Editing Dam (section)

==== Arch dams ====
{{main|Arch dam}}

[[File:Gordon Dam.jpg|thumb|upright|[[Gordon Dam]], [[Tasmania]], is an [[arch dam]].]]

In the arch dam, stability is obtained by a combination of arch and gravity action. If the upstream face is vertical the entire weight of the dam must be carried to the foundation by gravity, while the distribution of the normal [[Fluid pressure|hydrostatic pressure]] between vertical [[cantilever]] and [[arch action]] will depend upon the [[stiffness]] of the dam in a vertical and horizontal direction. When the upstream face is sloped the distribution is more complicated. The [[Normal (geometry)|normal]] component of the weight of the arch ring may be taken by the arch action, while the normal hydrostatic pressure will be distributed as described above. For this type of dam, firm reliable supports at the [[abutment]]s (either [[buttress]] or [[canyon]] side wall) are more important. The most desirable place for an arch dam is a narrow canyon with steep side walls composed of sound rock.<ref name="pbsarch">{{cite web |url=https://www.pbs.org/wgbh/buildingbig/dam/basics.html#arch |title=Arch Dam Forces |publisher=[[PBS]] |access-date=7 January 2007}}</ref> The safety of an arch dam is dependent on the strength of the side wall abutments, hence not only should the arch be well seated on the side walls but also the character of the rock should be carefully inspected.

[[File:Barrage Daniel-Johnson2 edited.jpg|thumb|left|[[Daniel-Johnson Dam]], [[Quebec]], is a multiple-arch buttress dam.]]

Two types of single-arch dams are in use, namely the constant-angle and the constant-radius dam. The constant-radius type employs the same face radius at all elevations of the dam, which means that as the channel grows narrower towards the bottom of the dam the central angle subtended by the face of the dam becomes smaller. [[Jones Falls Dam]], in Canada, is a constant radius dam. In a constant-angle dam, also known as a variable radius dam, this subtended angle is kept constant and the variation in distance between the abutments at various levels is taken care of by varying the radii. Constant-radius dams are much less common than constant-angle dams. [[Parker Dam]] on the Colorado River is a constant-angle arch dam.

A similar type is the double-curvature or thin-shell dam. [[Wild Horse Reservoir|Wildhorse Dam]] near [[Mountain City, Nevada]], in the United States is an example of the type. This method of construction minimizes the amount of concrete necessary for construction but transmits large loads to the foundation and abutments. The appearance is similar to a single-arch dam but with a distinct vertical curvature to it as well lending it the vague appearance of a concave lens as viewed from downstream.

The multiple-arch dam consists of a number of single-arch dams with concrete buttresses as the supporting abutments, as for example the [[Daniel-Johnson Dam]], Québec, Canada. The multiple-arch dam does not require as many buttresses as the hollow gravity type but requires a good rock foundation because the buttress loads are heavy.