DAMS AND ITS IMPORTANCE IN WATER STORAGE

DR.S.M.ALAM
Apr 06 - 12, 2009

Dam is a barrier, which is built across a watercourse of a normally big river of a country to hold back firmly the mighty water, often forming a big reservoir. Dams are made of earth, rock, concrete, or combinations of these materials. Dams have been constructed from early times to provide sufficient and ready supply of water for irrigation and other numerous purposes. The earliest recorded dam is believed to be a masonry structure 15 meters high that built across the Nile River in Egypt around 2900 BC. A rock fill dam built around 1300 BC in what is now modern Syria is the oldest dam still in use. Half of the Egypt's population mainly composes of farmers, who rely entirely on irrigation. Virtually, Egypt is a rainless country, depends for its existence entirely on the Nile, the second longest river in the world.

Irrigation means artificially watering the soil to initiate the growth of crops. The first large irrigation project was developed in the Eastern Hemisphere about 4000 B.C. It made possible the cultural advancement in Egypt, Syria, Persia, India, Java, and Ceylon. All agricultural land in Egypt is irrigated while it is about half in China, Japan, and Pakistan and about 33 million acres in USA and large parts of Europe. No country, in fact, is without irrigation projects. Irrigation requires enormous quantities of water. For instance, one ton of sugar beets need 1000 tons of water during its period of growth while wheat requires 1500 tons and rice 4000 tons.

Many modern dams are built for multi-purposes i.e., to provide irrigation water, water for human consumption, conserve occasional flood, check flow of water, and furnish power for hydroelectric plants. Many dams in the United States have constructed in Central Valley Project, Tennessee Valley Authority (TVA) Project, and Missisippi Missouri River Basin Project. Some largest dams are Bonneville dam, Grand Coule dam, and Hoover dam. A large dam of the Panama forms Gatun Lake. Notable dam in Egypt built on river Nile is Aswan (364 ft high and 3,280 ft). Yellow and Zambezi-Kariba dam (Zambia) is 420 ft high and 1900 ft long. Many important dams are built on important rivers such as the Tigris, Euphrates, Indus, etc.

Dams are categorized according to their profiles and construction materials. The principal types are earth fill, rock fill, gravel fill, solid-masonry gravity, solid masonry arch, arch gravity, structural masonry, and steel. Modern engineers make use of materials and designs that are best suited for a particular dam, also depending on complicated analyses of foundation conditions, load strains, temperature and pressure changes, the chemical characteristics of local groundwater, and the probability of seism activity.

Modern dams generally fall into two categories: embankment (earth fill) and masonry (concrete). Embankments are usually used in retaining water across broad rivers and are liked in part because large amounts of earth and rock are usually more available and less expensive. The profile of an embankment dam, such as the Aswan High Dam across the Nile in Egypt, looks like a broad-based triangle. Although these dams are generally known most for their length rather than their height, a dam in the Soviet Union is more than 330 meters high.

Once dams have retained water, it can be utilized. Outlets called gates that allow enough water through for irrigation, water supply, or power generation. They can also control the level of water in the riverbed below the dam for ecological reasons. Special gates in the form of stepped pools, locks, or fish ladders are built into many dams for the upstream and downstream passage of migratory fish such as salmon. Sluices are also used to drain the silt that accumulates behind a dam. Besides causing additional pressure on the structure, accumulated silt can eventually fill a reservoir. The most important auxiliary structure of a dam is a spillway the lack of which greatly increases the probability of structural failure. The spillway automatically discharges any water in excess of the capacity of the dam, because of heavy rain or landslide. Spillway water is usually diverted along the side of the dam or made to shoot out from the dam in what is called a ski jump spillway. In this way, the dam's foundation gets out of danger.

In Pakistan, three major dams i.e. Mangla, Tarbela and Chashma were constructed for the purpose of generating electricity, storing water, and irrigating agricultural lands. In addition, there are 23 barrages/head works/siphons. Main irrigation canals are 45, which have extended up to 40,000 miles. Similarly, there are 90,000 water courses, which are extended up to one million miles.

Mangla Dam (on river Jhelum, completed in 1967) is earth fill and has height 380 ft above riverbed and length 10300 ft. It has gross storage capacity of 5.85 MAF and main spillway 870,000 cusecs. It has hydropower installed generation capacity of 1000 MW. Tarbela Dam (on river Indus, completed in 1983) is earth and rock fill, has height 485 ft above riverbed and length 9000ft with gross storage capacity of 11.3 MAF and spillway capacity of 650,000 cusecs. It has lake area of 100 square miles and hydropower installed generation capacity of 1728 MW.

At present, only 11% of the total water resources are being stored in Pakistan, as no new water reservoir was constructed after Tarbela, while countries like China has built 6000 water reservoirs. Since the per capita water availability is continued to drop, situation calls for a national consensus for developing small or big dams all over the country to meet the formidable challenges of water shortage.

In view of the mounting gravity of the developing situation, there is an urgency of expediting development of adequate water resources to meet the increasing need of the economy, without any more loss of time. There is a need to construct more small dams/barrages in the country at least six in the Punjab, four in Sindh, three in NWFP and two in the provinces of Balochistan. In fact, building new dams will help overcoming silting in the Mangla and Terbala dams. These dams will provide additional storage of water to meet the existing water shortages, generate a large chunk of hydropower for meeting the growing demand of agricultural, industrial, and domestic consumers through providing low cost option, reduce dependence on imported fuels, create employment for persons during construction and significant numbers after commissioning. A long debate is going on in the country over the issue. Practically, it takes about 12 years to build a dam. Thus, in order to properly manage the impending water and energy crisis, construction of dams should be started with immediate effect. If dams are not built immediately, it would have disastrous effect especially for the people of Sindh.

Country is today suffering in the form of drought and acute water shortage throughout the country specially Sindh and Balochistan. The inadequate management of our water resources is one of the prime causes of our poverty. The issue of water distribution and water management was extremely complex. Water distribution and allocation among the provinces remain a contentious issue. Punjab province has plenty of sweet subsoil water and in case of shortage, it can meet its requirement by tube wells. However, Sindh has brackish subsoil water, which cannot be used for irrigation purposes. The water shortage in Sindh province will be much more acute in the coming years and this disaster can be averted only by undertaking construction of new dams on war footing basis.

Agriculture sector consumes about 75% of water but the industry, commerce, and public health are also largely dependent on the availability of water. Water use for irrigation purposes in the country is quite high and inefficient, rather wasteful. Industries are larger user of water of different quality depending upon the manufacturing process and the number of people employed.

According to WAPDA's Vision 2025, there would be acute shortage of water during the next 10 years and Pakistan would become an extremely water-starved country by 2012 with per capita water availability of only 1000cm. The per capita water availability was 5650cm in 1951 and 1400cm in 2000, which will go down further 885cm by 2025. With such shortage of water, it is anticipated that agricultural production may decrease by 16 million tons by the year 2020, leading towards a grave famine-like situation, if timely action is not taken. It simply means that by 2025, we will need to have 3-4 large reservoirs, otherwise we will be short of drinking water and agrarian growth will suffer, the economy will be adversely affected and the poor will remain poor. Last year, water shortage in Sindh alone destroyed 5000 lakes, left one million unemployed, and forced 100,000 families to migrate to other areas.

The country in the next 10 years will loss water storage capacity equivalent to Mangla dam as 5 x 105 tons of sedimentation is flowing into dam every day. According to an estimate within 10 years, the country's water storage capacity would deplete by over six million acre feet (MAF), which is equal to water stored in Mangla dam and this shortage will continue to increase with every passing year and the biggest sufferer will be the province of Sindh. The gigantic dams built earlier at Warsak, Mangla and Tarbela on rivers Kabul, Jhelum, and the Indus, respectively, have continued emitting signals of wearing down and now they have to be supplemented with new dams to meet the needs of the times. In the meantime, the growing need of water for irrigation and other purposes has acquired alarming proportions. To save and utilize the available water, construction of additional water reservoirs is essential for sustainable agriculture, the backbone of the country's economy.

Pakistan is fortunate in the soils, topography and climate, which are suitable for year round agriculture. Major agricultural areas tie within the plains formed by Indus River and its tributaries, namely Kabul, Chenab, Ravi, Jhelum, and Sutlej. Indus plains are like a tunnel with number of water sources at the top, converging into single stream, which flows into the Arabian Sea near the city of Karachi. After independence in 1947, many more developments in the canal systems were made. Different barrages/canals i.e. (Kotri barrage-1956, Taunsa barrage-1958 and Guddu barrage-1962), link canals (Marala-Ravi (MR), Bambanwala-Ravi-Bedian-Dipalpur (BRBD) and Balloki-Sulimanki (BS) were constructed.

Turkey (780,576 square kilometers) has constructed at least 40 dams on Tigris River within last five decades for agricultural development. While, Pakistan (796,095 square kilometers), during the same period built only four dams (Mangla, Tarbella, Chashma, Warsak etc,). If we will not construct dam, then water shortage within 10 years will rise to 6 MAF and up to 2015 by 8-10 MAF. No wonder, today, the country is facing a serious water crisis.

Iraq has many dams such as Qadisiya on the Euphrates, Saddam dam on the Tigris, Hamreen dam on Diyala, Bakhma dam on upper Zab and Fathab on Tigris. In Libya, there are no permanent watercourses. However, several seasonal watercourses run northward. Seventeen out of 36 planned dams have been built on the watercourses. In Syria, there are many small dams, which are fulfilling many important requirements of the country. Similarly, there are more than 200 small dams in Saudi Arabia.

Pakistan is an agricultural country, but it has only four dams. Recurrent failure of the monsoon system has become a regular feature in Pakistan for the past many years with the result that reservoirs/dams have lowered their actual height of water. There is a dire need to find a solution to the water shortage problem, as the country's population is increasing at an alarming rate of three percent per annum. The water situation seems to getting worst every year with reduced average rainfall. We have limited supply of water; the best option for us is to go for recycling of sewage water, now totally drained in Arabian Sea.

The government, public water utilities, and the people should take measures to augment availability of sweet water, minimize wastage, promote more efficient and economical water management, penalize those who pollute water by discharging untreated effluents in the rivers, canals or other waterways. Such water can be used for agriculture purposes. There is a dire need to introduce efficiency in the water distribution system to avoid any crisis-like situation. We have vast reserves of underground water in Balochistan, Sindh, and Punjab. The subsoil water can be used for drinking and agriculture purposes by purifying the water. Awareness for water conservation has to be developed and conservation actually be practiced at all levels. Presently, the country has 75.86 million acres cultivable land but out of it only 36 million acres is under irrigation while the rest is lying unused. In order to make use of this available land, dams should be immediately constructed..