In this article we will discuss about the non-modular, semi-modular and rigid modular irrigation outlets.
1. Non-Modular Outlets:
This outlet consists of a pipe circular or rectangular in shape. It may be in form of an open sluice also. Mostly pipe outlet is used as non-modular outlet. Its face, discharging water into the water course remains submerged into water. Pipes are laid in concrete with a masonry wall at its D/S face. Sometimes there may be masonry wall at U/S face of the pipe also.
Being embedded in concrete for full length and also because of masonry wall at its D/S face, the outlet cannot be tampered easily. This outlet does not distribute water equitably. Low lying fields lower the water level in the water course and thus discharge from the outlet is increased, more head being available. On the contrary higher fields do not get water of their share, as when water is issued to them the water level in water course increases and consequently head causing flow is reduced.
Discharge from this outlet is worked out from formula Q = CA√H , where C is constant whose value for average conditions is taken as 2.75, A is the sectional area of the pipe and H is the total loss of head, which is calculated by following formula-
where f is coefficient of friction, I is the length of the pipe in metres, d is diameter of the pipe in metres and V is the velocity of flow in m/ sec through the pipe. A table used by U.P. Irrigation Deptt. has been given in Table 21.1. In this table I has been considered 6 m. If length of pipe outlet exceeds 6 m next higher pipe diameter should be used. Silt discharge through the pipe outlet can be improved by keeping the D/S end at the same level but lowering the U/S end.
2. Semi-Modular Outlet:
In semi-module outlets, the discharge varies with water level in distributing channel, but is independent from water level in the water course.
The semi- modules are of following types:
1. Pipe outlet free fall
2. Kennedy’s gauge outlet or venturi flume outlet
3. Open flume outlet
4. Orifice semi-modules.
1. Free Fall Pipe Outlet:
It is one of the simplest types of semimodule outlets. The essential feature of this outlet is that it must discharge freely in air. It has high efficiency and its silt conducting power is good. Such an outlet can be provided only if there is sufficient difference between water levels in parent channel and water course. The best setting of this outlet is 0.3. But actually outlets are set lower than this and hence they are sub-proportional.
2. Venturi Flume Outlet:
This was invented by Mr. R.G. Kennedy and its modified form is known as Kennedy’s gauge outlet. It is made of cast iron and consists of an orifice with bell mouth entry, a long expanding delivery pipe, and an air vent pipe connected to the throat of the delivery pipe.
Because of air vent pipe, the jet of water at throat remains at atmospheric pressure and as such the fluctuations in the water level in the water course do not affect the discharge being issued from the throat.
The discharge of this outlet is found out by following formula:
Q = CA √2gH
Q = discharge of the outlet
C = constant of discharge whose value may be as high as 0.97
H = head measured from F.S.L. of the parent channel to the centre of the throat pipe.
A = area of cross-section of the pipe at throat.
In this particular outlet, the minimum head should be 0.22 H.
This outlet did not find much use as its discharge can be easily increased by the farmers by closing the airvent pipe. It is also a costly outlet, and easily tamperable by the users.
3. Open Flume Outlets:
This outlet consists of a weir, with long constricted throat and expanding flume on D/S side. This arrangement ensures formation of the hydraulic jump on D/S side and thus relieves the dependence of discharge on the water level in the water course and rendering the outlet semi-modular.
There are two types of open flume semi-modular outlets, namely:
(i) Crump’s Open Flume Outlet:
The main feature of this outlet is that wall lying on D/S of the flow in the parent channel is set projecting in the parent channel by d distance. This measure accelerates the silt conduction capacity of the outlet. Length of the constricted throat is kept 2.5 H. Discharge of this outlet is found out by following formula-
Q = CBtH3/2
where Bt = width of throat in metres
C = Coefficient of discharge whose theoretical value is 51.71
H = Head in metres over the crest
Q = discharge in cumecs.
This outlet works as proportional outlet when it is set at H/D =0.9. If set higher than this it works as hyper-proportional. It is a quite efficient outlet. Value of C varies from 1.60 for B varying from 6 cm to 9 cm width to 1.66 for B above 12 cm.
(ii) Punjab Open Flume Outlet:
It is slightly modified form the crump’s open flume outlet. In this outlet U/S wall of the outlet is curved for greater length and length of throat is reduced to 2H instead of 2.5 H. Formula for discharge is same as given for crump’s open flume outlet.
4. Orifice Semimodule Outlet:
This outlet consists of an orifice, developed with the help of a roofing block. The main features of this outlet are similar to a flumed regulator. It consists of a horizontal crest and curved approach on U/S side. D/S side is gradually expanding flume type. The flow through the orifice is hyper-critical and thus develops a jump on D/S side.
The water level in the water course does not affect the discharge of the outlet because of jump formation. The earliest form of this type of outlet is crump’s adjustable proportional module (A.P.M.). This outlet was later modified and modified form is known as adjustable orifice semi-module (A.O.S.M.). It is one of the best outlets and is very much used in Punjab and Rajasthan.
The discharge of the outlet is found from the following formula:
Q = 4.04 B1 V √HS
where Q = discharge in cumecs
V = vertical height of the opening
B1 = width of the throat
Hs = Head measured from water surface to the soffit of the opening.
N.D. Gulati gave following relation for minimum modular head (M.M.H.).
M.M.H. = 0.82 Hs – 0.5 B1
For proportionality consideration, the setting of this outlet i.e. Hs/D, should be 0.3.
The roofing block of the outlet consists of a set of cast iron straps in which bolts are fitted. The opening of the outlet is adjusted with the help of C.I. straps. Because of straps, tampering of the roofing blocks becomes almost impossible and even if there is tampering it can be easily detected.
3. Rigid Modular Outlet:
There are several types of rigid modules but Gibb’s rigid module is the most common type. Although Khanna also developed a rigid module but that did not work and hence not used.
1. Gibbs Module:
The module consists of bell mouthed inlet pipe and a curved rising rectangular trough, known as Eddy chamber. The rising circular trough is semi-circular in plan. A number of baffles are provided in the eddy chamber with their lower edges sloping at the required height above the bottom.
The baffles are provided to prevent increased discharge passing through the module. The water, after entering the inlet pipe through bell mouth, is led to eddy chamber where it develops into a free vortex flow. The characteristic of free vortex flow is that the product of the velocity and radius remains constant at all the points in the circular motion of the eddy chamber. The water level at the outer surface of the eddy chamber remains higher and water surface thus remains sloping towards the inside.
When flow into the module is due to increased head, water banks up at the outer circumference of the eddy chamber, and strikes against the baffle walls. Thus excess energy due to increased head is dissipated and discharge through the module remains constant. The number of baffle walls coming into action depends upon the increase or decrease in head. The angle of eddy chamber varies from a semi-circle to 1 ½ turns, depending upon the discharge and range of working required for the module.
The discharge of the module is found from following formula:
where Q = discharge
r0 = outer radius of the eddy chamber
r1 =inner radius of the eddy chamber
m = r0 /r1
d1 =depth of water at inner circumference
h0 =head loss in inlet pipe.
This formula is valid for the design given by Mr. Gibbs. He adopted m = 2 and h0 / D =7.
D = difference in level measured from minimum water level in the distributing channel to the floor of eddy chamber.
2. Khanna’s Rigid Module:
It is nothing but an orifice semi-module outlet. In addition, it consists of inclined shoots in roof, which remain connected to the throat of the outlet. When head causing flow is small the outlet works as A.O.S.M. When level in the parent channel rises above predetermined level, the water level in the differential chamber also rises and water starts flowing in the inclined shoots.
The water flowing through inclined shoots, strikes against the flow passing through the throat and dissipates the extra energy and thus discharge through the outlet is maintained constant. If more rise in water lever, in distributing channel takes place more inclined shoots come into action and excess energy due to increased head, dissipated. Inclined shoots are sloped at 1 in 2 slopes. This rigid module is not much in use as shoots can be easily closed and increased discharges can be obtained by the farmers. Inclined shoots may even get clogged.