RESEARCH COMMUNICATIONS CURRENT SCIENCE, VOL. 103, NO. 10, 25 NOVEMBER 2012 1206 *For correspondence. (e-mail: nrkakani@yahoo.com) Role of bathymetry in tsunami amplification – evidences from Andhra Pradesh coast K. Nageswara Rao 1, *, P. Subraelu 1 , A. S. Rajawat 2 and Ajai 2 1 Department of Geo-Engineering, Andhra University, Visakhapatnam 530 003, India 2 Marine, Geo and Planetary Sciences Group, Space Applications Centre, Ahmedabad 380 015, India Interpretation of satellite images representing both the pre- and post-tsunami of 26 December 2004 revealed three isolated locations along the Andhra Pradesh coast where the tsunami amplified despite its overall diminishing strength northward from Tamil Nadu. The digital elevation models generated from the depth contours traced from the National Hydrographic Charts indicated landward convexities in the 5 m depth contour juxtaposing the three locations. Appar- ently, the tsunami amplified locally as it entered these narrow canyon-like zones and threw up sediment onto casuarina plantations, aquaculture ponds, saltpans and other land-use features. The study highlights the possible role of nearshore bathymetry in tsunami amplification. Keywords: Bathymetry, depth contour, landforms, tsumani amplification. THE impact of the 2004 Indian Ocean tsunami on the coastal communities has been extensively researched, surveyed and documented. One of the common inferences made from various tsunami-affected coastal sectors was that the local landforms rendered some areas more vul- nerable than others 1 . For instance, places like Nagapatti- nam and Cuddalore along the Tamil Nadu coast were among the worst affected by the tsunami because of their low-lying nature 2 , as the linear valley-like swales behind the shore-parallel sand-dune ridges claimed several lives due to lateral flows from tidal inlets or breaches in dune ridges 3 . Several studies from various other parts such as along the coasts of Kerala 4 , Andaman and Nicobar Islands 5 , Sri Lanka 6 , Thailand 7 and Myanmar 8 have high- lighted the role of landforms in tsunami impact. The coastal bathymetry, i.e. the topography of the inner conti- nental shelf region also played an important role in the tsunami intensity as observed from the Tamil Nadu coast 9 , Andaman and Nicobar Islands 10 , Sri Lanka 11 and Vietnam 12 . Field measurements at 19 stations along Tamil Nadu coast revealed that the tsunami reached variable distances ranging from 100 to 900 m and the increase in tsunami height even up to 7 m in the northern regions of Tamil Nadu coast is perhaps due to the offshore bathy- metry in that region, wherein the 200 m depth contour was concave seaward, which possibly lead to the conver- gence of tsunami 9 . Despite an overall decrease in its intensity northward from Tamil Nadu along the east coast, the tsunami ampli- fied at some isolated pockets in the Andhra Pradesh (AP) coast also. An attempt is made in this study to identify the coastal sectors that were significantly affected by the tsunami and to find out if there is any correlation between the offshore topography and the tsunami ampli- fication at such locations along the AP coast. IRS P6 LISS-3 satellite images of 23.5 m spatial reso- lution pertaining to the pre- and post-tsunami dates were compared for identifying the locations where the tsunami impact was significant in the entire 1030 km long AP coast. Although the pre-tsunami images used in this study pertain to different dates ranging from April to December 2004 and the post-tsunami images are from early 2005 to early 2006 depending on the availability of cloud-free data, the coastal changes considered in this study are caused by the 2004 tsunami as evident from the field observations and measurements made all along the AP coast within three months from the date of tsunami occur- rence 13 . Furthermore, there was no possibility of such coastal changes having been caused by storm surges, as no such events occurred in the region during 2005. A comparison of the pre- and post-tsunami satellite images in GIS revealed significant changes in the coastal Figure 1. Outline map of the peninsular part of India showing the locations of the three villages, namely Krishnapatnam, Gangapatnam and Chinnamailavanilanka (marked with solid triangles), along the eastern coast in Andhra Pradesh.