5. SPILLWAY It is a structure constructed to pass surplus flood water on the D/S of reservoir and Dam. Essential requirements of a spillway
(i) (ii) (iii) (iv) (v)
The spillway must have sufficient capacity. It must be hydraulically and structurally adequate. It must be so located that it provides safe disposal of water i.e., spillway discharge will not erode or undermine the D/S toe of the dam. The bounding surface of spillway must be erosion resistant to withstand high scouring velocity created by the drop from reservoir surface to tail water. Usually some devices are needed for energy dissipation on the D/S side of spillway.
Spillway Capacity
The required capacity of spillway i.e., maximum outflow rate through spillway must be determined by flood routing knowing (i) Inflow rate v/s time (ii) Reservoir capacity curve (reservoir surface elevation v/s reservoir storage) (iii) Discharge curve (out flow v/s reservoir water surface elevation). However the required capacity of a spillway depends on the following factors: (i) Inflow flood (ii) Available storage capacity (iii) Discharge capacity of outlet works (iv) Whether gated or un-gated spillway (v) Possible damage if a spillway of adequate capa city is not provided. Note that large dam with inhabited area on the D/S side needs large protection. Whereas, Small dams with uninhabited area on its D/S side needs limited protection. Size of Dam Small Intermediate Large
Inflow flood for safety of Dam 100 years of flood Standard Project flood Probable maximum flood (PMF)
However, floods of large or smaller magnitude may be used if the hazard involved in the eventuality of a failure of a particular dam is high or low. A reservoir with larger storage capacity will normally require a smaller out flow rate through the spillway. If the out flows through the spillway are supplemented by release through the outlets then the required capacity of spillway may be reduced. For gated spillway more water is stored. By proper operation of gate higher heads may be developed so that greater out flow through the spillway is possible to pass the flood.
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Types of Spillway
Classification based on the time when the spillways come into operation (a) Main or service spillway (b) Auxiliary spillway (c) Emergency spillway M ain or ser ser vice spil lway
Main spillway is the one which comes into operation and is designed to pass the entire spillway design flood. Au xil iar y spill way
It is provided as a supplement to the main spillway and its crest is so located that it comes into operation only after the floods for which the main spillway is designed is exceeded. Thus it is provided in conjunction with the main spillway. Conditions favorable for the provision of o f auxiliary aux iliary spillway are the th e existence of a saddle or depression d epression along the rim of the reservoir which leads into a natural water way. On the crest of auxiliary spillway instead of a control gate a fuse plug (earth dam or dyke) may be provided, which allows the water surface in the reservoir to rise above the crest of spillway and is so designed That when overtopped it automatically washes out thus releasing excess flood water. Eme Em er ge gency ncy spil l way
It is also provided in addition to main spillway but it comes into operation only during emergency which may arise at any time which may not have been considered in the normal design of main spillway. Some of the situations which may lead to emergency are: (i) an enforced shut down of the outlet works (ii) a malfunctioning of spillway gates. (iii) The necessity of bye passing the regular spillway because of damage or failure of some part of the structure (iv) Further an emergency may also arise if a recurring flood occurs before a previous flood is evacuated by the main spillway and outlet works. Emergency spillways are also provided in the saddles or depressions if available. It is often provided as breaching section which is a portion of the dam with its crest lower than the main dam and is so designed that it is washed out when the water in the reservoir reaches a predetermined elevation. The breaching section is also called fuse plug spillway
Classification according to flow through the spillway (i) controlled or gated spillway (ii) un controlled or un-gated spillway
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Classification based on the prominent features pertaining to the various component of the spillway:
The various components of a spillway are Control structure, Discharge channel Terminal structures – structures – energy energy dissipaters Entrance & outlet channels. With respect to control structures discharge channel etc. the spillway is classified into following types. (i) (ii) (ii) (iii) (iv) (v) (vi)
Free over-fall or straight drop spillway Over flow or Ogee spillway Chute or open channel or trough spillway Side channel spillway Shaft or morning glory spillway Conduit or tunnel spillway Siphon spillway
Free over fall or straight drop spillway A free over fall spillway is the one for which the control structure is low height narrow crested weir having its down face vertical or nearly vertical. The over flowing water may be discharged as in the case of a sharp crested weir. Wa ter flowing over the crest drops as a free jet clearly away from the down stream face of the spillway. Occasionally the crest of this spillway is extended in the form of an overhanging lip to direct small discharges away from the D/S face. Over flow or Ogee spillway Overflowing water in this case is guided smoothly over the crest of the spillway and is made to glide over the down do wn stream face of the spillway. The profile of the ogee weir is generally confined to the lower nappe that would be obtained for maximum head over spillway. The control structure is a weir which is ogee or S shaped. The shape of such a profile depends upon the Head The inclination of U/S face of the overflow section and The height of the overflow section above the floor of the entrance channel.
The ogee profile should provide maximum possible hydraulic efficiency, structural stability and economy and also avoid the formation of objectionable sub-atmospheric pressure at the crest. The ogee profile may be categorized into three groups:
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(i) Overflow dams with vertical U/S face
The following equation as given by U.S. corps of engineers may be used for finding coordinates (X,Y) for the D/S profile 1.85
0.85*Y
X = 2 (Hd) Where X & Y are coordinates as shown in the figure and Hd is the design head. For U/S profile following coordinates with origin at crest are recommended
(ii)
Over flow dams with sloping U/S face
The D/S profile may conform to the following eq uation: n
n-1
X = K (Hd)
Y
Where: n., K are variable parameters which depends on the inclination of U/S face of the dam. Hd is the design head. These are called standard “WES “WES standard spillway shapes” Values of constant K,n , R 1,R 2, a and b Shape of K N R 1/Hd R 2/Hd a/Hd B/Hd U/S face Vertical 2.000 1.850 0.5 0.20 0.175 0.282 3V: 1H 1.936 1.836 0.68 0.21 0.139 0.237 3V: 2 H 1.939 1.810 0.48 0.22 0.115 0.240 3V: 3H 1.873 1.776 0.45 0.00 0.119 0.000
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The curved profile of the crest section is continued till it meets tangentially the straight sloping portion of the overflow dam section (spillway).The slope of the d/s face of the overflow dam usually varies in the range of 0.7(H):1(V) to 0.8:1 and is basically decided on the basis of stability requirements. The location of the point of tangent depends upon the slope of the d/s face, where the value of dy/dx for the curved profile and the straight segment must be equal at the end of the sloping surface of the spillway. U/S profile of the Weir Crest (a) Vertical U/S face: The u/s profile should be tangential to the vertical face and should have zero slope at the crest axis to ensure that there is no discontinuity along the surface of the flow. The u/s profile should conform to the following equation: y = {0.724 (x+ 0.270 Hd)
/(Hd )0.85 }+ 0.126 Hd - 0.4315(Hd)0.375(x + 0.270 H d)0.625
1.85
Alternatively the u/s slope may be considered to consist of compound circular curve with values of R 1= 0.50 Hd and R 2 = 0.20 Hd The chord length up to R 1, 1, and R 2 is given as a = 0.175 Hd and b = 0.282 Hd Discharge Computation of over ogee crest
The discharge over ogee spillway is computed from the basic equation of weir given below: 1.5 Q = Cd Le H Where, Q = Discharge in cumec Cd = coefficient of discharge, its value varies from 2.1 to 2.5 H = Head of over flow in meters including velocity of approach head. H = Hd + Ha Le = effective length of overflow crest crest in meters Le = L -2(N*k p+k a)*H L = Net length of the crest N = Number of piers k p = piers contraction coefficient k a = abutment contraction coefficient Pier Contraction coefficient k p Pier condition k p Square nosed piers with corners Rounded on a radius equal to 0.1 of pier thickness 0.02 Rounded nose piers
0.01
Pointed nose piers
0.0
Abutment Contraction coefficient k a Abutment condition k a Rounded abutment where R>0.5Hd o & wall is placed not more than 45 to direction of flow 0.0 Rounded abutment with head wall at 0 90 to the direction of flow when 0.5Hd > R > 0.15Hd 0.10 o Square abutment with head wall at 90 to the direction of flow 0.2
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The discharge coefficient is influenced by a number of factors: (i) Height of spillway above stream bed or depth of approach (ii) relation of the actual crest shape to the ideal nappe shape (iii) upstream face slope (iv) downstream apron interference, and (v) down stream submergence (vi) Ratio of actual total head to the design head (vii) effect of head due to velocity of approach The effect of these factors is discussed below:
H eigh t of spil spil lway or Ef fec fectt depth depth of approach
The height of spillway above stream bed or approach channel affects the velocity of approach which in turn affects the coefficient of discharge. With increase in height of spillway the velocity of approach decreases and the coefficient of discharge increases. Model tests indicate that the coefficient of discharge becomes fairly constant when height of spillway > 3.0 Hd, where Hd is the design head including the head due to velocity of approach. Various text books give a plot of Cd versus Hd which is reproduced here in the form of a table P/Hd Cd P/Hd Cd P/Hd Cd 0.0 1.7 0.1 1.875 0.2 1.97 0.3 2.025 0.4 2.06 0.5 2.09 0.6 2.12 0.7 2.135 0.8 2.15 0.9 2.16 1.0 2.17 1.5 2.185 2.0 2.195 2.5 2.2 3.0 2.205 4.0 2.210 In the table P is the height hei ght of spillway. When u/s face is inclined the above Cd values gets multiplied by a factor from 0.995 (for P/ Hd = 1.5) to 1.04 (for (for P/ Hd = 0.2)
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Negative Pressure over spillway surface H > Hd the pressure on the crest will be negative (sub atmospheric) may lead to cavitation. Model tests show that till H < 1.25 Hd there is no harmful cavitation effect. H < Hd the pressure on the crest is positive. Lower nappe of the jet attaches itself to the boundary. Also when H > Hd lower nappe may leave the boundary for some distance on d/s face and may attach again as the air mass in between the boundary and the lower nappe gets sucked out. This may lead to the vibration of the structure.
Avoid negative pressure as: (i) It increases overturning moment at the crest (ii) It increases the force required for lifting of the gates (iii) It causes decrease in the capability for automatic control (iv) It causes vibration which extends all over the structure. The vibration also causes cracks in the mortar of stone lining of the crest.
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Problem: Design ogee spillway for the following data: (i) Height of the spillway crest = 100.0 m (ii) Number of spans = 10 (iii) Length of each span (clear) = 12.5 (iv) Thickness of each pier = 3 m (v) D/s slope of spillway = 0.8(H): 1(V) (vi) Rock condition = good 3 (vii) Design discharge = 8500 m /s Design (i) Head over crest & coefficient of discharge (a) Clear Water Way = 10x12.5 = 125 Let us assume Cd =2.21 maximum value (b) Effective length of spillway = Le = L - 2[N K p+ K a]Hd o Cut water (90 ) nosed piers are supposed to be used with rounded abutments. 3/2 K p = 0.01 and K a = 0.1 and hence Q= 2.21[125 - 2(9x0.01 + 0.1) Hd] Hd (c) By trial and error Head over the crest Hd =10.024 (d) P/ Hd = 100/ 10.024 = 9.976 > 4.0 Hence Cd =2.21
(e) D/S profile 0.85 1.85 0.85 = 2 (Hd) Y = 2(10.024) Y = 14.188 Y The profile recommended is X 1.85 or, Y = X /14.188 The calculated coordinates of d/s profile are Xm 1.0 2.0 3.0 4.0 5.0
Ym 0.0705 0.2540 0.5380 0.9159 1.3841
Xm 7.0 9.0 11.0 13.0 14.28
Ym 2.5793 4.1061 5.9519 8.1072 9.6455
The slope of d/s glacis = 0.8(H):1(V) Differential equation of d/s profile dy/dx = 1/0.8 = 1.25 1.85 0.85 d/dx [ X /14.188] = (1.85/14.188) X = 1.25 so, X= 14.28 and Y = 9.6455 (f) U/S profile R 1 = 0.5 Hd = 0.5x 10.024= 5.012 and R 2= 0.2 Hd = 0.2x10.024= 2.00048 b= 0.282 Hd = 2.827 a = 0.175 Hd = 1.754 Alternatively calculate the profile from the given equation. Equation for u/s profile 0.85 0.375 0.27 Hd = 2.706 Hd = 7.09 Hd = 2.373 0.375 0.126 Hd = 1.264 0.4315 Hd = 1.024 1.85 0.625 y = [{0.725 (x+2.706 ) }/ 7.09] + 1.264 - 1.024 (x + 2.706)
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Chute Spillway or Trough Spillway An ogee spillway is mostly suitable for concrete Gravity dam when the spillway is located within the body of dam. For Earth & Rock-fill dam, a separate spillway is generally constructed in a flank or saddle, away from main valley. Some times even for gravity dams a separate spillway is required because of the narrowness of the valley. In such circumstances a separate spillway may have to be provided. The trough spillway or chute spillway is the simplest type of spillway which can be easily provided independently and at low costs. It is lighter & adoptable to any type of foundation and hence provided easily on Earth & Rock-Fill dam. It is also called at times Waste Weir. If it is constructed in continuation of the dam at one end, it may be called a Flank weir. If it is constructed in a natural saddle in the bank of the river separated from the main dam by a high ridge it is called a Saddle Weir.
A chute spillway essentially consists of a steeply sloping open channel placed along a dam abutment or through a flank or saddle. It leads the water from the reservoir to the downstream channel below the base of the channel is usually made of reinforced concrete slabs 25 to 50 cm thick. Light reinforcement of about 0.25% of concrete area is provided in the top of the slab in both directions. The chute is some times of constant width but is usually narrowed for economy and then widened near the end to reduce the discharging velocity. Expansion joints are usually provided in the chutes at intervals of about 9to 12m in either direction. The expansion joint s should be made water tight so as to avoid any under seepage and its troublesome effects. Under drains are also provided, so as to drain the water which may seep through the trough bottom and side walls. These drains may be in the form of a perforated steel pipes, clay tiles or rock filled trenches. Slope of chute can conform to available topography leading to minimum excavation, but the slope should be steep enough to maintain supercritical flow to avoid unstable flow
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conditions. When a vertical curve is provided at a point where chute slope changes it must be gradual & designed to avoid any separation of flow. Side Channel Spillway In a side channel spillway, the crest of the control weir is placed along the side of the discharge channel. The crest is approximately parallel to the side channel at the entrance. Thus the flow after passing over the crest is carried in a discharge channel running parallel to the crest. Water flows over the crest into the narrow trough of the discharge channel opposite the weir; it turns approximately at right angle and then continues in the discharge channel. The side channel spillway is usually constructed in a narrow canyon where sufficient space is not available for an overflow spillway, and where there is neither a suitable saddle, nor there is a availability of a wide flanks to accommodate a chute spillway. In such cases, if a crest of length required for the flow to occur perpendicular to the crest is provided, heavy cutting would be required making the cost of constructing a chute spillway or an overflow spillway prohibitive. The crest of a side channel spillway is usually an ogee- shaped section made of concrete. Sometimes it consists of a flat concrete pavement laid on an earthen embankment or the natural ground surface.
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Saddle Siphon Spillway
The saddle Siphon spillway consists of an air tight reinforced concrete cover called “Hood” or “cowl” provided over an an ogee shaped body wall to form a siphon duct. duct. Hence this spillway is also known as „Hood siphon spillway‟ or „Hood siphon‟.The top o f the body wall forms the crest of the siphon spillway and hence it is located at full Reservoir level.The entrance and the exit lips of the hood are so shaped that siphon duct has bell mouthed entry and exit. The inlet of the siphon duct is kept submerged well below the full reservoir level so that floating debris etc., does not enter the siphon duct and also the formation of vortices and draw downs which might break the siphonic action is avoided. Thr outlet of the siphon duct is kept submerged in a cup like basin which forms a water seal so that air cannot enter the siphon duct from this end. However, instead a cup like basin a cistern may be created by constructing a low weir a little away from the outlet on the down stream of the outlet. A small de-primer hood is provided above the main hood and both these hoods are connected through an air vent. The inlet of the de-primer hood is kept slightly above the full reservoir level
At full reservoir level water stands up to the crest of the spillway and hence there is no flow. When the water level rises above the full reservoir level i.e., above the crest of spillway, water starts flowing over the crest and the inlet of the de-primer hood gets submerged with the result that entry for air into the de-primer hood and the main hood gets sealed. Thus air cannot enter from both the inlet and outlet of the siphon duct and the air remaining entrapped in the top portion of the siphon duct above the sheet of the water flowing over the crest is gradually sucked by the flowing water. As the air is sucked the pressure drops to less than atmospheric in the top portion the siphon duct which was having atmospheric pressure at the starting of the flow. Thus a difference of pressure develops between the outside atmosphere and the air inside the siphon duct which creates a suction pull and draws in more water over the crest. This in turn takes in more air along with it which further increases the suction pull and draws in more and more water over the crest. This sucking action which increases progressively and gradually creates the necessary pull for the commencement of the syphonic action and the siphon duct start to run full. The action of the siphon spillway from the moment the water just starts to flow
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over the crest to the instant when the siphon duct starts flowing full is known as „PRIMING‟, „PRIMING‟, (or it is said to have „primed‟). It may however difficult to expel the entrapped air from the siphon since it has to be forced through the water in the sealing basin and hence it may not get primed quickly. Moreover relatively large rise in water level above the crest of spillway may be needed before it gets primed. To assist quick priming many priming devices can easily be fitted along with the siphon.
The siphoning action once initiated will continue as long as water level in the reservoir is above the full reservoir level. The operating head for the spillway is then equal to the difference between water levels on the U/S and the D/S of the spillway. When the water level in the reservoir drops to such a level that the inlet of the de-primer hood gets
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exposed then the air enters the siphon duct and breaks the siphoning action thus stopping the flow. This action is called de-priming of the siphon spillway. Another form of saddle siphon spillway provided in the body of Dam is called „Tilted Outlet type‟. It has a vertical lower limb and hence the draught of falling water is enough to cause priming and no other device is needed for its priming. However the outlet is tilted upwards in order to develop water seal at the bend and thus facilitate early priming. For De-priming air inlet pipe is provided. Advantage The main advantages of the siphon spillway are (i) Its automatic operation without any mechanical device ,and Its ability to pass higher discharge at relatively low surcharge head resulting in (ii) lower height of dam as well as less surrounding area to be acquired for submergence. Disadvantage (i) It is an expensive structure and has limited capacity. (ii) It has a serious disadvantage due to occurrence of sudden surges and stoppages of out flow as a result of erratic siphonic action, thus causing severe fluctuations in the down stream river stage. (iii) A minor crack in the cover of the siphon would interfere with the siphon Therefore the siphon spillway is always constructed in batteries so that entire spillway is not affected even if cracks have developed either in one or few units. (iv) The structrure and foundation have to be strong enough to resist the vibration. (v) There exists a possibility of clogging of the siphon due to debris and floating material. (vi) Like other type of closed conduit spillways, a siphon spillway too is incapable of handling flows appreciably greater than the designed capacity. Hence, siphon spillways must be provided in conjunction with an auxiliary or emergency spillway. Shaft Spill Way In shaft spillway water enters a horizontal crest, drops through a vertical or a sloping shaft and then flows to the down stream river channel through a horizontal or nearly horizontal conduit or tunnel. A rock out crop projecting into the reservoir slightly upstream of the dam would be b e an ideal site for a shaft spillway. Depending on the level of out crop and the required crest level, a spillway may have to be either constructed or excavated. The diversion tunnels if available can be utilized for discharge tunnels of spillway. Radial piers provided on the spillway crest ensure radial flow towards spillway and also provides support to the bridge connecting spillway and the dam or the adjoining hill.
A shaft spillway with a funnel shaped inlet is called ' Morning Glory ' or Glory Hole spillway. One of the distinguishing characteristics is that near maximum capacity of the spillway is attained at relatively low heads. Therefore a shaft spillway is ideal when maximum spillway discharge is not likely to be exceeded. Because of this feature,
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however, the spillway becomes unsuitable when a flow larger than the selected inflow design flood occurs. This disadvantage can be got rid of by providing an auxiliary or emergency spillway and using the shaft spillway as service spillway. Depending upon the type of crest, the shaft spillway can either be standard crested or flat crested. In a standard crested spillway, the water begins its free fall immediately upon leaving the crest where as in flat crested spillway water approaches upon the crest on a flat slope before beginning its free fall. The standard crested spillway would have a smaller diameter crest since its coefficient of discharge is greater than a flat crest. Therefore if the shaft spillway is to be constructed in the form of a tower, it would be economical to have a standard crested spillway. However a flat crested shaft spillway has a smaller funnel diameter and is therefore more advantageous when the spillway is to be excavated in rock.
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