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Design of Hydraulic Structures
The document discusses diversion headworks, a type of hydraulic structure built across rivers to divert water into canals, contrasting their function with that of storage headworks or dams. It details component parts of diversion headworks, such as weirs or barrages, undersluices, fish ladders, and their design considerations. Additionally, it addresses potential modes of failure and historical theories related to the structural integrity and hydraulic behavior of these systems.
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Design of Hydraulic Structures
- 1. Diversion Headworks Introduction Component Parts Weirvs Barrage Modes of Failure Design Aspects Design of vertical drop weir CEL351: Design of Hydraulic Structures
- 2. Diversion Headworks Any hydraulicstructure which supplies water to the off-taking canal is called a headwork. Headwork may be 1. Storage headwork. 2. Diversion headwork. A Storage headwork comprises the construction of a dam on the river. It stores water during the period of excess supplies and releases it when demand overtakes available supplies. A diversion headwork serves to divert the required supply to canal from the river. Introduction
- 3. Diversion Headworks A diversionhead works is a structure constructed across a river for the purpose of raising water level in the river so that it can be diverted into the offtaking canals. Diversion headworks are generally constructed on the perennial rivers which have adequate flow throughout the year and, therefore, there is no necessity of creating a storage reservoir. A diversion head works differs from a storage work or a dam. A dam is constructed on the river for the purpose of creating a large storage reservoir. The storage works are required for the storage of water on a non-perennial river or on a river with inadequate flow throughout the year. On the other hand, in a diversion head works, there is very little storage, if any.
- 4. Diversion Headworks If thestorage on the upstream of a diversion head works is significant, it is called a storage weir. If a diversion headworks is constructed on the downstream of a dam for the purpose of diverting water released from the u/s dam into the offtaking canals, it is called a pickup weir. Generally, the dam is constructed in the rocky or the mountainous reach of the river where the conditions are suitable for a dam, and a pickup weir is constructed near the commanded area in the alluvial reach of the river.
- 5. It raises thewater level on its upstream side. It regulates the supply of water into canals. It controls the entry of silt into canals It creates a small pond (not reservoir) on its upstream and provides some pondage. It helps in controlling the vagaries of the river. Functions of a Diversion Headworks Diversion Headworks
- 6.
- 7.
- 8. Weir or barrage Undersluices Divide wall Fishladder Canal head regulator Approach channel Silt excluders River training works (Marginal bunds and guide banks) Component Parts Diversion Headworks
- 9. Undersluice sections areprovided adjacent to the canal head regulators. The undersluices should be able to pass fair weather flow for which the crest shutters on the weir proper need not be dropped. The crest level of the undersluices is generally kept at the average bed level of the river Undersluice Diversion Headworks
- 10. A divide wallis constructed parallel to the direction of flow of river to separate the weir section and the undersluices section to avoid cross flows. If there are undersluices at both the sides, there are two divide walls Divide Wall Diversion Headworks
- 11. A fish ladderis a passage provided adjacent to the divide wall on the weir side for the fish to travel from u/s to d/s and vice versa. Fish migrate u/s or d/s in search of food or to reach their sprawling places. In a fish ladder the head is gradually dissipated so as to provide smooth flow at sufficiently low velocity. Suitable baffles are provided in the fish passage to reduce the flow velocity. Fish Ladder Diversion Headworks
- 12.
- 13. A canal headregulator is provided at the head of the canal offtaking from the diversion headworks. It regulates the supply of water into the canal, controls the entry silt into the canal, and prevents the entry of river floods into canal. Canal Head Regulator Diversion Headworks
- 14. A silt excluderis a structure in the undersluices pocket to pass the silt laden water to the downstream so that only clear water enters into the canal through head regulator. The bottom layer of water which are highly charged with silt pass down the silt excluder an escape through the undersluices. Silt Excluder Diversion Headworks
- 15. Guide banks areprovided on either side of the diversion headworks for a smooth approach and to prevent the river from outflanking. Marginal bunds are provided on either side of the river u/s of diversion headworks to protect the land and property which is likely to be submerged during ponding of water in floods. Guide Banks & Marginal Bunds Diversion Headworks
- 16. Weir or Barrage DiversionHeadworks Barrage Weir
- 17. A weir isa raised concrete crest wall constructed across the river. It may be provided with small shutters (gates) on its top. In the case of weir, most of the raising of water level or ponding is done by the solid weir wall and little with by the shutters. A barrage has a low crest wall with high gates. As the height of the crest above the river bed is low most of the ponding is done by gates. During the floods the gates are opened so afflux is very small Weir or Barrage Diversion Headworks
- 18. A weir maintainsa constant pond level on its upstream side so that the water can flow into the canals with the full supply level. If the difference between the pond level and the crest level is less than 1·5 m or so, a weir is usually constructed. On the other hand, if this difference is greater than 1·50 m, a gate-controlled barrage is generally more suitable than a weir. In the case of a weir, the crest shutters are dropped during floods so that the water can pass over the crest. During the dry period, these shutters are raised to store water upto the pond level. Generally, the shutters are operated manually, and there is no mechanical arrangement for raising or dropping the shutters. On the other hand, in the case of a barrage, the control of pondage and flood discharge is achieved with the help of gates which are mechanically operated Weir or Barrage Diversion Headworks
- 19. Advantages: The initial costof weirs is usually low. Disadvantages: There is a large afflux during floods which causes large submergence. Because the crest is at high level, there is great silting problem The raising and lowering of shutters on the crest is not convenient. Moreover, it requires considerable time and labour. The weir lacks an effective control on the river during floods. A roadway can not be conveniently provided over the weir. Weir – Advantages and Disadvantages Diversion Headworks
- 20. Advantages: The barrage hasa good control on the river during floods. The outflow can be easily regulated by gates. The afflux during floods is small and, therefore, the submerged area is less. There is a good control over silt entry into the canal. There is a good control over flow conditions, shoal formations and cross-currents on the upstream of the barrage. There are better facilities for inspection and repair of various structures. A roadway can be conveniently provided over the structure at a little additional cost. Disadvantages: The initial cost of Barrage is very high. Barrage – Advantages and Disadvantages Diversion Headworks
- 21. Vertical drop weirs. Rockfillweirs. Concrete glacis or sloping weirs. Types of Weirs Diversion Headworks
- 22. • Wall typestructure on a horizontal concrete floor. • Shutters are provided at the crest, which are dropped during floods so as to reduce afflux. • Water is ponded upto the top of the shutters during the rest of the period. • Vertical drop weirs were quite common in early diversion headworks, but these are now becoming more or less obsolete. Vertical Drop Weirs Diversion Headworks
- 23. • In arockfill type weir, there are a number of core walls. The space between the core walls is filled with the fragments of rock. •A rockfill weir requires a lot of rock fragments and is economical only when a huge quantity of rockflll is easily available near the weir site. •It is suitable for fine sand foundation. The old Okhla Weir across the Yamuna river is a rockfill weir. •Such weirs are also more or less obsolete these days. Rockfill Weirs Diversion Headworks
- 24. •Concrete sloping weirs(or glacis weirs) are of relatively recent origin. •The crest has glacis (sloping floors) on u/s as well as d/s. There are sheet piles driven upto the maximum scour depth at the u/s and d/s ends of the concrete floor. •Sometimes an intermediate pile is also driven at the beginning of the u/s glacis or at the end of d/s glacis. •The main advantage of a sloping weir over the vertical drop weir is that a hydraulic jump is formed on the d/s glacis for the dissipation of energy. •Therefore, the sloping weir is quite suitable for large drops Sloping/Glacis Weirs Diversion Headworks
- 25.
- 26. The river sectionat the site should be narrow and well- defined. The river should have high, well-defined, inerodible and non- submersible banks so that the cost of river training works is minimum. The canals taking off from the diversion head works should be quite economical and should have a large commanded area. There should be suitable arrangement for the diversion of river during construction. The site should be such that the weir (or barrage) can be aligned at right angles to the direction of flow in the river. uniform flow and length of the weir - minimum. Location or Site Selection Diversion Headworks
- 27. There should besuitable locations for the undersluices, head regulator and other components of the diversion headworks. The diversion headworks should not submerge costly land and property on its upstream. Good foundation should be available at the site. The required materials of construction should be available near the site. The site should be easily accessible by road or rail. The overall cost of the project should be a minimum. Location or Site Selection Diversion Headworks
- 28. • Irrigation/hydraulic structuresfor the diversion and distribution works are weirs, barrages, head regulators, distributary head regulators, cross regulators, cross-drainage works, etc. • These structures are generally founded on alluvial soils which are highly pervious. • These soils are easily scoured when the high velocity water passes over the structures. • The failures of weirs constructed on the permeable foundation may occur due to various causes: 1. Failure due to- subsurface flow 2. Failure due to surface flow Modes of Failure Diversion Headworks
- 29. The failure dueto subsurface flow may occur by 1. Piping due to Exit Gradient 2. Rupture of floor due to uplift Failure by rupture of floor • The water percolating through the foundation exerts an upward pressure on the impervious floor, called the uplift pressure. • If the weight of the floor is not adequate to counterbalance the uplift pressure, it may fail by rupture. Failure due to Subsurface Flow Diversion Headworks
- 30. • Piping/undermining occursbelow the weir if the water percolating through the foundation has a large seepage force when it emerges at the d/s end of the impervious floor. • When the seepage force exceeds a certain value, the soil particles are lifted up at the exit point of the seepage. • With the removal of the surface soil particles, there is further concentration of flow in the remaining portion and more soil particles are removed. • This process of backward erosion progressively extends towards the upstream side, and a pipe-like hollow formation occurs beneath the floor. • The floor ultimately subsides in the hollows so formed and fails. • This type of failure is known as piping failure. Piping Failure Diversion Headworks
- 31. The failure dueto surface flow may occur by suction pressure due to hydraulic jump or by scouring of the bed. (a) Failure by suction pressure In the glacis type of weirs, a hydraulic jump is formed on the d/s glacis. In this case, the water surface profile in the hydraulic jump trough is much lower than the subsoil H.G.L. Therefore uplift pressure occurs on the glacis. This uplift pressure is known as the suction pressure. If the thickness of floor is not adequate, the rupture of floor may occur. (b) Failure by scour During floods, scouring occurs in the river bed. The bed of the river may be scoured to a considerable depth. If no suitable measures are adopted, the scour may cause damage to the structure and may lead to the failure. Failure due to Surface Flow Diversion Headworks
- 32. • In 1910,W.G. Bligh gave creep theory. • The percolating water creeps along the contact surface of the base profile of the structure with the subsoil. • The length of the path thus traversed by the percolating water is called the length of creep or the creep length. • As the water creeps from the u/s end to the d/s end, the head loss occurs. • The head loss is proportional to the creep distance travelled. Bligh’s Theory Diversion Headworks 2d1d1 d2 d3 lu ld lu Hs Hs 2d2 2d3 ld LT 231 222 dldldL duT ++++=
- 33. • The subsoilhydraulic gradient, which is the loss of head per unit length of creep, is constant throughout the seepage path. • If the seepage head (which is the difference of water levels on the u/s and d/s of weir) is Hs and LT is the total creep length, the loss of head per unit length is equal to Hs /LT • The hydraulic gradient is inverse of Bligh’s Creep coefficient C, which varies from 10 to 18 depending upon the bed material • Therefore for known seepage head and creep coefficient the required creep length or seepage path LT = C Hs. • The uplift pressure at any point can be determined by Bligh theory and then required thickness to counteract it. Bligh’s Theory Diversion Headworks
- 34. • Bligh madeno distinction between the creep in the horizontal direction below the floor and the creep in the vertical direction along the faces of the piles. • Bligh's theory is quite simple and convenient. • A large number of early irrigation structures were designed using this theory. • Some of these structures are existing even today, but unfortunately a few of them failed. • The theory is now rarely used for the design of large, important irrigation structures. • However, sometimes it is used for the design of small structures or for the preliminary design of large structures Bligh’s Theory Diversion Headworks
- 35. 1. It doesnot differentiate between the vertical creep and the horizontal creep and gives the same weightage to both, actually, the vertical creep is more effective than the horizontal creep. 2. It that the head loss variation is linear, while the actual head loss variation is non-linear. 3. No distinction is made between the head loss on the outer faces and that on the inner faces of the sheet piles. Actually, the outer faces are more effective than the inner faces. 4. It does not emphasise the importance of the d/s pile without which piping failure occurs. It considers the downstream pile only as a component of the total creep length and not as a controlling factor for the exit gradient and the piping. Bligh’s Theory: Limitations Diversion Headworks
- 36. 5. The theorydoes not give any theoretical or practical method for the determination of the safe gradient. 6. Bligh did not consider the effect of the length of the intermediate pile. Bligh’s Theory: Limitations Diversion Headworks • Lane analysed a large number of dams and weirs founded on pervious foundations which failed or did not fail. • He brought out deficiencies in Bligh's creep theory and gave a new theory on statistical basis known as Lane's weighted creep theory. • This theory gives the vertical creep three times more weightage as compared to the horizontal creep, thus Lane’s Theory 231 333 dldldL duT ++++=
- 37. • In 1926-27,some siphons constructed on the Upper Chenab Canal on the basis of Bligh's creep theory, had piping problems. • Uplift pressure measurements by Dr. A.N. Khosla, Dr. N.K. Bose and Dr. E.M. Taylor indicated that the actual uplift pressures were quite different from those computed on the basis of Bligh's theory. • These investigations showed that 1. the outer faces of the end sheet piles are much more effective than the inner faces and the horizontal length of floor, 2. the intermediate piles of smaller in length than the outer piles are ineffective except for some local redistribution of pressure. Khosla’s Theory Diversion Headworks
- 38. 3. undermining (piping)of the floors starts from the tail end when the hydraulic gradient at the exit is greater than the critical gradient for that particular soil. • The soil particles move with the flow of water, thus causing progressive degradation of the subsoil and resulting in cavities below the floor and ultimate failure. • It is absolutely essential to have a reasonably deep cutoff (or pile) at the d/s end of the floor to prevent piping failure. • Khosla et al provided a complete rational solution of the problem based on potential flow theory and Schwartz- Christoffel transformation. • The theory gives uplift pressure at various points of the structure, depending upon its profile. • It also gives the exit gradient and it should be safe. Khosla’s Theory Diversion Headworks
- 39. The thickness ofthe floor should be adequate to resist uplift pressure. The uplift pressure head at any point is where Hs = seepage head and b = horizontal distance between entry and exit points of impervious floor. Khosla’s expression for the exit gradient is where ; and d = depth of d/s pile. Khosla’s Theory Diversion Headworks ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ = − b xH P s uH 2 cos 1 π λπ d H G s E 1 = 2 11 2 α λ ++ = d b =α
- 40. • The practicalprofile of a hydraulic structure rarely conforms to any single elementary form for which the mathematical solution was obtained by Khosla et al. • For the determination of uplift pressure at the key points of a composite structure Khosla et al gave the theory of independent variables. • A composite profile is split into a No of simple elementary standard forms for which the mathematical solution can easily be obtained. • Each elementary form is then treated independent of the other and the pressures at its key points are obtained from the solution already available. • Then the solutions of these elementary forms are superposed to obtain the pressure distribution at all the key points of the entire structure. Khosla’s Theory Diversion Headworks
- 41. • Further thesepressures are to be corrected as the individual pressures have been obtained based on the assumptions 1. the floor is of negligible thickness 2. there is only one pile line 3. the floor is horizontal. • Therefore for any given profile of a weir/barrage/hydraulic structure on pervious foundation the following four steps may be adopted to compute uplift pressure at any point: 1. Decompose the general profile into elementary profiles 2. Assemble/superposition of uplift pressures at key points 3. Correction due thickness, slope and interference 4. Interpolation for uplift pressure between key points Khosla’s Theory Diversion Headworks
- 42. Uplift Pressures atKey Points in Elementary cases Diversion Headworks Case 1: Pile at downstream end: The uplift pressure heads at key points E, D and C are ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − = − λ λ π 2 cos 1s uHE H P ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − = − λ λ π 1 cos 1s uHD H P 0=uHCP 2 11 2 α λ ++ = λπ d H G s E 1 = d b =α and the Exit Gradient is given by where
- 43. Uplift Pressures atKey Points in Elementary cases Diversion Headworks Case 2: Pile at upstream end: The uplift pressure heads at key points E1, D1 and C1 are 2 11 2 α λ ++ = d b =αwhere ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − = − λ λ π 2 cos 1 1 s uHC H P ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − = − λ λ π 1 cos 1 1 s uHD H P suHE HP =1
- 44. Uplift Pressures atKey Points in Elementary cases Diversion Headworks Case 3: Intermediate Pile: The uplift pressure heads at key points E, D and C are where ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − = − 1 21 1 cos λ λ π s uHE H P ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ = − 1 21 cos λ λ π s uHD H P ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ + = − 1 21 1 cos λ λ π s uHC H P d b1 1 =α d b2 2 =α 2 11 2 2 2 1 1 αα λ +++ = 2 11 2 2 2 1 2 αα λ +−+ = ;
- 45. Uplift Pressures atKey Points in Elementary cases Diversion Headworks Case 4: Depressed floor The uplift pressure head at key points D1’ and D’ are ( ) 2 2 2' 3 3 253 3 2 b dHPPH PPPP suHEuHDs uHDuHEuHDuHD + − =+−−= α ''1 uHDsuHD PHP −=
- 46. Uplift Pressures atKey Points in Elementary cases Diversion Headworks Principle of Reversibility: It may be observed that some of expressions are similar except sign. For example u/s pile and d/s pile cases if Similarly if θ λ λ =⎟ ⎠ ⎞ ⎜ ⎝ ⎛ −− 2 cos 1 θ π s uHE H P = ( ) uHEs s uHC PH H P −=−= θπ π1 1uHCsuHE PHP −= θ λ λ =⎟ ⎠ ⎞ ⎜ ⎝ ⎛ −− 1 cos 1 θ π s uHD H P = ( ) uHDs s uHD PH H P −=−= θπ π 1 1uHDsuHD PHP −= uHCsuHE PHP −=1 andthen or then and or Also
- 47. Uplift Pressures atKey Points in Elementary cases Diversion Headworks Principle of Reversibility: • The pressure at key points in pile at u/s case can be obtained from the pressure at key points in pile at d/s case by subtracting from the seepage head provided the length of floor and the depth of pile are same in both cases. • This is based on the principle of reversibility. • According to this principle if the direction of flow is reversed but other conditions remain the same, the flow pattern below the structure does not change
- 48. Uplift Pressures: Corrections DiversionHeadworks In actual structure the floor has some thickness and it may not be horizontal and also there may be more than one line of piles, so the following corrections have to be applied to the superposed values of the uplift pressures 1. correction for thickness of floor 2. correction for mutual interference of piles 3. correction for slope of the floor
- 49. Corrections for thicknessof floor Diversion Headworks t d PP PP uHDuHE uHEuHE − −=1 t d PP PP uHCuHD uHCuHC − +=1 Simple linear interpolation is adopted for this correction e.g. E d C D t C1E1
- 50. Corrections for MutualInterference of Piles Diversion Headworks Interference is local, only on facing side of interfering pile given by d Q D t P D Q d t P 100' 19 s P H b D b Dd C ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + += b' b' 100' 19 s Q H b D b Dd C ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + −=
- 51. Corrections for MutualInterference of Piles Diversion Headworks The following points may be noted: 1. the correction is positive if the point is u/s to the interfering pile and negative if the point is u/s to the interfering pile 2. both D and d are measured below the level at which the interference is required 3. the effect of interference of a pile is determined only for that face of the pile which is towards the interfering pile 4. the mutual interference does not apply to find the effect of an outer pile on an intermediate pile if the intermediate pile is equal to or less than the outer pile and is at a distance less than the twice the length of the outer pile.
- 52. Corrections for SlopingFloor Diversion Headworks If the floor is not horizontal but sloping, the correction is applicable to key points at beginning and end of sloping floor as given by Q t P bs 100 ' ' ss H C b b C ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ±= b' • C' varies from 2 for slope 8:1 to 11.2 for slope 1:1. • The correction is negative for the upslope and is positive for the downslope. • It may be noted that the correction is applicable only to the key points of the pile lines which lie either at the beginning or at the end of the sloping floor.
- 53. Interpolation: Uplift Pressuresat Other Points Diversion Headworks • The corrected pressures at all key points can be determined through elementary cases, superposition and appropriate correction. • The uplift at any point on the floor between the two key points/pile lines is obtained by linear interpolation of the pressures at key points of these two piles
- 54. Surface Flow Diversion Headworks g V yEf 2 2 1 11+= g V yEf 2 2 2 22 += Lff HEE += 12 ( ) g q yyyy 2 2121 2 =+ ( ) 21 3 12 4 yy yy HL − = Hydraulic jump is formed to dissipate energy on d/s glacis. A suction pressure occurs on the floor due to hydraulic jump. For the determination of the suction pressure, the location of hydraulic jump and the profile of water surface on the u/s and d/s of the point at which the jump is formed are required. The basic relations for a hydraulic jump are The conjugate depths before and after the jump (y1 and y2) are related as
- 55. Surface Flow Diversion Headworks cy y y2 = cy y x 1 = c L y H z = ( ) 2=+ yxxy ( ) xy xy z 4 3 − = ( ) 3/12 gqyc = ( ) 368.0 93556.01 zy += ( ) 24.0 93556.01 zy += cf y x xE ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ += 21 2 1 cf y y yE ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ += 22 2 1 The computation of y1 and y2 for known values of q and HL is very cumbersome and inconvenient. To simply the process Blench, Montague and IS code provided curves. To overcome these curves Swamee’s presented the following method ; ; and then These two equations involve x and y as function of z. Swamee solved these equation varying z and then fitted into explicit equation for y for z < 1 and Once y is known other parameters x, y1, y2, Ef1 and Ef2 are calculated for z > 1 ; and
- 56. 1. The structureshould be designed such that the piping failure does not occur due to subsurface flow. 2. The d/s pile must be provided to reduce the exit gradient and to prevent piping. 3. An impervious floor of adequate length is provided to increase the path of percolation and to reduce the hydraulic gradient and the seepage force. 4. The seepage path is increased by providing piles and impervious floor to reduce the uplift pressure. 5. The thickness of the floor should be sufficient to resist the uplift pressure due to subsurface flow. Design Requirements: Subsurface Flow Diversion Headworks
- 57. 6. The criticalsection is d/s of the weir/crest wall. 7. A suitably graded inverted filter should be provided at the downstream end of the impervious floor to check the migration of soil particles along with water. 8. The filter layer is loaded with concrete blocks. Concrete blocks are also provided at the upstream end. Design Requirements: Subsurface Flow Diversion Headworks
- 58. 1. The piles(or cutoff walls) at the u/s and d/s ends of the impervious floor should be provided upto the maximum scour level to protect the main structure against scour. 2. The launching aprons should be provided at the u/s and d/s ends to provide a cover to the main structure against scour. 3. A device is required at the d/s to dissipate energy. For large drops, hydraulic jump is used to dissipate the energy. 4. Additional thickness of the impervious floor is provided at the point where the hydraulic jump is formed to counterbalance the suction pressure. 5. The floor is constructed as a monolithic structure to develop bending resistance (or beam action) to resist the suction pressure. Design Requirements: Surface Flow Diversion Headworks
- 59. The floor shouldhave appropriate thickness to counteract the uplift pressure acting on it. At selected point let the residual head is h which is the subsoil H.G.L. measured from the top surface of the floor. If h' is the head measured above the bottom surface of the floor, then h’ = h + t where t is the thickness of floor. Floor Thickness Diversion Headworks
- 60. For the determinationof the floor thickness let us consider the force acting on the unit area of the floor so as where Sc = specific gravity of the floor material. For plain concrete floor, the value of Sc usually varies from 2.0 to 2.3 depending upon the type of aggregates used. A value of 2.24 is usually adopted. Generally, a factor of safety of 4/3 is adopted. Thus where PuH = h is uplift pressure head at that point above the top surface of the floor Floor Thickness Diversion Headworks ( )1' −=⇒=+⇒= cccw ShttSthth γγ 13 4 13 4 − = − = c uH c S P S h t
- 61. Design of VerticalDrop Weir Diversion Headworks • A vertical drop weir consists of a masonry/concrete crest wall with its d/s face vertical (or nearly vertical). • In this type of weir, the energy is dissipated by the impact of water, as no hydraulic jump is formed. • On the top of the crest wall, shutters are provided, if necessary. • Vertical drop weirs are usually provided when the flood discharge is not very large. • It is suitable for all types of foundations. • The design is usually done by Bligh's theory. • Finally, the thickness and length of floor is checked by Khosla's theory.
- 62. Design of VerticalDrop Weir Diversion Headworks The following data should be collected. 1. Maximum flood discharge (Q) 2. H.F.L. before construction of weir. 3. Average bed level of the river. 4. F.S.L. of off·taking canals. 5. Lacey's silt factor (f) 6. Permissible exit-gradient (GE) 7. Permissible Afflux Lacey's silt factor (f) for the silt at the river site is generally decided by experience. It can also be determined from the average size of the particles as sdf 76.1= where ds is the average size (not radius) of particle (mm).
- 63. Design of VerticalDrop Weir: Afflux Diversion Headworks Afflux is the rise in water level on u/s of structure after the construction of the weir. The high flood level on the u/s is higher than that at the d/s. The area of submergence and the top levels of the marginal banks and guide banks depend upon the afflux. The location of the hydraulic jump on the d/s glacis also depends upon u/s TEL which is a function of afflux. If the afflux is very large, the length of the weir will be small because of high discharge intensity (q) over the crest. However, the cost of the river training works (guide banks, marginal bunds, etc) will increase. The risk of the failure of the structure due to outflanking will also increase. Further, the scour depth will be large and it will increase the cost of protection works on the u/s and d/s of the impervious floor. The afflux is usually limited to 1 m. IS: 6933-1973 recommends an afflux of 1 m for the alluvial rivers in the upper and middle reaches of the river and of 0.3 m in the lower reaches.
- 64. Design of VerticalDrop Weir: Discharge Eq Diversion Headworks • The design flood discharge (Q) will be able to pass over the crest without exceeding the afflux. IS: 6966-1973 recommends the discharge formula: where Cd is the coefficient of discharge which depends upon the type of crest, Le is the effective length, and He is the head over the crest, including the head due to velocity of approach. For sharp crested weir (top width < 2 He/3) Cd = 1.84 and for broad crested weir (top width > 2 He/3) Cd = 1.705. • The effective length Le.is determined as: where L' = clear length excluding total width of piers, N is the number of piers, Kp is the pier contraction coefficient and Ka is the abutment contraction coefficient. Coefficients Kp and Ka depend on the shape of piers and abutment respectively. 2/3 eed HLCQ = eape HKNKLL )(2' +−=
- 65. Design of VerticalDrop Weir: Steps Diversion Headworks There are FOUR broad design steps 1. hydraulic calculation to fix various levels 2. design for weir wall 3. design for impervious floor and piles 4. design for u/s and d/s protection
- 66. Hydraulic calculation tofix various levels Diversion Headworks QtoPL )3.65.4(== ( ) 3/12 35.1 fqR = gVha 22 = 1. Length of waterway (L) = regime perimeter P. From Lacey's regime theory, where Q is the design discharge. 2. Discharge intensity q = Q/L 3. Using Lacey's theory Normal Scour depth where f Lacey's silt factor 4. Determine the regime velocity of flow V = q/R and then the velocity head, 5. d/s TEL = HFL before construction + ha 6. u/s TEL = d/s TEL + afflux
- 67. Hydraulic calculation tofix various levels Diversion Headworks 7. u/s HFL. = u/s TEL - ha 8. Determine the head required over the crest (He) for passing the design intensity q, assuming that the weir acts as a broad or sharp crested weir ( ) ( ) 2/32/3 84.1705.1 ee HqorHq == 9. Crest level = u/s TEL - He (If the crest level is lower, afflux will be less because the head over the crest is increased and consequently, the discharge intensity is also increased. However, a low crest gives rise to an increased depth of water over the crest upto the pond level. It results in the increased height of gates, thickness of floor and the overall cost of the structure).
- 68. Hydraulic calculation tofix various levels Diversion Headworks 10. Pond level = FSL of offtaking canal + Head loss through head regulator (The head loss through the head regulator is usually taken between 0.5 m to 1.0 m, depending upon the type of regulator). The full supply level (FSL) of the canal depends upon a number of factors, such as the water requirements of crops, the topography, the head loss in the canal system and the cumulative fall in the water surface levels from the head to the tail of the canal. 11. Height of shutters (s) = Pond level - Crest level
- 69. Design of WeirWall Diversion Headworks •The top level of the weir wall is kept at the required crest level therefore Height of Weir H = Crest level - Bed level of river bed. •Top width (a) and Base width (B) of weir can be estimated considering elementary profile of a gravity dam. The top width of the weir wall is fixed as the largest of the three values 1)(;; += shuttersofheights S d S d a cc μ where d = water depth above crest = u/s HFL level - Crest level = He - ha ; Sc = specific gravity of weir wall material; μ = friction factor. The top width should be sufficient so that when the shutter is laid over it during floods, it does not project beyond the wall.
- 70. Design of WeirWall Diversion Headworks •Bottom/Base width (B) should be sufficient so that the maximum compressive stresses are within the allowable limits and the tension does not develop. For preliminary design, the base width may be taken as the largest of the three values Ha S dH S dH B cc 8.0;; + ++ = μ The last criteria is based on the slopes on u/s (1:0.3) and d/s (1:0.5) faces. •Stability analysis assuming weir a gravity dam should be performed under different conditions (eg No flow, High flood, Normal water at pond level etc) to determine the developed stresses, those must be within permissible values.
- 71. Design of ImperviousFloor and Piles Diversion Headworks 1. Seepage head is the difference between water levels u/s and d/s of weir due to which seepage takes place. During high flood the head difference is equal to Afflux. During lean period gates are raised and all the river water is diverted into canal so water level is at pond level at u/s and no tail water at d/s and hence seepage head is equal to pond level minus river bed level. Normally the worst condition occurs during lean period. Thus the maximum seepage head (Hs) = Max. of (Afflux, Pond level - river bed level) or Hs = Pond level - river bed level = H + s. 2. Total length of impervious floor LT = C Hs from Bligh’s theory where C = Bligh’s creep coefficient.
- 72. Design of ImperviousFloor and Piles Diversion Headworks 3. Depths of u/s and d/s piles are fixed based upon the maximum scour depth, which is 1.25 R to 1.5 R for u/s pile and 1.5 R to 2 R for d/s pile. Thus d/s Max scour depth = d/s HFL – 2R and u/s Max scour depth = u/s HFL – 1.5R . Therefore d2 = 2R – (d/s HFL - d/s bed level) and d1 = 1.5R – (u/s HFL - u/s bed level). where d1 and d2 are the depths of the u/s and d/s piles below the bed levels, respectively. 4. The length of the horizontal floor (b) = lu + B + ld = LT – 2d1. - 2d2 where lu = length of impervious floor u/s of weir and ld = length of impervious floor d/s of weir. 5. Certain minimum length of impervious floor d/s of weir is always required to dissipate energy and avoid scouring, which is from Bligh’s consideration 1321.2 sd HCl =
- 73. Design of ImperviousFloor and Piles Diversion Headworks 6. Length of u/s impervious floor lu = LT – 2d1 - 2d2 - B - ld if it is excessive provide Intermediate pile of depth d3 >d1 &d2 7. Thickness of u/s floor: The upstream floor is provided with a nominal thickness of about 0.6 m to 1.0 m, as the net uplift force is zero on the u/s floor. 8. The thickness of the d/s floor in ld length is determined by computing uplift pressures at selected points. The section just d/s of weir is critical where maximum thickness has to be provided. In the remaining part the thickness may be provided in suitable number of steps. Bligh's theory may be used to determine uplift pressures at selected points
- 74. Design of ImperviousFloor and Piles Diversion Headworks 9. Uplift pressure head just d/s of weir and just before d/s pile and hence thicknesses at these points are and respectively. The thickness at 3 to 5 points is generally found; depending upon the length of d/s floor. The thickness of floor from the weir wall to d/s end is reduced in steps for ease in construction. 10. Use Khosla’s method to determine uplift pressures and corresponding thicknesses and also exit gradient for tentative dimensions fixed using Bligh’s theory. The final dimensions of the impervious floor and piles must not be unsafe as well over safe. ( ) TsduH LHldP += 21 2 ( ) TsuH LHdP 22 2= ( )1333.1 11 −= cuH SPt ( )1333.1 22 −= cuH SPt
- 75. Design of d/sand u/s Protection Works Diversion Headworks • An inverted filter is provided immediately d/s of d/s impervious floor beyond the d/s pile to relieve the pressure along with filtering out foundation material so that washing out of fine particles does not occur. The filter is properly graded, with the finer layer at the bottom. The total thickness of filter is usually between 50 to 75 cm. The length of the inverted filter is generally kept equal to 1.5 d2 to 2 d2. • To prevent the damage and dislocation of the inverted filter due to surface flow and counteract uplift, it is generally loaded with concrete blocks or block stones of size 90 to 120 cm cube, generally 1 m × 1 m × 1 m. The joints between the concrete blocks are 10 cm thick filled with sand or bajri.
- 76. Diversion Headworks Design ofd/s and u/s Protection Works 22 3/10 dd On the d/s on the inverted filter, a launching apron of length 1.5 d2 to 2.5 d2 is provided. It consists of loosely packed stones. The apron is initially laid horizontal at the river bed level but when scouring occurs, it settles and takes an inclined position. The launching apron protects the impervious floor, d/s pile and inverted filter, as it forms a protective covering of stones over a certain slope below the river bed. It is generally assumed that the aprons launch at a slope of 2:1 to 3:1. The thickness of the apron in the launched position is usually specified as 0.9 m to 1.0 m. The thickness of the apron in the horizontal position can be found from the volume of stone in the launched position. 'For example. for slope of 3: 1 and the launched thickness of 1 m, the thickness in horizontal position =
- 77. Design of d/sand u/s Protection Works Diversion Headworks • U/s protection works Concrete blocks The concrete blocks of thickness 90 to 120 cm are laid over gravel on the upstream of the u/s impervious floor for a length = d1 to 1.5 d1 . • U/s Launching apron The horizontal length of the u/s launching apron is usually kept = 1.5 dl to 2dl the thickness is determined as for d/s launching apron.