Accumulations de blocs par le tsunami du 26 décembre 2004 à Lhok Nga, ouest de Banda Aceh

In this chapter, we present a description of boulder deposition induced by the 2004 tsunami in the Lhok Nga Bay, located to the West of Banda Aceh (northwest Sumatra). The 220 boulders measured range from 0.3 to 7.2 m large (typically 0.7-1.5 m), with weights from over 50 kg up to 85 tons. We found one boulder, less than 1 m large, at 1 km from the coastline, but all the others were transported less than 450 m (< 7 m a.s.l.). No fining landward boulder size distribution could be detected. The coincidence of different size modes, from boulders to fine sands, with independent spatial distributions, suggests that all the material was not transported in suspension, but rather by a combination of suspension and bed load transport. Finally, the spatial and size distributions of tsunami boulder deposits mostly depend on the location and characteristics of their source (coral reef, beach rock, platform, dams), together with clast and surface interference during transport