lecture 3 for printing

7/31/2019 Lecture 3 for Printing

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SPILLWAYS, GATES AND OUTLET WORKS

Some provision must be made in the design of almost every damto permit the discharge of water downstream.

A spillway is necessary to discharge floods and prevent the damfrom being damaged.

Gates on the spillway crest, together with sluiceways, permit theoperator to control the release of water downstream for variouspurposes.

In some cases facilities to regulate the flow in canals or pipelinesleading from the reservoir are also necessary.

Spillways

A spillway is the safety valve for a dam. It must have the capacity to discharge major floods withoutdamage to the dam or any appurtenant structures, at the sametime keeping the reservoir level below some predeterminedmaximum level.


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The required capacity (maximum outflow rate through thespillway) depends on the spillway design flood (inflow hydrograph tothe reservoir), the discharge capacity of the outlet works, and theavailable storage.

The selection of the spillway design flood is related to the degreeof protection that ought to be provided to the dam which, in turn,depends on the type of dam, its location, and consequences offailure of the dam.

A high dam storing a large volume of water located upstream of

an inhabited area should have a much higher degree of protectionthan a low dam storing a small quantity of water whosedownstream reach is uninhabited.

The probable maximum flood is commonly used for the formerwhile a smaller flood based on frequency analysis is suitable for the

latter.

Spillway Design Capacity:

A spillway may be controlled or uncontrolled; a controlled spillwayis provided with crest gates or other facilities so that the outflowrate can be adjusted.


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A determination of the area that would be flooded if the damwere to fail is helpful in determining the acceptable risk.

Computer programs that emit analysis of the flood wave resulting

from the breach of a dam are available. The National Weather Service Program DAMBRK, for example,simulates the flood wave that is created by the breach and routesthe wave downstream.

This permits an estimate of the bounds of flooding.

Overflow Spillway:

An overflow spillwayis a section of dam designed to permit waterto pass over its crest.

Overflow spillways are widely used on gravity, arch, and buttressdams. Some earth dams have a concrete gravity section designedto serve as a spillway.

The design of the spillway for low dams is not usually critical, anda variety of simple crest patterns are used. In the case of highdams it is important that the overflowing water be guided smoothly

over the crest with a minimum of turbulence.


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If the overflowing water breaks contact with the spillway surface,a vacuum will form at the point of separation and cavitation mayoccur.

Cavitation plus the vibration from the alternate making andbreaking of contact between the water and the face of the dam mayresult in serious structural damage.

Some implosive activity will occur at the surfaces of the passageand in the crevices and pores of the boundary material.

Under a continual bombardment of these implosions, the surfaceundergoes fatigue failure and small particles are broken away,giving the surface a spongy appearance. This damaging action ofcavitation is calledpitting.

The ideal spillway would take the form of the underside of thenappe of a sharp-crested weir when the flow rate corresponds to

the maximum design capacity of the spillway.

Figures 9.1 b and 9.1c show an ogee weir that closelyapproximates the ideal.

The reverse curve on the downstream face of the spillway shouldbe smooth and gradual. A radius of about one-fourth of the spillway

height has proved satisfactory.


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Structural design of an ogee spillway is essentially the same asthe design of a concrete gravity section.

The pressure exerted on the crest of the spillway by the flowing

water and the drag forces caused by fluid friction are usually smallin comparison with the other forces acting on the section.

The change of momentum of the flow in the vicinity of thereverse curve may, however, create a force that must be considered.

Recent developments in the design of overflow spillways showthat a ramp of proper shape and size when properly located (Fig.9.1e) will direct the water away from the spillway surface to form acavity.

To be effective, air must be freely admitted to the cavity. Theresult is that air is entrained in the water, the water bulks up, andwhen it returns to the spillway surface, there is no problem with

cavitations.

On very high spillways these ramps may be used in tandem. Byplacing a projecting corbel on the upstream face of the spillwaysection, (Fig. 9.1f) a saving in concrete can be effected.


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23LhCQ w

where,

Q= discharge, cfs or m3/sCw= discharge coefficient

L = length of the crest, ft or m

h = head on the spillway (vertical dist. from

crest to reservoir level), ft or m

The discharge of an overflow spillway is given by the weirequation.

The coefficient Cw varies with the design head. For the standardoverflow crest of Fig. 9.1c the variation ofCw is given in Fig.9.2.

Experimental models are often used to determine spillwaycoefficients.

End contractions on a spillway reduce the effective length belowthe actual length L.

Square-cornered piers disturb the flow considerably and reducethe effective length by the width of the piers plus about 0.2h foreach pier (Fig.9.3).


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23

2

2

g

VhLCQ owwhere,Vo = approach velocity

Example: An ogee spillway 16 ft long is designed according to

Figures 9.1c and 9.2 to pass 420 cfs when the watersurfaceelevation upstream of the spillway is 23.0 ft. The reservoir bottomis horizontal and at elevation 0.0 ft upstream of the spillway. Findthe flow when the water-surface elevation upstream of the spillwayis 21.5 ft. Assume no end contractions and neglect velocityapproach.

Streamlining the piers or flaring the spillway entrance minimizesthe flow disturbance.

If the cross-sectional area of the reservoir just upstream from thespillway is less than five times the area of flow over the spillway,

the approach velocity will increase the discharge a noticeableamount.

The effect of approach velocity can be accounting for by theequation.


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Chute Spillway:

A chute spillway, variously called as open channel or troughspillway, is one whose discharge is conveyed from the reservoir to

the downstream river level through an open channel, placed eitheralong a dam abutment or through a saddle.

The channel is usually constructed of reinforced-concrete slabs l0to 20 in.(0.25 to 0.50 m) thick. Such a structure is relatively lightand is well adapted to earth or rock-fill dams.

A chute spillway may be constructed around the end of any typeof dam when topographic conditions permit, and such a location ispreferred to earth dams to prevent possible damage to theembankment.

The chute is sometimes of constant width but is usually narrowedfor economy and then widened near the end to reduce dischargevelocity.

If the grade of the chute can conform to topography, excavationwill be minimized.

Vertical curves should be gradual and designed to avoidseparation of the flow from the channel bottom.


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The side walls of the chute must be of adequate height toaccommodate bulking of the water caused by the entrainment of airin the high-velocity flow.

Expansion joints are usually required in chute spillways at

intervals of about 30 ft (10m). If water penetrates under the slab, itmay cause troublesome uplift.

The expansion joints should therefore be as watertight aspossible, and drains under the spillway are necessary. These maybe rock-filled trenches or perforated steel pipe.

Suitable filters must be provided to control piping. The slabs of achute spillway should be keyed together in such a manner that theupstream end of a slab cannot rise above the block next upstream.


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Side-Channel Spillway:

A side-channel spillway is one in which the flow, after passingover the crest, is carried away in a channel running parallel to the

crest. The crest is usually a concrete gravity section, but it may consistof pavement laid on an earth embankment or the natural groundsurface.

This type of spillway is used in narrow canyons where sufficient

crest length is not available for overflow or chute spillways. Figure 9.7 shows a sketch of the flow in a side-channel. Analysisof flow in the side channel is made by application of the momentumprinciple in the direction of flow.

Residual energy of the water after passing the spillway crest is

ignored in the design of the side channel. In fact, a weir, or sill, isoften placed at the downstream end of the channel to create astilling basin to dissipate this energy.

After passing through the side channel, the water is ordinarilycarried away through a chute or tunnel.


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There are many spillways that change direction immediately afterthe crest and whose characteristics are intermediate between the

chute and the side channel.


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There are three possible conditions of flow in a shaft spillway asshown in Figure 9.9. At low heads the outlet conduit flows partlyfull, the perimeter of the inlet serves as a weir, and the discharge ofthe spillway varies as h1

3/2 .

As the head is increased, water rises in the shaft, and the outletmay flow partly full (weir flow) or full (orifice flow). When the shaftis completely filled with water and the inlet is submerged, thedischarge becomes approximately proportional to h1

1/2 (pipe flow)where h2 is the total head on the outlet.

In this third stage, an increase in h2 results in only a very slightincrease in discharge. This, in effect, places a limit on the capacityof a shaft spillway.

The relation between flow rate and water-surface elevation of aproperly designed shaft spillway is depicted by the solid line in Fig.9.9; if improperly designed, a throttling of the flow will occur when

the flow changes to pipe flow, as shown by the dashed line.

An abrupt transition between the shaft and outlet conduit mayresult in cavitation; hence a smooth transition is preferred in largestructures.

Hydraulic analysis of shaft spillways is difficult, and model tests are

often employed.


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Models must be used with caution, for the air pressure in themodel is not reduced to model scale.

An undesirable feature of shaft spillways is the hazard of cloggingwith debris.

Trash racks, floating booms, or other types of protection arenecessary to prevent debris from entering the inlet.


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Siphon Spillway:

A siphon spillway is a closed conduit system formed in the shapeof an inverted U, positioned so that the inside of the bend of the

upper passageway is at normal reservoir storage level. If a large capacity is not necessary and space is limited, thesiphon spillway may be a practical selection.

Siphon spillways have the advantage that they can automaticallymaintain water-surface elevation within very close limits. At low

flows, the siphon spillway operates like an overflow spillway with itscrest at Cas shown in Figure 9.10.

If the outlet of the siphon is not submerged (Figure 9.10a), thehead h is the vertical distance from the water surface in thereservoir to the end of the siphon barrel.

When the outlet is submerged, h is the difference in elevationbetween the headwater and tail water (Fig. 9.10b).

If air is prevented from entering the outlet end of the siphon, flowthrough the siphon will entrain and remove the air at the crown andprime the siphon.


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Entrance of air can be prevented by deflecting the flow across thebarrel in such a way as to seal it off or by submerging the outletend of the barrel.

Siphon action will continue until the water level in the reservoirdrops to the elevation at the upper lip of the siphon entrance unlessa vent is provided at a higher level.

A siphon may be designed so that variations in upstream waterlevel are small with respect to total head, and thus the discharge isnearly always at capacity when the siphon is primed.

This makes the siphon spillway particularly advantageous indisposing of sudden surges of water such as may occur in canalsand forebays when the outlet gates are closed rapidly.

As soon as a siphon is primed, a vacuum forms at the crown. Inolder to prevent cavitation, the siphon should be designed so thatthis vacuum never exceeds three-fourths atmospheric pressure.

Thus, at sea level the vertical distance from the crown of thesiphon down to the hydraulic grade line should not exceed about 25ft (7.5m).


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At higher elevations the limiting distance from crown to grade lineis still smaller.

If the entrance of the siphon remains submerged to a depth of 6

ft (2m) there is little likelihood of clogging from debris or ice, buttrash racks may be a wise precaution.

One disadvantage of the siphon spillway is the relatively high costof forming the barrel, but if the siphon can be made from pipe, thecost may not be high.

Service Spillways and Emergency Spillway:

On many projects a single spillway serves to discharge all rates ofoutflow. In some instances, however, it is economic to have morethan one spillway a service or auxiliary spillway to convey

frequently occurring outflow rates and one or more emergencyspillways that are used only rarely during extreme floods.

Often a saddle or low point on natural ground at the periphery ofthe reservoir will serve as the emergency spillway. In otherinstances an engineered structure is used.


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An example of a service-emergency spillway structure is shownin Fig. 9.11 in which the side-channel spillway with concretedischarge chute is designed to handle the outflow from the 50 yrflood.

Larger flows pass over the backside of the side-channel spillway(secondary weir) and are conveyed to the river through a naturaldepression.

Dynamic Forces on Spillways:

Newtons second law of motion states that force equals the timerate of change of momentum.

The resultant of the forces on an element of water is:

VQF = density of water

Q = flow rateV= change in velocity

in vector form,

xx

VVQFx 12 yy VVQFy 12


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This equations may be used to find the dynamic forces exerted bywater on spillways, deflectors, turbine blades, pipe bends, and otherhydraulic structures,

The forces Fx and Fy are those acting on a significant free body of

fluid which include gravity forces, hydrostatic pressures, and thereaction of any object in contact with the water.

Example: Given the ogee spillway of Figure 9.12a with Cw = 3.8,find the total force of the water on the curved section AB.

CREST GATES:

Additional storage above the spillway crest can be made availableby the installation of temporary or movable gates.

Such an increase in reservoir level is permissible in the low-water

season, when low flows may be permitted over the crest-controldevice.

If a large flood occurs, full spillway capacity may be madeavailable by removing the temporary barriers. These devices mustbe used with caution on spillway of earth dams, where operational

failure of the gates may result in overtopping of the dam.


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Flashboards:

The usual flashboard installation consists of wooden panelssupported by vertical pins placed on the crest of the spillway as

shown on Figure 9.13a. Such installations are temporary and aredesigned to fail when the water surface in the reservoir reaches apredetermined level.

A common design uses steel pipe or rod set loosely in sockets inthe crest of the dam and designed to bend and release theflashboards at the desired water level. Temporary flashboards of

this type have been used in heights up to 4 or 5 ft (1.3 or 1.7 m).

Since temporary flashboards are lost each time the supports fail,permanent flashboards are more economic for large installations.

Permanent flashboards usually consist of panels that can beraised or lowered from an overhead cableway or bridge as shown in

Figure 9.13b.

In this setup, the lower edge of the panels is placed in a seat orhinge on the spillway crest, and the panels are supported in theraised position by struts or by attaching the upper edge of the panelto the bridge.


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Stop Logs and Needles:

Stop logs consist of horizontal timbers spanning the spacebetween grooved piers as shown in Figure 9.14a.

The logs may be raised by hand or with a hoist.

There is usually much leakage between the logs, and considerabletime may be required for removing the logs if they become jammedin the slots.

Stop logs are ordinarily used for small installations where the costof more elaborate devices is not warranted or in situations whereremovable or replacement of the stop logs is expected only at veryinfrequent intervals.

Needles consists of timbers with their power ends resting in akeyway on the spillway crest and their upper ends supported by a

bridge as shown in Figure 9.14b.

Needles are somewhat easier to remove than stop logs but arequite difficult to place in flowing water. Consequently, they are usedmainly for emergency bulkheads, where they need not be replaceduntil flow has stopped.


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Vertical Lift Gates:

Simple timber or steel gates that slide in vertical guides on pierson the crest of the dam are used for small installations.

Their size is limited by the high friction force developed in theguides because of the hydrostatic force on the gate.

By placing cylindrical rollers between the bearing surfaces of thegate and guides, the frictional resistance can be much reduced.

The stoney gate has rollers that are independent of the gate orguides, thus eliminating axle friction.

The independent roller train of the stoney gate is difficult todesign and build, and the development of low-friction roller bearingshas led to the use of the fixed-wheel gate, which has wheelsattached to the gate and riding in tracks of the downstream side of

the gate guide. In the large sizes excessive headroom is required to lift the gateclear of the water surface, and large vertical lift gates are often builtin two horizontal sections so that the upper portion may be liftedand removed from the guides before the lower portion is moved.


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This design also reducesthe load on the hoistingmechanism. Discharge my

occur over either one orboth sections of the gate orover the spillway crest.

A gate 50 ft (15m)square may have to

support a water load ofover 2000 tons, and thegate itself may weigh 150tons.

Design of such a gate and its operating mechanism is a

structural and mechanical problem of considerable magnitude.

Accurate alignment of the rollers and guides is necessary so thatthe gate will operate satisfactorily.


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Tainter Gates/Radial Gates:

Tainter, or radial gate isthe most widely used type

of crest gate for largeinstallations; it is thesimplest and usually themost reliable and leastexpensive.

The face of the gate is acylindrical segment suppor-ted on a steel frameworkthat is pivoted on trunnionsset in the downstreamportion of the piers on the

spillway crest.

Hoisting cables are attached to the gate and lead to winches onthe platform above the gate.

The winches are usually motor driven, tough hand power may bused for small gates.


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Each gate may have its independent hoisting mechanism or acommon unit may be moved from gate to gate.

Flexible fabric or a rubber strip is used to form a water seal

between the gates and the piers and spillway crest. A movable flap is sometimes attached to the top of the gate topermit floating material to pass readily over the gate.

Tainter gates vary in size from 3 to 35 ft (1 to 11 m) in heightand 6 to 60 ft (2 to 18 m) in length.

One of the largest tainter-gate installations is at Cark Hill Dam inGeorgia, which as 23 Tainter gates 35 ft (11 m) high and 60 ft (18m) long.

Tainter gates have several advantages. Friction is concentrated atthe pin and is usually much les than for sliding gates.

Since the trunnion bears the part of the load, the hoisting load isnearly constant for all gate openings and is much less than forvertical-lift gates of the same size.

Counterweights are sometimes required for large gates of eitherradial or vertical-lift type.


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Roller Gates:

A rolling or roller gate consists of a steel cylinder spanningbetween the piers.

Each pier has an inclined rack that engages gear teeth encirclingthe ends of the cylinder.

When a pull is exerted on the hoisting cable, the gate rolls up therack.

The lower portion of the gate consists of a cylindrical segmentthat makes contact with the spillway crest and increases the gateheight.

Rolling gates are well adapted to long spans of moderate height.

Drum Gates:

Another type of gates adapted to long spans is the drum gate asshown in Figure 9.16. This gate consists of a segment of a cylinderwhich, in the open or lowered position, fits in a recess in the top ofthe spillway.

When water is admitted to the recess, the hollow drum gate is

forced upward to the closed position.


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The type developed by the U.S. Bureau of Reclamation as shownin Figure 9.16a is a completely enclosed gate, hinged at theupstream edge so that buoyant forces aid in its lifting.

This type of gate is adapted to automatic operation and also

conforms closely to the shape of the ogee crest when lowered.

A second type as shown in Figure 9.16b has no bottom plate andis raised by water pressure alone. Because of the large recessrequired by drum gates in the lowered portion, they are notadapted to small dams.


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Figure: Drum Gate


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Bear Trap Gate:

A bear-trap gate consists of two leaves of timber or steel hingedand sealed to the dam as shown in Figure 9.17.

When water is admitted to the space under the leaves, they are

forced upward. The downstream leaf is frequently hollow so that its buoyancy aidsthe lifting operation.

This type of gate is adapted to low navigation dams, since at high-river stages the gate may be lowered so as not to interfere withnavigation over it.


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Rubber Gate:

Inflatable rubber dams are rubberized fabric tubes which areanchored to a sill and inflated to form a dam.

These dams are limited to very low-head project usage, aresubject to puncturing and vandalism, and are not recommended formajor projects.

R bb G t /R bb D


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The dam is activated bypumping water into the rubber

bladder thus inflating it to forma barrier that stands out abovethe channel bottom thusblocking the channel.

The dam is deactivated byreleasing the water from inside

the bladder. In its inflated statethe rubber dam may be strongenough for use as a temporarybridge for pedestrians andlightweight vehicles.

Rubber Gate/Rubber Dam:

Inflatable rubber dams are rubberized fabric tubes which areanchored to a sill and inflated to form a dam.

These dams are limited to very low-head project usage, are subjectto puncturing and vandalism, and are not recommended for majorprojects.

lecture 3 for printing

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