Coral boulder transport and gravel bar formation by storms in Lumaniag village, Batangas, northwestern Philippines

Abstract The contribution of tropical storms to a carbonate gravel bar formation is presented for Lumaniag village, northwestern Philippines, based on field measurements, high-precision U-Th dating of Porites sp. coral boulders (longest axis > 1 m), historical records of tsunamigenic earthquakes and typhoons, and anecdotal accounts. Results of U-Th dating suggest that the events which transported the sampled fully exposed boulders found on the gravel bar occurred during the 1960s to early 1970s; the underlying rubble are presumed to have been deposited by older inundation events. The U-Th ages are consistent with anecdotal accounts and records of typhoon tracks and damage reports from typhoons Dading (international name Winnie; 1964), Welming (international name: Emma; 1967), and several other strong typhoons in early 1970s. Hydrodynamic calculations estimate a minimum flow velocity of 3.2 m/s to initiate translocation of the largest coral boulders in the area. Potential shift in storm frequency and intensity over the next few decades suggests that the gravel bar will further accumulate carbonate sediments. With the mangroves contributing to its stability, the storm-deposited island-like feature contributes to the dissipation of approaching storm waves and provides natural protection for the nearby coastal community. The ridge will likely provide an effective buffer for extreme waves in the term unless destroyed by anthropogenic activities or an oversized extreme event.