Canal headworks

presented By ATUL HAJONG
Institute -NIT Meghalaya

1. CANAL HEADWORKS PRESENTED BY: KHLAINBORLANG MALNGIANG(B14CE013) ATUL HAJONG (B14CE014) ABISHEK KUMAR (B14CE016) SHATRUGHAN SHIRA(B14CE017) NEERAJ KUMAR YADAV(B14CE019)

2. CANAL HEADWORKS – Any Hydraulic Structures/works constructed across river and supplies water to the off taking canal is called canal headwork. Canal head works Diversion head works Weir or barrage is constructed across a perennial river to raise water level and divert the required quantity . Storage head works Dam is constructed across a river valley to form storage reservoir divert the required quantity.

3. TYPES OF HEADWORKS Fig1: DIVERSION HEADWORK Fig 2: STORAGE HEADWORK

4. CANAL HEADWORKS Purposes : Raises water level in the river Regulates supply of water into the canal Controls the entry of silt into the canal Provides some storage for a short period Reduces the fluctuations in the level of supply in river

5. TYPES OF DIVERSION HEAD WORKS 1. Temporary diversion head works Consists of a bund constructed across river to raise the water level in the river and will be damaged by floods . 2. Permanent diversion head works Consists of a permanent structure such as a weir or barrage constructed across river to raise water level in the river .

6. B14CE014

7. LOCATION OF CANAL HEAD WORKS Depends on the stages of flow (reaches) of river (i) Rocky stage (ii) Boulder stage (iii) Trough stage or alluvial stage (iv) Delta stage Both rocky and delta stages are not suitable for location of diversion head works

8. Rocky stage In this stage river is in the hills. The bed slope and velocities are high. It is generally not suitable for the location of a diversion headworks. Advantages: A sound rocky foundation is usually available at the site. Due to high velocities of flow no silting problem. High heads are available for hydroelectric work. Disadvantages: The land in the hilly area is not suitable for agriculture. The discharge in the river is low because of small catchment.

9. Boulder Stage In this stage the bed and banks of the river are composed of boulders and gravels. Advantages: L ength of weir is generally shorter in boulder stage. Construction materials such as stone, aggregates, sand, gravel are locally available. Disadvantages: S eepage losses are high. D emand of irrigation water is low because the land is not fertile.

10. Trough or Alluvial stage In this stage the cross section of river is made up of alluvial sand and silt. The bed slope and velocity are small. Advantages: D emand of irrigation water is high. Seepage losses are less in the canal. The water contains silt which has manurial value. Disadvantages: Construction material is usually not available locally. C ost of headworks is usually more due to poor foundation. P roblem of silt in the canal.

11. delta stage The river passese on to the delta stage as it approaches the ocean. The bed slope and velocities are reduce so much that it is unable to carry its sediments load. It drops down the sediments and gets divided into channels on either sides of the deposit resulting in the formation of a delta. Not Suitable for location of a diversion headworks because the river section is excessively wide.

12. Neeraj B14CE019

13. Site selection for diversion head work River section at the site should be narrow and well-defined. Should have a large commanded area. Site should be such that the weir (or barrage) can be aligned at right angles to the direction of flow in the river. Good foundation should be available at the site. Site should be easily accessible by road or rail. Overall cost of the project should be a minimum .

14. COMPONENTS OF DIVERSION HEADWORKS

15. 1. Weir or Barrage 2. Divide wall or divide groyne 3. Fish ladder 4. Pocket or approach channel 5. Under sluices or scouring sluices 6. Silt excluder 7. canal head regulator 8. River training works such as marginal bunds and guide bunds COMPONENTS OF DIVERSION HEADWORKS

16. WEIR OR BARRAGE Weir is a structure constructed across river to raise the water level and divert the water into the canal .Weir aligned at right angle to the direction flow. Shutters are provided at the crest of the weir so that part of raising up of water is carried out by shutters.

17. Abhisek B14Ce016

18. Types of weirs: 1.Masonry weirs with vertical drops 2.Rock fill weirs with slopping apron. 3.Concrete weirs with d/s glacis

19. Divide wall L ong wall constructed at right angles in the weir or barrage,with stone masonry or cement concrete. On the upstream side, the wall is extended just to cover the canal head regulator and on the downstream side, it is extended up to the launching apron. Functions of the divide wall are as follows: F orm a still water pocket in front of the canal head in which helps in settling of silt. C ontrols the eddy current or cross current in front of the canal head. Provides a straight approach in front of the canal head. It resists the overturning effect on the weir or barrage caused by the pressure of the impounding water.

20. Fish ladder C onsists of an inclined channel with a slope not exceeding 1 in 10. The compartment of bays of fish ladder should be sufficiently large so that the fish do not collide with the sides of the bay when ascending. Grooved gates are provided at the entrance and at the exit of the fish ladder for closing it. The water supply to the fish ladder should be adequate for the fish to travel at all times.

21. Under sluices Functions of under sluices:  M aintain a clear and well defined river channel in front of the headregulator .  U sed to scour away the silt deposited in front of the head regulator.  They pass low floods without dropping the weir crest shutters.  They control the silt entry into the canal.  They provide greater waterway for floods, thus lowering the flood levels .

22. Silt Excluder D evice to exclude silt from water entering the canal. Consists of a number of rectangular tunnels The tunnels are of different lengths . The length of the tunnels gradually decreases as the distance of the head regulator.

23. Canal head regulator Functions of Canal Head Regulator: It regulates the supply of water entering the canal. It prevents the river-floods from entering the canal The entry of silt into the canal is controlled Silt gets deposited in the pocket, and only the clear water enters the regulator bays.

24. Canal head regulator Constructed at the head of canal to regulate flow . Consists of a number of piers which divide the total width of the canal into a number of spans which are known as bays. The piers consist of number tiers on which the adjustable gates are placed . The gates are operated form the top by suitable mechanical device.

25. SHATRU B14CE017

26. Causes of Failure of weir or barrage on permeable foundation The combined effect of surface flow and surface flow may cause the failure of the weir or barrage . 1 . Failure due to subsurface flow : By piping or undermining: The water from the upstream side continuously percolates through the bottom of the foundation and emerges at the downstream end of the weir or barrage floor . The force of percolating water removes the soil particles by scouring at the point of emergence as a result depression is formed through the bottom of the foundation. A hollow pipe like formation thus develops under the foundation due to which the weir or barrage may fail.

27. Undermining or piping

28. 1. Failure due to subsurface flow ( b)By uplift Pressure: The percolating water exerts upward pressure on the foundation of the weir or barrage. If this uplift is not counterbalanced by the self weight of the structure, it may fail by rapture.

29. By Hydraulic Jump: When the water flows with a very high velocity over the crust of the weir then hydraulic jump develops. This hydraulic jump causes a suction pressure or negative pressure on the downstream side which acts in the direction of uplift pressure. If the thickness of the impervious floor is not sufficient, then the structure fails by rapture. 2 . Failure by Surface Flow :

30. (b ) By Scouring During floods: The gates of the barrage are kept open and the water flows with high velocity. The water may also flow with very high velocity over the crest of the weir. Both the cases can result in scouring effect on the downstream and on the upstream side of the structure. Due to scouring effect on the downstream and on the upstream side of the structure, its stability gets endangered by shearing . 2. Failure by Surface Flow :

31. Precautions Against Failure L ength should be carefully designed to reduce the exit gradient . Sheet piles should be provided on the upstream side and the downstream side to reduce the uplift pressure considerable. T hickness of the impervious floor should be sufficient to counterbalance the uplift pressure. Inverted filter should be provided with concrete blocks on the top so that the percolating water does not wash out the soil particles. Energy dissipater blocks like friction blocks, impact blocks, should be provided .

32. Energy Dissipater Blocks

33. Reference Linked slide shared. Irrigation engineering and Hydraulic. .structured. www.civilenggforall.com