Spillways and Flood Control Works


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Spillways and Flood Control Works

  1. 1. Spillways and Flood Control Works Unit-V BTCI09007
  2. 2. Syllabus • Factors affecting design of spillway, types of spillways, design principles of ogee spillway, trough spillway, siphon spillway and shaft spillway. Design of bucket type energy dissipater and stilling basin, flood mitigation reservoirs. Crest gates, types, advantages, choice, design of radial gate. Outlet works through dams, intake structures
  3. 3. Spillway • The various discharging facilities provided in a storage dams are spillway, intake and outlet works. Spillway is the escape provided in conjunction with a dam to dispose of surplus flood water from the reservoir. The surplus water is drawn from the reservoir and conveyed back to the river downstream or some natural drainage. • The spillway thus acts as the safety valve in the reservoir against failure of dam due to overtopping.
  4. 4. Spillway • Spillway are important auxiliary works of dams, provided to dispose of surplus floodwater safely which cannot be stored in the reservoir. It is necessary to provide a spillway of sufficient capacity so as to avoid water from overtopping the dam. Overtopping of dam may lead to failure of dam resulting in serious damage to the property.
  5. 5. Spillway
  6. 6. Spillway Requirements of a spillway • A Spillway should fulfill the following requirements • The spillway should have sufficient capacity • The location of spillway should provide safe disposal of water without toe erosion. • Spillway should be hydraulically and structure sufficient • Usually spillway should be accomplished by an energy dissipation work on its downstream side.
  7. 7. Spillway
  8. 8. Essentials of A Spillway The essentials of a spillway are • adequate capacity to serve as moderation of floods. • While conveying the excess flows downstream the tail water maintained is such that the purpose and protection of dam and its appurtenant work are fully ensured. • Providing safe & regulated release of the surplus water in excess of the reservoir capacity.
  9. 9. Essentials of A Spillway
  10. 10. Essentials of A Spillway • Hydraulically and structurally the spillway’s bounding surfaces are adequate and are erosion resistant to withstand high scouring velocities created by drop of flow from reservoir surface to tail water, • Device for the dissipation of energy at the bottom of the dam is provided.
  11. 11. Essentials of A Spillway
  12. 12. Essentials of A Spillway • The spillway size and type depends on the best combination of storage and spillway capacity to accommodate the selected inflow design flood which in turn depends on • (i) The character of the flood hydrography. • (ii) Effect of various dams and spillways combination on probable increase or decrease or damage above or below the dam • (iii) Relative cost of increasing spillway capacity • (iv) Use of combined outlet facilities to serve as control of release and control or passage of floods.
  13. 13. Spillway Components • Spillway may be constructed as an integral part of the dam or as independent structure but essentially comprises components discussed as under: Approach Channel • It is an entrance structure to draw water from the reservoir and convey it to the control structure. • The approach channel may be a straight or curved in plan. Its banks may be parallel, convergent, divergent or combination of these and may be vertical or sloping. • It may insure minimum head loss through the channel and to obtain uniformly of flow over the control structure, the entrance velocities are limited and channel curvatures and transitions, if any are gradual.
  14. 14. Approach Channel
  15. 15. Components Parts of a Spillway Control Structure • Control Structure consists of a weir which may be sharp. It is a major component of a spillway. It regulates and controls the surplus water from the reservoir. It does not allow the discharge of water below from reservoir level and allows the discharge of water below from reservoir level and allows only when water surface in the reservoir rises above that level.
  16. 16. Components Parts of a Spillway
  17. 17. Spillway Components Control Structure • It Consists of any overflow crest provided with a bridge and gates to regulate and control the overflow from the reservoir. The control limits the overflows below fixed reservoir levels and also regulated when the reservoir rises above that level. The length of the crest is finalized from economic considerations and hydraulic model studies as for a given level of the crest, the height of dam increases if the crest is made of short length.
  18. 18. Control Structure
  19. 19. Spillway Components Discharge Carrier • It is the waterway provided to convey the flows released from the control structure to the river below the dam, except in the case of arch dam wherein the discharge falls free from the crest or where the flow is released directly along the abutment hill side to cascade down the abutment face. • The structure of the discharge carrier may be the downstream face if a concrete dam, an open excavated channel, a closed cut and cover conduit placed through or under a dam or a tunnel excavated through an abutment. • The profile may be variable flat or steep, cross section may be variable rectangular, trapezoidal, circular or of other shape and the waterway may be wide or narrow, long or short.
  20. 20. Discharge Carrier
  21. 21. Components Parts of a Spillway Discharge Channel • Discharge Channel It is provided to convey the surplus water released through control structures to the stream bed below the dam safely. The discharge channel may be the downstream face of spillway itself, or open channel excavated along the ground surface or a closed conduit placed through or under a dam.
  22. 22. Components Parts of a Spillway
  23. 23. Spillway Components Terminal Structure • At the downstream end of the discharge carrier when flow from reservoir to downstream river level manifests itself in the form of high velocities enough to cause scour, erosion and subsequent damage to adjacent structures, a terminal structure usually in the form of stilling basin is provided for dissipation of residual energy.
  24. 24. Terminal Structure
  25. 25. Components Parts of a Spillway Energy Dissipaters • They are usually provided on the downstream side of the spillway. High Voltage water coming through Spillway may cause serious damage to the toe of dam and to the adjacent structures. This high energy of flow must be dissipated before it flows back to river. For this, energy dissipaters are provided
  26. 26. Components Parts of a Spillway
  27. 27. Components Parts of a Spillway
  28. 28. Energy Dissipaters
  29. 29. Energy Dissipaters
  30. 30. Spillway Components Types & Design of Spillways • The site, shape and type of spillway and the arrangement of its components depend upon the various factors as stated below: Topographical • Site conditions such as steepness of terrain traversed by the spillway control and discharge carrier, type and amount of excavated material, possibility of utilization of the excavated material for the dam embankment, chances of scour of the bounding surfaces and the need for lining, permeability and safe bearing capacity of the foundation, stability of the excavated slopes, and geological site conditions.
  31. 31. Spillway Components
  32. 32. Spillway Components Hydrological • Inflow and reservoir storage conditions such as inflow discharges, its frequency and shape of hydrography, reservoir capacity at various levels, and length and height of crest.. Purpose of Storage • Purpose of storage determines whether the spillway is required to be gated and the type of gates.
  33. 33. Spillway Components Types of Storage • Types of dam governs the spillway design flood. Spillway of certain types are eminently suitable for earth and rock-fill dams, while the other are adapted for concrete dams. Outlet facilities • Possibility of combined outlet facilities to serve more than one function, such as control of release and control or passage of floods.
  34. 34. Spillway Components
  35. 35. Spillway Components Downstream flow conditions • Certain types of spillways greatly alter the shape of flood hydrograph downstream of the spillway. Siphon spillway, for example, gives rise to a wave travelling downstream in the river which is detrimental to navigation & fishing.
  36. 36. Spillway Components Safety Considerations • Spillway of inadequate capacity or improperly designed spillway may cause failure of the dam. Economic • Several study of alternative types of spillway are undertaken to finalize the most economical and hydraulically efficient spillway suited to the discharging requirement and site conditions
  37. 37. Components Parts of a Spillway Entrance and Outlet Channels • They are not required in case of an overflow spillway. However entrance channels are provided to draw water from reservoir and convey it to control structure. Similarly Outlet channels are provided to carry the spillway flow in river channel below the dam. Similarly, outlet channels are provided to carry the spillway flow to river channels below the dam.
  38. 38. Types of Spillways • According to the prominent features related to various components of spillway viz. control Structures, discharge channel etc. The spillways may be classified in the following types. • Free Overfall • Ogee or overflow spillway • Trough Spillway • Shaft Spillway • Side Channel Spillway • Tunnel Spillway • Siphon Spillway.
  39. 39. Types of Spillways Free Over-fall • This is the simplest type of Spillway which consists of a low height narrow crusted Weir having downstream face either vertical or nearly vertical. This type of Spillway is suitable for low earth dam, low concrete masonry gravity dam or low thin arch dam. It is not suitable for high Dam.
  40. 40. Free Over-fall
  41. 41. Types of Spillways Ogee or Overflow Spillway • It is the modified form of drop spillway suitable for high gravity dam, arch dam and buttress dam. The overflow water is guided smoothly over the crest and profile of spillway. This type of spillway is more preferable on valleys where width of river is more to provide sufficient crest length and river bed can be protected from scouring at reasonable cost.
  42. 42. Ogee or Overflow Spillway
  43. 43. Types of Spillways Trough Spillway • Trough spillway Is provided when it is not possible to provide an overflow spillway such as in case of embankment dam or due to erodible nature of stream bed in case of concrete masonry dams. It discharges the surplus flood through a steep sloped open channel. The crest of spillway is kept normal to its centre line. It consists of a discharge channel to the river.
  44. 44. Trough Spillway
  45. 45. Types of Spillways Side Channel Spillway • It is the Spillway in which, the flow after passing over a weir or ogee crest, is carried away by a side channel. It is best suited for non rigid dam like earthen dam.
  46. 46. Side Channel Spillway
  47. 47. Types of Spillways Siphon Spillway: When available space is limited and surplus discharge is not large siphon spillway is often preferred. It is based on siphon action in the shape of an inverted pipe. Usually siphon spillway is provided in concrete gravity dam through its body.
  48. 48. Siphon Spillway
  49. 49. Siphon Spillway
  50. 50. Energy Dissipation Below Spillway • Water flowing over a spillway has high potential energy as it glides along spillway. This large kinetic energy rises to high velocity of flow which may cause large scale scour/erosion at the downstream toe, if proper arrangements are not made to dissipate this high energy. The arrangements provided to dissipate this energy are known as energy dissipaters. • In general the dissipation can be achieved in two ways • By developing a Hydraulic jump • By directing the jet of water using a deflector bucket.
  51. 51. Energy Dissipation Below Spillway
  52. 52. Energy Dissipation Below Spillway • A. Roller Basin B. Deflector Bucket C. Flip Bucket D. Non-radial spillway and sluice buckets E. Schoklitsch Dissipater
  53. 53. Spillway Crust Gates • Gates provided over the crest of a controlled spillway are termed as spillway crest gates. An additional storage can be obtained by providing gates on the spillway crest. However, during floods, these gates are lifted to make full use of spillway capacity. Great care is needed to be taken while operating crest gates of earth dams so as to avoid the overtopping of earth dam.
  54. 54. Spillway Crust Gates
  55. 55. Spillway Crust Gates • The following are some of the common types of crest gates. (i) Dropping shutters or flash boards • They are some sort of temporary gates used only for smaller spillway of minor importance. They consists of wooden panels usually 1.0 to 1.25 m high. They are hinge at bottom and supported by struts to resist water Pressure. These wooden panels can be raised or lowered from an overhead cableway or a bridge.
  56. 56. Dropping shutters or flash boards
  57. 57. Spillway Crust Gates Radial Gates or Tainter Gates • Radial gate has a curve water supporting face which is made of steel. Its shape is just as sector of a circle properly braced and hinged at the pivot. • The gate is thus made to rotate above a horizontal axis. The load of the gate, water etc. is carried on bearing which are mounted on piers. An operating plateform is provided to lift the the gate by means of ropes and chain
  58. 58. Radial Gates or Tainter Gates
  59. 59. Radial Gates or Tainter Gates
  60. 60. Drum Gates • This gate is developed by United States Bureau of Reclamation (USBR) this gates is suitable for longer span of the order of 40 or 50 m. The drum gate consists of circular sector in cross section formed by skin plates attached to internal bearing. The entire section may be raised or lowered such that upper surface coincides with the crest of spillway. The buoyant force due to head water pressure underneath the drum assists in its lifting. The drum Gate needs large recess and hence is not suitable for smaller spillway. Some other types of spillway gates are also used namely Vertical lift gates, rolling Gates, Tilting Gate or Flap Gate, Bear Trap gate.
  61. 61. Drum Gates
  62. 62. Drum Gates
  63. 63. Intake and Outlet Works • The stored water in a reservoir is essentially drawn to meet the intended purpose of irrigation, hydro power generation, public water supply, etc. through the discharge facilities termed as intake structure. An intake structure may take on many forms depending on the functions it is to serve, the range in fluctuations of reservoir water levels, quality of water drawn, discharge to be released, frequency and amount of reservoir drawdown, trash removal requirement, wave action in the reservoir and other such considerations such as sedimentation control devices, etc.
  64. 64. Intake and Outlet Works
  65. 65. Intake and Outlet Works • The general requirement of an Intake Structure are: Structural Stability • The Intake structure is stable to resist water and wave thrust besides wind pressure when reservoir is empty as also against the shock of earthquakes. Hydraulic efficiency • There is smooth entry into the water conductor system to ensure gradual transformation of static head to conductor system to ensure gradual transformation of static head to conduct velocity so as to involve hydraulic losses.
  66. 66. Intake and Outlet Works
  67. 67. Intake and Outlet Works Velocity Limitation • The velocity through trash rack gates and ports is within economic and safe limits. Operational efficiency • The intake and the equipments are such as to prevent/ minimize ice, floating trash and coarse sediment entering the water conductor system to ensure good operational efficiency.
  68. 68. Intake and Outlet Works • The main components of an irrigation intake structure are (i) Trash rack and supporting structure (ii) Anti-Vortex devices, (iii) Bell mouth entrance with transition and rectangular circular opening, and (iv) Gate slot closing devices with air vents.
  69. 69. Trash rack
  70. 70. Anti-Vortex devices
  71. 71. Bell mouth entrance
  72. 72. Gate slot closing devices with air vents.
  73. 73. Intake and Outlet Works Function of Intakes • Intake structure serve to permit withdrawal of water in the reservoir over a predetermined range of reservoir levels to the outlet. • The outlet may release water at a retarded rate than the incoming flow from the intake or release the impounded water at controlled rate constituent with the requirement of downstream such as feeding the power turbines, feeding the irrigation channels, discharging heavily silt laden inflow, depleting the reservoir to facilitate inspection channels, discharging heavily silt laden inflow, depleting the reservoir to facilitate inspection, as flood control regulator to release water temporarily impounded in flood control storage space. The other functions served by an intake are to support necessary auxiliary appurtenances such as trashrack, fish screens and bypass devices, etc,.
  74. 74. Intake and Outlet Works
  75. 75. Fish Ladders
  76. 76. Fish Ladders
  77. 77. Intake and Outlet Works
  78. 78. Intake and Outlet Works Location of Intakes • The various factors influencing the choice of location of intake structure are • (i) Type of storage reservoir • (ii) Location & type of dam/weir. • (iii) Type of water conductor system that is canal or tunnel, • (iv) Topographical features of the river.
  79. 79. Location of Intakes
  80. 80. Intake and Outlet Works Quality Water • Location of intake is required to be such as to draw the best quality of water from the reservoir. Depth of water at intake is important. Quality of water varies at different levels in the reservoir and it is necessary to draw water from different elevations of the reservoir at different seasons of the year for which multi-level intakes are frequently provided.
  81. 81. Intake and Outlet Works
  82. 82. Intake and Outlet Works Optimum water Utilization • Intake is located in the deepest part of the impounding reservoir to enable full utilization of the capacity of the reservoir and to protect intake from sediments in the reservoir. In the reservoir with wide variations in the water level. The intake is better located at the lowest stage so that one inlet is always submerged and operative to draw supply and minimum operating head is always available.
  83. 83. Intake and Outlet Works
  84. 84. Intake and Outlet Works Minimum Silt Entry • In order to prevent silt from the reservoir being carried into the outlet system, location of intake at low points or pockets in the reservoir is avoided. Bottom sediments are kept out of the intake if the lowest entrance ports are sufficiently above the reservoir floor.
  85. 85. Intake and Outlet Works
  86. 86. Intake and Outlet Works Wave action • The Intake structure is placed sufficiently removed from the currents that might threaten the safety of the structure. Waves exert pressure on the superstructure of the intake. Waves may also stir up the bottom sediments if the intake is not so located deep in the water below the water surface. Locations where winds may drift debris into the intake are avoided.
  87. 87. Intake and Outlet Works Ice Pressure • Ice may push down and clog ports. Ice troubles are reduced in case entrance velocities are kept between 7 to 10 cm/sec. low velocities do not transport ice and hold the entry of leaves and debris into the intake as also fish can escape from being drawn into by intake currents. Leaves and debris into the intake as also fish can escape from being drawn into by intake currents.
  88. 88. Intake and Outlet Works Types of Dams • Usual location of intake in an earthen dam is in deep water near the upstream toe of the dam while in concrete or masonry dam, the intake may be in the structure itself. Sewage Pollution • The intake is located to avoid sewage pollution. Locations where stagnant water or algae can accumulate are unsuitable for intake structure. Shore location • Usual location of the intake for public water supply is some distance away from the shore to avoid turbid water from surface wash along the banks.
  89. 89. Intake and Outlet Works
  90. 90. Intake and Outlet Works Types of Intakes • Depending on the function served and the range in reservoir head under which it is to operate, the discharging capacity and the frequency of the reservoir drawdown, intakes for hydroelectric projects may be relatively simple submerged intakes or more elaborate structure raised as a tower above maximum reservoir level. • Broadly the Intakes are classified as • (i) Run-of-river type • (ii) Reservoir type.
  91. 91. Intake and Outlet Works Run-of-River Intakes • In a run-of-River plants, intake is apparent to power house and draws water from the river without any appreciable storage upstream of the diversion structure. Characteristics of river flows., the intake is designed to withstand high peaks and short duration flood flows and high sediment loads. The bell mouth entrance is essentially provided with trash racks.
  92. 92. Run-of-River type
  93. 93. Intake and Outlet Works Canal Intake • It is also a variant of the run-of-river intake, that is provided adjacent to the diversion weir/ barrages to admit water into the canal. It is designed to function under low heads and the topography and geology permits straight reach suitable for it. Sediment excluder is an essential component of the intake. The crest of the intake is generally raised to prevent entry of coarse fraction of bed load into the canal.
  94. 94. Intake and Outlet Works
  95. 95. Canal Intake
  96. 96. Intake and Outlet Works Reservoir Type Intakes • Intake tower classified as Submerged, dry and wet intakes fall in this category. (i) Submerged Intake • An Intake Structure which remains entirely under water during its operation is termed as submerged intake. It is provided where the structure serves only as an entrance to the outlet conduct and where ordinarily cleaning of the trash is not required. The conduct intake may be inclined, vertical or horizontal in accordingly with the intake requirements. . An Inclined Intake may be provided with gates and operated on the upstream slopes of a low dam.
  97. 97. Reservoir Type Intakes
  98. 98. Submerged Intake
  99. 99. Intake and Outlet Works • The submerged Intakes remain entirely under water and possess the advantages • (i) Simple Structure, less costly to Construct, • (ii) No Obstruction to navigation and little obstruction to river flow • (iii) Little danger from floating material and a minimum of trouble from ice. • (iv) Trash rack cleaning is not generally required, • (v) Particularly suitable as public water supply intake from a river.
  100. 100. Intake and Outlet Works Intake tower • An Intake tower is used to draw water from the reservoir in which there are huge fluctuations in water level or quality water is to be drawn at the desirable depth or both. It Consist of an elaborate exposed or tower like structure rising above maximum reservoir level and closely located to the dam body or the bank of the stream so as to be approached by a connecting bridge of minimum span.
  101. 101. Intake and Outlet Works
  102. 102. Intake and Outlet Works • The Intake tower consist of circular concrete structure provided with openings or ports for water entry fitted with trash racks to prevent the entry of debris and ice large enough to injure the equipment and gates that control the flow through intakes into the feeding conduct outlet. • It has a merit that best quality of water available at different depths at different seasons of the year can be drawn through port openings at different elevations.
  103. 103. Intake and Outlet Works
  104. 104. Intake and Outlet Works • There are two types of intakes as under • (i) Dry Intake Tower • In dry intake tower the entry ports are directly connected with the withdrawal conduit and water inside the tower when gates are in a closed position. Dry Intake tower has a merit that the intake tower being dry is made accessible for inspection and operation besides that the water can be withdrawn from any level by opening the port at that level. • However, dry intake tower is massive in structure, than wet intake to withstand additional buoyant forces to which it is subjected when the port gates are closed.
  105. 105. Dry Intake Tower
  106. 106. Dry Intake Tower
  107. 107. Dry Intake Tower
  108. 108. Intake and Outlet Works Wet Intake Tower • A wet intake tower has entry ports at various levels and the vertical shaft is filled with water up to reservoir level. It differs from the dry intake tower is that the water enters from the ports into the tower and then into the withdrawal conduict through separate gated openings. As such it consists of a circular shell made of concrete filled with water up to reservoir level, housing another inside shaft directly connected to the withdrawal conduit. It is less costly to construct and is usually not subjected to flotation and certain other stress may not be the consideration.
  109. 109. Wet Intake Tower
  110. 110. Low Pressure Intake • It is the usual form of intake used for relatively smaller drawdowns as in the case of hydel power plants of smaller units catering for daily and weekly water surface variations. • Low Pressure Intakes usually performs the function of a dam and is designed accordingly based on the theory of stability of a dam. • The maximum velocity through the gross cross sectional area of the track is about 0.75 m/sec which may be of the order of 1.5 m/sec in case of large units and mechanical racks of wider rack bars spacing.
  111. 111. High Pressure Intake High Pressure Intake • High Pressure Intake, in general, is used in a reservoir which serves as a storage reservoir and head water for hydel power generation involving considerable drawdown under normal operation conditions. The Intake is constructed in a variety of forms depending on the type of dams whether of concrete or earthen dam. In concrete dam, high pressure intake varies little in detail from low pressure intake except in the type of gates installed. In earthen or rock-fill dam, the intake is generally a tower built near the foot of the upstream slope of the dam, the structural details of which vary greatly from the low pressure intake.
  112. 112. Trash Racks • Trash rack is defined as a screen or grating provided at the entrance of intake to prevent entry of debris. Trash racks usually consists of trash sections 1.5 to 2 m wide and not too long for handling, made up of mild steel flats on edge 5 to 15 cm. Coarse trash racks are provided near the ports to prevent large drift, such as cakes of ice, roots, trees and timber from being drawn into the intake.
  113. 113. Trash Racks • In some part of the intake fine trash racks are provided to protect the machine & machine parts through which water flows. In cold region, trash racks is often clogged with fragile ice. Electrical heating for small trash racks are provided to prevent ice formation on the racks. • The floating debris accumulated, as are denied entry into the intake, are removed with the help of power driven rack-rakes.
  114. 114. Trash Racks
  115. 115. Outlet Works • Outlet works comprise discharging facilities for conveyance of water from a dam for power generation and irrigation . Outlet work for power generation is termed as penstock and that as a means for releasing water to meet the demand of irrigation water downstream is called irrigation outlet. • The outlets are placed sufficiently below minimum reservoir level to provide necessary head for effective flows. Occasionally the outlet may be placed at a higher level to deliver water to a canal. In a multipurpose reservoirs, the irrigation outlets are operated rarely, the release being usually made through penstock to serve for power generation as well as to meet irrigation demands.
  116. 116. Outlet Works
  117. 117. Classification of Outlet Works • Outlet works are classified according to their purpose, their physical and structural arrangement, or their hydraulic operation, as follows • Classification according to their purpose (i) River Outlet • An outlet work which implies directly into a river (ii) Canal outlet • An outlet which discharges into a canal (iii) Pressure Pipe Outlet • An outlet which delivers water into closed pipe system
  118. 118. River Outlet
  119. 119. Canal outlet
  120. 120. Classification of Outlet Works • Classification according to their physical and structural arrangements. Open Channel • It consists of a conventional open flume or rectangular channel with a gate similar to that of a spillway. Open Channel outlet work is suitable for a low dam from release of water to a canal or to the water.
  121. 121. Open Channel Outlet
  122. 122. Classification of Outlet Works Cut & Cover conduict • Cut and cover conduit outlet is suitable where the water way is too small to justify a minimum size tunnel or where the foundation conditions are not suitable for a tunnel. Tunnel • It is closed conduit waterway. It is suitable for high earth fill dams, where open channel outlets cannot be provided. It may be carried through, under or around the dam as a cut-and –cover conduit or as a tunnel through dam abutment.
  123. 123. Classification of Outlet Works • Classification according to their hydraulic Operation Gated Outlet • It is outlet provided with operating gates and regulating gates to control and regulate flow. The gates may be located at the upstream end of the conduit, at an intermediate point along its length or at the downstream end of the structure. Ungated Outlet • Ungated Outlet flow in the conduit, as in the case of detention dams, is similar to that in a culvert spillway.
  124. 124. Outlet Works
  125. 125. References • Modi P.N. (2011), “Irrigation water resources and water power engineering”, Standard Book House • Garg S.K. (2010), “Irrigation Engineering and Hydraulic Structures”, Khanna Publishers • Internet Websites
  126. 126. Thanks….