Ancient high-energy storm boulder deposits on Ko Samui, Thailand, and their significance for identifying coastal hazard risk
Abstract Coastal geomorphic processes associated with high-energy storm events are difficult to estimate over recent geological history, though their frequency and magnitude are important to assess in order to understand present-day coastal vulnerability. Studying ancient coastal boulder deposits can shed light on the previous physical conditions necessary for their deposition. In this study, we estimated the physical processes required to move reef-derived coral boulders on the east coast of Ko Samui, a rapidly developing tourist island off eastern peninsular Thailand. The position and dimensions of 97 coral boulders (weight: mean 2.9 t, max. 12.7 t; transport distance: max. 125 m) were measured at two sites and dated using uranium/thorium methods. Flow velocities of 2.3–8.6 m/s were required to transport the measured boulders, with individuals deposited up to 4.7 m above mean sea level. Age-dating suggests that events capable of the highest flow velocities occurred around AD 1600 and AD 1750. These were probably driven by tropical cyclones (typhoons). Boulder transport by events of similar magnitude has not been detected within the last 250 years. The non-occurrence of similar events in living memory has implications for hazard perceptions at this important tourist destination. However, there is also evidence of substantial Holocene sea-level changes in the Gulf of Thailand, as observed at nearby Ko Phaluai. This potentially offers a challenge for the interpretation of older boulders dating from the mid-Holocene, as sea level may have been more than 2 m higher than present. Thus, studies using coral boulders as a proxy for past storm-wave conditions must consider the broader sea-level history, and are probably best limited to the period post-2000 BP in the Gulf of Thailand.