Dam
Barrier that stops or slows the flow of water
A dam is a structure that impounds water or restricts its flow. Dams are classified into four basic types: gravity dams are massive structures made of concrete or masonry that rely on their weight to resist the force of impounded water; embankment dams are large earthworks consisting of rocks, clay, sand, soil, or gravel; buttress dams consist of a sloped, concrete face supported on the downstream side by a series of triangular buttresses; and arch dams use a curved concrete wall to transfer the weight of the water to the surrounding valley walls.
Dams provide for irrigation, hydropower, water supply, flood management, recreation, inland navigation, and fish farming. Irrigation is a critical application of dams: about 20% of the world's arable land is irrigated using water from reservoirs impounded by dams. Dams generate hydropower, providing a clean and renewable source of electricity, and also supply water for household and industrial needs.
An early dam was Jawa Dam in modern Jordan, built around 3000 BCE. The Hittite Empire built several dams in modern Turkey between the 17th and 13th centuries BCE. In the 1st century CE, the Roman Empire began building masonry gravity dams – typically with vertical faces on both upstream and downstream sides. In medieval Europe, dams impounded water that powered water wheels for milling and mining. The process of designing dams began to transform in the late 18th century from an informal practice to an engineering discipline rooted in science. In the 20th century, the widespread availability of concrete and heavy construction machinery led to the creation of many large-scale dam projects worldwide.
The most visible part of a dam is the barrier that retains water, but most dams contain additional structures that perform important functions. When a dam blocks a navigable river, locks may be incorporated into the project to allow ships to pass through. Fish ladders are included in many dams to enable fish to migrate upstream. Spillways are often included to safely release excess reservoir water downstream and prevent catastrophic overflows. Dam outlets allow reservoirs to be partially drained to purge sediment, perform maintenance, or increase water flow downstream.
A dam may be deliberately removed for various reasons: if it poses a safety hazard, if the dam no longer fulfills its original purpose, to restore fish migration routes, or to improve the health of downstream rivers by improving sediment flow. Dams occasionally fail, resulting in flooding and loss of life. Many principles governing the design of safe dams have been developed based on lessons learned from dam failures.
Definition and etymology
Broadly, the word "dam" refers to any structure that retains water or prevents flooding. In the context of civil engineering, the word is typically limited to structures that impound water or raise the level of a body of water. This article uses the latter definition; therefore, it excludes dikes and levees on the banks of a river, and diversion dams which divert a portion of a flowing river without raising water levels.
The English word "dam" is found in Middle English, and traces back to dam in the Germanic languages – Middle Low German, Middle Dutch, and Old Norse. The word's roots include Gothic faur-dammjan ('to stop up'), and the Indo-European base *dhē- ('to set, put in place').
Types
Human-made dams are classified by their structural type: embankment, gravity, buttress, arch, and composite. Natural dams include rockslide, volcanic, and beaver dams.
Embankment dam
The most common type of dam is an embankment dam, constructed from compacted natural materials – such as rock, clay, sand, gravel, and soil – forming a broad, mound-like barrier. They are classified as rockfill or earthfill, depending on the primary material used. Among the largest embankment dams are Tarbela Dam, Nurek Dam, and Chicoasén Dam.
Embankment dams can be built from locally available materials, which make them less expensive to build than concrete dams requiring imported rocks and costly cement. They can also be built on softer soils because their broad base spreads their weight over a greater area (as opposed to heavy gravity dams that require bedrock foundations).
The primary drawback of embankment dams is that they are inherently permeable, so water can seep through or beneath the dam. Mitigation techniques to reduce seepage include placing a drainage system beneath the dam, injecting grout into the soil below the dam, and including a vertical layer of impervious material within the dam. If an impervious layer is included, it may be made of clay, cement, or asphalt. Failure to properly mitigate seepage can lead to dam failure caused by "piping" – water starts to flow through (or under) the dam in a small channel, which gradually enlarges until a large hole is pierced in the dam.
Early embankment dams were often built of a single type of earth, but starting in the mid-16th century, engineers began to use several types of material layered in zones. A typical zone pattern is a clay center (a vertical wall, extending from the riverbed to the crest of the dam), with gradually sloping banks of soil on both upstream and downstream sides, and both faces covered with large rocks. Large rocks on the upstream face protect the structure from wave action. In the 1970s, the Concrete Face Rockfill Dam (CFRD) design was invented, which is a rock-filled embankment dam with concrete slabs on the upstream face, sealed with waterproof joints.
Gravity dam
Gravity dams rely on their weight to resist the force of upstream water. Historically, gravity dams were built of masonry – stone, brick, or rubble – held together by mortar, but nearly all modern examples are made of concrete. The cost of concrete is much higher than earthfill used in embankment dams, so gravity dams are generally more expensive. An approach to reduce cost is to incorporate hollow chambers inside the dam – provided the dam's stability and strength are not compromised. Notable gravity dams include Grande Dixence Dam and Grand Coulee Dam.
The crest (top) of a gravity dam is generally a straight line stretching between the walls of the valley it crosses. When the crest is curved (the convex side of the curve always faces upstream) it is called an arch-gravity dam (discussed below). The cross-section of gravity dams is roughly triangular, with a flat bottom resting on the valley floor, and two inclined faces (upstream and downstream) that meet at the crest. To ensure that the dam is stable and will not tip over, the profile must conform to the middle-third rule, which states that the forces acting on the dam (gravity, water pressure, etc.) must produce a net force that is directed at the middle portion of the base (rather than directed near the downstream edge of the base). The thickness and inclination of a gravity dam must also be carefully designed to ensure stability. For non-overflow gravity dams, the thickness of the base should be about 70 to 85% of the height. The inclination (steepness, measured as run/rise) of the downstream face is typically 0.75 to 0.8, and inclination of the upstream face should be vertical or at most 0.05.
Because gravity dams are so heavy, they must rest on bedrock; a gravity dam built over soil would compress the soil, cause the dam to settle, and perhaps crack and fail. If the bedrock has cracks or defects, it must be prepared by injecting grout or placing concrete plugs. A concern that designers must address is "uplift": if water seeps under the dam structure, the water pressure can apply extreme upward force on the dam structure, which may lead to leaks or even dam failure. This risk can be mitigated with the use of grout curtains under the dam (which prevent water from seeping under the dam) and drainage systems under the dam, which lead water away when pressure increases.
Buttress dam
A buttress dam consists of an inclined upstream face supported on the downstream side by numerous triangular buttresses. Most buttress dams are made of concrete. Unlike a gravity dam (where the upstream face is nearly vertical) the upstream face of a buttress dam is sloped, typically with an inclination between 0.2 and 1.67. The inclination of the downstream face typically ranges from 0.09 to 0.8. The triangular profile of the reservoir's water pushes downward onto the dam, forcing it into the ground and increasing its stability. Notable buttress dams include Itaipu Dam, Lucendro Dam, and Al Massira Dam.
Buttress dams use much less concrete than comparable gravity dams, but the cost savings are offset by a more complex construction process. Buttress dams are not as strong as gravity dams, and are suited only for lower heights. Because buttress dams have a much smaller footprint (the area of ground the dam structure rests upon) than gravity dams, the risks associated with uplift forces (from water beneath the dam) are lower in buttress dams.
The individual buttresses may experience slight movements relative to each other. If the upstream face of the dam were a solid piece of concrete, the movements of the buttresses could introduce large stresses, resulting in cracking of the upstream dam face. To mitigate this, the upstream face is divided into multiple pieces, one per buttress, called the "buttress heads". Adjacent buttress heads are typically separated by a gap, and the gaps are filled with flexible seals.
Arch dam
An arch dam is a curved concrete structure that transfers the force of the impounded water horizontally into the valley walls. This design differs from gravity or buttress dams, which transfer the force downward into the foundation. Arch dams are relatively thin: the thickness of their base is less than half their height, and they are made of masonry or concrete. The central angle subtended by an arch dam can be relatively shallow or nearly semicircular, ranging from 46 to 140 degrees. Notable arch dams include Hongrin Dam, Flaming Gorge Dam, Moiry Dam, and Contra Dam.
All arch dams are curved, but there are a variety of shapes they may assume. Most older arch dams used a "constant radius" shape, which resembles a section of a vertical cylinder. A more complex shape is the "constant angle" shape, which gradually reduces radius from the crest to the base. Research into optimizing dam shapes for maximum strength led dam engineers to adopt the constant angle shape for many arch dams, beginning in 1914. Another shape is "double curved", which resembles a section of a dome and is defined by incorporating curvature in the vertical – as well as horizontal – direction.
Regardless of the shape of an arch dam's curvature, the arch must transfer the weight of the reservoir water into the valley walls. Arch dams can only be built in narrow valleys with strong, steep, rock walls that can withstand tremendous forces. In some dams, concrete abutments must be constructed between the arch and the valley walls to safely transfer the load.
Other dam structures
Some dams combine features from two of the basic dam structures. An arch-gravity dam combines features from arch dams and gravity dams: the overall shape is an arch, but it is not a true arch dam because the thickness of the dam's base is more than half of its height – giving it a weight and footprint that is characteristic of gravity dams.
A multiple-arch dam combines features of arch dams with buttress dams. It is similar to a buttress dam, but the upstream face is not flat – rather, the face consists of a number of small arch dams: each arch connects one pair of adjacent buttresses.
A barrage is a dam that has a spillway integrated into the dam structure, with multiple gates regulating the flow over the spillway. Dikes and levees – which share the same design as embankment dams – are not true dams because they generally line the banks of a river or sea, whereas dams are placed crosswise in a valley.
Natural dams
Not all dams are made by humans: natural dams are barriers created by processes such as rockslides, lava flows, or by the dam-building behavior of beavers.
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