This article throws light upon the five main types of regulation structures of a canal. The types are: 1. Canal Falls 2. Distributary Head Regulator 3. Cross regulator 4. Canal Escape 5. Canal Outlets.
Type # 1. Canal Falls:
A canal fall is a hydraulic structure constructed across a canal to lower down its water level. This is achieved by negotiating the change in bed elevation of the canal necessitated by difference in ground slope and canal slope.
The necessity of a fall arises when the available ground slope exceeds the design bed slope of the canal and which condition is rather usual. Thus, an irrigation channel which is in cutting in its head reach soon meets a condition when it has to be entirely in filling.
An irrigation channel in embankment i.e., in filling has the disadvantages of:
(i) Higher construction and maintenance cost,
(ii) Higher seepage and percolation losses;
(iii) Adjacent area being flooded due to any possible breach in the embankment, and
(iv) Difficulties in irrigation operations.
Hence, an irrigation channel should not be located in high embankments. Falls are, therefore, introduced at appropriate places to lower the supply level of an irrigation channel.
The canal water immediately downstream of the fall structure possesses excessive kinetic energy which, if not destroyed, may scour the bed and banks of the canal downstream of the fall. This would also endanger the safety of the fall structure. Therefore, a canal fall is always provided with measures to dissipate surplus energy which, in fact, is the consequence of constructing the fall.
The location of a fall is primarily influenced by the topography of the area and the desirability of combining a fall with other masonry structures, such as bridge, regulator, etc. In case of main canals, economy in the cost of excavation is to be considered.
Besides, relative economy of providing large number of smaller falls (achieving balanced earth work and ease in construction) compared to that of smaller number of larger falls (resulting in reduced construction cost and increased power production) is also worked out.
In case of channels which irrigate the command area directly, a fall should be provided before the bed of the channel comes into filling. The full supply level of a channel can be kept below the ground level for a distance up to about 500 metres downstream of the fall as the command area in this reach can be irrigated by the channels off taking from upstream of the fall.
Types of Canal Falls:
Canal falls are generally one of the following types:
(i) Notch Falls:
The shape of the notch in a notch fall is either trapezoidal or rectangular. The rectangular notch or low weir is economical and more suitable for discharge measurement. In trapezoidal notch fall, a number of trapezoidal notches are made in a high breast wall across the channel.
This arrangement provides an opening for flow right up to the bed level and, thus, eliminates silting in the channel upstream of the fall. When the subsidiary channels upstream of the fall have to be fed with minimum supply level in the parent channel, the fall should be capable of controlling the upstream water level.
Such falls consist of rectangular notches combined with one of the following three types of regulators:
(a) Sluice gates:
Raising or lowering the gate helps in controlling the upstream level.
(b) Horizontal stop logs inserted into grooves:
Their removal or insertion causes the required change in the upstream level.
(c) Vertical strips (or needles):
These change the effective width (i.e., the width of opening) of the channel and do not cause silting.
(ii) High-crested Falls:
These falls nearly maintain a fixed water surface level in the upstream channel. This is useful when either a subsidiary channel takes off upstream of the fall, or the fall is combined with a hydroelectric plant, and it becomes desirable that the water surface in the parent channel is maintained at a fixed level as far as possible.
High-crested falls are usually not flumed so as to keep the discharge per metre length of fall, q small. Smaller discharge intensity, q requires smaller head and, hence, the water level upstream of the fall can be maintained at a relatively fixed level to a considerable extent. Smaller value of q also makes energy dissipation easier.
Such falls are, therefore, relatively cheaper. Generally, the length of a fall is limited to the width of the channel, but can be increased by providing an expansion followed by contraction in the channel.
However, this type of provision would increase the construction cost of the fall. Depending upon the type of the weir crest, viz., broad or narrow, and the flow condition, viz., free or submerged, one can use these falls as metering devices after suitable calibration.
A raised crest fall with vertical impact was first used on the Sarda canal system in U.P. and is known as Sarda fall (Fig. 7.1.) Sarda fall may have either a rectangular crest (for canal discharge less than 14 m3/s) or a trapezoidal crest (for canal discharge more than 14 m3/s).
The amount of the drop at any fall in this canal system does not exceed 1.80 m. Large number of smaller falls were necessitated on this system because of the stratum of pure sand lying below the thin stratum of clay sand.
As such, the depth of excavation for channel construction had to be kept low so as to keep the seepage losses to the minimum. Another type of raised-crest fall can be a glacis fall (Fig. 7.2). In case of glacis fall, the energy is dissipated through the hydraulic jump which forms at the toe of the glacis.
Canal falls can, alternatively, be divided on the basis of their capability to measure discharge. Accordingly, they may be either meter falls or non-meter falls.
As a result of the flow passing over a fall, the potential energy of the flow gets converted into kinetic energy. This excess kinetic energy, if not destroyed properly, will result in undesirable scour of the bed and sides of the downstream channel. Hence, provision of suitable means to dissipate the surplus kinetic energy is essential in all types of canal falls, and it is provided in a portion of the canal fall which is known as cistern element.
The cistern element or simply cistern located at the downstream of the crest of a fall structure forms an important part of any canal fall. The cistern element, is defined as that portion of the fall structure in which the surplus energy of the water leaving the crest is destroyed and the subsequent turmoil stilled, before the water passes into the lower level channel.
The cistern element includes glacis, if any, devices for ensuring the formation of hydraulic jump and deflecting the residual high velocity jets, the roughening devices and the pool of water in which hydraulic impact takes place.
In other words, the cistern element includes the complete structure from the downstream end of the crest to the upstream end of the lower channel. Most of the cisterns employ hydraulic impact of the supercritical stream of the falling water with the subcritical stream of the lower channel for the dissipation of surplus energy.
Roughening Measures for Energy Dissipation:
Hydraulic impact is the best means of energy dissipation in case of most of the hydraulic structures including canal falls. Out of the three possible types of impact cisterns viz., vertical impact, horizontal impact and inclined impact cisterns—the vertical impact cistern is the most effective, while the inclined impact cistern is the least effective.
However, even in the case of efficient vertical impact cistern, the high turbulence persists downstream of the cistern and means for dissipating the residual energy are essential. In case of cisterns with no hydraulic impact, the roughening devices (provided on the cistern floor) are the only means to destroy the surplus kinetic energy.
Artificial roughness increases the actual wetted area which, in turn, increases the boundary friction. Besides, if correctly shaped and placed, the roughness increases the internal friction by increasing the interaction between high speed layers of the stream. Projections from the bed and sides of the channel are the most effective means of energy dissipation. Following roughening devices are generally used.
(i) Friction Blocks:
Rectangular concrete blocks properly anchored into the cistern floor and projecting up to one fourth the full supply depth are simple, effective and commonly used devices for dissipating surplus kinetic energy in hydraulic structures. The spacing between the blocks in a row is kept about twice the height of the blocks. Depending upon the need, two or more staggered rows of these friction blocks may be provided (Fig. 7.5).
The ‘arrows’ are specially shaped friction blocks (Fig. 7.6). The plan form of these arrows is approximately an equilateral triangle with rounded comers. The back face of the arrows is vertical. The top of arrows is sloped from the front rounded corner to the back edge to give an upward deflection to stream filaments.
(ii) Ribbed Pitching
Projections on the sides of the channel for the purpose of dissipating surplus energy of the flow can be provided in the form of ribbed pitching which consists of bricks laid fiat and on edge alternately (Fig. 7.7). Bricks laid on edge project into the stream and, thus, increase boundary friction and dissipate the surplus energy.
Provisions at the Downstream End of Cistern:
If the high velocity stream continues up to the end of the cistern, a baffle wall or a deflector [Fig. 7.8(a)], or a dentated sill [Fig. 7.8(b)] or a biff wall [Fig. 7.8(c)] may be provided at the downstream end of the cistern. The baffle wall provides a deep pool of water upstream of itself in the cistern.
This pool of water is helpful in the dissipation of residual energy. Other devices (i.e., deflector, dentated sill, biff wall) produce a reverse roller which results in a limited scour away from the toe and piles up material against the toe of the structure. A dentated sill, in addition, breaks up the stream jet.
Type # 2. Distributary Head Regulator:
The distributary head regulator is constructed at the upstream end, i.e., the head of a channel where it takes off from the main canal or a branch canal or a major distributary. The distributary head regulator should be distinguished from the canal head regulator which is provided at the canal head works where a canal takes its supplies from a river source.
The distributary head regulator serves to:
(i) Divert and regulate the supplies into the distributary from the parent channel,
(ii) Control the silt entering the distributary from the parent channel, and
(iii) Measure the discharge entering the distributary.
For regulating the supplies entering the off-taking channel from the parent channel, abutments on either side of the regulator crest are provided. Piers are placed along the regulator crest at regular intervals. These abutments and piers have grooves (at the crest section) for the purpose of placing planks or gates.
The supplies into the off-taking channel are controlled by means of these planks or gates. The planks are used for small channels in which case manual handling is possible. The span of hand-operated gates is also limited to 8 m. Mechanically-operated gates can, however, be as wide as 20 m.
An off-taking channel tends to draw excessive quantity of sediment due to the combined effects of the following:
(i) Because of their smaller velocities, lower layers of water are more easily diverted into the off-taking channels in comparison to the upper layers of water.
(ii) The sediment concentration is generally much higher near the bed.
(iii) The sediment concentration near the banks is usually higher because of the tendency of the bottom water to move towards the banks due to the difference in central and near-bank velocities of flow.
As such, if suitable steps are not taken to check the entry of excessive sediment into the off taking channel, the off taking channel will soon be silted up and would require repeated sediment removal.
Sediment entry into the off taking channel can be controlled by causing the sediment to concentrate in the lower layers of water (i.e., near the bed of the parent channel upstream of the off taking point), and then letting only the upper layers of water enter the off taking channel. Concentration of sediment in lower layers can be increased by providing smooth bed in the parent channel upstream of the off taking point.
The smooth channel bed reduces turbulence which keeps the sediment particles in suspension. In addition, the steps which accelerate the flow velocity near the banks would also be useful. It should also be noted that the alignment of the off taking channel also affects the sediment withdrawal by the off taking channel.
Hence, the alignment of the off taking distributary channel with respect to the parent channel needs careful consideration. The angle of off take may be kept between 60° and 80° to prevent excessive sediment withdrawal by the off taking channel. For all important works, the alignment of off taking channels should be fixed on the basis of model studies.
For the purpose of regulating the discharge in the distributary, it is essential to measure the discharge for which one can use the gauge-discharge relationship of the distributary. However, this relationship is likely to change with the change in the channel regime. Hence, it is advantageous to use the head regulator as a metering structure too.
Type # 3. Cross Regulator:
Cross regulator is a structure constructed across a canal to regulate the water level in the canal upstream of itself and the discharge passing downstream of it for one or more of the following purposes:
(i) To feed off taking canals located upstream of the cross regulator.
(ii) To escape water from canals in conjunction with escapes.
(iii) To control water surface slopes in conjunction with falls for bringing the canals to regime slope and section.
(iv) To control discharge at an outfall of canal into another canal or lake.
A cross regulator is generally provided downstream of an off taking channel so that the water level upstream of the regulator can be raised, whenever necessary, to enable the off taking channel draw its required supply even if the main channel is carrying low supply.
The need of a cross regulator is essential for all irrigation systems which supply water to distributaries and field channels by rotation and, therefore, require to provide full supplies to the distributaries even if the parent channel is carrying low supplies. Cross regulators may be combined with bridges and falls for economic and other special considerations.
Type # 4. Canal Escape:
Canal escape is a structure to dispose of surplus or excess water from a canal. A canal escape essentially serves as a safety valve for the canal system. It provides protection of the canal against possible damage due to excess supplies which may be on account of either a mistake in releasing water at head-works, or a heavy rainfall due to which there may be sudden reduction in demand and the cultivators, therefore, close their outlets.
The excess supply makes the canal banks vulnerable to breaches or dangerous leaks and, hence, provision for disposing of excess supply in the form of canal escapes at suitable intervals along the canal is desirable. Besides, emptying of canal for repairs and maintenance and removing a part of sediment deposited in the canal can also be carried out with the help of the canal escapes.
The escapes are usually of the following types:
(i) Weir or surface escape:
These are weirs or flush escapes constructed either in masonry or concrete with or without crest shutters which are capable of disposing of surplus water from the canal.
(ii) Sluice escapes:
Sluices are also used as surplus escapes. These sluices can empty the canal for repairs and maintenance and, in some cases, act as scouring sluices to facilitate removal of sediment. Location of escape depends on the availability of suitable drains, depressions or rivers with their bed level at or below the canal bed level for disposing surplus water through the escapes, directly or through an escape channel.
Type # 5. Canal Outlets:
When the canal water has reached near the fields to be irrigated, it has to be transferred to the watercourses. At the junction of the watercourse and the distributary, an outlet is provided. An outlet is a masonry structure through which water is admitted from the distributary into a watercourse.
It also acts as a water-measuring device. The discharge though an outlet is usually less than 0.085 m3/s. It plays a vital role in the warabandi system of distributing water. Thus, an outlet is like a head regulator for the field channel.
The main objective of providing an outlet is to provide ample supply of water to the fields, whenever needed. If the total available supply is insufficient, the outlets must be such that equitable distribution can be ensured.
The efficiency of irrigation system depends on the proper functioning of canal outlets which should satisfy the following requirements:
(i) The outlets must be strong and simple with no moving parts which would require periodic attention and maintenance.
(ii) The outlets should be tamper-proof, and in case there is any interference in the functioning of the outlet, it should be easily detectable.
(iii) The cost of outlets should be less since a large number of outlets have to be installed in an irrigation network.
(iv) The outlet should be able to draw sediment in proportion to the amount of water withdrawn so that there is no silting problem in the distributary downstream of the outlet.
(v) The outlets should be able to function efficiently even at low heads.
Choice of type of an outlet and its design are governed by factors, such as water distribution policy, water distribution method, method of water assessment, source of supply and the working of the distributary channel.
Water may be distributed on the basis of either the actual area irrigated in the previous year or the actual culturable commanded area. The discharge from the outlet should be capable of being varied in the first case, but can remain fixed in the second.
The method of water distribution may be such that each cultivator successively receives water for a duration in proportion to his area. Or, alternatively, all the cultivators share the outlet discharge simultaneously. The first system is better as it results in less loss of water. The outlet capacity is decided keeping in view the method of water distribution.
If the assessment is by volume, the outlet discharge should remain constant and not change with variation in the water levels of the distributary and the watercourse. On the other hand, if water charges are decided on area basis, the variation in the outlet capacity with water levels of the distributary and the watercourse is relatively immaterial.
With a reservoir as supply source, the cultivators can be provided water whenever needed and, hence, the outlets should be capable of being opened or closed. The outlets would generally remain open if the supply source is a canal without storage so that water is diverted to the field when the canal is running.
At times, the amount of water in the main canal may not be sufficient to feed all the channels simultaneously to their full capacity. As such, either all the channels may run with low discharge or groups of channels may be supplied their full capacity by rotation.
In the first case, the outlets must be able to take their proportionate share even with large variations in the discharge of the distributary channel. In the second case, the outlets must be such that the required amount of water is available for all the channels being fed with their full capacity.
It should be noted that whereas the cultivator prefers to have outlets capable of supplying constant discharge, the canal management would prefer that the outlets supply variable discharge depending upon the discharge in the distributary channel so that the tail end of the channel is neither flooded nor dried. Obviously, both these requirements cannot be fulfilled simultaneously.