Strongly aligned coastal boulders on Ko Larn island (Thailand): a proxy for past typhoon‐driven high‐energy wave events in the Bay of Bangkok

Abstract

Recent studies of elevated carbonate boulder deposits on several rock islands near Bangkok have indicated that Thailand's capital city may not be as protected from typhoon strikes as previously thought. Here, new evidence is presented for past high‐energy wave (HEW) events in the form of statistically significant patterns of boulder alignment on exposed rocky shorelines of Ko Larn island. The long‐axis orientations of 193 coastal sandstone boulders were analysed across four study sites. Several scenarios for the unimodal, bimodal, and polymodal patterns found can be envisaged. Either the most recent HEW event was the strongest—in which case most clasts were rearranged unimodally (one observation site), or the strongest HEW event was earlier and subsequent weaker ones realigned only smaller boulders to produce bimodal or polymodal patterns (three observation sites). Inferred northeastward or eastward onshore flow directions are consistent with palaeo‐typhoons penetrating into the Bay of Bangkok on northwestward curving tracks. The calculated minimum flow velocities required to transport all sampled boulders are 5.5–7.8 m s−1, similar to other findings throughout the Asia‐Pacific region. It was observed that the absence of a fitted boulder geomorphology lends credence to the earlier proposed time frame of 150–200 years between typhoon phases in the upper Gulf of Thailand. The current work has provided additional insights into the characteristics of past HEW events that have a possibility of reoccurring again at some time in the future. Our findings continue to raise awareness for a reassessment of the risks of coastal hazards for the Chao Phraya River delta and densely populated Bangkok, for which storm surge modelling should be an urgent priority, so as to give better perceptions on how typhoon‐driven marine incursion would impact the city.

Annual Coastal Boulder Mobility Detected in 2017–2021 Remote Sensing Imagery and Its Relation to Marine Storms (Gulf of Taranto, Mediterranean Sea)

“…Values of V up to 14 m/s were calculated to explain boulder displacements due to inundation events driven by typhoons in the Asia-Pacific region [39,79,[94][95][96]. Instead, for the Mediterranean region, V values up to 12 m/s are reported in the literature to displace SA or SB boulders by severe storms [1,48].…”

Clustered, Stacked and Imbricated Large Coastal Rock Clasts on Ludao Island, Southeast Taiwan, and Their Application to Palaeotyphoon Intensity Assessment

“…Distinctive depositional patterns are often seen in coastal boulders transported by waves. Reported styles of deposition include 1. isolated individual or small numbers of clasts (Frohlich et al, 2009;Terry et al, 2021); boulder fields comprising many clasts that are dispersed across wide areas (Kato and Kimura, 1983;Lau et al, 2018;Kennedy et al, 2019); groups of boulders clustered closely together, sometimes stacked one against another (Terry and Goff, 2019); boulders piled into elongated berms or ridges aligned in shore-parallel orientations (Switzer and Burston, 2010;Cox et al, 2012;Lario et al, 2020). Combinations of such depositional patterns are also possible.…”

“…to initially dislodge (pluck or quarry) boulders from rock platforms or reefs (Nott, 2003;Noormets et al, 2004;Morton et al, 2006;Terry et al, 2013). Even for boulders produced by non-marine processes, however, such as rockfall, high-energy flow is needed for their subsequent movement and reorganization (Terry and Goff, 2019). For coastal boulders showing evidence of landward transport, it is normally assumed that wave action of some form was responsible (Noormets et al, 2002;Nott, 2004;Iwai et al, 2019).…”

Coastal Boulder Deposits of the Neogene World: A Synopsis

“…On the other hand, boulders are regarded as precious evidence of present and ancient rocky shore facies and extreme events (such as storms, tsunamis, hurricanes, typhoons, and cyclones). This evidence has been examined by Autret et al [5], Bhatt et al [6], Biolchi et al [7], Cox et al [8][9][10], Dawson [11], Engel et al [12], Erdmann et al [13], Hearty and Tormey [14], Herterich et al [15], Hongo et al [16], Johnson et al [17,18], Kennedy et al [19], Kortekaas and Dawson [20], Lau et al [21], Olsen et al [22], Paris et al [23], Pepe et al [24], Scheffers et al [25], Schneider et al [26], Shah-Hosseini et al [27], Suanez et al [28], Terry and Goff [29], Terry et al [30], Trenhaile [31], Watanabe et al [32], and Weiss and Sheremet [33]. Most probably, the devastating catastrophes like the 2004 Indian Ocean tsunami [34] and the 2011 Tohoku tsunami [35] have fueled the interest of researchers in coastal sedimentology and, particularly, large clasts [36].…”