*4.2. Aringay River Delta*

Figure 11 shows a satellite image of the rectangular area enclosing the Aringay River delta, taken on 15 April 2006. Here, a slender sandbar extended southward from the north riverbank, closing the river mouth. A sandy beach continuously extended southward while enclosing a lagoon behind it. Furthermore, a slender sand spit of 308 m length elongated southward from the south end of the river mouth sandbar. The elongation direction of the sand spit was at a large angle of 43◦ relative to the direction normal to the coastline immediately south of the river mouth. In the following figures, the shoreline configuration measured on 15 April 2006, as shown in Figure 11, is drawn for comparison.

**Figure 11.** Satellite image of shoreline around Aringay River mouth (15 April 2016).

By 28 October 2009, a slender sand spit elongated to a length of 770 m, while rotating counterclockwise and enclosing a lagoon behind the sand spit, although the shoreline north of the river mouth was stable (Figure 12).

**Figure 12.** Satellite image of shoreline around Aringay River mouth (28 October 2009).

This southward elongation of a sand spit implies that the entire volume of sand supplied from the Aringay River was transported southward. Because longshore sand has not been supplied to A located on the opposing shore downcoast of the tip of the sand spit during the elongation period of the sand spit, the shoreline of 1.5 km in length between A and B was eroded with a maximum shoreline recession of 115 m at a location 600 m south of A. For comparison, A and B of Figure 12 are shown in the following Figures 13–17.

**Figure 13.** Satellite image of shoreline around Aringay River mouth (11 January 2011).

**Figure 14.** Satellite image of shoreline around Aringay River mouth (14 February 2014).

**Figure 15.** Satellite image of shoreline around Aringay River mouth (11 June 2015).

**Figure 16.** Satellite image of shoreline around Aringay River mouth (11 April 2016).

**Figure 17.** Satellite image of shoreline around Aringay River mouth (15 April 2017).

By 11 January 2011, sand was deposited near A located south of the sand spit (Figure 13). On the other hand, shoreline recession started at B located at the south end of the erosion area, and the erosion zone expanded until B' 135 m south of it forming an erosion wave. The maximum shoreline recession reached 154 m at a location 580 m south of A. Thus, the shoreline recession increased on the south side of the sand spit with the elongation of the sand spit. In addition, another sand spit with approximately the same size as that in 15 April 2006 started to extend from the left bank of the river.

By 14 February 2014, a barrier island was formed by the extension of a sand spit offshore of A, forming an enclosed lagoon inside (Figure 14). In this period, the tip of the sand spit elongated 730 m south of A, as well as the elongation of another sand spit from the river mouth. With erosion, the eroded area expanded southward, and the south end of the erosion area moved by 670 m south of B. The maximum shoreline recession was 230 m at a location 750 m south of A.

By 11 June 2015, A located at the north end of the erosion zone in 2009 was enclosed by a slender barrier island with the elongation of a sand spit (Figure 15). Simultaneously, the erosion area expanded up to B' located 1030 m south of B until this year. Even though the erosion expanded over time, the shoreline recession was relatively small near B because of the construction of five stone groins of 80 m in length in its vicinity. These groins prevented the shoreline from receding locally but caused downcoast erosion.

By 11 April 2016, the shoreline changed similarly to that shown in Figure 15, and another sand spit started to extend offshore of the barrier island formed by the extension of a sand spit near A (Figure 16).

Furthermore, the erosion zone extended up to B' 1180 m south of B, and the shoreline recession downcoast of B reached 256 m. Finally, by 15 April 2017, A was enclosed by double barrier islands, and the distance between A and the shoreline increased until 280 m (Figure 17). Although the south end of the erosion zone is located at the same point of B' 1180 m south of B, the shoreline recession downcoast of B reached 308 m.

In the vicinity of point B south of the Aringay River, the shoreline upcoast of the groin was locally fixed, whereas the shoreline receded downcoast of the groins because of the blockage of longshore sand transport owing to the groins, as shown in Figure 18; an enlarged satellite image of the rectangular area shown in Figure 17.

**Figure 18.** Enlarged satellite image of the shoreline in the rectangular area in Figure 17.

In this area, six groins were installed to protect the village, whereas the shoreline markedly retreated south of groin No. 6. For example, the village houses between groins Nos. 4 and 5, made of large boulders with a crown height of 2 m above mean sea level, were protected by a sandy beach between groins, as shown in Figure 19. Similarly, the intertidal beach was left north of groin No. 6.

**Figure 19.** Picture of groin No. 4 taken from behind groin No. 5.

In contrast, the shoreline south of this groin retreated as a water area can be identified landward (left) end of the groin (Figure 20). Thus, the construction of the six groins was successful in locally stabilizing the shoreline, but their impact further expanded downcoast.

**Figure 20.** View of groin No. 6 taken from groin No. 5.
