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This article throws light upon the four regulating devices generally used for controlling water reservoirs. The regulating devices are: 1. Flashboards and Stop-Logs 2. Vertical Lift Gates 3. Radial Gates 4. Drum Gates.
1. Flashboards and Stop Logs:
Flashboards and stop logs raise the reservoir storage level above a fixed spillway crest level when the spillway is not required to release flood. The flashboards usually consist of individual boards or panels of 1.0 to 1.25 m height which are hinged at the bottom and are supported against water pressure by struts.
Stop logs are individual beams or girders set one upon the other to form a bulkhead supported in grooves at each end of the span. To increase the spillway capacity, the flashboards or stop logs are removed prior to the flood.
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Alternatively, they are designed and arranged so that they can be removed while being overtopped. Flashboards and stop logs are simple and economical type of regulating device which provide an unobstructed crest when these are removed.
However, they have several disadvantages:
(i) They present a hazard, if not removed in time to pass floods, especially where the reservoir area is small and the stream is subject to flash floods,
(ii) They require the attendance of an operator or crew to remove them, unless they are designed to fall automatically,
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(iii) Ordinarily they cannot be restored to position while flow is passing over the crest,
(iv) If they are designed to fail when the water reaches certain stage, their operation is uncertain, and when they fail they release sudden and undesirably large outflows, and
(v) If the spillway functions frequently, the repeated replacement of flashboards may be costly.
2. Vertical Lift Gates:
These are usually rectangular and made of steel which span horizontally between guide grooves in supporting piers and move vertically in their own plane. The gates are raised or lowered by an overhead hoist and water is released by undershot orifice flow for all gate openings. Sliding gates offer large sliding friction due to water pressure and, therefore, require large hoisting capacity.
The use of wheels (along each side of the gate) would reduce the amount of sliding friction and thereby permit the use of a smaller hoist. Vertical lift gates have been used for spans and heights of the order of 20 and 15 m, respectively. At larger heights, however, the problem of raised operating platform becomes important.
3. Radial (or Tainter) Gates:
These are made of steel plates which form a segment of a cylinder which itself is attached to supporting bearings by radial arms. The cylindrical plate is kept concentric to the supporting pins so that the entire thrust of the water-load passes through the pins and only a small amount of moment needs to be overcome in raising or lowering the gate.
The hoisting loads then include only the weight of the gate, the sliding friction and the frictional resistance at the pins. The small hoisting effort required for the operation of the radial gates makes hand operation at small installations possible. Besides, they require less head rooms compared to that required by vertical lift gates. All these advantages make the radial gates more adaptable.
4. Drum Gates:
Drum gates are hollow (and, therefore, buoyant), triangular in section and made of steel plates. The drum gate is hinged at the upstream lip of a hydraulic chamber in the weir structure in which the gate floats. Water introduced into or drawn from the hydraulic chamber causes the gate to swing upwards or downwards.
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The inflow or outflow of water to the chamber is governed by controls located in the piers adjacent to the chambers.
