Beach recovery after 2004 Indian Ocean tsunami from Phang-nga, Thailand

The 2004 Indian Ocean tsunami devastated the coastal areas along the Andaman western coast of Thailand and left unique physical evidence of its impact, including the erosional landforms of the pre-tsunami topography. Here we show the results from monitoring the natural recovery of beach areas at Khuk Khak and Bang Niang tidal channels of Khao Lak area, Phang-nga, Thailand. A series of satellite images before and after the tsunami event was employed for calculating the beach area and locating the position of the changed shoreline. Field surveys to follow-up the development of the post-tsunami beach area were conducted from 2005 to 2007 and the yearly beach profile was measured in 2006. As a result, the scoured beach areas where the tidal channel inlets were located underwent continuous recovery. The return of post-tsunami sediments within the beach zone was either achieved by normal wind and wave processes or during the storm surges in the rainy season. Post-2004 beach sediments were derived mainly from near offshore sources. The present situation of the beach zone has almost completed reversion back to the equilibrium stage and this has occurred within 2 years after the tsunami event. We suggest these results provide a better understanding of the geomorphological process involved in beach recovery after severe erosion such as by tsunami events.     

Tsunamis versus storm deposits from Thailand

Along the Andaman (west) coast of Thailand, the 2004 tsunami depositional features associated with the 2004 tsunami were used to describe the characteristics of tsunamis in a place far away from the effect of both recent and ancient storms. The current challenge is that a lack of precise sedimentological characteristics have been described that will differentiate tsunami deposits from storm deposits. Here, in sedimentological senses, we reviewed the imprints of the sedimentological characteristics of the 2004 tsunami and older deposits and then compared them with storm deposits, as analyzed from the deposits found along the eastern (Gulf of Thailand; GOT) coast of Thailand. We discuss the hydraulic conditions of the 2004 tsunami and its predecessors, on the Andaman coast, and compare them to storm flows found on the coast of the GOT. Similar to an extensive tsunami inflow deposit, a storm flow overwash has very similar sedimentary structures. Well-preserved sedimentary structures recognized in sand sheets from both tsunami and storms include single and multiple normal gradings, reverse grading, parallel, incline and foreset lamina, rip-up clasts, and mud drapes. All these sedimentary structures verify the similarity of tsunami and storm inflow behavior as both types of high-energy flow start to scour the beach zone. Antidunes are likely to be the only unique internal sedimentary structures observed in the 2004 tsunami deposit. Rip-up clasts are rare within storm deposits compared to tsunami deposits. We found that the deposition during the outflow from both tsunami and storms was rarely preserved, suggesting that it does not persist for very long in the geological record.     

Flow conditions of the 2004 Indian Ocean tsunami in Thailand,inferred from capping bedforms and sedimentary structures

The 2004 Indian Ocean tsunami deposited a sheet of sand with surficial bedforms at the Andaman coast of Thailand. Here we show the recognition of bedforms and the key internal sedimentary structures as criteria of the tsunami supercritical flow condition. The presence of well‐preserved capping bedforms implied a dominant tsunami inflow. Sets of internal sedimentary structures including parallel lamination, seaward and landward inclined‐laminations, and downstream dipping laminae indicated antidune structures that were generated by a supercritical flow current in a depositional stage during the inflow. A set of seaward dipping cross‐laminations containing sand with mud drape on the surface of one depositional layer are a unique indication of an outflow structure. A majority of deposits show normal grading, but in some areas, localized reverse grading was also observed. The recognition of these capping bedforms and determination of the internal sedimentary structures provides new key criteria to help derive a better understanding of tsunami flow conditions.