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 structures form part of most major water engineering schemes, for irrigation, water supply, drainage, sewage treatment, hydropower. It is convenient to group the structures under three headings:

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

For most of these structures the depth-discharge relationship is based on the Bernoulli (or specific energy) equation. 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) weirsThis type of device is formed from plastic or metal plate of a suitable gauge. The plate is set vertically and spans the full width of the channel. The weir itself is incorporated into the top of the plate. The geometry of the weir depends on the precise nature of the application. In this section we will concentrate on two basic forms, the rectangular weir and the vee (or triangular) weir. However, other forms are available, such as the compound weir.

The primary purpose of a weir is to measure discharge. Once the upstream water level exceeds the crest height PS, water will flow over the weir. As the depth of water above the weir (h1) increases, the discharge over the weir increases correspondingly. Thus, if there is a known relationship between h, and Q, we need only to measure h, in order to deduce Q. The 'ideal' relationship between h, and Q may readily be derived for each weir shape on the basis of the Bernoulli equation. If these relationships are compared, it is evident that the triangular weir possesses greater sensitivity at low flows, whereas the rectangular weir can be designed to pass a higher flow for a given head and channel width.

Rectangular weirsThere are two types of rectangular weir (see Fig.1):

(a) 'uncontracted' or full width weirs comprise a plate with a horizontal crest extending from one side of the channel to the other - the crest section is as illustrated in the figure.

(b) a 'contracted' weir, by contrast, has a crest width which is less than the channel width.

Since the operation of a weir is based on the use of a gauged depth to estimate the discharge, we must know how these two quantities are related. The actual flow over a weir is quite complex, involving a three-dimensional velocity pattern as well as viscous effects. The simplest method of developing a numerical model which represents a weir is to use the Bernoulli equation as a starting point. An idealised relationship between depth and discharge is obtained. This relationship can then be modified to take account of the differences between ideal and real flows.

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Long-based weirsIn contrast to plate weirs, long-based weirs are larger and generally more heavily constructed (e.g. from concrete). They are usually designed for use in the field, and consequently may have to handle large discharges. An ideal long-based weir has the following 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 requires a minimum of maintenance.

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

The rectangular ('broad-crested') weirRectangular weirs are solid weirs of rectangular cross section, which span the full width of a channel (Fig 2.). The following facts apply:

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

(b) Given a hump of sufficient height, critical flow will be produced in the flow over the hump. There is then a direct relationship between Q and h, i.e. the flow is modular. Long-based weirs are designed for modular flow.

By definition, a rectangular weir is not streamlined. This, in turn, implies that the streamlines at the upstream end of the weir will not be parallel, since the flow will be accelerating. If frictional resistance is ignored, then the streamlines will become parallel and the flow become critical given a sufficient length of crest. It is then possible to derive a straightforward performance under submerged conditions than other long-based weirs.

Crump WeirsE.S. Crump published details of a weir with a triangular profile, which has been developed at the HRS. This was claimed to give a wide modular range, and also give a more predictable performance under submerged conditions than other long based weirs. See Figure 3.

Crump proposed upstream and downstream slopes of 1:2 and 1:5. respectively, which were based on sound principles. The upstream slope was designed so that sediment build-up would not reach the crest. The downstream slope was shallow enough to permit an hydraulic jump to form on the weir under modular flow conditions, thus providing an integral energy dissipator. Also, under submerged conditions, losses are not too high and the afflux is minimised. The primary gauging station is upstream of the weir. However, there is a second gauging point on the weir itself, just downstream of the crest. The second reading is used when the weir is submerged. The accuracy of the weir depends on the sharpness of the crest, so some weirs incorporate a metal insert at the crest. The secondary gauging tappings are drilled into the insert. The discharge equation for a Crump weir is in the form:

Which is clearly based on the same concepts as the corresponding equation for a rectangular weir. The value of Cd is about 0.63. The value of CV varies with the ratio h'/(h' + Ps).

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Fig 3

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FlumesThis 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)

The first type of flume is known as a venturi flume, Figure 4. Flumes are usually designed to achieve critical flow in the narrowest (throat) section, together with a small afflux. Flumes" are "specially applicable where deposition of solids must be avoided (e.g. in sewage works or m irrigation canals traversing flat terrain).A general equation for the ideal discharge through a flume may be developed on the basis of the energy and continuity principles.

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SpillwaysThe majority of impounding reservoirs are formed as a result of the construction of a dam. By its very nature, the stream flow which supplies a reservoir is variable. It follows that there will be times when the reservoir is full and the stream flow exceeds the demand. The 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 catastrophic failure of the structure. For this reason, carefully designed overflow passages - known as 'spillways' (Fig 5.) are incorporated as part of the dam design. The spillway capacity must be sufficient to accommodate the 'largest' flood discharge (the Probable Maximum Flood or 1 in 10000 year flood) likely to occur in the life of the dam. Because of the high velocities of flow often attained on spillways, there is usually some form of energy dissipation and scour prevention system at the base of the spillway. This often takes the form of a stilling basin.

There are several spillway designs. The choice of design is a function of the nature of the site, the type of dam and the overall economics of the scheme. The following list gives a general outline of the various types and applications:

(a) overfall and 'Ogee' spillways are by far the most widely adopted: they may be used on masonry or concrete dams which have sufficient crest length to obtain the required 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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Gravity ('Ogee') spillwaysThese are by far the most common type, being simple to construct and applicable over a wide range of conditions. They essentially comprise a steeply sloping open channel with a rounded crest at its entry. The crest profile approximates to the trajectory of the nappe from a sharp crested weir. The nappe trajectory varies with head, so the crest can be correct only for one 'design head' Hd. Downstream of the crest region is the steeply sloping 'face', followed by the 'toe', which is curved to form a tangent to the apron or stilling basin at the base of the dam. The profile is thus in the form of an elongated 'S' (Fig. 6). Profiles of spillways have been developed for a wide range of dam heights and The discharge relationship for a spillway is of the same form as for other weirs:

This equation is not dimensionless, and its magnitude increases with increasing depth of flow. C usually lies within the range 1.6 < C < 2.3 in metric units. The breadth, b, does not always comprise a single unbroken span. If control gates are incorporated in the scheme, the spillway crest is subdivided by piers into a number of 'bays'. The piers form the supporting structure for the gates. The piers have the effect of inducing a lateral contraction in the flow. In order to allow for this effect in the discharge equation, the total span, b, is replaced by bc, the contracted width:

bc = b - knH

where n is the number of lateral contractions and k is the contraction coefficient, which is a function of H and of the shape of the pier.

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Some other important aspects of spillway hydraulics are summarised below.

If H < 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. On the other hand, if H > Hd, then the nappe trajectory is higher than the crest profile, so negative pressure zones tend to arise. Frictional shear will accentuate this tendency, and cavitation may occur.

However, in practice, this pressure reduction is not normally a serious problem unless H > I.5 Hd. Indeed, recent work suggests that separation will not occur until H>3 Hd.

(a) Conditions in the flow down the spillway face may be quite 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

For these reasons, the usual equations for non-uniform flow. If it is necessary to make estimates of flow conditions on the spillway, then empirical data must be used. Each feature will now be examined in greater detail (Fig 7).

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(i) In a region of rapidly accelerating flow, the specific energy equation (or Bernoulli's 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 tam is incorporated. Nevertheless, in the light of (ii) and (iii), below, conditions on the spillway are far from those which under the energy equations.

(ii) A boundary layer will form in the spillway flow, commencing at the leading edge of the crest. The depth of the boundary layer will grow with distance downstream of the crest. Provided that the spillway is of sufficient length, at some point the depth, will meet the free surface of the water.The flow at the crest is analogous to the flow round any fairly streamlined body. This may imply flow separation, eddy shedding, or both. Such conditions may be instrumental in bringing about cavitation at the spillway face. There have been a number of reported incidents of cavitation in major dams.

(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 an increase in the gross volume 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 (Henderson 1966, Keller 1t Rastogi 1975, Cain 1' Wood 1981). The entrainment mechanism appears to be associated with the emergence of streamwide vortices at the free surface. Such vortices would originate in the spillway crest region. A rough estimate of the concentration of air may be made using the following equations:

C = 0.743 log (S0/q0.2) + 0.876

where C = (volume of entrained air)/(volume of air and water) and SO is the i bed slope, or

C = (x1/y1)2/3/74

where x1 and y1 are measured from the point at which entrainment com-mences. C may vary considerably over the width of the spillway.

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(iv) Cavitation arises when the local pressure in a liquid approaches the ambient vapour pressure. Such conditions may arise on spillways for a variety of reasons, especially where the velocity of flow is high (say V > 30 m/s). For example, if H > 3Hd, separated flow will probably arise at the crest. Irregularities in the surface finish of the spillway may result in the generation of local regions of low pressure. Cavitation has been observed in severely sheared flows (Kenn 1971, Kenn and Garrod 1981).

At the toe of the spillway, the flow will be highly supercritical. The flow must be deflected through a path curved in the vertical plane before entering the stilling zone or apron. This can give rise to very high thrust forces on the base and side walls of the spillway. A rough estimate of these forces may be made on the basis of the momentum equation.

The high velocity and energy of flow at the foot of the spillway must be dissipated, otherwise severe scouring will occur and the foundations at thetoe will be undermined. |

Siphon spillwaysA siphon spillway is a short enclosed duct whose longitudinal section is curved as shown in Fig 8. When flowing full, the highest point in the spillway lies above the liquid level in the upstream reservoir, and the pressure at that point must therefore be sub-atmospheric, this is the essential characteristic of a siphon. A siphon spillway must be self-priming.

The way in which this type of spillway functions is best understood by considering what happens as the reservoir level gradually rises. When the water level just exceeds the crest level, the water commences to spill and flows over the downstream slope in much the same way as a simple Ogee spillway. As the water level rises further, the entrance is sealed off from the atmosphere. Air is initially trapped within the spillway, but the velocity of flow of the water tends to entrain the air (giving rise to aeration of the water) and draw it out through the exit. When all air has been expelled, the siphon is primed (i.e. running full) and is therefore acting as a simple pipe.

There are thus three possible operating conditions depending on upstream depth:(a) gravity spillway flow; (b) aerated flow; (c) pipe ('backwater') flow.

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Operational problems with siphon spillways. The aerated condition is unstable and is maintained only for a short time while the siphon begins to prime, since (theoretically, at least) air cannot enter once the entry is covered. Therefore, in a simple siphon a small change in H produces a sharp increase or decrease in the discharge through the spillway. This can lead to problems if the discharge entering the reservoir is greater than the spillway flow but less than the backwater flow, since the following cycle of events is set in train:

(a) If the spillway is initially operating with gravity flow, then the upstream (reservoir) level must rise;

(b) when the upstream level has risen sufficiently, the siphon primes and the spillway discharge increases substantially;

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

The cycle (a) to (c) is then repeated.

Obviously, this can give rise to radical surges and stoppages" in the downstream flow. This problem can be overcome by better design. Other potential problems encountered with siphon spillways are:

(a) blockage of spillway by debris (fallen trees, ice, ac.);(b) freezing of the water across the lower leg or the entry before the reservoir level

rises to crest level;(c) the substantial foundations required 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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Shaft (morning glory) spillwaysThis type of structure consists of four parts (Fig 9):(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 horizontal (or gently sloping) outlet shaft.

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

(a) When the head is low the discharge will be governed at the crest. This is analogous to weir flow and the discharge may therefore be expressed as

Q = C L H3/2

where L is usually referred to the arc length at the crest. C is a function of N and the crest diameter, it is therefore not a constant. The magnitude of C is usually in the range 0.02.2, m. Below the crest, the flow will tend to cling to the wall of the transition and of the drop shaft. The outlet shaft will flow only partially full and is therefore, in effect, am open channel.

(b) As the head increases, so the annular nappe must increase in thickness. Eventually, the nappe expands to fill the section at the entry to the drop shaft. The discharge is now being controlled from this section, and this is often referred to as 'orifice control'. The outlet tunnel is not designed to run full at this discharge.

(c) Further increase in head will induce backwater flow throughout the drop and outlet shafts. The Q-H relationship must now conform to that for full pipe flow, and the weir will in effect be 'submerged'. The head over the weir rises rapidly for a given increase in discharge, with a consequent danger of overtopping the dam.

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The complete discharge-head characteristic is therefore as shown in Figure 9. The design head is usually less than the head required for backwater flow. This is done to leave a margin of safety for exceptional floods. Even so, the discharge which can be passed by a shaft or siphon spillway is limited, so great care must be exercised at the design stage. An auxiliary emergency spillway may be necessary. It is also worth noting that as the flow enters the transition it tends to form a spiral vortex. The vortex pattern must be minimised in order to maintain a smoothly converging flow, so anti-vortex baffles or piers are often positioned around the crest.

It may be undesirable for sub-atmospheric pressure to occur anywhere in the system, since this can lead to cavitation problems. To avoid such problems the system may (a) incorporate vents, (b) be designed with an outlet shaft which is large enough to ensure that 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 bank of siphon spillways around the crest to reduce the rise in reservoir level for a given discharge. Trash grids must be installed around the entrance to a shaft spillway to exclude large items of debris.

Energy dissipatorsThe flow discharged from the spillway outlet is usually highly supercritical. If this flow were left uncontrolled, severe erosion at the toe of the dam would occur, especially where the stream bed is of silt or clay. Therefore, it is necessary to dissipate much of the energy, and to return the water to the normal (sub critical) depth appropriate to the stream below the dam. This is achieved by a dissipating or 'stilling' device. Typical devices of this nature are:

(a) stilling basin; (b) submerged bucket; (c) ski jump/deflector bucket.

Stilling basinA stilling basin consists of a short, level apron at the foot of the spillway (Figs 10 and 11). It must be constructed of concrete to resist scour. It incorporates an integrally cast row of chute blocks at the inlet and an integral sill at the outlet. Some designs also utilise a row of baffle blocks part way along the apron (Fig 12).

The function of the basin is to decelerate the flow sufficiently to ensure the formation of an hydraulic jump within the basin. The jump dissipates much of the energy, and returns the flow to the sub critical state. The chute blocks break the incoming flow into a series of jets, alternate jets being lifted from the floor as they pass over the tops of the blocks. The sill (or baffle blocks and sill) provide the resistance required to reduce energy and control the location of the jump. Baffle blocks are not usually used where the velocity of the incoming flow exceeds 20 m/s, due to the likelihood of cavitation damage.

Provided that the Froude Number of the incoming flow (Fr2) exceeds 4.5, a stable jump can be formed. However, if 2.5 < Fr2 < 4.5, then the jump conditions can be less well defined and disturbances may be propagated downstream into the tail water. Based on a combination of operational and empirical data, a number of standard stilling basin designs have been proposed. A typical set of designs were published by the US Bureau of Reclamation.

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It is usually assumed that air entrainment makes little difference to the formation of the hydraulic jump in a stilling basin. The stage-discharge characteristic of the tail water is a function of the channel downstream of the dam. The stilling basin should therefore give an hydraulic jump with a downstream depth-discharge characteristic which matches that of the tail water.

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The submerged bucketA submerged bucket is appropriate when the tail water depth is too great for the formation of an hydraulic jump. The bucket is produced by continuing the radial arc at the foot of the spillway to provide a concave longitudinal section as shown in Fig13.

The incoming high velocity from the spillway is thus deflected upwards. The shear force generated between this flow and the tail water leads to the formation of the 'roller' motions. The reverse roller may initially slightly scour the river bed downstream of the dam. However, the material is returned towards the toe of the dam, so the bed rapidly stabilises. The relationship between the depths for the incoming and tail water flows cannot readily be derived from the momentum equation. Empirical relationships of an approximate nature may be used for initial estimates.

Some submerged buckets are 'slotted'. This improves" energy dissipation, but may bring about excessive scour at high tail water levels.

The ski jump/deflector bucketThis type of dissipator has a longitudinal profile which resembles the submerged bucket (Fig 14). However the deflector is elevated above the tail water level, so a jet of water is thrown 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. This breaks up the jets, and is a very effective means of energy dissipation.

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Control gatesWhenever a discharge has to be regulated, some form of variable aperture or valve is installed. These are available in a variety of forms. When gates are installed for channel or spillway regulation, they may be designed for 'underflow' or 'overflow' operation. The overflow gate is appropriate where logs or other debris must be able to pass through the control section. Some typical gate sections are shown in Figure 15.

The choice of gate depends on the nature of the application. For example, the vertical gate has to be supported by a pair of vertical guides. The gate often incorporates roller wheels on each vertical side, so that the gate moves as smoothly as possible in the guides. Even so, once a hydrostatic load is applied. a considerable force is needed to raise or lower the gate. Furthermore, in severe climates, icing may cause jamming of the rollers.

The radial (Tainter) gate consists of an arc-shaped face plate supported on braced radial arms. The whole tructure rotates about the centre of arc on a horizontal shaft which transmits the hydrostatic load to the supporting structure. Since the vector of the resultant hydrostatic load passes through the shaft axis, no moment is applied. The hoist mechanism has therefore only to lift the mass of the gate. Tainter gates are economical to install, and are widely used in overflow and underflow formats. Other gates (such as the drum or the roller gate) are expensive, and have tended to fall out of use.

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