Abstract No : 110

Sustainable development concerns, indicators, sustainable development of coastal and marine resources

Land Subsidence- A New Dimension In Coastal Hazard Of Bangladesh

Md. Bazlar Rashid Rashid (1) and Mohammad Feruj Alam Alam (2)(1) Geological Survey of Bangladesh, 1000 Dhaka, Bangladesh
Telephone: +880-01720614921 Email: bazlarrashid@ymail.com(2) GSB, 100 Dhaka, Bangladesh
Telephone: +88-01720614921 Email: bazlarrashid@ymail.com

Abstract No : 110 Sustainable development concerns, indicators, sustainable
development of coastal and marine resources Land Subsidence- A New Dimension In
Coastal Hazard Of Bangladesh Md. Bazlar Rashid Rashid (1) and Mohammad Feruj Alam
Alam(2) (1) Geological Survey of Bangladesh, 1000 Dhaka, Bangladesh Telephone:+880-01720614921 Email: bazlarrashid@ymail.com (2) GSB, 1000 Dhaka, Bangladesh
Telephone: +88-01720614921 Email: bazlarrashid@ymail.com A number of fractures anddepressions have been created in the south-western coastal part of Bangladesh, createdpanic among the local people. All the media reported this phenomenon with outmost
important. Geoscientists discussed about the causes and ultimate results of theseoccurrences. In this connection, the area has been investigated from 26 to 31 July, 2011
to unveil the hidden facts of the occurrences. Topographically, the area is flat with low
relief. Average elevation of the area is about 1 meter AMSL which slopes gently towards
south. The area is blanketed with recent alluvial deposits. The deposits are consisted ofabout 6 meters thick clayey silt/silty clay layer which is underlain by a peat/peaty clay
layer of about 4 meters thick. Peat is comprised with fully to partly decomposed and soft
vegetal matters. Two types of depressions, Circular depressions and Elongateddepressions- are found. The radius of the circular depressions is generally 1.5 meters andthe depth is 2.5 meters. These depressions are commonly funnel-shaped and are found tooccur in bamboo bushes. The elongated depressions are about 6 meters long and 1 meterwide with an average depth of about 1 meter. These depressions are occurred in big tree
gardens. The possible factors responsible for land subsidence are : sub-surface geologicalconditions, long prevailing summer, excess withdrawal of ground water and heavy
rainfall after long summer. There is no relation of the formation of these depressions toany neo-tectonic activity. Meteorological data confirm that, there is no considerable
amount of rainfall in the dry season. As a result, ground water table dropped naturally
well below the Peat/Peaty clay layers, might have caused lowering of water table for along period. As a result, the Peat/Peaty clay layer became totally dry creating a gap
between the peat layer and the overlying Clayey silt/Silty clay layer. Besides, numerous
fractures in the Peat/Peaty clay layer as well as in the overlying Clayey silt/Silty clay layer
might also have created. With the heavy rainfall, surface water was possibly intruded tothe previously formed subsurface gap through the big tree roots, rat/fox holes andsurface fractures. The intruded water began to flow through the gaps and fractures. As aresult, subsurface erosion of peat and sediments was taken place by the flowing water
which drastically reduced the total sediment volume leading to the occurrences of theseland subsidences. The shapes and sizes of the subsidence depend on the subsurface
erosional pattern. It is noticed that these types of depressions are local phenomena.
There is no possibility of large depression on regional scale. This is an ongoing process ofthe delta building activities.