Spillway Control Structures

Labyrinth spillways are commonly installed to increase weir length and the associated discharge capacity.

(Image Source: Gannett Fleming)

“A major component of a spillway is the control device, which regulates the outflows from the reservoir. A control device limits or prevents outflows below fixed reservoir levels and regulates releases when the reservoir rises above that level. A control structure may consist of a sill, weir, orifice, or pipe. The discharge-head relationship may be fixed, as in the case of a simple overflow crest, or unregulated port, or it may be variable, as with a gate crest or a valve- or gate-controlled pipe”.^[1]

“A controlled crest is one that includes gates which are used to control the flow; the uncontrolled crest is one unencumbered by gates. The crest and/or gates located near the crest provide the flow control through the spillway system. The capacity of the spillway is dependent upon the crest shape, crest length, and the hydraulic head. The hydraulic head is modified by approach conditions, pier and/or abutment effects, and submergence. To provide a high-efficiency spillway and yet produce a safe, low-maintenance structure, the crest shape must provide a high discharge coefficient and fairly uniform and predictable pressures on the crest boundary”.^[2]

“Ideally, the spillway should be designed to operate with crest control throughout the entire expected range of discharge. However, the range of expected discharge is based on the current hydrologic data. Spillway flood flow rates may change due to updated probable maximum precipitation quantities; changes in the basin runoff characteristics could vary significantly with time; and the project operation may be revised at a future date, which may result in an increase above the original spillway design flow. Any of these factors, separately or in combination, could be sufficient to cause a spillway designed for crest control to shift to conduit control in the upper range of expected discharge”.^[3]

“The value of an uncontrolled fixed crest spillway in providing an extremely reliable operation and a very low-cost maintenance facility is undeniable. Topographical, geological, economical, and political considerations at many dam sites may restrict the use of an uncontrolled fixed crest spillway. The solution to these problems is usually the inclusion of crest gates; however, the uncontrolled fixed crest spillway should be used regardless of these considerations when the time of concentration of the basin runoff into the reservoir is less than 12 hours. When the time of concentration is between 12 and 24 hours, an uncontrolled fixed crest spillway should be given preference over a gated spillway. Basically, the inclusion of crest gates allows the spillway crest to be placed significantly below the maximum operating reservoir level, in turn, permitting the entire reservoir to be used for normal operating purposes; and results in a much narrower spillway facility, avoiding the problems associated with high unit discharge/high-velocity flow and increased operation and maintenance costs. A gated spillway must include, as a minimum, two or preferably three spillway gates in order to satisfy safety concerns” ^[2]