Contents preface (VII) introduction 1—37

Fig. 13.9 Typical plan of a head regulator (6)

CANAL HEADWORKS

455

The width of waterway in the canal head regulator should be such that the canal can be fed its full supply with about 50 per cent of the working head provided (4). If the required width of waterway at the head regulator is more than the bed width of the canal, a converging transition is provided downstream of the regulator to attain the required canal width. The required head over the crest, H, for passing a discharge Q with an overall waterway L is worked out from Eq. (13.2).

The height of the gates is equal to the difference of the pond level and the crest level of the regulator. But during high floods, the water level in the river would be much higher than the pond level, and the flood water may spill over the gates. It would, obviously, be very uneconomical to provide gates up to the HFL. Besides the cost of heavier gates, the machinery required to operate them under large water pressures would also be expensive. To prevent such spilling of flood water into the canal, an RCC breast wall (Fig. 13.10) between the pond level and HFL, and spanning between adjacent piers is always provided. With this provision, the gate opening between the crest level and pond level is fully open when the gate is raised up fully, i.e., up to the pond level. The opening is fully closed when the gate is lowered to the crest of the regulator.

	Space for gate	Gate groove
	hoisting platform
	Breast wall	Axis of head
		regulator
		Baffle block
Loose	Upstream	Downstream	Inverted
	sheet pile	sheet pile	filter
apron
	Concrete floor
		Block	Block
		protection
			protection
	Toe wall
		Toe wall
				Loose
			apron

Fig. 13.10 Typical section of a head regulator (6)
Once the crest level, waterway, number of spans, and thickness of piers have been fixed, the head regulator is designed using the principles of weir design. The canal is generally kept closed when the highest flood passes through the river. This, obviously, would be the worst static condition and, hence, the floor thickness must be able to resist uplift pressures under this condition. The exit gradient for this condition should also be within safe limits. For economic reasons, the floor is designed such that it can support the uplift pressure by its weight and also the bending strength. For this purpose, the piers may have to be extended up to the floor to provide necessary support to the upward bending slab.
In the jump trough region, the worst condition of uplift may occur when some discharge is passing into the canal. The safety of this part of the floor should, therefore, be checked for different discharges including the maximum one also. The extension of the concrete floor upstream of the undersluices up to the end of the head regulator also reduces the uplift pressures on the downstream floor of the regulator.
A bridge and a working platform (for the operation of gates) are also constructed across the head regulator.