Shoreline movements around small reef islands of the Kalayaan Island Group, West Philippine Sea

Reef islands have highly dynamic beaches, with shorelines that shift easily due to changes in wave patterns influenced by wind conditions, typhoons, and sea-level. Habitat conditions within and around reef island systems also impact shoreline integrity. Mapping shoreline movement patterns is essential for understanding their evolution and potential responses to climate and human-induced changes.

Shoreline positions for several islands in the Kalayaan Islands, West Philippine Sea—including Pag-asa, Parola, Kota, Likas, and Lawak—were traced from satellite images obtained via Google Earth, covering the period from 2005 to 2023. These are complemented by field-derived shoreline positions acquired during 2022. Statistics were calculated using the Digital Shoreline Analysis System (DSAS). Patterns and rates of shoreline change vary among the reef islands. Net accretion has taken place in Parola Island while the rest of the islands have net erosion. Erosion rates are generally high on the eastern sections of the islands, while the southwest sections experience the highest accretion rates. Islands positioned more east of their atolls show greater erosion. The overall shoreline patterns are attributed to the reduced southwest (SW) monsoon and relatively constant northeast (NE) monsoon intensities during the period covered by this study. Consequently, the southwest sediment drift driven by the NE winds dominated over the northeastward sediment drift driven by the SW winds.

Shorelines also respond to sudden natural and anthropogenic stressors. Episodes of high wave energy conditions, Typhoon Hagupit in 2014 and frequent typhoons in 2017, caused substantial losses in island area. Vegetation Index Green (VI_green) was also computed to assess the extent of vegetation coverage. Losses in vegetation cover also correlate with losses in the island area. A disturbance that likely tempered erosion, temporarily, is the elevation of sediment input to the coast caused by extensive dredging on the reef flat between 2013 and 2014. Dredging exposed and released sediment to the reef. However, this activity reduced the living coral cover and effectively increased the water depth on the reef flat. Consequently, this will decrease the reef's ability to sustain sediment supply to the island in the long term. These various natural and anthropogenic impacts are vital in managing limited sediment resources in light of rising sea levels on small reef island systems.