CHAPTER 5- Water Conveynance and Control-1.pptx

irrigation

1. CHAPTER :FIVE 1 WATER CONVEYANCE &CONTROL

2. 5 .1 Irrigation distribution systems The purpose of any irrigation network is water intake, conveyance, distribution and field application to the crops. The conveyance and distribution of water will be affected by canals. Irrigation canals are irrigation structures (open channels) through which irrigation water is conveyed and distributed with in the irrigation system . Drainage canals are open channels which evacuate excess water (either excess irrigation water or excess water from rainfall) away from the irrigation area. 2

3. Con… The basic differences between irrigation and drainage canals are: Irrigation canals carry clean water drainage canals carry generally dirty (saline or alkaline) water ; The flow in irrigation canals is from the high level to the lower level canals. flow in drainage canals is from the lower level canals to the higher level canals. 3

4. Preliminary layout of irrigation and drainage canals The layout on irrigation and drainage systems should take into consideration: The physical (technical) feasibility and The economic feasibility As a very general principle for the layout of irrigation and drainage canals: drainage canals are located in the lower part of the area and irrigation canals on the higher parts of the area. The reason for this principle is that from irrigation canals water has to flow out by gravity to the fields located at the lower areas while water has to flow into the drainage canals located at the lower parts of the area. 4

5. Cont… 5 Irrigation canal Drains

6. Classification of irrigation canals Based on the alignment, irrigation canals can be classified into three: Contour canal Watershed canal (ridge canal) Side slope canal Main (Primary canal) Branch canal Secondary canals Tertiary canals Field canals 6 Based on the sizes and importance, irrigation canals can be classified as:

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8. Design of lined and unlined canals An irrigation and drainage network should be designed and operated in such a way that: the required discharge flows are passed at design water levels; no erosion of canal bottoms and banks will occur, and any sediment that enters the system will not settle in the network but will be carried along and discharged through the outlets, to the fields or to the natural drainage system. Design discharge Design discharge also called canal capacity is the maximum discharge that the cross section of a canal reach is designed for. This discharge is the maximum expected flow in the canal during peak periods of peak flow. 8

9. Determination of deign discharge The design discharge of a canal can be determined as follows: Determine the total area irrigated from the canal under consideration; Based on the crop water requirement, determine the water demand for each unit irrigated in l/s/ha; For each outlet in the canal reach, determine the outflow by multiplying the area by the water requirement; Determine the outflow into the next reach of the canal; Add all the outlet discharges and the outflow into the next reach of the canal to determine the design discharge of the canal reach under consideration. 9

10. Cont… The outlet discharges are; Where:- A= Area to be irrigated from the outlet q= peak water demand rate (l/s/ha) The design discharge of the reach is thus, Q= Q1+Q2+Q3+…+ Qo+ losses Where: Q1, Q2, Q3… are outlet discharges downstream of the reach Qo outflow into the next reach. 10

11. Canal cross section Irrigation and drainage canals can be constructed of earth (unlined canals) or lined canals. Irrigation canals are usually designed as trapezoidal sections; however lined irrigation canals can also be designed as rectangular sections. 11 b d 1 m Canal depth

12. Cont… 12

13. Cont… Freeboard: Vertical distance between the highest water level anticipated in the design and the top of the retaining banks. It is a safety factor to prevent the overtopping of structures. Side Slope (m): The ratio of the horizontal to vertical distance of the sides of the channel. m= e/d = e’/D 13 Sand, Soft Clay 3: 1 (Horizontal: Vertical) Sandy Clay, Silt Loam, Sandy Loam 2:1 Fine Clay, Clay Loam 1.5:1 Heavy Clay 1:1 Stiff Clay with Concrete Lining 0.5 to 1:1 Lined Canals 1.5:1

14. Cont… The design of irrigation canal cross section basically means fixing appropriate bottom width b, depth of flow d, total depth and canal side slope m. The other parameter used in the design canals is the longitudinal slopes of the canal. The following longitudinal slopes can be adopted for preliminary design of canals: Large canals, Q > 15 m 3 /s, …………….0.10 to 0.30 % Intermediate canals,0.3< Q<15 m 3 /s …0.20 to 0.40 % Small canals, Q < 0.3 m 3 /s,…………….0.30 to 0.50 % 14

15. Design of lined canals In lined canals the cross section of the canals is covered (lined) with some kind of harder material than earth (soil) to provide resistance against erosion and avoid seepage losses. Design of lined canals is usually done based on the permissible velocity approach. It is defined as the mean velocity at or bottom which the channel bottom and sides are not eroded. 15

16. Recommended Permissible Velocities ∗ . 16 Material V(m/s) Fine sand 0.6 Coarse sand 1.2 sandy silt 0.6 Silt clay 1.1 clay 1.8

17. Cont…. For the design of lined canals, uniform flow equations for open channel flow can be used. This can be the Chezy equation or Manning’s formula: Continuity equation Chezy equation; Manning’s Formula 17 Where:- Q = design discharge, m 3 /s A = the x-sectional area of flow C = Chezy constant R = hydraulic radius, m S = longitudinal slope of the canal n = Manning’s coefficient

18. Cont… The Manning’s n and the Chezy C are functions of the kind of lining and the condition (roughness) of the surface. The following are recommended values of the Manning’s n for different linings. Lining material value of n Concrete lining………………..…………0.013 to 0.018 Masonry lining with random stone………0.017 to 0.020 Brick lining……………………………….0.014 to 0.017 Asphalt lining……………………………..0.013 to 0.016 18

19. Design procedure for lined canal For a known permissible velocity V, Manning’s n, and longitudinal slope S of the canal, determine the hydraulic radius from Manning’s or Chezy equation; Form two equations with two unknowns, bottom width b and depth of flow y. Trapezoidal canal: Rectangular canal 3. In the equations * and **, there are two unknowns b and y, which can be solved simultaneously to determine the flow depth y and bottom width b. 19

20. Cont… The best hydraulic section is the one with minimum wetted perimeter for a given discharge. The best hydraulic section for a trapezoidal canal is one when R= y/2. The bed width/depth ratio, b/y, is usually unity. Table: Values for m and b/y for lined trapezoidal canals; for rectangular: b/y=2 20 Q(m3/s) 0.3 0.3 – 2.0 2.0 – 30.0 >30.0 m 1 or 1.25 1 or 1.25 1.25 or 1.5 1.5 – 2.0 b/y 1 0.03 * Q + 1.0

21. Design of unlined canals Unlined canals can be classified into two classes based on the stability of the boundaries of the canal for design purposes: Canals with stable (non-erodible) bed and Canals with erodible bed (Alluvial canal) Design of non-erodible (stable) canals Non-erodible canals are canals with fairly stable boundary . When a channel conveying clear water is to be lined, or the earth used for its construction is non-erodible in the normal range of canal velocities, Manning's equation is used. 21 Manning's n can also be got from Tables or estimated using the Strickler equation: n = 0.038 * d 1/6 ,d is the particle size diameter (m)

22. Cont… There are two approaches for the design of such canals: The recommended (b/y ratio) approach and The Tractive force (permissible velocity approach) Design of canals on b/y ratio approach The highest flow velocity in a section will occur when the hydraulic radius R is maximized. For any given hydraulic gradient S and side slope m, an infinite range of, b/y , could be selected. Minimizing the wetted perimeter by equating the first derivative on y to zero results in a ‘best’ hydraulic cross section if: 22

23. Cont… 23 In earth canals, the best hydraulic section is seldom applied because: The cross section would not be stable; The excavation would be too deep ; A change of flow heavily affects the depth of water and velocity distribution in deep and narrow canals .

24. Cont… For a preliminary design, recommended side slopes that are stable under normal conditions are as follows: Table: Side slopes in canals; m = cotangent of side slope 24 Material m Rock Stiff clay 0.5 Cohesive medium soils 1.0-1.5 Sand 2 Fine sand, porous clay, soft peat 3

25. Cont… The bed width/depth ratio of a small earthen irrigation canal is often close to unity; the ratio is gradually increased for larger canals. The recommended ratios as related to the design discharge of the canal Q are as follows: In most small and medium size earth canals, the freeboard varies from 50% to 60% of the depth y , with a minimum of 0.15 or 0.20 m. 25 ( b/y) recom. =1.76*Q 0.35 , for Q > 0.2 m 3 /s (b/y) recom. = 1.0, for Q ≤ 0.2 m 3 /s

26. Cont… Recommended values for m and b/y for unlined irrigation canals constructed in earth (cohesive medium soils) 26 Q (m 3 /s) 0.2 0.2 – 0.5 0.5 – 10.0 >10.0 m 0.8 – 1 1 – 1.5 1.5 - 2 > 2 b/y 1 1.76 * Q 0.35

27. Design of canals on Tractive force (Permissible velocity) approach The balance of forces on a sediment flow is an important aspect that often controls the cross section of earthen irrigation canals. Sediment transport in irrigation canals is a function of velocity of flow. Tractive force theory Tractive force or shear force is the force applied by the flowing water on the canal bed and sides in the direction of flow. This force per unit area is called Unit tractive force or shear stress. 27

28. Design of Erodible Channels Erodible canals are canals with movable bed. In canals designed on erodible (alluvial deposits), not only have erosion problem but also in most cases the water carries sediments with it. The design of such canals can be made based on the maximum and minimum permissible velocities Procedure For Design i) Determine the maximum permissible velocity . ii) With the permissible velocity equal to Q/A, determine A. iii) With permissible velocity = 1/n S 1/2 R 2/3 for known value Slope, s and n. iv) R = A/P , so determine P as A/R v) Then A = b y + m y 2 and P = b+ 2 y (m 2 + 1) 1/2 , s olve and obtain values of b and y 28

29. Cont… 29 Material Clear water Loaded water Velocity, m/s Tc (N/m 2 ) Velocity, m/s Tc (N/m 2 ) Fine sand, 0.46 1.30 0.76 3.61 Sandy loam, 0.53 1.78 0.76 3.61 Silt loam, 0.61 2.31 0.91 5.29 Alluvial silts, 0.61 2.31 1.07 7.22 Volcanic ash 0.76 3.61 1.07 7.22 Stiff clay, 1.14 12.51 1.52 22.13 Alluvial silts, 1.14 12.51 1.52 22.13

30. 30 ERODIBLE CHANNEL

31. Regime channels Canals designed for non-silting and non-scouring velocity are called regime canals. A channel is in state of regime means that whatever sediment entering the canal at the head is kept in suspension so that it will not settle and local sediments are not produced by erosion of the canals beds. There are two researchers called R.G. Kennedy and Lacey from India who have done a remarkable research for finding a solution for design of stable (non-silting and non-scouring) alluvial canals . 31

32. Kennedy’s Theory Kennedy selected some straight reach of a canal which had not caused serious silting and scouring for the previous more than 30 years. He concluded that whether a sediment particle will be kept in suspension or will settle down is a function of generation of eddies that rise to the surface. According to Kennedy, a critical velocity is the velocity which will just keep the canal free from silting and scouring. Where: V o = the critical velocity y= the depth of flow, m 0.55 and 0.64=constants which depend on silt charge . 32

33. Cont… 33 Sediment type m Light sandy silt 0.9 to 1.1 Sandy, loamy silt 1.2 Hard soil 1.3 As this formula is purely empirical depending on observations, in order to apply it to other canals reaches, some factor which takes into account the soil type. He introduced a factor called critical velocity ratio (m), which depends on the size of the silts .

34. Procedure for design of regime canals on Kennedy’s theory The following procedure can be used for canal design: Assume a trial depth of flow y and determine the critical velocity Vo; Determine the area of flow, A from A=Q/Vo; workout the canal cross sectional parameters; calculate the actual mean velocity V in the canal from the Kutter’s formula, Manning’s formula or Chezy equation; Compare V and Vo. If the same, the assumed depth of flow y is right if not the same assume another y and repeat steps 1 through 4. 34 Kutter’s Formula Where: V is flow velocity, m/s R is Hydraulic radius, m S is Slope of the canal n is roughness coefficient

35. Lacey’s Regime Theory Lacey carried out investigations for the design of regime canals on alluvial deposits. He came up with three kinds of regimes called initial, true and final regimes. He mentioned that the regime theory can be applied to only channels in true regime or final regime. A canal in true regime is a canal in which discharge, velocity, depth of flow, amount of silt and size of silt are constant. 35

36. Cont… Procedure for design of regime canals on Lacey’s Theory 1 . Evaluate the flow velocity from Where V is in m/s Q is design discharge in m 3 /s f is silt factor where d is mean particle size, mm 2. Determine the hydraulic radius, R from 36

37. Cont… 3. Calculate the area of flow, 4. Calculate wetted perimeter, P 5. Workout y and b from the known, A, P and R.   6. Compute the canal bed slope, S or S = 0.0003 f 5/3 /Q 1/6 37

38. 6.2 Flow measurement Critical-flow flumes and broad-crested weir are devices used to measure flow in open channels. These devices are adaptable to a variety of measurement applications in both natural and man-made channels, and both new and existing canal systems. Weir It is a barrier (structure) constructed across a river to raise the water level in the river behind it so as to enable regulated diversion of water. There are two types of weirs in common use: Sharp-crested weirs and the broad-crested weirs. The sharp-crested weirs are commonly used in irrigation practice. 38

39. 6.3 Related Hydraulic structure Water levels in canals need to be lowered in some cases for topographical reasons. Lowering of water levels will be attended by a loss of energy of the flow. Structures used for this purpose are called canal drops or falls. Falls (Canal drops) A canal drop is a RC or masonry structure provided in canal to lower down the water level and the bed level of a canal. A canal drop consists of a water level lowering structure and an energy dissipating structure . It is required when the natural ground slope along the alignment is steeper than the bed slope of the canal. The location of this structure is depend on topography and economy. 39

40. Chute A chute is a pipe or open channel lowering the water level from a higher canal to a lower one. It is used when the required crop height is large (5m and so) and water has to be conveyed over a longer distance. Cross Drainage Structures Cross- drainage structure is structures which is constructed at the crossing of a canal and natural drainage channels like river and stream, so as dispose of drainage water with out interrupting the continuous canal supplies. As cross drainage structures are expensive, the alignment of the canal should be that which minimizes the cross drainage works as much as possible. 40

41. Cont… Based on the relative position of the canal and drainage, cross drainage works can be classified as:- 1 . Canal over the drainage: by passing the canal over the drainage. This is when bed level of the canal is well above the High Flood Level(HFL) of drainage. Ex. Aqueduct, Siphon aqueduct 2. Canal below the drainage : by passing the canal below the drainage. provided when the bed level of drain is well above the Full Supply Level (FSL) of the canal. Ex. Super passage or inverted (canal) siphons 3 . Canal and drainage at the same level: by passing the drain through the canal. Provided when HFL and FSL of drain and canal are at the same level. In this case the canal water &the drainage water are allowed to mix to each other. 41

42. QUIZ(10%) Attempt Any One Question Design an irrigation channel(D,B,T,&S)to carry a discharge of 30cumecs by Kennedy's theory . Take B/D=8.0,n=0.0225 and m=1 Design regime channel (using lacy`s theory ) carrying discharge of 27.67cumecs and the average particle size in 0.323mm. 42

43. THE END!!!! 43