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

Abstract

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.

Introduction

The huge Indian Ocean tsunami of 26 December 2004 devastated 175 km of the Aceh coast of northwestern Sumatra, from Banda Aceh to Meulaboh (Fig. 1). The tsunami removed almost the entire suite of depositional landforms of beaches, low sand dunes and wetlands, and eroded the coast back for about 500 m. A new coast started to form within weeks, closely resembling the pre-tsunami version (Fig. 2). The new suite of depositional forms successfully masked the erosional effect of the tsunami within a very brief period. The destructive effects, however, persisted in locations where the coastal morphology had been altered earlier by sinking large fish tanks (locally known as tambaks) into the wetland.

We record in detail the recovery of the Aceh coast using several sets of high-resolution satellite images and field visits. To the best of our knowledge, no such account of a coastal recovery has ever been documented. The tools that made this study feasible were not available earlier, as high-resolution satellite images became available only a decade ago. It is now possible to study a large area at high resolution (175 km of coastline at 1 m resolution in this case), and also to repeatedly measure changes in specific landform, e.g., coastal retreat and width and length of beaches. This is not logistically possible in the field. In that sense this paper illustrates the integration of satellite imagery in mainstream geomorphology.

We used five sets of IKONOS images at 1 m resolution. The first three sets were taken (a) about a year before the tsunami, (b) several days after the tsunami, and (c) about a year after the tsunami. Two more sets at yearly intervals were used to follow the recovery over time (Table 1). The findings were checked by four field visits. We also observed the effect of the tsunami on the Khao Lak coast, Thailand (inset of Fig. 1) for comparison. This identification of a fast rebuilding process carries important implications regarding the role of tsunamis in coastal morphogenesis and coastal management. A brief introduction to the project has been published earlier (Liew et al., 2008), but this is the complete report.

Tsunamis have been invoked to explain uncommon coastal landforms (Bryant and Nott, 2001, Nott, 2004, Scheffers, 2004). We demonstrate that tsunami-eroded coasts naturally and rapidly return to their pre-tsunami morphology in Aceh, Sumatra and Khao Lak, Thailand. The effect of this tsunami varied among different coasts of the Bay of Bengal and Indian Ocean. The recovery process may also have different manifestations from place to place. We suggest that (a) in certain cases the optimal management of a coast after a tsunami is to leave it to nature, and (b) sedimentary deposits are better indicators of past tsunamis than coastal morphology. The destructive effect of this tsunami and the sediments left by it on land have been described several times (http://walrus.wr.usgs.gov/tsunami/sumatra05/; Borrero, 2005, Moore et al., 2006, Paris et al., 2007, Paris et al., 2009, Jankaew et al., 2008, Monecke et al., 2008). This paper draws on such information, but essentially, it is an account of a post-tsunami rebuilding of the coast.