Surcharge factor (S) is ratio of dam length (x + t) to buttress thickness (t).
S = (x + t)/t
The resultant impact can be considered by increasing unit weight of water by multiplying actual value by a surcharge factor.

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?.

A counterfort or a buttress is a structure built against wall to prevent overturning or to increase its bending strength. This term is synonymous with buttress and is often used when referring to dam, retaining walls and or structures.

In case of an arch dam, ends being fixed distance between m does not change and this restriction of change in span causes additional stresses known as rib-shortening stresses. These stresses are small in a long thin arch but in case of a thick arch of small-angle, y are appreciable.

The suggested economic buttress spacing in m for a normal ? value of 40° to 50° for different dam heights ?

Free slab type dams are generally preferred for smaller heights and multiple arch dams are used for higher dam heights. Manicougan-5 dam in Canada is highest multiple arch dam of world having a height of 210 m.

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.

The uplift pressure in an arch dam is generally neglected because it is small due to narrow base width of its body. Ice load, Temperature stresses and yield stresses are quite important in such dams and are thoroughly examined.

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.

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.