Ch-3.pptx

Hydraulic Structure-II, Lecture� CHAPTER Three

1. Chapter-3 3. DIVERSION HEAD WORKS 1

2. INTRODUCTION Diversion head works are structures constructed across a river to facilitate a regulated and continuous diversion of water into the off-taking canal. In rivers, it is hardly possible to divert a regulated and continuous flow into main canal without such headwork. This is due to the fact that the flow in the river is never uniform and varies from season to season. 2

3. INTRODUCTION Thus, there is a need to regulate the flow into the canal system in order to ensure a continuous diversion of water. There is practically no storage provided by a diversion structure. The purpose is to raise and keep the water level more or less constant (reduce the fluctuation of water levels) at the head of the canal. 3

4. TYPES OF DIVERSION STRUCTURES Diversion head works can be classified as weirs and barrages based on the structures provided at the crest. Weir: A weir 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. A weir has a raised crest behind which a small ponding of water will take place. Can be constructed with or without shutters on the crest of the weir. 4

5. Weir Types of weirs: The following types based on the geometry of the crest and materials used for construction Masonry weirs with vertical or slightly sloping u/s and d/s face Rock fill weirs Concrete weirs with sloping glacis Ogee crest weirs 5

6. Masonry vertical drop weirs Have a horizontal floor constructed of masonry and a crest wall with vertical or slightly sloping downstream face. The height of the crest depends on the actual site conditions and head required behind the weir. Sheet pile walls are driven at the upstream and downstream of the floor. Are suitable whenever the drop in water level is small. 6

7. Vertical drop weirs 7

8. 2. Rock fill weirs Are constructed of rocks with extremely sloping downstream face. Are suitable whenever there is excess stone available for construction. 8

9. 3 . Concrete sloping weir The crest of this weir has sloping glacis both on the upstream and downstream. Cutoff sheets are provided at the upstream, intermediate and downstream of the floor to the depth equal to the scour depth. Hydraulic jump is formed on the downstream slope for energy dissipation. These weirs are suitable whenever the drop in water level is large. 9

10. Glacis weirs 10

11. Ogee crest weirs Is a weir whose crest wall is rounded to increase the discharge coefficient. It consists of a concrete weir wall with vertical upstream face and rounded top and downstream. It is designed as gravity section similar to vertical drop weir. 11

12. Gravity and non-gravity weirs Seepage water causes uplift force on the base of the weir. Whenever the weight of the weir is sufficient to balance the uplift pressure caused by seeping water, it is called gravity weir. When the concrete slab (floor) is designed continuously with the weir body to keep the structure safe against uplift, it is called as non-gravity weir. 12

13. Barrage A barrage is also an obstruction constructed across a river for raising the water level and regulate the diversion of water to canals. However, the crest wall of a barrage is low and ponding of water takes place by gates. The gates are fitted on the top of the crest wall and can be closed and opened as required based on the flow in the river. 13

14. Barrage 14 Piers Gates

15. Advantages and disadvantages of weirs and barrages Weir: Advantage Low initial cost Disadvantage High afflux (increase in water level) during floods; Siltation or sedimentation problem due to relatively high crest; Lack of effective control during floods. 15

16. Barrage Advantage Effective control of flow is possible; Afflux and thus flooding is small during floods; Silt inflow into the off-taking canal can be effectively controlled. Disadvantage It has a disadvantage that its initial cost is high. 16

17. Some technical considerations for diversion headwork's When planning a new diversion headwork, investigations to be made can be classified into: Reconnaissance study Preliminary investigation Detailed investigation The technical considerations include: Location of headwork's Construction materials and resources Topographic survey Soil investigation Hydrological data 17

18. Location of Headworks For best site of diversion headwork, one has to have clear information of the site. Generally topographic maps are required for the purpose. However, one can also have a walk along the river to find out possible sites for the headwork during the reconnaissance study. 18

19. Factors when selecting site for diversion weirs. Location of the Irrigated Area If the area is too far away from the headwork, it necessitates construction of long canals with high cost If close to the headwork, some area located on the upper reach of the canal may not be commanded. Stability of the river bank Affects the cost and the performance. Ideal site: straight reach of a river with stable and narrower section . River banks are unstable in shallow and wide cross sections ; thus larger and costlier structure is needed. Flow velocity at these sections is small and results in more sedimentation and problem on the performance . 19

20. 2. Construction materials and resources Most important factor for the selection of construction materials is the economic factor . Questions to be made during site visit of diversion headwork's. What are the construction materials available in the area? Is there a shortage of required construction material in the local market? Is it possible to hire construction machinery in the area? What is the availability of skilled labor in the area and the rates? 20

21. 3. Topographic survey After site selection, the designer has to be able to have a detailed in formation of the cross section and profile of the river at this section. Thus a topographic survey of the site is needed. Moreover, the topography of the command area is needed to determine whether the highest spot points can be irrigated by gravity from the selected site or not. Particularly, this is important in flat areas where head is not available. Whenever, the site of the diversion is sufficiently higher than the command area, loss of head is a not a problem. 21

22. 4. Soil investigation Preliminary soil investigation is needed during the first visit of the site. The soil can be visually tested and its physical characteristics described. Shallow pits can be dug to describe the soil profile. The investigation is important to judge the suitability of the soil for foundation , its seepage condition and bearing capacity. 22

23. 5. Hydrological data Is needed in order to determine design discharges. The size of the structure depends on the maximum flood discharge that has to pass over the structure. Moreover, the minimum flow in the river is also needed for the design. The design engineer visiting the site for the first time has to find out if there are river gauging and meteorological stations in the area. If not, the local people can provide useful information on the maximum and minimum flows. 23

24. Components and Layout of Diversion Headworks Diversion headwork's generally consist of the following components: Weir wall/Barrage Under sluices Divide wall Canal head regulator Silt excluder Guide banks Wing walls 24

25. Components and layout… 25

26. Under sluices Adjacent to the canal head regulators , under sluice section is provided. When canal intake is only in one direction, the under sluice is provided on that side only. There is a divide wall between the weir body and the under sluice section to separate the two portions and to avoid cross flows. Its crest is at lower level than that of the crest of the weir (usually at river bed.) 26

27. Functions of under sluice Maintains well defined river channel near the head regulator; To scour (remove) away the silt deposited in front of the head regulator; To pass small floods of 10% to 20% Q d during rainy season; To quickly lower the u/s high flood level because the discharge intensity over the sluice portion is greater than that in the weir portion; To minimize the effect of main river water current on the head regulator. 27

28. Divide wall This is a wall placed paralle l to flow direction in river. Separates the weir section from the under sluice section of the headwork to avoid cross currents. On the upstream, it extends to little upstream of the head regulator and on the downstream it usually extends to the end of loose protection. 28

29. Functions of divide wall Separates the floor of the under sluices and weir (floor of under sluice at a lower level); Provides a clear pocket near the head regulator where silt can accumulate; Isolates the silt accumulation pocket to ensure scouring ; Helps to avoid cross currents which might cause deep scour of the river bed; Helps to concentrate the scour action of the under sluices on only the silt accumulation pocket; Minimizes the effect of river current on the head regulator. 29

30. Canal head regulator A structure provided at the head of the off-taking canal to regulate and control the inflow into the canal. Usually provided at one or both banks of the river with its axis making an angle 90 to 120 to the weir axis. It will be sized in such a way that it can pass the required design discharge of the canal when the water level on the upstream is at the pond level. 30

31. Layout of head regulator 31

32. Functions of head regulator Regulates the supply of water into the off-taking canal; Controls silt entrance into the canal; Prevents flood water from entering the canal; Used to stop the water supply into the canal for: maintenance and when highly silt-laden water flows in the river. 32

33. Section through head regulator… 33

34. Head regulator 34

35. Silt excluder Provided in the under sluices portion to pass highly silt-laden water through the under sluices. It enables only relatively clear water to enter the canal. Aligned at right angle to the axis of the canal. They are small lined tunnels through which the bottom silt-laden water will be passed down to the scouring sluices. 35

36. Alignment of silt excluder 36

37. Guide banks Are rigid structures provided on either side of the headwork to: guide the river flow directly to the headwork and to avoid scouring and meandering of the river near the work. Particularly important when the headwork is located near alluvial banks of the river where bank scouring and meandering are evident. Wing walls (Marginal bunds): are used to protect valuable areas and property from flooding. 37

38. PRINCIPLES FOR DESIGN OF DIVERSION HEADWORKS   The design of any hydraulic and irrigation structures have to consider the hydraulics of surface and subsurface flow . Subsurface flow (Seepage) Theory seeping water under the structure causes upward uplift pressure on the base of the structure. For safe design of hydraulic and irrigation structures on permeable foundation, the hydraulic gradient should be less than some allowable limit called critical hydraulic gradient. There are some theories developed on seepage of water under the foundation of hydraulic and irrigation structures.   38

39. Bligh’s Creep Theory This theory assumes that the seeping water creeps from the upstream to the downstream of the structure along the contact base of the soil with the structure. The length of seepage path traversed by the seeping water is called creep length (L). One of the shortcomings of the Bligh’s theory is that it does not make differences between vertical and horizontal creep. According to Bligh’s creep theory, the hydraulic gradient (i) is constant throughout the seepage path. 39

40. Bligh’s Creep Theory 40

41. Bligh’s Creep Theory 41 Considering the above figures, the hydraulic gradient of seepage is given by: Where H is the seepage head (difference in water level between upstream and downstream) L is the creep length: The uplift pressure u at any point along the seepage path is given by:

42. Bligh’s Creep Theory 42 Where ; ɣ is unit weight of water and h is the residual head at the point. The residual head at any point p is determined from: h= H-Head loss from upstream end to p Or it is equal to the head loss between point p and the downstream end, Where l’ is floor length from point p to the downstream end.

43. Bligh’s Creep Theory Design criteria based on Bligh’s creep theory   The hydraulic gradient should be less than permissible value 43

44. Bligh’s Creep Theory The floor thickness and weight should be sufficient to withstand the uplift pressure . 44

45. 2. Khosla’s Theory A.N. Khosla and his associates investigated the actual pressure on the base of structures and they found out that the actual up lift pressures were different from those determined on the basis of Bligh’s theory. According to him, the uplift pressure at any point, at distance X from the entry point of the impervious floor is given by; 45

46. Khosla’s Theory 46 Uplift pressure at key points Pile at d/s end

47. Khosla’s Theory Pile at u/s end 47

48. Khosla’s Theory Intermediate pile 48

49. Example: determine the uplift pressure heads at points E, D & C in the following figure. Determine the exit gradient (GE) . 49

50. Khosla’s Theory 50

51. Khosla’s Theory of Independent Variables The pressures obtained for each individual standard forms are then super-imposed to determine the pressure at the key points for the whole structure. Assumptions made : The following assumptions are made in the solution of the elementary forms. The floor thickness is negligible Only one pile at a time Horizontal floor Because of the above assumptions, corrections to the super-imposed pressures are applied 51

52. Correction for floor thickness 52 Where ø E ø C’ ø D are the pressure head or seepage head expressed in percentage. For example; ø E = The pressures at E’ and C’ can be obtained assuming linear variation of pressure from top to bottom of pile.

53. con’t… E’ is on d/s of E along the flow path indicated ØE’ < E C’ is on u/s of C along the flow path, ØC’ > C Thickness correction at E Thickness correction at C = Generally: - correction positive – for C & Negative for E. 53

54. Correction for Mutual interference of pile 54 when there are more than one pile line, there will be interference between the pile correction for mutual interference between pile is given by ; Where: C = is percentage correction in pressure b 1 = is the distance between the two piles b = is the total length of impervious floor . d = is the depth of the pile on which effect is required D = is the depth of the pile whose effect is required

55. Correction for slope of the floor 55

56. Con’t…. 56

57. Causes for Failure of weirs on permeable foundations Failure of weir can be due to sub – surface flow & surface flow. Failure due to sub – surface flow : Failure can be by piping or rupture due to uplift. Piping failure- is a failure when the seeping water takes place under high hydraulic gradient ic and thus more seepage force Failure by uplift: the seeping water under the floor exerts an upward take (pressure) on the floor called uplift pressure 57

58. Con’t…. Failure due to surface flow Such failure can be either due to suction pressure or scour. Suction pressure failure: when hydraulic jump forms on the d/s glacis the water surface in the hydraulic jump trough is lower than the subsoil HGL and thus additional thickness of floor is required to balance this pressure. Failure by scour : during high flow, scouring of the river bed occurs both on the u/s and d/s of the weir .Thus sheet piles have to be provided to a depth of maximum scour depth and appropriate protection works should be provided on both sides. 58

59. Design of weir and under sluice bay sections   The discharge which passes over the weir and sluice bay sections has to be determined first. Discharge over the sluice bay should at least equal to the larger of the following: Twice the Q design of the off taking canal 20% of the design discharge of the wire ( 80% over weir bay)   59

60. Procedure for design Fix the discharge over the sections of weir and under sluice bays Fix the crest levels of the weir and under sluice sections. Fix the water ways of the weir and under slice sections Determine the characteristics of the hydraulic jump with and with out retrogression . When high flood passes over the weir , the d/s bed of the river erodes and lowering takes place called retrogression Calculate the normal scour depth, R and determine the upstream and downstream pile depths. Fix the total length of the impervious floor from exit gradient consideration. Fix the levels of the floors on upstream & downstream 60

61. Procedure for design 7. calculate the uplift pressures at the key points of the piles for No flow condition (Khosla's theory is generally used for sloping glass weirs) High flood condition   8. Determine the uplift pressure from the subsoil HGL for no flow condition and suction pressure in high flood condition. Determine the floor thickness at various points for the determined uplift pressure in step8 Provide protection works (block protection and launching apron) both on the u/s and d/s side. 61

62. Vertical drop weir Such a weir consists of a crest wall with nearly vertical d/s face. Generally hydraulic jump is not formed and energy is dissipated by vertical impact of water .   Design data - Design discharge - High flood level before construction - Bed level of river - FSL of off taking canal - Silt factor (f) - critical exit gradient(GE) -Retrogression - stage –discharge r/ship at the site The design of a vertical drop weir is generally made by Bligh’s theory and the thickness of floor checked by Khosla’s theory. 62

63. Procedure Determine the water way from Lacey’s perimeter L= P = 4.75 * Q, Q = design discharge 2. Determine discharge intensity , q = Q/L 3. Determine the normal scour depth from, R = 1.35 ( q 2 /f ) 1/3 4 . Regime velocity of flow. V = q/R Determine velocity head from ha = V 2 /2g. 5. TEL upstream of the weir and d/s of the weir from: D/s TEL = HFL before construction + ha U/s TEL = D/s TEL + Afflux U/s HFL = U/s TEL – ha 63

64. 6 . Head over the crest can be determined with a broad crested weir formula : q = 1.70 *(He) 3/2 7. Determine the crest level: crest level = u/s TEL –He 8. Pond level can be determined from pond level = crest level = u/s TEL –He = FSL of canal + modular head . 9.Determine the depth of u/s and d/s piles from u/s pile depth = 1.5 * R d/s pile depth = 2.0* R 10. Determine the maximum seepage head for the worst condition ( WL on u/s at pond level and no tail water ) from : Hs = pond level - d/s bed level 64 Procedure

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72. 72 The end Thank you