Canal outlets are of the following three types: 1. Non-Modular Outlets 2. Semi-Modular Outlets 3. Modular Outlets.
Type # 1. Non-Modular Outlets:
In non-modular canal outlets, discharge capacity depends on the difference of water levels in the distributary and the watercourse. The discharge through non- modular outlets fluctuates over a wide range with variations in the water levels of either the distributary or the watercourse. The non-modular canal outlet is controlled by a shutter at its upstream end. Loss of head in non-modular outlet is less than that in a modular outlet.
Hence, non-modular canal outlets are very suitable for low head conditions. However, in non-modular canal outlets, the discharge may vary even when the water level in the distributary remains constant. Hence, it is very difficult to ensure equitable distribution of water at all outlets at times of keen demand of water.
The non-modular canal outlet is usually in the form of a submerged pipe outlet or a masonry sluice which is fixed in the canal bank at right angle to the direction of flow in the distributary. The diameter of the pipe varies from 10 to 30 cm. The pipe is laid on a light concrete foundation to avoid uneven settlement of the pipe and consequent leakage problems.
The pipe inlet is generally kept about 25 cm below the water level in the distributary. When considerable fluctuation in the distributary water level is anticipated, the inlet is so fixed that it is below the minimum water level in the distributary. Figure 7.9 shows a pipe outlet.
Obviously, the discharge through non-modular outlets varies with water levels in the distributary and the watercourse. In case of fields located at high elevations, the watercourse level is high and, hence, the discharge is relatively small. But in case of fields located at low elevations, the discharge is relatively larger due to lower water course levels.
Further, depending upon the amount of withdrawal of water in the head reaches, the tail reach may be completely dry or get flooded. The discharge through pipe outlets can be increased by deepening the watercourse and thereby lowering the water level in it. The discharge varies from outlet to outlet because of flow conditions and also at different times on the same outlet due to sediment discharge in the distributary channel.
As such, proper and equitable distribution of water is very difficult. These are serious drawbacks of pipe outlets. The non-modular outlets can, however, work well for low heads too and this is their chief merit. Pipe outlets are adopted in the initial stages of distribution or for additional irrigation in a season when excess supply is available.
Type # 2. Semi-Modular Outlets:
The discharge through a semi-modular canal outlet (or semi-module or flexible outlet) depends only on the water level in the distributary, and is unaffected by the water level in the watercourse provided a minimum working head required for its working is available.
A semi-module is more suitable for achieving equitable distribution of water at all outlets of a distributary. The only disadvantage of a semi-modular canal outlet is that it involves comparatively greater loss of head.
The simplest type of semi-modular canal outlet is a pipe outlet discharging freely into the atmosphere. The pipe outlet described as non-modular outlet works as semi-module when it discharges freely into the watercourse. The exit end of the pipe is placed higher than the water level in the watercourse.
In this case, working head H is the difference between water level in the distributary and the centre of the pipe outlet. The discharge through the pipe outlet cannot be increased by the cultivator by digging the watercourse and, thus, lowering the water level of the watercourse. Other types of flexible outlets include Kennedy’s gauge outlet, open flume outlet and orifice semi-modules.
(i) Kennedy’s Gauge Outlet:
This outlet was developed by R.G. Kennedy in 1906. It mainly consists of an orifice with bellmouth entry, a long-expanding delivery pipe and an intervening vertical air column above the throat (Fig. 7.10). The air vent pipe permits free circulation of air around the jet.
This arrangement makes the discharge through the outlet independent of the water level in the watercourse. The water jet enters the cast iron expanding pipe which is about 3 m long and at the end of which a cement concrete pipe extension is generally provided. Water is then discharged to the watercourse.
This outlet can be easily tampered with by the cultivator who blocks the air vent pipe to increase the discharge through the outlet. Because of this drawback and its high cost, Kennedy’s gauge outlet is generally not used.
(ii) Open Flume Outlet:
An open flume outlet is a weir with sufficiently constricted throat to ensure supercritical flow, and long enough to ensure that the controlling section remains within the throat at all discharges up to the maximum. A gradual expansion is provided downstream of the throat. The entire structure is built in brick masonry, but the controlling section is generally provided with cast iron or steel bed and check plates.
This arrangement ensures the formation of hydraulic jump and hence the outlet discharge remains independent of the water level in the watercourse. Figure 7.11 shows an open flume outlet which is commonly used in Punjab. The discharge through the canal outlet is proportional to H3/2.
(iii) Orifice Semi-Modules:
An orifice semi-module consists of an orifice followed by a gradually expanding flume on the downstream side (Fig. 7.12). 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 very much. The roof block is fixed in its place by means of two bolts which are 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. However, 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.
Type # 3. Modular Outlets:
In modular canal outlets, the discharge is independent of the water levels in the distributary and the watercourse, within reasonable working limits. These outlets may have moving parts or may be without moving parts. In the latter case, these are called rigid modules. The modular canal outlets with moving parts are not simple to design and construct and are, hence, expensive.
A modular canal outlet supplies fixed discharge and, therefore, enables the farmer to plan his irrigation accordingly. However, in case of excess or deficient supplies in the distributary, the tail-end reach of the distributary may either get flooded or be deprived of water. This is due to the reason that the modular outlet would not adjust its discharge according to the level in the distributary.
But, if an outlet is to be provided in a branch canal which is likely to run with large fluctuations in discharge, a modular outlet would be an ideal choice. The outlet would be set at a level low enough to permit it to draw its due share when the branch is running with low supplies.
When the branch has to carry excess supplies to meet the demands of the distributaries, the discharge through the modular outlet would not be affected, and the excess supplies would reach up to the desired distributaries.
Similarly, if an outlet is desired to be located upstream of a regulator or a raised crest fall, a modular outlet would be a suitable choice. Most of the modular outlets have moving parts which make them costly to install as well as maintain.
Following two types of modular outlets (also known as rigid modules), however, do not have any moving part:
(i) Gibb’s rigid module
(ii) Khanna’s rigid module
(i) Gibb’s Rigid Module:
This module has an inlet pipe under the distributary bank. This pipe takes water from distributary to a rising spiral pipe which joins the eddy chamber (Fig. 7.13). This arrangement results in free vortex motion. Due to this free vortex motion, there is heading up of water (due to smaller velocity at larger radius—a characteristic of vortex motion) near the outer wall of the rising pipe. The water surface, thus, slopes towards the inner wall.
A number of baffle plates of suitable size are suspended from the roof of the eddy chamber such that the lower ends of these plates slope against the flow direction.
With the increase in head, the wafer bank up at the outer wall of the eddy chamber and impinges against the baffles and spins round in the compartment between two successive baffle plates. This causes dissipation of excess energy and results in constant discharge. The outlet is relatively more costly and its sediment withdrawal is also not good.
(ii) Khanna’s Rigid Orifice Module:
This canal outlet is similar to an orifice semi-module. But it has, in addition, sloping shoots fixed in the roof block (Fig. 7.14). These shoots cause back flow and, thus, keep the outlet discharge constant.
If the water level in the distributary is at or below its normal level, the outlet behaves like an orifice semi-module. But when the water level in the parent channel is above its normal level, water level rises in chamber A and enters the first sloping shoot. This causes back flow and dissipates additional energy.
This results in maintaining a constant discharge. The number of sloping shoots and their height above the normal level can vary to suit local requirements. The shoots are housed in a chamber so that these cannot be tampered with. If the shoots are blocked, the outlet continues to function as a semi-module.