Design of Canals

LONGITUDINAL SECTION OF A CANAL

  • The points which should be consider in fixing alignment of the canal.
  • Actually, the whole of the area where irrigation is propose, is surveyed, and contour plans prepared.
  • The other features of the area are also marked over the plans.
  • The contour plan on which other features of the area are also marked in know shajra sheet.
  • The alignment of the main canal is fixed on the main ridge.
  • The area proposed to be irrigate so that irrigation is possible on both sides of the canal.
  • Branch and distributory channels are aligned along the main ridges of the area allotted for their command.
  • In this way, the whole of the area to be irrigated is divide into several parts and each part is command by a branch, distributory or minor depending upon the extent of the area.
  • The area under.
  • The command of each distributory or minor, is further subdivide into small areas.
  • Surround by small drainages and each area is know chak.
  • The chak is that area that is generally surround by minor drainages.
  • One outlet for each chak is provide from the distributory.
  • The outlet is locate in such a way that water may flow to all the areas of a chak under gravity.
  • The size of the outlet depends upon the command areas available in a chak for irrigation.
  • After having fixed the canal alignment, detaile leveling is done along the alignment, and.
  • The longitudinal section is plott on a drawing sheet.
  • For preparing maps of the area, a Horizontal scale of 1 cm 160 mand vertical scale of 1 cm = 1/2 m is use.
  • If the area is very much undulate, the vertical scale may be different.
  • After drawing the longitudinal section, along the alignment, the bed level of the proposed canal is marked.
  • While markmg bed level it should be ensured, that it will involve either too much cutting nor filling, and secondly the full supply level of the canal will remain above the ground level so that irrigation is possible on both the sides, along the alignment. Bed level and F.S.L. of the off-taking channels should be decided in relation to the Bed level and F.S.L. of the parent channel.
  • When water is diverted to the offtaking channel, some head loss is bound to occur.
  • To overcome this head loss, F.S.L. off-taking channel should be about 30 cm below the F.S.L. of the parent channel.
  • All the irrigation channels are given some longitudinal slope as these are gravity channels and water can flow only if some longitudinal slope is given to them.
  • The slope of the canal is also decided in relation to the general slope of the area, in which the canal is to run.
  • If the slope of the channel is almost the same the general slope of the area, no fall will have to be constructed.
  • But such ideal conditions are seldom available.
  • If this slope is changed to suit the general slope of the area, the canal will not remain a regime canal.
  • This fixed slope given by Lacey may not be same as general slope of the area.
  • The generally designed slope of the channel is much less than the general slope of the area, and, as such a number of falls have to be constructed.
  • If somewhere, the general slope of the ground is smaller than the designed slope.
  • The slope of the channel is changed to the general slope of the ground.
  • Section of the canal is accordingly modified.
  • After deciding the longitudinal slope and also the bed level at the head regulator.
  • The bed levels of the channel at all the points are know.
  • F.S.L. is marked parallel to the bed level.
  • F.S.L. should always remain above the ground level so that water may flow to fields under gravity.
  • After having marked Bed levels on the longitudinal section of the proposed channel, mark the lengths where cuttings or filling are to be done.
  • Extent of cutting or filling at a particular point is determined from the difference of ground level and Bed level.
  • In order to economize the work, cutting should be just equal to filling at a particular point.

Following points should be taken care of, while drawing longitudinal section of canal.

  1. Cutting and filling at all the points should be equal.
  2. In other words cutting and filling should balance each other.
  3. F.S.L. of off-taking channel should be below the F.S.L. of parent channel.
  4. The difference in F.S.L. of off-taking channel.
  5. Parent canal should be minimum 30 cm for distributory, 70 cm for branch and 1 m for main canal.
  6. F.S.L. of the channel should remain above the ground level for most of the length.
  7. At isolated high spots, it may remain below the ground level.
  8. F.S.L. should be above ground level only by 15 to 30 cm.
  9. This is consider sufficient because canals being aligned on water shed.
  10. Will develop sufficient cross-slope and water will be flowing to fields under sufficient head.
  11. Canals should not be too much in filling. The canals are always subjected to breaches at such reaches.
  12. Bed slope as obtained by Lacey’s theory if equals general slope of the ground it will be an ideal situation.
  13. If the design slope of the canal is less than a general slope of ground.
  14. Canal falls will have to be provide, at suitable intervals.
  15. The fall or drop structure should be such that F.S.L. of the canal.
  16. D/S of fall remains below G.L. for about 1 2 km distance and then emerges out of G.L.
  17. The high ground on both the sides, D/S of fall is irrigated by taking outlets from U/S of the fall.
  18. If the designe slope of the channel is greater than a general slope of the ground.
  19. The channel would go deep in cutting after running for a short distance running.
  20. In such a case general ground slope should be adopte and a section of the channel should be accordingly amended.

 

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