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1、International Standard 4371 INTERNATIONAL ORGANIZATION FOR STANDAROIZATION*MEKI1YHAPOlHAR OPTAHM3AUMR no CTAHI1APTH3AUMbl*ORGANISATlON INTERNATIONALE DE NORMALISATION Measurement of liquid flow in open channels by weirs and flumes - End depth method for estimation of flow in non-rectangular channels
2、 with a free overfall (approximate method) Mesure de dbit des liquides dans les canaux dcouverts au moyen de dversoirs et de canaux jaugeurs - Mthode dvaluation du dbit par dtermination de la profondeur en bout des chenaux non rectangulaires dversement dnoy (mthode approximative) First edition - 198
3、4-12-15 ui - UDC 532.532 Ref. No. IS0 4371-1984 (E) z Descriptors : liquid flow, water flow, open channel flow, weirs, flow measurement. 5 s Price based on 11 pages COPYRIGHT International Organization for Standardization Licensed by Information Handling Services COPYRIGHT International Organization
4、 for Standardization Licensed by Information Handling Services 4371-84 Foreword IS0 (the International Organization for Standardization) is a worldwide federation of national standards bodies (IS0 member bodies). The work of preparing International Standards is normally carried out through IS0 techn
5、ical committees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, govern- mental and non-governmental, in liaison with ISO, also take part in the work. Draft International Sta
6、ndards adopted by the technical committees are circulated to the member bodies for approval before their acceptance as International Standards by the IS0 Council. They are approved in accordance with IS0 procedures requiring at least 75 YO approval by the member bodies voting. International Standard
7、 IS0 4371 was prepared by Technical Committee ISO/TC 113, Measurement of liquid flow in open channels. International Organization for Standardization, 1984 O Printed in Switzerland - 1 4853903 0033437 E! COPYRIGHT International Organization for Standardization Licensed by Information Handling Servic
8、es COPYRIGHT International Organization for Standardization Licensed by Information Handling Services 4373-84 4 4851903 0033438 4 INTERNATIONAL STANDARD IS0 4371-1984 (E) Measurement of liquid flow in open channels by weirs and flumes - End depth method for estimation of flow in non-rectangular chan
9、nels with a free overfall (approximate method) O Introduction Free overfall occurs in many hydraulic structures when the bot- tom of a horizontal channel (or gently sloping channel) is ab- ruptly discontinued. Such an overfall forms a control section and offers an approximate means for the estimatio
10、n of flow. The flow at the brink is curvilinear and, therefore, the depth at the drop is not equal to the critical depth as computed by the principle based on assumption of parallel flow. However, the ratio between the end depth and the critical depth (as in the case of the assumption of parallel fl
11、ow) has an almost constant value. Therefore, from the depth measured at the drop, the discharge can be estimated. 1 Scope and field of application This International Standard specifies a method for the esti- mation of subcritical flow of clear water in smooth, essentially horizontal, straight open c
12、hannels with a vertical drop and discharging freely. Gentle positive slopes not greater than 1 in 2 o00 are admissible. This International Standard covers chan- nels with the following types of cross-section, the nappe being unconfined: a) trapezoidal; b) triangular; ci parabolic; d) circular. Using
13、 the measured depth at te end, the flow can be estimated. 2 References IS0 772, Liquid flow measurement in open channels - Vocabulary and symbols. IS0 143/1, Water flow measurement in open channels using weirs and venturi flumes - Part I : Thin-plate weirs. IS0 3846, Liquid flow measurements in open
14、 channels by weirs and flumes - Free overfall weirs of finite crest width lrec- tangular broad-crested weirs). IS0 3847, Liquid flow measurement in open channels by weirs and flumes - End-depth method for estimation of flow in rec- tangular channels with a free overfall. 3 Definitions For the purpos
15、e of this International Standard, in addition to the definitions given in IS0 772, the following definition shall apply: unconfined nappe: The jet formed by the flow where the guide walls of the structure end at the crest (or edge) and permit free lateral expansion of flow and where the nappe is suf
16、ficiently ventilated to ensure atmospheric pressure below the nappe. 4 Units of measurement The units of measurement used in this International Standard are SI units. 5 Selection of site A preliminary survey shall be made of the physical and hydraulic features of the proposed site to check that it c
17、on- forms (or may be made to conform) to the requirements necessary for measurement by the end depth method. Particular attention should be paid to the following features in selecting the site and ensuring the necessary flow conditions: a) an adequate straight length (at least 20 he where he is the
18、end depth corresponding to the maximum discharge anticipated) of channel of regular cross-section should be available upstream of the drop; 1 COPYRIGHT International Organization for Standardization Licensed by Information Handling Services COPYRIGHT International Organization for Standardization Li
19、censed by Information Handling Services 4373-84 IS0 4371-1984 (E) 7.2 Trapezoidal channels b) velocity distribution seen by inspection or measurement should be normal; 7.2.1 figure 1. The geometry of channel cross-section is shown in c) the channel bottom should be horizontal. Gentle positive slopes
20、 not greater than 1 in 2 O00 are admissible; 7.2.2 The ratio helhc (that is, end depth to critical depth) is a function of the parameter d) the side walls as well as the bottom should be smooth as far as possible (in this specification a smooth surface shall correspond to a neat cement finish); he B
21、 O NOTE - The finish of the structure shall be well maintained; changes in wall roughness due to erosion and various forms of deposition will change the discharge relationship. where e) the end of the channel shall be cut off normal to its centreline and the water shall be allowed to fall freely bey
22、ond this point; m is the side slope; Bo is the bottom width; he is the depth of flow at the end; f) the flow shall be sub-critical and normal upstream of the drop; and the value of h,lh, can be obtained from figure 2. Knowing the value of he, the value of h, can be computed and used to compute the d
23、ischarge from equation (1). g) the sides to permit unrestricted spreading. the nappe should be fully aerated and completely free at 7.2.3 As an alternative to equation (11, the discharge in terms of h, is given by the following equation : 6 Measurement of depth The depth shall be measured midstream
24、exactly at the end (drop) with a point gauge or other suitable measuring device. NOTE - The flow at the drop is fully curvilinear and any small error in the location of gauge will result in large errors in measurement of discharge. 7 Computation of discharge Figure 3 may be used directly for computi
25、ng the discharge, based on equation (21, in view of the simplicity of calculations. 7.1 Critical depth, h, 7.3 Triangular channels From the measured value of the end depth he, using the rela- tionship for the respective channel cross-section, the critical depth h, is computed and the discharge in te
26、rms of the critical depth is given by the following equation (for a channel cross- section) : 7.3.1 figure 4. The geometry of channel cross-section is shown in 7.3.2 The ratio h,lh, (that is, end depth to critical depth) is 0,795. 7.3.3 With a triangular channel of semi-apex angle 8, as an alternati
27、ve to equation (1 1, the discharge in terms of h, is given by: Q = g is the acceleration due to gravity; 7.4 Parabolic channels A, is the cross-sectional area at the critical section; 7.4.1 figure 5. The geometry of channel cross-section is shown in B, is the surface width of flow at the critical se
28、ction. From a knowledge of the critical depth h, and the geometry of the channel, A , and B, can be obtained and by using equation (11, the discharge can be computed. 7.4.2 The ratio h,lh, (that is, end depth to critical depth) is 0,772. 2 COPYRIGHT International Organization for Standardization Lic
29、ensed by Information Handling Services COPYRIGHT International Organization for Standardization Licensed by Information Handling Services 7.4.3 With a parabolic channel of the form, x2 = 4ay . . . (4) As an alternative to equation (I), the discharge in terms of h , is given by: Q = 2,175 b) the foll
30、owing limitations based on experiments should be satisfied : m he in the case of trapezoidal channels, the ratio - Bo the drop to tailwater level, d, should be equal to or 1) should be between 0,5 and 7,O; 2) in the case of triangular channels, the semi-apex angle f 3 should be between 2 5 O and 4 5
31、 ; 3) in the case of parabolic channels, the semi-latus rec- tum “2a“ should lie between 0,019 and 0,033 m; 4) in the case of circular channels, the ratio helr (that is, the end depth to the radius of the channel) should lie between 0,19 and l,O; IS0 4371-1984 (E) 9 Uncertainties in flow measurement
32、 9.1 General 9.1.1 The total uncertainty of any flow measurement can be estimated if the uncertainties from various sources are com- bined. In general, these contributions to the total uncertainty may be assessed and will indicate whether the rate of flow can be measured with sufficient accuracy for
33、 the purpose in hand. This clause is intended to provide information for the user of this International Standard to estimate the uncertainty in a measurement of discharge. 9.1.2 The error may be defined as the difference between the true rate of flow and that calculated in accordance with the equati
34、on of the type of channel at a site selected in accordance with this International Standard. The term “uncertainty“ will be used to denote the deviation from the true rate of flow within which the measurement is expected to lie some nineteen times out of twenty (95 % confidence limits). 9.2 Sources
35、of error 9.2.1 identified by considering the appropriate discharge equation. The sources of error in discharge measurement may be 9.2.2 The sources of error which need to be considered fur- ther are: a) the ratio helh,; b) the dimensional measurement of the channel (for example, Bo in the case of tr
36、apezoidal channels, f3 in the case of triangular channels, a in the case of parabolic chan- nels and r in the case of circular channels); c) the measured end depth, h , . 9.2.3 The uncertainties in dimensional measurements and in he shall be estimated by the user. The uncertainties in dimen- sional
37、measurement will depend on the precision to which the channel as constructed can be measured; in practice, this uncertainty may prove to be insignificant in comparison with other uncertainties. The uncertainty in the end depth will depend upon the accuracy of the depth-measuring device, the determin
38、ation of the gauge zero, the precise location of the instrument and upon the technique used. 9.3 Kinds of error c) the method is recommended for use when he is greater than 0.05 m; d) the width of flow at the top should be greater than 0,3 m. 9.3.1 Errors may be classified as random or systematic, t
39、he former affecting the reproducibility (precision) of measurement and the latter affecting its true accuracy. 3 COPYRIGHT International Organization for Standardization Licensed by Information Handling Services COPYRIGHT International Organization for Standardization Licensed by Information Handlin
40、g Services IS0 4371-1984 (E) 9.3.2 The standard deviation of a set of n measurements of a quantity Y under steady conditions may be estimated from the equation y y = n - I J . . . (7) where Y is the arithmetic mean of n measurements. The stan- dard deviation of the mean is then given by: . . . (8) S
41、Y SF= J“ and the uncertainty of the mean is fs,(to 95 % confidence level) 1). This uncertainty is the contribution of the observations of Y to the total uncertainty. 9.3.3 A measurement may also be subject to systematic error; the mean of very many measured values would thus still differ from the tr
42、ue value of the quantity being measured. An error in setting the zero of a water level gauge to invert level, for example, produces a systematic difference between the true mean measured head and the actual value. As repetition of the measurement does not eliminate systematic errors, the actual valu
43、e could only be determined by an independent measure- ment known to be more accurate. 9.4 Uncertainties of the ratio helh, 9.4.1 The values of the ratio helh, quoted in this International Standard are based on an appraisal of experiments, which may be presumed to have been carefully carried out, wit
44、h sufficient repetition of the readings to ensure adequate precision. However, when measurements are made on other installations, systematic discrepancies between coefficients of discharge may well occur, which may be attributed to variations in sur- face finish, the approach conditions, the scale e
45、ffect between model and site structures, etc. 9.4.2 The uncertainty in the ratios quoted in the preceding clauses of this International Standard is based on a consider- ation of the deviation of experimental data from the equations given. The suggested uncertainties thus represent the ac- cumulation
46、 of evidence and experience available. 9.4.3 The maximum systematic uncertainty in the ratio h,lh, is likely to be f 5 % from the specified values, with 95 % con- fidence limits. 9.5 user Uncertainties in measurements made by the 9.5.1 Both random and systematic errors will occur in measurements mad
47、e bv the user. 9.5.2 Since neither the methods of measurement nor the way in which they are to be made are specified, no numerical values for uncertainties in this category can be given; they shall be estimated by the user. For example, consideration of the method of measuring the channel width shou
48、ld permit the user to determine the uncertainty in this quantity. 9.5.3 The uncertainty of the gauged depth shall be deter- mined from an assessment of the individual sources of error, for example the zero error, the gauge sensitivity, backlash in the indication mechanism, the residual random uncert
49、ainty in the mean of a series of measurements, etc. The uncertainty on the gauge depth is the square root of the sum of the square of the individual uncertainties. 9.6 Combination of uncertainties to give total uncertainty on discharge 9.6.1 The total uncertainty is the resultant of several con- tributory uncertainties, which may themselves be composite uncertainties. When partial uncertainties, the combination of which gives the total uncertainty, are independent of one another, are small and numerous and have a Gaussian distribution, there is a probability