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Monitoring changes in Sunderban Mangrove Forest using RS/GIS
Mapping of Sunderbans: -
According to Mr. Curtis, IFS, who wrote the Third Working Plan of Sunderbans Division (1931-32 to 1950-51), the first survey of Sunderbans was done by M/s. Titchie, Richards and Martin during 1769-1773. In their maps (1" = 5 miles), only the largest waterways were recorded. Lt. Prinsep surveyed the boundary between forest and cultivation during 1821-23. Lt. Hodges continued this survey in 1829 and by 1831, prepared and published a map in 1" = 2 miles scale. In 1841, Capt. Lloyd surveyed the sea face and in 1850, Capt. Smith resurveyed and demarcated the earlier surveyed boundary lines. Revenue Surveys were made for these areas between 1851-63 and on the basis of all these survey data, Mr. James Ellison published a complete map of Sunderbans in 1873 on 1" = 2 miles scale. During 1905-08, the forests were surveyed in detail by the Bengal Provincial Survey Department at an estimated cost of Rs.2,15,236/= at that time. The cost was borne by the Forest Dept. and maps on 1" = 1 mile scale were prepared and published by Col. Longe in 1909. These maps were subsequently enlarged to 2" = 1 mile scale which was made available by Survey of India. The latest revised sheets of these areas were published in 1924, incorporating the accretion and erosion that had taken place meanwhile. The District Land Revenue Office had subsequently published an updated map in 1964. Forest Survey of India had carried out an aerial survey of Sunderbans and had mapped the saline blanks within the forest areas, which was the available latest authentic map till 1999 when detailed mapping of Sunderbans was taken up using satellite imageries.
Forest stock maps :-
The most authentic and detailed stock map0s of forests were prepared by Mr. Curtis for writing the Third Working Plan (1931-32 to 1950-51) of Sunderbans. He was supposed to have spent an amount of Rs.1,71,000/= in those days for carrying out the detailed surveys in an extremely hostile and inaccessible terrain. Later, the Working Plans were revised twice, once in 1949-50 and again in 1997-98 both of which adopted the stock maps prepared by Mr. Curtis in 1924-28 as the basis of management prescriptions. The major difficulties faced in regular updation of forest stock maps and topographical details were the (i) inaccessibility of the terrain due to presence of innumerable creeks and water-bodies, (ii) presence of man-eating tigers in the forests and the crocodiles in the rivers (iii) exorbitant costs involved in detailed manual surveys (iv) extremely long duration in carrying out such manual surveys in the hostile terrain, (v) lack of infrastructure and manpower in the forest Dept. for undertaking such massive jobs.
The forest areas in Sunderbans could be mapped in some detail only in 1988-89, after a gap of nearly six decades, when the State Forest Department took up the project of forest cover mapping of the entire state using IRS-1A satellites, in collaboration with Regional Remote Sensing Service Centre, Kharagpur, Dept. of Space. The study showed that a lot of changes had taken place in the quality of the forest cover of Sunderbans and had indicated the prospect of application of the Remote Sensing Technology in detailed and real-time mapping of the terrain.
Geology and River Systems: -
The geological formation of Sunderbans is of comparatively recent origin (Curtis, 1924). Till a few thousand years back, the whole tract was under the sea. The deposition of debris and formation of Sunderbans delta occurred recently with the change of the main course of river Ganga from the Bhagirathi to Padma towards the east between the 12th and 15th century A.D. This was the result of Bengal basin suffering some neotectonic movement (Morgan & Melntire, 1959, Mgmt. Plan of S.T.R. during 1997-98) and an easterly tilt. During 16th Century, the flow of Ganga shifted almost totally eastwards into river Padma (now in Bangladesh) and the Matla/ Bidyadhari rivers which had formed innumerable network of creeks in the delta, get completely cut off from the sweet water sources. These rivers are mostly fed by the backwater of sea. The erosion of the coastline between the Raimangal and Saptamukhi rivers went on continuously from the later part of nineteenth century to the beginning of 20th century, while accretions occurred only at the mouth of Muriganga and Hooghly.
Construction of a large number of dams and barrages in the Damodar river catchments and the Ganges has resulted into decreased silt load and lesser deposition of the debris in the down stream and into the estuaries. Most of the rivers draining into the Sunderbans estuary have lost contact with their original sources, and there is hardly any inflow of fresh/ sweet water with silt load into the Sunderbans. During the peak high tide, highly saline sea water inundates the numerous islands while during other times, the water fails to reach the interior of the islands due to raising of bank levels, as mentioned earlier, and due to lack of normal fresh water flow in the river systems. On the other hand, during the ebb tide, the receding water from the interior of the islands scour the top soil and from channels connecting with the rivers / creeks. These eroding actions of ebb tides are more prominent in some of the islands of Sunderbans as compared to others. With passage of time, these eroded channels keep on extending further inwards and result into formation of muddy blanks. As these blanks are not regularly flooded by high tides, capillary action of the clayey soil and excessive heat during dry time results into deposition of salt crust at the surface and turns them into saline blanks and prevents natural regeneration of mangrove spp.
Working Plan: -
Working Plan is the document of the forest Department, written for each forest division, which incorporates the spatial, quantitative and qualitative information of the forests under the control of the Division. The Working Plan is an extremely vital document since it presents the various strategies for the systematic management of forest, to be followed for the next ten years or so. Working Plan prescriptions are prepared on the basis of forest stock map, which contain information on extent, density, spp composition, forest blanks, topography, soil etc.
The earliest official effort to preserve and manage the forests of Sunderbans deltas dates back to 1878 when the forests were declared as Protected Forests following the efforts of Mr. A.L.Home, DCF, Dr. Schlich and Sir Richard Temple (1st Working Plan of 24 Parganas Division.).
Due to inaccessibility of the hostile Sunderbans terrain, it is almost impossible even for the Forest Department staff to carry out land-based survey of the mangrove forest and enumeration of the crop. To overcome these physical problems, remote sensing technique was tried out for the region to test the effectiveness of the technology in monitoring the mangrove ecosystem of Sunderbans.
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