A solid wall curved in plan standing across entire width of river valley in a single span is an arch dam. The dam body is usually made of cement concrete. When multiple or number of arches are used supported between intermediate piers, dam is known as buttress dam.

The economy in dam thickness is furr increased by making dam body not only curved in plan but also in section. This non-vertical dam is known as double curvature arch dam or shell-arch dam. These dams are designed as shell structures which are quite complex.

The curved wall will structurally behave as a cantilever retaining wall standing up from its base and partly load will be transferred to two ends of arch span. The load on cantilever wall reduces as it is distributed to side walls which in turn reduces thickness.

A special type of variable radius arch dam in which central angles of horizontal arch rings are of same magnitude at all elevations is called a constant angle arch dam. Such a dam proves to be economical as design can be made by adopting best central angle of 133°-34?.

The simplest in design as well as construction is a constant radius arch dam but uses maximum concrete. An intermediate choice is a variable radius arch dam using around 58% of concrete used by constant radius arch dam. A constant angle arch dam uses about 43% of concrete used by a constant radius arch dam.

Shell arch dams are much more economical than constant angle arch dams as ir sections are quite thin. The Idduki dam in India is only 45 m thick at its base even with 170.7 m height whereas famous Hoover dam of USA which is a constant radius arch dam is 201m thick at its base with only 222 m in height.

The most economical central angle is equal to 1330-34? and such an angle can be adopted only at one place in constant radius arch dam since as re is considerable variation with height due to narrow V-shape of valley. It is refore considered to keep economical angle of 1330-34? at about mid-height and angle at top will be more than this value due to topographical conditions.

The arch rings corresponding to upstream face is called extrados curve. The arch rings corresponding to downstream face are intrados curve.

The variable center arch dam is also preferred for V-shaped valleys as compared to constant radius arch dams. The constant radius arch dam may be preferred for comparatively wider U-shaped valleys.

In a constant radius arch dam, centres of extrados, intrados as well as centrelines of horizontal arch rings lie on a straight vertical line at various elevations that passes through centre of horizontal arch ring at crest. The centre of arch rings is not at one point but lies along on vertical line at different heights where in variable centre arch dam centres do not lie on same vertical line.