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CLASSIFICATION OF CANALS

Classification of canals

CANAL

  • The canal is an artificial channel, usually trapezoidal in section.
  • It is construct on the surface of the ground.
  • It is use to convey water from the 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 a protective canal is to protect the areas most prone to famines.
  • The canals are construct 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 clear by employing labor 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 instalment of initial investment plus 6 1 4% interest on the total investment.
  • Most of the irrigation canals pertain to this category of 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 know 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 know 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 that 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 a weir or barrage.
  • They however have a head regulator. Inundation canals will be discussed in detail in this chapter a little later.

3. Classification based upon the purpose of the canal.

  • The 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 purposes.
  • Power generating canals carry water to run generating units and Navigation canals are used for the purpose of augmenting inland transportation.
  • Carrier canals do irrigation and side by side carry water for other canals.
  • Feeder canals are constructed to feed two or smaller canals.

4. Classification based upon the relative position in a given network of canals.

  • An elaborate network of irrigation canals consists of the following categories of canals:

(i) Main canal.

(ii) Branch canal.

(iii) Distributory.

(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.

  • The 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 a specified area.
  • The discharge of each branch canal is decided to depend 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 1 2 to 7 cumecs.
  • They usually take off from the branch but they can also be taken from the main canal.
  • But their discharge has to be smaller than the 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 distributaties.
  • In that case, a small minor is also taken off from the headworks 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 the area may be located quite far off from the distributary.
  • In that case such areas may be irrigated by providing minors from the 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.
  • Watercourses are also sometimes know gools.
  • They may be Pucca or lined. Nowadays stress is being given to the lining of canals.
  • Water courses, a 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.
  • The main canal taking off from a river is hostly contour canal for some length near the diversion headworks.
  • Even branch and distributaries can be contour canals.
  • The contour choosen 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 the other side are higher and irrigation water cannot flow under gravity.
  • However, irrigation facilities can be provided to the area lying on the higher side of the contour canal by lift canals.

Water shed canal

  • Contour canals have only one bank.
  • The 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 the longitudinal slope, the 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 a minimum length of the canal.
  • They do not cross any drainage and hence the construction of cross-drainage works is obviated.
  • If ridge takes a very sharp turn the canal should be aligned straight.
  • This reduces the length of the canal but involves construction of cross drainage work to pass run-off from the 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 water to bye-pass the towns and villages locate on the watershed.
  • 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 slopes and the 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 know 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.
  • A large amount of water is lost by percolation.
  • Most of the canals in India are Kucha canals.
  • But the government is aware of the shortcomings of such canals and laying more and more emphasis on lining the existing as well as new canals.
Side slope channel
Side slope channel

(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 limit and to provide irrigation facilities to larger areas, judicious use of available water is very essential.
  • Hence lined canals is the need of the time.
  • If we analyze the benefits of the lining of canals on a long-term basis, we can easily conclude that benefits would out weight the expenditure incurred for the lining.

 

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