barrier reef and it is, hence, difficult to estimate its wave climate as well as the variability around the island; outside of the lagoon, the main wave direction is East to West, parallel to the trade wind direction [13].

**Figure 1.** Geographical location of Bora Bora in French Polynesia and satellite imagery of Bora Bora (image from 20 July 2019) highlighting the main topological locations discussed in the article. The white rectangle on the image of Bora Bora above Vaitape corresponds to the location of the images of Figure 2. Letters A–E indicate the location of the coastal habitats featured in Figure 3. Imagery from CNES/Airbus 2019 and Google Earth 2021.

> Bora Bora's lagoon is characterised by a majority of medium to coarse sediments in the shallow areas near the barrier reef (over 98% of the total fraction is >125 μm) and fringing reefs (30% of the total fraction > 125 μm), and a larger fraction of fine sediments at depth, below 22 m (>95% of fine grains of sizes < 125 μm). Most sediments are derived from the breakdown of skeletal fragments from reef organisms [10]; a fraction of sediments are modern cemented non-skeletal grains such as ooids that arise from the precipitation of calcium carbonate on the shallow areas between the motu and the lagoon where currents are weak and allow for carbonate super-saturation [10].

> The absence of sustained riverine input of sediments (no permanent rivers on Bora Bora apart from a stream in the north-western Faanui bay) leads to a mostly carbonate sedimentology with few siliciclastic components (mostly clay) derived from the volcanic island [10].

**Figure 2.** Very high-resolution imagery of the same location at the centre of Vaitape in 1955 (aerial photograph) and 2019 (CNES/Airbus imagery), highlighting the infilling of fringing reefs for constructions and urban development of the area.

**Figure 3.** Examples of ground view of coastal classes. (**A**) sandy beach, (**B**) grass, (**C**) mangrove, (**D**) road embankment, (**E**) trees.

#### 2.1.2. Human Presence

Bora Bora's first infrastructures were constructed during the 1940s, with an airport, roads, quays, embankments. Since then, Bora Bora has been the stage of an important demographic boom (2215 inhabitants in 1971, 10,549 in 2017 [14]; average population density in 2017: 350/km2) and economic development, most notably through tourism and recreational activities (over 80% of international tourists coming to French Polynesia visit the island [15]).

This demographic and economic development is associated with the rapid urbanisation of the coastline, as most of the inner island is constituted of steep cliffs and dense forest. Faanui and Vaitape (Figure 2), on the western side, house more than two thirds of the island's population while Anau in the east has a population of around 2000. This urbanisation is accompanied by coastal modifications (dredging and removal of sand to build roads, authorised and unauthorised land fillings to extend gardens and houses over the fringing reef) and structures (roads, quays, embankments, seawalls).
