Surf beat-induced overwash during Typhoon Haiyan deposited two distinct sediment assemblages on the carbonate coast of Hernani, Samar, central Philippines

Abstract

Wave set-up steepened and accentuated the storm surge during Typhoon Haiyan in November 2013 resulting in bore-like flooding with surge heights of 7 m and flow velocities reaching 5 m s− 1 on the open-sea coastal plain near Hernani. This study investigates two distinct sediment assemblages left behind by the coastal flooding associated with this surge. The first assemblage consists of numerous coastal boulders that now occupy the reef flat, and the second pertains to a laterally extensive sand sheet that blanketed the coastal plain up to ~ 300 m inland. The majority of the boulders has b axes between 1 and 2 m, weigh no > 10 t, and were originally submerged on the reef edge. The boulders found more landwards of the reef edge were potentially lying loosely on the reef flat prior to Haiyan. In contrast, the coarse carbonate sand sheet starts at ~ 10 m inland of the current shoreline and has a maximum thickness of 10 cm and gradually thins to 3 mm at ~ 300 m inland. The Haiyan sand contains moderate concentrations of foraminifera (Amphistegina spp., Baculogypsina sphaerulata, and Peneroplis spp.) that were both abraded and unaltered, pointing to a reef flat and beach source for the sand. Sediment transport inverse modeling complements previous flow velocity estimates using numerical modeling, which altogether indicate sustained high-velocity overland flow despite the presence of coral reefs and mangroves as natural defenses to extreme waves.