Subsidence
Subsidence a type of mass movement that involves principally a downward movement/ displacement of surface material caused by natural or artificial removal of underlying support. It is the net sum of tectonic activity, isostatic adjustment, sediment compaction, fluid withdrawal and sea level rise. Most deltaic areas experience relatively great subsidence balanced by large input of river borne sediments under natural conditions. If the river is channelled, diverted or damaged, subsidence may be uncompensated. In some areas groundwater withdrawal can lead to accelerated consolidation of subsurface materials and hence increase subsidence.
The bengal delta occupies most of the bengal basin and 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 which is now buried under thick Mio-Pliocene deltaic sediments. The rate of subsidence of the Bengal Basin and the Ganges-Brahmaputra delta varies with time and place and is influenced by the plate motion and sediment supply in the basin from the rising Himalayas. Subsurface data from the Hazipur well on the northwest of Dhaka indicate that at-least a part of the basin is subsiding at a rate of 2.4 cm/year. The Hizla-Muladi Well, located on the southern side of Bangladesh, gives an approximate rate of subsidence of 2.0 cm/year. The Hatiya Trough, Faridpur trough and Sylhet trough are possibly subsiding at the above rate or even faster. The Khulna area is presumably subsiding at rate about 4 mm/year. The rate of subsidence in Dhaka Depression around 1.88 mm/year. The depressed areas of the metropolis Dhaka are subsiding at a rate of 0.96 mm/year in Mirpur and 0.60 mm/year in Rampura. The average rate of subsidence in Dhaka city area is about 0.65 mm/year.
Evidence of subsidence of geological strata should not be considered as a general lowering of the land surface. It is a natural process in areas of unconsolidated sediments because it reflects the gradual compaction of deeply buried sediments in response to overburden pressure. Generally equilibrium exists between sediment supplied to the surface and subsidence so that the land levels do not significantly change. Activities by man, however, can create subsidence at the land surface by reducing the sediment supply and by accelerating compaction of the sediments. Large scale flood control and drainage schemes or river diversion can interrupt sediment supply while extensive groundwater withdrawals could significantly reduce subsurface ground water pressure leading to increased vertical compaction of sediments. Well-known cases of subsidence caused by groundwater withdrawals have occurred in Bangkok, Tokyo, Mexico City, Venice, and in the San Joaquin Valley of California. Large declines of groundwater levels have taken place beneath Dhaka City, but there is insufficient data to determine if significant subsidence has resulted.
The risk of subsidence and soil loss by winnowing is high in the Gopalganj-Khulna peat basins if groundwater levels are lowered. Consequently, proposal for groundwater development will have to be limited in these areas. [Md Sazzad Hossain]
Differential Subsidence when subsidence takes place at a different rate at different points. If the displacement is sudden it is referred to as collapse. Subsidence and collapse are important where removal of rock or fluids such as water or hydrocarbons takes place on a large scale. Underground mining is the most obvious cause of ground subsidence and causes much damage to land and property. However, the extent of land subsidence due to groundwater or hydrocarbon withdrawal is not up to the extent caused by underground mining. Examples of ground subsidence due to mining are numerous whereas groundwater withdrawal has caused problems in Mexico City, Venice, Bangkok, Shanghai, Taiwan, San Francisco, San Joaquin valley of California, etc.
In Bangladesh ground subsidence is not a major problem so far. As there has been no underground mining, there was no report of subsidence due to mining activities. However, very recently it has been reported in the local newspapers that ground subsidence has been taking place in the Bogra area due to the mining of sand from the subsurface using shallow pumps. A number of large ponds have developed over the last ten years in different places of Bogra. Although there has been apprehension about the ground subsidence in Dhaka due to large-scale groundwater withdrawal, there are no visible evidences of such subsidence. A study conducted in the early nineties concluded that the subsidence caused by groundwater withdrawal in Dhaka is balanced by the tectonic upliftment of the same. The reports of cracks developed in buildings from different parts of the city could be the result of soil or organic matter subsidence in swamps or peat bogs. [Kazi Matinuddin Ahmed]
