Unit 3 spillway
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The document provides an overview of spillways, which are crucial structures for controlling water flow from reservoirs to prevent dam overtopping. It outlines various types of spillways, such as straight drop, overflow, chute, and siphon spillways, along with their design principles and requirements for effective water management. The document also discusses energy dissipation methods and the role of stilling basins and spillway gates in ensuring safe and efficient operation.
![Tail Water Curve (TWC):
• It is a curve representing tail water depth (D) with discharge (q)
• Tail water rating curve The tail water rating curve (TWRC) gives the
relation between the tail water depth y2' (i.e. the actual water
depth in the river on the downstream) as ordinate and the
discharge intensity q as abscissa [Fig. 2 (b)]. The actual tail water
depth corresponding to any discharge intensity q depends upon
the hydraulic characteristics of the river downstream. The values
of y2' corresponding to different values of q are obtained by actual
stream gauging. If there is a suitable control somewhere
downstream of the spillway where the depth of water and
discharge can be accurately measured, the tail water depth y2' at
the spillway can also be determined by backwater computation.
While plotting the tail-water rating curve, an allowance for
channel retrogression, which is likely to occur, must be made.](https://image.slidesharecdn.com/unit-3spillway-210203022246/85/Unit-3-spillway-24-320.jpg)
![TYPES OF STILLING BASINS
• [A] U.S.B.R. Stilling basin
1. Type – 1 Basin
2. Type – 2 Basin
3. Type – 3 Basin
• [B] Indian Standard Basin
1. Horizontal Apron Type -1
2. Horizontal Apron Type -2
3. Sloping Apron Type -3
4. Sloping Apron Type -4](https://image.slidesharecdn.com/unit-3spillway-210203022246/85/Unit-3-spillway-29-320.jpg)
Unit 3 spillway
- 2. INTRODUCTION • A passageway through which surplus water escapes from a reservoir, lake, or the like. • Discharges the flood water to downstream of river without overtopping the dam. • A structure which will not let the water rise above the maximum reservoir level. • A spillway is essentially a safety valve for a dam. Location of Spillway • Within the body of the dam • At one end of dam. • Entirely away from it, independently in a saddle.
- 4. Requirements of Spillway Provide structural stability to dam under all conditions. Able to pass the designed flood without raising the water level above H.F.L. Safe disposal of water without toe erosion. Should have an efficient operation and should be economical. Types of Spillways • Straight drop spillway • Overflow spillway • Chute spillway • Side channel spillway • Shaft spillway • Siphon spillway
- 5. Straight drop Spillway • In this type of spillway, the water freely drops down from the crest. • It is a low weir and simple vertical fall type structures. • The water falls freely from the crest under the action of gravity. • To prevent scouring at the downstream, an auxiliary dam of Artificial pool is to be constructed at the place of fall of water.
- 7. Overflow spillway • It is generally known as Ogee spillway. • It represents the S-shape curve so, it is called ogee spillway. • It is an improved form of straight drop spillway. • It is mainly used in gravity dams. • It has got the advantage over other spillways for its high discharging efficiency.
- 9. Chute spillway • It is often called as trough or open channel spillway. • For earthen and rockfill dams, spillway is to be constructed separately away from the main valley. • Chute Spillway is the simplest type of a spillway which can be easily provided independently and at low costs. • It is lighter and adaptable to any type of foundations.
- 11. Side channel spillway • The flow in this spillway is turned 90 ̊ after passing the crest such that the flow is parallel to the weir crest. • Best suitable for non rigid dams like earthen dams. • It is preferred where space is not available for providing sufficient crest width for chute spillway. • The discharge carrier may be an open channel type or a conduit type.
- 13. Shaft spillway • The water from the reservoir enters into a vertical shaft which conveys this water into a horizontal tunnel which finally discharges the water into the river downstream. • This type of spillway is preferred where the space is not available for providing the above type of spillways If the inlet leg is provided in shape of a funnel, it is called Morning Glory Spillway. • It has maximum discharge even at low heads.
- 15. Siphon Spillway • It works on the principle of siphonic action. • It consists of a siphon pipe whose inlet leg is kept just below the normal pool level and an air vent kept at normal pool level is connected to the crown of siphon. • When the water raises the pool level, siphonic action starts automatically and the water discharges to downstream side. • When the water level falls below the pool level, air is entered through air vent and the discharging of water stops. There are two types of siphon spillways: 1. Saddle siphon spillway 2. Volute siphon spillway 1. Saddle siphon spillway: Saddle siphon spillway can be constructed in many designs but basic concept for all of them is the same. One type of saddle siphons is shown in Figure below. The diagram consists of a U-shaped siphon pipe whose D/S end remains submerged in tail water. An air inlet is provided near the top of the bend.
- 16. Figure 1 Figure 2
- 19. Energy Dissipation Hydraulic Jump • When flood discharge passes over the spillway crest, it has high potential energy which gets converted into kinetic energy as it glides along it. • This high energy has to be dissipated otherwise it would cause erosion at the downstream toe.
- 20. Energy Dissipation • When flood discharge passes over the spillway crest, it has high potential energy which gets converted into kinetic energy as it glides along it • Water flowing over a spillway acquires a lot of kinetic energy because of the conversion of the potential energy into kinetic energy. • If the water flowing with such a velocity is discharged into kinetic energy. • If the water flowing with such a high velocity is discharged into river it will scour the river bed. • If the scouris not properly controlled, it may extend backward and may endanger the spillway and the dam. • In order to protect the channel bed against scour, the kinetic energy of the water should be dissipated before it is discharged into the d/s channel. • For the dissipation of the excessive kinetic energy of water, the following two methods are commonly adopted. 1.By developing a hydraulic jump
- 21. • Hydraulic jump • Hydraulic jump is the sudden rise of water that takes place when the flow changes from supercritical flow state to the subcritical state. • When a stream of water moving with a high velocity and low depth strikes another stream of water moving with low velocity and high depth, a sudden rise in the surface of water place. • This phenomenon is called hydraulic jump. • This is generally accompanied by a large scale turbulence, dissipating most of the kinetic energy of supercritical flow. • Such a phenomenon may occur in a canal below a regulating sluice, at the bottom of the spillway, or at a place where a steep channel slope turns flat.
- 23. JHC vs TWC Jump Height Curve (JHC): • It is a curve representing post jump depth (d2) with discharge (q)
- 24. Tail Water Curve (TWC): • It is a curve representing tail water depth (D) with discharge (q) • Tail water rating curve The tail water rating curve (TWRC) gives the relation between the tail water depth y2' (i.e. the actual water depth in the river on the downstream) as ordinate and the discharge intensity q as abscissa [Fig. 2 (b)]. The actual tail water depth corresponding to any discharge intensity q depends upon the hydraulic characteristics of the river downstream. The values of y2' corresponding to different values of q are obtained by actual stream gauging. If there is a suitable control somewhere downstream of the spillway where the depth of water and discharge can be accurately measured, the tail water depth y2' at the spillway can also be determined by backwater computation. While plotting the tail-water rating curve, an allowance for channel retrogression, which is likely to occur, must be made.
- 25. Refer text book (B.C.PUNMIA) for more explanation REGARDING JHC vs TWC
- 28. STILLING BASINS • Stilling basins are external energy dissipators placed at the outlet of a culvert, chute or rundown. • These basins are characterized by some combination of chute blocks, baffle blocks and sills designed to trigger a hydraulic jump in combination with a required tail water condition. • With the required tail water, velocity leaving a properly designed stilling basin is equal to the velocity in the receiving channel. • While various stilling basin designs ODOT practice is to use the St. Anthony Falls (SAF) stilling basin, which can operate over a range of approach flow Froude numbers from 1.7 to 17
- 29. TYPES OF STILLING BASINS • [A] U.S.B.R. Stilling basin 1. Type – 1 Basin 2. Type – 2 Basin 3. Type – 3 Basin • [B] Indian Standard Basin 1. Horizontal Apron Type -1 2. Horizontal Apron Type -2 3. Sloping Apron Type -3 4. Sloping Apron Type -4
- 31. U.S.S.R. Type -1 Stilling Basin
- 32. U.S.S.R. Type -2 Stilling Basin
- 33. U.S.S.R. Type -3 Stilling Basin
- 37. I.S. Type -3 and Type - 4 Basin
- 38. SPILLWAY GATE Spillway gate, also called stop gates, are adjustable gates used to control water flow in reservoir, river, stream systems. →They also acts as barrier for the storage of additional water. →By these the height of the dam will be more such that land acquisition will be more. → Gates can be provided to all types of spillways
- 39. TYPES OF SPILLWAY GATES 1. DRIPPING SHUTTERS OR PERMENENT FLASH BOARDS 2. STOP LOGS AND NEEDLES 3. RADIAL GATE OR TAINTER GATES 4. DRUM GATES 5. VERTICAL LIFT GATES OR RECTANGLE GATES
- 40. DRIPPING SHUTTERS OR PERMENENT FLASH BOARDS • Consists of wooden panels of usually 1.0-1.25m HIGH • Hinged at bottom and are supported against water pressure by structs. The shutters falls flat on the crest when the downstream supporting structs are tripped. Hence they are not suitable for curved crests. • These shutters can be raised or lowered from an over head cable way or a bridge. • Various automatic gates which drop themselves have been designed these days.
- 41. STOP LOGS • They consists of wooden beams or planks placed one upon the other and spanning in the grooves b/w the spillway piers . • →they can be romed either by hand or with hoisting mechanism. It takes lot of time for removing them, if they become jammed in slots. • →Leakage bw logs is a big problem hence they are used in minor works.
- 42. NEEDLES • Needles are wooden logs kept side by side with their lower ends resting in a keyway on the spillway and upper ends supported by a bridge. • It is difficult to handle these at the time of flow hence these are not used on any major works. • They are sometimes used for emergency bulk heads, where they need not be replaced until the flow has stopped.
- 44. RADIAL GATE OR TAINTER GATES • Radial gates are rotary gates consisting of cylindrical sections. • They may rotate vertically or horizontally. Tainter gates are a vertical design that rotates up to allow water to pass underneath. • Low friction trunnion bearings, along with a face shape that balances hydrostatic forces, allow this design to close under its own weight as a safety feature.
- 46. Drum Gates • Drum gates are hollow gate sections that float on water. They are pinned to rotate up or down. Water is allowed into or out of the flotation chamber to adjust the dam's crest height. • These are desirable to longer spans in order of 40 or so and medium heights of 10 or so.