Quaternary International xxx (xxxx) xxx–xxx

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Quaternary International

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Major typhoon phases in the upper Gulf of over the last 1.5 millennia, determined from coastal deposits on rock islands

∗ James P. Terrya,d, , James Goffb,c,d, Kruawun Jankaewd,1 a College of Natural and Health Sciences, Zayed University, PO Box 19282, Dubai, United Arab Emirates b PANGEA Research Centre, School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, Australia c CNRS, Laboratoire Chrono-Environnement, Université de Bourgogne-Franche-Comté, Besançon, France d Steering Group on Natural Hazards and Disaster Risk, International Council for Science Regional Office for Asia and the Pacific (ICSU ROAP), Kuala Lumpur, Malaysia

ARTICLE INFO ABSTRACT

Keywords: Rapid growth of Asian megacities, exemplified by the megacity of , Thailand, with a population of over Typhoons 10 million inhabitants, means that an increasing number of people are living in low-lying coastal areas exposed Thailand to hazards such as typhoons. While Bangkok has always been considered not to be at risk from typhoon strikes, Rock coasts recent discoveries of elevated carbonate boulder deposits have started to question this assumption. This work Boulders reports on findings from the islands of Ko Khang Khao and Ko Phai, farther north and west than earlier studies, and adds to the existing body of evidence for prehistorical typhoon-driven high energy marine inundation (HEMI) events penetrating northwards into the Bay of Bangkok. Elevated carbonate boulder deposits up to 6 m amsl indicate that these were emplaced by waves generating onshore minimum flow velocities between 3.0 and 5.5 m/s, consistent with typhoon-impacted coastlines elsewhere in the tropical Asia-Pacific region. When viewed in conjunction with other data from the Bay of Bangkok their chronologies indicate age clusters around four major phases of activity in AD 600–700, AD 900–1000, AD 1150–1250 and AD 1400–1650. At 250 years duration, the last phase of activity was the longest, but it has also been followed by the longest period of quiescence that has coincided with rapid urban growth in and around the city of Bangkok. The re-occurrence of typhoon-driven HEMI events on the scale of the prehistorical events reported here would threaten all the low- lying coasts in the Bay of Bangkok, including the Chao Phraya delta, and as such these results indicate an urgent need to re-evaluate coastal hazards for the region.

1. Introduction and Tachikawa, 2012). Yet, Bangkok is assumed to be at a relatively low risk from high energy marine inundation (HEMI) events driven by 1.1. Vulnerability of Bangkok and the Chao Phraya Delta storm waves and storm surge, based on the premise that the city “is not subject to direct hits from tropical typhoons or cyclones” (World Bank, Rapid growth and ongoing development of Asian megacities such as 2010: 25). , , Dhaka and Bangkok, mean that an increasing Such “low risk” assessments are invariably based upon historical number of people are living in low-lying coastal areas exposed to ha- data. In the case of Bangkok, the information on tropical storm tracks zards such as storms and tsunamis (World Bank, 2010). Such exposure dates back to 1951, as part of the record for the western North Pacific, is exemplified by the megacity of Bangkok, the capital and commercial including the and the (GoT) (JMA, centre of Thailand, with a population of over 10 million inhabitants. At 2017; NOAA, 2017). Indeed, over the six decades since 1951, only nine an elevation of just 1.5 m above mean sea level (amsl), Bangkok sits tropical storms have tracked through the GoT (Fig. 1), mostly on east- astride the 1500 km2 delta of the Chao Phraya River at the head of the to-west trajectories from the South China Sea. The northern end of the Gulf of Thailand (Fig. 1). The city has a recognised flooding hazard upper GoT narrows into the Bay of Bangkok, which is approximately associated with monsoonal rainfalls in the upper Chao Phraya basin in 100 km wide. As tropical storms traverse the GoT from east to west, the northern Thailand (Ziegler et al., 2012), and future maximum flood reasonably narrow width of the Bay of Bangkok therefore provides discharges may increase in response to climate change (Hunukumbura limited fetch for storm wave and storm surge generation. This limited

∗ Corresponding author. College of Natural and Health Sciences, Zayed University, PO Box 19282, Dubai, United Arab Emirates. E-mail address: [email protected] (J.P. Terry). 1 Formerly, Department of Geology, Chulalongkorn University, Bangkok, Thailand. https://doi.org/10.1016/j.quaint.2018.04.022 Received 15 October 2017; Received in revised form 5 February 2018; Accepted 9 April 2018 1040-6182/ © 2018 Elsevier Ltd and INQUA. All rights reserved.

Please cite this article as: Terry, J.P., Quaternary International (2018), https://doi.org/10.1016/j.quaint.2018.04.022 J.P. Terry et al. Quaternary International xxx (xxxx) xxx–xxx

Fig. 1. Typhoon and tropical storms tracks in the Gulf of Thailand since 1951. Bracketed values indicate the minimum pressure attained while the storms transited the Gulf. fetch has ensured that no significant storm-driven coastal flooding has exceptionally high-magnitude but low-frequency events that occur over occurred over the historic record. centennial timescales. Thus, the modern archive alone does not provide One of the most intense storms to affect the region since 1951 was an unequivocal data source to guide assessments of future typhoon and Typhoon Linda in 1997. It managed to produce offshore wave heights storm surge risks for the Bay of Bangkok. Second, rare but atypical up to about 4 m near Huahin about 100 km south west of Bangkok tropical storms can follow a northward trajectory into the upper GoT. (Aschariyaphotha et al., 2006). However, the Bay of Bangkok to the Two particular storms serve to illustrate this. In late October 2003, north remained relatively sheltered compared to the more exposed Tropical Storm 23 W formed within the GoT and tracked north north- western GoT coastline (Aschariyaphotha et al., 2011; Wannawong and westwards, intersecting the Thai Peninsula just outside of the Bay of Ekkawatpanit, 2012). Earlier, Typhoon Harriet in 1962 and Typhoon Bangkok. In late November 2004, , originally following Gay in 1989 each caused significant casualties and damage to homes an east-to-west path, then recurved northwards to skirt along the and infrastructure in Thailand's southern provinces (Phantuwongraj western GoT coastline, before dissipating about 250 km south of and Choowong, 2012), but left Bangkok more or less unscathed. The Bangkok. Neither storm caused extensive damage. But a scenario can be perception that Bangkok is protected has therefore been enhanced by envisaged where more a powerful and longer-lived system might the recognition that historically documented east-to-west trajectory manage to penetrate into the Bay of Bangkok by continuing further typhoons entering the GoT from the South China Sea have mostly im- along a similar northwardly oriented track. In such a scenario, the tri- pacted the southern provinces of Thailand on the exposed eastern coast angular and enclosed configuration of the GoT with the Bay of Bangkok of the Thai Peninsula that faces the western GoT (Phantuwongraj et al., at its head, the shallow bathymetry of the bay, and the longer fetch 2013), as opposed to the coastal provinces further north surrounding from the south, might funnel a storm surge into the narrow confines of the Bay of Bangkok. the bay. Third, in recent decades a combination of deltaic subsidence In spite of the aforementioned evidence apparently to the contrary, from excessive groundwater pumping, a reduction in fluvial sediment several arguments can be put forward to cast doubt on the low-risk delivery and aggradation, eustatic sea-level rise, the removal of coastal assessment for Bangkok. First, available tropical storm databases (JMA, mangroves and shoreline retreat (Vongvisessomjai et al., 1996; 2017; NOAA, 2017) provide only a short decade-scale ‘snapshot’ for the Rokugawa et al., 2006; Saito et al., 2007; Uehara et al., 2010) have GoT. Like many similar collections worldwide, they do not capture caused the Chao Phraya Delta to be recognised as a “delta in great peril”

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