Definition and Functions:
Regulatory works are the hydraulic structures constructed across the canals to facilitate complete control over the flow of water in the irrigation canals.
The regulatory works are constructed to perform following functions:
1. A regulator regulates the flow of a canal by releasing measured quantity of water in the canal.
2. A fall or a rapid corrects the bed slope of a canal and prevents the canal from going into excessive filling.
3. An escape is a surplussing channel which takes away excess flow from an irrigation canal.
4. A silt ejector or a sluice removes the deposited silt from an irrigation canal and keeps it clean.
5. An outlet releases measured discharge from a canal into a field channel for irrigating crops.
6. A flume and a gauge well helps in measuring the canal discharge at a desired point.
Regulators:
For equitable and efficient distribution of irrigation water it is very essential to regulate the supply. A hydraulic structure constructed to regulate the water supply is called a regulator. The regulators not only regulate the irrigation water supply but also control the silt entry into the canal.
To regulate the water supply it is essential to have various vent-ways. The ventways are provided by constructing abutments and piers in the canal cross section. The grooves are made in the piers and abutments in which shutters operate. They control the openings. To control silt entry the shutters are provided in tiers. The lower tier is usually kept closed. Thus sill of the regulator is raised. As a result only top layers of silt free water enters the canal through the regulator.
For efficient and successful regulation it is essential that a regulator should serve the purpose of a meter also.
For economy a road or a rail bridge, if any, should be combined with the regulator. As then the piers, abutments and the foundation work is common to both structures.
Depending upon the location of a regulator following broad classification of regulators may be recognised:
(i) Canal head regulator.
(ii) Canal cross regulator.
(iii) Distributary head regulator.
(i) Canal Head Regulator:
It is a hydraulic structure constructed at the head of a canal system to regulate the irrigation supplies.
(ii) Canal Cross Regulator:
It is a hydraulic structure constructed across a canal to regulate irrigation water supplies. It may be constructed across any type of canal, main, branch or a distributary.
Following considerations make it necessary to construct a regulator across the canal:
(i) When due to inadequate supply the water level is lowered the off-taking channels do not get their proper share. A cross regulator is provided to raise the water level.
(ii) Sometimes it becomes necessary to carry out some repair work on a canal. A cross regulator if existing above, the canal can be closed and repairs can be done efficiently.
(iii) Sometimes it is necessary to close the canal below a particular point. Say when there is no demand for irrigation water during a particular period.
(iv) Cross regulators divide a long canal reach into smaller ones and make it possible to maintain the reach successfully and efficiently. For efficient functioning they should be spaced 10 to 13 km apart on the main canal and 7 to 10 km on the branches.
A cross regulator is often combined with a rail or a road bridge. When there is a fall available on the canal the cross regulator is constructed as a fall-regulator (Fig. 17.1). The cross regulator may be flumed at the site. It is similar in construction to the head regulator.
(iii) Distributary Head Regulator:
It is a hydraulic structure constructed, at the head of a distributary. This regulator performs the same functions as that of a head regulator. That is regulation of supply of a distributary. It can be used many times as a meter. It is also a silt selective structure. Only difference is that distributary head regulator is much smaller in magnitude as compared to the head regulator. Fig. 17.2 shows sectional end view of a distributary head regulator.
Falls:
A canal is given uniform bed slope. However, natural ground does not have uniform slope. When the ground has a steep slope heavy earth filling is required to construct the canal with a flatter bed slope. It is a very costly method. As an alternative, vertical fall or a drop may be provided at a suitable section. It brings down the canal bed line. In this process water comes down the fall with a great force. All the excess potential energy is converted into kinetic energy. Excess energy of flow is destroyed with some suitable energy dissipation method.
Following points should be considered while selecting a site for a fall:
(i) Possibility of combining some other structure with a fall e.g. a cross regulator, road bridge, etc. It reduces the cost of the project.
(ii) Command should not be reduced due to lowering of F.S.L. The fall may be located below the outlets.
(iii) Cutting and filling required below and above the fall should be equal.
Types of Falls:
(a) Ogee Fall:
In early stages of fall design an ogee fall was commonly constructed. The body of the fall was given the shape of a falling nappe. It was given to provide smooth changeover of water levels (Fig. 17.3). In this type protection of the bed below the fall was found difficult because the descending water used to acquire excessive kinetic energy.
(b) Rapid or Glacis:
Second stage of development of a fall was a rapid or glacis. It was nothing but an inclined fall with steep slope (Fig. 17.4).
The slope ranged from 1 in 10 to 1 in 20. To protect bed and sides of the canal stone pitching was done.
In this fall towards the foot of the slopping fall a standing wave or a hydraulic jump forms. It destroys the excess kinetic energy of flow. The disadvantage of this fall is that it occupies quite a large length.
(c) Stepped Fall:
It was constructed with small drops. Short horizontal step was given after every drop. The fall is something like a staircase. It also takes sufficient length.
(d) Vertical Fall:
It consists of a body wall constructed across the canal. Its crest is kept in level with the upstream canal bed. The body wall is given slight batter on the downstream side. It is constructed with masonry. Gates are generally provided on the top to regulate the flow. Then it acts as a regulator also. Some energy dissipation device is provided below the fall. (Fig. 17.5)
(e) Notch Fall:
It consists of a body wall constructed across the canal. On the body wall there are notches in between the piers. The notches may be trapezoidal or rectangular in shape. The sill of the notches is in level with the upstream canal bed above the fall.
Thus depth discharge relationship of normal canal section is maintained at the fall also. Hence the fall can be used for measuring the discharge of a canal. As the sill of the notch is at bed level there is no silting. Some energy dissipation device is provided below the fall. (Fig. 17.6)
(f) Sarda Type Fall:
It is a fall with a raised crest. The body wall is constructed like a weir (Fig. 17.7). Below the fall suitable device is provided for dissipating excess energy of falling water. This type of falls were constructed on the Sarda canal in Uttar Pradesh and hence the name. As the crest of the fall is raised silting of the upstream canal is possible.
(g) Flumed Fall:
In this type the length of the body wall of a fall is less than the normal canal width. The section is restricted at site of the fall. The narrowing of the section is done gradually. Three main falls come under this category.
They are the following:
(i) C.D.O Type Fall:
It is widely constructed in Punjab. It is a drowned type of fall.
(ii) Montague type fall.
(iii) Inglis type Fall.
Both are very similar in features. In both the falls hydraulic jumb occurs on the downstream sloping face. It destroys the energy.
(h) Fall Regulator:
It is designed as a fall cum regulator. Generally cross regulator is very well combined with a fall. It is constructed in such a way that the regulating gates can be arranged to suit the water level upstream of the fall. The fall regulator is shown in Fig. 17.1.
Canal Escape:
They are nothing but outlet structures provided in a canal bank with a side channel to relieve irrigation canals of excess discharge, if any.
The outlet structure is of two types:
(i) Weir type, and
(ii) Sluice type.
The crest of a weir or the sill of a sluice is kept at F.S.L. to allow withdrawal of flow in excess of full supply discharge only. The openings are generally controlled with gates.
Although maximum discharge of an irrigation canal is always fixed, the canal discharge may increase in a particular reach due to any one of the following reasons:
(a) Excessive rainfall in the upstream reach.
(b) Faulty regulation at the head of the canal.
(c) Sudden closure of outlets in the upstream reach.
It is clear that if discharge in a canal is allowed to increase above the design discharge irrigation canal is likely to be damaged. Hence provision of escapes is essential. The capacity of an escape may be kept about 50 per cent of the design discharge of the irrigation canal. The excess water may be taken through side channel to a natural drain or a river for proper disposal. Location of escape is done in arbitrary manner. But after an interval of 60 to 70 km an escape may be provided on long canals. The interval may be smaller if situation so demands.
The escapes may be provided on the canal at intermediate points and also at the tail of a canal.
Then they are called by different names viz.:
(a) Surplussing escape, and
(b) Tail escape, respectively.
The escape may also be used to scour out silt from a canal. Then it is called silt escape (Fig. 17.16).
Silt Ejector and Sluices for Silt Removal:
That though silt has manurial property heavy silt concentration in the irrigation canal creates troubles. It is very essential to keep perfect control over silt concentration.
Following two steps are essentially required to keep control over silt:
(i) Prevention of silt entry into the canal; and
(ii) Removal of silt which has entered into the canal.
The silt which has entered the canal and is likely to disturb the regime condition may be removed in various ways:
(a) Silt Ejector:
This is a structure which extracts the bottom layers of silt laden water. It consists of a series of tunnels constructed on the bed of a channel. They are parallel to the flow at entrance. The tunnels are then turned through 90 degrees to take out the water from the canal (Fig. 17.13).
The tunnels are formed by constructing the piers on the bed of a canal and then covering the compartments so formed by a slab. To increase the efficiency of an ejector the canal is widened just up-stream of the ejector. When the waterway is increased, the velocity of flow decreases.
As a result the silt load remains confined to the bottom layers. The bed of the tunnels may be depressed below the bed of the canal for achieving efficiency in extracting. The tunnel bed may be given suitable slope for maintaining self-cleaning velocity. Once the silt laden water comes out of the canal it may be disposed off suitably in some natural drain.
(b) King’s Vanes:
They are also termed as silt vanes. They are nothing but curved vanes (8 cm thick, made of R.C.C.) of low height (height ranges from 1/3 to 1/4 depth of flow) constructed on the bed of a parent channel in front of an off-taking channel. They are generally spaced 1.5 times the height. They extend for half the width of the parent channel.
The opening of the vanes is at right angles to the flow of water. In other words the vanes are parallel to the flow of water on the upstream side. The main aim of this structure is to deflect the silt laden bottom layers of water. Thus only clear water is allowed to enter the off-taking channel (Fig. 17.14).
The radius of the vanes ranges from, 7.5 to 12.25 m. Down-stream end of the vanes are generally inclined at 30 degrees to the direction of flow. The length of the vanes should be sufficient to protect full width of the off-taking channel. Generally the vanes extend 0.6 to 1.5 m beyond an imaginary line drawn from down-stream junction point of the off-taking channel at 2: 1 slope (longitudinal: lateral).
(c) Gibb’s Groyne Wall:
It is a solid and slightly curved wall which extends from the downstream junction point of an off-taking channel into the parent channel (Fig. 17.15).
As the wall extends in the parent channel it divides the flow of the parent channel into two compartments. As a result the silt charge of the flow is also divided.
Thus the groyne wall allows only part of the silt charge to enter into the off- taking channel.
(d) Silt Escapes or Sluices:
They are nothing but scouring sluices provided in the body of a weir constructed in the bank at the mouth of some natural drain as shown in Fig. 17.16.
The silt level of the sluices may be fixed below the bed of the canal for efficient working. The accumulated silt is flushed through the sluices at intervals. The escapes are provided all along the canal reach where possible. It is true that for flushing the silt large quantity of water is required. Hence this procedure can only be used when irrigation water is available in excess.
(e) Silt Traps:
Sometimes pits of suitable size may be dug in the bed of a channel. When the canal runs silt goes on accumulating in these pits. When the pits are filled and when the canal is dry accumulated silt may be removed manually.
Irrigation Outlets:
They are the openings constructed in the banks of distributaries and minors. A field channel takes off from this point and gets the irrigation water through the outlet. The outlet is also called a turn-out or a sluice in some parts of India. In a canal system there are numerous outlets. The channels below the outlets are maintained by the cultivators. Once the water comes out of an outlet Irrigation Department has no control over it.
Hence a perfect outlet should fulfill following requirements:
(i) Cost of construction of the outlet should be low.
(ii) The outlet should be simple and the construction should be speedy.
(iii) Depth of flow in minors and distributaries is quite low. Hence the outlet should work under low head.
(iv) The outlet should draw the silt in proportion to its discharge.
(v) It should be difficult for cultivators to tamper with the outlet.
(vi) There should not be moving or loose parts in the outlet. Moving parts are damaged early.
(vii) The outlet should discharge a constant quantity of water. It should be independent of water depths in distributary and the field channel.
(viii) The outlet should work equally well when head is high and when it is low.
There are three classes of outlets. They are:
(i) Non-modular outlet;
(ii) Modular outlet or rigid module; and
(iii) Semi module or flexible module.
A non-modular outlet is an ordinary type of outlet in which discharge is directly dependent on the working head. That is, it is dependent on the difference of water level in the parent channel and in the field channel. Thus when working head is less discharge is less. On the contrary when the head is more discharge is more.
A modular outlet is also called a rigid module. In this type of outlet, the outlet discharge is maintained constant and it is not at all dependent upon the water levels in the parent and the field channels.
Sometimes semi-modular outlet is also termed as flexible module. In this type of outlet the discharge is directly dependent on the water level in the parent channel. But it is independent of the water level in the field channel.
A proportional module is a type of semi-module. In this type the discharge fluctuates in the same ratio as that of full supply discharge of the parent channel.
The outlets of each type mentioned above are described below:
a. Pipe Outlets:
It is provided in the form of a simple opening made in the canal bank to lead water from the parent channel to the field channel. Fig. 17.17 shows longitudinal section of a non-modular pipe outlet.
b. Pipe Outlet (Non-Modular Type):
The opening is generally drowned. The pipe line or barrel is generally laid in horizontal position. For channels in which discharge variation is more sill of the opening is kept at the bed level of the channel. To regulate the discharge through the outlet shutter may be provided at entrance with some type of locking arrangement.
c. Gibb’s Module:
It is a modular outlet. Irrigation water is taken through an inlet pipe to a rising pipe. The rising pipe is generally semi-circular.
The rising pipe is connected to an eddy chamber. Fig. 17.18 gives a plan and longitudinal section of the Gibb’s module.
The eddy chamber is rectangular in section but semi-circular in plan with horizontal floor. It takes back the water in the original direction of flow. In the eddy chamber the baffles are provided at equal distances to dissipate excess energy of flow, and to maintain a constant discharge. The baffles do not rest on the bottom of the eddy chamber but there is an opening left between the floor of the chamber and the lower end of the baffles.
This bottom opening is not rectangular in shape but the depth of opening goes on reducing towards the inner side of the chamber. This arrangement helps in maintaining a constant discharge. From the eddy chamber water goes to a spout. The spout is joined to a field channel by 1: 10 diverging section. The Gibb’s module requires complicated arrangements and it is costly. In alluvial tracts it gets choked with silt.
d. Kennedy’s Semi Module:
It consists of a bell mouth orifice. It is made of cast iron. The orifice is fixed in a truncated cone which is slightly bigger in diameter than the orifice. An air vent pipe is fitted at the junction of the cone and the orifice. The air vent pipe is kept sloping and is protected by an angle iron on the outer side. An enameled gauge is fixed on the angle iron (Fig. 17.19).
The air vent pipe is fitted to permit the orifice to discharge into free air, at atmospheric pressure. The air vent pipe is connected to an air inlet pipe at the top. The air inlet pipe is a horizontal perforated pipe laid on dry ballast. It makes the discharge independent of the water level in the field channel so long as the minimum modular head is available. Minimum modular head is 0.22 H. Where, H is depth of water over the centre of the orifice.
The outlet is cast in definite sizes for fixed discharge. The intermediate discharges may be obtained by raising or lowering the outlet orifice.
Crump’s Adjustable Proportional Module:
Generally abbreviation A.P.M. is used for this type. It is also called Adjustable Orifice Semi Module (A.O.S.M.). Fig. 17.20 gives a plan and longitudinal section of A.P.M.
In this type a cast iron roof block is provided at the entrance end. The roof block is screwed to the masonry entrance by the bolts fixed in the masonry. This block is given lemniscate curve at the lower end on the entrance side. At the sill also a cast iron base is provided. A 0.3 m wide check plate is also provided.
To facilitate the smooth water entry the upstream wing wall is made smaller in length. There is a throat of uniform width for about 0.6 metres. Then the side walls diverge out with a radius of 7.6 m. The bed of the outlet is laid with a slope of 1 in 15 till it joins the bed of a water course. Whole outlet is constructed with masonry.
Thus it is perfectly rigid once the roof block is fixed. But at the same time after dismantling the masonry slightly the opening can be adjusted by lowering or raising the roof block. The velocity of water in the outlet barrel is above critical. Hydraulic jump occurs on the sloping bed of the outlet downstream of the crest.
This type of outlet is very popular because of its merits over other types.
Tail Clusters:
A structure constructed at the tail end of an irrigation canal is called a tail cluster. It is nothing but an assemblage of more than one similar outlet. When a distributary or a minor carries 0.14 m3/sec or less water distribution is done by the tail cluster. It assures equitable distribution.
The tail cluster consists of similar outlets. They are always arranged symmetrically in a group. The most suitable type of outlet in such situation is an open flume semi-modular outlet. Figs. 17.21, 17.22 and 17.23 show the schematic arrangement of tail clusters for two, three and tour outlets respectively.
The crest of all the outlets of a tail cluster, are kept at the same level. Then the water entry in each outlet is equally affected due to the change in the discharge of the parent channel. The side modules are aligned at equal inclination with the centre line of the parent channel.
Flumes and Gauge Wells:
A flume is that portion of a canal which is made narrow than the normal canal width. Fluming is done by building a converging masonry. It is then followed by a short throat. It is nothing but a narrow rectangular water-way of short length. A diverging masonry is built to join the throat to the normal canal section below. To prevent losses at entry and exit splay is given to the transitions.
The objects served by the flumes are the following:
(i) Metering:
By reading the water level in a gauge well at the entrance of the throat discharge of the canal can be measured.
(ii) Fluming of important hydraulic structures is done to reduce the cost of construction. Aqueducts, falls, cross regulators, etc. are generally flumed to reduce the cost.
The flumes can be divided as a Venturi flume or a meter flume and a standing wave flume.
Standing Wave Flume:
It is very much similar to a venturi flume in construction. A standing wave or a hydraulic jump forms on the downstream slope in the divergent transition. To create a jump on the bed of throat a hump is given (Fig. 17.24).
The throat length is 2 to 3 times the depth of flow in the throat.
Discharge of the canal can be obtained using a formula:
Q = Cd × 3.13 × b × H3/2
where b is width of the throat, H is depth of water in the throat. The depth is measured in a gauge well. It is constructed by the side of a throat entrance.