reservoir
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What is a reservoir?
A reservoir is typically an open-air storage area, often created by damming rivers or using natural or artificial depressions, where water is collected and stored in large quantities for later use. They are crucial components of water supply systems worldwide, helping to manage the variations in river flows caused by changing weather patterns. During periods of high flow, reservoirs impound water, which helps prevent flooding. They then release water gradually during low-flow periods.
Besides storing water, reservoirs can support biodiversity, promote tourism, and grow regional economies. However, reservoirs can also have negative environmental impacts, such as destroying fish habitats and ecosystems, and they can lead to the displacement of entire communities.
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Why are reservoirs important for water storage?
Reservoirs are vital components of water supply systems globally, serving as open-air storage areas, typically created using masonry or earthwork, where water is amassed and stored for future utilization. They play a crucial role in managing the fluctuating flows of streams and rivers caused by changing weather patterns.
During periods of high flow, reservoirs impound water, preventing potential flood disasters. Conversely, during periods of low flow, they enable the gradual release of water. Additionally, reservoirs can be single-purpose, designed for functions such as irrigation, power generation, or flood control, or multiple-purpose, serving at least two principal functions.
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How are reservoirs created?
Reservoirs, crucial for global water supply systems, are typically created by constructing dams across rivers. These dams trap water, preventing floods by holding back excess flows and allowing controlled release during low-flow periods.
Off-channel reservoirs can also be made by diverting water from a river to natural or artificial depressions using canals or pipelines.
However, reservoirs face challenges such as sedimentation, reducing their storage capacity over time, and environmental impacts, including habitat destruction and displacement of communities.
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What are the main uses of water stored in reservoirs?
Reservoirs serve several crucial functions in water management. They store water during periods of high flow, which helps prevent flooding, and they ensure a consistent water supply during droughts. Reservoirs are also essential for irrigation, hydroelectric power generation, and navigation. Additionally, they can support recreation and regulate low flows in rivers. These multiple uses make reservoirs vital components of water supply systems, especially in regions where natural water availability fluctuates.
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How do engineers control the water level in a reservoir?
Engineers use several methods to control water levels in reservoirs, which are crucial components of water supply systems. Dams create most reservoirs by blocking water flow, forming an artificial lake that stores water from wet periods for use during droughts or low streamflow.
To manage water levels, engineers use spillways, which allow the automatic, uncontrolled discharge of surplus water past the dam. Gates are also essential; they facilitate drawing water for irrigation, water supply, maintaining minimum river flow, power generation, and silt removal. These openings are fitted with screens to prevent debris entry.
Additionally, drum gates can automatically control the reservoir level without mechanical power. These gates consist of a shaped-steel caisson held in position by hinges and supported in a flotation chamber.
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What are some environmental impacts of building reservoirs for water storage?
Reservoirs, crucial for water storage, can have adverse environmental effects. Constructing them can lead to the destruction of fish habitats and ecosystems. Large-scale projects may also require the submergence of cities and towns.
When streamflow is impounded, the flow velocity decreases, and sediment is deposited. Erosion of the stream channel can occur below a reservoir due to the released water's renewed transporting capacity. Also, water in a reservoir may be lost through surface evaporation and seepage. Despite their negative impacts, reservoirs can be lifelines for biodiversity.
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reservoir, an open-air storage area (usually formed by masonry or earthwork) where water is collected and kept in quantity so that it may be drawn off for use. Reservoirs are an important feature of many water supply systems around the world.
Changes in weather cause the natural flow of streams and rivers to vary greatly with time. Periods of excess flows and valley flooding may alternate with low flows or droughts. The role of water-storage reservoirs, therefore, is to impound water during periods of higher flows, thus preventing flood disasters, and then permit gradual release of water during periods of lower flows. Simple storage reservoirs were probably created early in human history to provide water for drinking and for irrigation. From southern Asia and northern Africa the use of reservoirs spread to Europe and the other continents.
Reservoirs ordinarily are formed by the construction of dams across rivers, but off-channel reservoirs may be provided by diversion structures and canals or pipelines that convey water from a river to natural or artificial depressions.
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When streamflow is impounded in a reservoir, the flow velocity decreases and sediment is deposited. Thus, streams that transport much suspended sediment are poor sites for reservoirs; siltation will rapidly reduce storage capacity and severely shorten the useful life of a small reservoir. Even in larger reservoirs, sedimentation constitutes a common and serious problem. Because removal of the deposited sediments from reservoirs is generally too costly to be practical, reservoirs on a sediment-laden stream are characteristically planned to provide a reserve of storage capacity to offset the depletion caused by sedimentation. Despite this, the life expectancy of most reservoirs does not exceed 100 years at present sedimentation rates.
An associated problem is erosion of the stream channel below a reservoir when water is released. Because the sediment load is deposited in the reservoir, the released water has renewed transporting capacity, and channel erosion results.
Water in a reservoir may be lost by surface evaporation, by seepage into the surrounding soil or rocks, and by seepage through dam foundations. Seepage losses ordinarily can be reduced, but evaporation losses are often of major consequence. Gross evaporation from water surfaces in the temperate and tropical climates may amount to a few metres a year. In humid regions this loss is offset by direct precipitation, and the net surface loss may be moderate or negligible. In regions of lower rainfall the net loss may be substantial, amounting to 1.5 metres (5 feet) or more annually in some desert areas. There are a number of physical, chemical, and biological methods aimed at reducing evaporation from reservoirs, each with their own advantages and disadvantages. Floating or suspended covers can save a large percentage of water but are impractical or too costly for large reservoirs. Chemical treatments are widely used to reduce evaporation rates, though these require frequent reapplication and do not substantially reduce water loss. Biological methods include wind breakers and floating plants and can significantly reduce evaporation, though their use is limited to reservoirs with favourable conditions.
Water reservoirs range in size and complexity from small single-purpose impoundments to huge and complex multiple-purpose impoundments. A single-purpose reservoir is designed to fulfill only one function, such as irrigation, hydroelectric power generation, navigation, flood control, water supply, recreation, or low-flow regulation. The trend has been toward construction of multiple-purpose reservoirs designed to serve at least two principal functions.
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Reservoirs can cause adverse environmental impacts—e.g., destruction of fish habitats and ecosystems—and large-scale reservoir projects may require the submergence of cities and towns. For example, construction of the Three Gorges Dam on the Yangtze River (Chang Jiang) in China, designed primarily for flood control and as a source of electric power, displaced almost two million people from their homes. Social and environmental impacts must be taken into account during the planning stages of new reservoirs.
Relatively small enclosed reservoirs are familiar sights in most towns and cities. Often constructed on hills or supported in steel tanks on towers, these reservoirs are integral parts of most local water distribution systems. They usually provide a storage volume equal to the community’s average water demand for a single day. The tanks are elevated to provide adequate water pressures in the pipelines. The water is used for emergencies such as power blackouts, pump station failures, and fire control. In addition, these tanks serve to satisfy the peak hourly water demands in the community. When water demand exceeds the average daily demand, water flows out of the tanks into the distribution network. When water demand is very low (i.e., late at night), high-lift pumps refill the tanks.