17.6. TYPES OF GATES FOR SPILLWAY CRESTS
Free or uncontrolled overflow crest of a spillway is the simplest form of control as it automatically releases water whenever the reservoir water level rises above the crest level. For such crests, there is no need of constant attendance and regulation of the control devices by an operator. Besides, the problems of maintenance and repair of the controlling device also do not arise. However, when sufficiently long uncontrolled crest or large surcharge head for the required
596 IRRIGATION AND WATER RESOURCES ENGINEERING
spillway capacity cannot be obtained, a regulation gate may be necessary. Such regulating devices enable the spillway to release storages even when the water level in the reservoir is below the normal reservoir water surface. Gates can be provided on all types of spillways except the siphon spillway. The installation of gates involves additional expenditure on initial cost, and on their repair and maintenance. The selection of type and size of the controlling device depends on several factors, such as: (i) discharge characteristics of the device, (ii) climate, (iii) frequency and nature of floods, (iv) winter storage requirements, (v) the need for handling ice and debris, and (vi) special operating requirements such as presence of operator during periods of flood, the availability of electricity, operating mechanism, and so on. In addition, economy, reliability, efficiency, and adaptability of the regulating device must also the looked into.
The following types of regulating devices are generally used: (i) Flashboards and stoplogs,
(ii) Rectangular lift gates, (iii) Radial gates, and
(iv) Drum gates.
These may be controlled either manually or automatically through mechanical or hydraulic operations.
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