Coastal sedimentation associated with the December 26, 2004 tsunami in Lhok Nga, west Banda Aceh (Sumatra, Indonesia)

With wave heights of 30 m and runups reaching 50 m a.s.l. and 6 km inland, the December 26, 2004 tsunami in Sumatra was one of the largest tsunamis in recorded human history. In this paper, we present a description of the event in the Lhok Nga Bay (west coast of Banda Aceh) and an interpretation of the tsunami sand deposits, mostly based upon grain-size analysis. The 3.5 km long transect of Lampuuk displays two landward-fining, thinning and sorting sequences. The thickest cross-sections of the first sequence of deposits (0-1.5 km inland) suggest a deposition by three consecutive runups (inflows) and a final backwash (return flow or outflow). From 1.5 to 3.5 km inland, the record is dominated by the second and highest wave (15-30 m). Normally-graded couplets or triplets of layers were used to identify the runup of each wave. The topmost layers, interpreted as the backwash deposition, describe a seaward sequence of increasing mean grain-size and decreasing degree of sorting. The local effects of the topography could be identified: thickest deposits in the topographic lows (50-80 cm), great spatial variations in thickness and upper laminated texture when the sedimentation was limited by steep slopes, landward coarsening and very poor sorting at the wave breaking point, bimodal grain-size distributions reflecting different sources of sediments. Finally, by coupling the longitudinal and vertical trends of the 26 cross-sections, we propose a model of sediment transport and deposition in a large tsunami wave. (c) 2007 Elsevier B.V. All rights reserved.