This article deals with structures that regulate the discharge and maintain the water levels within a canal network (Fig. 4.1).
These structures may be described as follows:
(a) Drops and falls to lower the water level of the canal.
(b) Cross regulators to head up water in the parent channel to divert some of it through an off take channel, like a distributary.
(c) Distributary head regulator to control the amount of water flowing in to off take channel.
(d) Escapes, to allow release of excess water from the canal system.
The flow of a main canal bifurcating into a branch canal with the rest flowing downstream is controlled with the help of a cross regulator across the parent canal and a head regulator across the branch canal. At times, the flow of a canal divides into two or three smaller branch canals without any regulating structure by designing the entrance of the canals in such a way that the flow enters each branch canal proportionate to its size.
Again, from a canal, outlet structures may take out water for delivery to the field channel or water courses belonging to cultivators. These outlet works, of course, are generally not provided on the main canal and branches, but are installed in the smaller distributaries.
Apart from these, there could be a need to measure the flow in a canal section and different structures have been tried, mostly based on the formation of a hydraulic jump and calibrating the discharge with the depths of flow. Typical structures of these kinds are graphically represented in Figure 4.2.
A canal conveying water from the head works has to run for large distances and has to maintain the water levels appropriately, as designed along its length. It has to run through terrains which generally would have a different slope small than the canal. The surrounding areas would invariably have its own drainage system ranging from small streams to large rivers.
The canal has to carry the water across these water bodies. The main structures of a canal system for conveyance of canal flow and control of water levels are aqueducts, syphon aqueducts, super-passage, canal siphon or level crossings across natural drainage courses or other depressions.
Structures that are created as obstructions across rivers with an intention to store some of the water for future use are called storage dams. They are functionally slightly different from the structures used for flow diversion, called barrages or weirs. A diversion structure is primarily meant to create an elevation rise of the river water such that it may flow into a canal, perhaps throughout the year or at least during the lean flow period of the river.
During high flood in a river, the canal is kept closed in cases like irrigation canals for fear of high sediment entering and depositing on the canal bed. Even if some water is drawn into a canal during river flood season, the main bays of a diversion structures are usually kept wide open to let the flood water pass down the river with the minimum obstruction.
This is not so in case of storage dams, for which the storage of water especially a portion of the flood flow is primary concern. This storage of water is done with an intention to either reduce the impact of a flood downstream or to use the water beneficially in future. This is achieved by creating an obstruction of sufficient height which creates a reservation on the upstream of the structure.
Naturally, since the reservoir would have a finite capacity which would vary with height of the dam and the shape of the river valley on the up steam, any excess flood water has to be discharged through a spillway. Hence, the principal components of a storage dam would be a storage structure to obstruct river flow, a spillway for discharging excess flood water and outlets for allowing the storage water to be withdrawn for the rest of the year for some useful purpose or even let it flow downstream at a regulated quantity.
Sometimes sluice are provided in the body of the dam to lower the water level in the dam at the time of an emergency it is not necessary that all the principal components of a storage dam be located in the same structure. In fact, all three may be located separately. Of course, the spillway is usually made of reinforced concrete and sometimes combined with a concrete dam. But it may be economical or practically more feasible to construct an earthen or rock fill dam, in which case there has to be separate spill way made of concrete.
In fact, it is also not essential for the spillway to be adjacent to the main dam, and can be located any convenient position at the periphery of the reservoir, if that helps in some way. Similarly, out let works may be located at any suitable place in the reservoir and possibly connected to a canal or a tunnel.
1. Canal Drops and Falls:
A canal has a designed longitudinal slope but has to pass through an undulating terrain. When a canal crosses an area that has a larger natural surface slope, a canal drop, also called all in India, has to be provided suitably at certain intervals (Fig. 4.3).
The location of a fall has to be judiciously worked out such that there should be a balance between the quantities of excavation and filling. Further the height of the fall has to be decided, since it is possible to provide larger falls at longer intervals or smaller falls at shorter intervals! It may be observed that the portion of the canal which is running in filling (Figure 4.3) may be able to serve the surrounding area by releasing water by gravity. For the portion of the canal that is running in excavation, if surrounding areas have to be irrigated, it has to be done through pumping.
There are various types of fall structures, some of which are no more provided these days. However, there are many irrigation projects in India, which have these structures in the canal network, as they were designed many years ago. Many of these structures used boulder masonry as their construction material, whereas now brick masonry or, more commonly, mass concrete is being used commonly in modem irrigation projects.
2. Canal Regulators:
These include the cross regulator and the distributary head regulator structures for controlling the flow through a parent canal and its off-taking distributary as shown in Fig. 4.1. They also help to maintain the water level in the canal on the upstream of the regulator. Canal regulators, which are gated structures, may be combined with bridges and falls for economic and other considerations, like topography, etc.
The angle at which a distributary canal off-takes from the parent canal has to be decided carefully. The best angle is when the distributary takes off smoothly, as shown in Fig. 4.4(a). Another alternative is to provide both channels (off-taking and parent) at an angle to the original direction of the parent canal Fig. 4.4(6). When it becomes necessary for the parent canal to follow a straight alignment, the edge of the canal rather than the centre line should be considered in deciding the angle of off-take (Fig. 4.4(c)).
To prevent excessive entry of silt deposition at the mouth of the off-take, the entry angle should be kept to between 60° and 80°. The water entering into off-taking distributary canal from the parent canal may also draw suspended sediment load.
The distributary should preferably be designed to draw sediment proportional to its flow, for maintaining non-silltation of either the parent canal or itself.
3. Silt Vanes:
Silt vanes, or King’s vanes, are thin, vertical, curved parallel walled structures constructed of plain or reinforced concrete on the floor of the parent canal, just upstream of the off-taking canal. The height of the vanes may be about one-fourth to one-third of the depth of flow in the parent canal. The thickness of the vanes should be as small as possible and the spacing of the vanes may be kept about 1.5 times the vane height. To minimise silting tendency, the pitched floor on which the vanes are built should be about 0.15m above the normal bed of the parent channel. A general three- dimensional view of the vanes is shown in Fig. 4.5.
4. Canal Escapes:
These are structures meant to release excess water from a canal, which could be main canal, branch canal, distributary, minors, etc.
Though usually an irrigation system suffers from deficit supply in later years of its life, situations that might suddenly lead to accumulation of excess miter in a certain reach or a canal network may occur due to the following reasons:
(a) Wrong operation of head works in trying to regulate flow in along channel resulting in release of excess water than the total demand in the canal system downstream.
(b) Excessive rainfall in the command area leading to reduced demand and consequent closure of downstream gates.
(c) Sudden closure of control gates due to a canal bank breach.
The excess water in a canal results in the water level rising above the full supply level which, if allowed to overtop the canal banks, may cause erosion and subsequent breaches. Hence, canal escapes help in releasing the excess water from a canal at times of emergency. Moreover, when a canal is required to be emptied for repair works, the water may be let off through the escapes. Escapes as also built at the tail end of minors at the far ends of a canal network. These are required to maintain the required full supply level at the tail end of the canal branch.
The locations for providing escapes are often determined on the availability of suitable drains, depressions or rivers with their bed level at or below the canal bed level so that any surplus water may be released quickly disposed through these natural outlets. Escapes may be necessary upstream of points where canals takeoff from a main canal branch. Escape upstream of major aqueducts is usually provided. Canal escapes may be provided at intervals of 15 to 20 km for main canal and at 10 to 15 km intervals for other canals.
5. Structures for Crossing Canals across Natural Streams:
These structural elements are required for conveying the canals across natural drainage. When a canal layout is planned, it is usually seen to cross a number of channels draining the area, varying from small and shallow depressions to large rivers. It is not generally possible to construct cross-drainage structures for each of the small streams.
Some of the small drainage courses are, therefore, diverted into one big channel and allowed to cross the canal. However, for larger streams and river, where the cost of diversion becomes costlier than providing a separate cross-drainage work, individual structures to cross the canal across the stream is provided.
There could be a variety of combinations of the relative position of the canal with respect the natural channel that is to be crossed. These conditions are shown in Figures 4.6 to 4.10.
The notations used in the figures are as follows:
(a) CBL- Canal Bed Level;
(b) SBL- Stream Bed Level;
(c) FSL- Canal Full Supply Level; and
(d) HFL- Stream High Flood Level.
Figure 4.6 shows the relative position of canal (shown in cross-section) with respect to a natural stream (shown in longitudinal section), when canal bed level is higher than stream high flood level.
Figure 4.7: Shows the relative position of a canal whose bed level is below but full supply level is above the stream high flood level.
Figure 4.8: Shows a canal with full supply level almost matching the high flood level of the natural stream.
Figure 4.9 Shows a canal full supply level and bed levels below the levels of flood level and bed level of stream, respectively.
Figure 4.10: Shows the relative position of canal with respect to the natural stream where the canal full supply level is below the stream bed level.
In general, the solution for all the illustrated conditions possible for conveying an irrigation canal across a natural channel is by providing a water conveying structure which may:
(a) Carry the canal over the natural stream;
(b) Carry the canal beneath the natural stream; or
(c) Carry the canal at the same level of the natural stream.
(a) Structures to Carry Canal Water over a Natural Stream:
Conveying a canal over a natural watercourse may be accomplished in two ways:
i. Normal canal section is reduced to a rectangular section and carried across the natural stream in the form of a bridge resting on piers and foundations (Figure 4.11). This type of structure is called a through type aqueduct.
ii. Normal canal section is continued across the natural stream but the stream section is flumed to pass through ‘barrels’ or rectangular passages (Figure 4.12). This type is called, barrel type aqueduct.
For the aqueducts, it may be observed that the HFL of the natural stream is lower than the bottom of the trough (or the roof of the barrel). In this case, the flow is not under pressure, that is, it has a free surface exposed to atmospheric pressure.
In case the HFL of the natural stream goes above the trough bottom level (TBL) or the barrel roof level (BRL), then the flow in the natural watercourse would be pressured and the sections are modified to form which is known as syphon aqueducts.
It may be observed that the trough type aqueduct or syphon aqueduct would be suitable for the canal crossing a larger stream or river, whereas the barrel type is suitable if the natural stream is rather small. The relative economics of the two types has to be established on case-to-case basis.
Further, the following points maybe noted for the two types of aqueducts or siphon aqueducts:
i. Trough Type:
The canal is flumed to not less than 75 percent of the bed width keeping in view the permissible head loss in the canal. Transitions 3 : 1 on the upstream and 5 : 1 on the downstream side are provided to join the flumed section to the normal canal section. For the trough-type syphon aqueduct the designer must consider the upward thrust also that might act during high floods in the natural stream when the stream water flows under pressure below the trough base and for worst condition, the canal may be assumed to be dry at that time.
The dead weight of the trough may be made more than that of the upward thrust or it may be suitably anchored to the piers in order to may be counteract the uplift condition.
ii. Barrel Type:
The barrel may be made up of Reinforced Cement Concrete (RCC), which could be single or multicell, circular or rectangular in cross-section. Many of the earlier structures were made of masonry walls and arch roofing. Precast RCC pipes may be economical for small discharges. For barrel-type syphon aqueducts, the barrel is horizontal in the central portion but slopes upwards on the upstream and downstream side at about an inclination of 3 H: IV and 4 H : 1 V; respectively. A self-cleaning velocity of 6 m/s and 3 m/s is considered while designing RCC and masonry barrels, respectively.
(b) Structures to Carry Canal Water below a Natural Stream:
A canal can be conveyed below a natural stream with the help of structures like a super-passage or a siphon. These are exactly opposite in function to that of the aqueducts and siphon aqueducts, which are used to carry the canal water above the natural stream. The natural stream is flumed and made to pass in a trough above the canal. If the canal water flows with a free surface, that is, without touching the bottom of the trough, it is called a super-passage. Else, when the canal passes below the trough as a pressure flow, then it is termed as a syphon or a canal syphon.
Instead of a trough, the canal flow may be conveyed below the natural stream using small pre-cast RCC pipes (for small discharges) and rectangular or circular barrels, either in single or multiple cells, may be used (for large discharges), as shown in Fig. 4.12.
(c) Structures to Carry Canal Water at the Same Level as a Natural Stream:
A structure in which the water of the stream is allowed to flow into the canal from one side and allowed to leave from the other; known as a level crossing, falls into this category (Fig. 4.13).
This type of structure is provided when a canal approaches a large sized drainage with high flood discharges at almost the same level. The flow control is usually provided on either side of the canal and on the outlet side of the drain. As such, this type of arrangement is very similar to canal head-works with a barrage.
Advantage may be taken of the flow of the natural drainage to augment the flow of the outgoing canal. The barrage type regulator is kept closed during low flows to head up the water and allows the lean season drainage flow to enter the outgoing canal. During flood seasons, the barrage gats may be opened to allow much of the silt-laden drainage discharge to flow down.
Another structure, called an inlet, is sometimes provided which allows the entry of the stream water into the canal through an opening in the canal bank, suitably protected by pitching the bed and sides for a certain distance upstream and downstream of the inlet. If the natural stream water is not utilized in the canal then an outlet, which is an opening on the opposite bank of the canal is provided. The canal bed and sides suitably pitched for protection.
6. Canal Outlets:
Canal outlets, also called farm turnouts in some countries, are structures at the head of a water course or field channel. The supply canal is usually under the control of an irrigation authority under the state government. Since an outlet is a link connecting the government owned supply channel and the cultivator owned field channel, the requirements should satisfy the needs of both the groups.
Since equitable distribution of the canal supplies is dependent on the outlets, it must not only pass a known and constant quantity of water, but must also be able to measure the released water satisfactorily. Also, since the outlets release water to each and every farm watercourse, such structures are more numerous than any other irrigation structure.
Hence it is essential to design an outlet in such a way that it is reliable and be also robust enough such that it is not easily tampered with. Further the cost of an outlet structure should be low and should for efficiently with a small working head, since a larger working head would require higher water level in the parent channel resulting in high cost of tile distribution system. Discharge through an outlet is usually less than 0.085 cumecs. Various types of canal outlets have been evolved from time to time-out none has been accepted as universally suitable.
It is very difficult to achieve a perfect design fulfilling both the properties of ‘flexibility’ as well as ‘sensitivity’ because of various indeterminate conditions both in the supply channel and the watercourse of the following factors:
(a) Discharge and silt,
(b) Capacity factor,
(c) Rotation of channels, and
(d) Regime condition of distribution channels, etc.
These modules are classified in three types, which are as follows:
i. Non-Modular Outlets:
These outlets operate in such a way that the flow passing through them is a function of the difference in water levels of the distributing channel and the, watercourse. Hence, a variation in either affects the discharge. These outlets consist of regulator or circular openings and pavement.
ii. Semi-Modular Outlets:
The discharge through these outlets depends on the water level of the distributing channel but is independent of the water level in the watercourse so long as the minimum working head required for their working is available.
iii. Module Outlets:
The discharge through modular outlets is independent of the water level in the distributing channel and the watercourse, within reasonable working limits. This type of outlets may or may not be equipped with moving parts. Though modular outlets, like the Gibb’s module, have been designed and implemented earlier but they are not very common in the present Indian irrigation engineering scenario.
7. Spillway:
Spillways or passages for letting out flood waters when the reservoir, is over flowing has three major components:
(a) Entry to the spillway, which may or may not be controlled using gates.
(b) A channel for conveying the water from the reservoir side to the downstream of the dam.
(c) Energy dissipating arrangement for the water flowing down the spillway channel as it reaches a lower elevation near the outlet of the channel.
The capacity of the water conveyance of the spillway should be such that it must safely pass the maximum design flood. More than one type of spillway may be provided in a particular dam. For example in the Indira Sagar Dam on rivet Narmada, two sets of spillways have been provided, one called the main spillway and the other auxiliary spillway.
All the spillways are gated, but the crest level of the auxiliary spillways, are slightly higher than that of the main spillways. Under normal flood situations, the main spillways may be operated but if the flood is excessive, auxiliary spillways can be brought into operation. Spillways can be integrated into any type of concrete dam, if so desired but for embankment dams, separated passage has to be designed.
8. Energy Dissipators:
The water flowing down from the spillways possess a large amount of kinetic energy that is generated by virtue of its losing the potential head from the reservoir level to the level of the river on the downstream of the spillway. If this energy is not reduced, there are danger of scour to the riverbed which may threaten the stability of the dam or the neighbouring river valley slopes. The various arrangements for suppressing or killing of the high energy water at the downstream toe of the spillways are called Energy Dissipators.