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Sea-level Rise, Subsidence and Impact of Climate Change in the Sunderbans

Gautam Kumar Das

The waters of the Bay of Bengal intrude into the Sunderbans over flooding the creeks and tidal courses for years as a result of sea level rise due to climatic changes. This situation causes ecological disaster for the man and environment. Erosion and inundation, as a result of sea level changes removes sediments from the flood plain and river banks by the current and wave actions. Islands of Sunderbans face inundation that sometimes results flood in the agricultural lands and gradually become a low lying land because of submergence.

Hazards as a result of climatic changes affect the geographical location, structure and function of biotic community, livelihood of the rural islanders of Sunderbans. Further, changes of ecosystem of the Sunderbans have the major impact on the tourism, fisheries, biodiversity, fresh water supplies, and ground water availability due to intrusion of salinity.

Sea-level Rise
Continuous subsidence of the Bengal basin results much more sea level rise than that of eustatic rates of 2.2 mm/year. Global climatic change causes the sea-level rise, although the sea-level rise girdling the coastal areas of the Sunderbans is alarming. Current trend of sea-level rise along the Indian Ocean is 2.5 mm/year against the long-term sea-level rising trend of 1.0 mm/year.  IPCC (Intergovermental Panel on Climate Change) predicts their expectation on sea-level rise of about 15 - 38 cm by 2050 and 46 - 59 cm by 2100 around Indian subcontinent. World Bank Report (2000) projected the sea-level rise for the Sunderbans step by step by mentioning the percentage of inundation for the Sunderbans which is published by Lund University, Sweden (2000).

Projected height of the sea level rise and percentage of inundation of the Sunderbans


Sea-level rise (in cm)

Inundation (%)

10 cm
25 cm
45 cm
60 cm

15
40
75
100

Bangladesh faces about the double of the sea-level rise of 30 - 50 cm by 2050, on contrary to the 15-38 cm of the global average. Much greater sea-level rise of about 1 m for the Bangladesh Sunderbans coincides with the local sea-level rise of 10 cm due to subsidence with the 90 cm sea-level rise for global warming due to climatic change. About 20 islands of both Bangladesh and Indian Sunderbans including emerging New Moore Island (Purbasha) are to be undergone submergence.

Subsidence
It was not a tale of a tub when everybody who was anybody at all was at the gathering of experiences of finding in-depth tree-trunks in the Sunderbans in the past century. The local inhabitants at present are going out of their depth observing these sediment-sunk tree trunks over the centuries. Series of tree trunks are seen scattered in the most of the excavated ponds in the depth of eight to eighteen feet in the blocks of Canning, Patharpratima, Gosaba, Mathurapur, Namkhana etc. of the Sunderbans region. And they are all identified as the tree trunks of the Sundari trees. The similar incident of the occurrence of tree trunks at the depth of 30 feet below the surface level was experienced during excavation at Khulna and Sealdah in the British period. The then Director of the Botanical Gardens confirmed identifying those in situ tree trunks as Sundari trees. Experts describe both of these incidents of past and present as the evidences of subsidence. Evidence of subsidence is the existence of Sundari trees at a much lower level (3 to 6 m) than the present surface near Calcutta (presently Kolkata) and Khulna proves that the old forest areas must have subsided to that depth where tree trunks are available. Burial of earlier mangroves layers by subsequent alluvial deposits is a common and routine phenomenon in the Sunderbans. Further, tilting of landmass eastward as a result of a crack developed in the continental shelf of the Bengal basin during the Cretaceous period has created a shallow marine condition at the southern boundary of the Sunderbans and the mangroves tend to shift northwards due to natural pressure. Shifting of mangroves about 130 km north was happened to be occurred around 6500 BP as a result of transgression. Sunken tree trunk of mangroves found in the Sealdah and Khulna during excavation is the evidences for both subsidence and shifting of mangroves from the south to the north.

The reality of the facts of subsidence phenomenally is correct with respect to the geology of the Bengal Basin. The Sunderbans occupies the lower (tidal) delta plain of the Ganges-Brahmaputra Delta (GBD) of the Bengal Basin which is slowly subsiding as a result of isostatic adjustment of the crust due to rise of the Himalayas and dewatering of the Proto-Bengal Fan sediments. Further, in the Sunderbans, subsidence and uplift by means of sediment accumulation happen to be a simultaneous process. And for these processes to be verified as acid test there are numbers of methodology available and they are worked out for the determination of the phenomena of subsidence and sediment accumulation. For the interpretation of these processes, the geology of the Bengal Basin is considered where some experts in this field infers that at least a part of the Bengal Basin is subsiding at a rate of 2.2 mm/year as revealed by the well-log data analysis. On contrary, radiocarbon measurements indicate that the sediment accumulation in the Bengal Basin is taking place at a rate of 1.1 mm/year. Although average rate of subsidence and sediment accumulation all over the Bengal Basin almost run parallel to each other having rapid sediment accumulation at a rate of 0.7 cm/year marked with the land subsidence rate 0.5 cm/year.  Sediment of the lower delta plain is derived from an offshore source after having originally been supplied by the Ganges-Brahmaputra-Meghna (GBM) River systems. The Ganges has a mean annual discharge 196 to 480 million tons having suspended sediment concentration about 190-1400 mg/liter, annual sediment discharge of the Brahmaputra ranges from 387 to 650 million tons with suspended sediment concentration approximately 220-1600 mg/liter, and Meghna has the sediment discharge of about 50 million tons per year. Amount of combined sediment discharge of the three rivers (GBM) is about 1 billion tons annually. An estimated quantity of 17% sediments out of 1 billion tons supplied by the Ganga-Brahmaputra rivers is carried westward by prevailing currents and advected inland by monsoonal coastal set up and events of cyclonic storms. This amount of sediments mixing with the water enters into the tidal inlets and creeks during flood time and spill over the river flood plains. Sedimentation from stagnant water situation during the transitional period in between flood and ebb time results elevation of river bed through accumulation of sediments. It is observed in many occasions that the river bed along with the flood plains are much more elevated than that of the adjacent human habitation zone and agricultural land inside the reclaimed area where sediment-laden water does not spill over due to protected measures taken through the construction of the embankments. The embankment restricts the spilling over of sediment-laden water of the Sunderbans Rivers that carries an average of suspended solids amounting 15 mg/liter. Further, the accreting active deltaic region of the Sunderbans faces sediment compaction-induced subsidence simultaneously with the plate tectonics results subsidence of the lower delta plain. To make along story of subsidence told over the centuries short, the venture of the same old story ended beyond any emphasis. Thus, the story goes upon the Sunderbans, a part of the Bengal Basin, be on the straight, has been subsiding since its formation simultaneous with the uplift i.e. elevation as a result of sediment accumulated from the suspended solids carried by the sediment-laden river waters.

Let allow only hundred years to come when a little different story of reversible habitat from the present one will come up where human habitat zone would be water-logged area and elevated non-reclaimed zone (forest and water courses) would be befitted for the human habitat and agriculture. Sedimentation gradually clogs the creeks and tidal courses of any active delta under formation. Sunderbans is not out of that situation or an exception. Subsidence due to tectonic movement all over the Bengal Basin might be a special providence to the destiny of the Sunderbans.

Impact of Climatic Change
Climatic change has a major impact on sea level rise and the rise of the sea level is observed quite much more at Sunderbans as a result of continuous natural subsidence of the lower part of the Ganges- Brahmaputra Delta (GBD) of which Sunderbans is a part of it. The rate of sea level rise in the Sunderbans is 2.2 mm per year in comparison to the global average of 1 mm per year. There is a climatic change alert over the tigers of the Sunderbans. Prediction of only 28 cm sea-level rise from 2000 will decline about 96% of the mangrove swamp tiger habitat of the Sunderbans. Experts identify about eleven islands that will disappear in the next three decades due to rising sea level. Bulchery, Bangaduni and Dalhousie are among these three islands which are the tiger habitat under the Sunderbans tiger reserves. Saline water will gradually intrude into the coastal hinterland which will reduce the fresh water availability. Both water and soil get salinised with time. Production of crops, fruits and vegetables will be decreased. Fresh water fishes will be disappeared, although brackish water fish species will be produced more that leads to the conversion of fresh water aquaculture into the brackish water aquaculture. Luxuriant growth of mangroves will be stunted due to the adverse effects of increased salinity and humidity due to sea level rise. Saline water intrusion changes the mineral content of soil and water and this situation helps to generate series of chak-keora (S. caseolaris), a typical mangrove species alongside the riverbank of Hugli starting from Budge budge to Milleneum park of the megacity, Kolkata although the city of joy, Kolkata is straight away 130 km far from the Sunderbans. Inundation of saline water covering more area means the inundation of the habitat of benthic animals. Most of the mangroves vegetation will be lost due to permanent inundation of pneumatophores of the mangroves as a result of sea-level rise. True mangroves like Sundari (H. fomes) have been gradually reducing from the world’s largest mangrove canopy of the Sunderbans. Sea level rise due to climatic change in reality is a major threat to the biodiversity and fragile ecosystem of the Sunderbans.

Frontier
Jan 2, 2019


Gautam Kumar Das ektitas@gmail.com

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