DAMS — SOURCES FOR WATER STORAGE
At present only 11 per cent of the total water resources are being stored in Pakistan, as no new water reservoirs had been constructed after Tarbela
By Dr. S.M. ALAM
Jan 17 - 30, 2005
Dam is an embankment to restrain water, the water thus confined. Dam is therefore a barrier, which is built across a water course of a normally giant river of a country to hold back firmly the mighty water, often forming a big reservoir or lake. Dams are made of timber, earth, rock, masonry, concrete or combinations of these materials. Timber is seldom used in a dam because, timbers are impermanent and their height is not reasonable. Dams made from rock, earth, masonary are all useful. Dams have been constructed from early times to provide sufficient and ready supply of water for irrigation and other numerous purposes. For centuries, dam design was based upon previous experience. Remains of ancient earth fill dams still exist in India and Sri Lanka. The earliest recorded dam is believed to be a masonry structure 49 feet (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. A very important dam is the Aswan High Dam, started in Egypt in 1960 near the Aswan dam built in 1902. Half of the Egypt's population are farmers, who rely entirely on irrigation. Virtually, Egypt 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 projects developed in the Eastern Hemisphere about 4000 B.C and made possible the advanced cultures of Egypt, Syria, Persia, India, Java and Ceylon. All agricultural land in Egypt is irrigated, about half in China, Japan and Pakistan, about 33 million acres in US and large parts of Europe. No country, in fact, is without its 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; wheat 1500 tons and rice 4000 tons. At least half of it is lost by evapotranspiration. The rest is incorporated into the plants themselves (which are over 60 percent water), and drains down into the subsoil.
One of the earliest large dams for this purpose was a marble structure, built in 1660 by a ruler in the sub-continent. Many modern dams are built for multi-purposes i.e., to provide for irrigation water, water for human consumption to aid occasional flood occurs in a country, check flow of water and to furnish power for hydroelectric plants. Many dams in the United States have constructed in Central Valley Project, Tennessee Valley Authority (TVA) Project and Misisippi Missouri River Basin Project. Among the largest are Bonneville dam, Grand Coule dam, and Hoover dam. A large dam of the Panama forms Gatun lake. Notable dams in Egypt are built on river Nile are Aswan (364 ft high and 3,280 ft long), and Sennar dams, and many important dams are built on important rivers such as the Drieper, Tigris, Euphrates, Indus, Ganges (Damodar), Yellow and Zambezi-Kariba dam (Zambia), which is 420 ft high and 1900 ft long.
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 or timber. The first four types have been used from antiquity, while the others have been developed in the 19th and 20th centuries. The choices made by modern engineers regarding the materials and design that are best suited for a particular dam depend upon 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 retain water across broad rivers; in part this is because such large amounts of material is needed, and 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 dams are generally known most for their length than their height, a Dam in the Soviet Union is more than 330 meters high.
There are several different design possibilities for a masonry dam. The gravity dam utilizes the downward force for the weight of the construction materials to resist the horizontal force of the water. The base, where the force of the water is greatest, is made of concrete its width is roughly three-fourths the height of the dam. Concrete buttress dams have materials in the wall itself through the use of support buttresses around the outside base. Extra rigidity is possible if the buttresses can be linked together, as in the multiple-arch dam, but such a structure, is only possible where no movement is anticipated in the buttress foundations. The Daniel Johnson Dam in Quebec has 14 buttresses across in crest (length) of 4,297 feet. The arch dam is built in a convex arch, facing the reservoir. It has the unique advantage of reinforcement from the water pressure itself, which keeps the masonry joints closed tight. The strength of this design was tested in Italy, in 1963, when the Vaiont Dam remained basically undamaged after a huge quantity of soil and rock slided into the reservoir, causing water to surge over the top of the dam.
Once water has been retained by dams, it can be utilized. Outlets called gates, allow enough water through for irrigation, water supply, or power generation; they can also control the level of water in the river bed 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 such migratory fish 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 is not eroded.
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): Earth fill, height 380 ft above river bed, length 10,300 ft, gross storage capacity 5.85 MAF, main spillway 870,000 cusecs. Lakes are 100 square miles, hydropower generation — 1000 MW capacity, completed in 1967.
TARBELA DAM (ON RIVER INDUS): Earth and rock fill, height 485 ft above river bed, length 9000 ft, gross storage capacity 11.3 MAF, spillway capacity 650,000 cusecs, lake area 100 square miles hydropower generation 1728 MW, completed in 1983.
At present only 11 per cent of the total water resources are being stored in Pakistan, as no new water reservoirs had been constructed after Tarbela, while countries like China had built 6000 water reservoirs during this period. 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. The dams have been constructed for the purpose of depleting capacity of the existing water reservoirs call for at least several small dams in the country to meet the water requirements. In view of the mounting gravity of the developing situation, there is an urgency of initiating a timely more to expedite development of adequate water resources to meet the increasing need of the economy adequately, without any more loss of time. There is 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. As a matter of 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, generating a large chunk of hydropower for meeting the growing demand of agricultural, industrial and domestic consumers through low cost option. Reducing dependence on imported fuels, creating 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 10-15 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 sub water and in case of shortage, it can meet its requirement by sinking more tube wells. But 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. 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. Industry 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-short country by 2012 with per capita water availability off only 1000cm, it 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 shortfall 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, 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 are 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 suffer 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 lie 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. First canal were constructed some 5 to 6 centuries ago and extended under the great Moghul emperors. In earlier 19th century, there were numerous inundation canals leading from Indus and its tributaries. World's largest contiguous irrigation project was started in 19th century. 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 consructed.
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 on four dams (Mangla, Tarbella, Chashma, Warsak etc). If we will not construct dam, then water shortage within 10 years will rise to 6 million MAF and up to 2015 by 8-10 millions 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 water courses. However, several seasonal water courses run north ward. Seventeen out of 36 planned dams have been built on the water courses. 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 basically 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 get 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 it from Arabian Sea. Chances are it would deteriorate fast in future in the absence of concerted campaigns for water conservation at every level and in all sectors. The country must guard jealously its water resources including seawater. The government, public water utilities and the people should take measure to augment availability of sweet water, minimize wastage, promote more efficient and economical, penalize those who pollute water by discharging untreated effluents in the rivers, canals or other water ways. Such water can be used for agriculture purposes. Mismanagement of the distribution system, where justice comes in. 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 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. From extensive investigations, it has been proved that Pakistan is rich in basic resources, but sincere and concerted efforts are needed to develop them for the service of the country. In agricultural sector, the International Food Policy Research Institute has declared that Pakistan is one of the only two countries in Asia which has the capacity of exporting food on a sustainable basis in the 21st century, provided this potential is realized by improving the water and farm management practices and agricultural inputs. The growing shortage of water requires intensive efforts which should be made to preserve water and to utilize available water resources if we are to become self-sufficient in food, embark upon socio-economic development and alleviate poverty. But unfortunately, new dams construction has fallen prey to political controversies, afflicting the development of all four provinces.