Monitoring Coastal and Seafloor Changes in The Gulf of Mannar, Southeast Coast of India Using Remote Sensing and GIS Technology
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.
Shoreline Change Mapping
Geocoded FCC of IRS LISS III (May 1998) satellite data on 1:50,000 scale belonging to low tide period and Survey of India Topographic map (SOI 1969) on 1: 50000 scale were used to prepare shore line change map. The low tide line (as shore line) from SOI Topographical map and satellite image were extracted and mapped on 1: 50, 000 scale by visual techniques. After identification and delineation, coastal villages, some monuments, light house etc were selected as sample points on SOI topographic map for ground truth checking. During the time of ground truth study the entire coast was checked with local people and some necessary corrections were carried out on primary shoreline map. The georeferenced multi date shore line maps were carefully digitized in ARC/INFO and were overlaid using TIC coordinates of the study area. Digitized maps were edited and labels assigned to the polygons. Finally a temporal shoreline change map was generated using intercept operation of ARC/VIEW, and summary statistics was generated for erosional and accretional areas using STATISTIC program of Arc/INFO. The changes were estimated for a period of 30 years between 1969 and 1998.
RESULTS AND DISSECTION
The coastal plain between Dhanuskodi and Tuticorin has various geomorphic units with different types of configurations (Figure 2). The geomorphic units, interpreted from remotely sensed data and checked subsequently through fieldwork, have been categorized into four genetic classes marine, fluvio-marine, fluvial, aeolian and biogenic landforms.
Figure 2. Costal geomorphology map of Gulf of Mannar derived from IRS LISS-III satellite
In the coastal zone various marine landform features such as beaches, spit, beach ridges, swales and backwater zone, mudflat, offshore islands, coral reefs, wave cut platform, sea cliff, sea cave, water logged land and strand lines have been identified.
Beaches are extensively developed along the entire coastline of Gulf of Mannar with an average width of 105.35 m. The shore between Tuticorin and south of Sippikulam (2.04 kmē), Vaippar River and Gundar River (2.56 kmē), Gundar River and Palar River, (2.64 kmē), Palar River and Kottakkarai River (2.189 kmē), Kottakkarai River and Marakkayarpattanam (2.18 kmē), southern coastal parts of the Rameswaram Island (2.91 kmē) and the western part of the Rameswaram Island from Pamban to Peikkarumbu have important beach areas in Gulf of Mannar coast. All along the shore the beach is observed to be gently sloping and marked with altered crusts and troughs that are formed due to wave action.
Among the various depositional landform features the formation of spit is a significant feature of recent age. Normally the formation of spit has been attributed commonly to the movement and deposition of materials by long shore current. South of Tuticorin coastal area two spit formations have been observed with 0.75 to 2 km long and tongue shaped. It appears to have been built by the sediments brought by long shore current during southwest monsoon. As the Gulf of Mannar is on the lee of the northeast monsoon, there is no long-shore drift from the northeast that might be the cause for the inward curving of this spit (Ahmad 1972). It can be explained that the Tuticorin spit might have been the result of the long shore currents during monsoon and the sediments discharged by Tamiraparani River. Geocoded IRS LISS-III imagery taken in the year of 1998 has shown a well-developed spit near Valinokkam (Figure 3a), but the toposheet of the year 1969 does not show any indication of spit. This spit maybe formed due to the long shore current from south west, probably under the influence of the south west monsoon. The southwestern shore of Rameswaram has a tongue shaped spit. SOI toposheet of the year 1969 does not show any spit but recent IRS LISS-III imagery (1998) clearly shows the spit (Figure 3b).It may be assumed that these spits are recently formed. It can be explained that the Rameswaram spit may have been the result of littoral current from Palk Bay to Gulf of Mannar during northeast monsoon period.
Figure 3. a) Spit neat Valinokkam, b) Spit at southwesr of Rameswaram Island
Well-developed twelve beach ridges are seen between Mandapam and East of Vaippar River. Almost all beach ridges in this area are parallel to each other, and cover an area of 155.49 kmē and trend from east to west and northeast to southwest direction. On the basis of the nature and dispositions of beach ridges, it can be grouped into (i) Beach ridges south of Vaigai River, (ii) Beach ridges between Kotangudi River and Palar River, (iii) Beach ridges between Palar River and Gundar River, (iv) Beach ridges between Gundar River and Vaippar River and (v) Beach ridges south of Vaippar River. These ridges are clearly indicating the emerging of coastal land in Gulf of Mannar area.
Swales and backwater zones are seen between Mandapam and Kottakkarai River. These are branched and arranged in series of linear patterns and situated almost parallel to the present coastline. Prominent backwater zones have been observed in the coastal plains between Valinockkam and Vaippar River, Mandapam and Southeast of Tiryppullani.
The coastal areas between Mandapam and Tinaikkulam, Valinokkam and Krishnapuram and North of Terku Mukkaiyur and Tukukankulam consist of prominent and wide backwater zones. These three backwater zones are connected by small, linear and narrow swales to the sea by means of few creeks, which supply water from sea to backwater channels during high tide. The basin bed is composed of silt and mud. The adjacent low lying area, as a part of swale zone is used at present for salt production. Mudflats are very common feature in Vaippar River mouth, around Valinokkam backwater lagoon, Kallar River mouth and Gundar River mouth. The area covered by mudflat has been estimated to be 14.50 kmē.
A chain of 21 low islands have been observed along the offshore region of Gulf of Mannar. They extends from south of Rameswaram to Tuticorin. 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. Around all offshore islands, well-developed coral reefs (Figure 4) have been noticed. Geomorphologically, coral reefs in this area are of fringing type, though some patchy corals are also observed in between Appa Island and Pilliyarmuni Island, and in some areas like Bharathi nagar coast and southeast coast of Kariya Shuli Island.
Figure 4. Coral reef at northeast of Single Island
Along the rocky beaches, frequent wavecut platforms are observed. These features indicate marine erosional formative processes and represent flat to moderately undulating platform, predominantly made up of beach rocks and sometimes-calcareous rocks. Wavecut platforms have been observed along the coast of Mandapam, Ramaswami Madam, Pudumatam, Valinokkam etc. At Pudumadam coast, hard and tough sandstone platform occupies the intertidal zone.
Along the coast of Gulf of Mannar, sea cliffs have been observed in Mandapam, Rameswaram, Pudumatam and Appa Island coastal areas. Generally the sea cliff and caves are made up of calcareous sandstone and located at the high water level. Due to intensive action of waves on cliffs at some places sea caves are formed. Such caves have been observed near Mandapam coastal area and Southwestern and Southern coastal areas of Appa Island. At some places, these features have been destroyed due to slumping of upper cliff materials.