**8. Concluding Remarks**

The formation of shoreline undulations due to high-angle wave instability was actually observed at two river deltas and around a sand spit at Santo Tomas in Lingayen Gulf. The sand spit at Santo Tomas extended, similarly to a sinusoidal function due to high-angle wave instability, resulting in large shoreline recession and advance. South of the Balili River, the elongation of a sand spit was observed on a coast with an abrupt change in shoreline configuration, because waves were obliquely incident to the shoreline at a large angle. These observation results were explained by the mechanism described by San-nami et al. [4].

The morphologies of the east coast in Lingayen Gulf and the west coast in Suruga Bay are very similar (Figure 1 vs. Figure 29). A protruded shoreline develops north of the Ohi River, similarly to south of the Balili River, and the shoreline orientation abruptly changes. Continuous transport of sand was disrupted in a 20 km stretch between the Aringay River and the sand spit at Santo Tomas because of the construction of many groins to locally protect the coast, causing downcoast erosion. Although the sand supplied from the Abe River was transported northward forming Mihono-matsubara sand spit at a location where the shoreline orientation abruptly changes [21], severe beach erosion occurred on the Shizuoka and Shimizu coasts north of the river mouth owing to the decrease in sediment supply from the river caused by excess riverbed mining in the Abe River [20]. On the Suruga coast north of the Ohi River and the Shizuoka and Shimizu coasts located north of the Abe River, the beaches were severely eroded. As a measure, sand bypassing from the riverbed to the coasts was adopted. Taking into account the fact that the east coasts in Lingayen Gulf have the same condition as those in Suruga Bay, overall management of sand movement is required instead of local optimization using groins. A soft measure, such as sand bypassing, should be preferred along the coasts in Lingayen Gulf instead of hard measures against erosion to prevent an artificial, protected coastline from expanding, similarly to the case of coasts in Suruga Bay in Japan, where sand bypassing has now been adopted as a measure for sediment management.

**Author Contributions:** Data curation, T.U.; Formal analysis, Y.N. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not Applicable.

**Informed Consent Statement:** Not Applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
