The design and operation of dams and reservoirs
In the present day large dams are becoming a common feature along river profiles they are especially useful for production of electrical energy and to act as water catchment areas for agricultural activities. This is especially effective in areas where rainfall is intermittent and unreliable. In these areas rains come down in torrents at one time and go completely off during another time of the year. The reservoirs harness the overflows during the rainy day for the dry one. After the water has been held it can be easily released in manageable amounts and used as deserved. It can be released as desired to water turbines to produce hydroelectric power.
The site of construction for a dam is determined by a number of factors. Several are cited forthwith.
§ The availability of land
§ The purpose for which it’s been constructed.
§ The nature of the underlying rock. If it is permeable or not. A permeable one allows water to infiltrate.
§ The gradient of the land. Dams are hard to construct on hilly areas. The water won’t stand.
§ The population density of the area. Dams pose a health risk and should be far away from human settlements ( Golze, 1977)
Generally, the design of a dam radically depends on the purpose for which it’s been built for. After their construction they alter the hydrology system upstream and downstream. . A second major and critical constraint on dam operation is exerted by affluent flow against the dam but this is of course not under human control. The construction of a dam entails the following facts
Crest elevation: This is the highest possible dam height. This should be chose carefully to prevent floods and hold the right amount of water.
Discharge structures: Turbine intakes should be positioned at the highest possible point while spillways should be in an ideal position to avoid.
Discharge water reoxygenation: Equipment to oxygenate the deeper water layers in the vicinity of the dam should be installed to ensure that the downstream river channel receives oxygenated water at all times
Reservoir water level fluctuation: Water level management at dams is guided by a target type curve. If well used if tells about the trends about the water cycle and can easily predict floods and
Downstream discharge: Changes in the magnitude of downstream discharge should not be abrupt. During initial reservoir filling discharge magnitude must never drop below a minimum safety limit. The release of an artificial flood annually is desirable as it refreshes the water in the reservoir. Termination of the flood must be gradual (Thomas, 1976 )
The assignment requires me to explain the design and operation of a dam and also give an example of one. The following is my case study.
Design and Operation of Hume Dam
Hume Dam structure is simply comprised of four walls and the total length of Hume Dam is about 1.5 kilometers. The height of the spillway hoist bridge above the bed of the storage is slightly over 50 meters. The total quantity of concrete and earthwork in the dam is 440 000 and 3 300 000 cubic meters respectively. The capacity of the Reservoir is over 3 000 gigalitres at a full supply. The total surface area when full is 200 square kilometers. The surface area is important both for the available space for recreational use, and the amount of evaporation. The main earthen walls have a central concrete core wall. This core wall is 1.8 meters thick at its base and 0.6 meters thick at the crest. On both sides of the core wall there is a narrow zone of selected clay plus a vertical drainage zone on the downstream side. The core wall is not intended to provide major structural support; this is provided by the earth, the core wall acts as an impervious layer, providing the “water proof” component of the wall. There are four main modes of operation of Hume Dam: (Powling, 2007)
Filling mode: During the filling mode, minimum releases are maintained when water requirements can be met from downstream tributaries. Minimum releases are maintained to enable the reservoir to refill typically during winter and spring. However, high inflows at other times of the year may sometimes lead to increases in storage. (Powling, 2007)
Pre-release mode: A ‘pre-release’ is a release at a rate higher than downstream requirements. Pre-release only occurs when the storage level and inflows are sufficiently high that taking into account the current conditions, and expected demands, the storage will just fill under lowest (?) on record inflows. (Thomas, 1976)
(http//www.Design and operation of Hume dam/ river Murray water .com, last updated 2007)
Spilling mode: When the reservoir is full or near full, the spillway gates at the Dam are used , to pass inflows. The aim of flood operation is to pass floods so that the magnitude of the flood downstream is no greater than would have occurred without the Dam in place. If sufficient airspace is available prior to the arrival of a flood, significant flood mitigation is possible. Overall, Hume Reservoir provides a large degree of flood protection downstream.
Release Mode: Releases are made from the storage (at rates no greater than river’s channel capacity) to meet downstream water requirements. (Thomas,1976 )
While engaging in the above described processes the dam usually comes across some limitations some of which are described below: – fluctuations, silting and social problems.
A dam built to produce electricity requires a constant amount of water of times. This is quite hard to maintain given the dry weather in most parts of the world nowadays. Siltation happens when silt is deposited along the river channel causing blockages resulting in floods. Social problems relate to the human population around the water body. Dangerous creatures reside in the water and can be dangerous to humans. Mosquitoes breed there and spread diseases. The vast waters have been known to drown many. (Thomas,1976 )
A dam or reservoir built across a river has some advantages and disadvantages to its name.
On the bright side a storage reservoir collects water during the rainy season for use when its dry. This is cheap way of storing water. A regulatory reservoir regulates the amount of water flowing to avert floods. Either of the dams described above can be used to run turbines to produce electricity. They can also be used as places for practicing fishery.( Golze, 1977)
A dam built across a river profile does not require much for its maintenance. Provided its been build strong the only sort of maintenance it requires is regular dredging to remove silt that usually accumulates in the spillway. This when not removed usually blocks the spill way and results in floods.( Golze, 1977)
Golze, A.R., 1977 Handbook of dam engineering. New York, Van Nostrand Reinhold Co., 793 p.
. Thomas, H.H., 1976 The engineering of large dams. London, John Wiley and Sons, 2 vols. 777 p
Burns, F.L. , 1981 Desertification of lakes and reservoirs to improve water quality. Canberra, Australian Government Publishing Service, 915 p.
I.J. Powling.2007 Design and Operation of Hume Dam, Australian Government Publishing Service
http//www.Design and operation of Hume dam/ river Murray water .com, last updated 2007
Bater, S.1998,design of dams and reservoirs, published by scot pressmen