Dhaka, Geology

Dhaka, Geology the capital city of Bangladesh, was founded about 400 years ago by the side of the river buriganga. The earliest available map shows Dhaka extending over an area of about 1.5 sq km near the junction of the Dholai Khal and Buriganga river. Large-scale urbanisation was initiated by the British Raj in 1904 when Dhaka was made the capital of East Bengal, a newly created province of British India. Dhaka gained city status in 1947 when it was made the capital of East Pakistan and by that time stretched over an area of about 40 sq km. The importance of Dhaka increased exponentially after 1971, when it became the capital of independent Bangladesh. As a result the city expanded phenomenally and according to the census of 1991 the area and population of Dhaka Megacity or Dhaka Statistical Metropolitan Area (DSMA) were 1,600 sq km and 6.83 million respectively. According to the same census the area under the dhaka city corporation (DCC) was 360 sq km, with a population of 3.39 million. The present population of DSMA is about 9.0 million (2001).

Dhaka is situated between latitudes 23°42' and 23°54'N and longitudes 90°20' and 90°28'E. The city is bounded by the rivers Buriganga to the south, turag to the west, balu to the east and Tongi Khal to the north. The city has three distinct seasons: winter (November-February), dry with temperature 10° to 20°C; the pre-monsoon season (March-May), some rain and hot with temperature reaching up to 40°C; and the monsoon (June-October), very wet with temperatures around 30°C. Dhaka experiences about 2,000 mm rain annually, of which about 80% falls during the monsoon.

Urbanisation in Dhaka is restricted mostly to the north bank of the river Buriganga. The four-hundred-year history of Dhaka city can be divided into 5 different stages of development: Pre-Mughal period, Mughal period, British period, Pakistan period and Bangladesh period.

Rapid urbanisation without considering the geological aspects has brought significant changes in the geo-environment of the city area. Waterlogging, pollution, change in the hydrogeological system, localised land subsidence and building collapse are the hazards associated with these changes in the geo-environment. groundwater withdrawal has increased more than 900% over the last 30 years resulting in groundwater mining and lowering of the water level by 20m. Water resources of the city are being polluted by the indiscriminate disposal of untreated industrial and municipal wastes in swamps and natural channels in and around the city.

Geology Dhaka is situated at the southern tip of a Pleistocene terrace, the madhupur tract. Two characteristic geological units cover the city and surroundings, viz Madhupur Clay of the Pleistocene age and alluvial deposits of Recent age. The Madhupur Clay is the oldest sediment exposed in and around the city area having characteristic topography and drainage. The major geomorphic units of the city are: the high land or the Dhaka terrace, the low lands or floodplains, depressions and abandoned channels. Low lying swamps and marshes located in and around the city are other major topographic features.

The subsurface sedimentary sequence, up to the explored depth of 300m, shows three distinct entities: one is the Madhupur Clay of the Pleistocene age, characterised by reddish plastic clay with silt and very fine sand particles. This Madhupur Clay unconformably overlies the dupi tila formation of the Plio-Pleistocene age, composed of medium to coarse yellowish brown sand and occasional gravel. The incised channels and depressions within the city are floored by Recent alluvial floodplain deposits and are further subdivided into Lowland Alluvium and Highland Alluvium.

Structure and Lineaments The Dhaka city area does not show any surface folding. However, a large number of faults and lineaments have N-S, E-W. NE-SW, NW-SE trends recognised from air photo interpretation and the nature of the stream courses. All four sides of the city are bounded by major faults.

Seismicity and Neotectonics Bangladesh is divided into 3 seismic zones based on the vulnerability to earthquakes and possible severity of damages. In all the classifications Dhaka city and its surroundings are shown to be situated in the seismic zone 2, the medium risk/hazard zone. Neotectonic movement in and around the city has been reported widely. The Madhupur Tract as a whole is a structural high in which the Dhaka-Tongi block is the most uplifted part. The boundaries of the tract to the west, south and east are characterised by step faulting.

Geotechnical Characteristics of the Madhupur Clay in Dhaka city and its surroundings vary significantly both aerially and vertically. The evaluated parameters, particularly its low strength and high compressibility values indicate that the clay, to some extent, is problematic for engineering construction. The moisture content and plastic limit results show that Madhupur Clay is normally consolidated to over-consolidated. The clay is normal to active and has intermediate to high plasticity. The compressibility values suggest that the clay is very low to highly compressible at different locations.

Hydrogeology The Dupi Tila sands aquifer is the main source of water in Dhaka city. Madhupur Clay overlies the aquifer with a thickness of 8 to 45m (averages 10m). The aquifer varies in thickness from 100 to 200m (averages 140m). The groundwater level has dropped by 6m in the last seven years. The water table was 52 meters below mean sea level in 2011. Under the present conditions the peripheral rivers act as sources of recharge as the Dupi Tila sands are exposed along the riverbeds. Other sources of recharge are vertical percolation of rain and flood water, leakage from water mains and the sewer system and seepage from standing water bodies within the city. [Sifatul Quader Chowdhury]