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Monitoring Changes in Seafloor Morphology Using Multi-date Bathymetry data: A Case Study of the Gulf of Mannar, Southeast Coast of India
3. Materials and Methods Naval Hydrographic Chart (1975), Survey of India Topographic Sheet (SOI), Ecosounder (ODEM) and Global Position System (GPS) were used in this study. Bathymetry map of study area on 1: 50,000 scale was prepared manually using 1975 Naval Hydrographic Chart. The prepared bathymetry map was digitized into ARC-INFO and a digital elevation model was prepared using ERDAS imagine software. During April 1999 bathymetry survey was conducted using eco-sounder (ODEM) and Global Position System (which is used to locate the sample points) along Mandapam and Tuticorin coastal area (within 10 m depth) in the Gulf of Mannar. The depth values are recorded at a particular location with reference to chart datum (1975). The measured depths were tide corrected with respect to time and then converted with respect to chart datum. Measured tide table from the Tuticorin port was used for final data conversion to chart datum. 4. Result and Discussion 4.1 Continental Shelf Morphology In Gulf of Mannar, the slope and width of the continental shelf is approximately the same as the average for the eastern coast of India (Ahmad 1972). The total width of the shelf is around 30 km having a slope of about 21’. The slope near the shore is about 4’. Shelf morphology (Figure 2 and 3) of the study area has been described in detail by dividing the study area into four segments viz (1) Tuticorin to Vaippar River, (2) Vaippar River to Gundar River, (3) Gundar River to Palar River and (4) Palar River to Dhanushkodi shelf regions.  Figure 2. Three dimensional model for Gulf of Mannar sea floor  Figure 3. Bathymetry Map of Gulf of Mannar (1975) In the shore between Tuticorin to Vaippar River, the sea floor is sloping gently down to 5m depth while in Sippikkulam coast where the sea floor is very steep down to 2m depth and it extending up to 0.129 km from the coast. In between 4 and 5m depths, some elevated rises and islands are noticed. At a distance of 4.30 km (78° 12'16"E-8 49'N) from Tuticorin coast, a 3.9 m elevated rise was observed. Just northeast of this riise a 6m elevated island (78° 12'28" E-8 49'35"N) called Van Island, which is situated 5 km away from Tuticorin coast. Around this island the sea floor is sloping very gently down to 2m depth in southeast, east, northeast and north directions. Where as west, southwest, and northwestern parts of the island, the sea floor slope is very steep. On the northeastern, eastern and southern sides of the island, the sea floor is covered by fringing coral reef, extending down to 2 m depth with an average distance of 1.50 km from the coast of the island. Between the depths of 3m and 4m, there is another island called Koswari Island, which is located at 78°13'22" E-8°52'2" N, with a distance of 6.09 km from Taruvaikkulam coast. In between 5 and 10m depths, the sea floor slopes moderately. From the depth of 10m to 20 m, the sea floor slopes gently, having a distance of 16.65km between them. At a depth of 20m, the seafloor falls suddenly with a very steep slope (continental slope) extending till 30m depth. The width of this slope has been calculated as 7.27 km. The total width of continental shelf in Tuticorin region has been calculated as 26.75 km. In the coast from Veppaloadi to Sippikkulam, between 4m and 6m depths, elevated islands namely Kariya Shuli and Vilangu Shuli Islands are situated at a distance of 4.77 km and 6.56 km from Sippikulam coast respectively. The sea floor is found to be sloping gently towards north, east and south from the shore of Vilangu Shuli Island, whereas west of the the sea floor tends to slope very steeply. An extensive well-developed fringing reef has been identified around Vilangu Shuli and Karia Shuli Islands extending to 3 meters depth. In the shore between Vaippar and Muttiapuram, the sea floor is at a depth of 3 m., which extends to an average distance of 3.51 km from the shore. In the area near the mouth of Vembar the sea floor topography is plain with a depth of 2 m. Between 2 m and 20 m the sea floor slopes very gently and extends to a distance of 19.75 km. A sudden steep slope is encountered at 20m depth, this area is the starting point of the continental slope and it extends to the depth of 30m. The width of the slope has been calculated as 5.45 km and the width of the continental shelf has been calculated as 19.75 km. In the area from Terkku Narippeyur to Gundar River, the shelf is found at a depth of 3m, extending to 0.36 km south and southeast of Narippeyur coast and 0.88 km south of Gundar River mouth. Between the depths of 7 and 8 km, the sea floor is very broad and the slope is very gentle. An elevated island is found exposed above sea level viz., Uppu Tanni Island situated at 6.72 km from Gundar River mouth. The sea floor is plain up to 1m depth in all directions around the island except in the north and northeastern directions where the slope is very steep. |