Monitoring Coastal and Seafloor Changes in The Gulf of Mannar, Southeast Coast of India Using Remote Sensing and GIS TechnologyAll along the coast, strandlines are so common especially in between Tiruppullani to Mandapam, eight series of strandlines in curvilinear form have been observed. The general trend of the strandline is in the east to west direction. In the south of Rameswaram area also, curvilinear strandlines have been observed. Deltaic plains are very common landform along the coast of Gulf of Mannar, which is predominantly controlled by Fluvio-Marine processes. These deltaic plain are considered to be of Pleistocene to Recent age (Loveson 1993). The areas around the river courses of Vaippar, Gundar, Palar and Kottakkarai Rivers, had vast deltas, but at present they are found to be inactive. The deltaic plains are marked by flat and vast areas, having vegetation cover. Number of tanks have been noticed on the deltaic plains. The total area has been estimated to be about 221.69 kmē. In the coastal zone of Gulf of Mannar various fluvial landform features such as floodplains, natural levees and Flood basin have been identified. Well-established rivers usually have their floors covered with alluvium, in which the normal flow channel is covered. The surface of low relief on the alluvium from the banks of the low-water channel to the base of the valley walls is called the flood plain of a river. Flood plain and their major morphologic subdivisions are primarily deposited landforms. Floodplains have been very clearly observed along the riverbanks of Kallar, Vaippar and Gundar Rivers in study region. These flood plains are inactive, which is covered with thick vegetation. The total estimated area of flood plain in study region is 24.43 kmē. Land forms in deltaic regions include natural levees bordering river channel and backed by lowering of the swamp or flooded depression in the lower parts of the river valleys (Bird 1984).The over bank deposits are located more or less parallel to the riverbank. Generally, the levees are mainly seen adjoining the meandering course of the rivers. A natural levee has been observed in the Gundar River basin. The width of these levees encountered ranges from 2 to 3 km. and covers an area of 1.10 kmē.Flood basin occupying the shallow reaches of the flood plain consists of mostly gray and brown sandy, silty-clay and clay. Kottakkarai River surface has a flood basin with an area of 26.83 kmē. Sand dunes and Teri dunes are the most common aeolian land forms in the coastal zone of Gulf of Mannar area. Almost entire coastal plains in the study area are covered by sand dune. The area in between swale system and shoreline is marked by dune complex. The area covered by dune complex has been estimated to be about 469.10 kmē. However, extensive spread is observed around Rameswaram Island, Mandapam, Mangudi, Bharathinagar, Valinokkam, Thunamariyur, Terku Nerippaiyur ,Tuticorin, Taruvaikkulam and Sippikkulam. In other places the areal spread is less. Aeolian process is dominating in this zone and its migrating dunes are seen in Tuticorin coast. Thruvaikkulam and the other above-mentioned areas also exhibit features similar to those in Tuticorin area. In the coastal plains between south of Vaippar River and Tuticorin near Maravanmadam seven patches of teri dunes have been observed with a thick cover of vegetation. North of Panaiyur, oval shaped teri dunes with sparse vegetation have been observed. The areal extent of this dune has been calculated to be 6.27 kmē. Another two teri dunes have been observed near Kumarapuram. They cover an area of 2.67 kmē. Near Pandiyapuram, rounded dune complex has been observed with thick vegetation. The area of this dune has been estimated as 4.27 kmē. Near Milavittam small rounded patches of teri dune complex covering an area of 2 kmē has been observed. Two other teri dune complexes have been observed near Maravanmatam area with thick vegetation. They cover an area of 3.08 kmē. All teri dune complexes in this area are trending in the northeast to southeast direction. Back swamp is very common biogenic landform along the coast of Gulf of Mannar. It occurs in marshy areas along the coast; they particularly occur at the edge of the tropical or sub-tropical seas, in bays lagoons and estuarine regions (Gerlech 1973). Small back swamp areas have been observed in the areas near the mouth of Korampallam odai around Tuticorin coast and west of Rameswaram Island. They cover a total area of 1.87 kmē. These swamps are covered by mangrove vegetation. Seafloor Changes 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 5 and 6) 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. Any changes in seafloor may be the result of sea-level variation or due to a change in the elevation of land surface. Changes in absolute water-surface levels are worldwide due to the interconnectivity of the oceans and are termed as eustatic changes. Changes in the absolute level of the land are localized. They may be due to tectonic adjustments or due to adjustments caused by the distribution of weight on the land surface. As and when sedimentation or ice build-up occurs, such changes happen and are known as isostatic. A rise in the sea level or down warping of land would involve the opposite movements of sea and land. Synonymous with positive and negative changes are the forms of sea-level transgression and regression, although in many cases these terms also refer to the horizontal movement of the shoreline associated with vertical changes of sea level.  Figure 5. Three dimensional model for Gulf of Mannar sea floor  Figure 6. Bathymetry Map of Gulf of Mannar (1975) The sea floor depth contours with reference to chart datum (1975) measured at Tuticorin and Mandapam coasts during April 1999 are shown in the Figures 7 and Table 1. Recent depth contour map (1999) has been compared with bathymetry map of 1975; it reflects that the seafloor level decreased along the coastal and around the islands in the study area. It may be due to emerging of land due to tectonism. Many authors have reported that the coast of Gulf of Mannar is on an emerging phase due to tectonic movement (Foot, 1888; Ahmad, 1972; Stoodart and Pillai, 1972; Loveson and Rajamanicam, 1988; Ramasamy, Ramasamy, (1996), has build up a post collision tectonic model for the southern part of Indian and in which he has observed a series of geoenvironmental problems being caused due to such ongoing tectonic movement. In very few places particularly at river mouths and in island areas, the sea floor level has increased, which may be due to erosion caused by anthropogenic activities. The removal of corals in Tuticorin group of island by coral mining the depth has increased to about 1m (Thanikackalam and Ramachandran 2002a).  Figure 7. Bathymetry map of Tuticorin Neare shore area during 1999 Recent bathymetry (1999) data have been compared with bathymetry data of 1975. It reflects that the depth of seafloor decreased along the mainland coast and around the islands in the study area. It may be due to emerging of land due to tectonic movement. In very few places particularly at river mouths and in island areas, the sea floor level has increased, which may be due to erosion caused by anthropogenic activities. The average depth reduction of seafloor along the coast of the study area has been estimated as 0.51m over a period of 24 years. Assuming that the rate of change of depth of sea floor is uniform over a year, the rate of decrease of depth is estimated as 0.02 m/year along the coast, this may be due to emerging of land or lowering of sea level by tectonic activities. Some of the field evidence such as seaward migration of shoreline, increasing the areal extent of mangroves swamp and mudflat, new spit formation, occurrence of bunch of beach ridges, swales and strandlines in the Gulf of Mannar coast proved that the coast is going on emerging by tectonic movement 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 8a and 8b. 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 (Figure 8c & 8d). 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, 2002d). 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.   Figure 8. a) Sea floor changes at Tuticorin and Van Island b) Sea floor changes at Veppaloadi and Vilangu Shuli Island c) Sea floor changes at Marakkayarpattinam Coast d) Sea floor changes at Gundugul and Kursadi Island 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. Coastal Erosion and Accretion In the coastline between Dhanuskodi and Tuticorin, erosion and accretion areas have been identified. The areas of erosion and accretion have been estimated as 4.34 and 23.49 kmē, respectively, over a period of 30 years (1969 to1998). For coastal erosion and accretion studies the study area has been classified into (1) Shoreline between Tuticorin and Vaippar River, (2) Shoreline between Vaippar River and Gundar River, (3) Shoreline between Gundar River and Palar River, (4) Shoreline between Palar River and Kottangudi River, (5) Shoreline between Kottangudi River and Thoniturai and (6) Shoreline of Rameswaram Island. Along the shore between Tuticorin and the Vaippar River, twelve accretion and seven erosional sites have been identified (Figure. 9). The estimated average rates of accretion and erosion in this area are in the order of 4.3 and 2.73m/year, respectively. In the area between the Vaippar River and the Gundar River, the entire shore has been an observed to be accretion zone. Some creeks and backwater zones are also observed along this shore. Mouths of these creeks and rivers are closed by muddy sand and mudflat occurs most of the time in a year except rainy season. The area of accretion is estimated as 3.35 kmē. The estimated average rate of accretion is 6.42 m/year.  Figure 9. Shoreline change map between 1969 and 1998 In the region between the Gundar River and the Palar River mouth, three accretional and two erosional sites were observed. Most part of this shore is covered by sandy materials except south of Valinokkam coast, where it is covered by calcareous sandstone. The estimated rates of erosion and accretion in between the Gundar and the Palar coast are 3.35 and 6.42 m/year, respectively, during the period 1969 to 1998. Along the shore between the Palar River and the Kottangudi River, four erosion and seven accretion sites were observed. The areal extent of erosion and accretion were approximately 0.125 and 0.876 kmē. The rates of erosion and accretion are 2.97 and 3.73 m/year, respectively. The shore between the Kottangudi River and Thoniturai has five erosional and accretional sites. The lengths of erosion and accretion sites along the coast were 12.61 and 2.49 km, respectively. The areas of erosion and accretion were approximately 0.34 and 2.11 kmē, respectively. The rates of erosion and accretion are 4.19 and 4.87 m/year, respectively. Along the Shore of Rameswaram Island both accretional and erosional features were observed. The areas of erosion and accretion along the coast were approximately 4.66 and 2.01 kmē respectively. The average rates of erosion and accretion are 6.23 and 6.54 m/year. Changes in Islands There are 21 islands between Tuticorin and Rameswaram. All islands are made up of a calcareous framework of dead reef and sand. They have a low and narrow sandy coast and some of them have rocky coast. Fringing reef along the windward side of the islands protects the islands from direct wave action. Morphology of sandy islands is very dynamic, Dhandapani, (1992) investigated the morphological characters of some of the islands in the Gulf of Mannar. Ramanujam et al., (1995) and Ramanujam and Mukesh, (1998) have studied the morphological variation of Tuticorin group of islands. The morphological variations of islands occur due to natural and anthropogenic stress. The natural agents include erosion, accretion, wave, current, sea level variation, neo-tectonic activity etc. Anthropogenic impacts are construction of breakwaters, discharging of effluents, mining of coral reef, etc. The comparison of 1969 and 1998 maps of islands of Gulf of Mannar showed changes in their shape, size and location and these have been caused by erosion and accretion of shore. The total area of erosion and accretion were calculated as 4.16 kmē and 3.31kmē, respectively, during the 30-years period. Island erosion and accretion are caused mainly by the action of waves and wave-induced current and long shore currents along the shores of islands. In the study region the waves are in the northeast and southwest direction and wind direction is similar to the wave direction. According to Chandrasekar, (1996) coral reefs are destroyed by siltation, logging and illegal mining in Tuticorin group (Van, Koswari, Kariya Shulli and Vilangu Shulli Islands) of islands. Mining of stony corals from the reef area, especially from, Tuticorin group of islands, for building, industrial and chemical purposes have also destabilised the formation of Tuticorin group of islands (Ramanujam et al., 1995). Hence the waves hit directly on south, southeast and southwest shores of these islands, causing erosion. These eroded sediments are then transported by wave-induced currents and deposited at the leeward sides of these islands. By such repeated processes, the windward sides of the island get reduced and leeward sides of the islands are accreted. The total areas of erosion and accretion were estimated as 0.35 and 0.13 kmē, respectively. Hence size, shape and location of these islands have changed. All islands in Tuticorin group have been migrating towards mainland (Figure. 9). It is estimated that (1) 528.74 m of Van Island, (2) 118 m of Koswarai Island and (3) 137 m of Kariya Shulli migrated towards mainland between 1969 to 1998, According to Ramanujam et al., (1995) the landward migrationof islands in Tuticorin region are caused by sea level variation and mining of reef material. Vilangu Shuli Island is one of the islands in Tuticorin group, situated at 6.25 km from Sippikkulam. The entire part of this island was eroded due to the direct action of waves; it may be caused by coral reef mining. Ramanujam et al., (1995) showed that the direct wave attack on this island eroded the whole area below the sea level. The area of erosion was estimated as 0.06 kmē. Keelakkarai and Mandapam groups of islands, have moved towards seaward side.The island erosion has been mostly identified along the northern shore of these islands (landward side). Some of the evidences such as submerged trees and sharp edged coasts are found along the northern shores of these islands. This is because the long shore current and tidal current are flowing north to south along the northern shore of islands, then erode the coast, this eroded materials are transported and deposited on seaward side of the island coast. While high velocity waves are moving towards south shores of islands with the littoral sediments and coming across the coral reefs, when these sediments are be dropped on the coral reefs; wave speed reduces and turns into a wave-induced current. By repeated action of such processes the area between Islands and reef edge get shallow and reefs have submerged. Submerged reefs in southeast of Kursadai Island are evidence for accretion of sediment at seaward side of the islands. The areas of erosion and accretion along the shore of these islands were estimated to be 3.81 and 3.18 kmē, respectively, for the last 30 years between 1969 and 1998. Puvarasanpatti Island is one of the islands in Keelakkarai group, located 6.90 km from Kalachimundal coast. The entire island was eroded due to the direct attack of the waves, and its level is below the sea level. The area affected by erosion is 0.05 kmē. CONCLUSION Remote sensing and GIS techniques are most useful for coastal landform, shore line change and bathymetry mapping. Interpretation of IRS LISS-III imagery aids in demarcating various coastal geomorphologic features like beach, spit, beach ridges, swales, mudflat, back swamp, dune complex, teri sand, natural levee, flood plain, deltaic plain, flood plain, strandline, etc. All the coastal geomorphologic features such as shoreline changes, bathymetry changes and coastal landform, in particular, spit, beach ridges, strandlines, swales and backwater system, back swamp, mud flat etc. indicate that the Gulf of Mannar coast is going on emerging. REFERENCES
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