Dr. S. M. Ramasamy
Professor & Director, Centre for Remote Sensing
Bharathidasan University, Tiruchirappalli
The coastal eco system is very complex and fragile because of the multivariate dynamically active and pulsatory tectonic and geomorphic processes. As remote sensing provides potential information on the pattern and the splay of geomorphic features, which stand as a testimony for the past and present eco system, fabrication of the eco system was made for Tamil Nadu coast using the remote sensing techniques. These information were subsequently animated using PC Arc/info.
The coastal eco system is unique, self styled and divergent owing to the multivariate tectonic, fluvial, coastal and aeolian geomorphic processes. These processes which have acted in varying degrees and duration during the Quaternary period have left their imprints in the form of various geomorphic features along the coast such as deltas of different morphology (lobate, arcuate, cuspate, digitate and estuarine), beach ridge complexes, backwater-lagoon systems, estuary and creek systems, spit systems etc. Hence, once these features and there from the geomorphic processes are mapped using technologies like remote sensing not only the present eco system but also the palaeo eco system right from approximately 1.5 million years to today can be fabricated. In such a fabrication spatial technology plays significant role.
The Tamil Nadu coast is one such a coast which stands as a testimony for the coastal eco system of the Quaternary period and its temporal changes during the past nearly one million years or so. As remote sensing provides excellent information on the pattern and the spatial distribution of these geomorphic features and the spatial technology has unique capabilities, these were utilised in mapping the eco system of Tamil Nadu coast and some of the salient cases are briefly presented in this paper.
The studies carried out by various workers in general and the present author and his research team in particular have brought out detailed information that the Indian Peninsular is tectonically active with
- E-W trending chains of alternating cymatogenic arches and deeps
- N-S trending Quaternary extension fractures / faults
- NE-SW trending Quaternary sinistral fractures / faults
- NW-SE trending Quaternary dextral fractures / faults and
- E-W trending crestline fractures along cymatogenic arches (Fig. 1)
These tectonic forces which have generated well defined arches in Chennai and Ramanathapuram – Rameswaram areas (Fig. 1) with intervening deeps in Ponnani – Manamelkudi area and the N-S faulting in Tanjore region have played a significant role in controlling the geomorphic processes, evolution and therefrom the eco system of the area. As a result deltas with multivariate morphology were formed in Palar, Ponnaiyar, Cauvery, Vaigai and other rivers. (Fig. 2) along Tamil Nadu coast.
In areas of cymatogenic arching in Chennai and Ramanathapuram – Rameswaram region (Fig. 1, 2) beach ridges are found in wider breadth of 3 to 40 km. On the contrary, the beach ridges found to a breadth of 50 kms in Point Calimere / Vedaranniyam region (Fig. 1, 2) is attributed to the tectonic emergence of the land along the N-S faults in the area east of Tanjore. That is because of the tectonic emergence, the land has slowly prograded pushing away the sea gradually. In addition gradual shrinkage in backwater, withdrawal of creeks and estuaries, heavy sedimentation of backwater, sediment discharge into the ocean are found. Whereas in areas of tectonic subsidence (Pondicherry and Manamelkudi) flooding, gradual increase in tidal activities, mangrove spreading and storm surges are witnessed.
The fragile eco system of the Tamil Nadu coast is thus well recorded and appreciated from the geomorphic features.
For example, in tectonically rising Vedaranniyam (Point Calimere) region the study of satellite imagery (Fig. 3) and the followed up C14 dating of the beach ridges (Fig. 4) shows that the sea has receeded or the land has prograded to the tune of 55 kms (from Chettipulam to Kodiaykkarai) in between 6000 and 1000 years BP (Before Present) i.e. at the rate of overall 11 Mts. per year. The digitally processed IRS 1A imagery (Fig. 3) shows that rapid sand building activity is going on and off shore shoals have been built unto 22 km from Kodiyakkarai in the northwest towards Jaffna (Sri Lanka) in the south east during the last 1000 years. Such rapid land progradation and the sand building activity is attributed to land emergence along N-S faults in the area east of Tanjore and the gradual envelope of sand bodies by the violently active littoral currents.
Similarly, along E-W trending Ramanathapuram – Rameswaram cymatogenic arching, because of such land emergence not only the sea has receeded over 40 – 50 kms leaving bundles of beach ridges but the Rameswaram island has also tilted towards northerly.
Such critical observations made on the over all ecological systems of Tamil Nadu coast in general and Vedaranniyam and Rameswaram area in particular were animated using PC ARC/INFO.
The mapping of the beach ridges from satellite data (Fig. 3) and the C14 dating of the beach ridges (Fig. 4) have shown that the land progradation is to the tune of 11 meters per year during the last nearly 6000 years. The same has been animated using ARC / INFO GIS. It has shown that India will get welded with Jaffna (Sri Lanka) through sand banks in another 300-350 years (Fig. 5). The probable sand accumulation pattern at of every 100 years have been shown in different colours.
Similarly, the land – water distribution pattern were analysed between 1919 and 1990 for Rameswaram island by analysing oldest toposheets, aerial photographs and IRS images. These were amalgamated using PC ARC/INFO (Fig. 6). The same has shown that
- a river was earlier flowing in between Ramanathapuram and Rameswaram island which has gradually migrated towards northerly and got totally choacked due to sand advance
- the land – water distribution of four periods from 1919 to 1990 and their GIS integration shows that (Fig. 6) the water bodies are gradually shifting towards northerly indicating the gradual tilting of Rameswaram island towards northerly
Thus, the paper narrates the eco systems of Tamil Nadu coast, its modifications and the feasibility of animating the same using remote sensing and spatial technology for futuristic planning.