Canal Regulation and Lining

The amount of water which can be directed from a river into the main canal depends on: (i) the water available in the river,

(ii) the canal capacity, and

(iii) the share of other canals taking off from the river.

The flow in the main canal is diverted to various branches and distributaries. The distribution of flow, obviously, depends on the water demand of various channels. The method of distribution of available supplies is termed canal regulation. When there exists a significant demand for water anywhere in the command area of a canal, the canal has to be kept flowing. The canal can, however, be closed if the water demand falls below a specified quantity. It is reopened when the water demand exceeds the specified minimum quantity. Normally, there always exists a demand in some part of the command area of any major canal. Such major canals can, therefore, be closed only for a very small period (say, three to four weeks in a year). These canals run almost continuously and carry discharges much less than their full capacity, either when there is less demand or when the available supplies are insufficient. If the demand is less, only the distributaries which need water are kept running and the others (including those which have very little demand) are closed. In case of keen demand, but insufficient supplies, either all smaller channels run simultaneously and continuously with reduced supplies, or some channels are closed turn by turn and the remaining ones run with their full or near-full capacities. The first alternative causes channel silting, weed growth, increased seepage, water-logging, and low heads on outlets. The second alternative does not have these disadvantages and allows sufficient time for inspection and repair of the channels. A roster is usually prepared for indicating the allotted supplies to different channels and schedule of closure and running of these channels. It is advantageous to have flexible regulation so that the supplies can be allocated in accordance with the anticipated demand. The allocation of supplies is decided on the basis of the information provided by the canal revenue staffs who keeps a close watch on the crop condition and irrigation water demand. The discharge in canal is usually regulated at the head regulator which is usually designed as a meter. When the head regulator cannot be used as discharge meter, a depth gauge is provided at about 200 m downstream of the head regulator. The gauge reading is suitably related to the discharge. By manipulating the head regulator gates, the desired gauge reading (and, hence, the discharge) can be obtained.
Lining of Irrigation Channels
Most of the irrigation channels in India are earthen channels. The major advantage of an earth channel is its low initial cost. The disadvantages of an earth channel are:
(i) the low velocity of flow maintained to prevent erosion necessitates larger cross-section of channels,

(ii) excessive seepage loss which may result in water-logging and related problems such as salinity of soils, expensive road maintenance, drainage activities, safety of foundation structures, etc.,

(iii) favourable conditions for weed growth which further retards the velocity, and

(iv) the breaching of banks due to erosion and burrowing of animals. These problems of earth channels can be got rid of by lining the channel.
A lined channel decreases the seepage loss and, thus, reduces the chances of water-logging. It also saves water which can be utilised for additional irrigation. A lined channel provides safety against breaches and prevents weed growth thereby reducing the annual maintenance cost of the channel. Because of relatively smooth surface of lining, a lined channel requires a flatter slope. This results in an increase in the command area. The increase in the useful head is advantageous in case of power channels also. The lining of watercourses in areas irrigated by tube wells assumes special significance as the pumped water supply is more costly. As far as practicable, lining should, however, be avoided on expansive clays. But, if the canal has to traverse a reach of expansive clay, the layer of expansive clay should be removed and replaced with a suitable non-expansive soil and compacted suitably. If the layer of expansive clay is too thick to be completely excavated then the expansive clay bed is removed to a depth of about 60 cm and filled to the grade of the underside of lining with good draining material. The excavated surface of expansive clay is given a coat of asphalt to prevent the entry of water into the clay.
The cost of lining a channel is, however, the only factor against lining. While canal lining provides a cost-effective means of minimising seepage losses, the lining itself may rapidly deteriorate and require recurring maintenance inputs if they are to be effective in controlling seepage loss. A detailed cost analysis is essential for determining the economic feasibility of lining a channel. The true cost of lining is its annual cost rather than the initial cost. The cost of lining is compared with the direct and indirect benefits of lining to determine the economic feasibility of lining a channel. Besides economic factors, there might be intangible factors such as high population density, aesthetics, and so on which may influence the final decision regarding the lining of a channel.
Types of Lining
Types of lining are generally classified according to the materials used for their construction. Concrete, rock masonry, brick masonry, bentonite-earth mixtures, natural clays of low permeability, and different mixtures of rubble, plastic, and asphaltic materials are the commonly used materials for canal lining. The suitability of the lining material is decided by:
(i) economy, (ii) structural stability, (iii) durability, (iv) repair-ability, (v) impermeability, (vi)
hydraulic efficiency, and (vii) resistance to erosion.

The principal types of lining are as follows:
(i) Concrete lining,
(ii) Shotcrete lining,
(iii) Precast concrete lining,
(iv) Lime concrete lining,
(v) Stone masonry lining,
(vi) Brick lining,
(vii) Boulder lining,
(viii) Asphaltic lining, and
(ix) Earth lining.
Concrete Lining
Concrete lining is probably the best type of lining. It fulfils practically all the requirements of lining. It is durable, impervious, and requires least maintenance. The smooth surface of the concrete lining increases the conveyance of the channel. Properly constructed concrete lining can easily last about 40 years. Concrete linings are suitable for all sizes of channels and for both high and low velocities. The lining cost is, however, high and can be reduced by using mechanised methods. The thickness of concrete depends on canal size, bank stability, amount of reinforcement, and climatic conditions. Small channels in warm climates require relatively thin linings.
Channel banks are kept at self-supporting slope (1.5H: 1V to 1.25H: 1V) so that the lining is not required to bear earth pressures and its thickness does not increase. Concrete linings are laid without form work and, hence, the workability of concrete should be good. Also, experienced workmen are required for laying concrete linings.
Reinforcement in concrete linings usually varies from 0.1 to 0.4% of the area in the longitudinal direction and 0.1 to 0.2% of the area in the transverse direction. The reinforcement in concrete linings prevents serious cracking of concrete to reduce leakage, and ties adjacent sections of the lining together to provide increased strength against settlement damage due to unstable sub-grade soils or other factors. The reinforcement in concrete linings does not prevent the development of small shrinkage which tend to close when canals are operated and linings are water soaked. The damage due to shrinkage and temperature changes is avoided or reduced by the use of special construction joints. Reinforced concrete linings may result in increased water tightness of the lining. However, well-constructed unreinforced concrete linings may be almost equally watertight.
The earlier practice of using reinforced concrete linings is now being replaced by the employment of well-constructed unreinforced concrete linings. However, reinforcement must be provided in: (a) large canals which are to be operated throughout the year, (b) sections where the unreinforced lining may not be safe, and (c) canals in which flow velocities are likely to be very high. Proper preparation of sub-grade is essential for the success of the concrete lining which may, otherwise, develop cracks due to settlement. Natural earth is generally satisfactory for this purpose and, hence, sub-grade preparation is the least for channels in excavation. Thorough compaction of sub-grade for channels in filling is essential for avoiding cracks in lining due to settlement. Some cracks usually develop in concrete linings. These can be sealed with asphaltic compounds. The lining may be damaged when flow in the canal is suddenly stopped and the surrounding water table is higher than the canal bed. This damage occurs in excavated channels and can be prevented by providing weep holes in the lining or installing drains with outlets in the canal section.
Shotcrete Lining

Shotcrete lining is constructed by applying cement mortar pneumatically to the canal surface. Cement mortar does not contain coarse aggregates and, therefore, the proportion of cement is higher in shotcrete mix than in concrete lining. The shotcrete mix is forced under pressure through a nozzle of small diameter and, hence, the size of sand particles in the mix should not exceed 0.5 cm. Equipment needed for laying shotcrete lining is light, portable, and of smaller size compared to the equipment for concrete lining. The thickness of the shotcrete lining may vary from 2.5 to 7.5 cm. The preferred thickness is from 4 to 5 cm. Shotcrete lining is suitable for: (a) lining small sections, (b) placing linings on irregular surfaces without any need to prepare the subgrade, (c) placing linings around curves or structures, and (d) repairing badly cracked and leaky old concrete linings. Shotcrete linings are subject to cracking and may be reinforced or unreinforced. Earlier, shotcrete linings were usually reinforced. A larger thickness of shotcrete lining was preferred for the convenient placement of reinforcement.
The reinforcement was in the form of wire mesh. In order to reduce costs, shotcrete linings are not reinforced these days, particularly on relatively small jobs.
Precast Concrete Lining
Precast concrete slabs, laid properly on carefully prepared sub-grades and with the joints effectively sealed, constitute a serviceable type of lining. The precast slabs are about 5 to 8cm thick with suitable width and length to suit channel dimensions and to result in weights which can be conveniently handled. Such slabs may or may not be reinforced. This type of lining is best suited for repair work as it can be placed rapidly without long interruptions in canal operation. The side slopes of the Tungabhadra project canals have been lined with precast concrete slabs.
Lime Concrete Lining
The use of this type of lining is limited to small and medium size irrigation channels with capacities of up to 200 m3/s and in which the velocity of water does not exceed 2 m/s. The materials required for this type of lining are lime, sand, coarse aggregate, and water. The lime concrete mix should be such that it has a minimum compressive strength of about 5.00 kN/m2 after 28 days of moist curing. Usually lime concrete is prepared with 1 : 1.5 : 3 of kankar lime : kankar grit or sand : kankar (or stone or brick ballast) aggregate. The thickness of the lining may vary from 10 to 15 cm for discharge ranges of up to 200 m3/s. Lime concrete lining has been used in the Bikaner canal taking off from the left bank of the Sutlej.
Stone Masonry Lining
Stone masonry linings are laid on the canal surface with cement mortar or lime mortar. The thickness of the stone masonry is about 30 cm. The surface of the stone masonry may be smooth plastered to increase the hydraulic efficiency of the canal. Stone masonry linings are stable, durable, erosion-resistant, and very effective in reducing seepage losses. Such lining is very suitable where only unskilled labour is available and suitable quarried rock is available at low price. This lining has been used in the Tungabhadra project.
Brick Lining
Bricks are laid in layers of two with about 1.25 cm of 1:3 cement mortar sandwiched in between. Good quality bricks should be used and these should be soaked well in water before being laid on the moistened canal surface. Brick lining is suitable when concrete is expensive and skilled labour is not available. Brick lining is favoured where conditions of low wages, absence of mechanisations, shortage of cement and inadequate means of transportation exist.
Brick linings have been extensively used in north India. The Sarda power channel has been lined with bricks. The thickness of the brick lining remains fixed even if the sub-grade is uneven. Brick lining can be easily laid in rounded sections without form work. Rigid control in brick masonry is not necessary. Sometimes reinforced brick linings are also used.

Boulder Lining

Boulder lining of canals, if economically feasible, is useful for preventing erosion and where the ground water level is above the bed of the canal and there is a possibility of occurrence of damaging back pressures. The stones used for boulder linings should be sound, hard, durable, and capable of sustaining weathering and water action. Rounded or sub-angular river cobbles or blasted rock pieces with sufficient base area are recommended types of stones for boulder lining.
Asphaltic Lining
The material used for asphaltic lining is asphalt-based combination of cement and sand mixed in hot condition. The most commonly used asphaltic linings are: (a) asphaltic concrete, and (b) buried asphaltic membrane. Asphaltic linings are relatively cheaper, flexible, and can be rapidly laid in any time of year. Because of their flexibility, minor movements of the subgrade are not of serious concern. However, asphaltic linings have short life and are unable to permit high velocity of flow. They have low resistance to weed growth and, hence, it is advisable to sterilise the sub-grade to prevent weed growth. Asphaltic concrete is a mixture of asphalt cement, sand, and gravel mixed at a temperature of about 110°C and is placed either manually or with laying equipment. Experienced and trained workmen are required for the purpose. The lining is compacted with heavy iron plates while it is hot. A properly constructed asphaltic concrete lining is the best of all asphaltic linings. Asphaltic concrete lining is smooth, flexible, and erosion-resistant. Since asphaltic concrete lining becomes distorted at higher temperatures, it is unsuitable for warmer climatic regions. An asphaltic concrete lining is preferred to a concrete lining in situations where the aggregate is likely to react with the alkali constituents of Portland cement.
Buried asphaltic membrane can be of two types:
(a) Hot-sprayed asphaltic membrane, and
(b) Pre-fabricated asphaltic membrane.
A hot-sprayed asphaltic membrane is constructed by spraying hot asphalt on the Sub-grade to result in a layer about 6 mm thick. This layer, after cooling, is covered with a layer of earth material about 30 cm thick. The asphalt temperature is around 200°C and the spraying pressure about 3 × 105 N/m2. For this type of lining, the channel has to be over-excavated.
The lining is flexible and easily adopts to the sub-grade surface. Skilled workmen are required for the construction of this type of lining. Pre-fabricated asphaltic membrane is prepared by coating rolls of heavy paper with a 5mm layer of asphalt or 3 mm of glass fibre reinforced asphalt. These rolls of pre-fabricated asphaltic membrane are laid on the sub-grade and then covered with earth material. These linings can be constructed by commonly available labour. Materials used for covering the asphaltic membrane determine the permissible velocities which are generally lower than the velocities in unlined canals. Maintenance cost of such linings is high. Cleaning operations should be carried out carefully so as not to damage the membrane.
Earth Linings
Different types of earth linings have been used in irrigation canals. They are inexpensive but require high maintenance expenditure.

The main types of earth linings are: (a) stabilised earth linings, (b) loose earth blankets, (c) compacted earth linings, (d) buried bentonite membranes, and (e) soil-cement linings.
Stabilised earth linings: Stabilised earth linings are constructed by stabilizing the Sub-grade. This can be done either physically or chemically. Physically stabilised linings are constructed by adding corrective materials (such as clay for granular subgrade) to the subgrade, mixing, and then compacting. If corrective materials are not required, the subgrade can be stabilised by scarifying, adding moisture, and then compacting. Chemically stabilised linings use chemicals which may tighten the soil. Such use of chemicals, however, has not developed much.
Loose earth blankets: This type of lining is constructed by dumping fine-grained soils, such as clay, on the subgrade and spreading it so as to form a layer 15 to 30 cm thick. Such linings reduce seepage only temporarily and are soon removed by erosion unless covered with gravel. Better results can be obtained by saturating the clay and then pugging it before dumping on the subgrade. The layer of pugged clay is protected by a cover of about 30 cm silt. This type of lining requires flatter side slopes.
Compacted earth linings: These linings are constructed by placing graded soils on the subgrade and then compacting it. The graded soil should contain about 15% of clay. The compacted earth linings may be either thin-compacted or thick-compacted. In thin-compacted linings, the layer thickness of about 15 to 30 cm along the entire perimeter is used. Thick compacted linings have a layer about 60 cm thick on the channel bed and 90 cm thick on the sides. If properly constructed, both types are reasonably satisfactory. However, the thick linings are generally preferred. Compacted-earth linings are feasible when excavated materials are suitable, or when suitable materials are available nearby. Compaction operations along the side slopes are more difficult (particularly in thin-compacted linings) than along the channel bed. The lining material should be tested in the laboratory for density, permeability, and optimum moisture contents. The material must be compacted in the field so as to obtain the desired characteristics. Buried Bentonite Membranes: Pure bentonite is a hydrous silicate of alumina. Natural deposits of bentonite are special types of clay soil which swell considerably when wetted. The impurities of these soils affect the swelling and, hence, the suitability of these as canal lining material. Buried bentonite linings are constructed by spreading soil-bentonite mixtures over the subgrade and covering it with about 15 to 30 cm of gravel or compacted earth. Sandy soil mixed with about 5 to 25 per cent of fine-grained bentonite and compacted to a thickness of 5 to 7.5 cm results in a membrane which is reasonably tough and suitable for lining.
Soil-cement Linings: These linings are constructed using cement (15 to 20 per cent by volume) and sandy soil (not containing more than about 35 per cent of silt and clay particles). Cement and sandy soil can be mixed in place and compacted at the optimum moisture content. This method of construction is termed the dry-mixed soil-cement method. Alternatively, soil-cement lining can be constructed by machine mixing the cement and soil with water and placing it on the subgrade in a suitable manner. This method is called the plastic soil-cement method and is preferable. In both these methods, the lining should be kept moist for about seven days to permit adequate curing. The construction cost of soil-cement linings is relatively high. But these resist weed growth and erosion and also permit velocities slightly higher than those permitted by unlined earth channels. The use of soil-cement linings for irrigation canals is restricted to small irrigation canals with capacities of up to 10 m3/s and in which the velocity of water does not exceed 1 m/s.
Failure of Lining
The main causes of failure of lining are the water pressure that builds up behind the lining material due to high water table, saturation of the embankment by canal water, sudden lowering of water levels in the channel, and saturation of the embankment sustained by continuous rainfall. The embankment of a relatively pervious soil does not need drainage measures behind the lining. In all situations requiring drainage measures to relieve pore pressure behind the lining, a series of longitudinal and transverse drains satisfying filter criteria are provided. The growth of weeds on canal banks and other aquatic plant in channels may not result in failure of the lining but would affect the conveyance of channels which may be lined or unlined. Weeds and aquatic plants consume water for their growth and thus the consumptive use of irrigation water increases. Weed growth increases channel roughness and, hence, reduces the flow velocity thereby increasing evaporation losses. The cleaning of channels having excessive weed growth is, therefore, a vital maintenance problem. Cleaning operations can be carried out manually or by mechanical devices, such as used in dragline excavation and tractor-drawn cranes. Commonly used methods are pasturing, mowing, burning, and applying chemical weed killers.