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

Rapid growth of Asian megacities, exemplified by the megacity of Bangkok, Thailand, with a population of over 10 million inhabitants, means that an increasing number of people are living in low-lying coastal areas exposed to hazards such as typhoons. While Bangkok has always been considered not to be at risk from typhoon strikes, recent discoveries of elevated carbonate boulder deposits have started to question this assumption. This work 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 lowlying 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.