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Dimanche 26 décembre 2004. La terre tremble au large des côtes de Sumatra. C’est l’un des plus puissants séismes jamais enregistrés (magnitude 9.3). S’en suit un tsunami cataclysmique, le plus meurtrier de l’histoire avec près de 230 000 victimes. Les vagues ont ravagé les côtes indonésiennes proches, avant de se propager vers la Thaïlande, le Sri Lanka, l’Inde puis les côtes africaines. La province d’Aceh, au nord-ouest de Sumatra, est de loin la plus touchée. La moitié de la ville de Banda Aceh est rasée par des vagues de plus de 15 m de haut, l’eau ayant même atteint une hauteur de 50 m sur les falaises de la côte ouest de Lhok Nga. De nombreux villages sont rayés de la carte. Durant près de deux ans (2005-2006), les scientifiques du programme de recherche « Tsunarisque » se sont relayés sur le terrain pour essayer de décrypter, reconstituer le tsunami dans ses moindres détails, afin de tirer les enseignements d’une catastrophe hors norme et de proposer des stratégies de prévention plus efficaces. Cet ouvrage retrace tout ce travail en trois volets : tout d’abord la reconstitution et la modélisation du tsunami, de l’extension des vagues à la cartographie des dégâts infligés au bâti, ensuite l’impact du tsunami sur l’environnement, et notamment les formes d’érosion et de dépôts, et enfin les aspects socioculturels, des facteurs ethniques aux enjeux de la reconstruction et de la prévention. « Toute l'histoire du monde ne me paraît souvent rien d'autre qu'un livre d'images reflétant le désir le plus violent et le plus aveugle des hommes : le désir d'oublier. » Herman Hesse (1943), Le Voyage en Orient

Large tsunamis are major geomorphic crises, since they imply extensive erosion, sediment transport and deposition in a few minutes and over hundreds of kilometres of coast. Nevertheless, little is know about their geomorphologic imprints. The December 26, 2004 tsunami in Sumatra (Indonesia) was one of the largest and deadliest tsunamis in recorded human history. We present a description of the coastal erosion and boulder deposition induced by the 2004 tsunami in the Lhok Nga Bay, located to the West of Banda Aceh (northwest Sumatra). The geomorphological impact of the tsunami is evidenced by: beach erosion (some beaches have almost disappeared); destruction of sand barriers protecting the lagoons or at river mouths; numerous erosion escarpments typically in the order of 0.5-1.5 m when capped by soil and more than 2 m in dunes; bank erosion in the river beds (the retreat along the main river is in the order of 5-15 m, with local retreats exceeding 30 m); large scars typically 20-50 cm deep on slopes; dislodgement of blocks along fractures and structural ramps on cliffs. The upper limit of erosion appears as a continuous trimline at 20-30 m a.s.l., locally reaching 50 m. The erosional imprints of the tsunami extend to 500 m from the shoreline and exceed 2 km along riverbeds. The overall coastal retreat from Lampuuk to Leupung was 60 m (550,000 m2) and locally exceeded 150 m. Over 276,000 m3 of coastal sediments were eroded by the tsunami along the 9.2 km of sandy coast. The mean erosion rate of the beaches was ~30 m3/m of coast and locally exceeded 80 m3/m. The most eroded coasts were tangent to the tsunami wave train, which was coming from the southwest. The fringing reefs were not efficient in reducing the erosional impact of the tsunami. One year after, the coastal environment in northwest Sumatra is still in a post-tsunami dynamic. Thus, the difference between the largest tsunamis (height > 30 m) and the moderate tsunamis (height

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.