In this article we will discuss about:- 1. Classification of Canals 2. Points to be Considered While Fixing Canal Alignment 3. Alignment of Water Courses.
Canal is an artificial channel, usually trapezoidal in section. It is constructed on the surface of the ground. It is used to convey water from river, lake, reservoir, etc., to fields for irrigation, for water supply schemes, for power generating units etc. They always flow under gravity. The canal may be Kucha or Pucca. Pucca canals are known as lined canals.
Classification of Canals:
The canals can be classified in several ways. All the possible classifications are given as follows:
1. Classification Based on Financial Output:
Under this classification canals may be divided into two types:
(i) Protective canal.
(ii) Productive canal.
(i) Protective Canal:
The purpose of protective canal is to protect the areas most prone to famines. The canals are constructed having all the permanent works required for their regulation. No discharge of water is left in them under normal conditions. But whenever famine conditions are anticipated due to shortage or no rains, these canals are cleared by employing labour at a short notice and water is run in them to provide water for drinking as well as irrigation purposes. These canals do not give any revenue to the state.
(ii) Productive Canals:
These are such canals, which after deducting repair, maintenance, and supervision charges, yield revenue to the state. The revenue they yield should be installment of initial investment plus 6 ¼ % interest on the total investment. Most of the irrigation canals pertain to this category of the canals.
2. Classification Based Upon the Nature of Source of Supply:
Under this classification, the canal can be divided into two categories:
(i) Permanent canals.
(ii) Inundation canals.
(i) Permanent Canals:
When canals are fed regularly or continuously, from a permanent source, such canals are known as permanent canals. Permanent canals have a regular, well defined section. They have permanent concrete masonry regulation works. Such canals run practically throughout the year. Such canals are also sometimes known as Perennial canals. These canals are closed only when either some construction is to be carried out over them or silt clearance is to be done. These canals always take off from ice fed perennial rivers.
(ii) Inundation Canals:
These are such canals which run only for the duration, during which water level in the river remains above some specified level. These canals do not have a very regular section and structures like falls etc. They are not provided with any diversion works in the river, in form of weir or barrage. They however have a head regulator.
3. Classification Based Upon the Purpose of the Canal:
Following types of canals come under this category-
(i) Irrigation canals.
(ii) Water supply channels.
(iii) Power generating canals.
(iv) Navigation canals.
(v) Carrier canals.
(vi) Feeder canals.
All these canals are made for some specific purpose. Irrigation canals supply irrigation water to fields and water supply channels supply water to cities for drinking purpose. Power generating canals carry water to run generating unit and Navigation canals are used for the purpose of augmenting the inland transportation. Carrier canals do irrigation and. side by side carry water for other canals. Feeder canals are constructed to feed two or more smaller canals.
4. Classification Based Upon the Relative Position in a Given Network of Canals.
An elaborate network of irrigation canals consists of following categories of canals:
(i) Main canal.
(ii) Branch canal.
(iii) Distributary.
(iv) Minor.
(v) Water course.
(i) Main Canal:
This canal takes off directly from a river or reservoir. It is generally very big. Being too big, direct irrigation is generally not done from it except in exceptional circumstances. It acts as a carrier to feed branch canals or major distributaries.
(ii) Branch Canals:
Irrigation area for big canals is generally very large. It may not be possible to supply irrigation water from one canal. In such circumstances the main canal is bifurcated into two or more parts, which are known as branch canals. Each branch canal is assigned to the task of irrigating specified area.
Discharge of each branch canal is decided depending upon the area to be irrigated by each. Branches also carry quite large discharges and as such direct outlets should be given to lonely higher spots only lying along the alignment which cannot be irrigated from the distributaries. Branches act as feeder canals for distributaries.
(iii) Distributaries.
Distributaries are channels carrying small discharges of say ½ to 7 cumecs. They usually take off from branch but they can also be taken from main canal, but their discharge has to be smaller than branch canal, otherwise they will become branches. The most of the irrigation is carried out by distributaries. Outlets are located at regular intervals and water is supplied to the fields.
(iv) Minors:
They are also sometimes called minor distributaries. They take off either from branch or distributaries. Mostly they take off from distributaries. Mostly area lying along the branches is quite high and cannot be irrigated by distributaries. In that case, a small minor is also taken off from the head works of some distributary and this minor is run along the branch canal.
Outlets to the area lying in the vicinity of the branch are given from the minors. There may be some areas lying very low or area may be located quite far off from the distributary. In that case such areas may be irrigated by providing minors from distributary. Minors carry hardly discharge for 10-15 outlets. Hence its discharge may be from 0.25 to 0.50 cumec.
(v) Water Course:
They are small channels that ultimately carry water to the fields from outlets. Water courses are also sometimes known as gools. They may be Pucca or lined. Nowadays stress is being given for lining of canals and water courses, as lot of precious irrigation water is otherwise lost in percolation. Outlets are usually taken from distributaries and minors, but they can be taken from branches also but only in special circumstances.
5. Classification Based on the Alignment.
Depending upon the alignment they follow, the canals can be classified into following three categories:
(i) Contour canal.
(ii) Ridge or water shed canal.
(iii) Side slope canal.
(i) Contour Canals:
These canals run nearly parallel to the contours of the country. Main canal taking off from a river is hostly contour canal for some length near the diversion head works. Even branch and distributaries can be contour canals. The contour chosen for the alignment should include all the contours of the area it has to irrigate.
Contour canals, provide irrigation on one side only as contours of other side are higher and irrigation water cannot flow under gravity. However irrigation facilities can be provided to the area lying on higher side of the contour canal by lift canals.
Contour canals have only one bank. Other side being higher does not require the second bank. These canals are also sometimes known as single bank canals. They may however be having two banks also. They do not follow the same contour all along. Some longitudinal slope has to be given to cause flow in the canal. Because of longitudinal slope, contour canal slowly leaves the higher contour and adopts the next lower contour.
(ii) Ridge Canal:
The canal which follows the ridge of the country is known as ridge canal. It generally takes off from a contour canal. It irrigates on both sides. Since this canal can irrigate areas along both the banks it commands the largest area with minimum length of canal. They do not cross any drainage and hence constructions of cross-drainage works are obviated. If ridge takes a very sharp turn the canal should be aligned straight.
This reduces the length of the canal but envolves construction of cross drainage work to pass run-off from enclosed area, to the other side of the canal. Also irrigation in this enclosed area cannot be done. Canals may also have to leave the watershed to bye-pass the towns and villages located on the water shed. Most of the irrigation canals are ridge canals.
(iii) Side Slope Canals:
The side slope channels are aligned roughly at right angles to the contour canals, along the slope between the ridges and the valleys. They are roughly parallel to the natural drainage of the country. They do not intercept any cross-drainage and hence no cross-drainage works have to be constructed. Side slope canals have to be lined, as they have very steep bed slope and Kucha canal may not withstand the erosive effect of increased velocities.
6. Classification Based Upon the Material of Construction:
Under this category the canals may be-
(i) Kucha or unlined canals.
(ii) Lined canals.
(i) Kucha or Unlined Canals:
The canal which runs through the natural soil of the region, is known as Kucha canal or unlined canal. The section of such a canal is trapezoidal. The side slope of the banks depends upon the nature of soil. Slopes vary from 1:1 to 2:1 in cutting and 2:1 to 3:1 in filling for general soils like soft clay, alluvial soil, sandy loan etc.
These canals have to be run with restricted velocity so that erosion or sour may not take place. Large amount of water is lost by percolation. Most of the canals in India are Kucha canals. But government is aware of the shortcomings of such canals and laying more and more emphasis on lining the existing as well as new canals.
(ii) Lined Canals:
The section of such a canal in pucca section, made of some strong and impervious material. Lined canals can be run with large velocities and as such section of the canal can be considerably reduced, thus causing economy in the earth work. Lined canals do not allow any percolation loss, and wore ever smaller areas being exposed; evaporation losses are also considerably reduced.
The irrigation water saved by lining of the canals can be used to provide irrigation facilities to additional areas. Lined canals have more bed slope and thus lot of command, is lost as the level of water is depressed faster.
Sources of water are limited and to provide irrigation facilities to larger areas, judicious use of available water is very essential. Hence a lined canal is the need of the time. If we analyse the benefits of lining of canals on long term basis, we can easily conclude that benefits would out weight the expenditure incurred for lining.
Points to be Considered While Fixing Canal Alignment:
Following points should be considered while fixing the alignment of a canal:
1. The canal should be straight. Such a canal would be minimum in length. Length being minimum losses due to percolation and evaporation are bound to be minimum.
2. The canal should follow the ridge as far as possible. This will cause irrigation of the areas lying on both the sides of the canal. If somewhere canal leaves the ridge, our effort should be to catch the ridge again as soon as possible.
3. Cross-drainage works should be minimum, as such works are very costly.
4. The canal should not pass through a village or town; but by the side of it.
5. Canal taking off from a river is a contour canal for some length. Every effort should be made to mount if the main water shed, as soon as possible.
6. Alignment should avoid deep cuttings and high embankments.
7. Idle length of the canal i.e. length of the canal not providing any irrigation, should be minimum.
8. The alignment should avoid rock, fissured, and brakish formations. Fissured formations cause loss of water through fissures and brakish formations render water useless. Rocky tracks involve lot of labour for the construction of canals.
9. Suitable foundations for works like falls, cross regulators, head regulators etc. should be available. Besides this, materials of constructions should be available in the vicinity.
10. Unnecessary curves in the canals should be avoided. The curves should be of as large radius as possible.
Radii of curves for canals, of various discharges should be as follows:
11. The canal section should be partly in cutting and partly in filling. Cutting should equal filling if work has to be most economical.
12. Diversion works should be so located that idle length of the canal is minimum. Moreover the elevation of water being diverted to the canal, should be such that maximum possible available area is brought under command of the proposed canal.
13. To avoid excessive percolation losses the canal should not be passed through sandy tracks.
Alignment of Water Courses:
Though the responsibility of maintenance of the water courses is that of farmers, still in new irrigation works, the alignment, of the water courses is fixed by the government. This work is being done these days by canal area development (C.A.D.) department or collonisation department.
Following points should however be considered while fixing the alignment of water courses:
1. Separate water courses should be provided for high lands and low lands of the same village.
2. They should not cross through field but should be laid along the boundaries of the fields.
3. Sufficient water should be available to fields lying at the farthest point from the outlet.
4. The water courses should be aligned along the ridges of the area of the village.
5. If low lying area is near the outlet and high land is at the further end of the village, the water course which has to carry water for high lands should be made in heavy embankments and such water courses should preferably be lined.
6. In order to ensure equitable distribution of water to all the fields, the water course levels for each square of land, should be fixed and made permanent. If this is not done, the farmer whose land is low dig the water courses deep and when a farmer whose land is high takes water from the same deep water course, has to fill the water curse by the water from his own time of watering, because otherwise water will not rise to higher fields.
This causes less availability of water to high fields. Again if the turn of watering of a former whose land is low, he gets water course full for water and this causes advantage to him. To curb this injustice, it is very important to fix the water course levels for each square and these levels should be made pucca. This will make irrigation water available to all the fields at desired levels. Deep excavation of water courses will not give any advantage to the farmers whose land is low.