Arch Dam

An arch dam is a dam curved so that it is convex toward the upstream reservoir, an arrangement that carries the horizontal thrust of the impounded water outward into the rock walls of the canyon or valley it spans, rather than relying on the dam's own weight alone to resist the water as a gravity dam does. This lets an arch dam be built far thinner, and so with far less material, than a gravity dam of the same height, provided it sits in a narrow valley with rock strong enough to take the thrust. Roman engineers built small arch dams for water supply as early as the first centuries CE, and the design matured into large modern structures during the twentieth century as reinforced concrete and improved stress analysis let engineers build much taller and thinner arch dams for major hydroelectric and water-storage projects.