Disclosure to Promote the Right To Information Whereas the Parliament of India has set out to provide a practical regime of right to information for citizens to secure access to information under the control of public authorities, in order to promote transparency and accountability in the working of every public authority, and whereas the attached publication of the Bureau of Indian Standards is of particular interest to the public, particularly disadvantaged communities and those engaged in the pursuit of education and knowledge, the attached public safety standard is made available to promote the timely dissemination of this information in an accurate manner to the public. इंटरनेट मानक “!ान $ एक न’ भारत का +नम-ण” Satyanarayan Gangaram Pitroda “Invent a New India Using Knowledge” “प0रा1 को छोड न’ 5 तरफ” Jawaharlal Nehru “Step Out From the Old to the New” “जान1 का अ+धकार, जी1 का अ+धकार” Mazdoor Kisan Shakti Sangathan “The Right to Information, The Right to Live” “!ान एक ऐसा खजाना > जो कभी च0राया नहB जा सकता ह ै” Bhartṛhari—Nītiśatakam “Knowledge is such a treasure which cannot be stolen” IS 14974 (2001): Liquid flow Measurement in Open Channels by Weirs and Flumes - Rectangular Broad-crested Weirs [WRD 1: Hydrometry]
Transcript
Disclosure to Promote the Right To Information
Whereas the Parliament of India has set out to provide a practical regime of right to information for citizens to secure access to information under the control of public authorities, in order to promote transparency and accountability in the working of every public authority, and whereas the attached publication of the Bureau of Indian Standards is of particular interest to the public, particularly disadvantaged communities and those engaged in the pursuit of education and knowledge, the attached public safety standard is made available to promote the timely dissemination of this information in an accurate manner to the public.
इंटरनेट मानक
“!ान $ एक न' भारत का +नम-ण”Satyanarayan Gangaram Pitroda
“Invent a New India Using Knowledge”
“प0रा1 को छोड न' 5 तरफ”Jawaharlal Nehru
“Step Out From the Old to the New”
“जान1 का अ+धकार, जी1 का अ+धकार”Mazdoor Kisan Shakti Sangathan
“The Right to Information, The Right to Live”
“!ान एक ऐसा खजाना > जो कभी च0राया नहB जा सकता है”Bhartṛhari—Nītiśatakam
“Knowledge is such a treasure which cannot be stolen”
“Invent a New India Using Knowledge”
है”ह”ह
IS 14974 (2001): Liquid flow Measurement in Open Channelsby Weirs and Flumes - Rectangular Broad-crested Weirs [WRD1: Hydrometry]
Is 14974:2001ISO 3846:1989
hxiian Standard
LIQUID FLOW MEASUREMENT IN OPEN CHANNELSBY WEIRS AND FLUIVIES — RECTANGULAR
BROAD-CRESTED WEIRS
ICS 17,120.20
@ BIS 2001
BUREAU OF IN DIANSTA ND ARDSMANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG
This Indian Standard which is identical with ISO 3846:1989 ‘Liquid flow measurement in openchannels by weirs and flumes — Rectangular broad-crested weirs’ issued by the InternationalOrganization for Standardization (ISO) was adopted by the Bureau of Indian Standards on therecommendations of the Fluid Flow Measurement Sectional Committee (WRD 01) and approval of theWater Resources Division Council.
In the adopted standard, certain conventions are, however, not identical to those used in IndianStandards. Attention is especially drawn to the following:
a) Wherever the words ‘International Standard’ appear referring to this standard, they should beread as ‘Indian Standard’.
b) Comma (,) has been used as a decimal marker while in Indian Standards, the current practiceis to use a point (.) as the decimal marker.
CROSS REFERENCES
In this adopted standard, the following International Standards have been referred. Read in theirrespective places, the following Indian Standards:
International Corresponding Degree ofStandard /ndian Standard Equivalence
1s0 748 Liquid flow IS 1192:1981 Velocity-area methods Identical with elucidation inmeasurement in open for measurement of flow of water in Indian Standard (/S f f 92:1981channels — Velocity-area open channels (first revision) is under revision based onmethods /S0 748:1997 ‘Measurement
of liquid flow in open channels- Velocity-area mefhods~
1s0 772 Liquid flow IS 1191:1971 Glossary of terms and Technically equivalent (1Smeasurement in open symbols used in connection with the 1191 .“ 1971 is under revisionchannels — Vocabulary and measurement of liquid flow with a based on /S0 772:1996symbols free surface (first revision) ‘Hydrometric determinations—
Vocabulary and symbols?
ISO 1100-1 Liquid flow IS 2914:1964 Recommendations for Technically equivalent (/Smeasurement in open estimation of discharge by establish- 2914:1964 is under revisionchannels — Part 1: Esta- hing stage-discharge relation in based on /S0 1fOO-l:f996blishment and operation of a open channels ‘Measurement of liquid flow ingauging station open channels — Part 1.”
Establishment and operationof a gauging station’ and 1S01100-2:1998 ‘Measurement ofliquid flow in open channels —Part 2 : Determination of thestage-discharge relation’)
ISO 5168 Measurement of IS (under preparation based on /S0/fluid flow — Estimation of TR5168:1998 Measurement of ‘fluid
—
uncertainty of a flow-rate flow — Evaluation of uncertainties)measurement
(Continued on third cover)
IS 14974:2001
ISO 3846:1989
lndian Standard
LIQUID FLOW MEASUREMENT IN OPEN CHANNELSBY WEIRS AND FLUMES — RECTANGULAR
BROAD-CRESTED WEIRS
1 Scope and field of application
This International Standard lays down requirements for the useof rectangular broad-crested weirs for the measurement of flowof clear water in open channels under free flow conditions.
Annexes A, B and C form an integral part of this InternationalStandard.
2 References
ISO 746, Liquid flow measurement in open channels –
Velocityarea methods.
1SO 772, Liquid flow measurement in open channels –
Vocabulary and symbols.
ISO 11(X-1, Liquid flow measurement in open channels –
Part 1: Establishment and operation of a gauging station.
/S0 5166, Measurement of fluid flow – Estimation of uncer-
tainty of a flow-rate measurement.
1SO 6366, Liquid flow measurement in open channels –
Guidelines for the selection of flow gauging structures.
3 Definitions
For the purposes of this International Standard, the definitionsgiven in 1S0 772 apply. The symbols used in this InternationalStandard are giveri in annex A.
4 Installation
The conditions regarding the preliminary survey, selection ofsite, installation, approach channel, maintenance, measure-
ment of the head, and stilling or float wells which are generallynecessary for flow measurement are given in 4.1, 4.2, clause 5and clause 6. The particular requirements for the rectangularbroad-crested weir are given separately in clause 7.
4.1 Selection of site
A preliminary survey shall be made of the physical ai]dhydraulic features of the proposed site to check that it con-forms (or may be made to conform) to the requirementsnecessary for flow measurement by the weir.
Particular attention shall be paid to the following features inselecting the site for the weir:
a) the availability of an adequate length of channel ofregular cross-section;
b) the existing velocity distribution;
c) the avoidance of a steep channel, if possible (see 4.2.2);
d) the effects of any increased upstream water level due tothe measuring structure;
e) the conditions downstream, including influences suchas tides, confluences with other streams, sluice gates, milldams and other controlling features, which might causedrowning;
f) the impermeability of the ground on which the structureis to be founded, and the necessity for piling, grouting orother means of controlling seepage;
g) the necessity for flood banks to confine the maximumdischarge to the channel;
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IS 14974:2001
1S0 3846:1989
h) the stability of the banks, and the necessity for trim-ming and/or revetment in natural channels;
i) the clearance of rocks or boulders from the bed of theapproach channel;
j) the effects of wind, which can have a considerableeffect on the flow ina river, orover a weir, especially whenthe river or weir is wide and the head is small and when theprevailing wind is ina transverse direction.
If the site does not possess the characteristics necessary forsatisfactory measurements, the site shall .be rejected unlesssuitable improvements are practicable.
If an inspection of thestream shows that the existing velocitydistribution is regular, then it maybe assumed that the velocitydistribution will remain satisfactory after the construction of theweir.
If the existing velocity distribution is irregular and no other sitefor a gauge is feasible, due consideration shall be given tochecking the distribution after the installation of the weir and toimproving it if necessary.
Several methods are available for obtaining a more preciseindication of irregular velocity distribution. These includevelocity rods, floats or concentrations of dye, which can beused in small ~hannels; the last is useful to check the conditionsat the bottom of the channel. A complete and -quantitativeassessment of the velocity distribution may be made by meansof a current-meter. More information about the use of current-meters is given in ISO 748.
4.2 Installation conditions
4.2.1 General requirements
The complete measuring installation consists of an approachchannel, a measuring structure and a downstream channel.The conditions of each of these three components affect theoverall accuracy of the measurements.
Installation requirements include features such as the surfacefinish of the weir, the cross-sectional shape of the channel, thechannel roughness, and the influence of control devicesupstream or downstream of the gauging structure.
The distribution and direction of velocity have an importantinfluence on the performance of a weir, these factors beingdetermined by the features mentioned above.
Once a weir has been installed, the user shall prevent anychanges which could affect the discharge characteristics.
4.2.2 The approach channsl
On all installations the rlow in the approach channel shall besmooth, free from cwturbance and have a velocity distributionas normal as possible over the cross-sectional area. This canusually be verified Dy inspection or measurement. In the case ofnatural -streams or rivers, this can only be attained by having a
long straight approach channel free from projections into theflow. The following general requirements shall be compliedwith.
a) The altered flow conditions owing to the constructionof the weir might cause a build-up of shoals of debr%upstream of the structure, which in time might affect theflow conditions. The likely consequential changes in thewater level shall be taken into account in the design ofgauging stations.
b) In an artificial channel the cross-section shall beuniform and the channel shall be straight for a length equalto at least 10 times its water-surface width.
c) In a natural stream or river the cross-section shallbe reasonably un[form and the channel shall be straightfor a sufficient length to ensure a regular velocity distri-bution.
d) If the entry to the approach channel is through a bend,or if the flow is discharged into the channel through a con-duit or a channel of smaller cross-section, or at an angle,then a longer length of straight approach channel may berequired to achieve a regular velocity distribution.
e) Baffles shall not be installed closer to the points ofmeasurement than a distance 10 times the maximum headto be measured.
f) Under certain conditions, a standing wave may occurupstream of the gauging device, e.g. if the approach chan-nel is steep. Provided that this wave is at a distance of notless than 30 times the maximum head upstream, flowmeasurement is feasible, subject to confirmation that aregular velocity distribution exists at the gauging station arrdthat the Froude number in this section is less than 0,3.
If a standing wave occurs within this distance the approachconditions and/or the gauging device shall be modified.
4.2.3 The measuring structure ‘
The structure shall be rigid and watertight and capable ofwithstanding flood flow conditions without distortion or frac-ture. It shall beat right angles to the direction of flow and shallconform to the dimensions given in the relevant clauses.
4.2.4 Downstream of the structure
The nappe shall not be ventilated in order to maintain waterunderneath ti I? nappe when it separates from the crest, par-ticularly for high values of hi/L. This condition can only be metif the downstream channel is rectangular and of the same widthas the weir for a distance equal to twice the maximum headdownstream of the downstream face of the weir.
The channel further downstream of the structure is usuallyof no importance as such provided that the weir has beendesigned toensure that the flow is modular under all operatingconditions.
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ISO 3846:1989
However, the water level maybe raised sufficiently to drown The connecting pipe or slot shall, however, be as small asthe weir if the altered flow conditions due to the construction of possible consistent with ease of maintenance. Alternatively thethe weir cause the build-up of shoals of debris immediately connecting pipe or slot shall be fitted with a constriction todownstream of the structure or if river works are carried out at damp out oscillations due to short amplitude waves.a later date.
The well and the connecting ~ipe or slot shall be watertight.
Any accumulation of debris downstream of the structure shall The well shall be of adequate diameter and depth to accom-
therefore be removed. mod ate the f Ioat of a level recorder, if used.
The well shall also be deep enough to accommodate any sedi-
5 Maintenance – General requirementsment, which may enter, without the float grounding. The floatwell arrangement may include an intermediate chamber,between the stilling well and the approach channel, of similar
Maimenance of the measuring structure and the approach proportions to those of the stilling well to enable sediment tochannel is important to secure accurate continuous measure- settle out. For ease of maintenance, the pipework may be fittedments. with valves.
It is essential that the approach channel to weirs be kept clean More detailed information on the stilling well may be obtainedand free from silt and vegetation as far as practicable for at least from ISO 1100-1,the distance specified in 4.2.2. The float well and the entry fromthe approach channel shall also be kept clean and free fromdeposits. 6.3 Zero setting
The weir shall be kept clean and free from clinging debris and A means of checking the zero setting of the head measuring
care shall be taken in the process of cleaning to avoid damage device shall be provided, consisting of a datum related to the
to the weir crest. level of the w-eir.
A zero check based on the level of the water when the flowceases is liable to serious errors from surface tension effects
6 Measurement of head(s) and shall not be used.
6.1 Genera! requirementsWith decreasing size of the weir and the head, small errors inconstruction and in the zero setting and reading of the head
The head upstream of the measuring structure may bemeasuring device become of greater importance.
measured by a hook gauge, point gauge or staff gauge wherespot measurements are required or by a recording gauge wherea continuous record is required. In many cases, it is preferable 7 Rectangular broad-crested weirsto measure heads in a separate stilling well to reduce the effectsof surface irregularities.
7.1 Specification for the standard weir
The discharges calculated using the working equation are The crest of the standard weir shall be a smooth, horizontal,volumetric figures, and the liquid density does not affect thevolumetric discharge for a given head provided that the
rectangular plane surface (in these specifications a “smooth”
operative head is gauged using a liquid of identical density. Ifsurface shall have a surface finish equivalent to that of rolled
the gauging is carried out in a separate well, correction for thesheet metal ). The width of the crest perpendicular to the direc-
difference in density maybe necessary if the temperature of thetion of flow shall be equal to the width of the channel in which
liquid in the well is significantly different from that of the flow-the weir k located. The upstream and downstream end faces of
ing liquid. However, it is assumed herein that the densities arethe weir shall be smooth, plane surfaces and they shall be
equal.perpendicular to the sides and the bottom of the channel inwhich the weir is located. The upstream face, in particular,
It shall, however, be ensured that the gauge is not located in ashall form a sharp right-angle corner at its intersection with theplane of the crest.
pocket or still pool, but that it measures the piezometric head.
If the upstream corner of the weir is slightly rounded, the
6.2 Stilling or float welldischarge coefficient can increase significantly.
Where prowaed, the stilling well shall be vertical and shallA typical sketch of the weir is shown in figure 1.
extend at least 0,6 m above the maximum estimated water levelto be recorded in the well. 7.2 Location of the head gauge section
It shall be connected to the approach channel by an inlet pipe, Piezometers or a point-gauge station for the measurement ofor slot, large enough to permit the water in the well to follow the head on the weir shall be located at a sufficient distancethe rise and fall of the head without significant delay. The level upstream from the weir to avoid the region of surfaceof the inlet pipe shall be at least 0,1 m below the crest level. drawdown, ‘They (or it) shall, however, be close enough to the
3
IS 14974:2001
ISO 3846:1989
weir for the energy loss between the section of the measure-ment and the control section on the weir to be negligible. It isrecommended that the head measurement section be locatedat a distance equal to three to four times the maximum head(i.e. 31rl, m,X to 4/rl, ~ax) upstream from the upstream face ofthe weir.
7.3 Provision for modular flow
Flow over a rectangular broad-crested weir is not affected bytailwater levels if the crest level is chosen such that thesubmergence ratio does not exceed the modular limit. Themodular limit is given in annex B.
8 Discharge relationships
8.1 Discharge equation
The equation of discharge is based on the use of a gaugedhead:
()Q = + 3’2gl/2K~13/2 . . . (1)
where
Q is the discharge;
g is the acceleration due to gravity;,.,
b k the width of the weir perpendicular to the directionof flow;
c is the gauged head discharge coefficient;
h, is the upstream gauged head related to the crest eleva-tion.
8.2 Discharge coefficient
The gauged head discharge coefficient C is given in figure 2and the table as a function of hi/L and hi/p, where L k thelength of the weir in the direction of flow and p is-the height ofthe weir with respect to the bottom of the approach channel.
Intermediate values of C may be obtained by linear inter-polation.
NOTE – The recommended limits of application are those values which appear within the bold rules
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ISO 3846:1989
The coefficient of discharge Chasa constant value of0,85intherange O,l < hi/L < 0,3andforh1/p < 0,15.
On the basis of the variation in C with /sl/L, distinction can bemade between the following types of flow (see figure 3).
a) Broad-crested flow, 0,1 < hi/L < 0,4: the flow acrossthe weir is parallel to the crest fora certain portion.
b) Short-crested flow, 0,4 < hi/L < 1,6: the flow istotally curvilinear.
NOTE - The distinction between the gauged head discharge coef-ficient andthetotal head discharge coefficient isexplained in annex C.
8.3 Limitations
The following general limitations are recommended.
To avoid surface tension and viscous effects, h, >0,06 m,b > 0,30 mandp > 0,15 m.
There are no calibration data available beyond the practicallimits 0,1 < L/p <4,0 and 0,1 < hi/L < 1,6.
To avoid unstable water levels, hllp < 1,6.
“These limitations have been indicated on figure 2 by dashedlines.
8.4 Accuracy
8.4.1 The relative accuracy of flow measurements made withthese weirs depends on the accuracy of the head measurementand the measurements of the dimensions of the weir, and theaccuracy of the coefficient as it applies to the weir in use.
a) Broad-crestedweir0,1 < hi/L <0,4
8.4.2 With reasonable care and skill in the construction andinstallation of these weirs, the systematic uncertainty (in percent) in the coefficient of discharge may be deduced from
x; = t [1,5 + (hl/p)4
The random uncertainty, as derived from th~ research used todetermine the coefficient, may be taken as XC = i 1 ‘%. in thiscase.
8.4.3 The method by which the uncertainties in the coef-ficients shall be combined with other Souices of errors is givenin clause 9.
9 -Uncertainties in flow measurement
This clause is intended to provide sufficient information for theuser of this International Standard to estimate the uncertaintyin a measurement of ‘discharge.
9.1 General
9.1.1 Reference should be made to ISO 5168.
9.1.2 The total uncertainty in any flow measurement can beestimated if the uncertainties from various sources are com-bined. In general, these contributions to the total uncertaintymay be assessed and will indicate whether the discharge can bemeasured with sufficient accuracy for the purpose in hand.
9.1.3 The error may be defined as the difference between theactual rate of flow and that calculated in accordance with theequation for the weir, which is assumed to be constructed andinstalled in accordance with this International Standard.
The term “uncertainty” will be used to denote the deviationfrom the true rate of flow within which the measurement isexpected to lie some 19 times out of 20 (for 95 ‘A confidencelimits).
9.2 Sources of error andthe uncertainty in the mean is2sp(atthe95 0/0 confidencelevel). This uncertainty is the contribution of random errorsin
9.2.1 Thesources oferror inthedischarge measurement may any series of experimental measurements to the total uncer-
be identified byconsidering the discharge equation tainty.
() ~ ‘2~~A13/2NOTE – The factor of 2 assumes that n is large. For n = 6 the factor
Q ,= :..32 1 should be 2,6; n = 8 requires a factor of 2,4; n = 10 requires a factorof 2,3; n = 15 requires a factor of 2,1.
where
[-)2 3/2
3is a numerical constant not subject to error;
,x k the acceleration due to gravity (this varies fromplace to place but, in general, the variation is smallenough to be neglected in flow measurements).
9.2.2 The only sources of error which need to be consideredfurther are
a) the discharge coefficient C (numerical estimates of theuncertainty in C are given in 8.4);
b) the dimensional measurement of the structure, e.g. the
width h of the weir;
c) the measured head, h,.
9.2.3 The uncertainties in b and h must be estimated by theuser. The uncertainty in their dimensions will depend on theaccuracy to which the device as constructed can-be measured;in practice this uncertainty may prove to be insignificant incomparison with other uncertainties. The uncertainty in thehead will depend on the accuracy of the head measuringdevice, the determination of the gauge zero, and the techniqueused. This uncertainty may be small if a vernier or micrometerinstrument is used, with a zero determination of comparableprecision.
9.3 Types of error
9.3.1 Errors may be classified as random or systematic, theformer affecting the reproducibility (precision) of measurementand the latter affecting its true accuracy,
9.3.2 The standard deviation of a set of n measurements of aquantity Y under steady conditions maybe estimated using thefollowing ‘equation:
[1n 112
z( Yi – W
,=1Sy =
7-1
where Y is the arithmetic mean of the n measurements.
The standard deviation of the mean is then given by
SYsF=—
fi
9.3.3 A measurement may also be subject to systematic error;the mean of very many measured values would thus still differfrom the true value of the quantity being measured. Forexample, an error in setting the zero of a water-level gauge tothe crest level produces a systematic difference between thetrue mean of the measured head and the actual value. A rep-etition of the measurement does not eliminate systematicerrors; the actual value can only be determined by an inde-pendent measurement which is known to be more accurate.
9.4 Uncertainties in coefficient values
9.4.1 The errors in this category are both random andsystematic.
9.4.2 The values of the discharge coefficients C quoted in thisInternational Standard are based on an appraisal ofexperiments, which may be presumed to have been carefullycarried out, with sufficient repetition of the readings to ensureadequate precision. However, when measurements are madeon other similar installations, systematic discrepancies betweencoefficients of discharge may well occur, which may beattributed to variations in the surface finish of the device, itsinstallation, the approach conditions, the scale effect betweenmodel and site structures, etc.
9.4.3 The uncertainties in the discharge coefficients, quotedin 8.4, are calculated on the basis of the deviation of theexperimental data (from various sources) from the theoreticalequations given. The suggested uncertainty values thus rep-resent the accumulation of evidence and experience available.
9.5 Uncertainties in measurements made by the
user
9.5.1 Both random and systematic errors will occur inmeasurements made by the user.
9.5.2 Since neither the methods of measurement nor the wayin which they are to be made is specified, no numerical valuesfor uncertainties in this category can be given; they shall beestimated by the user. For example, consideration of themethod of measurement of the width of the weir should permitthe user to determine the uncertainty in this quantity.
9.5.3 The uncertainty in the value of the gauged head shall bedetermined from an assessment of the separate sources ofuncertainty, e.g. the uncertainties in the zero setting, the pre-vailing wind characteristics, the gauge sensitivity and thebacklash in the indicating equipment (where appropriate), andthe residual uncertainty in the mean of a series of measure-ments (where appropriate).
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IS 14974:2001
ISO 3846:1989
9.6 Combination of uncertainties 9.6.5 Thepercentage systematic uncertainty XQinthe rateofflow may be calculated from the foliowing equation:
9.6.1 The total systematic or iandom uncertainty is the re-sultant of severai contributory uncertairlties, which may ..——— —.—
themselves be composite urrcertai,wies. Pr:>vi:ded that theXl -= f J.Y;;2+ x;;2 + I,@ Xj;12
contributing uncertainties are independal~t, small andnumerous, they may be comb!ned togetk!et to give an overall where
random (or systematic) uncertainty at the 95 YO confidencelevel. X; is thepercentage systematic uncertainty in C’;
9.6.2 All sources contributing uncertainties will have both~;;
istbe percentage systematic uncertainty in b;
random and systematic components, However, in some caseseither the random or the systematic component may be x;;, is the percent~ges ystematic uncertainty in h,.
predominant and the other component can be neglected bycomparison. In the above
9.6.3 Because of the differem nature of random and x;;, = (, X;:,2 + 2X;:12 + .,, )1’2
systematic uncertainties, they should not normallv be com-bined with each o?her, How&er, with the provisa of 9.6.1,
, ,,1$2X;;,, .where X“ are percentage systematic uncertaintiesrandom uncertaintiesfmm different sources may be combinedtogether by l.he rcmt-surn-of squares wle; systematic uncer- in the head measurement (see 9.5.3).
tainties from different sources rnav be similarly combined
9.7 Presentation of results9.6.4 The percentage random uncertainty X; in the rate of
flow may becalculatedfmm the following equation: Although it is desirable, and frequently necessary, to list tlwtottd random and total systematic uncertainties separately, i[ ,,5
X;j= L ~ X;2 + 1,52X;; appreciated that a simpler presentation of results ITIay be r{:quired. For this purpose, random and systematic uncerlaintl ~~nmy be combined as shown in ISO 5168:
where
A“;. is the percentage random uncertainty in C; XQ = ?. @ + .Y;’2.
X;, is thfl percentage random uncertainty in b;
X;, IS the percentage random uncertainty in /?l\
10 Example
l-he following is an example of the computation of the dis-In ll~e above ctiarge and the associated uncertainty in a single measurernw?t
~Jfilow using a rectangular broad-crested weir for modular flowconditions. The crest height p above the bed of the approach
x;> =- 100::chailnel is 0,3 m and the gauged head h, is 0,4 m. The width bof the weir crest and the width B of the approach are botb
and equal to 10 m, The length L of the weir in the direction of flowis 0,5 rn. A digital punched tape recorder operating at intervdls
X;ll = (l.q + ~.x;,~ -t + X:,:)1 ‘2 of 1 mm is assumed to be used.
where 10.1 The discharge is calculated using equation (1), givenin 8.1,
(’}, is the random uncertfiintv in the breadth Ineasurernent;
,, ~l’~hl, 2X[11, ‘“ are percentage !anfj(.inl urlcel lainties in the 10.2 The value of the gauged head discharge coefficient C
head measurement (see 9.5.3); fur the corresponding values of /?,/1.= 0,8, /r,//) = 1,333 and[./[> = 1,667 is determined from figure 2 to be C = 1,043.
x;,, is the percentage rando,m uncertainty in the mean if.aseries of readil~gs of bead meastjrement are taken at con- 10.3 Using equation (1) :stant water level,
The term X;,, is easily estimated if, for example, a point gauge is()
Q = .: 3’2g1i2 bc~13/2
~sed for water level measurement, For continuous or digitalrecording equipment, the random uncertainty in reading a = 1,705 x 10 x 1,043 x 0,432given water level -can be assessed by using laboratory tests onthat equipment, = 4,50 m3/s
8
10.4 To calculate the uncertainty in this value of Q, the
uncertainties (in per cent) in the coefficient value are first deter-
mined as follows :
X; = + 1 Y. (from 8.4)
X: = t [1,5 + (hl/p)2] (from 8.4)
– + 3,28 %—
10.5 If it is assumed that several measurements of the width
are taken, the random component of uncertainty in the widthmeasurement can be considered to be negligible. Thesystematic uncertainty in the width measurement is assumed inthis case to be 0,01 m.
Accordingly,
X;l =0
0,01X;; = i — x 10I3= i 0,10 70
10
10.6 The magnitude of the uncertainty associated with the
head measuring device depends on the particular equipment
used. It has been demonstrated that the gauge zero of a digitalpunched tape recorder can be set to an accuracy of f 3 mm.This is a systematic uncertainty. There is no random uncer-tainty associated with the zero setting error because, until thezero is reset, the true zero will have the same magnitude andsign.
Therefore,
x.–()1 ill –
,, = + 0,0031X121 — x 100= * 0,75 %
- 0,4
10.7 Uncertainties associated with different types of waterlevel observation equipment can be determined using carefultests under controlled conditions. The random component ofuncertainty can be determined by taking a series of readings ata given water level; however, to distinguish this uncertaintyfrom other -sources of -uncertainty it is necessary that thesetests be carried out with the water level always rising (orfalling), For the equipment used in this example, the randomcomponent of uncertainty in water level measurement is
approximately + 1 mm. Systematic uncertainties in water levelmeasurement occur owing to backlash, tape stretching, etc.
Where possible, corrections should be applied, but controlled
tests for given types of equipment will indicate the magnitude
Is 14974:2001
ISO 3846:1989
of the residual systematic uncertainty. In this case, when adigital punched tape recorder is used, this value is approxi-mately * 2,5 mm.
Accordingly
0,0012X:, . + –— X 100= + 0,25 yO0,4
0,00252X;, = k —
0,4X 100= t (),63%
10.8 The combination of individual uncertainties to obtainthe overall uncertainty in discharge can be carried out asfollows.
Assuming that X;, is negligible the uncertainties in wate!measurement are
– f 0,25 Yo—
X;l = f (1X;12 + 2X;12)”2 = f (0,752
= + (3,98 %
The total random uncertainty in the disharge
XQ = * (x~2 + x;2 + I,52x;:)112
—— + (12 + O + 2,25 X 0,252)1/2 Yo
= 1,07 %
+ 0,632)’/2
level
‘=/0
measurement is
The total systematic uncertainty in the discharge measurementis
XQ = ~ (X;* + X~2 + l,52x~7z)l/2
—— * (3,282 + 0,1’2 + 2,25 x 0,w2)l/2 %
= * 3,60 %
To facilitate a simple presentation, the random and systematicuncertainties can be combined by the root-sum-of-squares ruleas follows:
XQ = + (x; + XQ2)”2
—— f (l, @72 + 3,602)1/2 %
== f 3,76 %
The flow rate “Q is therefore 4,50 ins/s * 3,8 ‘7.. The randomuncertainty is + 1,07 O/O.
9
IS 14974:2001ISO 3846:1989
Annex A
Symbol
A
B
b
c
CD
.Cv
eb
g
HI
h,
h2
L
n
P
Qs
s,
Sy
SF
VIx
x~
xc
x~l
Xm
XQ
Nomenclature
(This annex forms an integral part of the standard. )
area of the approach charmel
width of the approach channel
width of the weir crest perpendicular to the flow direction
discharge coefficient (gauged head)
discharge coefficient (total head)
velocity of approach factor
random uncertainty in the width measurement
acceleration due to gravi~,
upstream total head above crest level
upstream gauged head above crest level
downstream gauged head above crest level
length of the weir in the direction of flow
number of measurements in a set
height of weir (difference between mean bed level and crest level)
total discharge
submergence ratio, h2/hl
modular limit
standard deviation of quantity Y
standard deviation of the mean ~
mean velocity in the approach channel
overall percentage uncertainty
percentage uncertainty in b
percentage uncertainty in C
percentage uncertainty in h,
percentage uncertainty in the mean of a set of head measurementreadings
percentage uncertainty in Q
Superscripts
random component of uncertainty
,, systematic component of uncertainty
Unit
m2
m
m
non-dimensional
non-dimensional
non-dimensional
m
mlsz
m
m
m
m
non-dimensional
m
m3/s
non-dimensional
non-dimensional
1)
1)
mls
70
0/0
%0
%
70
%0
1) The unit is the same as that of the quantity Y,
10
IS 14974:2001
ISO 3846:1989
Annex B
Submerged flow
(This annex forms an integral part of the standard. )
Sufficient data are not available to predict submerged flow. if S < S1, the flow is free flow, and
if .S > S1, the flow is submerged flow.Submerged flow will not occur as long as the modular limit isnot exceeded. /rz is defined as the downstream gauged head above crest level.
Figure 4 shows the modular limit.The modular limit S1 is defined as the submergence ratioS = /rJ/r, for which the deviation between the submersed Theoretically the modular limit also de~ends on h, /~. Therefore
flow c~lculated with the free-flow head discharge equa~on,.
the values for S, given in figure 4 should be considered to be
[equation (1)1 and the real flow is 1 Yo, i.e. minimum values.
2,C
1,:
(y
Free flow
Io0 0,2
Figure 4 –
ISubmerged flow
0,4 0,6 0,8
Submergence ratio S = h2/hl
Modular limit S, as a function of hi/L
Z Modular limit S1
—
IS 14974:2001
1S0 3846:1989
Annex C
Gauged head discharge coefficient and total head
(This annex forms an integral part of the standard.)
Distinction should be made between the gauged headdischarge coefficient and the total head discharge coefficient.
a) The gauged head discharge coefficient is given by
c= Q
(-)2 3/2~1/2~~13/2
3
The head h, is measured in the rectangular approach
channel, as indicated in figure 1.
b) The total head discharge coefficient is given by
CD = -—— Q_
()’2 3’2gl/2~cv~1312T
where Cv is the velocity of approach
correct the gauged head, given by
factor, introduced to
Cv = (H1/h1)3’2
where HI is the upstreamcrest elevation, i.e.
total head with respect to the
2Hl=hl+$
where v, is the mean velocity in the approach channel.
The head h, can be measured in any shape of approachchannel.
The relation between the gauged head coefficient C and thetotal head coefficient CD is given by
c = c~cv
The gauged head discharge coefficient C is given in figure 2.
(Continued from second cover)
International CorrespondingStandard /ndian Standard
1s0 8368 Liquid flow IS 12752:1989 Guidelines for themeasurement in ooen selection of flow gauging structureschannels — Guidelines forthe selection of flow gaugingstructures
REFERENCES TO ERRORS AND CLARIFICATIONS IN TEXT
Degree ofEquivalence
Technically equivalent (/s
12752: f 989 is under revisionbased on ISO 8368:1999‘Liquid flow measurement in
open channels — Guidelinesfor the se/ection of flowgauging structures)
The Technical Committee while adopting the text of this International Standard identified certaintextual errors to the following clauses and felt necessary to correct these in the Indian context:
Clause CorrectionsReference
4.l(e) The words ‘mill dams’ are not used in Indian context.
6.1 The words ‘surface irregularities’ appearing in the first para may be replaced by‘water surface turbulence’ for clarity.
6.2 A reference to ISO 4373:1995 ‘Measurement of liquid flow in open channels —Water level measuring devices’ in the last para may also be made for its relevancein providing information on ‘stilling wells’.
9.1,2 The words ‘and will’ in the fourth line may be read as ‘to’.
Gen For the units used for various symbols, refer Annex A.
●
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This lndial] Standard has been developed from Dot: No. WRD 1 (246).
Amendments Issued Since Publication
Amend No. Date of Issue Text Affected
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