A dam is a structure that impounds or restricts the flow of water. 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. Arch dams use a curved concrete wall to redirect the force of the water outwards into the surrounding valley walls.
An early dam was the 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. By the first century CE, the Roman Empire began building masonry gravity damstypically with vertical faces on both upstream and downstream sides. In medieval Europe, dams 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. Organized opposition to dam projects emerged in the modern era, including protests in the 1870s against the Thirlmere Dam in Britain.
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 by water that originated in reservoirs impounded by dams. Dams generate hydropower, providing a clean and renewable source of electricity, and also supply water for household and industrial needs. Dams that support flood management reduce downstream water levels to a safe limit.
Dams are often a component of a larger project. Many dams incorporate power plants that run water through generators to produce electricity. Spillways are often included to safely release excess reservoir water downstream and prevent catastrophic overflows. Dam outlets are structures which permit the reservoir to be partially drained to purge sediment, generate electricity, or increase water flow downstream. When a dam blocks a navigable river, locks may be incorporated into the project to allow ships to pass through.
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. Dams can fail for many reasons: earthquakes, weak rocks at the abutments, water leaking within or under the dam, or the dam sliding over its foundation. Beyond structural issues, dams also present political challenges caused by water scarcity, population growth, and the impacts of climate change. Dams built in countries such as Turkey, India, Ethiopia, and China have led to international disputes with downstream nations.
Etymology
The English word "dam" is found in Middle English, and traces back to the word dam in Germanic languages Middle Low German, Middle Dutch, and Old Norse. Roots of the word include Gothic faur-dammjan ('to stop up'), and the Indo-European base *dhē- ('to set, put in place').
Types
Dams can be classified by their structural type: embankment, gravity, buttress, and arch. Other forms include composite dams and rockslide dams.
Embankment dam
The most common dam structure is an embankment dam. These consist of a pile of earth (rocks, clay, sand, gravel, soil, etc) formed into the shape of a large levee with a broad trapezoidal cross-section. Embankment dams are categorized as rockfill or earthfilldepending on the primary material used in construction.
Embankment dams can be built from locally available materials, which tend to make them less expensive to build than concrete dams requiring imported rocks and 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 to embankment dams is that they are inherently porous, so water can seep through or underneath the dam. Mitigation techniques to reduce seepage include placing a drainage system underneath 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 through 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. The resistance to water seepage varies widely between the various materials: clay resists water seepage 10 times more than silt, 10,000 times more than sand, and 100 million times more than gravel.
Gravity dam
Gravity dams rely on their weight to resist the forces exerted by the upstream water. Historically, gravity dams were built of masonry (stone, brick, or rubble) with mortar filling the joints but nearly all modern examples are made of concrete. The cost of concrete is much higher than dirt or rocks, so concrete gravity dams are generally more expensive than embankment or buttress dams. An approach to reduce cost is to incorporate large hollow chambers inside the damprovided the dam's stability and strength is not compromised.
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).