1. The outlet should be strong, with no moving parts liable to be damaged or requiring periodic attention and maintenance.
2, It should be so designed that the farmer cannot temper with its functioning and any interference from him should be easily detectable.
3. Since a large number of out4ets are fixed. on a distributing channel, the moat essential requirement is that it should e cheap.
4. The design should be simple so that It can be constructed or fabricated by local masons or technicians,
5, It should be possible for the outlet to work efficiently with a small working head.
6 The outlet should draw Its fair share of silt carried by the distributing channel
7. From the farmers point of view, the outlet should give a fairly constant discharge However, from the and regulation point of, view, the outlet should draw proportionately more or on discharge with the varying supply in the distribution channel. Types Outlet Outlet may be classified under the following three heads:
1. Non modular Outlet
2. Semi module or Flexible Module
3. Rigid Module Non-modular Outlet A non-modular outlet is the one in which the discharge depends upon the difference in level between the water levels in the distributing channel and the water course. The discharge through such an outlet varies in wide limits with the fluctuations of the water levels in the distributing and the field channels. The common examples under this category arc : submerged pipe outlet, masonry sluice and orifices and wooden shoots.
Semi-module or Flexible outlet: A flexible outlet or semi-module is the one in which the discharge is affected by the fluctuations in the water level of the distributing channel while the’ fluctuations in the water levels of the field channel do not have any effect on its discharge. The various outlets in common use that fall under this category are pipe outlet Kennedy’s gauge outlet, Crump’s open flume outlet and pipe-cum-open flume outlet. Rigid Module A rigid module is the one which maintains constant discharge within limits, irrespective of the fluctuations in the water levels in the distributing channel and/or field channel. The most common outlet that falls under this category is th1e Gibb’s rigid module.
To understand the criteria for judging the behavior and functioning of outlets, the following definitions are useful: 1. Flexibility It is ratio of rate of change of discharge of an outlet to the rate of change of the discharge of the distribution channel. 2. Proportionality A proportional outlet is the one on which the flexibility (F) is equal to unity. Thus, in a proportional outlet, the rate of change of its discharge is equal to the rate of change of the discharge of the distributing channel. For proportionality.
The ratio HID is known as the setting. In a proportional outlet therefore, setting is equal to the ratio of the outlet and the canal indexes.
From the point of view of proportionality, an outlet is classified into three types:
(a) Proportional outlet
(b) Hyper-proportional outlet
(c) Sub-proportional outlet.
These will be explained both with respect to their Flexibility as well as setting. 3. Setting Setting is the ratio of the depth of the soil or the crest level of the module below the full supply of the distributing channel to the full supply depth of the distributing channel. 4. Hyper proportional outlet A hyper proportional outlet is the one in inch the flexibility is greater than one, i.e. the discharge in the outlet changes by larger percentage than the percentage change in the discharge of the distributing channel. Thus, for a hyper proportional outlet 5. Sub-proportional outlet A sub proportional outlet is the one in which the flexibility is lesser than one. i.e. the discharge in the outlet changes by a smaller percentage than the percentage change in the discharge of the distributing channel. Thus, for a sub-proportional outlet: 6. Sensitivity It is defined as the ratio of the rate of change of discharge of an outlet to the rule of’ change in the level of the distributing surface referred to the normal depth of the channel. Thus, 7. Efficiency Efficiency is the measure of a conservation of head by an outlet. It is defined as the ratio of the head recovered to the head put in. Less is the working head required for the functioning of the outlet, more will be its efficiency. 8. Drowning ratio It is ratio between the depths of water level over crest on the downstream and upstream of the module. Minimum modular head 9. The minimum modular head The minimum modular loss is the minimum loss of head or the difference between the upstream and downstream water levels which is absolutely necessary to he maintained to enable the module to pass its design discharge 10. Modular limits and modular range The modular limits of an outlet are the upper and the lower limits of any one or more factors beyond which an outlet is incapable of acting as a module or semi-module. Modular range is the range between the modular limits. It is the range of various factors which a module or semi-module works as designed. Non Modular Outlets Pipe Outlet The most common types of non-modular outlets are: submerged pipe outlet, masonry sluice and orifices, and wooden shoots, shows a pipe outlet. The pipes vary from 10 to 30 cm diameter, and are frequently laid on a light concrete foundation to prevent uneven settlement and consequent leakage. They are generally fixed horizontally at angles to the direction of flow.
(i) Pipe outlet discharging freely in the atmosphere,
(ii) Kennedy’s gauge outlet,
(iii) Open flume outlet,
(iv) Orifice semi-modules. (1) Pipe outlet If the pipe outlet is so set that it discharges freely in the atmosphere, the discharge through it becomes independent of the water level in the field channel and hence it acts as semi-module. The discharge cannot be increased by the cultivators by digging the water course. The outlet, therefore, worked well. Later, however, the farmers in invited a method of increasing the discharge by constructing a ramp in the water-course thereby heading up the water to the top level of the pipe and drowning it. (2) Kennedy’s gauge outlet This was the earliest types of semi module, invented by R.G. Kennedy, Chief Engineer, Irrigation, Punjab, sometime in 1906. This outlet is made of cast iron and consists of three main parts:
1. An orifice with bellmouth entry.
2. A long expanding delivery pipe.
3. An intervening vertical air column above the throat.
Water from the distributing channel enters the outlet through bell mouthed approach. From the narrowest diameter at thc throat the jet shoots across the air in the base of the vertical air vent pipe. The base of the air vent pipe is conical and allows tree circulation of air around the jet. Thus the jet is exactly in the same condition as if it were discharging into open air, making the discharge independent of the water-Level m the water course. The jet is then received by a cast iron expanding pipe, about 3 m long. At the end of the G.L pipe, cement concrete pipe extension is usually added and the water is then discharged in the field channel.
It has been found by experiments that the losses of head by friction, eddies. etc., are about 0.22 Ho and that the available working head or the minimum modular head should not be less than this value.
This module has, however; been superseded because its discharge could very easily be increased by closing air vents of the vent pipe. After the air vents are closed, the jet will suck the air of the chamber and its pressure drops below the atmosphere. This increases the discharge. Some safeguards like enclosing the air holes in a chamber and fixing the pipe to angle irons embedded in ground to prevent tampering were provided.
This worked for sometime, but the farmer, later, found a method of plugging the vent pipe through the bell mouth entry Due to this difficulty, and also due to high cost of its manufacture, this outlet is no longer in use.
(i) Cramp’s open flume outlet
(ii) Punjab open flume outlet
Fig shows the plan and section of Crump’s open flume outlet. The upstream face will is set back by a distance x equal to the width of the opening of the module. This enables the outlet to take a fair share of silt. This throat is constricted to ensure hypercritical velocity. The length of the throat is kept equal to 2.5.
1. An orifice with bellmouth entry.
2. A long expanding delivery pipe.
3. An intervening vertical air column above the throat.
Water from the distributing channel enters the outlet through bell mouthed approach. From the narrowest diameter at the throat the jet shoots across the air in the base of the vertical air vent pipe. The base of the air vent pipe is conical and allows tree circulation of air around the jet. Thus the jet is exactly in the same condition as if it were discharging into open air, making the discharge independent of the water-level in the water course. The jet is then received by a cast iron expanding pipe, about 3 m long. At the end of the G.L pipe, cement concrete pipe extension is usually added and the water is then discharged in the field channel. (3) Open flume outlet: An open flume outlet is a sort of weir with constricted throat at expanding flume on the downstream. Due to this, hypercritical velocity is ensured and hydraulic jump is formed. This formation the hydraulic jump makes the discharge through the outlet independent of the water-level in the water course, making it a semi-module. There are two types of open flume outlets:
(i) Crump’s open flume outlet
(ii) Punjab open flume outlet
Shows the plan and section of Crump’s open flume outlet. The upstream face will is set back by a distance x equal to the width of the opening of the module. This enables the outlet to take a fair share of silt. This throat is constricted to ensure hypercritical velocity. The length of the throat is kept equal to 2.5. (4) Orifice semi-module: An orifice semi-module consist of an orifice provided with a gradually expanding flume on the downstream side of the orifice. The flow through the orifice is hypercritical, resulting in the formation of hydraulic jump. This makes the discharge independent of the flow conditions in the watercourse. There are various forms of orifice semi module-the earliest of these being Crumps adjustable proportional module (commonly known as Crumps APM) Fig. shows Its modified form, known as adjustable orifice semi-module (A,O.SM), commonly used In Punjab and is, considered to be one of the best forms of the outlet The essential feature of the module is a roof block shaped to a lemniscates curved with a tilt of 1 In 7, fixed by bolts, Other features are similar to a flume regulator with horizontal crest.
1. Gibb’s rigid module.
2. Khanna’s rigid module.
3. Foote module.
Glbb’s module is described below: GIbb’s module
The outlet was designed by A.S,Gibb’s, formerly Executive Engineer, Punjab Irrigation.fig. shows the plan and the section of the module. The essential feature of the outlet is an eddy chamber, semi-circular in plan, round which water flows giving rise to a free vortex flow. Water enters through an, inlet pipe having bell mouth entry and is directed to the eddy chamber through a 180° rising pipe in which free vortex flow is developed. The characteristic feature of the free vortex flow is that the product of the velocity and radius is constant for all filament (V x r —constant). Thus the water at the outer circumference of the chamber has greater radius and hence lesser velocity, resulting in rise of water level there. Thus the water surface in the eddy chamber slopes down towards the inner circumference. A series of baffles are suspended from the roof of the eddy chamber, with their lower edges sloping at the required height above the sill of the module. If the head causing flow increases, water banks up at the outer circumference of the eddy chamber and impinges against the baffles imparting an upward, rotational, direction of flow to the water, which spins round in the compartment between two successive baffles and finally drops on the oncoming stream of water, thus dissipating excess energy. This keeps discharge constant for a wide range of variation in the ‘head. The number of baffles coming into action depends upon the variations in the head causing flow, The angle f eddy chamber varies from a semi-circle to 1=1/2 complete turns depending upon the discharge and the range of working required for the module.
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