*4.3. Formation of a Sand Spit at Santo Tomas*

The shoreline changes in the rectangular area including a sand spit at Santo Tomas, shown in Figure 1, were investigated. A satellite image taken on 25 October 2003 is shown in Figure 21.

**Figure 21.** Satellite image around the sand spit at Santo Tomas (25 October 2003).

When setting A at the protruded shoreline in front of Narvacan, a concave shoreline is present near the location 880 m southeast of A, and then the shoreline protrudes again at a location 1400 m southeast of A. Since the shoreline undulation was not so large in 2003, the sand spit was considered to have been simply extending, owing to the continuous sand supply by southward longshore sand transport. However, 19 groins had been constructed until 25 October 2003, north of Narvacan, and continuous sand supply to the tip of the sand spit gradually became difficult.

By 15 April 2006, a significant shoreline change started to occur (Figure 22). By setting points B, C, and D, we found that the shoreline receded between B and C, and advanced between C and D. Until 15 March 2010, dominant shoreline recession occurred between B and C with a maximum shoreline recession of 155 m (Figure 23).

**Figure 22.** Satellite image around the sand spit at Santo Tomas (15 April 2006).

**Figure 23.** Satellite image around the sand spit at Santo Tomas (15 March 2010).

In contrast, the shoreline advanced by 86 m maximum between C and D. Furthermore, as shown in Figure 23, new sand spits of a small size developed near Narvacan, and simultaneously, the shoreline downcoast of the sand spit retreated because of the discontinuity in longshore sand supply at the tip of the sand spit, causing local shoreline recession.

Figure 24 shows a satellite image taken on 27 October 2013. In this figure, wave crestlines can be clearly observed offshore of the coastline. By setting point P 1.5 km offshore of B, we estimated the incident wave angle at P to be S113◦W. Because the angle of the direction normal to the shoreline between B and C is S63◦W, the wave incidence angle relative to the direction normal to the mean shoreline becomes 50◦, satisfying the occurrence condition of high-angle wave instability. In Figure 24, the critical point where the shoreline recession started was located at B' 240 m south of B, and the shoreline markedly receded between B' and C with a maximum shoreline recession of 210 m across transect a–a'. In contrast, the shoreline advanced between C and D with a maximum shoreline advance of 190 m across transect b–b'.

**Figure 24.** Satellite image around the sand spit at Santo Tomas (27 October 2013).

By 28 February 2015, the shoreline retreated between B' and C with a maximum shoreline recession of 210 m at transect a–a', whereas the shoreline advanced between C and D with a maximum shoreline advance of 190 m at transect b–b' (Figure 25). As mentioned above, accretion occurred in the areas of AB' and CD with erosion in the area of B'C separating these accretion areas, and the shoreline undulated as a periodic function with increasing amplitude. These shoreline changes are similar to those observed in the area south of the Aringay River mouth, and the amplitude was extremely large at 220 m in the area of B'C.

**Figure 25.** Satellite image around the sand spit at Santo Tomas (28 February 2015).
