Sedimentological characteristics of storm deposits induced by the tropical storm Pa Buk along the western coast of the gulf of Thailand

Abstract

Tropical storm Pabuk (TS-Pabuk) attacked the western coast of the Gulf of Thailand (W-GOT) on January 4th, 2019, generating high storm surge levels, high wave height and leaving washover sediments along the coastal areas. Soon after the storm, an initial survey was set up for 44 localions to described sedimentary characteristics, measured beach topography, hydrodynamic intensities and geomorphological conditions. Then, the 4 main locations including Chao Samran beach (CSR), Ban Thung Noi (BTN), Thung Tako (TTK), and Talumphuk (TLP) were selected to study sedimentary detail, ground penetrating radar (GPR) with 3 frequencies and diatom analysis on their storm deposits. These 4 locations vary in depositional environments and distances in ascending order to the landfall site. Here the first modern record of tropical storm deposits in Thailand landfalling on the W-GOT in the past 57 years is presented. The conclusion is that distance from landfall site has less influence than local factors; local high tide level, storm surge height, wave runup height, elevations of dune crest, artificial structure and sediments supply. The TS-Pabuk sediments range from very fine to very coarse sand with mainly medium sand, poorer sorted than the underlying original sediment, thinning landward, maximum thickness is 40 cm and average is 25-30 cm. They show at least 3 units reflecting 3 sets of flow. The lower unit A is planar horizontal laminations of finer grains with reverse grading that deposited in bedload transportation of grain flow during an initial stage of low energy. The middle unit B is coarser than others and contains much and large shell fragments and valves. Inclined structures can be found only in this unit such as low-angle (3°- 9°) to high-angle (20°) cross laminations and antidune. Lateral changing of horizontal structure to inclined structure was also observed. This unit was formed in bedload transportation during the peak intensity or maximum inundation level. The uppermost unit C is semi-massive to horizontal laminations of finer grain with normal grading in most locations. This reflects the late stage of the storm inundation regime allowing the sediments to settle down. Thin mud contents at the top are also presented. Some large sedimentary structures cannot be noticed. However, Ground penetrating radar (GPR) can solve this problem helpfully. The 200 and 400 MHz frequency provided 5-6 m and 2.5 m depth, respectively. Whereas, the 900 MHz GPR gave the best resolution with 75 cm depth which is suitable for shallow deposits of storm. GPR were classified into 3 radar surfaces and 7 radar facies. The signals provide landward inclined structure within storm deposits at TTK that was invisible in the narrow sediment pits. Moreover, foreset bedding of paleostorm from previous storm surge events at TLP was also captured. Diatom diversity ranging from marine to freshwater were identified at least 22 species. The freshwater diatoms indicate heavy precipitation that causes overbank river flooding.