Twenty years after the Indian Ocean tsunami - Andaman Sea coast of Thailand revisited

The Indian Ocean tsunami of 26 December 2004 had a devastating impact on Thailand's Andaman Sea coast, claiming more than 5,000 lives, causing widespread destruction and posing significant environmental challenges.The event also left extensive tsunami deposits, documented at hundreds of sites by international post-tsunami field surveys conducted in the months and years that followed. In November 2024, two decades after the disaster, we revisited these sites to assess the long-term evolution of the post-tsunami landscape and the preservation of tsunami deposits. Our study covered a range of sites, from rapidly urbanising tourist centres to areas abandoned after the tsunami. The main findings relate to long-term coastal change, preservation of tsunami deposits, finding of younger storm deposits, and recommendations for future research in the area.Intensive development of tourist infrastructure and housing was observed in areas such as Phuket Island, where wave heights were lowest. In contrast, abandoned areas inundated by higher tsunami waves showed new soil formation and vegetation growth in tsunami-inundated zones. Tsunami evacuation roads, memorials, vertical evacuation structures and information signs have been installed throughout the affected areas.However, other ongoing hazards have been identified, such as coastal erosion, which continues along significant stretches of coastline, such as between Khao Lak and Nham Kem, with erosion rates exceeding 2 metres per year.The preservation of tsunami deposits was assessed at several dozen sites. Tsunami deposits thicker than 10 cm and located on contrasting sedimentary substrates within depression-like terrains were relatively well preserved. However, soil development and post-depositional processes have significantly obscured their macroscopic internal structure. At other sites, tsunami deposits were absent or unidentifiable by visual inspection. Laboratory analyses are underway to determine whether sedimentological and geochemical signatures remain detectable where macroscopic evidence is absent. Boulder deposits left by the tsunami have either been relocated or show signs of ongoing karstification. In addition, post-2004 storm deposits have been documented, consisting of up to 1 metre thick layers of laminated sands extending tens of metres inland.The study highlighted the need for further research into coastal hazards and long-term environmental change along the Andaman Sea coast. Despite progress, significant gaps remain in understanding the region's Holocene coastal evolution, the effects of sediment supply and climate change.The area's anthropogenic history, including centuries of tin mining and its abrupt interruption - possibly related to palaeo-tsunamis - also warrants further investigation. Emerging concerns about critical resources such as sand and gravel need to be addressed alongside wider environmental and sustainability issues. Future research should therefore take a holistic, multi-hazard approach, integrating tsunami geology with studies of other geohazards such as storms, landslides, coastal erosion and saltwater intrusion. These efforts should also prioritise sustainable coastal development under increasing anthropogenic pressures, using interdisciplinary methodologies.This research was funded by the Polish National Science Centre grant No. 2020/37/B/ST10/03677 (TSUNASTORM).