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Dam a structure built across a stream, river, or estuary to store water. A reservoir is created upstream of the dam to supply water for human consumption, irrigation, or industrial use. Reservoirs are also used to reduce peak discharge of floodwater, to increase the volume of water stored for generating hydroelectric power, or to increase the depth of water in a river so as to improve navigation and provide for recreation. Dams are usually of two basic types - masonry (concrete) and embankment (earth or rock-fill). Masonry dams are used to block streams running through narrow gorges, as in mountainous terrain; though such dams may be very high, the total amount of material required is much less. The choice between masonry and earthen dam and the actual design depend on the geology and configuration of the site, the functions of the dam, and cost factors.

Auxiliary works for a dam include spillways, gates, or valves to control the discharge of surplus water downstream from the reservoir; an intake structure conducting water to a power station or to canals, tunnels, or pipelines for more distant use; provision for evacuating silt carried into the reservoir; and means for permitting boats or fish to cross the dam. A dam therefore is the central structure in a multipurpose scheme aiming at the conservation of water resources.

Water levels in the reservoir upstream is controlled by opening and closing gates of the spillway which acts as the safety valve of the dam. In addition to spillways, openings through dams are also required for drawing off water for irrigation and water supply, for ensuring a minimum flow in the river for riparian interests downstream, for generating power, and for evacuating water and silt from the reservoir. sediment in rivers seriously influences the effective life of a reservoir and therefore the financing of a dam. Some modern dams have been rendered useless for storing water because the reservoir has been filled up with silt. In many others, effective storage capacity has been seriously reduced. Usually the heavy silt-laden floodwater is allowed to pass through the sluices and only the cleaner water at the end of the flood season is stored.

The history of dams has been traced to about 2900 BC when a 49-ft (about 15m) high masonry structure was built on the Nile at Kosheish to supply water to King Menes' capital at Memphis. Evidence exists of a masonry-faced earthen dam built about 2700 BC at Sadd-el-Kafara, about 19 miles (about 30.5 km) south of Cairo. This dam failed shortly after completion when it was overtopped by a flood in the absence of a spillway. The oldest dam still in use is a rock-fill structure about 20 ft (6.1m) high on the Orontes in Syria, built about 1300 BC.

Dam construction evolved quite independently in Asia. In 240 BC a stone crib was built across the Gukow river in China; this structure was 98ft (29.89m) high and about 985ft (300.23m) long. Many earthen dams of moderate height (in some cases, of great length) were built by the Sinhalese in Ceylon after the 5th century BC to form reservoirs or tanks for extensive irrigation works. India and also Bengal were experienced in dam construction since that time. But none of those dams exist. In Bangladesh, a number of dams have been constructed for different purposes. Kaptai dam, Meghna crossdam and Feni closure dam are the most important. In the recent past Bangladesh has also installed a few rubber dams.

Kaptai Dam The most notable dam of the country which has been constructed on the karnafuli river at Kaptai in Rangamati district, 65 km upstream of Chittagong. It is 670.56m long and 45.7m high. The width at the foundation level is 45.7m and at crest level 7.6m. The crest level is 36m above mean sea level (MSL). The quantity of total excavation was 113,400 cu m (4.05 million cubic ft). Since it is an earthen dam a 16-gated spillway (each 12.2m by 11.3m) with discharge capacity of 625,000 cusec has been constructed on the left side of the main dam. The catchment area of the reservoir is 11,000 sq km. Average annual rainfall is 2,500 mm over the catchment and average annual flow to the reservoir is 12 million ac-ft. The full reservoir level above mean sea level is 33.23m and the reservoir area and capacity at this level are 777 sq km and 5.25 million acr-ft, respectively. The dead storage at 23.16m above MSL is 1.18 million ac-ft. Flood storage capacity is 0.83 million ac-ft.

The generation of hydroelectric power is the main purpose of the lake. Other utilities include navigation, flood control, fishery, recreation and tourism. The present generation capacity of the karnafuli hydro power station is 230 MW during peak load hours. The downstream flood peak in the Karnafuli river can be reduced by 50% by storing water in the reservoir. Usually all floodwater during May to August is stored. A cargo transfer system with overhead trolleys facilitates transfer of boats, timber logs and other commodities from the reservoir side to the downstream side in the river. The panoramic view and boating facilities of the reservoir upstream of the dam offer a good recreation area and attract many visitors.

The Karnafuli Hydro Power Station was first contemplated in 1906 and a brief reconnaissance was carried out. The study was taken up again in 1923. In 1946, EA Moore submitted a report recommending a dam site at Barkal about 65 km upstream of the present dam site at Kaptai. In 1950, Merz Rendal Vatten, Consulting Engineers, proposed a site for the construction of a dam at Chilardak, about 48 km upstream of Kaptai and in 1951 government engineers proposed a site at Chitmoram about 11 km downstream of Kaptai. Finally, in 1951 under the guidance of Khawja Azimuddin, then Chief Engineer (Irrigation), the dam site was selected at Kaptai. Preliminary work started in 1951 and the government engineers did some physical work. The US Government agreed to assist in the implementation of the project and in 1952 the International Engineering Co Inc (IECO) was engaged for a study of the project through the International Cooperation Administration of the USA. On the basis of these studies the project was taken up and IECO was appointed for the engineering services of the project. Utah International Inc was engaged as the construction contractors and started working in October 1957.

In 1962, the initial phase of construction was completed, comprising the dam, spillway, penstock and powerhouse with a generation capacity of 80 MW by means of two units, each with a capacity of 40 MW. The third generating unit with an installed capacity of 50 MW was completed in November 1981. The project was financed by government, ICA and DLF loan and the total cost including the dam with ancillary structures, Units 1, 2 and part of Unit 3 was Rs 503 million with a foreign exchange component of Rs 171.6 million. The government and USAID financed the remaining cost of the third unit. The total cost was Tk 274 million with a foreign exchange component of Tk 156 million. In October 1988 construction of the 4th and 5th units of the Kaptai station was completed raising the total generation capacity to 230 MW. The total cost of the extension work was Tk 1,900 million, which included a foreign exchange component of Tk 1,080 million.

Although the Kaptai dam has provided increased economic opportunities which include flood control and power generation, many local inhabitants have lost their homes and means of livelihood owing to inundation, making way for a storage reservoir. There has also been a general destruction of wilderness and open spaces, and an accompanying loss of wildlife and wildlife habitats.

Meghna Cross Dams A major channel of the meghna used to flow between Ramgati and the Noakhali mainland. The channel gradually silted up over a period of 20 to 30 years. In 1957 the Irrigation Department constructed a 12-km long embankment, known as Meghna Cross Dam 1, across the channel to divert the flow westward. The construction cost of this dam was Rs 1.4 million. As a result a new land area of 207 sq km was formed in the slack water adjacent to the cross-dam up to 1964. In 1964 the bangladesh water development board constructed another cross-dam having a length of 16 km, Meghna Cross Dam 2, between Char Jabbar and mainland of the Noakhali at a cost of Tk 9 million. This led to reclamation of another 563 sq km of land up to 1985 and the total reclaimed land area increased to 717 sq km.

Feni Closure Dam A 3.41-km closure dam constructed across the feni river in 1965-66 at a cost of Tk 593.5 million to divert the flow through the Feni regulator. This closure dam is located in Mirsharai and Sonagazi, respectively in the Chittagong and Feni districts. The Feni regulator with 40 gates was constructed to prevent saline water intrusion from downstream and retain fresh water upstream for use in the Muhuri Irrigation Project.

Rubber Dams These are made of reinforced rubber bags inflated either by water or air and anchored to the channel bed for retention of water in small rivers and stream channels. The body of the rubber dam is made of rubber reinforced by woven synthetic fabric. The reinforcement provides the tensile strength with rubber acting as the adhesive and water-proofing element. The fabric reinforcement is used in layers as decided by the design strength requirement. Rubber for the dam is produced in long strips of fixed widths dependent on machines producing the fabric. The strips are then joined by using glue and pressing by machine. Using steel clamps or concrete wedge blocks the dam bag is anchored to a concrete base prepared on the river bottom.

Compared to concrete barrage and regulators rubber dams are cheaper and more flexible and can be fully deflated during the flood season providing unobstructed passage to flood water. Rubber dams can be as long as 100m and used for irrigation and rural water supply. These can also be used for small-scale hydropower generation in hilly streams. Recently the Local Government Engineering Department of the Bangladesh government has constructed two rubber dams on a pilot basis in Cox's Bazar district with the technical assistance of IWHR of China. Construction of one dam at Bakkhali and the other at Eidgaon was started in February 1995 and completed in May 1995.

The bakkhali river at the dam site is tidal and located at Jhilwanja union about 3 km from Cox's Bazar. The dam conserves fresh water upstream and prevents saline water intrusion from downstream. The retained water is used for irrigation of Boro rice by lowlift pumps from January to May. The Eidgaon Khal is non-tidal at the dam site near Eidgaon Bazar on the Chittagong-Cox's Bazar highway, about 30 km from Cox's Bazar. The dam is operated to retain stream flow in channel storage, which is used for irrigation by gravity for growing boro rice. [M Fazlul Bari]

See also barrage; embankment.