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

Highlights

• UTh dates of coral boulders indicate the times of extreme inundation in the area.

• Anecdotes and records of storms and earthquakes narrowed down specific HEMI events.

• Right combination of storm strength, track, and position prompts boulder transport.

• Continuous storm events created an emergent land feature near the coast.

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 UTh dating of Porites sp. coral boulders (longest axis > 1 m), historical records of tsunamigenic earthquakes and typhoons, and anecdotal accounts. Results of UTh 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 UTh 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.

Introduction

High-energy marine inundation (HEMI) events, such as tsunamis and intense storms, have been known to prompt sudden impacts which drive the reshaping of the coast with long-term geomorphic consequences (Chorley et al., 1984). An indicator of these HEMI occurrences are coastal boulder deposits (Forbes et al., 2004; McInnes et al., 2009; Etienne and Terry, 2012; Lau et al., 2016). With careful interpretation, coral boulders can be effective tools for backtracking strong wave events in an area. Detailed investigation of their size, location, orientation, and age may provide clues about the intensity, timing and frequency, spatial extent and inundation direction of the event(s) that produced them (Goto et al., 2009; Araoka et al., 2010; Barbano et al., 2010). Such possibilities in this field of study can help coastal geomorphologists in ascertaining the role of past extreme events to the area's geomorphology. This can also be used as a tool for hazard planning and vulnerability assessment of the area which can contribute to the preparedness of communities.

Discriminating the type of high-energy wave event that caused boulder deposits - whether they are of tsunami or storm origin - has been a challenging task ever since (Goto et al., 2010; Switzer and Burston, 2010; Weiss, 2012). Tsunamis are more capable of casting ashore large boulders (Young and Bryant, 1992; Nakata and Kawana, 1995; Mastronuzzi and Sansò, 2004; Scheffers and Kelletat, 2005; Scheffers et al., 2008; Pignatelli et al., 2009; Goff et al., 2010; Etienne et al., 2011) since they have greater wave power (Nott, 2003). However, other studies have also proven that even storms, which mostly deliver sediment and coral rubble to the reef flat, can also be powerful enough to transport large boulders (Bayliss-Smith, 1988; Morton et al., 2006; Goto et al., 2009; Etienne and Paris, 2010; Etienne and Terry, 2012; May et al., 2015; Soria et al., 2018), and even megaclasts (Noormets et al., 2004; Williams and Hall, 2004; Terry and Goff, 2014). Characteristic features of boulders themselves that distinguish whether they are transported by tsunami or typhoon remain poorly understood.

Resolving the origin of wave which deposited these boulders have important implications. A large percentage of the human population lives within a kilometer of the coast, but most areas have no historical records of past inundations. Features which are consequences of HEMI events might be perceived as either negative or advantageous to the community. They are worthy of study as we might always get lessons from an area's geological past (Terry et al., 2013).

Gravel bars and emergent island-like features are found on the coast of Batangas, a province in northwestern Philippines. Many of these coastal features are not evident in previous National Mapping & Resource Information Authority (NAMRIA) topographic maps within the last century. Strewn in an emergent gravel bar in Lumaniag village, Batangas, are numerous large boulders of corals. The origin of these large boulders is not accounted in history, but they certainly have been transported by past HEMI events from the adjacent reef to their present locations. Here we aim to establish the nature of emplacement of the large coral boulders on an emergent gravel bar in Lumaniag village by (1) determining their timing of emplacement (through geochemical age-dating and anecdotal accounts), (2) establishing the nature of inundation events (whether tsunami or storm) which emplaced the boulders, and (3) estimating the minimum height and speed (through field measurements and hydrodynamic calculations) of the waves responsible for the boulder transport.