Dams and irrigation

Dams are designed for a number of purposes, including conservation and irrigation, flood control, hydroelectric power generation, navigation, and recreation; most major dams have been constructed to serve more than one of these purposes.

Background

A dam is an artificial facility that is constructed in the path of a flowing stream or river for the purpose of storing water. Historically, dams are the oldest means of controlling the flow of water in a stream. The primary function of most dams is to smooth out or regulate flows downstream of the dam. Generally dams are permanent structures. In some cases, however, temporary structures may be constructed to divert flows, as from a construction site. Cofferdams are used in this regard—not to store water but to keep a construction area free of water. Such temporary structures are designed with a higher assumed risk than are permanent dams. Dams date (in recorded history) to about 2600 to 2900 B.C.E.

Uses of Dams

Dams are primarily used for four purposes: conservation, navigation, flood control, and generation of hydroelectric power. A fifth but somewhat less important use is recreation. Conservation purposes include water supply (including irrigation) and low-flow augmentation (to achieve wastewater dilution requirements). Flood-control objectives dictate that a dam’s reservoir be as empty as possible so that any excess water from the watershed (the area upstream of the dam that sheds water to it) can be detained or retained in the to reduce any potential flood-related damage of property and/or loss of life. A single dam can be used for all the foregoing purposes. In that case, the reservoir is called a multipurpose reservoir. Rarely can the cost of a major dam be justified for one purpose.

Types of Dams

Dams are classified according to the type of material used in their construction and/or the structural principles applied in their design (for structural integrity and stability). Generally, dams are constructed with concrete or earthen materials readily available at the construction site. In some cases the nature of the construction site with regard to underlying formation, climate, topography, and sometimes the width and load-carrying capacity of the valley in which the dam is constructed significantly influences the type of dam selected. Common types include earthfill or earthen dams, rockfill dams, and concrete dams, which include gravity dams (their structural stability depends on the weight of the concrete), arch dams, and buttress dams. Buttress dams are further categorized as flat-slab (also called Ambursen dams after Nils Ambursen, who built the first of this kind in the United States in 1903), multiple-arch, and massive-head dams. Arch dams are designed to take advantage of the load-bearing abutments in the valley or gorge where the dam is constructed. The structure is designed so that all the loading is transmitted to the abutments. Therefore these abutments must be rock of high structural integrity and strength capable of sustaining substantial thrust loading with little displacement. Arch dams are typically thinner than gravity dams and are constructed from reinforced concrete. Arch dams (as the name implies) are curved to ensure that compressive stresses are maintained throughout the dam. The basic buttress dam consists of a sloping slab supported by buttresses at intervals over the length of the reservoir. Earthen (or earthfill) dams are constructed as earth embankments. To avoid the destructive effects of seepage through the dam (especially “piping,” a slow leak that develops into destructive through the core of the dam), an impervious core is constructed to prevent seepage. For example, the use of compacted core is common. Rockfill dams are like earthfill dams but use crushed rock as fill material. The impermeable core is usually constructed using concrete.

Dams can be further classified as major (with a storage capacity of more than 60 million cubic meters), intermediate (with a storage capacity between 1 and 60 million cubic meters), and minor (with a storage capacity of less than 1 million cubic meters). Major dams are designed to handle the probable maximum precipitation (PMP). PMP is the estimated maximum precipitation depth for a given duration which is possible over a particular geographical region at a certain time of year. Intermediate dams are designed to withstand the flood from the most extreme rainfall event considered to be characteristic of its watershed or basin. Floods that occur every fifty to one hundred years are used in the design of minor dams.

Dams associated with hydropower plants can be categorized by the height of the water surface at the plant intake above the tailwater (the water surface at the discharge end of the hydropower plant). This vertical difference in height is called the “head.” Three categories exist: low (head between 2 and 20 meters), medium (head between 20 and 150 meters), and high (head above 150 meters).

Sizing of Dams

The useful or service storage volume of dams is determined by an analysis of streamflows occurring at the proposed dam site and of the expected average releases or demand flows from the reservoir. Several methods have been developed, ranging from the Rippl method (attributed to Wenzel Rippl in 1883, also called the stretched-thread method) to optimal reservoir sizing schemes which employ more sophisticated operations research techniques. Rippl’s method is simply a mass diagram analysis technique. Another less cumbersome method for estimating the active (service) storage of a dam is the sequent peak method. Reservoir storage can be divided into three components: flood storage capacity for flood damage mitigation, dead storage needed for sediment storage, and the active storage for the regulation of streamflow and water supply.

Reservoir Benefits and Cost

The construction of a dam can be justified only if it is cost-effective. That is, the total benefits resulting from its construction must be greater than the direct or indirect costs incurred in its construction and operation. Benefits accrue from hydropower sales, water supply and flow augmentation, recreation activities, and flood damage mitigation. Costs (or disbenefits) include net loss of streamflow because of evaporation, loss of water to seepage, the inundation of areas upstream from the dam, destruction of aquatic habitat, and the prevention of migration of fish in the stream, to mention a few.

Siltation

One of the factors that can shorten the useful life of a dam is the unavoidable siltation that will occur during the projected life (service period) of a reservoir. In the design of any reservoir, the portion of the reservoir earmarked or set aside for reservoir siltation is the part of the reservoir storage referred to as dead storage. Siltation occurs in the reservoir as the sediment load carried by flow entering the reservoir is trapped in the reservoir because of decreased flow velocities within the reservoir. This siltation can lead to other problems, especially if organic debris is carried in the sediment load. Because organic material will degrade, typically resulting in the depletion of dissolved oxygen in the stored water, water quality can easily be affected by such loads. In addition, because the sediment load in the water discharged from a reservoir may be decreased because of siltation, unnaturally clarified water in the channel downstream may lead to above-normal erosion of the downstream streambed. Further, sediment and buildup in a dam can submerge and choke benthic communities (bottom animal and plant life), thereby changing the character of reservoir bottom plants. In order to control the level of siltation in the reservoir, it may be necessary to undertake programs to reduce bank erosion. Activities which lead to increased sediment load, including construction activities, must be minimized and their effects carefully monitored. Bank stabilization schemes are very important first-line defense measures against the reduction of reservoir capacity by siltation.

Hydropower Production

Hydroelectricity is produced when flow from a reservoir is passed through a turbine. A turbine is the direct reverse of a pump. In the case of a pump, mechanical energy is converted to fluid energy. For example, in a typical pumping station, a pump is supplied with electric power that is converted to mechanical energy—usually to turn a motor. The mechanical energy is then converted to energy which is imparted to the fluid being pumped, resulting in increased fluid energy. A turbine is used to generate hydroelectricity in a directly opposite manner. In this case, water from the reservoir travels through special piping called penstocks and impinges on the turbine wheels (which can be rather large—up to 5 meters in diameter), causing them to spin at very high rotational speeds. A significant proportion of hydropower installations are used for “peaking.” Peaking is the practice of using hydropower plants to supply additional electric power during peak load periods. Because hydropower plants can be easily brought online in an electric power grid (in contrast to fossil-fuel powered plants), they are often used in this manner. Hence hydropower plants are used in most cases as part of an overall power supply grid.

There are two types of hydropower plants: storage and pumped storage. In a storage hydropower plant water flows only in one direction. In contrast, in a pumped-storage plant, water flow is bidirectional. In pumped-storage facilities, power is generated during peak load periods, and during off-peak (low load) periods water flow direction is reversed—water is pumped from the tailwater pool (downstream) to the headwater pool (the upstream reservoir). This is economically feasible only because the price of energy is elastic and time dependent. During peak load periods, energy is relatively expensive, so the use of a hydropower plant to meet demand requirements is cost-effective. During off-peak periods, it is economical to pump water upstream, and water is stored (or re-stored) as potential energy. The amount of hydroelectricity generated for a unit flow of water is directly proportional to the volumetric rate of flow, the hydraulic head (approximately the difference between the elevation at the headwater pool and the surface water elevation at the tailwater pool), and the mechanical efficiency of the hydropower plant (turbines).

Researchers continue to explore the most cost-effective way to incorporate hydropower production in the operation of multipurpose reservoir systems. Many operational research techniques have been reported in the literature. In general, mathematical formulations (models) that describe important interactions and constraints necessary to model and evaluate operational objectives are developed and solved by efficient methods. The value of such work lies in the fact that substantial benefits could be reaped from efficient use of existing reservoirs (dams) in contrast to the capital-intensive construction of new ones.

Hydropower production is a nonconsumptive use of water. This means that the water that passes through the turbines can be used (without undergoing any treatment) for other purposes. This nonconsumptive nature is one of the most attractive aspects of hydropower production; hydropower generation does not result in the significant degradation of water quality. However, problems with hydropower production do exist, most notably dissolved oxygen reduction and the adverse effects on aquatic life sensitive to changes in dissolved oxygen. Highly sensitive marine species which require a relatively stable aquatic system may suffer shock and undue stress from the drawdown of reservoir pool levels. Furthermore, pool elevation changes (swings) to sustain hydropower production may result in the significant reduction of recreational benefits. On the other hand, smaller downstream flows (during reservoir filling times and low water-demand periods) may cause water quality to degrade by increasing the temperature of water and pollutant concentrations. Significant temperature increases can be devastating to some species. For example, it is known that even slight temperature increases affect trout and salmon. The optimum temperature range for salmonids is about 6° to 13° Celsius. Adult fish will die when the water temperature exceeds 28° Celsius, while the juveniles will die when the temperature exceeds 22° Celsius. Unfavorable thermal conditions may discourage fish migration and cause the death of marine life because increasing temperatures delay or postpone the migration of adult fish, encouraging or promoting the development of fungus and other disease organisms. Eventually the balance of the is modified, since predator or competitive species are favored, adversely affecting the salmon or trout population.

Other Ecological Effects

With the construction of a dam, several permanent or temporary changes may occur to an ecosystem. These include changes attributable to the pool of water behind the dam, such as temperature increases because the water is relatively stagnant. In addition, salts and hydrogen sulfide could accumulate. These altered conditions could lead to the decimation of sensitive stream-type organisms, stressed marine life, diseases or disablement, and displacement of native marine and aquatic organisms. Increased temperature of the reservoir pool could also lead to increased evaporation and other ecological problems. In areas, the lowering of the rate of river flow (and volume) by upstream dams may result in saltwater intrusion.

Bibliography

"Dams." National Geographic Education, 21 June 2024, education.nationalgeographic.org/resource/dams/. Accessed 26 Dec. 2024.

Fritz, Jack J., ed. Small and Mini Hydropower Systems: Resource Assessment and Project Feasibility. New York: McGraw-Hill, 1984.

Henry, J. Glynn, and Gary W. Heinke. Environmental Science and Engineering. 2d ed. Upper Saddle River, N.J.: Prentice Hall, 1996.

Jansen, Robert B., ed. Advanced Dam Engineering for Design, Construction, and Rehabilitation. New York: Van Nostrand Reinhold, 1988.

Jobin, William R. Sustainable Management for Dams and Waters. Boca Raton, Fla.: Lewis, 1998.

Leslie, Jacques. Deep Water: The Epic Struggle over Dams, Displaced People, and the Environment. New York: Farrar, Straus and Giroux, 2005.

Linsley, Ray K., et al. Water Resources Engineering. 4th ed. New York: McGraw-Hill, 1992.

McCully, Patrick. Silenced Rivers: The Ecology and Politics of Large Dams. Enlarged and updated ed. New York: Zed Books, 2001.

Mays, Larry W. Water Resources Engineering. Hoboken, N.J.: John Wiley & Sons, 2005.

Scudder, Thayer. The Future of Large Dams: Dealing with Social, Environmental, Institutional, and Political Costs. London: Earthscan, 2005.

Vischer, D. L., and W. H. Hager. Dam Hydraulics. New York: Wiley, 1998.