Sediment distribution on the inner continental shelf off Khao Lak (Thailand) after the 2004 Indian Ocean tsunami

Abstract

The coastline of Khao Lak (Thailand) was heavily damaged by the 2004 Indian Ocean tsunami. Onshore tsunami deposits and satellite images, which show large amounts of sediment transported offshore, indicate that the seafloor was impacted by tsunami run-up and backwash. In this study, high-resolution maps of sediment distribution patterns and the geological development of the seafloor are presented. These maps are based on multibeam, side-scan sonar and seismic profiling surveys offshore Khao Lak. Paleoreefs, with associated boulder fields and sandy sediment dominate the inner continental shelf. Patches of fine-grained (silt to fine sand) sediments exist in water depths of less than 15 m. The sediment distribution pattern is stable between 2008 and 2010, apart from small shifts regarding the boundaries of the fine-grained sediment patches. In one sediment core and several grab samples an event layer was documented, situated below a cover of modern sediments which is only a few cm thick. The event-layer is traced down to 18 m water depth. It consists mostly of sand and contains compounds of terrigenous origin. It is interpreted as a 2004 Indian Ocean tsunami deposit. However, over large areas of the study-site, the impact of the tsunami is hardly identifiable by seafloor morphology or sediment distribution.

Internal structure of event layers preserved on the Andaman Sea continental shelf, Thailand: tsunami vs. storm and flash-flood deposits

“…Enhanced sedimentation due to anthropogenic impacts has been observed in different settings such as tropical estuaries in Papua New Guinea, Vietnam, Australia and Indonesia (Wolanski and Spagnol, 2000); the Pearl River in Hong Kong (Owen and Lee, 2004;Owen, 2005); and the Waiapu River in New Zealand (Wadman and McNinch, 2008). Offshore Khao Lak, Feldens et al (2012) found sediment distributions composed of silt and fine sand deposited above 15 m water depth that were orientated parallel to the shoreline. These fine sediments offshore Khao Lak, which show little cohesiveness due to a high percentage of coarse silt and fine sand, were deposited during a multitude of smallscale events (Table 3a-V, 3c-IV, 3d-V and 3e-VI and Fig.…”

“…Since then, several studies (Di Geronimo et al, 2009;Sugawara et al, 2009;Feldens et al, 2009Feldens et al, , 2012 have focused on Table 1. History of nine tropical storms and typhoons that approached within a 180-nautical-mile radius of Phuket during the 52-year period .…”

“…Based on geomorphological evidence from sandy hooks and spits, northwarddirected current-induced longshore sediment transport occurs in the study area Brill et al, 2014b). Maps of the nearshore bathymetry and sediment distribution have been created following the 2004 Indian Ocean Tsunami (e.g., Di Geronimo et al, 2009;Feldens et al, 2009Feldens et al, , 2012, but these maps cover only a small percentage of the Andaman Sea Shelf. According to these studies, mud patches are widespread in water depths between 5 and 15 m north and south of PC.…”

“…History of nine tropical storms and typhoons that approached within a 180-nautical-mile radius of Phuket during the 52-year period . Modified from the Typhoon Havens Handbook (Brand, 2009 (Di Geronimo et al, 2009;Sugawara et al, 2009;Feldens et al, 2012;Sakuna et al, 2012;Milker et al, 2013). Due to the reworking of shelf sediments and the re-establishment of the coastline Grzelak et al, 2009) following the tsunami, the remaining offshore tsunami deposits have been chiefly found in locally sheltered positions adjacent to granitic outcrops, within an incised channel system and in areas of locally higher sediment accumulation rates Sakuna et al, 2012;Milker et al, 2013).…”

“…The behavior of tsunami waves is controlled by the source earthquake or landslide and the ocean basin's morphology on a larger scale, and by the inner shelf and coastal bathymetry as well as the hydrological conditions on a local scale (Cheng and Weiss, 2013;Spiske et al, 2013;Goto et al, 2014). Coastal and inner-shelf bathymetry is the most important factor controlling backwash flow (Le Roux and Vargas, 2005;Paris et al, 2010;Feldens et al, 2012;Spiske et al, 2014).…”