SPILLWAY

The channel or passageway around or over a dam through which excess water is diverted. If the flow is controlled by gates, it is a controlled spillway; if the elevation of the spillway crest is the only control, it is an uncontrolled spillway. The following lists some typical spillways: (1) Auxiliary Spillway (Emergency Spillway): A secondary spillway designed to operate only during exceptionally large floods; (2) Fuse Plug Spillway: An auxiliary or emergency spillway comprising a low embankment or a natural saddle designed to be overtopped and eroded away during normal inflow and flood flows; (3) Primary Spillway (Principal Spillway): The principal or first-used spillway during normal inflow and flood flows; (4) Shaft Spillway (Morning Glory Spillway): A vertical or inclined shaft into which flood water spills and then is conducted through, under, or around a dam by means of a conduit or tunnel; if the upper part of the shaft is splayed out and terminates in a circular horizontal weir, it is termed a "bellmouth" or "morning glory" spillway; (5) Side Channel Spillway: A spillway whose crest is roughly parallel to the channel immediately downstream of the spillway; (6) Siphon Spillway: A spillway with one or more siphons built at crest level; this type of spillway is sometimes used for providing automatic surface-level regulation within narrow limits or when considerable discharge capacity is necessary within a short period of time.