An arch dam is a concrete dam that is curved upstream in plan. The arch dam is designed so that the force of the water against it, known as hydrostatic pressure, presses against the arch, causing the arch to straighten slightly and strengthening the structure as it pushes into its foundation or abutments. An arch dam is most suitable for narrow canyons or gorges 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. Classification In general, arch dams are classified based on the ratio of the base thickness to the structural height (b/h) as: Thin, for b/h less than 0.2, Medium-thick, for b/h between 0.2 and 0.3, and Thick, for b/h ratio over 0.3. Arch dams classified with respect to their structural height are: Low dams up to , Medium high dams between , High dams over . History 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 high and in length. Its radius was about , 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 high and , with a radius of . 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