Types of dams

A dam holds back water to store it, raise its level for diversion, generate power or control floods. The choice of type depends on the valley shape, the foundation rock or soil, available materials, the flood to be passed and seismic conditions.

Gravity dams of concrete or masonry resist the water by their own weight; they need sound rock and suit sites where a spillway can sit on the dam itself. Arch dams curve upstream and pass the load into the valley sides; they need narrow valleys with strong abutments and use far less concrete. Buttress dams use a sloping slab supported by a row of buttresses.

Embankment dams are built of earth or rockfill. They tolerate weaker foundations and use local materials, but they must never be overtopped, so the spillway is usually a separate structure. They control seepage with a clay core or an upstream membrane, plus filters and drains.

Many large dams combine types: a concrete spillway section in the river channel flanked by earth embankments across the wider valley.