Dam Features—Outlets, and Spillways
Typical features of various types of dams are depicted in the diagrams and illustrations. Two that come in for mention are the “outlets” and the “spillway”. The outlets provide for the draw-off of required water and often constitute the “intakes” of other installations associated with use of the water; in particular the intakes to pipelines or penstocks of hydro-electric power projects may be quite large. In some cases the intakes may be part of a structural unit that forms a separate dam from the main barrier. Similarly, the facilities for the discharge of floodwaters may sometimes be provided in a separate structure where circumstances are appropriate. Since power development on a national scale involves damming of the larger catchments, the flood spillage provision must usually be correspondingly large. Often the spillway is a part of the dam where the crest is made lower than it is elsewhere; for thin dams (e.g., arch type) the outflowing cascade of water may be arranged to spring clear of the structure (e.g., Manorburn concrete arch dam), though it is more usual for the spillway lip and outflow surface to be specially shaped to conform to the natural overflow jet profile, known as an “ogee” type.
Spillway openings in dam crests (or as separate structures) may be relatively narrow and deep and fitted with gates—the more usual case in modern practice; or they may be made much wider and shallower and be unobstructed, these being known as the free-overflow type (e.g., Waitaki dam).