International Journal of Applied Earth Observation and Geoinformation
Impact of the tsunami and earthquake of 26th December 2004 on the vital coastal ecosystems of the Andaman and Nicobar Islands assessed using RESOURCESAT AWiFS data
Introduction
Tsunami waves struck the Indian coast on 26th December 2004 affecting the Andaman and Nicobar group of islands, Tamil Nadu, Andhra Pradesh and Kerala coasts. A tsunami is not a single wave but a series of traveling ocean waves generated by the geological changes near or below the ocean floor. The recent tsunami was set off as a result of a massive earthquake in the region registering a magnitude of 9.3, with its epicentre under the sea (more than 8.9 km below the sea bed), off the northern tip of the Indonesian archipelago near Aceh. These waves traveled as far as 6400 km from the epicentre. The tsunami moves rapidly across the ocean (900 km/h) and takes the form of destructive high waves along shallow coastal waters (10 m height and a speed of 40 km/h).
A tsunami can damage marine ecosystems through its initial surge and the ensuing backwash, the latter of which may carry sediments and debris from land. The sediments and debris can smother and scrape seafloor habitats, such as coral, while introducing toxins and disease agents to the ecosystem. The physical forces of a tsunami are powerful enough to cause meter-wide coral boulders to tumble across the seafloor.
Satellite data was used to rapidly assess the damage caused by the recent Asian tsunami, particularly on the coastal wetlands of the affected areas. Selected coral reefs were also monitored to study the status of the ecosystems after 8–11 months.
The Andaman and Nicobar Islands bore the maximum brunt as they are located closest to the epicentre and were in the closest direct path of the tsunami. This paper describes the status of the coral reefs and the coastal wetlands of the Andaman and Nicobar group of islands after the tsunami struck. The Andaman and Nicobar groups of islands located in the southeast of the Bay of Bengal (Fig. 1) between 6°45′N and 13°14′N latitudes and 92°12′E and 93°57′E longitudes consist of about 350 islands, of which only 38 are inhabited along with a number of exposed islets and rocks, and the land area is about 8500 km2 (Anon, 1987). The general orientation of the islands is north south. The islands are characterized by three interconnected natural ecosystems – Forest, Mangrove and Marine ecosystems that are the result of typical topographical, climatological and soil characteristics of the area.
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Methods and material studied
AWiFS data of RESOURCESAT-1 was used for rapid assessment of the damage to ecosystems and subsequent monitoring. LISS III data of RESOURCESAT-1 with a resolution of 23 m were used along with AWiFS data for selected areas to study the status of the coral reefs currently. Details of the data used for the study are given in Table 1.
Data of the period October–December 2004 helped to assess the status of the coastal habitats just before the tsunami event. For studying the condition of these habitats
Results and analyses
Sai et al. (2004) and Sharma (2004) have pointed the usefulness of RESOURCESAT AWiFS data in monitoring the natural resources due to its improved quantization level (10-bit data) as compared to the LISS III data of Indian Remote Sensing Satellite 1C and 1D and RESORCESAT LISS III and LISS IV data. The inclusion of SWIR band helped in monitoring the vegetation and the increased temporal capability was an edge over the other Indian Remote Sensing Satellites in monitoring the coastal resources
Discussion and conclusion
RESOURCESAT AWiFS data helped in assessing the impact of the natural hazard on coastal resources rapidly. Among the coastal ecosystems the coral reefs have suffered the maximum damage. The Nicobar reefs have been totally eroded at most of the places. Fig. 6 aptly summarizes the impacts of tsunami and earthquake of 26th December 2004. Out of total 223.7 km2 area occupied by Nicobar reefs prior to tsunami, only 10 km2 of reef remained just after tsunami with 154 km2 eroded and 65.8 km2 damaged due to
Acknowledgements
The authors are thankful to the Director, Space Applications Centre, and Deputy Director, Remote Sensing Applications and Image Processing Area, Space Applications Centre, Ahmedabad, for providing guidance and facility to carry out this work. We are also thankful to Central Agricultural Research Institute, Port Blair for providing us logistics support and expertise for carrying out ground observations.
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