2. Inundation canal.
A canal is said to be perennial when it is fed by a permanent source of supply. The canal is a well made up regular graded channel. It has also permanent masonry works of regulation and distribution of supplies. A permanent canal is also sometimes known as perennial canal when the sources from which canal takes is an ice fed perennial river.
Inundation Canals usually draw their supplies from rivers whenever there is a high stage in the river. They are not provided with any headworks for diversion of river water to the canal. They are, however, provided with a canal head regulator. The head of the canal has to be changed sometimes to suit the changing pattern of river course. (b) Classification based on financial output (1) Productive canal.
(2) Protective canal.
Productive Canals are those which yield a net revenue to the nation after full development of irrigation in the area. Protective canal is a sort of relief work constructed with the idea of protecting a particular area from famine. (c) Classification based on the function of the canal (1) Irrigation canal.
(2) Carrier canal.
(3) Feeder canal.
(4) Navigation canal.
(5) Power canal.
An irrigation canal carries water to the agricultural fields. A carrier canal besides doing irrigation, carries water for another canal. Upper Chenab canal in West Punjab (Pakistan) is the example of one such canal. A feeder canal is constructed with the idea of feeding two or more canals. Examples of such canals are Rajasthan feeder canal and Sirhind feeder. (d) Classification based on the discharge and its relative Importance in a given network of canals (1) Main canal.
(2) Branch canal.
(3) Major distributary.
(4) Minor distributary.
(5) Water course.
Main canal generally carries water directly from the river. Such a canal carries heavy supplies and is not used for direct irrigation except in exceptional circumstances. Main canals act as water carriers to feed supplies to branch canals and major distributaries,
Branch canals are the branches of the main canal in either direction taking off at regular intervals. In general, branch canals also do not carry out any direct irrigation, but at times direct outlets may be provided. Branch canals are usually feeder channels for major and minor distributaries. They usually carry a discharge of over 5 cumes.
Major distributaries usually called Rajbha, take off from a branch canal. They may also sometimes take off from the main canal, but their discharge is generally lesser than branch canals. They are real irrigation channels in these that they supply water for irrigation to the field through outlets provided along them. Their discharge varies from 1/4to5 cumecs.
Minor Distributaries called minors take off from branch canals or from distributaries. Their discharge is usually less than 1/4 cumecs. They “supply water to the water courses through outlets provided along them.
A water course is a small channel which ultimately feeds the water to irrigation fields. Depending upon the size and extent of the irrigation scheme, a field channel may take off from a distributary or minor. Sometimes, it may even take off from the branch canal for the field situated very near to the branch canal. (e) Classification based on canal alignment According to the aligilment, a canal may be classified as under: (1) Contour canal.
(2) Watershed canal
(3) Side slope canal
The characteristic features of these canals are discussed in the next article. Factors Governing alignment of a Canal: 1. The alignment of the Canal should be such as to ensure
(i) The Most economical way if distributing the water to the land
(ii) as high a command as possible and
(iii) minimum number of Cross drainage works.
2. The alignment of a canal on a watershed, being the most economical, is preferred. As a general rule, all tile watershed lying in a command should be occupied by distributaries
3. The length of the main canal from the point where it takes off from a river to a point where it mounts on a watershed should be minimum.
4. The contour alignment should be changed this was or that way in order to reduce the number of cross-drainage works to a minimum.
5. The alignment should avoid villages, roads, cart tracks, cremation places, places of worship and other valuable properties
6. The alignment should pass through the balanced depth of cutting. If not, it should involve minimum depth of cutting or minimum height of filling.
7. The number of kinks and acute curves should be minimum.
8. Ideal length of canal should be minimum and branches etc. should be economically planned.
9. The alignment should not be made in a rocky, brackish or cracked strata, 1. Lining:
1. Necessity 1. To minimize the seepage losses in canal.
2. To increase the discharge in canal section by increasing the velocity.
3. To prevent erosion of bed and side due to high velocities.
4. To reduce maintenance of canal.
The following are the important types of concrete lining used in Pakistan: (a) Hard surface type lining: 1. Cement concrete lining.
2. Shotcrete lining
3. Precast concrete lining.
4. Cement mortar lining.
5. Brick lining.
6. Stone blocks, or undressed stone lining.
7. Asphaltic lining.
8. Earth type lining:
9. Soil cement lining.
10. Clay puddle lining.
11. Sodium carbonate lining.
(C) Buried and protected membrane type lining:
12. Prefabricated light membrane lining.
13. Bentonite soil and clay membrane lining.
14. Road oil lining.
15. Concrete lining
Concrete lining has found only a limited use in Pakistan because of the high initial cost. Concrete as a lining material has excellent hydraulic properties.
Thickness of concrete lining: The thickness o lining is governed by the requirement of imperviousness and structural strength. Usual thickness provided is from 5 to 10 cm for M 15 concrete and id 7.5. cm to 15 cm for M 10 concrete. Bank slope Earthen banks of canals upon which lining has to rest are made at a self supporting slope so that no earth pressure is exerted upon lining. Reinforcement: Reinforcement is not provided except where it may be particularly required. Reinforcement interferes with the wording of mechanical equipment but if provided it prevents formation of wide gap and possible faulting in case of failure of such grade. It also reduces width of temperature crack. Joints Construction joints localize volume changes due to shrinkage of concrete and avoid cracking. Lining is usually provided in panels not more than 3 m. Different types of joints provided are shown in Fig. 16.5. Where lining operations are continuous, transverse grooves or longitudinal and transverse grooves both, at a spacing of 3 to 5 m are formed. Construction of concrete lining Prepare a subgrade and compact it fully. Saturate subgrade to a depth of 30 cm in sandy soil and 15 cm in other soils and then treated in any of the following manner before laying concrete
(a) Lay a base coal of 1: 4 cement sand slurry on the subgrade.
This is cheapest and best treatment or
(b) Spread oil paper on the subgrade or
(c) Spread crude oil on subgrade.
The concrete used for lining generally has a mix ratio 1: 4:8 or 1 :3:6 or 1 :4:6.The concrete is usually laid in alternate blocks. The form work is removed after seven days and the remaining blocks are laid then. After the concreting is over, the entire lined surface is properly cured. 2. Shotcrete lining In this type of lining a mixture of cement and sand (1:4)is shot a; the subgrade through a nozzle The thickness of this type of lining varies from 25 to 6,5 cm2 Shotcrete consumes large amount of cement. Shotcrete can be placed on irregular sub-grade and hence fine dressing of sub-grade is not required. Shotcrete is also used for repair of old but sound concrete lining. 3. Precast concrete lining This type of concrete lining consist, of precut slabs usually 90 cm x 30 cm in size. The thickness of each slab Is from 5 to 6.5 cm. The blocks ire manufactured with an interlocking arrangement. The slabs are laid on well prepared and compacted subgrade. The joint is grouted with cement or Is sealed with aaphalt to prevent any seepage through joint 4. Cement mortar lining: Cement mortar lining has never boon used u such It Ii usually used u and which material between brick layers Thickness for this type of lining is kept from 1 to 4 cm. The method of construction is similar to that of concrete lining. A large amount of cement is consumed In this type of lining and hence it Is quit, costly. 5. Brick lining This type of lining consists of a single or double layer of brick masonry or a layer of brick masonry followed by a layer of tiles laid in mortar. The first layer is laid on a 12 mm layer of 1: 6 cement mortar The mortar Is first spread on a properly compacted and wetted iu1gradc A 12 mm thick layer of plaster In 1: 3 cement mortar is laid over the first layer. Then the second layer Is laid over it In 1 : 3 cement mortar. The size of brick especially made is restricted to 30 x 15 x 5 cm for convenience of handling.
This type of lining has been used In Punjab on Bhakra and Hivell canals. On Bhakra canal its performance has been found to be extremely satisfactory, This lining is hydraulically as efficient u that of concrete lining. In cue of any settlement of subgrade, the mortar joints between bricks provide for numerous cracks which are so fin, that they do not permit any seepage through them. In cue of any abnormal settlement the failure in this type sing will be local and lining can be repaired easily. 6. Asphaltic lining Asphaltic lining is carefully controlled mixture of asphalt and grade aggregate mixed and placed at a high temperature of 200°C and covered with a 30 cm layer of earth material for a protection. The mix is placed either by hand or by an equipment similar to that of concrete. 7. Soil cement lining; Soil cement is a mixture of cement and soil. Soil used in soil cement should not contain more than 35% of fine fraction passing 200 sieve. The mixture, in proportion of 90 to 95 % soil and 10 to 5% cement is thoroughly mixed dry and is then added with water to raise the moisture content to optimum. It is, therefore placed on the subgrade and properly compacted. After laying in a reach is complete, it is covered with wet sand or wet straw for seven days for curing. In U.S.A. plastic soil cement has been used with a higher consistency to get a faster rate of construction. 8. Clay puddle lining Clay puddle is produced from clay by first exposing clay to weathering. it is then mixed with water to bring it to the saturation and is pugged thoroughly by trampling under man’s or cattle’s feet. This thickness of lining is 30 cm. It is then protected by a layer of earth material. 9. Sodium carbonate lining. The mixture consists of clayey soil and sodium carbonate in a proportion of at least 10% clay and 6% sodium carbonate. Thick of lining is kept as 10cm. This type of lining is used on small canal and water coarse and not found to be durable. 10. Stone block lining This consists of undressed stone block set in mortar and laid over a prepared subgrade. The lining is able to check seepage effectively but has a considerable resistance to flow o water. Dressed stone block lining is effective but costly. 11. Pre-fabricated light weight membrane They are matted fibres of asbestos or jute and is coated with asphalt. It is laid on a smooth and prepared subgrade, and is covered with a layer of earth material. 12. Bentonite and clay membrane This consists of bentonite or clay blanket 4 cm thick hid a prepared subgrade, and covered with earth. 13. Road oil lining The road oil sprinkled on subgrade in a thickness of about 1.5 mm is sufficient enough to saturate subgrade to a depth of 8 cm. The subgrade is then rolled so that oil enters the soil pores. Two types of lined canal sections: (i) triangular shaped or curved channel and (ii) trapezodial shaped or flat bottom shaped channel. The side slopes are so selected that they are nearly equal to the angle of repose of the soil so that no earth pressure is imposed on the lining. The corners are rounded off to improve the hydraulic efficiency. Triangular shaped section is adopted for small discharges, as it is the best discharging section. For higher discharges, trapezoidal section may be adopted. TI the limiting velocity is taken as 1.8 m/sec then circular section can be adopted only if the discharge is less than 85 cumecs. However, Pakistan Standard recommends only trapezoidal section for all discharges.
In order to design a lined canal section following data is required:
(1) discharge Q, (ii) rugosity coefficient N, (iii) longitudinal slope S. (iv) side slopes, and (v) limiting velocity V Advantages of Lining Merits The main advantages derived by canal lining arc mentioned below:
1. The lining of canals prevents seepage loss and thus more area can be Irrigated by the water so saved. The cost of irrigation is therefore, reduced.
2. The lining of canal is an important anti-water logging measure as. It reduces seepage to the adjoining land.
3. Th. lining provides a smooth sur1co. The rugosity coefficient, therefore decreases. The resistance to flow also decreases and hence the velocity of flow In ho lined canal Increases.
4. The Increased velocity minimizes the losses due to evaporation.
5, The increased velocity help to provide a narrow cross section for lined channels
6. Higher velocity helps In providing a flatter hydraulic gradient or bid slop. Thus bitter command can be obtained.
7. Higher velocity prevents silting of channel.
8. Lining makes the banks more stable in light textured soil.
9. Lining reduces maintenance costs and possibility of breaching due to increased stability of section,
10. Lining of a canal prevents or reduces weed growth.
11. Lining of a canal Increases- available head for power generation as a flatter gradient can be provided.
12. Canal lining assures economical water distribution.
13. Canal lining prevents water to come in contact with harmful salts during transit. Disadvantages of Canal Lining The canal lining has certain disadvantages although the advantages far out-weigh the meager disadvantages. The disadvantages of canal lining are mentioned below:
1. Canal lining requires a heavy initial investment.
2. Lining being permanent, it is difficult to shin the outlets very often.
3. It is very difficult to repair the damaged lining.
4. A lined channel section i without a berm. The additional safety provided by the berm for vehicular and pedestrian traffic is, therefore, absent in a lined channel. KENNEDY’S THEORY Kennedy selected a number of sites on Upper Ban Doab Canal system, one of the oldest in Punjab (Pakistan) for carrying out investigations about velocity and depth of the channel. The sites selected by him did not require any silt clearance fore more than thirty years and were thus supposed to be flowing with non- silting non-scouring velocity.
Kennedy’s study revealed the following: 1. The flowing water ha to counteract some amount of friction against the bed of the canal, This gives rise to vertical eddies rising up gently to the surface. These eddies are responsible for keeping most of the silt in suspension. Some eddies may start from sides but these are for most of its horizontal and so do not have any silt supporting power. The silt supporting power is therefore, proportional to the bed with of the stream and not to its wetted perimeter. Lacy’s Regime Theory Dimension width, depth and slope of a regime channel to carry a given discharge loaded with a given silt charge are all fixed by nature. This idea was first put forward by Lacey. Lindley’s theory is also based on the same concept. Lacey succeeded in evolving more generally applicable equations based on his own experiments und the experiments of the put Investigators.
Regime channel: Lacey defined regime channel as a stable channel transporting a regime still charge. A channel will be in regime If It flows in coherent unlimited alluvium of the same character as that transported and the silt grade and sill charge are all constant.
Incoherent alluvium: It is a soil composed of loose granular graded material which can be scoured with the same ease with which it is deposited.
Regime Slilt charge: It is the minimum transported load consistent with fully active bed.
Regime slit grad.: This indicates the gradation between the / small and the big particles It should not be taken to mean the average mean diameter of a particle,
Regime conditions: A channel is said to be in regime when the following conditions are satisfied.
1. The channel is flowing in unlimited Incoherent alluvium of’ the same character as that transported.
2. Silt grad and silit charge is constant
3. Discharge is constant
If the above three conditions are met with fully, then the channel is said to be in true regime. However, it is seldom that the above conditions are realized in field. Hence, Lacey gave the idea of initial and final regime for actual channel. Initial regime One of the conditions of attaining regime of a channel is that there should be freedom for the channel to form its own section. Initial regime is the state of channel that has formed its section only and yet not secured the longitudinal slope. Final regime When a channel is constructed with defective slope, it tries to throw off the incoherent silt on the bed to increase their slopes. To attain the final regime the channel forms its section first before the final slope. The channel after attaining its section and longitudinal slope, will be said to be in final regime. Permanent regime: When a channel is protected on the bed and side with some kind of protecting material the channel section cannot be scoured up and so there is no possibility of change of section or longitudinal slope; the channel will then be said to be in permanent regime. Regime theory is not applicable to such channels.
There is only one section and only one longitudinal slope at which the channel will carry a particular discharge with a particular silt grade. Natural silt transporting channels have a tendency to assume a semi-elliptical section. The coarser the silt, greater is the waterway of such a channel and narrower the depth. The finer the silt, greater is the depth and the channel closely approximates a semi-circle: Channel section according to Laceys theory If a channel is constructed with too small a cross-section for a particular discharge and the slope steeper than required, scour will occur till final regime is attained, Similarly silting will occur in a channel till final regime is attained in a channel with a widet cross section and flatter slope than required.
Lacey also states that the silt is kept in suspension due to the force of vertical eddies. According to him, the eddies are generated from bed, and sides, both normal to surface of generation. Hence, vertical component of eddies generated from sides will also support the silt. Lacey, therefore, assumed hydraulic mean depth (R) as variable unlike Kennedy who assumed depth D as variable. Since Lacey assumed a semi-ellipse as the cross-section of a regime channel assumption R as a variable seem to more logical. Lacey’s Regime Equations: On the basis of arguments mentioned above, Lacey plotted a large mass of data to obtain a relationship between (i) V vs. R and (i) A vs. V. Lacey recognized the importance of silt grade in the problem and introduced a function f known as silt factor in the regime relationship. The first two equations, originally suggested by Lacey in F.P.S. units, Converted M.K.S., units.
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