2004 Indian Ocean tsunami inflow and outflow at Phuket, Thailand

At Bangtao Beach, Phuket, the 2004 Indian Ocean tsunami produced a repeated sequence in which rapid inflow of turbulent water was followed by ponding and then by gradual outflow. Photographs and eyewitness accounts show an initial withdrawal followed by series of inflows. The tsunami left behind a sand sheet as much as 25 cm thick that contains parallel, inclined, landward and seaward lamina in addition to the normal grading commonly reported from tsunami deposits. The sheet contains evidence for two times of vigorous inflow. Each of these is marked by mud rip-ups, medium to coarse sand that grades upward to fine, landward-inclined laminae, and a sharp basal contact. The top of the sand sheet, when observed in the first days after the tsunami, abounded in current dune and ripple bedforms of mostly landward orientation. The tsunamiʹs first positive wave left no onshore sedimentary record in this pitting area. The second wave deposited sand that is much less extensive and slightly finer than that of the third wave. The deposit of both these waves contains multiple fining-upward sequences possibly due to multiple surges in one wave train. The depth-averaged flow velocity estimated from thickness and grain size are in the range 7–21 m/s, whereas, a near bottom threshold velocity calculated from bedforms reveals the order of magnitude difference from 1.74 to 1.03 m/s.