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ENERGY LINE

6

Fig.6c3.5%0.035) = 0.46

0.820.921.80HmaxH=1.5 mH=2 m

GATED SPILLWAY

Morning Glory

STEPPED SPILLWAYService spillways from top to bottom, Pineview Dam, Utah; Monticello Dam, California; and Upper Stillwater Dam, UtahSPILLWAYAuxillaryThis are designed for infrequent use and may sustain limited damage when used.This are used in combination with service spillway and sometimes also with flood outlets.It is designed to function automatically when required without aggravating downstream floods.

GATED AUXILLIARY SPILLWAYGATED SERVICE SPILLWAY

OGEE SERVICE SPILLWAYFUSE PLUG AUXILLIARY SPILLWAYAuxiliary spillways from top to bottom, Stewart Mountain Dam, Arizona; New Waddell Dam, ArizonaSPILLWAYEmergencyThis are designed to provide a reserve protection against overtopping of a dam and are intended for use under extreme conditions, such as misoperation or malfunction of a service spillway or other emergency conditions.

GATED EMERGENCY SPILLWAYGATED SERVICE SPILLWAYEmergency spillway , Folsom Dam, CaliforniaControlled (or gated) spillway Uncontrolled (or ungated) spillway 2Classifications based on CONTROLSPILLWAYControlledThese spillways enable storage to be maximized by controlling water levels. Generally more complex and more costly to build and maintain than uncontrolled spillways.It should be backed up by auxiliary spillways as the gates may be subject to automatic operation malfunction, human error and debris lockageSPILLWAYUncontrolledMost commonly used at small dams because of their reliability, simplicity and ability to pass debris and to reduce the magnitude of incoming flood peaks, as well as being cheaper to build and maintain.Ogee spillway Chute (or open channel or trough) spillway Side-channel spillway Shaft (or morning glory) spillway Siphon spillway Conduit (or tunnel) spillway Cascade spillway 3Classifications based on PROMINENT FEATURESPILLWAYSTypes ofSIPHONSHAFTTUNNELCHUTEOGEESIDE-CHANNELSPILLWAYOgeeOgee spillways are also called Overflow spillwaysThis type of spillway allows the passage of the flood wave over its S-shaped crest.Can be classified under controlled or uncontrolled.Widely used on Gravity dams, Arch dams, and Buttress Dams.

SPILLWAYOgee

EMBANKMENT DAMUNGATED

3 GATED GRAVITY DAMOgee spillways from top to bottom, Fier Dam, Pankshin; and Takato Dam, NaganoSPILLWAYChuteChute spillways are common and basic in design as they transfer excess water from behind the dam down a smooth decline into the river below.The spillways slope and its side are lined with concrete.In case of having sufficient stiff foundation conditions at the spillway location, a chute spillway may be used instead of overflow spillway due to economic consideration

UNGATED CHUTE SPILLWAYROCK-FILL DAM

ROCK-FILL DAM3 GATED CHUTE1 UNGATEDChute spillway from top to bottom, Mohale Dam, Africa; and Pantabangan Dam, PhilippinesSPILLWAYSide ChannelIf a sufficient crest length is not available for an overflow or chute spillway in narrow valleys, excess water is removed from the reservoir through a side channel spillway.The side channel through which water is discharged can also be lined with concrete to prevent erosion and subsequent sedimentation in dams on the course of the river.

SIDE CHANNEL SPILLWAYGRAVITY DAMSPILLWAYShaftIt discharges excess water from a reservoir through a shaft that is constructed near the crest of the Dam with height less than that of the crest.The shaft spillway is constructed when the other types of spillways cannot be constructed due to a lack of space.When the shaft is completely submerged, further increased in head will not result in appreciable increase in discharge.It is not suitable for large capacity and deep reservoirs because of stability problems.

ARCH DAMMORNING GLORYMonticello Dam, CaliforniaSPILLWAYSiphonA siphon spillway is similar to a shaft spillway but instead is incorporated into the damThe presence of a siphon spillway weakens a dam at certain points, so the dam has to be reinforced at these weak points incurring extra cost.Maintenance of this spillway is very difficultSiphon spillways comprise usually of five components which include an inlet, an upper leg, a throat or control section, a lower leg and an outlet.

SPILLWAYConduitConduit spillway or tunnel spillway is the one in which a closed channel is used to convey the discharge around or under a dam. The closed channel may be in the form of a vertical or inclined shaft, a horizontal tunnel through earth dam or a conduit constructed with open cut and backfilled with earth materials. These spillway are designed to flow partly full. To ensure free flow in the tunnel, the ratio of flow area to the total tunnel area is often limited to 75% and air vents are provided at critical points along the tunnel or conduit to ensure an adequate air supply which will avoid unsteady flow through the spillway

Depending on the site conditions and hydraulic particularities an overflow structure can be of various designs:Frontal overflow,Side-channel overflow, andShaft overflow.

Other types of structures such s labyrinth spillway use a frontal overflow but with a crest consisting of successive triangles or trapezoids in plan view.Still another type is the orifice spillway in the arch dam.

STRUCTUREOverflowMain Types of Overflow Structures

Frontal OverflowSide OverflowShaft OverflowThe frontal type of overflow is a standard overflow structure, both due to simplicity and direct connection of reservoir to tailwater. It can normally be used in both arch and gravity dams. The frontal overflow can easily be extended with gates and piers to regulate the reservoir level, and to improve the approach flow to spillway. Gated overflows of 20 m gate height and more have been constructed, with a capacity of 200 m3 /s per unit width. Such overflows are thus suited for medium and large dams, with large floods to be conveyed to the tailwater. Particular attention has to be paid to cavitation due to immense heads that may generate pressure below the vapor pressure in the crest domain. STRUCTUREOverflowfrontal overflowCrest ShapesOverflow structures of different shapes are:Straight (standard)

Curved

Polygonal

Labyrinth

The labyrinth structure has an increased overflow capacity with respect to the width of the structure. Plan viewOVERFLOWFrontalLabyrinth spillway

When the flow over a structure involves curved streamlines with the origin of curvature below the flow, the gravity component of a fluid element is reduced by the centrifugal force. If the curvature is sufficiently large, the internal pressure may drop below the atmospheric pressure and even attain values below the vapor pressure for large structures. Then cavitation may occur with a potential cavitation damage. As discussed, the overflow structure is very important for the dam safety. Therefore, such conditions are unacceptable.For medium and large overflow structures, the crest is shaped so as to conform the lower surface of the nappe from a sharp-crested weir.CREST SHAPESStandardThe transverse section of an overflow structure may be rectangular, trapezoidal, or triangular.In order to have a symmetric downstream flow, and to accommodate gates, the rectangular cross section is used almost throughout.

The longitudinal section of the overflow can be made either;Broad-crested.Circular crested, orStandard crest shape (ogee-type)

For heads larger than 3 m, the standard overflow shape should be used.Although its cost is higher than the other crest shapes, advantages result both in capacity and safety against cavitation damage.

Broad CrestedCircular CrestedOgee Crested

Fig. 1Crest Shape of Overflow SpillwaysThe lower surface of a nappe from a sharp-crested weir is a function ofthe head on the weir, the slope or inclination of the weir surface, the height of the crest, which influences approach velocity.

On the crest shape based on a design head HD, when the actual head is less than HD, the trajectory of the nappe falls below the crest profile, creating positive pressures on the crest, thereby reducing the discharge. On the other hand, with a higher than design head, the nappe-trajectory is higher than crest, which creates negative pressure pockets and results in increased discharge. Accordingly, it is considered desirable to underdesign the crest shape of a high overflow spillway for a design head HD, less than the head on the crest corresponding to the maximum reservoir level, He. However, with too much negative pressure, cavitation may occur. The U.S. Bureau of Reclamation (1988) recommendation has been that He/HD should not exceed 1.33. The Corps of Engineers (COE) has accordingly recommended that a spillway crest be designed so that the maximum expected head will result in an average pressure on the crest no lower than (- 4.50m) of water head (U.S. Department of Army, 1986). Pressures of (-4.50m) can be approximated by the following equations (Reese and Maynord, 1987). For He, HD 10 m, HD = 0.43He1.22 (without piers) HD = 0.39He1.22 (with piers)

For He, HD < 10 m, HD =0.70He (without piers) HD = 0.74He (with piers) Another empirical equation given for the maximum head on the crest for no cavitation is, Hmax = 1.65HDThe U.S. Bureau of Reclamation described the complete shape of the lower nappe by separating it into two quadrants, one upstream and one downstream from the crest (apex), as shown in previous figure. The equation for the downstream quadrant is expressed as,

Where HD = Design head excluding the velocity approach head. x, y = Coordinates of the crest profile, with the origin at the highest point (O)K = Constant that depends on upstream inclination and approach velocity. Constant K can be varied from 2.00 for a deep approach to 2.20 for a very shallow approachEq. 1where xDT = Horizontal distance from the apex to the downstream tangent point = Slope of the downstream face.In a high-overflow section, the crest profile merges with the straight downstream section of slope , as shown in Fig. (1) (i.e., dy/dx = ). Differentiation of Eq. (1) and expressing that in terms of x yield the distance to the position of downstream tangent as follows:

Eq. 2

Fig. 2 Coordinate coefficients for spillway crest (U.S. Department of the Army, 1986) The discharge efficiency of a spillway is highly dependent on the curvature of the crest immediately upstream of the apex. To fit a single equation to the upstream quadrant had proven more difficult. An ellipse, of which both the major and minor axes vary systematically with the depth of approach, can closely approximate the lower nappe surfaces.

where x = Horizontal coordinate, positive to the right y = Vertical coordinate, positive downward A, B = One-half of the ellipse axes, as given in Fig. (2.b and c) for various values of approach depth and design head. With respect to origin at the apex, the equation of the elliptical shape for upstream quadrant is expressed as, Eq. 3For a inclined upstream face of slope FS, the point of tangency with elliptical shape can be determined by the following equation.

Eq. 4The lifespan of a dam is of the order of 100 years. The design discharge may be related to the maximum flood discharge that may occur within this period. Probability of occurrence of a discharge that can seriously damage the system should be minimum. As an example (depending on project size and country regulations) Q100 For optimum flow conditions observed. Q1000 For some adverse flow conditions may be tolerated, but there should be no damage. Q10000 For minor damage may be tolerated but system should not fail. Another approach is based on the concept of the possible maximum flood (PMF). Accordingly, a rainfall-runoff model with the most extreme combination of basic parameters is chosen, and no return period is specified.SPILLWAYDesign Discharge of Discharge CharacteristicsThe discharge over an ungated ogee crest is given by the formula:

Where:Q=discharge,C=discharge coefficient,L=effective length of crest,He=total head on the crest, including the velocity of approach head, ha.The discharge coefficient, C, is influenced by a number of factors:The depth of approach,Relation of actual crest shape to the ideal nappe shape,Upstream face slope,Downstream apron interface,Downstream submergence.

Eq. 5

Coefficient of discharge for ogee crests with vertical faces (Roberson, Cassidy, Chaudhry, 1998)Fig. 3

Coefficient of discharge for ogee crests with vertical faces (Roberson, Cassidy, Chaudhry, 1998)Fig. 4Overflow Gates:The overflow structure has a hydraulic behavior that the discharge increases significantly with the head on the overflow crest..The height of the overflow is usually a small portion of the dam height.Further, gates may be positioned on the crest for overflow regulation.During the floods, if the reservoir is full, the gates are completely open to promote the overflow.A large number of reservoirs with a relatively small design discharges are ungated.

Currently most large dams are equipped with gates to allow for a flexible operation.The cost of the gates increases mainly the magnitude of the flood, i.e.: with the overflow area.Improper operation and malfunction of the gates is the major concern which may lead to serious overtopping of the dam.In order to inhibit floods in the tailwater, gates are to moved according to gate regulation.Gates should be checked against vibrations.The Advantages and Disadvantages of GatesThe advantages of gates at overflow structure are:Variation of reservoir level,Flood control,Benefit from higher storage level.The disadvantages are:Potential danger of malfunction,Additional cost, and maintenance.Depending on the size of the dam and its location, one would prefer the gates for:Large dams,Large floods, andEasy access for gate operation.

Three types of gates are currently favored:Hinged flap gates,Vertical lift gates,Radial gates.

Flap GateVertical GateRadial Gate

FLAP GATESVERTICAL LIFT GATERADIAL GATEThe flaps are used for a small head of some meters, and may span over a considerable length.

The vertical gate can be very high but requires substantial slots, a heavy lifting device, and unappealing superstructure.

The radial gates are most frequently used for medium or large overflow structures because of their simple construction, the modest force required for operation and absence of gate slots.They may be up to 20m X 20m, or also 12 m high and 40 m wide. The radial gate is limited by the strength of the trunnion bearings.

The risk of gate jamming in seismic sites is relatively small, if setting the gate inside a stiff one-piece frame.For safety reasons, there should be a number of moderately sized gates rather than a few large gates.For the overflow design, it is customary to assume that the largest gate is out of operation.The regulation is ensured by hoist or by hydraulic jacks driven by electric motors.Stand-by diesel-electric generators should be provided if power failures are likely.

FLASHBOARDNEEDLESRUBBER DAMROLLING GATESOverflow spillways frequently use undershot radial gates for releases over the dam. The governing equation for gated flows,

Where C is a coefficient of discharge, and H1 and H2 are total heads to the bottom and top of the gate opening. The coefficient C is a function of geometry and the ratio d/H1, where d is the gate aperture. Fig. (7.3). Eq. 6

Fig. 5Crest piers and abutments cause contraction of the flow, reduction in the effective length of the crest, and cause reduction in discharge.

where L = Effective length of the crest for calculating discharge L = Net length of the crest N = number of piers Kp = Pier contraction coefficient Ka = Abutment contraction coefficient He = Total head on the crest

Eq. 6.1Pier and Abutment effects

The nose of piers and abutments should be rounded sufficiently to minimize the hydraulic disturbance.Piers may extend downstream on the chute as a dividing wall in order to suppress shock waves.Abutments are extended towards the reservoir to facilitate gentle flow conditions at the entrance of spillway.

Piers on overflow structures are provided:to improve approach flow conditionsto mount overflow gatesto divide the spillway into subchannelsto aerate the chute flow at the pier ends

PierDivide WallAbutmentEffective Flow widthBetween PiersPDAIe

EFFECTIVE Crest Length

Fig. 2

Eq. 1

2a

2Eq. 4Eq. 3

Fig. 4

Submerged Discharge on Overflow Spillways The coefficient of discharge decreases under the condition of submergence. Submergence can result from either excessive tailwater depth or changed crest profile. The effect of tailwater submergence on the coefficient of discharge depends upon the degree of submergence defined by hd/He and the downstream apron position, (hd+d)/He shown in Fig. (6). For a value of (hd+d)/He up to approximately 2, the reduction in the coefficient depends on the factor (hd+d)/He and is independent of hd/He as shown in Fig. (6.a), i.e., it is subject to apron effects only.Submerged Discharge on Overflow Spillways

Fig. 6 Reduction of discharge coefficient for submerged spillway: a) apron effects, when (hd+d)/He> 5 Submerged Discharge on Overflow Spillways Figure6. (c) Reduction of discharge coefficient for submerged spillway when (hd+d)/He is between 2 and 5.When (hd+d)/He is above 5, the reduction depends only on hd/He as shown in Fig. (6.b), i.e., tailwater effects control. For (hd+d)/He between 2 and 5, the reduction of the coefficient depends on both factors, given in Fig. (6.c). The effect on the discharge due to crest geometry is not well defined. Model studies are the best way to determine the coefficient.

DYNAMIC FORCE on Spillways

DYNAMIC FORCE on SpillwaysEq. 7aEq. 7bFig. 7 (a) shows an ogee spillway which discharges water with a head of 1.2 m over the crest. Taking the coefficient of discharge as 2.2, compute the dynamic force on the curved section AB which has a constant radius of 3 m.

Solution. The discharge over the spillway is given by,Q = CLH3/2or q=Q/L = CH3/2 = 2.2 (1.2)3/2 =2.9 cumecs/m

Let d1 and d2 be the depth of sheet of water at A and B and v1 and v2 be the velocities. Assuming that there is no loss of energy and neglecting approach velocity, we may apply Bernoulli's theorem at u/s water surface, and at sections A and B.

Thus we get11.2 = 1.5 + d1 cos 60 + v12/2g = d2 + v22/2g (1)

But V1d1 = q = 2.9 = V2d2

Hence V1 = 2.9/d1 and V2 = 2.9/d2Side channelsSide channels are often considered at sites where: a narrow gorge does not allow sufficient width for the frontal overflow, impact forces and scour are a problem in case of arch dams,a dam spillway is not feasible, such as in the case of an earth dam, when a different location at the dam site yields a simpler connection to the stilling basin.

Side channels consist of a frontal type of overflow structure and a spillway with axis parallel to the overflow crest.The specific discharge of overflow structure is normally limited to 10 m3 /s/m, but for lengths of over 100 m. The overflow head is limited to say 3 m. Not equipped with gates.The shaft type spillway has proved to be economical, provided the diversion tunnel can be used as a tailrace. The main elements are: The intake,The vertical shaft with a bend,The almost horizontal spillway tunnel, and,Energy dissipator.

Air by aeration conduits is provided in order to prevent cavitation. Also, to account for flood safety, only non-submerged flow is allowed such that free surface flow occurs along the entire structure, from the intake to the dissipator. Used for dams with small to medium design discharges Morning Glory

Morning Glory

Overfall is advantageous when: seismic action is small, the horizontal spillway may be connected to the existing diversion channel, floating debris is insignificant,space for the overflow structure is limited, geologic conditions are excellent against settlement, and

Location of the Morning Glory The intake is prone to rotational approach flow, which should be inhibited with a selected location of the shaft relative to the reservoir topography and the dam axis. The radial flow may be improved with piers positioned on overfall crest.Crest shape The shape of the Morning Glory overfall is a logical extension of the standard overfall crest. Experiments were performed on circular sharp crested weir.

All quantities referring to the weir are over barred. The overflow head relative to the sharp crest is and the coordinate system ( , ) is located at the weir crest.Discharge The discharge over a Morning Glory overfall structure is in analogy with the straight-crested overfall

for the range of

An initial value of H or R may be assumed for a fixed H/R ratio to start the computations.

Shaft radius Rs can be determined from

Rs = 1 + 0.1R (in meters)

3.5%2.9%