Prograding Shoreline, Emerging Gravel Bars, and Mangrove Recolonization in 40 Years on a Mixed Siliciclastic-Carbonate Coast in Southwestern Luzon, Philippines

Abstract

Changes in shoreline positions provide a historical record for identifying causal mechanisms and adaptive responses of coastlines to a wide spectrum of sea-level change that arises either from natural or anthropogenic disturbances. This paper presents a case study of coastal change during a period of the steady rise in sea level that is coupled with frequent storm events. Observations from multiple sources including field survey, topographic map, and series of satellite images were combined and analyzed to determine shoreline changes along the western coast of Calatagan, Batangas from 1979–2018. The Digital Shoreline Analysis System (DSAS) extension tool of ArcGIS was used to estimate magnitudes of shoreline changes, either erosion or accretion. Surface sediment samples were acquired in March and April 2010 to assess the predominant grain size and composition. Historical shoreline positions show that the western coast of Calatagan exhibits discrete erosion, but predominantly coastal accretion trend over a period of nearly 40 years. Both land progradation and the emergence of sand-gravel bars after 1992 occurred in a period of rising sea level across the South China Sea basin, including the West Philippine Sea. Land progradation is attributed to an increase in sediment input to the coast from agricultural activities in unconsolidated volcanic terrain. Notably, overlapping events of storms between 1993–1995 and the 1994 tsunami in the Verde Island Passage (VIP) may have emplaced and reworked a large amount of reef front material onto the reef flat forming emergent bars. Newly accreted lands and emergent bars were subsequently colonized by Avicennia and Sonneratia mangroves. This study highlights that in certain areas, local factors such as increased sediment supply from agricultural activities, storms, and possibly tsunami can modulate shoreline changes in response to modern-day sea-level rise.