hydraulic design of flow measuring structure

8/2/2019 Hydraulic Design of Flow Measuring Structure

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21.1 INTR DU TI

Experienced water providers and users can use this chapter as a quick review of hydraulicprinciples related to water measurement and its relation to hydraulic design for environ-mental considerations.

The hydraulic design of flow measuring structures usually confronts the engineer withtwo opportunities. One is the design of measurement structures in a retrofit situation andthe other is in original project design. The retrofit mode is usually difficult and requiresmuch innovation just to obtain passable function within the space and sizing limitationsand other constraints usually imposed. Because of the increasing emphasis on quantifyingflow rates and volumes in most aspects of water resource planning and management, theretrofit applications currently dominate the design problems.

Most textbooks deal with recommending ideal installation situations and retrofit pro-jects appear to be unable to comply without great economic impact. This too frequentlycan lead to arbitrary compromises that produce poor measurement performance. Evennew installations may be limited by space requirements. This may force design decisionsinto the final construction that compromise accuracy. This chapter will strive to show thedesign concepts available, particularly those useful for designing both new and retrofitinstallations, and will point out measurement behaviors to be expected from various com-promises. This chapter suggests those deviations that cause least impact and guides thedesigner to choices that may be hydraulically acceptable and still meet structural goals.

Of the numerous flowmetering methods available to the hydraulic engineer, most are

based on well-established hydraulic principles and are amenable to design manipulationsof size, shape, and response. While this aspect of flow measurement is documented in sev-eral handbooks and texts, the design and retrofit of sites to accommodate and facilitatemeasurement is not as well described or is described in a scattered assortment of booksand articles.

Pipeline flows of water are usually less complicated to measure than open-channelflows, most obviously because the flow area does not change significantly with flow rate.

CHAPTER 21

HYDRAULIC DESIGN OFFLOW MEASURING

STRUCTURES

21.1

John A. Replogle and Albert J. ClemmensUSDAARS Water Conservation Laboratory

Phoenix, Arizona

Clifford A. PughUS Bureau of Reclamation,

Denver, Colorado

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.

Any use is subject to the Terms of Use as given at the website.

Source: HYDRAULIC DESIGN HANDBOOK


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Consequently, many applications of pipeline flows are held to stricter accuracy standardsthan channel flows can reasonably achieve. Thus, channel flows and their measurementare usually limited to large delivery volumes and to accuracies acceptable to the related

activities, such as sewer flows and irrigation deliveries.The purpose of this chapter is to consolidate design information for evaluating a flow

measurement site, selecting a flow measuring system, and adapting the measuring site tooptimize measuring and other functions that may be desired from the site. Emphasis willbe on open-channel flow measurements because that is a likely need of the hydraulic engi-neer. Pipe flowmeters in water supply will also be discussed in lesser detail because themajor application of the many types of pipe flowmeters is well covered in the chemicaland petroleum industry literature.

Experienced readers may wish to further investigate and seek more advanced refer-ences in hydraulics and fluid mechanics. Extensive information on fluid meter theory and

detailed material for determining coefficients for tube-type meters is given in AmericanSociety, of Mechanical Engineers (ASME) (1959, 1971) and revisited with modernupdates in books by Spitzer (1990) and Miller (1996). Brater and King (1982) have a thor-ough discussion of general critical depth relations and detailed relationships for mostcommon hydraulic flow section shapes in open channels. Bos (1989) covers a broad seg-ment of open channel water measurement devices.

21.2 HYDRAULIC CONCEPTS RELATED TO WATER

MEASUREMENT

21.2.1 Basic Concepts for Pipe and Channel Flows

Flow can be classified into closed conduit flow and open-channel flow. Open-channelflow conditions occur whenever the flowing stream has a free or unconstrained surfacethat is open to the atmosphere. Flows in canals or in vented pipelines that are not flowingfull are typical examples.

In hydraulics, a pipe is any closed conduit that carries water under pressure. The filledconduit may be square, rectangular, or any other shape, but is usually round. If flow isoccurring in a conduit but does not completely fill it, the flow is not considered pipe orclosed conduit flow, but is classified as open-channel flow.

Flow rate in a pipeline responds mainly to the pressure gradient or head difference thatexists between two points along the pipeline, modified by the frictional resistance to flowcaused by pipe length, pipe roughness, bends, restrictions, changes in conduit shape andsize, the nature of the fluid flowing, and the cross-sectional area of the pipe.

In open-channel flows, the pressure gradient, or energy grade line, is controlledmainly by the force due to gravity, which is influenced by the channel slope, resistancefrom the channel wall roughness, the channel shape, and the flow area. The fluid is usu-ally water.

Basic flow metering in both pipe flow and open channels depends on determining an

average flow velocity by some means and combining it with the flow cross-sectional area.For open channels, a common means involves current meter measurements where meteredpoint velocities are applied to their applicable subareas and summed over a flow cross sec-tion. Exceptions include tracer-dilution techniques that do not require flow area or veloc-ity. The uses of tracer techniques are applicable to special pump calibrations and some dif-ficult channel flows (mountain streams). They are avoided for most city water distributionsystems, sewer flows and irrigation applications because of the general expense with han-

21.2 Chapter Twenty-One



HYDRAULIC DESIGN OF FLOW MEASURING STRUCTURES


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dling the equipment and doing the analysis. The most used techniques applicable to open-channel systems, including sewer flows and irrigation canal flow measurements, dependon exploiting the special velocity properties of critical flow, as discussed in a section

21.2.3.

ontinuity equation. The first basic equation for water flowing in either pipes or chan-nels is the continuity equation, which simply states that discharge rate (volumetric flowper unit time), Q, is equal to flow cross-sectional area,A, times flow mean velocity, V,through the flow cross section, or

Q AV (21.1)

Bernoulli energy equation. Another basic equation involves energy relations and isalso applicable to both pipe and channel flows. The most familiar form is for closed pipeflow, wherein the basic energy principles are described by the Bernoulli energy equation.For two locations along a pipe at stations 1 and 2,(Fig. 21.1), the Bernoulli equation canbe expressed as

z1 h1 V

g12

z2 h2 V

g22

constant (21.2)

where the terms are expressed in length dimensions as z the height from an arbitraryreference plane (datum) h the pressure head V average velocity through the pipecross-section at the designated location V

2/2g the velocity head the gravitational

constant 1 ,2 subscripts denoting the respective locations along the pipeline.

This equation is based on uniform velocity across the conduit area and no energy loss-es. However, in real fluid flows, nonuniform velocities exist and friction causes energyconversion to heat. Typically, these velocities are zero at the walls and reach a maximumprofile velocity near the center of the flow. If the flow is viscous flow in a round pipe, theflow profile is parabolic, that is, bullet-shaped. If the velocity is fully turbulent, the bul-let-shape is much flattened, with steep velocity gradients near the wall and nearly uniformprofile across the remainder of the pipe. These idealized profiles can be skewed drastical-ly by regulating valves, structures, conduit bends and other flow obstructions. Therefore,

application of these equations depends on knowing, or controlling, the velocity profile sothat the average velocity in the conduit cross section can be inferred.

Hydraulic Design of Flow Measuring Structures 21.3

FIGURE 21.1 Energy balance in pipe flow.





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Equation (2.2) requires some adjustments to convert it to the energy equation, whichis useful in analyzing flows in pipes or open channels with a small slope (Chow, 1959).First we introduce correction factors, 1 and 2, called the velocity distribution coeffi-cients, to account for the computational expediency of using the average velocities, V1 andV2, to compute the kinetic energy term, V/2g, at the respective locations 1 and 2 along achannel. These values for the usual range of turbulent flows in water usually range fromabout 1.01 to 1.05, although for thick petroleum products in pipe flows and low velocityflows, the value can approach a value of 2. Second, a term, h , for the loss of energybetween the two points is included. The result is

z1 h1 1 V

g12

z2 h2 V

g22

h (21.3)

21.2.2 Pipe Hydraulics

Reynolds number. The behavior of flow in pipes is governed primarily by the viscosity ofthe fluid. In pipeline flows, the ratio between the dynamic forces and the viscus forces isimportant for defining the limits between laminar and turbulent flows and other functionsof pipe flow. This ratio is called theReynolds number,Rn, and is defined as

Rn V

v

L (21.4)

where V the velocity of the flow,Lc characteristic length, typically the pipe diame-

ter,D and v the kinematic viscosity.

Headloss characteristics in pipes. The Reynolds number, Rn, defined above, repre-sents the effect of viscosity relative to inertia and is used to define appropriate flow rangesfor headloss equations in pipe flow. For example, headloss is proportional to the square ofthe velocity, when the velocities and pipe size combinations defined by a pipe-diameter-based Reynolds number,Rn, greater than about 1000. Most of the flows of interest in gen-eral hydraulic engineering have Reynolds numbers greater than 1000. Some exceptionsare found in drip or trickle irrigation systems common in agricultural and urban landscapesettings.

The headloss, hf forRn greater than the minimum value of about 1000 is traditionallyexpressed in terms of a friction factor, the pipe diameter,D, pipe length,L and the veloc-ity head, V/2g, where g is the gravitational constant, and Vis the average velocity, as

h fDL

Vg

2

(21.5)

The value forfis usually obtained from a Moody diagram which is a graphical repre-sentation of thefvalue in terms of the Reynolds number, the roughness height of the pipewall material, , and the pipe diameter,D. TheMoody diagram is a graphical solution ofthe Colebrook function

1

f log

3/D

R

2

n51

f

(21.6)

The values range from 0.0000015 m for smooth plastic pipe to 0.00026 m for castiron pipe. Concrete pipe ranges from about 0.0003 m to 0.003 m (Daugherty andIngersoll, 1954). The equation can be readily solved by iteration techniques using a com-puter spreadsheet.






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21.2.3 Channel Hydraulics

Hydraulic mean depth. The hydraulic mean depth, Dm [U.S. Bureau of Reclamation

(USBR), 1997] is the flow cross-sectional area,A, divided by the flow surface width, T,

Dm AT (21.7)

For conduits such as pipes flowing nearly full, the surface flow width may be narrow,and Dm may be a larger value than the physical water depth. For the usual natural chan-nels and most canals, Dm is interchangeable with average depth. Sometimes it is simplycalled the ydraulic depth (Chow, 1959).

Froude number. Open-channel flow behavior is governed primarily by gravity forces.

The ratio of the inertial forces to the gravity forces is called the Froude number, Fr, andis defined by

Fr

V

Dm(21.8)

where V the velocity of the flow, g the gravitational constant, Dm the hydraulicmean depth.

The Froude number applies to most open channel flows and is used for defining modelscale ratios and estimating stable flow characteristics in open channels.

Specific energy. It is useful to define the energy equation in terms of the local channelbottom instead of an arbitrary datum. This is called the specific energy, E, and is given by:

Ey 2Vg (21.9)

That is, the specific energy is equal to the sum of the depth of flowy and the velocityhead (Fig. 21.2).

Critical flow and critical depth. In open channels a flow phenomenon occurs thatdoes not happen in closed pipe flows. The process is called critical flow. Critical flow isdefined for open channel flows as the maximum discharge for the minimum specific

energy, that is, critical flow represents the minimum combination of potential energy(depth of flow,y) and kinetic energy (velocity head, V

2/2g) for the given discharge (Chow,

1959). The depth of flow then is the critical depth. By virtue of the continuity equation,for a constant discharge at critical flow, an increase in depth must necessarily be accom-panied by a decrease in velocity, which is called subcritical velocity. Conversely, adecrease in depth for the same flow rate necessarily requires an increase in velocity, whichis called supercritical velocity.

When critical flow occurs in an open channel it can be shown (Chow, 1959) that

V

gc

Dm (21.10)

where V mean flow velocity, g gravitational constant,Dm hydraulic mean depth,and velocity distribution coefficient (Chow, 1959).

This can further be combined with the continuity equation, Eq. (21.4), to express thecritical flow discharge rate, Qc, as






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Q A D

m

g (21.11)

In practice, the water surface slope in a contraction is relatively steep and the pre-cise plane of the critical flow section is not easily or reliably located. Thus, the datafor accurately evaluating the hydraulic depth, Dm, is not readily obtained. For criticalflow flumes, the flow depth is therefore not measured at this critical section, butinstead a depth is measured in the upstream channel, where the velocity head is com-putable or is minimal. The critical depth is then mathematically derived based on ener-gy principles described by Bos et al. (1991). These flumes, sometimes referred to asthe computable flumes that rely on critical flow theory, will be discussed in more detailin Section 21.7.

For maximum discharge for minimum energy, the condition described above for criti-cal flow in open channels, it can be shown that

V2g

c2

A

Dm(21.12)

where:A = the channel flow area, T= the top width of the channel flow,Dm = the hydraulicmean depth, and Vc = the critical velocity.

Thus, the velocity head at critical flow is equal to half the hydraulic mean depth, some-

times called hydraulic depth Dm A/T(Chow, 1959).From the above,

V

D

c

1 Fr (21.13)

where Fr is the Froude number defined above. Thus, at critical flow the Froude number isunity. Also note that the Froude number can be defined by Fr V Vc for velocities otherthan critical.

Normal depth. Yet another depth is associated with open channel flows, the nor-mal depth. When the flow in an open channel does not change from station to station,

the flow is said to be uniform and the bottom slope, the ydraulic grade line, and theenergy grade line are all parallel to each other. Figure 21.2 shows the condition whenthe flow is not uniform.

Modular limit. If the downstream depth in a channel is too deep, the backwater willprevent critical depth from occurring. The flow is considered to be submerged wheneverthe downstream water surface exceeds the crest elevation of a channel control, such as a


FIGURE 21.2 Specific energy balance.





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weir or flume. For flumes, particularly, this submergence has little effect on critical depth,and free flow exists until a certain limiting submergence for that particular flow modulecalled the modular limit is reached. At some point of submergence, the upstream flow

depth is affected. and the modular limit is exceeded, and free flow does not occur. Themodular limitis defined as that limiting submergence ratio, and is based on the ratio ofthe downstream depth to upstream depth. The modular limit occurs when the downstreambackwater causes more than1 percent change in the calculated discharge in a particularflow module, or device (Bos, 1989). When the modular limit is exceeded, the flow iscalled nonmodular.

21.2.4 Energy Balance Relationships in Channels

Hydraulic problems concerning fluid flow are commonly described in terms of conserv-ing kinetic and potential energy, and are conveniently expressed using the classicalBernoulli equation in combination with the Continuity equation. The applications of theseequations are generally well documented, particularly for pipe flows, in texts and hand-books and are not repeated here (Brater and King, 1982; Miller, 1996). The case for openchannels is less complete, but is given considerable treatment in Brater and King (1982),Chow (1959), and Herschy (1985). The computational uncertainties evolve from theeffects of friction and viscosity that distort the classic assumptions of a uniform velocityprofile across the fluid stream. When accountings for friction and flow profile are suc-cessfully applied, the results for discharge computations are usually good to excellent forboth pipes and open channels (Bos et al., 1991).

Headloss characteristics in channels. In terms of frictional headlosses, the wettedperimeter, Pw, of the flow is important.Hydraulic radius, R , is defined as the area of theflow section,A, divided by the wetted perimeter, Pw,

R P

A

w

(21.14)

Conversely, the wetted perimeter times the hydraulic radius is equal to the area of anirregular flow section. The hydraulic radius of a channel can be compared to the radius ofa pipe, r, with a cross-sectional areaA r2 and a circumference or wetted perimeter Pw

r. Under these conditions, the hydraulic radius compares to the pipe radius, and tothe pipe diameter,D, as

Rh 2

r

D (21.15)

The Mannings formula. anal and stream discharge rates are usually estimated withuse of the Mannings formula. Many open-channel flow equations have been proposed,but the most used is the Mannings formula. This expression is partly rational and uses anempirical coefficient, , that is used in both the SI and American unit systems. In generalform it becomes

V

C

R h2/3

Se1/2

(21.16)

where V average velocity, n the Mannings roughness coefficient,Rh the hydraulicradius, Se the energy-line slope, and Cm conversion of units: 1.0 for metric units and1.486 for American units.

The factor S is the slope of the energy line. Note that the bed slope of the channel, So,and the slope of the water surface, S , are not to be used. These parameters are, however,equal to S when uniform flow, with the resulting normal depth occurs. As defined above,






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normal depth occurs when a channel flow approaches uniformity from station to stationalong the channel (Chow, 1959).

For design purposes, the value for concrete lined canals is usually about 0.014. Agood finish can lower it to 0.012, while concrete in poor condition and channels con-

structed with shot crete or gunite, usually have n values from 0.016 to 0.018. In some

instances, concrete lined canals, with significant algae growth, have experienced n values

as high as 0.032. This latter value approaches the values usually experienced with unlined

channels, 0.030.04. Thus, for reliable application, the use of Mannings formula requires

field experience and on-site inspection of the channel being computed.

21.2.5 Modeling Characteristics for Open Channels

For flowing water in open channels, fluid friction is a factor as well as gravity and iner-

tia. This would seem to present a problem for hydraulic scale modeling, because both

dynamic and kinematic similarity are difficult to achieve simultaneously. Fortunately for

most open-channel flows, there is usually fully developed turbulence. Thus, the fluid fric-

tion losses are nearly proportional to V2, and are nearly independent of Reynolds number,

Rn, with rare exceptions.

This means that in openchannel flows, inertia and gravity forces dominate over vis-

cous forces (associated with pipe flows) and are a function of the Froude number, Fn,

alone. Geometric similarity between a model and a prototype then provides kinematic

similarity. For kinematic similarity the ratios of the respective velocities are everywherethe same. The velocity ratio, Vr, is the velocity in the prototype, Vp, divided by the veloc-

ity in the model, Vm, or

Vr V

V

m

p (21.17)

For Froude modeling, and from the definition for Fn, we note that Vis proportional

to the square-root of a length, L (for open channels we used the hydraulic depth, Dm)

with the gravitational constant, g, assumed to be constant. Thus, the above equation can

be written as

Lr VV

m

(21.18)

whereLr = the length ratio between prototype and model dimensions, Lp: L

Because the velocity varies as Lr and the cross-sectional area as Lr2 it followsthat

Qp Qm Lr5/2 : 1 (21.19)

or

Q

Q

m

p

L

L

m

p

/2(21.20)

This equation is valid when all the physical structure dimensions and the heads are ofthe same ratio. For example, it can be used to convert a flume rating for one size to that






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of a similar flume of another size. Scale modeling works best for determining calibrationsin a range ofLp:Lm less than about 10:1, although ranges exceeding 50:1 have sometimesbeen used for studying special situations.

21.3 BASIC PRINCIPLES OF WATER MEASUREMENT

Flow is usually measured by determining an average flow velocity and using the flow area

to compute the volume discharge. Flow meters then have the function of detecting this

velocity and combining it with the physical information of the conduit to produce a use-

able readout. This is easily demonstrated for closed conduits. Propeller meters, ultrasonic

meters, laser-Doppler velocimeters, electromagnetic meters, Venturi meters, and orifice

meters all are based on inferring a basic velocity measurement applied to a flow area for

a discharge rate.

For open channels, many flumes depend on determining the velocity based on energy

principles of critical flow. Weirs are usually described in terms of orifice flow integrated

over the weir width and the crest depth. Again these are basically velocity expressions for

flow through a defined area.

Dilution techniques applicable to both closed pipe and open channel flows depend on

detecting the amount of fluid added to a known starting amount of tracer material. The

dilution ratio determines the discharge ratio, in the case of constant injection of a tracer.

The tracer may be a chemical or even injected heat or heated fluid.

Electromagnetic meters depend on generating voltages by flowing a conductive fluid,

usually water, through a magnetic field to produce a velocity indication.

21.3.1 Water Meter Classification

Flow measuring devices are commonly classified into those that are rate meters and mea-

sure discharge rate as the primary reported indication and those that are quantity meters

and measure volume as the primary indication. The latter include weighing tanks and

batch volume tanks and are used mostly in laboratory settings as flow rate standards.

Devices in either of these broad classes can again be divided according to the physicalprinciple that is used to detect that primary indication (ASME, 1959). The meter part that

interacts with the flow to produce the primary indication is referred to as the primary

device. This interaction exploits one or more of a few physical principles, such as pressure

force, energy conversion, weight, electrical properties, mixing properties, sonic proper-

ties, and so on, to generate a signal. Primary devices are thus limited in number and vari-

ety. Secondary devices convert the primary interaction into useable readout. These sec-

ondary devices are numerous and relatively unlimited in configuration and variety. The

function of one class can be converted into the response of the other with suitable sec-

ondary devices.

Some water measuring devices particularly suitable to municipal water supply, waste-water treatment, agricultural irrigation, and drainage applications are the historical rate

meters that are treated in most hydraulic text. These include (1) weirs, (2) flumes, (3) ori-

fice meters, and (4) Venturi meters.

Head, h, or upstream depth, commonly is used for the open channel devices such as

flumes and weirs. Either pressure,p, head, h, or differential head, , or differential pres-

sure, p, is used with tube-type devices, such as Venturi meters and orifice meters.






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Venturi meters in pipelines and long throated flumes in open-channel flows are exam-

ples where the energy principles and the flow accountings mentioned above give good to

excellent computational results with minor dependency on empirical coefficients (Bos,

1989; Bos et al., 1991).

21.3.2 Installation Requirements

Special difficulties arise in applying velocity profile and friction accountings when insuf-ficient pipe or channel exists upstream from a flow measuring device. This is needed toensure that predictable and acceptable velocity profiles are presented to the meter.Frequently pipe or channel lengths can be significantly shortened by special structuralflow conditioners. These structural measures then become a design option. Some of these

are discussed below and in section 21.3.3Designs for pipe discharges are well described in textbooks and in standard hand-

books. The design difficulties center around selecting appropriate metering candidates foraccomplishing the measuring function and in providing an appropriate environment foreconomical, accurate, and serviceable operation.

In the case of pipe flows, recommended straight pipe lengths, in terms of pipe diame-ter, are to be provided upstream of the meter to assure reasonable operating accuracy. Theselengths depend on the flow pattern presented to the meter primarily caused by valves andpipe elbows upstream from the meter. The number and orientation of elbows greatly influ-ence the circulation patterns and flow profile distortions presented to the meter.

Open-channel flow water measurement generally requires that the Froude number ofthe approach flow be less than 0.5 to prevent wave action that would hinder or possiblyprevent an accurate head determination.

Energy concepts are used to describe Venturi meters in pipe flows based on theBernoulli equation in which part of the pipe forms a contracted throat that necessarilychanges the flow velocity and hence converts some of the static pressure to velocity head.The decrease in static pressure is the basis for flow detection. A similar concept can beapplied to open channels. A historical version is the socalled Venturi flume (Brater andKing, 1982) that detects the change in water surface elevation between an upstream sta-tion and in a contracted section. However, this small change is difficult to accuratelydetect, so the direct concept is not used. Rather, contractions are designed to be severe

enough to force critical flow velocities in the contracted section. Thus, only an upstreamhead is needed to define the flow energy and flow area which can be converted to dis-charge rate. These are generally called critical-flow flumes. The flow condition whereonly one head measurement is needed is called ree flow.

The critical-flow flumes themselves consist of those called long-throated flumes thatforce parallel flow in the contracted, or control, section, called the throat, and those thathave curvilinear flow in the throat and are called short-throated flumes. The limitingthroat control section is the sharp-crestedweir consisting of a thin plate. Thus, forflumes and weirs one unique head value exists for each discharge, simplifying the cali-bration procedure.

However, if the downstream flow level submerges critical depth enough to affect theupstream reading, the modular limit is exceeded, and free flow does not occur. Whenexceeded, separate calibrations at many levels of submergence are then required, and twohead measurements are needed to measure flow. This condition generally is to be avoid-ed in meter site design because it reduces the accuracy of the measurement and increasesthe difficulty of flow determination. The modular limit for sharp-crested weirs, in prac-tice, is less than zero, requiring full clearance of the overfall nappe of at least 3 cm, whileshort-throated flumes can usually tolerate 65 percent to 70 percent submergence. Long-






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throated flumes can tolerate from 70 percent to 90 percent depending on flow conditionsand flume size.

Designing flumes for submerged flow beyond the modular limit decreases the accura-

cy of the flow measurement. Sometimes flumes and weirs can be overly submerged unin-tentionally by poor design, construction errors, structural settling, attempts to supplyincreased delivery needs with increasing downstream heads, accumulated sedimentdeposits, or weed growths. Sometimes use of the submerged range beyond the modularlimit is an economic compromise.

Approach flow conditions for pipes. Water measurement devices are generally cali-

brated with certain approach flow conditions. The same approach conditions must be

attained in field applications of measuring devices. Poor flow conditions in the area just

upstream of the measuring device can cause large discharge indication errors. For open

channels, the approaching flow should generally be subcritical. The flow should be fullydeveloped, mild in slope, and free of curves, projections, and waves.

Pipeline meters commonly require 10 or more diameters of straight pipe approach.

Fittings and combinations of fittings, such as valves and bends, located upstream from a

flow meter can increase the number of required approach diameters. Several references

(ASME, 1971; ISO, 1991) give requirements for many pipeline configurations and

meters. These are discussed in detail by Miller (1996).

Flow conditioning options. Many installations, especially in retrofit situations, do

not provide for sufficient lengths of straight pipe to remove velocity profile distortions and

swirl to an acceptable level. Therefore, the designer may need to use flow conditioners incombination with straight pipe lengths. Swirl sensitivity varies widely. Some meters are

particularly sensitive to swirl, such as the propeller and turbine meters. Magnetic flow

meters are somewhat less sensitive to radial velocities than single-path ultrasonic flow

meters. Venturi meters are less sensitive than orifice meters. For a swirl angle of 20 , the

discharge coefficient changes by about 1 percent for a Venturi meter with 0.32 ( isthe ratio of meter throat diameter to the pipe diameter) and about10 percent for a sim-

ilar orifice. Thus a swirl can increase the discharge through an orifice for the same differ-

ential head reading (Miller, 1996).

In pipeline flows, contractions can produce a central jet and also increase an incoming

swirl, while expansions tend to slow swirls and produce enough secondary flow to restoreflow profiles to some semblance of acceptability. These characteristics can modify the

straight pipe lengths needed or the type of flow conditioner to recommend (Miller, 1996).

Rough pipes also tend to reduce a swirl.

For flows, such as that encountered in sewage discharges and irrigation pipeline deliv-

eries that originate from open channels, many of the tube-bundle types of flow condition-

ers can gather trash and cause maintenance problems. Many meter providers in these sit-

uations use fins or vanes that protrude from the wall and have sloped upstream edges that

shed trash. The vanes protrude about one-fourth of the pipe diameter into the flow, leav-

ing the center core of the flow open. While these vanes can vary in number and length, the

logic being that the fewer the vanes the longer they should be in the direction of flow,common configurations are four vanes that are about two or three pipe diameters long.

Vanes in themselves do not condition wall jets well. Field experience, has shown that trou-

blesome flow profiles can be conditioned significantly by inserting an orifice into the pipe.

The orifice diameter is about 90 percent of the pipe diameter and is used to control wall

jets and force them to mix with the general flow. The orifice in itself tends to cross-mix






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the jets and would appear to reduce spin. However, if the jets are symmetrical and an ini-

tial swirl exists, orifices tend to increase the swirl. Inserting an orifice appears to be sup-

ported by recent recommendations of Miller (1996) where it is stated: To achieve a fully

developed profile, it is important that the flow be blocked or restricted close to the wall,

with the central core having the larger flow area.

The addition of vanes when space permits is recommended. Because orifices, in gen-

eral, tend to force the flow to the pipe center while increasing spin, it appears best to place

the vanes upstream from the orifice. If they are placed downstream, the spin not only may

be increased, but the spinning central flow may not be touched by the vanes.

21.3.3 Examples of Flow Conditioning in Field Situations

Flow conditioning in an irrigation delivery pipeline. As mentioned previously, measur-ing devices frequently must be installed in flow situations that are less than optimal. A fieldexample occurred in Arizona where a large pipe was used as an outlet to a secondary canaland a single-path ultrasonic meter placed in it was subjected to flow profile distortions. Thepipe was about 0.75 m in diameter and delivered approximately 400 L/s. The flow ratereadout was unstable, with fluctuations varying by about 15 percent. The problem appearedto be caused by slowly spiraling flow induced by the bottom jet from a partly open pipeinlet gate and a 45

oelbow. This is similar to two closely spaced pipe elbows that are not in

the same plane, which can cause a spiral flow pattern (ASME, 1971).A successful attempt to modify the jet and cause it to cross mix so that the jet effects

and the strength of the spiral flow were reduced, was accomplished by inserting a large -ratio orifice in the pipe (Fig. 21.3). This consisted of an annular metal ring with the out-side radius approximately that of the pipe and an inside diameter about 10 percent less, oran orifice with 90 percent. The orifice was installed about three diameters down-stream from the elbow. The slight increase in headloss was compensated by increasing theupstream gate opening. The orifice can be constructed by cutting notches from an appro-priately sized piece of angle iron or aluminum and bending it to a polygon that approxi-mates the circle diameter of the pipe interior. Some leakage around the ring is acceptable.For propeller meters, additional vanes projecting from the walls may be needed to further


FIGURE 21.3 An orifice plate with a large opening is used to condition a flow profile.





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reduce spiral flow. These vanes would be placed upstream from the orifice. In this instal-lation, the fluctuation was reduced to within about 3 percent.

Flow conditioning in channels. By analogy and using a minimum of 10 pipe diame-ters of a straight approach channel, open channel flow would require 40 hydraulic radii ofstraight, unobstructed, unaltered approach, based on the calculation of hydraulic radius forcircular pipes being equal to one-fourth the pipe diameter, (Eq. 21.15). This would trans-late for very wide channels into approximately 40 times the flow depth. For narrow chan-nels that are as deep as they are wide, this would compute to be about 13 channel depthsor top widths.

Other recommendations on approach channel criteria are presented by Bos (1989)and USBR (1997). Major features of that criteria follow:

If the control width is greater than 50 percent of the approach channel width, 10 aver-age approach flow widths of straight, unobstructed approach are required.

If the control width is less than 50 percent of the approach width, 20 control widths ofstraight, unobstructed approach are required.

If upstream flow is below critical depth, a jump should be forced to occur. In this case,30 measuring heads of straight, unobstructed approach after the jump should be pro-vided.

If baffles are used to correct and smooth out approach flow, then 10 measuring heads(10 h1) should be placed between the baffles and the measuring station.

Approach flow conditions should be continually checked for deviation from these con-ditions as described in Bos (1989) and USBR (1997).

The baffles described above can become unacceptable maintenance problems in openchannels. Some field expediencies are therefore described that have been found to work inspecific instances, but have not been studied for assured design generalizations. Nevertheless,these constructions are but small extensions to currently accepted practices in pipe flows.

Applications for open channel flow conditioners include abrupt channel turns, sluicegate outflows, and channels downstream from a hydraulic jump. The abrupt turns maybenefit from floor and wall mounted vanes or fins. Based on pipe flow experience, andassuming the channel is half of a closed conduit, these fins or vanes would probably be

about 10 percent to 15 percent of the channel depth.As in pipe flow, wall jets that can develop downstream from sluice gates appear to needtreatment. This can be in the form of a structural angle bolted on the channel floor and upthe walls. Suggested size, based on the pipe flow analogy, is for the angle to be about 5percent into the channel flow depth. Whether the sidewalls need larger angles when thechannels are wide has not been tested.

21.3.4 Wave Suppression

Of special concern in open channels is wave suppression downstream from a sluice gate,hydraulic jump, or an abrupt turn. Thus, the flow conditioners in channels have the addi-tional task not present in pipe flows of surface wave suppression. Excessive waves in irri-gation canals make reading sidewall gages difficult. These waves are usually caused by a

jet entry from a sluice gate or by a waterfall situation. The unstable surface can be 1020cm high and extend for tens of meters downstream.

Wave suppression in canals. A surface wave suppressor was tested by Schuster asreported in USBR (1997). It basically was a constructed roof over the canal for a distance






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equal to about four times the flow depth. The roof structure is inserted into the flow aboutone-third the flow depth. All flow is forced to pass under the structure. Wave suppression isbetween 60 percent and 93 percent (Fig. 21.4). For canals that usually flow at one level, thiswave-suppression method is appropriate. The wave suppressor shown in Figure 21.4 hasbeen successfully used in both large and small channels (USBR, 1997). An important aspectis that the structure is fixed and not allowed to float. Floating suppressors are not effective.

Successful field applications of wave suppressors include some installations in trape-zoidal irrigation channels, with 1:1 side slopes and 60-cm bottom width. They were flow-ing about 400 L/S at about 45 cm deep. While the velocity was not high, about 0.8 m/s,the agitation from a flow entry gate was producing waves about 15 cm high. The sup-pressor roof was only about 60 cm in the direction of flow, and penetrated the flow byabout 15 percent.

Another version that has worked in small channels is illustrated in Figure 21.5.

This can work with a single crossmember if the flow is usually at a fixed dischargerate and becomes similar to the suppressor described above. In severe jet cases anadditional floor sill, about 10 percent of the flow depth in height, has been used suc-cessfully.

The length of the roof in the flow direction has not been well studied, but field obser-vations seem to support a length greater than two lengths of the surface wave, if that canbe estimated, otherwise, use two to four times the maximum flow depth as described above.

To suppress waves in canals that do not always flow at the same depth, a staggered setof baffles may help (Replogle, 1997). Because these will be submerged part of the time,they must have a thickness that overlaps slightly to accommodate the vertical depth ofinterest. To avoid obstructing the channel severely, these baffles probably should notobstruct more than about 20 percent of the channel at any particular location. Staggeringthem as shown in Fig. 21.5 would accomplish this without excessive obstruction.Rounding the upstream edges will help shed trash, but may be less effective in suppress-


FIGURE 21.4 Wave suppresor design (From USBR, 1997).

FIGURE 21.5 Wave suppressor for variable-depth flows in a canal. (FromReplogle, 1997)





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ing waves. Observe in the sequence of drawings in Fig. 21.5 that the staggering is upwardin the downstream direction. Note that the next baffle slightly overlaps the horizontal flowlines so that flow passing over the top of one baffle is not allowed to free-fall and start

another wave. Fig. 21.5 ac illustrate, the general behavior as the flow becomes less deep.

21.4 MEASUREMENT ACCURACY

Accurate application of water measuring devices generally depends upon standard designsor careful selection of devices, careful fabrication and installation, good calibration dataand adequate analysis. Also needed is proper user operation with appropriate inspectionand maintenance procedures. During operation, accuracy requires continual verificationthat all measuring systems, including the operators, are functioning properly. Thus, good

training and supervision are required to attain measurements within prescribed accuracybounds. Accuracy is the degree of conformance of a measurement to a standard or truevalue. The standards are selected by users, providers, governments, or compacts betweenthese entities. All parts of a measuring system, including the user, need to be consideredin accessing the system's total accuracy.

As mentioned above, a measurement system usually consists of a primary element,which is that part of the system that creates what is sensed, and is measured by a sec-ondary element. For example, weirs and flumes are primary elements. A staff gage is asecondary element.

Designers, purchasers, and users of water measurement devices generally rely on stan-

dard designs and manufacturers to provide calibrations and assurances of accuracy. A fewwater users and providers have the facilities to check the condition and accuracy of flowmeasuring devices. These facilities have comparison flow meters and/or volumetric tanksfor checking their flow meters. These test systems are used to check devices for compli-ance with specification and to determine maintenance needs. However, maintaining facil-ities such as these is not generally practical.

Various disciplines and organizations do not fully agree on some of the definitionsrelated to measuring device specifications, calibration, and error analysis. Therefore, it isimportant to verify that a clear and mutual understanding of the specifications, calibrationterminology, and the error analysis processes is established when discussing these topicswith others.

21.4.1 Definitions of Terms Related to Accuracy

Error. Error is the deviation of a measurement, observation, or calculation from thetruth. The deviation can be small and inherent in the structure and functioning of the sys-tem and be within the bounds or limits specified. Lack of care and mistakes during fab-rication, installation, and use can often cause large errors well outside expected perfor-mance bounds. Because the true value is seldom known, some investigators prefer to usethe term uncertainty. Uncertainty describes the possible error or range of error which

may exist. Investigators often classify errors and uncertainties into spurious, systematic,and random types.

Precision. Precision is the ability to produce the same measurement value withingiven accuracy bounds when successive readings of a specific quantity are measured.Precision represents the maximum departure of all readings from the mean value of thereadings. Thus, a single observation of a measurement cannot be more accurate than






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the inherent precision of the combined primary and secondary precision. It is possible tohave good precision of an inaccurate reading. Thus, precision and accuracy differ.

Spurious errors. Spurious errors are commonly caused by accident, resulting in falsedata. Misreading and intermittent mechanical malfunctions can cause discharge readingswell outside of expected random statistical distribution about the mean. Spurious errorscan be minimized by good supervision, maintenance, inspection, and training.Experienced, well-trained operators are more likely to recognize readings that are signif-icantly out of the expected range of deviation. Unexpected spiral flow and blockages offlow in the approach or in the device itself can cause spurious errors. Repeating measure-ments does not provide information on spurious error unless repetitions occur before andafter the introduction of the error. On a statistical basis, spurious errors confound evalua-tion of accuracy performance.

Systematic errors. Systematic errors are errors that persist and cannot be consideredrandom. Systematic errors are caused by deviations from standard device dimensions,anomalies to the particular installation, and possible bias in the calibration. Systematicerrors cannot be removed or detected by repeated measurements. They usually cause per-sistent error on one side of the true value. The value of a particular systematic error for aparticular device may sometimes be considered as a random error. For example, an instal-lation error in the zero setting for a flume might be + 1 mm for one flume and 2 mm foranother. For each flume the error is systematic, but for a number of flumes it would be arandom error.

Random errors. Random errors are caused by such things as the estimating requiredbetween the smallest division on a head measurement device and water surface waves ata head measuring device. Loose linkages between parts of flowmeters provide room forrandom movement of parts relative to each other, causing subsequent random outputerrors. Repeated readings decrease the average expected error resulting from randomerrors by a factor of the square root of the number of readings.

Total error. Total error of a measurement is the result of systematic and random errorscaused by component parts and factors related to the entire system. Sometimes, error lim-its of all component factors are well known. In this case, total limits of simpler systemscan be determined by computation (Bos et al., 1991). In more complicated cases, it maybe difficult to confidently combine the limits. In this case, a thorough calibration of theentire system as a unit can resolve the difference. In any case, it is better to do error analy-sis with data where entire system parts are operating simultaneously and compare dis-charge measurement against an adequate discharge comparison standard.

Expression of errors. Instrument errors are usually expressed by manufacturers aseither a percent of reading or a percent of full scale. The secondary devices based on elec-tronic outputs are more frequently expressed in terms of percent full scale. The designermust be aware that a probable error value of say 1 percent full-scale can exceed 10percent for small value readings on the output device. When used with weirs, for exam-ple, the head reading of 1.5 in the weir equation can increase this 10 percent head mea-surement error to a 15 percent flow measurement error.

21.4.2 Terms Related to Measurement Capability

Linearity. Linearity usually means the maximum deviation in tracking a linearly vary-ing quantity, such as measuring head, and is generally expressed as percent of full scale.






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Discrimination. Discrimination is the number of decimals to which the measuringsystem can be read. Precision is no better than the discrimination.

Repeatability. Repeatability is the ability to reproduce the same reading for the samequantities. Thus, it is related to precision.

ensitivity. Sensitivity is the ratio of the change of a secondary measurement, suchas head, to the corresponding change of discharge.

Range and Rangeability. Range is fully defined by the lowest and highest value thatthe device can measure without damage and comply within a specified accuracy. The upperand lower range bounds may be the result of mechanical limitations, such as friction at thelower end of the range and possible overdriving damage at the higher end of the range.Range can be designated in other ways: (1) as a simple difference between maximum dis-

charge (Qmax) and minimum discharge (Qm n), (2) as the ratio (Qmax/Qm n), called rangeabili-ty, and (3) as a ratio expressed as 1:(Qmin/ max). Neither the difference nor the ratios fullydefine range without knowledge of either the minimum or maximum discharge.

Additional terms (hysteresis, response, lag, rise time). Additional terms related more

to dynamic variability might be important when continuous records are needed or if the

measurements are being sensed for automatic control of canals and irrigation.Hysteresis is

the maximum difference between measurement readings of a quantity established by the

same mechanical set point when set from a value above and reset from a value below.

Hysteresis can continually get worse as wear of parts increases friction or as linkage free-

dom increases.Response has several definitions in the instrumentation and measurementfield. For water measurement, one definition for response is the smallest change that can

be sensed and displayed as a significant measurement.Lag is the time difference of an out-

put reading when tracking a continuously changing quantity. Rise time is often expressed

in the form of the time constant, defined as the time for an output of the secondary element

to achieve 63 percent of a step change of the input quantity from the primary element.

21.4.3 Comparison Standards

Water providers may want, or may be required, to have well-developed measurement pro-grams that are highly managed and standardized. If so, water delivery managers may wishto consult American Society for Testing Materials Standards (ASTM, 1988), Bos (1989),International Organization for Standardization (ISO, 1983: ISO, 1991), and the NationalHandbook of Recommended Methods for Water Data Acquisition (USGS, 1980).

Research laboratories, organizations, and manufacturers that certify measurementdevices may need to trace accuracy of measurement through a hierarchy of increasinglyrigid standards.

The lowest standards in the entire hierarchy of physical comparison standards arecalled working standards, which are shop or field standards used to control quality of pro-

duction and measurement. These standards might be gage blocks or rules used to ensureproper dimensions of flumes during manufacturing or devices carried by water providersand users to check the condition of water measurement devices and the quality of theiroutput. Other possible working standards are weights, volume containers, and stopwatch-es. More complicated devices are used, such as surveyors levels, to check weir staff gagezeros. Dead weight testers and electronic standards are needed to check and maintainmore sophisticated and complicated measuring devices, such as acoustic flow meters anddevices that use pressure cells to measure head.






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For further measurement assurance and periodic checking, water users and organiza-tions may keep secondary standards. Secondary standards are used to maintain integrityand performance of working standards. These secondary standards can be sent to govern-

ment laboratories, one of which is the National Bureau of Standards in Washington, D.C.,to be periodically certified after calibration or comparison with accurate replicas of pri-mary standards. Primary standards are defined by international agreement and maintainedat the International Bureau of Weights and Measurements in Paris, France.

Depending on accuracy needs, each organization should trace their measurement per-formance up to and through the appropriate level of standards. For example, turbineacceptance testing, such as in the petroleum industry, might justify tracing to the primarystandards level.

21.5 SELECTION OF PRIMARY ELEMENTS OF WATER

MEASURING DEVICES

21.5.1 General Requirements

Design considerations involve the selection of the proper water measurement device fora particular site or situation. Site-specific factors and variables must be considered inextended detail. Each system has unique operational requirements and installation con-cerns. Knowledge of the immediate measurement needs and reliable estimates on future

demands of the proposed system is advantageous. Possible selection constraints may beimposed by laws and compact agreements and should be consulted before selecting ameasurement device. Contractual agreements for the purchase of pumps, turbines, andwater measuring devices for water supply, sewage and drainage districts often dictate themeasurement system required for compliance prior to payment. These constraints maybe in terms of accuracy, specific comparison devices, and procedures. Bos (1989) pro-vides an extensive and practical discussion on the selection of open channel water mea-surement devices. Miller (1996) provides a recent compilation of selection criteria forpipe flow-meters suited to liquids and steam and other gas flows. Bos (1989) provides aselection flow chart and a table of water measurement device properties to guide the

selection process for the open channel devices. Miller (1996) describes each meter indetail for the pipe systems, but is more general in leaving the selection to the designer.Because the design engineers for civil engineering projects are most likely to be deal-

ing with irrigation water supply, waste water, or drainage and flood flows, the emphasis isplaced on the measuring systems deemed most appropriate to these processes. Largeclosed-pipe systems for water supplies are frequently encountered, so installation situa-tions appropriate to these will also be included. Gas flows, including steam, are more like-ly to be encountered by mechanical and chemical engineers and those readers are referredto Miller (1996) and ASME (1959, 1971).

21.5.2 Types of Measuring Devices

System operators for water supply, drainage, and waste water commonly use many typesof standard water measuring devices, usually in open channels with limited applicationsin closed conduits. Particularly prominent uses of open channel devices are found in irri-gation delivery systems and farm distribution systems, although these measuring devices






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are frequently used for sewer flows and even flood flows. However, the latter two areas ofapplication are frequently more difficult because of the likelihood of heavy bed loads andfloating debris.

In pipe flowmeters, the most commonly installed devices in industry are the orificemeters, accounting for up to 80 percent of all industrial meters (Miller, 1996). Venturimeters and flow tubes provide much of the remainder. In absolute numbers, the house-hold meters, based on various technologies from nutating disks to paddle-wheel tur-bines, dominate.

For openchannel flows, weirs, flumes, submerged and free orifices, and currentmeters dominate the flow measuring methods. Pipe flow meters, propeller and turbine,acoustic, magnetic, and vortex-shedding meters are used on large water supply wells suchas those used in irrigation and municipal water supply. Differential head meters, such asorifice meters, Venturi meters, and flow tubes, are also used in these applications. The

meters considered herein are

1. Open-channel flow devices

a. Current metering (cup, propeller, and electromagnetic probes)

b. Weirs

c. Flumes

. Acoustic (transonic and Doppler)

e. racers

. Miscellaneous

2. Pipe flow devices

a. Differential head meters

b. Acoustic (transonic and Doppler)

c. Tracers

. Turbine/propeller/other insert mechanical

e. Vortex-shedding

. Miscellaneous

The main factors which influence the selection of a measuring device include(USBR, 1997):

a. Accuracy requirements

b. Cost

c. Legal constraints

. Range of flow rates

e. Head loss

. Adaptability to site conditions

g. Adaptability to variable operating conditions

h. Type of measurements and records neededI. Operating requirements

j. Ability to pass sediment and debris

k. Longevity of device for given environment

l. Maintenance requirements

m. Construction and installation requirements






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n. Device standardization and calibration

o. Field verification, troubleshooting, and repair

p. User acceptance of new methodsq. Vandalism potential

r. Impact on environment

Accuracy requirements. The desired accuracy of the measurement system is an impor-tant consideration in the selection of a measurement method. Most water measurementinstallations, including the primary and secondary devices, can produce accuracies of5percent. Some systems are capable of 1 percent under laboratory settings. However, inthe field, maintaining such accuracies usually requires considerable expense or specialeffort in terms of construction, secondary equipment, calibration in-place, and stringent

maintenance. Selecting a device that is not appropriate for the site conditions can result ina nonstandard installation of reduced accuracy, sometimes exceeding10 percent.

Accuracies are frequently reported that relate only to the primary measurement methodor device. However, many methods require secondary measurement equipment that pro-duces the actual readout. This readout equipment typically increases the overall error ofthe measurement.

Cost. The cost of the measurement method includes the cost of the device itself, theinstallation, secondary devices, operation, and maintenance. Measurement methods varywidely in their cost and in their serviceable life span. Measurement methods are often

selected based on the initial cost of the primary device with insufficient regard for theadditional costs associated with providing the desired records of flows over an extendedperiod of time.

Legal constraints. Governmental or administrative water board requirements maydictate the water measurement devices or methods. Water measurement devices thatbecome a standard in one geographic area may not necessarily be accepted as a standardelsewhere. In this sense, the term standard does not necessarily signify accuracy orbroad legal acceptance. Many water agencies require certain water measurement devicesused within their jurisdiction to conform to their standard for the purpose of simplifyingoperation, employee training, and maintenance.

Flow range. Many measurement methods have a limited range of flow conditions forwhich they are applicable. This range is usually related to the need for certain prescribedflow conditions which are assumed in the development of calibrations. Large errors inmeasurement can occur when the flow is not within this range. For example, using a buck-et and stopwatch for large flows that engulf the bucket is not very accurate. Similarly,sharp-edged devices, such as sharp crested weirs, typically do not yield good results withlarge channel flows. These are measured better with large flumes or broad-crested weirs,which in turn are not appropriate for trickle flows.

Certain applications have typical flow ranges. Irrigation supply monitoring seldom

demands a low-flow-to-high-flow range above about 30, while this range on naturalstream flows may exceed 1000.In some cases, secondary devices can limit the practical range of flow rates. For exam-

ple, with devices requiring a head measurement, the accuracy of the head measurementfrom a visually read wall gage may limit the measurement of low flow rates. For somedevices, accuracy is based on percent of the full-scale value. While the resulting error maybe well within acceptable limits for full flow, at low flows, the resulting error may becomeexcessive, limiting the usefulness of such measurements. Generally, the device should be






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selected to cover the desired range. Choosing a device that can handle an unnecessarylarge flow rate may result in compromising measurement capability at low flow rates, andvice versa. This choice depends on the objective of the measurement. For example, in irri-

gation practice, usually choose a device that can measure the most common flow range atthe expense of poorly measuring extremes, such as flood flows. For urban drainage, theflood peak may be important.

For practical reasons, different accuracy requirements for high and low flows may bechosen. This is reasonable when an annual total is the primary goal and the low flows con-tribute a small percentage to that total. Also, if the inaccurate low flow readings are trulya random error then this error approaches zero with large accumulations of readings. Thusthe designer needs to know if management decisions are made from individual readingsor from long term averages.

Headloss. Most water measurement devices require a drop in head. On retrofit instal-lations, for example, to an existing irrigation project, such additional head may not beavailable, especially in areas that have relatively flat topography. On new projects, incor-porating additional headloss into the design can usually be accomplished at reasonablecost. However, a tradeoff usually exists between the cost of the device and the amount ofheadloss. For example, acoustic flow meters are expensive but require little headloss.Sharp-crested weirs are inexpensive but require a relatively large headloss. The head lossrequired for a particular measuring device usually varies over the range of discharges. Insome cases, head needed by a flow measuring device can reduce the capacity of the chan-nel at that point.

Adaptability to site conditions. The selection of a flow measuring device mustaddress the site of the proposed measurement. Several potential sites may be available forobtaining a flow measurement. The particular site chosen may influence the selection ofa measuring device. For example, discharge in a canal system can be measured within areach of the channel or at a structure such as a culvert or check structure. A differentdevice would typically be selected for each site. The device selected ideally should notalter site hydraulics so as to interfere with normal operation and maintenance. Also, theshape of the cross-sectional flow area may favor particular devices.

Adaptability to variable operating conditions. Flow demands for most water deliv-ery systems usually vary over a range of flows and flow conditions. The selected device

must accommodate the flow range and changes in operating conditions, such as variationsin upstream and downstream head. Weirs or flumes should be avoided if downstreamwater levels can, under some conditions, cause excessive submergence. Also, the infor-mation provided by the measuring device should be conveniently useful for the operatorsperforming their duties. Devices that are difficult and time consuming to operate are lesslikely to be used and are more likely to be used incorrectly.

In some cases, water measurement and water level or flow control are desired at thesame site. A few devices are available for accomplishing both (e.g., constant-head orifice,vertically movable weirs, and Neyrpic flow module; Bos, 1989). However, separate mea-surement and control devices are typically linked for this purpose and usually can exceed

the performance of combined devices in terms of accuracy and level control, if care isexercised to assure that the separate devices are compatible and achieve both functionswhen used as a system.

Type of measurements and records needed. An accurate measure of instantaneousflow rate is useful for system operators in setting and verifying flow rate. However,because flow rates change over time, a single (instantaneous) reading may not accuratelyreflect the total volume of water delivered. Where accounting for water volume is desired,






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a method of accumulated individual flow measurements is needed. Where flows aresteady, daily measurements may be sufficient to infer total volume. Most deliveries, how-ever, require more frequent measurements. Meters that accumulate total delivered volume

are desirable where water users take water on demand. Totalizing and automatic record-ing devices are available for many measuring devices. For large structures, the cost forwater-level sensing and recording hardware is small relative to the structure cost. Forsmall structures, these hardware costs remain about the same and thus become a majorpart of the measurement cost, and may often exceed the cost of the primary structure itself.

Many water measuring methods are suitable for making temporary measurements(flow surveys) or performing occasional verification checks of other devices. The methodchosen for such a measurement might be quite different from that chosen for continuousmonitoring. Although many of these flow survey methods are suited for temporary oper-ation, the focus here is on methods for permanent installations.

Operating Requirements. Some measurement methods require manual labor toobtain a measurement. Current metering requires a trained staff with specialized equip-ment. Pen-and-ink style water-stage recorders need operators to change paper, add ink,and verify proper functioning. Manual recording of flows may require printed forms to bemanually completed and data to be accumulated for accounting purposes. Devices withmanometers require special care and attention to assure correct differential-head readings.Automated devices, such as ultrasonic flowmeters and other systems that use transducersand electronics, require operator training to set up, adjust, and troubleshoot. Setting gate-controlled flow rates by simple canal level references or by current metering commonlyrequires several hours of waiting between gate changes for the downstream canal to fill

and stabilize. However, if a flume or weir is installed near the control gate, that portion ofthe canal can be brought to the stable, desired flow level and measured flow rate in a fewminutes, and the canal downstream of the flume or weir can then fill to the correct levelover a longer time without further gate adjustments. Thus, the requirements of the oper-ating personnel in using the devices and techniques for their desired purposes must beconsidered in meter selection.

Some measuring devices may inherently serve an additional function applicable to theoperation of a water supply system. For example, weirs and flumes serve to hydraulical-ly isolate upstream parts of a canal system from the influence of downstream parts. This

occurs for free overfall weirs and flumes flowing below their modular limits. Acoustic,propeller, magnetic, and vortex-shedding flowmeters do not provide this function withoutadditional structural measures such as a downstream overfall. If these meters are used, andthe isolation function is desired, then the designer should be made aware of the require-ment and provide a free overfall. Isolating the influence of upstream changes from affect-ing downstream channels, is less easily accomplished. However, it can be partly imple-mented with orifices that have a differential head that is large compared to the upstreamfluctuations.

The designer should be aware that a sharp-crested weir overfall requires a relatively

high head drop and may need to be excessively wide to provide the isolation function

with low absolute head drop. While a long board can be used downstream from a pro-

peller meter to provide the necessary width of flow that will pass a required quantity of

water at small head, that small head, and the crude board would not be well suited for

measuring flow rate.

The designer may wish to take advantage of broad-crested weir behavior and provide

a thick crest that can withstand in excess of about 80 percent submergence, which usu-

ally translates into low absolute head loss. When used with a propeller meter, for exam-

ple, the broad-crested weir need not be well defined and can be economically installed

(Replogle, 1997).






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Ability to pass sediment and debris. Canal systems often carry a significant amountof sediment in the water. Removal of all suspended solids from the water is usually pro-hibitively expensive. Thus, some sediment will likely be deposited anywhere the veloci-

ties are reduced, which typically occurs near flow measuring structures. Whether this sed-iment causes a problem depends on the specific structure and the volume of sediment inthe water. In some cases, this problem simply requires routine maintenance to removeaccumulated sediment; in others, the accumulation can make the flow measurement inac-curate or the device inoperative. Sediment deposits can affect approach conditions andincrease approach velocity in front of weirs, flumes, and orifices. Floating and suspendeddebris such as aquatic plants, washed out bank plants, and fallen tree leaves and twigscan plug some flow measurement devices and cause significant flow measurement prob-lems. Many of the measurement devices which are successfully used in closed conduits(e.g., orifices, propeller meters, and so on) are not usable in culverts or inverted siphons

because of debris in the water. Attempting to remove this debris at the entrance to culvertsis an additional maintenance problem.

Flumes, especially long-throated flumes, can be designed to resist sedimentation. Thedesign options available are to select a structure shape that will maintain velocities thatassure erosion of sediments, or at least continued movement of incoming sedimentsthrough the flume, at important flow rates. In large broad-crested weirs (a class of long-throated flumes) for capacities greater than 1 m3/s per m of flume width, velocities greaterthan 1 m/s can be achieved for the upper 75 percent of the flow range, and is usually ero-sive enough to maintain flume function even for high-sediment bed loads. At the lowerflow ranges and for heavy sediment bed loads, deposition is likely and frequent mainte-

nance may be required.Trapezoidal sections tend to retain low velocities into the upper ranges of flow and

are less sediment worthy. Long-throated flumes with flat bottoms throughout and sidecontractions maintain a high velocity for 0.5 m /s per m width, and higher, but musthave throat lengths that are 2 to 3 times the throat width in order to be accurately com-putable. The sediment worthiness of a flume design depends more on these absolutevelocities than on whether the flume floor is flat throughout or raised as in a broad-crested weir. This prompts the designer to select shapes that can provide these veloci-ties. One suggestion for broad crested weirs in a fixed sized channel is to construct afalse floor in the head gage area to increase the velocity there and prevent changes in

area of flow there. Also, sediments can accumulate in the upstream channel to a depthof the false floor without affecting the function of the flume. This can extend the timebetween mandatory channel cleaning.

Device environment. Any measurement device with moving parts or sensors is sub-ject to failure if it is not compatible with the site environment. Achieving proper operationand longevity of devices is an important selection factor. Very cold weather can shrinkmoving and fixed parts differentially and solidify oil and grease in bearings. Water canfreeze around parts and plug pressure ports and passageways. Acidity and alkalinity inwater can corrode metal parts. Water contaminants such as waste solvents can damagelubricants, protective coatings, and plastic parts. Mineral encrustation and biological

growths can impair moving parts and plug pressure transmitting ports. Sediment canabrade parts or consolidate tightly in bearing and runner spaces in devices such as pro-peller meters.

Measurement of wastewater and high sediment transport flow may preclude the use ofdevices that require pressure taps, intrusive sensors, or depend upon clear transmission of






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sound through the flow. Water measurement devices that depend on electronic devices andtransducers must have appropriate protective housings for harsh environments. Improperprotection against the site environment can cause equipment failure or loss of accuracy.

Maintenance requirements. he type and amount of maintenance varies widely withdifferent measurement methods. For example, current metering requires periodic mainte-nance of the current meter itself and maintenance of the meter site to assure that is has aknown cross section and velocity distribution. When the flow carries sediment or debris,most weirs, flumes, and orifices require periodic cleaning of the approach channel. Asmentioned above, design and meter selection can mitigate the maintenance problems withsediments, but are not likely to eliminate them. Electronic sensors need occasional main-tenance to ensure that they are performing properly. Regular maintenance programs arerecommended to ensure prolonged measurement quality for all types of devices.

Construction and installation requirements. In addition to installation costs, the dif-ficulty of installation and the need to retrofit parts of the existing conveyance system cancomplicate the selection of water measurement devices. Clearly, devices that can be easi-ly retrofitted into the existing canal system are much preferred because they generallyrequire less down time, and usually present fewer unforeseen problems.

Device standardization and calibration. A standard water measurement device infersa documented history of performance based on theory, controlled calibration, and use. Atruly standard device has been fully described, accurately calibrated, correctly construct-ed, properly installed, and sufficiently maintained to fulfill the original installation

requirements and flow condition limitations. Discharge equations and tables for standarddevices should provide accurate calibration. Maintaining a standard device usually onlyinvolves a visual check and measurement of a few specified items or dimensions to ensurethat the measuring device has not departed from the standard. Many standard devices havea long history of use and calibration, and thus are potentially more reliable. Commercialavailability of a device does not necessarily guarantee that it satisfies the requirements ofa standard device.

When measuring devices are fabricated onsite or are poorly installed, small devia-tions from the specified dimensions can occur. These deviations may or may not affectthe calibration. The difficulty is that unless an as-built calibration is performed, the

degree to which these errors affect the accuracy of the measurements is largelyunknown. All too frequently, design deviations are made under the misconception thatcurrent metering can be used to provide an accurate field calibration. In practice, cali-bration by current metering to within 2 percent is difficult to attain. An adequate cal-ibration for free-flow conditions requires many current meter measurements at severaldischarges. Changing and maintaining a constant discharge for calibration purposes isoften difficult under field conditions.

Field verification, troubleshooting, and repair. After construction or installation of adevice, some verification of the calibration is generally recommended. Usually, the meth-ods used to verify a permanent device (e.g., current metering) are less accurate than thedevice itself. However, this verification simply serves as a check against gross errors inconstruction or calibration. For some devices, errors occur as components wear and thecalibration slowly drifts away from the original. Other devices have components that sim-ply fail, that is, you get the correct reading or no reading at all. The latter is clearly pre-ferred. However, for many devices, occasional checking is required to ensure that they arestill performing as intended. Selection of devices may depend on how they fail and howeasy it is to verify that they are performing properly.






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User acceptance of new methods. Selection of a water measurement method mustalso consider the past history of the practice at the site. When improved water measure-ment methods are needed, proposing changes that build on established practice are gen-

erally easier to institute than radical changes. It can be beneficial to select a new methodthat allows conversion to take place in stages to provide educational examples and demon-strations of the new devices and procedures.

Vandalism potential. Instrumentation located near public access is a prime target forvandalism. Where vandalism is a problem, measurement devices with less instrumenta-tion, or instrumentation that can be easily protected, are preferred. When needed, instru-mentation can be placed in a buried vault to minimize visibility.

Impact on environment. During the selection of a water measurement device, con-sideration must be given to potential environmental impacts. Water measurement devices

vary greatly in the amount of disruption to existing conditions that is needed for installa-tion, operation, and maintenance. For example, installing a weir or flume constricts thechannel, slows upstream flow, and accelerates flow within the structure. These changes inthe flow conditions can alter local channel erosion, local flooding, public safety, localaquatic habitat, and movement of fish up and down the channel. These factors

hydraulic design of flow measuring structure

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