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.