An arch dam is a type of dam that is curved and commonly built with concrete. The arch dam is a structure that is designed to curve upstream so that the force of the water against it, known as hydrostatic pressure, presses against the arch, compressing and strengthening the structure as it pushes into its foundation or abutments. An arch dam is most suitable for narrow gorges or canyons with steep walls of stable rock to support the structure and stresses. Since they are thinner than any other dam type, they require much less construction material, making them economical and practical in remote areas. The development of arch dams throughout history began with the Romans in the 1st century BC and after several designs and techniques were developed, relative uniformity was achieved in the 20th century.
The first known arch dam, the Glanum Dam, also known as the Vallon de Baume Dam, was built by the Romans in France and it dates back to the 1st century BC. The dam was about 12m high and 18m in length. Its radius was about 14m and it consisted of two masonry walls. The Romans built it to supply nearby Glanum with water.
The Monte Novo Dam in Portugal was another early arch dam built by the Romans in 300 AD. It was 5.7m high and 52m long with a radius of 19m. The curved ends of the dam met with two winged walls that were later supported by two buttresses. The dam also contained two water outlets to drive mills downstream.
The Dara Dam was another arch dam built by the Romans in which the historian Procopius would write of its design: "This barrier was not built in a straight line, but was bent into the shape of a crescent, so that the curve, by lying against the current of the river, might be able to offer still more resistance to the force of the stream."
The Mongols also built arch dams in modern-day Iran. Their earliest was the Kebar Dam built around 1300, which was 26 m high and 55 m long, and had a radius of 35 m. Their second dam was built around 1350 and is called the Kurit Dam. After 4 m was added to the dam in 1850, it became 64 m tall and remained the tallest dam in the world until the early 20th century. The Kurit Dam was of masonry design and built in a very narrow canyon. The canyon was so narrow that its crest length is only 44% of its height. The dam is still erect, even though part of its lower downstream face fell off.
The Elche Dam in Elche, Spain was a post-medieval arch dam built in the 1630s by Joanes del Temple and the first in Europe since the Romans. The dam was 26 m high and 75 m long, and had a radius of 62 m. This arch dam also rest on winged walls that served as abutments.
In the 20th century, the world's first variable-radius arch dam was built on the Salmon Creek near Juneau, Alaska. The Salmon Creek Dam's upstream face bulged upstream, which relieved pressure on the stronger, curved lower arches near the abutments. The dam also had a larger toe, which off-set pressure on the upstream heel of the dam, which now curved more downstream. The technology and economical benefits of the Salmon Creek Dam allowed for larger and taller dam designs. The dam was, therefore, revolutionary, and similar designs were soon adopted around the world in particular by the U.S. Bureau of Reclamation.
Arch dam designs would continue to test new limits and designs such as the double- and multiple-curve. The Swiss engineer Alfred Stucky and the U.S. Bureau of Reclamation would develop a method of weight and stress distribution in the 1960s, and arch dam construction in the United States would see its last surge then with dams like the 143-meter double-curved Morrow Point Dam in Colorado. By the late 20th century, arch dam design reached a relative uniformity in design around the world.  Currently, the tallest and largest arch dam in the world is the 292 m Xiaowan Dam in China, which was completed in 2010. The longest multiple arch with buttress dam in the world is the Daniel-Johnson Dam in Quebec, Canada. It is 214 meters high and 1,314 meters long across its crest. It was completed in 1968 and put in service in 1970. China is currently constructing the Jinping 1 Dam in Sichuan, which when complete will be world's tallest arch dam at 305m.
Arch dams require a high level of stress and force analysis in order to create a sufficient design. The main force against an arch dam is the hydrostatic pressure provided by the reservoir behind it, uplift, which is water pressure beneath the dam, the weight of the dam itself, and all the forces combined. Other forces that affect a dam include, but are not limited to, temperature, chemical reactions, settling, silt accumulation, and earthquakes.
Most often, the arch dam is made of concrete and placed in a "V"-shaped valley. The foundation or abutments for an arch dam must be very stable and proportionate to the concrete. There are two basic designs for an arch dam: constant-radius dams, which have constant radius of curvature, and variable-radius dams, which have both upstream and downstream curves that systematically decrease in radius below the crest. A dam that is double-curved in both its horizontal and vertical planes may be called a dome dam. Arch dams with more than one contiguous arch or plane are described as multiple-arch dams. Early examples include the Roman Esparragalejo Dam with later examples such as the Daniel-Johnson Dam (1968) and Itaipu Dam (1982). At an increased cost, an arch dam can have steel rods or pre-stressed steel cable reinforcements and therefore require less concrete than does a gravity dam or arch-gravity dam. As arch dams are commonly constructed in remote mountainous areas, less material is beneficial when it has to be delivered with difficulty to the site.  However, as a result of the failure of the Gleno Dam shortly after it was constructed in 1923, the construction of new multiple arch dams has become less popular.
In general, arch dams are classified as thin, medium, or thick. If width of an arch dam's base is 2/10 of the dam's height or less, it is thin. Any base and height ratio greater than 2/10 but less than 3/10 is considered medium. Thick is a ratio of 3/10 or greater. In addition, an arch dam's base is historically just as thick as the crest, but often it is twice as thick as the crest. Contraction joints are normally placed every 20 m in the arch dam and are later filled with grout after the control cools and cures.
Examples of arch dams
- Buchanan Dam (example of Multiple-Arch type)
- Daniel-Johnson Dam
- Flaming Gorge Dam
- Glen Canyon Dam
- Hoover Dam
- Idukki Dam
- Inguri Dam
- Karun-3 dam
- Pensacola Dam (longest Multiple-Arch type)
- Victoria Dam
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