Drivers of changes in shoreline position of two small reef islands in the West Philippine Sea

Anne Drew Vergara Carrillo, Mary Rose Gabuyo, Denise Faye Janer, Paolo Co, Fernando P. Siringan

This study examines the variations in the shoreline position of two atoll reef islands in the West Philippine Sea, Pag-asa Island, and Lawak Island, and identifies the influence of potential drivers of erosion. Data from field observations, beach profiling, and Google Earth Pro satellite images from 2004 to 2022 are used to identify shoreline movement for each island. Shoreline change is calculated, and area change is measured for each island. Seasonal changes in shoreline position due to seasonal variations in wind regime are observed in Lawak Island, but not in Pag-asa Island. The lack of seasonal variation in the shorelines of Pag-asa Island are attributed to solid-based structures impeding sediment drift around the island. The overall trend of relative sea-level rise is correlated with the overall erosion and interannual changes in sea level which coincides with interannual variations in area change for both islands. Storm events are also found to lead to erosion and the raising of the island elevation through overwash deposition. The overall changes in shoreline position show that the northern and eastern coasts of Pag-asa Island and Lawak Island, eroded faster than their southern and western coasts, respectively. Such variations are attributed to the overall weakening of Southwest Monsoon winds and more consistent northeasterly winds, which are likely due to the negative Pacific Decadal Oscillation (PDO) during the period of study. Given the future scenarios on climate change and the threat it poses to reef islands, improving the health of the coral reefs is a must since the sediment that reefs provide dampens the erosive effects of sea-level rise and extreme-wave events.