Home Articles Monitoring changes in Sunderban Mangrove Forest using RS/GIS

Monitoring changes in Sunderban Mangrove Forest using RS/GIS

Atanu Kumar Raha

Introduction :
The famous “Sunderbans” region cover the southern parts of Gangetic delta bordering the Bay of Bengal. Upto 14th August 1947, the entire forest area of Sunderbans was under one single forest division, namely Sunderbans Division with Headquarters at Khulna, now in Bangladesh. After the partition of the country, the western part of Sunderbans, confined within the then 24-Parganas district of West Bengal, came to be known as “24-Parganas Division” with Headquarters at Alipore, Calcutta. Spatially, nearly one-third of the total Sunderbans forests lie within the Indian jurisdiction and the rest is under the control of Bangladesh. The Reserved Forests of Indian Sunderbans is bounded on the north and west by cultivated lands/settlements, on the east by the rivers Kalindi, Raimangal and Harinbhanga, and on the south by the Bay of Bengal. The islands in the rivers of Muriganga, Saptamukhi and Thakuran are also included in the Sunderbans forests. The Sunderbans forests are situated between the latitudes 22°15′ and 21°30′ North and 88°10′ and 89°51′ East. The total area of Sunderbans forests in Indian part is 4262 sq km, out of which 2585 sq km has again been reconstituted as Sunderbans Tiger Project from the year 1973. (Land area = 1680 sq.km and water area = 905 sq km). The river Matla separates the Tiger Project from the 24-Parganas(S) Forest

Division.

  • Chief Conservator of Forests and Director, Sunderbans Biosphere
  • Reserve, Forest Dept., W.Bengal, India

General Information of the area :-
A close network of rivers, channels and creeks intersect the entire Sunderbans forest area comprising hundreds of islands which get either partially or fully inundated during the diurnal high tides. The existing large rivers running from north to south are the remnants of the old courses of the Ganges. The main current of Ganga had gradually shifted eastwards over the last few centuries. A major tectonic movement in the sixteenth century might have caused lifting of the upper crust towards the west, thus forcing the Ganga to drain mostly through its eastern channel Padma, flowing through Bangladesh (Cartis, 1928). The rivers Matla, Saptamukhi and Thakuran have practically no connection with their original mother streams and are now inlets of the sea. The sources of all the rivers in the western part of Sunderbans have been progressively silted up, thus disconnecting the inflow of fresh/sweet water into the mangrove delta. This has resulted into increased salinity of the river waters and making them shallower over the years. On the other hand, during the ebb tides, the receding water level causes scouring and creates innumerable number of small creeks, which normally originate from the centres of the islands. The receding water, while draining into the Bay of Bengal, carries large volumes of silt load. These silt, on the other hand, get deposited along the bank of the rivers and creeks during high tide, resulting into increase in height of the banks as compared to interior of the islands. As a result, high tide cannot normally reach the interior of home of the islands.

Topography: –
Sunderbans delta is one of the most dynamic ecosystems of the world. Erosion of banks and formation of new islands is continuously going on and the mangrove vegetation is subjected to ever changing salinity of water, soil texture, tidal actions as well as ecological factors arising out of increasing biotic pressure in the western and northern fringes of the Reserved Forests. Lands in the sea faces are continuously denuded by tidal waves. Towards the west, new land formation is taking place due to heavy silt load deposition from Hooghly River and its affluent Muriganga at their confluence with Bay of Bengal.

   

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.

   

Methodology
IRS IC/ID, LISS-III data of December, 1999, March 2001 in 4 bands (Green, Red, Near IR and SW IR) were requisitioned from NRSA, Hyderabad for the scenes (row-path nos.) 107-57, 108-57& 108-56 which covered the entire Sunderbans mangroves of Indian portion and major parts in Bangladesh. After applying the histogram stretch and appropriate brightness contrast, the standard FCC was classified through supervised classification using “Maximum Likelihood classifier”, on the basis of known ground truth points (A.O.I.). The entire imagery was classified with the following classes: dense and open mangrove vegetation, water with high sediment, clear water, non-forest uses, sands and muddy channels/mud flats. Extensive Ground Truth Verification was carried out by the author with the help of field officers and staff of Sunderbans Tiger Reserve.

The Reserved Forest boundaries of 24-Parganas Forest Division and Sunderbans Tiger Project were digitised, geo-referenced using ARC/INFO software and overlayed on the classified imagery, both the raster and vector coverage being in polyconic projection with 88°E & 24°N as central meridian / projection origin.

Results & Discussions
The objective of the study was to clearly delineate the dense, open and degraded mangrove areas as well as to re-map the courses of rivers and creeks, which have undergone major shifts over the last few decades.

Analysis of the classified imageries shows that the effects of erosion by ebb tides and formation of muddy channels gradually extending to the interior of the islands, is very high in Goasaba 1, 2 and 3 blocks of STR. Goasaba-3 is the most affected compartment, which contains large patches of mud flats at the heads of these muddy channels. Though there is no harvesting or felling of trees in these parts of STR, yet considerable parts of the block is either devoid of trees, or is open type of forest with sporadic growth of bushes or dwarf trees like Ceriops decandra ( local name-Jhanti Goran ).

The density status of Sunderbans mangrove, including the Tiger Project and Territorial Forest Division is as follows: –

Class name   Dec 1999 March 2001
Dense Mangrove   = 1570.35 sq km  
Open mangrove   = 577.70 ,,  
Sand/ Grass/ Charland   = 180.59 ,,  
Non Forest use   = 26.57 ,,  
Clear Water   = 109.58 ,,  
Muddy Water   = 1817.59 ,,  
Total   = 4282.38 ,,  

Such detailed mapping of Sunderbans estuary, showing the periodic changes and the latest positions of river courses, landmasses and water bodies, as well as quantum of dense and open mangroves, could be completed on real-time basis, which was not possible in the past. While conventional survey allows such detailed mapping, perhaps once in five decades, the new technology as mentioned in this paper allows such detailed mapping of a highly dynamic, inaccessible and fragile ecosystem at least once a year. The cost involved was also reasonably moderate which includes the data cost, paid to NRSA, cost of hiring one Project personnel, office expenses and cost of field verification. The cost of such mapping and survey is also low as compared to conventional manual method. While calculating the cost, the overhead cost of computer hardware & GIS software has not been taken into account as the hardware and software have been in use for many other jobs.

Recommendations: –
It is recommended that regular mapping, at least biennially, of Sunderbans forests be carried out, using the GIS and Remote Sensing technology, to monitor the changes in Sunderbans ecosystem which is the Habitat of the highly endangered spp of Royal Bengal Tiger. The utility of this technology in real-time monitoring of the changes in the remote and mostly inaccessible habitat has been proved beyond doubt.

References

  1. Working Plan Division, West Bengal – First Working Plan for the 24 Parganas Forest Division, 1949-50 to 1958-59, Southern Circle, Forest Deptt.
  2. Field Director, Sunderban Tiger Reserve- Draft Management Plan of STR for the period 2000-2001 to 2005-2006.
  3. W.W.Hunter- A Stastical Account Of Bengal, Sunderbans- Vol. I, Pt. II, W.W.District Gazetteers, Deptt of Higher Education, G.O.W.B. ( 1998 )
  4. A.B.Chaudhuri- Wetland Ecology- Resources, Research and Conservation- An Indian Scenario- MEPS Publishers (1998 )