hydraulic structures notes

8/14/2019 Hydraulic structures notes

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Hydraulic structures

Hydraulic structures are devices which are used to regulate or measure flow. Some are of

fixed geometrical form, while others may be mechanically adjusted. Hydraulic structuresform part of most major water engineering schemes, for irrigation, water supply, drainage,sewage treatment, hydropower. It is convenient to group the structures under threeheadings:

(a flow measuring structures, e.g. weirs and flumes!(b regulation structures, e.g. gates or valves!(c discharge structures, e.g. spillways.

"or most of these structures the depth#discharge relationship is based on the $ernoulli (orspecific energy e%uation. However, some modifications have to be incorporated to

account for the losses of energy which are inevitably incurred in real flows.

Thin plate (sharp crested) weirs&his type of device is formed from plastic or metal plate of a suitable gauge. &he plate isset vertically and spans the full width of the channel. &he weir itself is incorporated intothe top of the plate. &he geometry of the weir depends on the precise nature of theapplication. In this section we will concentrate on two basic forms, the rectangular weirand the vee (or triangular weir. However, other forms are available, such as thecompound weir.

&he primary purpose of a weir is to measure discharge. 'nce the upstream water level

exceeds the crest height S, water will flow over the weir. )s the depth of water above theweir (h* increases, the discharge over the weir increases correspondingly. &hus, if thereis a +nown relationship between h, and , we need only to measure h, in order to deduce. &he -ideal- relationship between h, and may readily be derived for each weir shapeon the basis of the $ernoulli e%uation. If these relationships are compared, it is evidentthat the triangular weir possesses greater sensitivity at low flows, whereas the rectangularweir can be designed to pass a higher flow for a given head and channel width.

ectangular weirs&here are two types of rectangular weir (see "ig.*:

(a -uncontracted- or full width weirs comprise a plate with a hori/ontal crest extendingfrom one side of the channel to the other # the crest section is as illustrated in thefigure.

(b a -contracted- weir, by contrast, has a crest width which is less than the channelwidth.

Since the operation of a weir is based on the use of a gauged depth to estimate thedischarge, we must +now how these two %uantities are related. &he actual flow over aweir is %uite complex, involving a three#dimensional velocity pattern as well as viscouseffects. &he simplest method of developing a numerical model which represents a weir is

to use the $ernoulli e%uation as a starting point. )n idealised relationship between depthand discharge is obtained. &his relationship can then be modified to ta+e account of thedifferences between ideal and real flows.

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Long-based weirsIn contrast to plate weirs, long#based weirs are larger and generally more heavilyconstructed (e.g. from concrete. &hey are usually designed for use in the field, andconse%uently may have to handle large discharges. )n ideal long#based weir has thefollowing characteristics:

(a it is cheaply and easily fabricated (perhaps off#site! (b it is easily installed!(b it possesses a wide modular range!(c it produces a minimum afflux (i.e. increase in upstream depth due to the installation

of the weir!(d it re%uires a minimum of maintenance.

&here are a number of different designs, of which a selection is considered in detail below.

The rectangular ('broad-crested') weirectangular weirs are solid weirs of rectangular cross section, which span the full width ofa channel ("ig 0.. &he following facts apply:

(a ) hump placed on the bed of the channel results in a local increase in the velocityof flow and a corresponding reduction in the elevation of the water surface.

(b 1iven a hump of sufficient height, critical flow will be produced in the flow over thehump. &here is then a direct relationship between and h , i.e. the flow is modular.2ong#based weirs are designed for modular flow.

$y definition, a rectangular weir is not streamlined. &his, in turn, implies that thestreamlines at the upstream end of the weir will not be parallel, since the flow will beaccelerating. If frictional resistance is ignored, then the streamlines will become paralleland the flow become critical given a sufficient length of crest. It is then possible to derive astraightforward performance under submerged conditions than other long#based weirs.

Crump Weirs3.S. 4rump published details of a weir with a triangular profile, which has been developedat the HS. &his was claimed to give a wide modular range, and also give a morepredictable performance under submerged conditions than other long based weirs. See"igure 5.

4rump proposed upstream and downstream slopes of *:0 and *:6. respectively, whichwere based on sound principles. &he upstream slope was designed so that sediment

build#up would not reach the crest. &he downstream slope was shallow enough to permitan hydraulic jump to form on the weir under modular flow conditions, thus providing anintegral energy dissipator. )lso, under submerged conditions, losses are not too high andthe afflux is minimised. &he primary gauging station is upstream of the weir. However,there is a second gauging point on the weir itself, just downstream of the crest. &hesecond reading is used when the weir is submerged. &he accuracy of the weir dependson the sharpness of the crest, so some weirs incorporate a metal insert at the crest. &hesecondary gauging tappings are drilled into the insert. &he discharge e%uation for a4rump weir is in the form:

6.*

*

6.7

vd hgb44, =

8hich is clearly based on the same concepts as the corresponding e%uation for arectangular weir. &he value of 4d is about 7.95. &he value of 4varies with the ratioh-;(h- < s.

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"ig 5

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Flumes&his term is applied to device in which the flow is locally accelerated due to:

(a a streamlined lateral contraction m the channel sides!(b the combination of the lateral contraction, together with a streamlined hump m the

invert (channel bed

&he first type of flume is +nown as a venturi flume, "igure =. "lumes are usually designedto achieve critical flow in the narrowest (throat section, together with a small afflux."lumes> are >specially applicable where deposition of solids must be avoided (e.g. insewage wor+s or m irrigation canals traversing flat terrain.) general e%uation for theideal discharge through a flume may be developed on the basis of the energy andcontinuity principles.

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Spillways&he majority of impounding reservoirs are formed as a result of the construction of adam. $y its very nature, the stream flow which supplies a reservoir is variable. It followsthat there will be times when the reservoir is full and the stream flow exceeds thedemand. &he excess water must therefore be discharged safely from the reservoir. In

many cases, to allow the water simply to overtop the dam would result in a catastrophicfailure of the structure. "or this reason, carefully designed overflow passages # +nown as-spillways- ("ig 6. are incorporated as part of the dam design. &he spillway capacity mustbe sufficient to accommodate the -largest- flood discharge (the robable ?aximum "loodor * in *7777 year flood li+ely to occur in the life of the dam. $ecause of the highvelocities of flow often attained on spillways, there is usually some form of energydissipation and scour prevention system at the base of the spillway. &his often ta+es theform of a stilling basin.

&here are several spillway designs. &he choice of design is a function of the nature of thesite, the type of dam and the overall economics of the scheme. &he following list gives a

general outline of the various types and applications:

(a overfall and -'gee- spillways are by far the most widely adopted: they may be usedon masonry or concrete dams which have sufficient crest length to obtain there%uired discharge!

(b chute and tunnel (shaft spillways are often used on earthfill dams!(c side channel and tunnel spillways are useful for dams sited in narrow gorges!(d siphon spillways maintain an almost constant headwater level over the designed

range of discharge.

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ra!ity ('"gee') spillways&hese are by far the most common type, being simple to construct and applicable over a

wide range of conditions. &hey essentially comprise a steeply sloping open channel witha rounded crest at its entry. &he crest profile approximates to the trajectory of the nappefrom a sharp crested weir. &he nappe trajectory varies with head, so the crest can becorrect only for one -design head- Hd. @ownstream of the crest region is the steeplysloping -face-, followed by the -toe-, which is curved to form a tangent to the apron orstilling basin at the base of the dam. &he profile is thus in the form of an elongated -S-("ig. 9. rofiles of spillways have been developed for a wide range of dam heights and&he discharge relationship for a spillway is of the same form as for other weirs:

6.*6.*d Hb4Hbg4xttancons, ==

&his e%uation is not dimensionless, and its magnitude increases with increasing depth offlow. 4 usually lies within the range *.9 A 4 A 0.5 in metric units. &he breadth, b, does notalways comprise a single unbro+en span. If control gates are incorporated in the scheme,the spillway crest is subdivided by piers into a number of -bays-. &he piers form thesupporting structure for the gates. &he piers have the effect of inducing a lateralcontraction in the flow. In order to allow for this effect in the discharge e%uation, the totalspan, b, isreplaced by bc, the contracted width:

bcB b # +nH

where n is the number of lateral contractions and + is the contraction coefficient, which isa function of Hand of the shape of the pier.

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Some other important aspects of spillway hydraulics are summarised below.

If H A Hd, the natural trajectory of the nappe falls below the profile of the spillway crest,then there will therefore be positive gauge pressures over the crest. 'n the other hand, ifH C Hd, then the nappe trajectory is higher than the crest profile, so negative pressure/ones tend to arise. "rictional shear will accentuate this tendency, and cavitation mayoccur.

However, in practice, this pressure reduction is not normally a serious problem unlessH C I.6 Hd. Indeed, recent wor+ suggests that separation will not occur until HC5 Hd.

(a 4onditions in the flow down the spillway face may be %uite complex, since:(b the flow is accelerating rapidly, and may be -expanding- as it leaves a bay#pier

arrangement!(c frictional shear promotes boundary layer growth!(d the phenomenon of self#aeration of the flow may arise!(e cavitation may occur

"or these reasons, the usual e%uations for non#uniform flow. If it is necessary to ma+eestimates of flow conditions on the spillway, then empirical data must be used. 3achfeature will now be examined in greater detail ("ig D.

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(i In a region of rapidly accelerating flow, the specific energy e%uation (or $ernoulli-se%uation is usually applied. It is possible to obtain very rough estimates of thevariation of and y down the spillway on this basis, accuracy will be slightlyimproved if a head loss tam is incorporated. Eevertheless, in the light of (ii and (iii,below, conditions on the spillway are far from those which under the energye%uations.

(ii ) boundary layer will form in the spillway flow, commencing at the leading edge ofthe crest. &he depth of the boundary layer will grow with distance downstream of thecrest. rovided that the spillway is of sufficient length, at some point the depth, will

meet the free surface of the water.&he flow at the crest is analogous to the flowround any fairly streamlined body. &his may imply flow separation, eddy shedding,or both. Such conditions may be instrumental in bringing about cavitation at thespillway face. &here have been a number of reported incidents of cavitation in majordams.

(iii )eration has been observed on many spillways. It entails the entrainment ofsubstantial %uantities of air into the flow, which becomes white and foamy inappearance. &he additional air causes an increase in the gross volume of the flow.'bservations of aeration have led to the suggestion that the point at which aerationcommences coincides with the point at which the boundary layer depth meets the

free surface (Henderson *F99, Geller *t astogi *FD6, 4ain *- 8ood *F*. &heentrainment mechanism appears to be associated with the emergence ofstreamwide vortices at the free surface. Such vortices would originate in the spillwaycrest region. ) rough estimate of the concentration of air may be made using thefollowing e%uations:

4 B 7.D=5 log (S7;%7.0 < 7.D9

where 4 B (volume of entrained air;(volume of air and water and S' is the i bedslope, or

4 B (x*;y*0;5;D=

where x* and y* are measured from the point at which entrainment com#mences. 4 may vary considerably over the width of the spillway.

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(iv 4avitation arises when the local pressure in a li%uid approaches the ambientvapour pressure. Such conditions may arise on spillways for a variety ofreasons, especially where the velocity of flow is high (say C 57 m;s. "orexample, if H C 5Hd, separated flow will probably arise at the crest.Irregularities in the surface finish of the spillway may result in the generation oflocal regions of low pressure. 4avitation has been observed in severely

sheared flows (Genn *FD*, Genn and 1arrod *F*.

)t the toe of the spillway, the flow will be highly supercritical. &he flow must bedeflected through a path curved in the vertical plane before entering the stilling/one or apron. &his can give rise to very high thrust forces on the base andside walls of the spillway. ) rough estimate of these forces may be made onthe basis of the momentum e%uation.

&he high velocity and energy of flow at the foot of the spillway must bedissipated, otherwise severe scouring will occur and the foundations at thetoe will be undermined.

Siphon spillways) siphon spillway is a short enclosed duct whose longitudinal section is curved asshown in "ig . 8hen flowing full, the highest point in the spillway lies above theli%uid level in the upstream reservoir, and the pressure at that point must thereforebe sub#atmospheric, this is the essential characteristic of a siphon. ) siphonspillway must be self#priming.

&he way in which this type of spillway functions is best understood by considering whathappens as the reservoir level gradually rises. 8hen the water level just exceeds thecrest level, the water commences to spill and flows over the downstream slope inmuch the same way as a simple 'gee spillway. )s the water level rises further, theentrance is sealed off from the atmosphere. )ir is initially trapped within the spillway,but the velocity of flow of the water tends to entrain the air (giving rise to aeration ofthe water and draw it out through the exit. 8hen all air has been expelled, the siphonis primed (i.e. running full and is therefore acting as a simple pipe.

&here are thus three possible operating conditions depending on upstream depth:(a gravity spillway flow!(b aerated flow!(c pipe (-bac+water- flow.

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'perational problems with siphon spillways. &he aerated condition is unstable and ismaintained only for a short time while the siphon begins to prime, since (theoretically, atleast air cannot enter once the entry is covered. &herefore, in a simple siphon a smallchange in H produces a sharp increase or decrease in the discharge through the

spillway. &his can lead to problems if the discharge entering the reservoir is greater thanthe spillway flow but less than the bac+water flow, since the following cycle of events isset in train:

(a If the spillway is initially operating with gravity flow, then the upstream (reservoirlevel must rise!

(b when the upstream level has risen sufficiently, the siphon primes and the spillwaydischarge increases substantially!

(c the upstream level falls until the siphon de#primes and its discharge drops.

&he cycle (a to (c is then repeated.

'bviously, this can give rise to radical surges and stoppages> in the downstreamflow. &his problem can be overcome by better design. 'ther potential problemsencountered with siphon spillways are:

(a bloc+age of spillway by debris (fallen trees, ice, ac.!(b free/ing of the water across the lower leg or the entry before the reservoir level

rises to crest level!(c the substantial foundations re%uired to resist vibration!(d waves arising in the reservoir during storms may alternately cover and uncover

the entry, thus interrupting smooth siphon action.

problem (a may be ameliorated by installing a trash#intercepting grid.

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Sha#t (morning glory) spillways&his type of structure consists of four parts ("ig F:(a circular weir at the entry!(b a flared transition which conforms to the shape of a lower nappe for asharp#crested weir!(b the vertical drop shaft!

(d the hori/ontal (or gently sloping outlet shaft.

&he discharge control may be at one of three points depending on the head:

(a 8hen the head is low the discharge will be governed at the crest. &his is analogousto weir flow and the discharge may therefore be expressed as

B 4 2 H5;0

where 2 is usually referred to the arc length at the crest. 4 is a function of E and thecrest diameter, it is therefore not a constant. &he magnitude of 4 is usually in the range

7.70.0, m. $elow the crest, the flow will tend to cling to the wall of the transition and ofthe drop shaft. &he outlet shaft will flow only partially full and is therefore, in effect, amopen channel.

(b )s the head increases, so the annular nappe must increase in thic+ness. 3ventually,the nappe expands to fill the section at the entry to the drop shaft. &he discharge is nowbeing controlled from this section, and this is often referred to as -orifice control-. &heoutlet tunnel is not designed to run full at this discharge.

(c "urther increase in head will induce bac+water flow throughout the drop and outletshafts. &he #H relationship must now conform to that for full pipe flow, and the weir will

in effect be -submerged-. &he head over the weir rises rapidly for a given increase indischarge, with a conse%uent danger of overtopping the dam.

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&he complete discharge#head characteristic is therefore as shown in "igure F. &he designhead is usually less than the head re%uired for bac+water flow. &his is done to leave amargin of safety for exceptional floods. 3ven so, the discharge which can be passed by ashaft or siphon spillway is limited, so great care must be exercised at the design stage.)n auxiliary emergency spillway may be necessary. It is also worth noting that as the flowenters the transition it tends to form a spiral vortex. &he vortex pattern must be minimised

in order to maintain a smoothly converging flow, so anti#vortex baffles or piers are oftenpositioned around the crest.

It may be undesirable for sub#atmospheric pressure to occur anywhere in the system,since this can lead to cavitation problems. &o avoid such problems the system may (aincorporate vents, (b be designed with an outlet shaft which is large enough to ensurethat the outlet end never flows full or (c have an outlet shaft with a slight negative slope,sufficient to ensure that the outlet does not flow full (and can therefore admit air.Some recent shaft spillways incorporate a ban+ of siphon spillways around the crest toreduce the rise in reservoir level for a given discharge. &rash grids must be installedaround the entrance to a shaft spillway to exclude large items of debris.

$nergy dissipators&he flow discharged from the spillway outlet is usually highly supercritical. If this flowwere left uncontrolled, severe erosion at the toe of the dam would occur, especially wherethe stream bed is of silt or clay. &herefore, it is necessary to dissipate much of theenergy, and to return the water to the normal (sub critical depth appropriate to thestream below the dam. &his is achieved by a dissipating or -stilling- device. &ypicaldevices of this nature are:

(a stilling basin!(b submerged buc+et!(c s+i jump;deflector buc+et.

Stilling basin) stilling basin consists of a short, level apron at the foot of the spillway ("igs *7 and **.It must be constructed of concrete to resist scour. It incorporates an integrally cast row ofchute bloc+s at the inlet and an integral sill at the outlet. Some designs also utilise a rowof baffle bloc+s part way along the apron ("ig *0.

&he function of the basin is to decelerate the flow sufficiently to ensure the formation ofan hydraulic jump within the basin. &he jump dissipates much of the energy, and returnsthe flow to the sub critical state. &he chute bloc+s brea+ the incoming flow into a series ofjets, alternate jets being lifted from the floor as they pass over the tops of the bloc+s. &hesill (or baffle bloc+s and sill provide the resistance re%uired to reduce energy and controlthe location of the jump. $affle bloc+s are not usually used where the velocity of theincoming flow exceeds 07 m;s, due to the li+elihood of cavitation damage.

rovided that the "roude Eumber of the incoming flow ("r0 exceeds =.6, a stable jumpcan be formed. However, if 0.6 A "r0 A =.6, then the jump conditions can be less welldefined and disturbances may be propagated downstream into the tail water. $ased on a

combination of operational and empirical data, a number of standard stilling basindesigns have been proposed. ) typical set of designs were published by the JS $ureauof eclamation.

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It is usually assumed that air entrainment ma+es little difference to the formation of thehydraulic jump in a stilling basin. &he stage#discharge characteristic of the tail water is afunction of the channel downstream of the dam. &he stilling basin should therefore givean hydraulic jump with a downstream depth#discharge characteristic which matches thatof the tail water.

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The submerged buc%et) submerged buc+et is appropriate when the tail water depth is too great for theformation of an hydraulic jump. &he buc+et is produced by continuing the radial arc at thefoot of the spillway to provide a concave longitudinal section as shown in "ig*5.

&he incoming high velocity from the spillway is thus deflected upwards. &he shear forcegenerated between this flow and the tail water leads to the formation of the -roller-motions. &he reverse roller may initially slightly scour the river bed downstream of thedam. However, the material is returned towards the toe of the dam, so the bed rapidlystabilises. &he relationship between the depths for the incoming and tail water flowscannot readily be derived from the momentum e%uation. 3mpirical relationships of anapproximate nature may be used for initial estimates.

( )( )

=66.7

0;5

*s0;**sb 3hg

%3h799=.7y

++=

Some submerged buc+ets are -slotted-. &his improves> energy dissipation, but may bringabout excessive scour at high tail water levels.

The s%i &umpde#lector buc%et&his type of dissipator has a longitudinal profile which resembles the submerged buc+et("ig *=. However the deflector is elevated above the tail water level, so a jet of water isthrown clear of the dam and falls into the stream well clear of the toe of the dam.Spillways may be arranged in pairs, and it is then usual for the designer to angle the jets

inwards so that they converge and collide in mid#air. &his brea+s up the jets, and is avery effective means of energy dissipation.

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Control gates8henever a discharge has to be regulated, some form of variable aperture or valve isinstalled. &hese are available in a variety of forms. 8hen gates are installed for channelor spillway regulation, they may be designed for -underflow- or -overflow- operation. &heoverflow gate is appropriate where logs or other debris must be able to pass through thecontrol section. Some typical gate sections are shown in "igure *6.

&he choice of gate depends on the nature of the application. "or example, the verticalgate has to be supported by a pair of vertical guides. &he gate often incorporates rollerwheels on each vertical side, so that the gate moves as smoothly as possible in theguides. 3ven so, once a hydrostatic load is applied. a considerable force is needed toraise or lower the gate. "urthermore, in severe climates, icing may cause jamming of therollers.

&he radial (&ainter gate consists of an arc#shaped face plate supported on braced radial

arms. &he whole tructure rotates about the centre of arc on a hori/ontal shaft whichtransmits the hydrostatic load to the supporting structure. Since the vector of the resultanthydrostatic load passes through the shaft axis, no moment is applied. &he hoistmechanism has therefore only to lift the mass of the gate. &ainter gates are economical toinstall, and are widely used in overflow and underflow formats. 'ther gates (such as thedrum or the roller gate are expensive, and have tended to fall out of use.

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