Recovery from a large tsunami mapped over time: The Aceh coast, Sumatra

This is a record of the rapid recovery of the Aceh coast, northwest Sumatra after its destruction in the tsunami of 26 December 2004, using high-resolution IKONOS images and field visits. We mapped the same 175 km of coastline at 1 m resolution five times: 2003 (before the tsunami); 2004 and early 2005 (immediately after the tsunami); 2006; 2007; and 2008. The Aceh coast was eroded back for about 500 m in the tsunami except at the rocky headlands, and almost the entire suite of depositional landforms of beaches, low sand dunes and wetlands were removed. A new coast started to appear within weeks, closely resembling the pre-tsunami version. The new suite of depositional forms not only masked the erosional effect of the tsunami within a very brief period, but also mimicked the old coast. The post-tsunami coast was subjected to the normal coastal processes and the new coast was formed within the constraints of the prevalent coastal environment, thus resembling the old one. The new beaches tend to be bigger than the old one, although they have not yet reached their former seaward positions. On this coast, a tsunami appears to be an episodic destructive event at an interval of 500+ years, followed by a set of geomorphic processes that rapidly tend to remove or mask the evidence of such destruction. The recovery is hindered where anthropogenic influences have modified the coastal forms and configuration. We suggest that (a) the optimal management of coasts of this type after a tsunami is to leave it to nature, and (b) sedimentary deposits are better indicators of past tsunamis than coastal morphology. As demonstrated, high-resolution satellite images can be used not only for measuring changes caused by a high-magnitude event but also for tracing the recovery over time. This was impossible earlier, except for very small areas which could be mapped on foot.