*3.1. Coastline Changes*

The average annual horizontal coastline change rates for 2010–2020 ranged from over −25 to +15 m/yr (Figure 4). Erosion rates increased alongshore in the same direction as the

littoral drift (east–west). The only exceptions to the predominant erosive regime occurred on the westernmost segment of the study area where accretion rates larger than +10 m/yr were identified. Localized accretion was also observed on the east side of the mouth of the CGSM and next to two groins built west of the coastal town of Ciénaga in the early 2010s (see white and green colors in Figure 4).

**Figure 4.** Coastline changes (m/yr) as derived from DSAS for the period 2010–2020.

From the mouth of the CGSM to the 20th km, erosion ranged between −1 and −10 m/yr (see pink and yellow transects in Figure 4). This erosion peaked along the stretch between the 20th km and La Atascosa lagoon (orange and red transects in Figure 4) and reached local average values up to −25 m/yr. Erosion rates progressively decreased westward from this lagoon and changed to accretion west of the El Torno lagoon (see green colors in Figure 4), where a levee, built at the mouth of the Magdalena River, hampers the westerly movement of sediment. The most significant erosion rate values for 2010–2020 occurred seaward of La Atascosa lagoon. Upon inspection of the imagery, it was observed that this largest average erosion rate was produced from the migration of an inlet after a breach of the lagoon in 2016.
