Four ways to hold back a river, and why each dam shape works
Some dams stay put through sheer weight, while others lean on triangular props or push the water's load sideways into the rock of the valley. The shape engineers choose depends on the ground, the width of the gorge and the budget, and choosing wrong can let the water tunnel straight through.
People have been blocking rivers for a very long time. Jawa Dam in present-day Jordan dates to around 3000 BCE, the Hittites built several in Anatolia, and Romans raised masonry gravity dams from the 1st century CE. Only in the late 18th century did dam design start turning from rule of thumb into a science, and 20th-century concrete and heavy machinery unleashed a wave of giant projects. Reservoirs behind dams now water about 20% of the world's irrigated farmland.
The most common kind is the embankment dam, a broad mound of compacted rock, clay, sand, gravel or soil. Local materials keep it cheap, and its wide base spreads the load so it can sit on softer ground. Its weakness is seepage: if water finds a small channel through or beneath the mound, the channel can widen into a breach, a failure called piping. Engineers counter this with drains, grout injected into the soil and clay or asphalt cores.
Gravity dams, now nearly always concrete, resist the water by mass alone. Their roughly triangular profile must follow the middle-third rule so the combined forces point at the centre of the base and the structure cannot tip. They need bedrock, and water seeping underneath can push upward hard enough to cause leaks. Buttress dams save concrete with a sloped face braced by triangular supports, letting the reservoir's weight press the dam downward, but they suit only modest heights.
Arch dams are the slender option, with a base thinner than half their height. They curve upstream and shove the load horizontally into canyon walls, so they demand narrow valleys of strong, steep rock. Nature builds its own versions too, from rockslides and lava flows to beaver lodges.
Source: Dam
