*3.3. LOS Displacement Time Series*

To inspect the trends of the land displacement over time, the LOS displacement time series of four sites are depicted in Figures 8–11, whose locations are shown in Figure 6a. Figure 8 shows the LOS displacements for site 1 and exemplifies the anthropogenic impact on VLM (Figures 6a and 8). In this sector, to face coastal erosion on the western side of the mouth of the CGSM, two groins were built in 2012. These coastal structures produced a gain of sediment on the updrift side of the groins, reflected by a slope value of 0.9 cm/yr for the best fitting line to the associated time series.

**Figure 8.** Satellite images before (**a**) and after (**b**) the groins were installed. Location of time series is indicated by white dot; (**c**) LOS displacement time series from Sentinel-1A for 2017–2021 (line represents the best fitting line with a slope of 0.9 cm/yr), and (**d**) photograph of the groins taken in 2014. Source of imagery: GeoEye images provided by Planet Labs Inc.

Figures 9 and 10 illustrate the time series of an area that was densely vegetated with mangroves until the 1980s (site 2 and 3, Figure 6a). This sector became the location of frequent overwashes that left behind sandy deposits, buried vegetation, and vegetated dunes scarped by erosive processes [24] (Figure 9c). Typical time series show gradual downward displacement with slopes of approximately −0.5 cm/yr (Figures 9b and 10b).

The westernmost time-series transect was located seaward from El Torno lagoon (site 4, Figure 6a). Although the shorefront in this area is erosive, no breachings have been observed in this lagoon during the last decade, and limited overwash deposits appear on satellite imagery. The area between the lagoon and the coastline consists of beach sands and embryo dunes sparsely covered with pioneer vegetation. LOS velocities for this site revealed a negative slope of −1.2 cm/yr for the linear fitting regression.

**Figure 9.** (**a**) Aerial photograph taken in 1953 showing the former mangrove forest before the highway was built. Parabolic dunes are seen in the backshore. Location of time series is indicated by white dot; (**b**) LOS displacement time series from Sentinel-1A for 2017–2021 (line represents the best fitting line with a slope of −0.52 cm/yr), and (**c**) shows standing dead trees on a former wetland.

**Figure 10.** (**a**) Area affected by coastal erosion seaward of the highway as observed in 2013. Location of time series is indicated by white dot; (**b**) LOS displacement time series from Sentinel-1A for 2017–2021 (line represents the best fitting line with a slope of −0.71cm/yr), and (**c**) photograph of the dunes landward from the 20th km. Source of imagery: Digital Globe provided GeoEye image.

**Figure 11.** (**a**) El Torno lagoon showing stabilized washover in 2013. Location of time series is indicated by white dot; (**b**) LOS displacement time series from Sentinel-1A for 2017–2021 (line represents the best fitting line with a slope of −1.2 cm/yr); (**c**) photograph of the beach seaward, and (**d**) next to the north shore of the lagoon. Source of imagery: Digital Globe provided GeoEye image.
