33.1 Introduction Spillway is a passage in a dam through which the design flood could be disposed off safely to the downstream. The ogee-crested spillway, because of its superb hydraulic characteristics, has been one of the most studied hydraulic structures. Its ability to pass flows efficiently and safely, when properly designed, with relatively good flow measuring capabilities, has enabled engineers to use it in a wide variety of situations. Although much is understood about the general ogee shape and its flow characteristics, it is also understood that a deviation from the standard design parameters such as a change in upstream flow conditions, slightly modified crest shape, or construction variances can change the flow properties. These small changes often require engineers to evaluate the crest and determine whether or not the change or deviation will be detrimental to the spillway's performance. Such is the case when an updated probable maximum flood calculation requires a spillway to pass a larger flow than it was designed to handle.
In general, spillways comprise five distinct components namely: (i) an entrance channel, (ii) a control structure, (iii) a discharge carrier, (iv) an energy dissipator, and (v) an outlet channel. The entrance channel transfers water from the reservoir to the control structure, which regulates the discharge from the reservoir. Water is then conveyed from reservoir to the low-level energy dissipator on the riverbed by the discharge conveyor. An energy dissipator is required to reduce the high velocity of the flow to a nonscouring magnitude.
Most common types of spillway-control system used are roller, tainter, vertical-lift, and drum gates. In view of the varying conditions, the choice of suitable gate is bound by the cost , the head on the crest, the height of dam, and the hydraulic behaviour of the gate. Piers are located on the spillway crest for the purpose of supporting the control gates, the gate-operating mechanisms or a roadway. Their size and shape will vary accordingly with their function. The piers should be streamlined both in the upstream
and the downstream sides to reduce contraction of the overflowing jet and to provide a smooth water surface.
The element which introduces the energy-reducing action is generally known as " stilling basin." One of the most common methods out of several methods are dissipating the flow at the toe of a spillway, is the hydraulic jump. Other types used in conjunction with spillways are roller and trajectory buckets. Spillway outlets means the combination of structures and equipment required for the safe operation and control of the water released for different purposes for which the dam is planned. These structures may be river outlets, penstocks, canal outlets. The size and number of river outlets satisfy the discharge requirements at various stages of the reservoir . If the outlets are located in the overflow portion, the conduits should be aligned downwards to minimise disturbance to the flow over spillway. The discharge from an outlet, (gates, valves, or free-flow conduits) has a relatively high velocity. Flow must expend the energy in order to prevent scour of the bed and banks of the river channel. This may be accomplished by constructing a stilling basin immediately downstream from the outlet. The crest of the spillway is usually provided at F.R.L (Full Reservoir Level). However, in order to control floods the gates could be provided at the top and the water level could be increased upto maximum water level. The height between F.R.L and M.W.L is called the "Flood lift". Reservoir level should not cross MWL. Following are different types of spillways usually adopted in practice. 1. Overflow spillway. 2. Side channel spillway. 3. Shaft spillway. 4. Siphon spillway. 5. Chute. 6. Breaching section (emerging spillway).
Major dam will be usually provided with an overflow spillway with crest gates. However, the type and location of spillway depends on the site conditions of topography.
33.2 Ogee Type Spillway Profile This type of spillway is the most common type adopted in the field. It divides naturally into three zones. Crest, spillway face and the toe. The concept evolves from replacing the lower nappe of the flow over thin plate weir by solid boundary (Fig. 33.1).
Figure 33.1(a) - Flow over a thin weir
Figure 33.1 (b) - The fluid boundary bottom nappe) has been replaced by solid boundary
The frictional resistance comes into play in case of solid boundary. Hence, the analysis of flow profile is not aminable for analytical solutions. The high head spillways are designed for proposed design head for the given discharge. However, the spillway will also have to operate at lower heads and possibly higher heads as well. The former will result in above atmospheric pressures on the crest and the lower discharge coefficient. The latter behaves exactly in opposite manner namely sub atmospheric pressure, higher discharge coeffiicient. From the experimental investigations by Rouse and Reid it is found that the actual head may exceed the design head by atleast 50% with a 10% increase in the coefficient of discharge subject to local pressures do not fall below the cavitation level.
5.0
0.0 Separation
4.5
q Discharge intensity __ ___________________ = 1.5 Ha Actual Head 1.5
-1.0
p __ ( γ )min ________ Hd
p __ ( γ )min -2.0 ________ Hd
4.0 q __ 1.5 Ha
3.5
3.0
0
-3.0
2
1
3
-4.0 4
Ha Normalised Head (actual head to Design head). i.e., ____ Hd
However, in practice, this pressure reduction is not normally a serious problem unless H > 1.5 Hd. Indeed, separation will not occur until H
→ 3Hd. The acceptable range is 1.5
to 3.0 Hd. If H < Hd - positive gauge pressure on crest. If H > Hd - negative pressure develops on the surface. Conditions in the flow down the spillway face may be quite complex, since (i) the flow is accelerating rapidly, and may be 'expanding' as it leaves a bay-pier arrangement; (ii) frictional shear promotes boundary layer growth. (iii) the phenomenon of self - aeration of the flow may arise; (iv) cavitation may occur (Velocity may reach 30 m/s for occurance of cavitation).
For these reasons, the usual equations for non-uniform flow developed for Gradually Varied Flow cannot really be applied. If it is necessary to make estimates of flow conditions on the spillway, then empirical data must be used. (i) In a region of rapidly accelerating flow, the specific energy equation is usually applied. It is possible to obtain very rough estimates of the variation of V and y down the spillway on this basis, accuracy will be slightly improved if a head loss term is
incorporated. Nevertheless, in the light of (ii) and (iii), below, conditions on the spillway are far from those which underly the energy equations. (ii) A boundary layer will form in the spillway flow, commencing at the leading edge of the crest. The boundary layer thickness, δ , increases with the distance downstream of the crest. The depth of the boundary layer, δ , will meet the free surface of the water (Fig. 33.2).
Ha
Hd = design head Boundary layer P.I (point of inception)
h
point of tangency face m 1
toe
Figure 33.2 - Boundary Layer Growth on Ogee Spillway The flow of the crest is analogous to the flow round any fairly streamlined body. This may imply flow seperation, eddy shedding, or both. Such condtions may be instrumental in inducing cavitation at the spillway face. There have been a number of cases of occurance of cavitation in major dams. For example Tungabhadra Dam. (iii) Aeration has been observed on many spillways. It entails the entrainment of substantial quantities of air into the flow, which becomes white and foamy in
appearance. The additional air causes the bulkage of the flow. Observations of aeration have led to the suggestion that the point at which aeration commences coincides with the point at which the boundary layer depth meets the free surface called Inception point (Thandaveswara). The entrainment mechanism appears to be associated with the emergence of streamwise vortices at the free surface. Such vortices would originate in the spillway crest region. The geometric elements of an ogee spillway are shown in Fig. 33.3. A typical layout of the spillway is shown in Fig. 33.4.
axis (both quadrants) a
Hd
origin X
r2
r1 0.85
x1.85 = 2Hd y
h b
n -1
a = 0.175 H d xn = KH d y y r1 = 0.5 Hd r2 = 0.20 Hd b = 0.282 Hd Y
Hd = design head
H
X Y
Y = X*/K 0.6 1.0
h
Point of tangency Spillway face l
slope of face m = m:1
1 m = 1.67
toe
Figure 33.3 - Ogee Type Spillway Profile K and n depends on the slope of the upstream face. Upstream face slope Vertical 3 (v) on 1 (H) 3 on 2 3 on 3
K 2.00 1.936 1.939 1.873
n 1.85 1.836 1.810 1.776
Reservoir 2 1
8 3
7 4 5
6
Schematic representation of typical spillway 5. End sill 1. Spillway crest / bay 6. Armoured scour preventing bed 2. Pier 7. Power house 3. Spillway face 8. Sector gate 4. Energy dissipator - stilling basin
Figure 33.4 -Diagram of spillway layout
33.3 Spillway Discharge Equation Equation for estimating the spillway discharge, Q is given below. 2 Q = Cw L h 3/ m3 / s e
in which he is the effective head ( m) effective head = design head + velocity head he =
⎛ h ⎞ ⎟ ⎝ hd ⎠
C w = f ⎜
Hd
+
Ha
Effective length = measured length - end contraction L =
(L '
- 0.1 NH)
in which N is the number of contractions. C w = 1.804 + 0.40
⎛H ⎞ ⎜ h = 5 to 10⎟ ⎝ ⎠
H h 1.5
⎛ H⎞ C w = 2.952 ⎜ 1 + ⎟ ⎝ h⎠ Q=
2
( MKS)
⎛H ⎞ ⎜ h = 10 to 15 ⎟ ⎝ ⎠
Cd ( L − 0.1NH ) 2g H3/ 2
3 2
Cw = Cw
Cd 2g 3 is a function of depth of flow, ranges between 1.6 to 2.3 (MKS)
b
Pier shapes vary and has to be choosen carefully. Figure 33.5 shows typical pier shapes.
0.133 Hd
Pier Type II Nose Shape semi circular
Nose Shape circular
Nose Shape Parabolic
Nose Shape Rectangular with Rounded Corners
Figure 33.5 - Different Pier Shapes
The general equation for discharge is given by Q=
2 3
3
Cd 2gLHe 2
in which Q is the total discharge; L is the crest width; H e is the total head upstream from the crest; g is the gravitational constant; and C d is the discharge coefficient. It may be noted that He, the total head, includes the velocity head. Generally, this requires an iterative solution technique as the velocity head is unknown, as it depends on flow rate which is to be calculated. However, as the velocity head is generally small , the equation converges quickly.
The discharge coefficient C d is not constant. It depends on several factors such as the depth of approach flow, crest shape related to the ideal nappe shape, upstream face slope, downstream apron, and downstream submergence.
33.4 Spillway Structures Spillways provide controlled releases of flood / surplus water in excess of the reservoir capacity and convey it to the river channel downstream below the dam in such a manner that the dam and foundation are protected from erosion and scour. The object of spillway design is to provide a safe and adequate structure for the least combined cost of spillway and dam.
Basic considerations affecting the design of spillways include design flood, crest control (gates), control system, structural stability, and adequate dissipation of energy. The capacity of a spillway must be sufficient to accommodate the maximum discharge without allowing the reservoir surface to rise above a predetermined (maximum reservoir elevation).
Determination of the maximum flood to be used as a basis for spillway design results from hydrological studies and available flood peak data.
A spillway crest may be uncontrolled, thereby permitting water to spill from the reservoir whenever the water surface is higher than the crest level, or it may be controlled by gates installed on the crest. The length of the spillway crest affects the elevation of the crest and also the required control. The spillway length is decided based on other parameters such as cost, type of gate.
Choice of Spillway in the field Ogee Spillways Chute Spillways Shaft (Tunnel) Spillways Side Channel and Shaft Spillway Siphon Spillway
Used in concrete and Masonry dams Used in earthen and rock fill dams Used in earthen and rock fill dams When gorge is very narrow Almost constant head for design range of discharge
Reference Chow V.T., "open Channel Hydraulics", McGraw Hill Publciation, student edition, 1958.
33.5 Computation of Water Surface Profile Over Spillway With the rapidly changing advances in computational modelling for solving the governing equations of fluid flow, engineers now face the decision of which method(s) to use in evaluating existing and proposed spillway designs. The choice of a physical model, computational model, or interpolating/extrapolating the needed information from design/performance curves can be a tough task.
This type of study was completed at the Utah Water Research Laboratory (UWRL) to compare the discharge and crest pressures from flow over an uncontrolled ogeecrested spillway using a physical model, computational model, and design curves from the USBR and USACF.
To determine the shape of the crest of an overflow spillway, different methods are available that depend on the relative height and upstream face slope of the spillway . In 1888, a comprehensive laboratory investigation was first made to study the ogee shape.
Cassidy, in 1965 using potential flow theory and mapping into the complex potential plane, he obtained the solution for free surface and crest pressures and his results were in good agreement with experimental data. Better convergence of Cassidy's solution was obtained by Ikegawa and Washizu in 1973 and Betts in 1979 using linear finite elements and the variation principle. Li et al. in 1989 improved on the 2D irrotational gravity flow by using higher-order elements to model the curved water surface and spillway surface.
Quo et al. in 1998 extended the potential flow theory by using the analytical functional boundary - value theory. This method was applied successfully to spillways with a free drop. Biirgisser and Rutschmann in 1999 used finite elements and an eddy viscosity to iteratively solve the incompressible 2D vertical steady Reynolds-averaged NavierStokes (RANS) equations. Given a flow rate, they successfully computed the free surface and velocity and pressure fields using a finite-element grid that adapts locally for a changing water surface. Olsen and Kjellesvig in 1998 also included viscous effects by numerically solving the RANS equations in two and three dimensions, using the standard k − ε
equations to model turbulence. Olsen and Kjellesvig in 1998 showed
excellent agreement for water surfaces and discharge coefficients for a limited number of flows. However, pressure data were only recorded at five locations downstream from a nonstandard crest at one flow and showed some variability.
Savage and Johnson approached the problem numerically using the RANS equations. Crest pressures are compared at three different flow rates. Furthermore, the pressures are compared over the entire length of the spillway, including the flip bucket. Although there seems to be considerable data in the literature of crest pressures up to the tangent section located at x / H d = 1.4 and at flip buckets, there is a dearth of information on pressures extending from the tangent section to the flip bucket. These pressures are required if one intends to complete an overall stability analysis of the dam.
The commercially available CFD package Flow-3D uses the finite-volume method to solve the RANS equations. Fractional Area / Volume Obstacle Representation (FAVOR) method can be used for computing the free surface over uncontrolled spillway. To numerically solve the rapidly varying flow over an ogee crest, it is important that the free surface be accurately tracked. Tracking involves three parts: locating the surface, defining the surface as a sharp interface between the fluid and air, and applying boundary conditions at the interface. The VOF method is similar to the FAVOR method in defining cells. However, the VOF method allows for a changing free surface over time and space. VOF numerical techniques tend to be dissipative in nature. The general governing continuity and momentum equations for non compressible flows are solved. It has been established that the relative error of the numerical model agrees within 1% with the physical model for H e /Hd > 0.7.
For uncontrolled flow over an ogee spillway, numerical tools are sufficiently advanced to calculate discharge and pressures on the spillway. New numerical techniques provide practicing engineers with an additional tool in the design or analysis of spillways. This tool may be very useful when reevaluating a dam for higher flows or improved hydrologic event flood calculations.
Physical model studies are still considered the basis from which all other methods are compared. However, model studies cost more and take more time to complete than a numerical study. If only approximate discharge and pressures are required, design nomographs provide quick solutions. As an alternative, numerical methods may offer accurate solutions, within given parameters, at a cost and time that may be less than model studies. Also, numerical models have the advantage of providing more details of pressure and velocity.
33.6 Salient Features of Selected Spillways Ukai Dam Spillway The Ukai Dam on the river Tapi is an important Dam in Gujarat state. Against a design flood of 28,600 m3 / s a flood of 37,100 m3 / s was experienced in 1968, and there was practically no damage took place to the diversion channel and the performance of the diversion as predicted by the model.
The spillway has been designed for an outflow of 45,300 m3 / s . It has 22 radial gates of 15.35 * 14.71 m. and the total length of the spillway is 425.2 m. The F.R.L. is at R.L.105.15 m and the invert of the ski-jump bucket is at 51.82 m and the lip level at R.L.58.22 m. Immediately downstream of the lip a concrete apron 15.0 m long and 1.5 m thick has been provided horizontally.
Kadana Dam Spillway Main spillway in the Mahi river Kadana Dam across the Mahi river and located about 71 km upstream of Wanakbori weir. It has a maximum height of 76.5 m above the foundation with a gross storage of 1554 M m3 .
The spillway was designed for an outflow capacity of 31, 400 m3 / s . It has 21 gates of 15.54 * 14.02 m high. The energy dissipation is by a roller bucket. As there was foundation difficulty in spillway spans No. 9 to 11, the invert of the roller bucket has been kept 3 m above seperating the spillway in three parts.
In 1973 there were heavy floods throughout Gujarat and damage worth crores of rupees had taken place. Almost all the major rivers of Gujarat had unprecedented floods. The Tapi, the Narmada, the Mahi, the Sabarmati and the Banas had very heavy floods. As a result, the design floods of all the dams were revised. Accordingly the design flood of Kadana was revised from 31, 400 m3 / s to 44,900 m3 / s ; only two spans could be added
on the main river . As such an additional spillway with a capacity of 10, 000 m3 / s was planned in the adjoining saddle by cutting the hill beyond the right flank. Additional spillway has six radial gates of the same size as the main spillway i.e. 15.54 * 14.02 m. The width of the spillway is 113 m. Immediately downstream of the glacis the width converges to 52 m (46 %) in a short distance. A narrow and deep channel has been excavated in the hills about 50 m high width side slopes 4:1. It has a bed slope of 1 in 93. Coming out of the gorge it meets a small natural nala which drains in the Mahi at a short distance from the main dam.
Dantiwada Dam Spillway Dantiwada dam has been constructed on the river Banas at Dantiwada about 30 km from Palanpur in north Gujarat. The design discharge for the spillway had been fixed at 3 6654 m / s . It has been provided with 11 radial gates of 12.5 * 8.3 m. The original
design provided a stilling basin as an energy dissipator, which required deep excavations in hard rock. The F.R.L. was 183.0 m. Six spans on the right in the main river gorge are provided with a roller bucket whereas the remaining five spans on the left flank are be provided with a ski-jump bucket, with a divide wall 38 m long beyond the roller bucket. The invert of the roller bucket was kept at R.L.137.6 m with a radius of 21.94 m and exit angle of 35
whereas the ski-jump bucket invert was kept at
R.L.144.20 m with a radius of 15.24 m and exit angle of 35 . Thus Dantiwada spillway is
a unique combination of a roller bucket and a Ski-jump bucket.
Spillway in Panchet Tail Pool Dam on Damodar The lower pool on Damodar about 4 km downstream of Panchet dam is mainly an earthen dam with a concrete gravity spillway in the river bed near the left bank. This dam is having a design flood of 17 , 698 m3 / s . Out of this 9, 203 m3 / s have been taken care of in the 202 m long Ogee-shaped spillway and the remaining 8, 495 m3 / s is allowed over a breaching dyke of length 365.76 m.
The Salient Features of the Spillway (a) Discharge capacity (b) Total length (c) Total number of bays (d) Width of pier (e) Width of bay (f) H.F.L (g) Crest level (h) River bed level (i) Type of energy dissipator (j) Length of stilling basin (k) Elevation of basin floor level
Spillway design discharge Tarbella dam (Pakistan) 18500 to 24000 m3/s. Dennison (Canada) 21000 m 3 /s. Aldeadavila (Cuba) 10000 m 3 /s. Melones (Cuba) 8740 m 3 /s. Bhakra details
9 , 203 m3 / s
202 11 Nos. (one emergency) 2.44 m 16.15 m 106.8 m 97.53 m 94.49 m stilling basin 30.48 m 90.22 m
