Contents preface (VII) introduction 1—37

Fig. 5.9 Khanna’s module
Example 5.2 A semi-modular pipe outlet of diameter 15 cm is to be installed on a distributary with its bed level and full supply level at 100.3 and 101.5 m, respectively. The maximum water level in the watercourse is at 101.15 m. Set the outlet for maximum discharge and calculate the same. The coefficient C in the discharge equation, Eq. (5.12), may be taken as 0.62. Is the setting proportional, subproportional or hyper-proportional?
Solution: For maximum discharge, the pipe outlet must be at the maximum water level in the watercourse.
Therefore,
H = (101.5 – 101.15) − ^0.₂¹⁵ = 0.275 m

and	Q = CA	2 gH
	= 0.62 × 3.14 × (0.15)^2	2 × 9.81 × 0.275
							4
	= 0.0254 m^3/s
Flexibility	F =	m	×	h
n	H
and	m =	1		×	3	= 0.3
2	5
				n
Therefore,		F =		0.3 × 12.		= 1.309
0.275

Therefore, the setting is hyper-proportional.

CANAL IRRIGATION

187

Example 5.3 A distributary channel having bed width 5.00 m and full supply depth of 1.20 m carries 3.0 m³/s of discharge. A semi-modular pipe outlet in this channel has a command area of 15 ha growing rice with a kor depth 20 cm and kor period of three weeks. Determine the size of the outlet and set it for sub-proportionality with a flexibility of 0.9. Assume the length of the pipe as 3.0 m and friction factor as 0.03. The available diameters of the pipe are 150, 125, 100, and 75 mm.
How does this outlet behave if the distributary runs below FSL at 1.0 m depth ?
Solution:
Outlet discharge factor, D = ^{8.64 × 3 × 7} = 907.2 ha/m³/s 0.2
Therefore, outlet discharge = 15/907.2 = 0.0165 m³/s

From Eq. (5.5),	F =	m ×	h
H
					n
∴	H =		(1 / 2)		×	1.2
(5 / 3)		0.9

= 0.4 m

L

d(2 gH)

O1/ 2

π

2

From Eq. (5.12),

Q = M

P

d

4

N 15. d + fL Q

LF

π I

2

d^5

O1/ 2

= MG

J

2 gH.

P

MH

4K

15. d + fLP

N

Q

LF

π I

2

d^5

O1/ 2

= MG

J

2g (0.4)

P

M^H

4 K

15. d + (0.03) (3) P

N

O1/ 2

Q

L

4.84 d^5

∴

Q = M

P

N 1.5 d + 0.09 Q

Assuming d = 0.1 m, Q = 0.142 m³/s which is less than required. For d = 0.125 m, Q = 0.023 m³/s which is okay.
Therefore, recommended diameter of the outlet is 125 mm.
When the distributary is running at 1.0 m depth (i.e., 0.2 m below FSL) then H = 0.4 – 0.2 = 0.2 m and h = 1.0 m.

	F	1 / 2I	10.
∴	F = G		J	0.2	= 1.5
	H	5 / 3K

Therefore, the outlet behaves as hyper-proportional outlet.
5.10. CANAL REGULATION

The amount of water which can be directed from a river into the main canal depends on: (i) the water available in the river, (ii) the canal capacity, and (iii) the share of other canals taking off from the river. The flow in the main canal is diverted to various branches and distributaries.

188 IRRIGATION AND WATER RESOURCES ENGINEERING
The distribution of flow, obviously, depends on the water demand of various channels. The method of distribution of available supplies is termed canal regulation.
When there exists a significant demand for water anywhere in the command area of a canal, the canal has to be kept flowing. The canal can, however, be closed if the water demand falls below a specified quantity. It is reopened when the water demand exceeds the specified minimum quantity. Normally, there always exists a demand in some part of the command area of any major canal. Such major canals can, therefore, be closed only for a very small period (say, three to four weeks in a year). These canals run almost continuously and carry discharges much less than their full capacity, either when there is less demand or when the available supplies are insufficient.
If the demand is less, only the distributaries which need water are kept running and the others (including those which have very little demand) are closed. In case of keen demand, but insufficient supplies, either all smaller channels run simultaneously and continuously with reduced supplies, or some channels are closed turn by turn and the remaining ones run with their full or near-full capacities. The first alternative causes channel silting, weed growth, increased seepage, waterlogging, and low heads on outlets. The second alternative does not have these disadvantages and allows sufficient time for inspection and repair of the channels.
A roster is usually prepared for indicating the allotted supplies to different channels and schedule of closure and running of these channels. It is advantageous to have flexible regulation so that the supplies can be allocated in accordance with the anticipated demand. The allocation of supplies is decided on the basis of the information provided by the canal revenue staff who keep a close watch on the crop condition and irrigation water demand.
The discharge in canal is usually regulated at the head regulator which is usually designed as a meter. When the head regulator cannot be used as discharge meter, a depth gauge is provided at about 200 m downstream of the head regulator. The gauge reading is suitably related to the discharge. By manipulating the head regulator gates, the desired gauge reading (and, hence, the discharge) can be obtained.