29/03/2020
The Sundarbans lie across the outer deltas of the Ganges, Brahmaputra and Meghna rivers. At 10,000 sq.km, it forms the largest estuarine mangrove forest in the world, 40% in India, 60% in Bangladesh. The adjacent World Heritage sites in India and Bangladesh cover just over a quarter of the area. The forest is composed of small forested islands and mudflats intersected by an intricate network of tidal waterways. It exemplifies the ecological processes of monsoon rain flooding, delta formation, tidal influence and plant colonisation.The area has a wide range of rare fauna, including the Bengal tiger, estuarine crocodile and many reptiles and birds. The devastation caused by Cyclone Sidr, especially to the forest in Bangladesh, emphasised how important mangrove forests, flood refuges and an early warning system are to protection of the coastal population
Geographical Location
The Sundarbans mangrove forest lies among the creeks and distributaries of the Ganges and Meghna (Bramaputra) river.
Areas
139,500 ha. The total area of both World Heritage sites is 272,510 ha
Physical Features
The Sundarbans are part of the world's largest delta, formed from the sediments brought down by three great rivers, the Ganges, Brahmaputra and Meghna, which converge on the Bengal Basin. The forest extends over some 200 islands, separated by 15 major distributary rivers flowing north-south, and 400 interconnected tidal estuaries, creeks and canals. It forms an impenetrable saltwater swamp reaching 100-130km inland, which supports the largest tidal mangrove forest in the world, covering 10,200 sq. km. Of this 595,500ha (59.3%) are in Bangladesh and 426,200ha (42.5%) in India, 232,000ha of which is land. The landscape is one of low-lying forested alluvial islands, 56 on the Indian side, mudbanks with sandy beaches, and dunes along the coast. The area is approximately three-fifths of the 16,700 sq.km that existed 200 years ago having been cleared and converted to agriculture, especially in India (Hussain & Archarya, 1994). The forest swamp, extensively embanked and empoldered, is an essential buffer for inland areas against the ravages of frequent cyclones from the Bay of Bengal. The nutrient-rich waters also provide the most important nursery for shrimps and spawning grounds for crustaceans and fish along the whole coast of eastern India.
The landscape is dynamic, constantly moulded and altered by tidal action, with erosion along estuaries and deposition along the banks of inner creeks augmented by the discharge of silt from seawater (Sanyal & Bal, 1986). The wider rivers of this intricate network of waterways are over two kilometres wide, run north-south and tend to be long and straight. They are maintained largely by the two diurnal flow tides and ebb tides over a tidal range of 3-5m up to 8m, together with the erosion-resistance of the clay and silt of their banks (Ghosh & Mandal, 1989). Innumerable small channels drain the land at each ebb but major coastal deposition occurs primarily in Bangladesh. There is a deep submarine canyon-head depression on the eastern Bengal shelf which forms a sediment trap where sedimentation increases. The way that waves diverge over the delta undersea, especially during monsoon gales, largely depends on the changing deep wave fields accompanying them which are affected by this trough. Flood currents deflect waves more to the east, ebb currents deflect them more to the west, and are important in the redistribution of river-borne sediments along the coast (Ghosh & Mandal, 1989; Michels et al., 1998). On the coast, easily eroded sands collect at the river mouths forming banks which are blown into dunes above high-water level by strong south-west monsoon winds. Finer silts are washed out into the Bay of Bengal and where they are protected from wave action, form mud flats in the lee of the dunes. These become overlain with sand from the dunes, and develop into grassy flats. This island-building process continues for as long as the area on the windward side is exposed to wave action. With the formation of the next island further out, silt begins to accumulate along the shore of the island and sand is blown or washed away (Seidensticker & Hai, 1983). This process is most evident in the Bangladesh section of the coast.
The Sundarbans in Bangladesh are dissected by seven main north-south-flowing rivers, from the Rainagal, a branch of the Hariabhanga to the Baleswar, a mouth of the Meghna. These waterways, apart from the Baleswar River on the eastern edge of the East Sanctuary, now carry little fresh water as they are mostly cut off from the Ganges, the freshwater outflow of which over 400 years has shifted progressively eastwards from the Bhagirathi-Hooghly channels (Seidensticker & Hai, 1983). This shift is due to tectonic subsidence of the Bengal Basin during the 10th-12th centuries, and the continuing gradual eastward tilting of the underlying crust. Where the forest in India remains relatively stable and essentially land-locked, with rivers almost completely cut off from the main freshwater sources and therefore salt, the southeastern corner in Bangladesh is subsiding and is an area of active sedimentary deposition (Sanyal & Bal, 1986). The average salinity of water and soils therefore decreases markedly from west to east (Islam, 1973). The area has three main hydrological zones: brackish, moderately saline and saline, which influence the types of vegetation which dominate each. As with the rest of the Bengal Plain, the alluvial deposits are geologically very recent but deep, sediments of just the last few million years being as much as 1,000m thick (Seidensticker & Hai, 1983). The subsoil consists of alternate layers of sand and silty clay loam down to a depth of 1.1-1.4m and thereafter stiff black clay, gradually changing into shales and sandstone. The soils are saline, chiefly an alkaline clay with an excess of salt except on the seaward side of islands at the coastal limits, where sandy beaches occur (Lahiri, 1973). The pH ranges from 5.6 to 8.0 (Christensen, 1984). By comparison with the drier western Sundarbans the surface soil in the east is fertile silt loam, very suitable for tree growth (Choudhury, 1968).
Climate
The climate is humid sub-tropical, tempered by the sea. Temperatures rise from daily minima of 2-4°C in winter to over 32°C during the monsoon and a maximum around 43°C in March. The mean annual maxima and minima recorded at the Jhingakhali meteorological station were 34°C and 20°C respectively. Rainfall is heavy and the humidity averages 70-80% due to the nearness of the Bay of Bengal. The mean annual rainfall varies from about 1,800mm at Khulna, and 1,920mm at Jhingakali north of the Sundarbans, to 2002mm recorded at the observatory on the western coastal island of Sagar and 2,790mm on the Bangladesh coast. 80% of the rain falls during the monsoon between mid-June and October, purging saline soils of their salt. From then to mid March the weather is dry until mid-March, a period when evapotranspiration exceeds precipitation. During the monsoon over half the Sundarbans is submerged under water. Conditions are most saline in February to April, when the depletion of soil moisture occurs at the same time as freshwater flows from upstream are reduced. From 1983 to 2003 the annual sea level rise was 3.14cm compared with the world average of 2cm and the outer islands began to erode away. A 25cm rise in sea level would destroy 40% of the Sundarbans, and a 45cm rise by the end of the 21st century would destroy 75% (Colette, 2007). Rising sea levels also increase saltwater intrusion into aquifers.
The prevailing wind is from the north and northeast from October to mid-March, although January and February are calm. However, violent southwesterlies prevail from mid-March to September. Storms, funnelled up the shallow upper Bay of Bengal, are common in May and from October to November, sometimes developing into cyclones which can be accompanied by storm surges up to 7.5m high, causing enormous loss of life, damage to property and forests, as in 1970 and 1991. The effects on India of the 2004 tsunami, and of the mid-year monsoon floods and cyclone Sidr were less drastic than in Bangladesh. There the effects of the 2004 tsunami were very bad and those of the July and September monsoon floods and cyclone Sidr in November 2007, disastrous on a huge scale. The strongest effects were felt between the Passur and Baleswar rivers in the East Sanctuary. Cyclonic winds reached 220 kph with a 6.5m storm surge which took over 5,000 lives, affected up to twelve million people in the area, and damaged or destroyed some one million homes, livestock, rice fields, forests and the fishing industry (Bangladesh Forest Department, 2008; Indian Water Portal Blog, 2007). Such storms vividly emphasise the protective function of the coastal forest but also the vulnerability of the Sundarbans to the effects of climate change.
Vegetation
Mangrove swamp forest extends over half of the Sundarbans, the rest being largely brackish and salt water. The name comes from the dominant sundari tree Heritiera fomes, so called because of its elegance, and from ban, forest (Jain & Sastry, 1983). The vegetation consists of Malayan Peninsular and Polynesian elements, with some Indo-Chinese and Ethiopian elements, even a few from the New World. It is found nowhere else except in small parts of the Mahanadi and Godaveri deltas to the southwest and in the Bay Islands (Mukherjee, 1975). The mangrove flora of the Sundarbans, which contains 27 species, is unique in comparison with non-deltaic coastal mangrove forests. Unlike these, the Rhizophoraceae and Avicenneaceae are of only minor importance: most of the genera are from the Sterculiaceae and Euphorbiaceae (Hussain & Acharya, 1994). The dominant species are sundari Heritiera fomes, gewa Exoecaria agallocha, goran Ceriops decandra and keora Sonneratia apetala. The sundari dominate where the soil water is relatively fresh, especially in the northeast, and on higher ground, and forms 60% of the commercially useful timber. The reason for the difference is the strong influence of freshwater. Excoecaria agallocha dominates the zone of moderately saline soils; and Ceriops decandra, the saline soils. Other mangrove species include garjan or red mangrove Rhizophora mangle, R. mucronata and R. apiculata, kankra Bruguiera gymnorhiza, and baen or Indian mangrove Avicennia officinalis.
The Sundarbans are classified as moist tropical seral forest, comprised of a mosaic succession of four types of tidal forest communities: low mangrove forest, tree mangrove forest, salt-water Heritiera forest and freshwater Heritiera swamp forest, now much cleared for settlement. The pioneer vegetation on newly accreted sites is Sonneratia apetala, followed by Avicennia officinalis. The golpata or nypa palm Nypa fruticans grows on levee banks, especially if they are well established. As the ground rises with soil deposition, Exoecaria agallocha comes to dominate, and when the land is only occasionally flooded, Heritiera. Beach forest occurs on coastal islands of low very xerophytic sand-dunes due to the lime from disintegrating shells and salt. The sand dunes are partially covered with spear-grass Imperata cylindrica, with behind them, creepers and shrubs or trees such as jhao Tamarix troupii, palita Erythrina variegata and kulsi Aegiceras corniculatus. Wild rice Oryza coarctata, Nypa and speargrass Imperata cylindrica are prevalent on mud flats (Khan, 1986) The large stands of Sonneratia, which colonises new mudbanks, provide important wildlife habitat (R.Salter, pers. comm., 1987). Though mangrove forests are not very diverse, there are many climbers, creepers, algal and fungal species on the forest floor. Prain (1903) described the flora of the mangrove forest of the Ganges-Brahmaputra delta, recording 245 genera and 334 species of plant as did Seidensticker & Hai in 1983 in the Bangladesh section, listing the principal woody and herbaceous species. Chaffey & Sandom (1985) also give a detailed list of trees and shrubs in the Bangladesh section and Islam (1973) gives an account of the mangrove algal flora.
All four types of tidal forest are found here. Sundarbans West is in the salt-water zone, which supports a dense understory of Ceriops, with sparse Exoecaria and intermittent patches of hantal palm Phoenix paludosa on drier ground, riverbanks and levees. Carapa obovata, Bruguiera and passur Xylocarpus mekongensis grow sporadically throughout the area. Sundarbans South where there is the greatest seasonal variation in salinity levels has relatively longer periods of moderate salinity where Exoecaria is the dominant tree, often mixed with Heritiera, frequently associated with a dense understory of Ceriops and some Xylocarpus. Sundarbans East where freshwater and sundari predominate, has some Exoecaria and Xylocarpus, with Bruguiera in areas of more frequent flooding. The Nypa fruticans palm growing along the creeks on wet mud-banks is widespread along drainage lines. There is an understory of shingra Cynometra ramiflora where soils are drier, amur Amoora cucullata in wetter areas and Ceriops in saline soils
