1. SPILLWAYS Prepared By :- Shubham Modi (13BCL054) Pintu Nakrani (13BCL055) Grishma Parmar (13BCL057) 1 2/19/2017

2. INDEX  Introduction  Location of spillway  Design considerations for main spillway  Types of spillway 2 2/19/2017

3. INTRODUCTION  A spillway is a structure constructed at a dam site, for effectively disposing of the surplus water from upstream to downstream.  Every reservoir has a certain capacity to store water. If the reservoir is full and flood water enter the same, the reservoir level will go up and may eventually result in over-topping of the dam. To avoid this situation, the flood has to be passed to the downstream and this is done by providing a spillway which draws water from the top of the reservoir.  Spillways release floods so that the water does not overtop and damage or even destroy the dam. Except during flood periods, water does not normally flow over a spillway. 3 2/19/2017

4.  The flood water discharging through the spillway has to flow down from a higher elevation at the reservoir surface level to a lower elevation at the natural river level on the downstream through a passage, which is also considered as a part of the spillway.  At the bottom of the channel, where the water rushes out to meet the natural river, is usually provided with an energy dissipation device that kills most of the energy of the flowing water. These devices, commonly called as Energy Dissipators, are required to prevent the river surface from getting dangerously scoured by the impact of the outfalling water. In some cases, the water from the spillway may be allowed to drop over a free overfall. 4 2/19/2017

5. LOCATION OF SPILLWAY  Spillway can be located either within the body of dam, or at one end of it, or entirely away from it, independently in a saddle.  Main gorge (main valley)  Saddle  Hillock o level of hillock 5 2/19/2017

6.  Emergency spillway or Subsidiary spillway :- o Sometimes, a concrete or a masonry dam along with its spillway can be constructed in the main valley, while the flank or flanks are closed by earthen material or embankments. o Top level o Material & design o In excessive flood, provides extra outlet. o Prevents failure of dam. 6 2/19/2017

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8. DESIGN CONSIDERATIONS FOR MAIN SPILLWAY  Spillway should be properly designed so as to dispose of the excess water without causing damage to dam or appurtenant structures.  The required discharging capacity of the spillway should be estimated as closely as possible. o Underestimation o Overestimation  On large dams conservative view is preferred.  Failure of emergency spillway. 8 2/19/2017

9. TYPES OF SPILLWAY  There are two main types of spillways: 1.Controlled spillway 2.Uncontrolled spillway  These types are based upon the availability of gates on the spillway. 9 2/19/2017

10. CONTROLLED SPILLWAY  A controlled spillway has mechanical structures or gates to regulate the rate of flow.  This design allows nearly the full height of the dam to be used for water storage year-round, and flood waters can be released as required by opening one or more gates. 10 2/19/2017

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12. UNCONTROLLED SPILLWAY  An uncontrolled spillway, in contrast, does not have gates; when the water rises above the lip or crest of the spillway it begins to be released from the reservoir.  The rate of discharge is controlled only by the depth of water above the reservoir's spillway. Storage volume in the reservoir above the spillway crest can only be used for the temporary storage of floodwater; it cannot be used as water supply storage because it is normally empty. 12 2/19/2017

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14. 14 TYPES OF SPILLWAY ON THE BASIS OF SHAPE 1.Straight drop spillway 2.Overflow spillway 3.Trough spillway or Open channel spillway 4.Side channel spillway 5.Shaft spillway 6.Syphon spillway 2/19/2017

15. STRAIGHT DROP SPILLWAY OR OVERFALL SPILLWAY  This is the simplest type of spillway and may be constructed on small bunds or thin arch dam.  It is a low weir and simple vertical fall type structure.  The downstream face of the structure may be kept vertical or slightly inclined  The crest is sometimes extended in the form of overhanging lip, which keeps the small discharge away from the overfall section.  Since vacuum gets created in the underside portion of the falling jet,sufficient ventilation of nappe is required. 15 2/19/2017

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17. OGEE SPILLWAY OR OVERFLOW SPILLWAY  Ogee spillway is an improvement upon the ‘free overfall spillway’.  Such a spillway can be easily used on valleys where the width of the river is sufficient to provide the required crest length and river bed below can be protected from scour at moderate costs.  The profile of this spillwayis made in accordance with shape of the lower nappe of a free falling jet, over a duely ventilated sharp crested weir. 17 2/19/2017

18.  Normally, the upstream face of the spillway is kept vertical and the crest shape confirms to the lower nappe of a vertical sharp crested weir under maximum head.  If the upstream face of the spillway is kept sloping, the crest shape should also confirm to the lower nappe that would be obtained for an inclined sharp crested weir. 18 2/19/2017

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20. TROUGH SPILLWAY  Sometimes, gravity dams, a separate spillway is required because of the narrowness of the main valley.  The trough spillway is the simplest spillway which can be easily provided independently and at low costs.  It is lighter and adaptable to any type of foundations ; and hence provided easily on earth and rockfill dams. 20 2/19/2017

21.  A trough spillway is sometimes known as a waste weir.  If it is constructed in continuation of dam at one end, it may be called a flank weir.  If it is constructed in a natural saddle in a bank of a river separated from the main dam by a high ridge, it is called a saddle weir.  It leads the water from the reservoir to the downstream channel below.  The reinforced concrete slabs, 25-50 cm thick. 21 2/19/2017

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23. SIDE CHANNEL SPILLWAY  In this flow of water after spilling over the crest, is turned by 90˚ such that it flows parallel to the weir crest (AB).  This type of spillway is provided in narrow valleys where no side flanks of sufficient width to accommodate a trough spillway are available.  If a crest length equal to AB is provided along AC, heavy cutting shall be required.  It required for diverting the flow. 23 2/19/2017

24.  The slope of the side channel should be sufficient to overcome the frictional losses as well as to provide acceleration in the direction of flow against the mass of incoming water. 24 2/19/2017

25. SHAFT SPILLWAY  In a 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.  Sometimes, a vertical shaft may be excavated through some rocky natural island or rocky spur existing on the u/s of the river near the dam.  For small heights, the shafts may be constructed entirely of metal or concrete or clay tiles. 25 2/19/2017

26.  For large heights, RCC may be used.  The horizontal tunnel or the conduit may be taken either through body of the dam or below the foundations.  The diversion tunnels constructed for diversion of the river, may sometimes be planned and used for shaft spillways.  Preventions  The entry of debris and other materials floating in the reservoir must be prevented from entering the shaft, otherwise they will clog the shaft or the conduit or the at junction point.  Properly designed or tested trash racks may be provided for this purpose. 26 2/19/2017

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28. CAVITATION  The crest of the ogee spillway can be made to confirm only to one particular nappe that would be obtaioned at one particular head. This head is called the designed head and represented by, Hd  If the poerating head on the spillway is more then the designed head, the lower nappe of the falling jet may leave the ogee profile, thereby generating negtive pressure at the point of separation.  The generation of negative pressure may lead to formation of bubbles or cavies in the water. 28 2/19/2017

29. SIPHON SPILLWAY  A siphon spillway essentially consists of a siphon pipe, one end of which is kept on the upstream side and is in contact with the reservoir, while the other end discharges water on the downstream side  There are two types of siphon installations which are as below : 1.Tilted outlet type of siphon spillway 2.Hooded types of a siphon spillway 29 2/19/2017

30. TILTED OUTLET TYPE SIPHON SPILLWAY  When the vally is very narrow and no space is available for constructing a saperate spillway, the siphone pipes can be installed within the dam body.  An air vent may be connected with the siphon pipe.  The level of vent may be kept at normal pool level, while entry point of the siphone pipe may be kept at normal pool level, while the entry point of siphon pipe may be kept still lower so as to prevent the entry of debris, etc in the siphone.  The outlet of the siphon may be submerged so as to prevent the entry in the siphon from its downstream end. 30 2/19/2017

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32.  When the water in the reservoir is upto or below the normal pool level, air enters the siphone through the vent and siphonic action cannot take place.  When once the water level in the reservoir goes above the normal pool level, and if once the siphon is filled with water; the water will start flowing through the siphon by siphonic action.  The outflow will continue till the water level in the reservoir falls back to normal pool level.  As soon as, it happens, the air will enter the siphon through the new exposed air vent and the flow will stop. 32 2/19/2017

33.  Sometimes, it msy be possible to keep the outlet submerged in tail water because of limitations of negative pressure at the crown.  In such case, the outlet can be left open above the water level, but air entry through the outlet will have to be controlled by priming devices. 33 2/19/2017

34. HOODED TYPE OF A SYPHONE SPILLWAY  The construction of a hooded type of siphon spillway is more commonly adopted.  In the cases, a reinforced concrete hood is constructed over an ordinary overflow section of a gravity dam.  The inlet of this hood is kept submerged so as to prevent the entry of debris, ice, etc.  A small deprimer hood is kept above the main hood and both these hoods are connected by an air vent.  The inlet of the deprimer hood is kept at normal pool level. 34 2/19/2017

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36.  The principle of functioning of both types of installations is essentially the same, except for the initial filling up of the siphone or ‘priming’.  At normal pool level, the water stands up to the crest of the spillway. If now a flood enters the reservoir, the level would start rising and a sheet of water would start flowing over the spillway crest.  Since the water level is above the deprimer hood, the air entry at the inlet is sealled. The air entry at the inlet is sealed by tail water, etc.  Hence, the water spilling over the crest, sucks all the remaining air from the hood within minuites.  Siphonic action gets established after the air in the bend over the crest have been completely exhausted.  A photoview of this type of siphon spillway, called Sarala Sagar spillway, flanked by earth dam constructed on china vagu stream. 36 2/19/2017

37. PRIMING DEVICES  When the outlet cannot be sealed by tail water, some other devices, called priming devices,are used which lead to an automatic priming of the siphon at a certain rise of water level above the crest.  The maximum rise of water level is called priming depth.  In the figure, the priming is accomplished by means of a step calld joggle which deflects the sheet of water to strike against the lower end of the cover or hood, thus sealing the lower end from the atmoshphere.  When the water level reaches slightly above the crest, the baby siphon, which is an additional siphon, starts running full. 37 2/19/2017

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39. NEGATIVE PRESSURE  As soon as the siphon is primed, a vacuum is formed at the throat. The nehative pressure developed should be limited to such a magnitude that absolute pressure of water does not exceed the vapour pressure of water at the tempreture.  This is necessary to avoid cavitation and its ill effects. Hence, on an average, a maximum negative head equal to 7.5m or so can be allowed.  In other word, the vertical distance from crown of the siphon down to the discharging point should not exceed a value of 7.5m or so, under average conditions. 39 2/19/2017

40.  Both type of siphone describe in privious slides are known as Saddle Siphon Spillways.  A special type of siphon spillway caled Volute Spillway has been designed in india by ganesh iyar is shown in figure. 40 2/19/2017

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42. REFERENCES  Irrigation Engineering And Hydraulic Structures by S. K. Garg  https://en.wikipedia.org/wiki/Spillway https://en.wikipedia.org/wiki/Spillway  www.slideshare.net www.slideshare.net 42 2/19/2017

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