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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
The seafloor depth contours with reference to chart datum measured at Tuticorin coast during April 1999 along 10 transects perpendicular to the shoreline are shown in the Figures 4a and 4b. The average reduction in the depth has been calculated as 0.31m over a period of 24 years (1975-1999), it may be due to deposition of sediment and emerging of land (by tectonics). The depth of sea floor has reduced along 8 transects and increased along 7 transects around the Vilangu Shuli and Van Islands. This reduction of depth may be caused by deposition of sediment due to ocean currents, whereas the increasing of sea floor depth may be due to erosion caused by anthropogenic activities (Coral mining). Evidences show that the entire Shuli Island was eroded and lies below the sea level. The average amount of reduction and increase of sea floor depth around the Vilangu Shuli Island were calculated as 0.08 m and 0.35 m over a period of 24 years. Around Van Island the decreasing and increasing trend of sea floor depth have been calculated as 0.88 and 0.62 m over a period of 24 years. The sea floor has rise with reference to chart datum along 8 transects off the coast of Mandapam area may be due to emerging of land and sediment deposition. This part the of study area is very dynamic and during the southeast monsoon period a long shore current transports large amount of sediment from Palk Bay to Gulf of Mannar through Pamban Channel and deposit the sediment. In addition during the low tide period the tidal current move towards south and deposit some amount of sediment in this part. New spit formation near Kundugal is the evidence for deposition of sediment (Thanikachalam and Ramachandran, 2002). The average rising of sea floor along Mandapam coast is found to be 0.68m over a period of 24 years (1975-1999). Around the Mandapam group of islands, the raise of sea floor was noticed at the seaward side of the island. It may be due to sedimentation caused by wave induced current. At the seaward side of Kursadi, Kovi, Pumurichan, Musal, Manalli and Manalli Putti Islands the reef edge was found at an average distance of 1 km away from the shores of the island towards sea. The reef acts as a wave breaker and when the high velocity waves hit against the reef edge, it reduces the velocity of waves and the wave induced current moves towards north, distributing the littoral sediments on these coral reefs. By such repeated action, the area between islands and reef edge gets shallow. The average rising of sea floor at seaward side of Pumurichan, Kovi and Kursadi Islands was calculated as 0.23 km for the past 24 years and the rate of rising of sea floor is 0.009 m/year. Towards the seaward side of Musal, Manalli and Manalli Putti Islands the sea floor depth has decreased by 0.36m over a period of 24 years. The rate of rising of sea floor in this area is estimated as 0.015m/year. The sandbar formation between Manalli and Manalli Putti Islands, occurrence of spits along the coast and rising reefs in Mandapam and Keelakkarai groups of islands are the evidences for rising of the sea floor.
5. Conclusion
Geographic Information System and ERDAS imagine soft wares are very much useful for bathymetry mapping. Various shelf morphological features like channels, continental raise and islands, and their slopes and extents are identified. The present study suggests that in the study area sea floor has been gradually rising due to tectonic upliftment.
Acknowledgements
We are very thankful to Head of the Department, Principal and Correspondent of Mepco Schlenk Engineering College, Sivakasi, Tamil Nadu, India, for giving us this opportunity, constant encouragement and facilities to carry out this study.
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