In this article we will discuss about:- 1. Introduction to Irrigation Outlets 2. Requirements of a Good Outlet 3. Classification or Types 4. Tail Cluster Outlet 5. Selection.
Contents:
- Introduction to Irrigation Outlets
- Requirements of a Good Outlet
- Classification or Types of Outlets
- Tail Cluster Outlet
- Selection of Outlet
1. Introduction to Irrigation Outlets:
Outlet is a small structure or device, built at the head of the water course to facilitate flow of water from distributing channel to the water course. The water course receiving water from outlet leads it to the fields for irrigation purposes. We may call outlet, as a head regulator for the water course.
Government is responsible for maintenance and running of main canal, branch canals, distributaries and minors. Outlets are also controlled by the government. But responsibility of maintenance of water courses is of farmers. Outlet is a device which connects the government controlled channel with farmer controlled water course.
2. Requirements of a Good Outlet:
An outlet should fulfill the following requirements:
1. The outlet should be simple in design, construction and working.
2. Farmers should not be in a position to tamper with the outlets easily. If tampered, it should be easily detectable.
3. It should function efficiently with very small working head.
4. It should be cheap and easy in initial construction as well as in maintenance.
5. It should draw its fair share of sediments from the parent channel, which may be minor, distributary, or branch.
6. From farmer’s point of view, the outlet should give constant discharge, irrespective of change in discharge of parent channel.
7. From distribution point of view, the outlet should draw only the proportional discharge from the varying discharge of the parent channel.
8. It should not be easily clogged by floating or suspended matter, in the parent channel.
9. It should not weaken the bank of the canal.
3. Classification or Types of Outlets:
The outlets may be divided into following three categories:
1. Non-Modular Outlet:
It is such an outlet whose discharge is dependent upon the difference in levels of water in the distributary channel and the water course. In this, discharge varies with the water levels in the distributary as well as the water course. Submerged pipe outlet, masonry or orifice outlets, are the examples of non-modular outlets.
2. Semi-Modular Outlet:
It is also known as flexible outlet. The discharge of this outlet is dependent upon the water level in the supply channel only. Fluctuations in the water level and in the water course do not in any way affect the outlet discharge. Free discharging pipe outlet, Kennedy’s gauge outlet, crumps open flume outlet, are the common examples of semi-modular outlets.
3. Rigid or Modular Outlet:
It is such an outlet, which within minimum working limits of head, always gives constant discharge. Fluctuation in levels of water in distributing channel and water course do not affect the discharge whatsoever. Gibb’s rigid module is the example of rigid outlet.
4. Tail Cluster Outlet:
As the canal goes on flowing from head towards tail its discharge goes on reducing, as outlets are continuously withdrawing water. At the tail of the canal when canal carries only discharge for two or three outlets, the discharge is distributed to the area with the help of open flumed type bunch of outlets. This bunch of outlets is known as tail cluster.
The discharge in the canal at tail remains fluctuating. If some outlets in the head reach are taking excess discharge or somehow irrigation water is being stolen by siphons or otherwise, the water reaching tail will be very small. On the contrary when head reach outlets are drawing less than the design discharge, excess amount of water reaches the tails. Hence uncertainty about the availability of irrigation water at tail always remains and hence no farmer is prepared to purchase lands at tails.
In other words, the cost of the land is generally very low at tails in relation to upper regions. To deal with excess water reaching tails, a Tail escape is also provided. Tail cluster takes its designed discharge or even more if required, and the remaining excess water is let loose in the open low lying land through Tail escape.
Spanish Modules:
It is rigid module operated with the help of a float. The head of water over the throat or opening is maintained constant with the help of a float. The outlet consists of an entrance pipe and a float chamber. The discharge given by this outlet is A x V. The float remains connected to a needle valve.
Tail Reservoir:
Sometimes instead of directly tailing the canal by Tail cluster, a tail reservoir is constructed. Canal water and excess water reaching at the tail, both get collected in the reservoir. The outlets are given from this reservoir.
The reservoir eliminates the fluctuations in the discharge at tail cluster and excess stored water from the reservoir can be used to augment the discharge at the time of low discharges at tail. In this way, the water which would have escaped through tail escapes, and gone waste, is economically used.
5. Selection of Outlet:
While selecting the type of outlet the paramount consideration in the mind should be, that water is equitably distributed to the farmers. Besides this, assured suppliers of water should always be available. This develops confidence into the farmers and they can plan their crop patterns well in advance.
From this point of view rigid modules are considered as the best choice. But this is not practicable. When canal has full supplies it can feed all the outlets with designed discharge, but when supplies in the canal are less than designed, only head reach outlets will receive water and all the outlets on tail side will go dry. This causes unequal distribution of water.
Secondly on all the new canals, position of outlets may have to be frequently changed to achieve better locations. But rigid modules once set cannot be easily shifted. Hence from this consideration, non-modular outlets should be initially installed and after some years of run of the canal, when area gets stabilized and outlet positions are almost finally fixed, the non-modular outlets should be replaced by semi-modular outlets.
Rigid modules also do not draw the silt of their share. It is one of the biggest drawbacks with rigid modules. Rigid modules are practically not used anywhere. Mostly non-modular outlets are installed in the beginning and after some time when conditions become stabilized they are replaced by semi-modules, predominantly A.O.S.M. type.