The following points highlight the distribution channels that are used for discharging water into the canals for irrigation. The distribution channels are: 1. Standing Wave Flume 2. Crump’s Adjustable Proportional Semi-Module 3. Lindley Type Standing Wave Flume 4. Gibb’s Module 5. Pipe Outlets 6. Open Flume Outlets 7. Adjustable Proportional Module (APM) 8. Tail Clusters.
Distribution Channel # 1. Standing Wave Flume:
The standing wave flume is a semi-module measuring discharge with a high degree of accuracy (viz., 1.5 percent) besides having the advantage that a single gauge reading upstream is all that required. In the standard standing wave flume evolved at Poona- the head required can vary from 8 to 15 percent of the upstream depth of water over the sill without affecting the discharge; the modular ratio (i.e., the ratio of the downstream water depth to the total upstream depth, measured above the skill level) can be as high as 85 percent in small flumes and 92 percent in large flumes.
It can be best used when variable discharge needs to be measured accurately and also when facilities for supervision or for automatic recording for gauges are available.
This flume was evolved by Crump (Punjab) and Inglis (Bombay) after carrying out intensive model investigations and shown in Fig. 4.19 and Fig. 4.20 as shown below:
The flume comprises of:
1. An approach channel of suitable design,
2. A bell mouth entrance,
3. A throat with a horizontal bottom and vertical sides,
4. A downstream glacis, and
5. An expansion in which the flow is redistributed before it passes into the downstream channel and head is recovered.
It is essentially a broad-crested weir and its discharge is given by formula:
Q = CoCBH1.5
in which, B is the width of the throat, H is the total head (depth of water above crest plus the velocity head,) on the upstream side sill level, and C is a coefficient to allow for losses due to friction, eddies, impact shock, etc.
Values of C and adjusted values of the constant for properly designed flumes without piers are given in Table below:
More abrupt curves than in the standard design will slightly lower the coefficient. The coefficient C (= 0.99) for discharges from (1.4 to 14 m3/sec) was confirmed by actual observations carried out on the prototype in Sind. With piers, loss of energy due to shock which lowers the value of C. In Sind, falls and fall regulators were designed using the values shown in Table above, but observations showed that C was much lower, the average value of C for discharges 110 to 280 m3s on the Rohri Canal being about 6 percent lower.
Based on the experimental investigations carried out at the Central Water and Power Research Station, Poona, in 1933, the following formula is suggested with the piers:
Q = C(B – k n H) H1.5
In which, ‘k’ is the coefficient of contraction due to piers, (0.82 with standard piers), n is the number of piers, B is the waterway, C = 3.088, and H = total head (including velocity head).
Distribution Channel # 2. Crump’s Adjustable Proportional Semi-Module:
This semi-module can be either of the orifice type or of the open type and fixed at the head of the outlet. These have been used extensively in Punjab as showing Fig. 4.21.
Distribution Channel # 3. Lindley Type Standing Wave Flume:
This is a short throated flume with one side straight and the other curved. This is normally used as an outlet for water courses taking off at right angles from the distributary.
Distribution Channel # 4. Gibb’s Module:
The main disadvantage of a non-modular outlet is that cultivators can draw more water by tampering in large numbers on a canal system. Gibb module was found to be the only module which has no moving parts. As against modules whose working depends on floats or other moving mechanisms there are a few devices in which the discharge is automatically regulated by the velocity of the water itself without the necessity of any moving parts.
Gibb an Executive Engineer of the irrigation department, Punjab devised a module form of outlet, which was built for the first time on the Melay distributory of the Lower Thelam Canal. This module is named as Gibb module after its inventor and it gives an almost constant discharge over a considerable range, irrespective of the upstream and downstream water levels. It is one of the rigid modules without any moving parts. It does not need any supervision and cannot be easily tampered with.
Water is led through an inlet pipe (See Fig. 4.22) into a spiral rectangular trough (eddy chamber) in which free vortex flow is developed. The water on the outside of the curve rises in level and the water surface slopes towards the inner wall. A number of baffles are inserted in the eddy chamber with their lower edges sloping at the required height above the bottom.
As the head increases, the 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 on-coming stream of water, thus, dissipating excess energy and keeping the discharge constant. The degree of turn of the spiral depends on the volume of discharge and the working range required and generally varies from one semi-circle to one and a half complete circles.
Though this module gives constant discharge, it has the following disadvantages:
1. This module could be easily tampered with by breaking the baffles and eddy chamber.
2. It is costlier than other types of outlets.
3. Construction of this module is a very difficult process and needs higher technical skills.
4. It is said to have a lot of trouble regarding silt drawal. The vent is likely to be choked by the silt and floating materials coming in the channel and periodical cleaning may be difficult.
Under the circumstances stated above this module can be used in places where small drawals are required for small plots from main channels. For example in channel having 0.5 m3/s flow a plot of 40 hectares will be requiring 0.03 m3/s and the depth of flow in the main channel will less than 0.4 m. Under such circumstances this will ensure minimum losses due to the small branches taking off from main canal.
Distribution Channel # 5. Pipe Outlets:
This is a pipe with exit end submerged in the watercourse (Fig. 4.23). The pipes are placed horizontally and at right angles to the centre line of the distributing channel and acts as non-modular outlet.
Discharge through the pipe outlet is given by the formula:
Q = CA (2gh)1/2
In the above equation, Q is the discharge; A is the cross sectional area; g is the acceleration due to gravity; H is difference in water levels of supply channel and watercourse and C is the coefficient of discharge which depends upon friction factor (f), length (L) and diameter of the outlet pipe (d) related by the formula:
The coefficient f is the fluid friction factor and its value may be taken as 0.005 and 0.01 for clear and encrusted iron pipes respectively. For earthenware pipes, f may be taken as 0.0075. All other variables are in SI units, that is, meters and seconds.
It is a common practice to place the pipe at the bed of the distributing channel to enable the outlets to draw proportional amount of silt from the supply channel. The entry and exit ends of the pipe should preferably be fixed in ‘masonry to prevent tampering. Since the discharge through this type of outlet can be increased by lowering the water surface level of the watercourse (thus increasing the value of H in the discharge equation), it is possible for the irrigator to draw more than fair share of water.
A pipe outlet may also be designed as a semi-modular outlet, that ‘is, one which does not depend upon the water level in the watercourse by allowing it to fall freely in to the watercourse (Fig. 4.24).
Pipe outlets require minimum working head and have higher efficiency. It is also simple and economical to construct and is suitable for small discharges. However, these outlets suffer from disadvantages like the coefficient discharge which varies from outlet to outlet and at the same outlet at different times apart from the possibility of tampering in the non-modular type.
Distribution Channel # 6. Open Flume Outlets:
This is a smooth weir with a throat constricted sufficiently long to ensure that the controlling section remains within the parallel throat for all discharges up to the maximum. Since a hydraulic jump forms at the control section, the water level of the watercourse does not affect the discharge through this type of outlet. Hence this is a semi-modular outlet.
This type of structure is built in masonry, but the controlling section is generally provided with case iron or steel bed and check plates. The open flumes can either be deep and narrow of shallow and wide in which case it falls to draw its fair share of silt. Generally, this type of outlet does not cause sitting above the work, except when supplies are low for a considerable length of time. The silt which gets accumulated gets washed away during high supplies.
The open flume outlet is also cheaper than the Adjustable Proportional Module (APM). The discharge formula for the open flume outlet is given as:
Q = CB1 H3/2
Where Q (given in 1/s) is related to the coefficient of discharge C, as given in the table below, Bt is the width of the throat in cm; and H is the height of the full supply level of the supply channel above the crest level of the outlet in cm.
The minimum head required to drive the outlet is about 20 percent of H.
Distribution Channel # 7. Adjustable Proportional Module (APM):
There are various forms of these outlets but the earliest of them is the one introduced by E.S. Crump in 1992. In this type of outlet, a cast, a cast iron base, a cast iron roof block and check plates on either are side are used to adjust the flow and is set in masonry structure (Fig. 4.26). This outlet works as a semi-module since it does not depend upon the level of water in the watercourse.
The roof block is fixed to the check plates by bolts which can be removed and depth of the outlet adjusted after the masonry is dismantled. This type of outlet cannot be easily tampered with and at the same time be conveniently adjusted at a small cast.
The roof blocks may also be built of reinforced concrete. The face of the roof block is set 5 cm from the starting point of the parallel throat. It has a lamniscate curve at the bottom with a tilt of 1 in 7.5 in order to make the water converge instead of a horizontal base which would cause it to diverge. The cast iron roof block is 30cm thick.
As such, the APM is the best type of outlet if the required working head is available and is the most economical in adjustment either by raising or lowering the roof block or crest. However, it is generally costlier than the other types of outlets and also requires more working head.
The discharge formula for this type of weir is given as:
Q = C Bt H1 (H2)12
Where Q (given in e/s) is related to the coefficient of discharge, C, which is taken equal to around 0.0403; Bt is the width of the throat in cm; H1 is the depth of head available, that is the difference between the supply channel full supply level and the outlet bed (crest) level; and H2 is the difference between the supply channel full supply level and the bottom level of the roof block (Fig. 4.27).
The base plates and the roof block are manufactured in standard sizes, which with the required opening of the orifice are used to obtain the desired supply through the outlet.
Distribution Channel # 8. Tail Clusters:
When the discharge of a secondary, tertiary of quaternary canal diminishes below 150 l/s, it is desirable to construct structures to end the canal and distribute the water through two or more outlets, which is called a tail cluster. Each of these outlets is generally constructed as an open flume outlet (Fig. 4.28).
