Contents preface (VII) introduction 1—37

5.9.4.3. Orifice Semi-Modules
An orifice semi-module consists of an orifice followed by a gradually expanding flume on the downstream side (Fig. 5.7). Supercritical flow through the orifice causes the formation of hydraulic jump in the expanding flume and, hence, the outlet discharge remains independent of the water level in the watercourse. The roof block is suitably shaped to ensure converging streamlines so that the discharge coefficient does not vary much. The roof block is fixed in its place by means of two bolts embedded in a masonry key. For adjustment, this masonry can be dismantled and the roof block is suitably adjusted. After this, the masonry key is rebuilt. Thus, the adjustment can be made at a small cost. Tampering with the outlet by the cultivators would be easily noticed through the damage to the masonry key. This is the chief merit of this outlet.

Front elevation

Angle iron and air inlet pipe welded

Gauge on angle iron covering air pipe

F.S.L.
Supports

H0
Orifice
	Air chamber
1 m	Cast iron or
	sheet steel
	expanding pipe
Angle iron
	Angle iron

Ballast with thin layer of asphalt on top
Expanded metal around air inlet pipe

Dry ballast ^h1

Concrete pipe

182

IRRIGATION AND WATER

Fig. 5.5 Kennedy’s gauge outlet

RESOURCES ENGINEERING

CANAL IRRIGATION 183

57	HeighttobekeptaccordingtoH	SectionXX	45	HeighttobekeptaccordingtoH	SectionYY				shouldbeinlevel	upstreamwingwall
	120		39	120					Top	with
	75								to	1
							.5
								1
										precastRCslab
			Y	Y		Bankwidth					75mmthick
			10
			in
			1

		D2
	2H	H–	X	X
22.5	R=	R=		Berm

D		1	to
2
			57
			level
		Fullsupply
		Distributory

Fullsupplylevel	Bedlevel	27	15
22.5			50	AccordingtoD	andbankwidth

150	Floor	Concrete	incentimetres
2H	152H	Alldimensions
	12
		R=
2H

5.0

H

D

level	B
	u
Bed

Fig. 5.6 Plan of open flume outlet for distributary above 0.6 m depth and H less than full supply depth (3)

184 IRRIGATION AND WATER RESOURCES ENGINEERING

Gauge

Check plate

Tie rod

Bt

Bed plate

Y

³⁰ Tie rod
Details of roof block

Distributary

	^Bt	30
65
		Bed width
W		of canal
65	W
		7.6
		Plan

						Roof block	Top of bank
					15	1
									.
FSL									5:1
						H		Hs
	D			5:1		y	H
			.5:1
		.
				0		1
Bed level

R = 2H
Longitudinal section

All dimensions in centimetres

Water course

FSL
Bed level

Fig. 5.7 Crump’s adjustable proportional module (3)
The base plates and roof blocks are manufactured in standard sizes, such as B_t = 6.1, 7.6, 9.9, 12.2, 15.4, 19.5, 24.4, and 30.5 cm. B_t is the throat width. The base plates and roof blocks of these standard sizes with required opening of the orifice are used to obtain desired supply through the outlet.

CANAL IRRIGATION

185

The waterway in this type of outlet is either deep and narrow, that can easily get blocked, or shallow and wide in which case it does not draw its fair share of sediment. The discharge in this type of outlet is given by the formula (3):

Q = 4.03 Bt Y H

(5.13)

The ratio H_s/D should be between 0.375 and 0.48 for proportionate distribution of sediment and should be 0.8 or less for modular working (3). Here, H_s, D (= h), B_t, Y, and H are as shown in Fig. 5.7.