Is Sunderbans Subsiding?
Gautam Kumar Das
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 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. 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 those 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 infer that at least a part of the Bengal Basin is subsiding at a rate of 2.2 cm/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 cm/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 of 196 to 480 million tons having suspended sediment concentration about 190-1400 mg/litre, annual sediment discharge of the Brahmaputra ranges from 387 to 650 million tons with suspended sediment concentration approximately 220-1600 mg/litre, 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 affected 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 cases 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 embanknent restricts the spilling over of sediment-laden water of the Sunderbans Rivers that carry an average of suspended solids amounting 15 mg/litre. Further, the accreting active deltaic region of the Sunderbans faces sediment compaction-induced subsidence simultaneously with the plate tectonics resulting subsidence of the lower delta plain. 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 one 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.
Vol. 48, No. 45, May 15 - 21, 2016