Embankment

Embankment a ridge built with earth or rock to contain flood water or to construct a road, railway, canal. Embankments vary in nature and function under a variety of situations. Designed to control or prevent flooding, flood control embankment is one of several types of embankments on the floodplains. An embankment built to prevent flooding of low-lying land is also called a levee or dyke constructed along a riverbank and at some distance from the river to retain floodwater. It may or may not have an impervious core. The earliest recorded embankment in this subcontinent was built during the Sultani period (1213-1519 AD). Sultan Ghiyasuddin Iwaz Khilji built a series of embankments to protect his capital, Lakhnauti from floods. The Grand Trunk Road, which has a length of about 150 miles (240 km) and built during his time, also acted as a flood control embankment. The Mughal emperors constructed embankments along different large rivers.

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In Bangladesh coastal embankments were constructed as early as the 17th century on private initiative under the patronage of zamindars. Systematic development of large-scale embankments for flood control started in the 1960s. Since then hundreds of kilometres of embankments have been built along rivers and in the coastal areas of Bangladesh. These embankments provide a protected environment for agricultural and other economic activities.

During the last few decades, under the programme of flood control and drainage improvement, about 7,555 km of embankment (including coastal embankments of about 4,000 km), 7,907 hydraulic structures including sluices, and around one thousand river regulators, 1,082 river closures and 3,204 km of drainage channels have been built spending a thousand crore taka. Under the scheme a total of 332 projects, aimed at freeing 3.5 million ha of land from flood water, have been implemented. Thus, about 24% of the total land area and 39% of the net cultivated area have been protected. However, in this programme some form of natural detention basin, channel improvements, flow diversions and bank stabilisation and anti-erosive measures have been tried. Other than the flood control embankments on the floodplains, the railway and national road embankments constructed during the colonial period played a major role in flood mitigation. A brief account of major embankments in Bangladesh is given here.

Brahmaputra Right Bank Embankment One of the first embankments constructed in 1960s to provide flood protection to about 230,000 ha lying on the western side of the Brahmaputra-Jamuna and tista rivers. It is 217 km long and extends from Kaunia in Rangpur at the northern end up to Bera upazila in Sirajganj district at the southern end. Construction of the embankment started in 1963 and was completed in 1968 at a cost of about Tk 80 million. The average height is 4.5m, crest width 6m and side slope 1:3 on both sides. The embankment has been under constant threat of erosion by the jamuna river and needs relocation further away from the riverbank.

Gumti River Embankment located in Comilla district and comprises 67 km of embankment on the left bank of the river gumti from Katak Bazar and 64 km on the right bank from Golabari up to Gauripur of Daudkandi. The Gumti embankment protects some 37,440 ha of land from flood. The average height of the embankment is 4.5m, crest width 4.27m, side slope 1:3 on the riverside and 1:2 on the countryside. At first, this embankment was constructed during murshid quli khan's regime (1704-1727), but in the colonial and Pakistan periods it decayed due to lack of proper maintenance. However, the reconstruction of the Gumti embankment started in 1986-87 and was completed in 1991-92 at a cost of Tk 1,234 million.

Khowai River Embankment lies in habiganj district and provides flood protection to 25,790 ha of land. This 40 km embankment on the right bank of the river khowai extends from Habiganj to chunarughat and 47 km on the left bank up to Rajabazar.

The Coastal Embankment Project (CEP) covers the coastal districts of Bangladesh and includes Cox's Bazar, Chittagong, Feni, Noakhali, Lakshmipur, Bhola, Barisal, Patuakhali, Jhalokati, Barguna, Pirojpur, Khulna, Satkhira and Bagerhat districts. The CEP comprises a complex network of dikes and drainage sluices and was the first comprehensive plan for providing protection against flood and saline water intrusion in the coastal area. The project was implemented between 1961 and 1978 by the bangladesh water development board in two phases. Phase I comprises some 92 polders providing protection to one million ha of land. Phase II consists of 16 polders covering another 0.40 million ha. Polder is a Dutch word meaning an area enclosed by dikes. Within the CEP more than 4,000 km of embankment and 1,039 drainage sluices have been constructed. Embankments include sea dikes at locations facing the bay of bengal, wide rivers and at places where high waves occur; interior dikes along rivers where wave action is less severe; and marginal dikes are along channels where current and wave action is mild. Typical dimensions of three types of embankments are as follows:

Embankment type Side slope Crest width (m) Free board (m) Set back distance (m)
Country side Sea side
Sea dike 2:1 7:1 4.2 1.5 75
Interior dike 2:1 3:1 4.2 0.9 50
Marginal dike 2:1 2:1 2.4 0.9 40

These embankments are intended to protect land from tidal inundation but cannot prevent overtopping and damage from cyclonic surges and tidal bores. In addition to increased agricultural production, these embankments have provided good road communication and contributed towards improvement of the overall socio-economic condition in the coastal zone.

Embankments have also been constructed as integral part of large irrigation projects. For example, in Chandpur irrigation project there is a 100 km embankment along the meghna and dakatia rivers, 65 km in the Meghna-Dhonagoda irrigation project along the Meghna, 50 km along the manu and kushiyara rivers in the Manu river project, and 80 km along the Tista river in the Tista Barrage Project.

The embankments often altered the natural flow pattern, which induced siltation in tidal channels and at the outlet of sluice gates. Such an effect has been most severe in the khulna region where discharge from an upland area is less. This leads to prolonged waterlogging in several polder areas. A number of studies were conducted to investigate the causes of siltation and drainage congestion. On the basis of these studies rehabilitation work in the CEP is being done since 1988 to alleviate drainage congestion and waterlogging. [M Aminul Islam and M Fazlul Bari]

See also barrage; dam; flood control drainage and irrigation projects.