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Mapping on high-resolution satellite images and in the field shows differences in landforms and characteristics of tsunami flow on two contrasting coastal plains following the giant earthquake on December 26, 2004: the plain of Banda Aceh on the northern tip of the Sumatra island, Indonesia, and the Nam Khem plain in the Andaman Sea coast of Thailand. The landforms of the Banda Aceh coastal plain are characterized as deltaic lowland with tidal plains in the western and central parts, and strand plain with beach ridges in the eastern part. The run-up tsunami flow invaded areas about 3–4 km from the coast. Strong tsunami flow severely damaged the tidal plain and the landforms along the coast except coastal dunes in the east. Most of the landforms except sand dunes along the coast had almost no effect on the protection against the tsunami, but the higher micro-landforms such as beach ridges and natural levees prevented the flow of the tsunami from its invasion into the inland near the end of tsunami inundation. The tsunami inundation spread out over the entire Nam Khem coastal plain with an average depth of 4–5 m. The direction of run-up flow was almost perpendicular to the coastline, whereas backwash flow directions were controlled by topography. Backwash flow was concentrated in the lower portions of the plain, for example in small stream channels. Wedge-shaped channels in the lower parts of the streams were formed due to the concentration of backwash flow. The existence of the swales between parallel beach ridges corresponds well with the distribution of thick tsunami deposits. Coastal erosion of the plain was caused by the direct attack of tsunami waves, and the lower reaches of small rivers were eroded by strong backwash flow. |
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