Transferts sédimentaires syn- et post-tsunami à Kajhu, nord-est de Banda Aceh

Résumé 0

The district of Kajhu Perumnas, located north-east of Banda Aceh, is a flat area that allowed wave penetration far inland, until 5 km in some places. The topography was locally modified by the turbulent wave front and then buried by a layer of sediment translocated from the beaches and seafront dunes. In order to better understand the link between wave dynamics and sediment facies in that particular setting, the tsunami deposits were studied in the light of direct and indirect evidence concerning the number and behaviour of the waves involved. The respective roles of run up and backwash on the sedimentary signature were also studied. Near the sea, the deposits were found to be coarse and ungraded, whereas inland the frequency of stratified normally graded sequences increases. Once the water had completely retreated, the entire area affected by the tsunami, which is covered by unconsolidated sands, became prone to active wind deflation caused by an increase in wind speed. On flat zones located inland, the deflation caused an accumulation of sand, which now forms fields of small dunes that are rapidly growing in size Measurements carried out in August 2005 have estimated that ~1100 m3·ha– 1 of coastal sediment was transported and deposited inland.During the year following the event, the displacement by runoff back towards the sea of sediment that was initially spread inland by the tsunami wave train has been feeding the longshore drift. It contributed to beach progradation and the displacement of the coastline towards the sea (Fig. 1) at a rate of 150 m to 500 m during the year following the event.