Coastal vulnerability to typhoon inundation in the Bay of Bangkok, Thailand? Evidence from carbonate boulder deposits on Ko Larn island

- Coastal carbonate boulders (CCBs) occur on Ko Larn island in the Bay of Bangkok.
- CCBs >0.5 m in length are found at elevations up to 4 m above m.s.l.
- Minimum flow velocities of 4.7 m/s and 7.1 m/s are needed to transport 50% and 100% of measured CCBs.
- CCB deposits are signatures of past high-energy wave events.
- Prehistorical typhoons may have impacted the Bay of Bangkok.

At the head of the Gulf of Thailand, the subsiding Chao Phraya delta and adjacent low-lying coastlines surrounding the Bay of Bangkok are at risk of coastal flooding. Although a significant marine inundation event has not been experienced in historical times, this work identifies coastal depositional evidence for high-energy waves in the past. On Ko Larn island in eastern Bay of Bangkok, numerous coastal carbonate boulders (CCBs) were discovered at elevations up to 4+ m above sea level, the largest weighing over 1.3 tonnes. For the majority of CCBs, their karstified appearance bears testimony to long periods of immobility since original deposition, whilst their geomorphic settings on coastal slopes of coarse blocky talus is helpful in recognising lifting (saltation) as the probable mode of wave transport. In the absence of local tsunamigenic potential, these CCBs are considered to be prehistoric typhoon deposits, presumably sourced from fringing coral reefs by high-energy wave action. Application of existing hydrodynamic flow transport equations reveals that 4.7 m/s and 7.1 m/s are the minimum flow velocities required to transport 50% and 100% of the measured CCBs, respectively. Such values are consistent with cyclone-impacted coastlines studied elsewhere in the tropical Asia-Pacific region. Overall, the evidence of elevated carbonate boulder deposits on Ko Larn implies that typhoons before the modern record may have entered the Bay of Bangkok. The recurrence of a similar event in future would have the potential to cause damaging marine inundation on surrounding low-lying coastlines.