**2. Method of Study**

First, the morphological characteristics of Lingayen Gulf were briefly described using a satellite image and bathymetric survey data as well as the investigation of wave characteristics using the Global Wave Statistics [8]. Because of the shape of Lingayen Gulf with a large aspect ratio, oblique wave incidence with a large angle to the direction normal to the shoreline prevails in this gulf. Taking these characteristics into account, satellite images of Lingayen Gulf were collected and the shoreline changes along the east coast were determined, selecting the deltas of two rivers (Balili and Aringay Rivers) and a sand spit at

Santo Tomas with a supplementary field observation on 23 February 2018. Then, the wave field in Lingayen Gulf was calculated to explain the mechanism of occurrence of shoreline undulations along the east coast of the gulf due to high-angle wave instability [1] using the energy balance equation [9], based on the seabed contours of the gulf. Along the east coast of this bay, waves are obliquely incident to the direction normal to the shoreline over 45◦, causing the shoreline instability. In an area south of the Balili River delta, a sand spit has formed at a location where the shoreline configuration abruptly changes due to high-angle wave instability. Therefore, the results of the numerical simulation regarding the formation of a sand spit on a coast with a sudden change in shoreline configuration were shown and the comparison of the results with the measured data was made. Furthermore, the rate of longshore sand transport along the sand spit at Santo Tomas was estimated from the temporal change in sand volume by integrating the foreshore area of the beach. Finally, the morphological similarity of Lingayen Gulf to that of Suruga Bay in Japan, and the rate of longshore sand transport in both bays were compared to consider the future measures against beach erosion along the coasts in these bays.
