Unit 4-canal

jhbhb

1. Canals

2. INTRODUCTION A canal is defined as an artificial channel constructed on the ground to carry water from a river or another canal or a reservoir to the fields.

3. Types of Canals

4. TYPES OF CANALS (Based on Source of Supply)

5. TYPES OF CANALS ( Based on Function) Note, a canal can serve more than one purpose.

6. TYPES OF CANALS (Based on Alignment)

7. Watershed canal or Ridge canal The canal which is aligned along any natural watershed (ridge line) is called a watershed canal, or a ridge canal.

8. Watershed canal or Ridge canal (Contd.) Aligning a canal (main canal or branch canal or distributary) on the ridge ensures gravity irrigation on both sides of the canal. Since the drainage flows away from the ridge, no drainage can cross a canal aligned on the ridge. Thus, a canal aligned on the watershed saves the cost of construction of cross-drainage works.

9. Watershed canal or Ridge canal (Contd.) Pranamesh Chakraborty

10. Contour Canal Watershed canal along the ridge line are not found economical in hill areas . In hills, the river flows in the valley well below the watershed. In fact, the ridge line (watershed) may be hundred of meters above the river. It therefore becomes virtually impossible to take the canal on top of such a higher ridge line. In such conditions, contour canals are usually constructed. A contour canal irrigates only on one side because the area on the other side is higher.

11. Contour Canal (Contd.)

12. Side Slope Canal A side slope canal is that which is aligned at right angles to the contours; i.e. along the side slopes. Since such a canal runs parallel to the natural drainage flow, it usually does not intercept drainage channels, thus avoiding the construction of cross-drainage structures. It is a canal which is aligned roughly at right angle to contours of the country but not on watershed or valley.

13. Side Slope Canal (Contd.)

14. Types of Canals (BASED ON DISCHARGE)

15. MAIN CANAL Main Canal takes off directly from the upstream side of weir head works or dam. Usually no direct cultivation is proposed

16. BRANCH CANAL All offtakes from main canal with head discharge of 14-15 cumecs and above are termed as branch canals. Acts as feeder channel for major distributaries

17. MAJOR DISTRIBUTARY All offtakes from main canal or branch canal with head discharge from 0.25 to 15 cumecs are termed as major distributaries . MINOR DISTRIBUTARY All offtakes taking off from a major distributary carrying discharge less than 0.25 cumec are termed as minor distributaries WATER COURSE Small channels which carry water from the outlets of a major or minor distributary or a branch canal to the fields to be irrigated.

18. Types of Canals (Based on lining provided or not)

19. A PICTURE OF A UNLINED CANAL

20. A PICTURE OF A LINED CANAL

21. Canal Alignment General consideration for alignment:- It should be aligned in such a way that maximum area is served with the least length. And its cost including CD- work is minimum. A shorter length of canal has less loss of head due to friction and smaller loss of water due to seepage and evaporation, so that additional area can be brought under cultivation. The alignment should be kept straight as far as possible, it will result in minimum losses. It should have minimum CD work.

22. Canal Alignment General consideration for alignment:- It should not passes through the village, town, forest or costly lands, otherwise heavily compensation shall have to given . It should be such that heavy cutting and filling (embankment) are avoided. It should through passes the ridge so that both side of canal can be irrigated. The alignment should be such that as far as possible a balanced depth of cutting and filling is achieved. The alignment should not be made in rocky, brackish or cracked strata.

23. Cross-Section of Irrigation Canal

24. A canal is generally taken in such a way that its section is partly in cutting and partly in filling in order to approach close to balancing depth. Many times however the canal has to be carried through deep cutting or filling. A canal structure may, therefore, either : In Cutting In Filling In Partial Cutting and Filling. Cross-section of canal

25. Cross-section of canal:-

26. Cross-section of canal:-

27. Cross-section of canal:-

28. A canal section will be economical when earth work involved at a particular section has an equal amount of cut and fill. Usually a canal section has a part in cutting and part in filling as shown in fig. If the amount of cut is equal to the amount of fill, it has to be paid for once only. Definition:- For a given C/S there is always only one depth of cutting for which the cutting and filling will be equal. The depth is known as balancing depth. Balancing Depth

29. Balancing Depth:-

30. If :- h = vertical height of top of bank from the bed of canal. b = bed width of the channel. t = top width of the canal bank. n:1 = side slope of bank in filling. z:1 = side slope of canal in cutting. d = full supply depth of canal. y = depth of cutting. Balancing Depth:-

31. Area of the cut = by + zy 2 = y(b + zy) Area of fill = 2[(h – y)t + n(h-y) 2 ] Equating the area of cut and fill: y(b + z y) = 2[(h – y)t + n(h-y) 2 ] b y+ zy 2 = 2th + 2nh 2 – 2nhy – 2ty – 2nhy + 2ny 2 y 2 (2n – z) – (b + 4nh + 2t)y + 2h(t + nh) =0 From this equation the balancing depth of the canal may be determined. Balancing Depth

32. A canal is usually constructed with side slope of 1:1 in cutting and a slope 1.5:1 in filling. Putting n = 1.5 and z = 1 in above equation. We get; y 2 – (b/2 + 3h + t)y + h (t + 3/2 h)=0 Balancing Depth

33. Components of Cross- Section Side slope Berm Freeboard Bank Service road Back Berm or Counter Berm Spoil Bank Borrow Pit

34. Side Slope The side slopes should be such that they are stable, depending upon the type of the soil. A comparatively steeper slope can be provided in cutting rather than in filling, as the soil in the former case shall be more stable.

35. Berm Berm is the horizontal distance left at ground level between the toe of the bank and the top edge of cutting.

36. Berm (contd.) Purposes of Berms: They give additional strength to the banks and provide protection against erosion and breaches. They protect the banks from erosion due to wave action. They provide a scope for future widening of the canal.

37. Freeboard The margin between FSL and bank level is known as freeboard. The amount of freeboard depends upon the discharge of the channel.

38. Bank The primary purpose of banks in to retain water. This can be used as means of communication and as inspection paths.

39. Service Road Service roads are provided on canals for inspection purposes, and may simultaneously serve as the means of communication in remote areas. Dowla : As a measure of safety in driving, dowlas with side slopes of 1.5: 1 to 2:1, are provided along the banks.

40. Back Berm or Counter Berm Even after providing sufficient section for bank embankment, the saturation gradient line may cut the downstream end of the bank. In such a case, the saturation line can be kept covered at least by 0.5 m with the help of counter berms as shown in figure below.

41. Spoil Bank When the earthwork in excavation exceeds earthworks in filling, the extra earth has to be disposed of economically. Economical mode of its disposal may be collecting this soil on the edge of the bank embankment itself.

42. Borrow Pit When earthwork in filling exceeds the earthwork in excavation, the earth has to be brought from somewhere. The pits, which are dug for bringing earth, are known as borrow pits. If such pits are excavated outside the channel, they are known as external borrow pit s, and if they are excavated somewhere within the channel, they are known as internal borrow pits . Internal borrow pits are more preferred than external one.

43. Borrow Pit (Contd.) The inside borrow pit may be located at the centre of canal. The idea behind this is that the borrow pits will act as water pockets where the silt will be deposited and ultimately the canal bed will get levelled up.