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UDC 532.57: 532.543:627.133
IS0
I NT ERN AT I ON AL ORGAN I ZAT I O N FOR STAND AR D I Z AT I O N
IS0 R E CO M M EN DATl O N
R 74%
LIQUID FLOW MEASUREMENT IN OPEN CHANNELS
BY VELOCITY AREA METHODS
1st EDITION
June 1968
COPYRIGHT RESERVED
The copyright of IS0 Recommendations and IS0 Standards
belongs to IS0 Member Bodies. Reproduction of these
documents, in any country, may be authorized @herefore only
by the national standards organization of that country, being
a member of ISO.
For each individual country the only valid standard is the national standard of that country.
Printed in Switzerland
Also issued in French and Russian. Copies to be obtained through the national standards organizations.
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BRIEF HISTORY
The IS0 Recommendation R 748, Liquidflow measurement in open channels by velocity
area methods, was drawn up by Technical Committee ISO/TC 113, Measurement of liquid
flow in open channels, the Secretariat of which is held by the Indian Standards Institution
(ISI).
Work on this question by the Technical Committee began in 1956 and led, in 1965,
to the adoption of a Draft IS0 Recommendation.
In June 1966, this Draft IS0 Recommendation (No. 948) was circulated to all the
IS0 Member Bodies for enquiry. It was approved, subject to a few modifications of an
editorial nature, by the following Member Bodies :
Argentina Greece New Zealand
Belgium India Romania
Brazil Ireland South Africa, Rep. of
Canada Israel Switzerland
Chile Italy Turkey
Czechoslovakia Japan United Kingdom
France Korea, Rep. of
Germany Netherlands
No Member Body opposed the approval of the Draft.
The Draft IS0 Recommendation was then submitted by correspondence to the IS0
Council, which decided, in June 1968, to accept it as an IS0 RECOMMENDATION.
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IS0 / R 748 - 1968 (E)
CONTENTS
Page
1. Scope . 5
............................. 5
2. Terminology
3. Units of measurement . . 5
4. Principle of the methods of measurement .
5
5. Selection and demarcation of site .
6
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6. Measurement of cross-sectional area .
7
7. Measurement of velocity . 9
8. Computation of discharge . 17
9. Errors of flow measurement . 22
ANNEX A - Methods of correcting for sag, pull, slope and temperature in
measuring the width of the cross-section by tape or wire .
26
ANNEX B . Methods of measurement across the cross-section .
28
ANNEX C - Corrections for wetted length of wire when measuring depths
with the wire not normal to the surface . 31
ANNEX D - Correction for drift . 32
ANNEX E - Examples of the individual components of the overall error . . 33
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IS0 / R 748 - 1968 (E)
IS0 Recommendation R 748 June 1968
LIQUID FLOW MEASUREMENT IN OPEN CHANNELS
BY VELOCITY AREA METHODS
1. SCOPE
This IS0 Recommendation deals with the methods for determining the velocity and cross-
sectional area of water flowing in open channels, and of computing the discharge therefrom.
It covers methods of employing current meters and floats to measure the velocities. Although,
in most cases, these measurements are intended to determine the stage-discharge relationship
by means of readings at several stages, this IS0 Recommendation deals only with single measure-
ments of the discharge; the continuous recording of discharges over a period of time will be
covered in a separate IS0 Recommendation.
L
NOTE. - Measurements for the purpose of determining the discharge in efficiency tests of hydraulic turbines are
specified in Publication 41, International code for the field acceptance tests of hydraulic turbines, of the International
Electrotechnical Commission.
2. TERMINOLOGY
For the purposes of this IS0 Recommendation, the definitions given in IS0 Recommendation
R 772, Vocabulary of terms and symbols used in connection with the measurement of liquid flow
with a free surface, apply.
3. UNITS OF MEASUREMENT
The units of measurement used in this IS0 Recommendation are seconds, metres (or feet),
kilogrammes and degrees Celsius (or Fahrenheit).
4. PRINCIPLE OF THE METHODS OF MEASUREMENT
c
4.1 The principle of these methods consists in effectively measuring the flow velocity and cross-
sectional area. A gauging site is chosen conforming to the specified requirements; the width
is measured either by means of a steel tape or by some other surveying method, depending on
its magnitude, and the depth is measured at a number of verticals along the width, sufficient
to determine the shape and area of cross-section.
Velocity observations are made at the same time as measurement of depth, especially in the
case of unstable beds; they are made by any one of the standard methods by the use of
current meters, the principle of which is based on the proportionality between the local flow
velocity and the speed of the rotor. Velocity observations are also made by the use of surface
floats and velocity rods under certain circumstances. From the individual observations the
mean velocity is generally computed; however, in certain methods such as the integration
method, the mean velocity is directly obtained.
4.2 The discharge is computed by summing the products, either arithmetically or graphically, of
the velocity and corresponding area for a series of observations in a cross-section.
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5. SELECTION AND DEMARCATION OF SITE
5.1 Initial survey of site
it is desirable that approximate measurements of widths, depths and velocities should be
made in a preliminary survey to decide on the suitability of a site conforming as far as possible
with the conditions given in clause 5.2. It is only intended that these measurements should
serve as a guide that both the longitudinal and transverse bed profiles and the velocity dis-
tribution are acceptable for the purpose of discharge measurement.
5.2 Selection of site
The accuracy of the determination of the discharge by the velocity area method is increased if
(a) the conditions of flow do not change within the period of measurement;
(b) the velocities at all points are parallel to one another and at right angles to the
measuring cross-section ;
(c) the curves of the distribution of the velocities are regular in the vertical and hori-
zontal planes on which they are measured;
(d) the geometrical dimensions of the cross-section of the open channel are clearly
defined.
Hence, the most favourable conditions for accurate measurements are to be found where the
cross-section is in a straight reach. On this basis, therefore, the site should be selected such
that, as far as possible, the requirements and points given in clauses 5.2.1 and 5.2.2 are met.
5.2.1 The site selected should comply, as far as possible, with the following requirements:
(U) the open channel at the gauging site should be straight and of uniform cross-
section and slope, as far as possible, in order to avoid an abnormal velocity
distribution. When the length of the straight channel is restricted, it is recom-
mended for current meter measurements that the straight length upstream
of the measuring cross-section should be twice that downstream ;
(b) the depth of water in the selected reach should be sufficient to provide for the
effective immersion of either the current meters or floats, whichever are to
be used;
(c) the view of the gai sing site should be clear and unobstructed by trees or other
obstacles.
5.2.2 In addition to the requirements specified in clause 5.2.1, the following points should be
taken into consideration when selecting the gauging site:
(a) the bed of the reach should not be subject to changes during the period of
measurement ;
(b) irrespective of the flow, all discharges should be contained within a defined
channel or channels, or within an unobstructed floodway having substantially
stable boundaries, with well-defined geometrical dimensions;
(c) the site should be remote from any bend or natural or artificial obstruction if
disturbance of the flow is likely to be caused thereby;
(d) the gauging site should be kept clear of aquatic growth during the period of
measurement ;
(e) sites at which vortex, backward flow, or dead zones tend to develop should be
avoided ;
(f) measurement with converging, and more so with diverging, flow over an
oblique measuring section should be avoided as it is difficult to allow for the
systematic errors that can arise;
(g) the orientation of the reach should be such that the direction of flow is as
closely as possible normal to that of the prevailing wind.
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IS0 / R 748 - 1968 (E)
Where these requirements cannot be met (for instance, when in alluvial rivers the river
bed is changing during the period of measurement, or when, under flood conditions, the
river is not confined to a single channel in embankments), a gauging site should be
chosen such that the bed change and/or overflow is a minimum. Flood plains-if they
cannot be avoided-should be of minimum width, as smooth as possible, without a
distinct channel, and clear of bushes and trees.
In those instances where it is necessary to make measurements in the neighbourhood of
a bridge, it is preferable that the gauging site should be upstream of the bridge. However,
in special cases and where accumulation of ice, logs or debris is liable to occur, it is
preferable that the gauging site should be downstream of the bridge.
NOTE. - If, after the site has been selected, unacceptable changes occur in the channel conditions, another
site should be selected for the measurements.
5.3 Demarcation of site
The site, after selection, should be provided with means for demarcation of the cross-section
and for determination of the stage.
5.3.1 The position of each cross-section, normal to the mean direction of flow, should be
defined on the two banks by clearly visible and readily identifiable markers.
5.3.2 The stage should be read from a gauge at intervals throughout the peiiod of measurement
and the gauge datum should be related by precise levelling to a standard system of levels.
5.3.3 An auxiliary gauge on the opposite bank should be instaIled where there is likelihood
of a difference in the level of water surface between the two banks. This is particularly
important in the case of very wide rivers. The mean of the measurements taken from the
two gauges should be used as the mean level of the water surface and as a base for the
cross-sectional profile of the stream.
6. MEASUREMENT OF CROSS-SECTIONAL AREA
The cross-sectional profile of the open channel at the gauging site should be determined by
measuring, at the cross-section, the depth relative to the water surface at a sufficient number of
points to establish the shape of the bed.
The location of these points should be determined by measuring their horizontal distance to a
fixed reference point in the cross-section. In practice, these measurements determine the cross-
sectional area of the individual segments between successive verticals where velocities are mea-
sured.
6.1 Measurement of widths
The measurements of the width of the channel and the widths of the individual segments may
be obtained by measuring the distances from or to a fixed reference point, which should be
in the same plane as the cross-section at the gauging site.
6.1.1 Where the width of the channel permits, or when the surface is covered by ice, these
distances should be measured by direct means, e.g. a steel tape or suitable marked wire,
care being taken to apply the necessary corrections given in Annex A. The intervals
between the verticals, i.e. the widths of the segments, should be similarly measured.
6.1.2 Where the channel is too wide for the above methods of measurement, the distances
should be determined by optical or electrical distance meters, or by one of the surveying
methods given in Annex B.
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IS0 / R 748 - 1!HE (E)
6.2 Measurement of depth
6.2.1 Measurements of depth should be made at intervals close enough to define the cross-
sectional profile accurately. In general, the intervals should not be greater than 1/15
of the width in the case of regular bed profiles, and should not be greater than 1/20
of the width in the case of irregular bed profiles.
NOTE. - For small channels with a regular bed profile, the number of intervals may be reduced. This
may, however, affect the accuracy of the determination of the bed profile (see section 9).
6.2.2 The depth should be measured employing either sounding rods or sounding lines, or
other suitable devices. Where the channel is of sufficient depth an echo-sounder may be
used. If the velocity is high, it is preferable to use an echo-sounder or other device which
will not require large corrections.
NOTE. - When measuring the depth under an ice cover on the water, it is necessary to determine the
space, if any, between the water surface and the underside of the ice by using special devices such as
L-shaped scales. If there is a layer of frazii ice its thickness should also be determined.
6.2.3 When a sounding rod or sounding line is used, at least two readings should be taken at
each point and the mean value adopted for calculations, unless the difference between
the two values is more than 5%, in which case two further readings should be taken.
If these are within 5 %, they should be accepted for the measurement and the two earlier
readings discarded. If they are again different by more than 5%, no further readings
should be taken, but the average of all four readings should be adopted for the measure-
ment, noting that the accuracy of this measurement is reduced.
When an echo-sounder is used, the average of several readings should preferably always
be taken at each point, but regular calibrations of the instrument are required under
the same conditions of salinity and temperature of the water.
Where it is impracticable to take more than one reading of the depth, the error in
measurement may be increased (see section 9).
NOTES
I. Where measurements of the depths are made separately from the velocity measurements and the
water level is not steady, it should be observed at the time of each measurement of depth. If this is
not possible, the water level should be observed at intervals of 15 minutes and the value of the level
at the time of each determination of depth should be obtained by interpolation.
2. If the bed is composed of unstable material (shifting sand, silt) repeated measurements by a rod or
line of the depth at one point are not recommended with a view to avoiding disturbance of
the bed.
3. When, during the measurement of discharge, the bed profile changes appreciably, depth measure-
ments should be carried out by taking one reading at each point at the beginning and one at the end
of each measurement of velocity verticals, and the mean value of these two measurements should be
taken as the effective depth.
4. Inaccuracies in soundings are most likely to occur
(U) owing to the departure from the vertical of the sounding rod or line, particularly in deep
water, when the velocity is high;
(b) owing to the penetration of the bed by the sounding lead or rod;
(c) owing to the nature of the bed when using an echo-sounder.
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IS0 I R 748 - 1968 (E)
Difficulties due to (U) may be avoided by the use, where practicable, of an echo-sounder, or pressure-
measuring device. The effects of drag on a sounding line may be reduced by using a streamlined
lead weight at the end of a fine wire. A correction should be applied to the wetted length of wire if
the wire is not normal to the water surface. It is recommended that the angle should not be greater
than 300 in view of the inaccuracies involved. Methods of applying the correction are given in
Annex C.
Difficulties due to (b) may be overcome by fitting a baseplate to the lower end of the sounding rod,
or by fastening a disk to the end of the sounding line, when they will not cause scour of fine bed
material due to high velocities.
5. In certain cases, for example floods, it may be impossible to determine an adequate profile of cross-
section during the measurement. For those cases the full profile should be determined by surveying
methods, either before or after the measurement; however, it should be recognized that this method
is subject to errors due to possible erosions or deposits in the cross-section between the time the
profile is determined and the time of measurement.
7. MEASUREMENT OF VELOCITY
7.1 Measurement of velocity using current meter
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7.1.1 Calibration of current meter
The current meters should be individually rated to cover the range of velocities of the
flow in the channel, taking care that the calibration is carried out under similar condi-
tions of suspension to those used during the discharge measurement. If an individual
calibration is not possible, a mean rating may be adopted, determined from a number
of current meters of the same type, but particular regard should then be paid to the
question of the tolerance to be used in the estimation of errors.
NOTE. - The rating of the current meter can be effected by drawing it through still water and noting the
angular velocity of the rotor of the current meter for known speeds of the carriage. The rating of the
meter should be checked by repetition of the rating operation or by comparison with a standard current
meter at sufficiently frequent intervals to ensure that the rating has not changed.
7.1.1.1 The current meter should be kept in good condition by thorough cleaning after
each use, and by timely replacement of all worn or damaged parts. Where important
parts are replaced, recalibration of the current meters is necessary.
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7.1.1.2 Cup-type meters should be tested by the " spin test " as specified by the manu-
facturer before and after each use. The bucket wheel should come to rest slowly
and evenly and should revolve throughout in the same horizontal plane. Propeller-
type current meters should be tested before and after each use as specified by the
manufacturer; it may be useful to check, with the help of moulds or metal calipers,
that the propellers are not deformed since their calibration.
7.1.2 Measuring procedure
Velocity observations are normally made at the same time as measurements of the depth.
This method should be used in the case of unstable beds. Where, however, the two
measurements are made at different times, the velocity observations should be taken at a
sufficient number of places, ordinarily in fifteen to twenty verticals, and the intervals
should be measured as described in clauses 6.1.1 and 6.1.2.
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IS0 / R 748 - 1968 (E)
At least eight verticals should be used, provided that this does not necessitate the distance
between the verticals being less than one diameter of the current meter rotor. A smaller
number of verticals may be used if the width is less than eight diameters of the current
meter rotor, or the depth and velocity profile are uniform.
The measuring verticals, as well as the number of observation points on these verticals,
should be chosen such that each measuring point controls a fairly equal portion of the
flow.
NOTE. - If conditions arise in which water runs both over and under the ice, the measurement of
discharge should be carried out in the two parts separated by the ice.
7.1.2.1 The current meter should be held in the desired position in any vertical by means
of a wading rod in the case of shallow channels, or by suspending it from a cable
or rod from a bridge, trolley or boat in the case of deeper channels. When a boat
is used, the current meter should be held so that it is not affected by disturbances of
flow caused by the boat. After the current meter has been placed at the selected
point in the vertical, it should be permitted to become adjusted to the flow before
the readings are started.
NOTES
1. Care should be taken to ensure that the current meter observations are not affected by random
surface waves and wind.
2. When a number of segments having sensibly the same flow of water are to be explored, a
battery of current meters fixed to the same rod can be used to measure corresponding velocities
simultaneously, ensuring that there is no mutual interference.
3. If there is any appreciable deflection of the cable on which the meter is suspended, a correction
should be applied for the depth of the measuring point. No generally applicable correction
factor can be given, but it should be determined by the user for his particular instrument and
conditions of measurement.
7.1.2.2 The current meter should be exposed at each selected point for a minimum of
40 seconds. If the water velocity is known to be subject to periodic pulsations, it is
advisable that the current meter should be exposed at each selected point for at
least three consecutive periods of 1 minute, or for periods of sufficient duration
to cover at least two periods of the pulsation. The velocity at the point should then
be taken to be the average of all the separate readings, unless it is apparent that the
difference is due to some cause other than pulsation of the flow.
7.1.2.3 The current meter should be removed from the water at intervals for examination,
usually when passing from one vertical to another.
7.1.2.4 More than one current meter may be used in determining velocities in the individual
verticals, different current meters being used for consecutive verticals. The averaging
effect will tend to reduce the systematic error of measurement.
7.1.2.5 In channels where the flow is unsteady, it is useful to correct for the variations in
the total discharge during the period of the measurement not only by observing
the change in stage, but also by continuously measuring the velocity at some con-
veniently chosen point. Experience has shown that, provided the variations in
the discharge are small, the velocity distribution is not modified significantly.
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IS0 / R 748 - 1968 (E)
A reference current meter can be positioned in the main current and the velocity
at this point measured, if possible, at the same time as each local point velocity
measurement in the cross-section is made. Ail the individual measurements during
the test can then be referred to the corresponding reference reading. The mean
reference velocity, defined as the average of all the readings of the reference current
meter, can be calculated precisely. Each reading of the local point velocity can then
be corrected by multiplying by the ratio of the mean reference velocity to the
reference velocity reading at tht instant being considered. If the reference current
meter measurements are not made at the same time as the individual measurements,
they should be taken at regular intervals as frequently as possible throughout the
measurement and the reference reading appropriate to a local point velocity
measurement obtained by interpolation.
7.1.3 Obliqueflow
If oblique flow is unavoidable, the angle of the direction of the flow to the perpendicular
to the cross-section must be measured and the measured velocity corrected. Special
instruments have been developed for measuring the angle and velocity at a point simul-
taneously. Where, however, these are not available and there is insignificant wind, the
angle of flow throughout the vertical may be taken to be the same as that observed on
the surface. If the channel is very deep, or if the local bed profile is changing rapidly,
this assumption should not be accepted without checking.
If the measured angle to the perpendicular is y, then
7.1.4 Method for mean velocity measurement in each vertical
The mean velocity of the water in each vertical can be determined by any of the following
methods, depending on the time available and having regard to the width and depth
of the water, to the bed conditions, and to changing stage and whether there is ice
cover, as well as to the accuracy which is to be obtained.
(a) Velocity-distribution method (see clause 7.1.4.1).
(b) Reduced-point methods (see clause 7.1.4.2).
(c) Integration method (see clause 7.1.4.3).
(d) Other methods (see clause 7.1.4.4).
In the neighbourhood of the minimum velocity of use, the error in determining the
velocity increases appreciably. With normal current meters, the minimum velocity for
reliable measurements is O. 15 m/s. Special current meters giving reliable measurements
below this velocity may be used if they have been tested in this range of velocities for
repeatability and accuracy, prior to the measurement.
The horizontal axis of the current meter should not be situated at a distance less than
one and a half times the rotor height from the water surface, nor should it be at a distance
less than three times the rotor height from the bottom of the channel. Besides, no part
of the meter should break the surface of the water.
When the current meter is used close to the bed or surface, particularly when there are
ripples or waves, additional calibrations should be made under immersed conditions
similar to those encountered during the measurement.
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IS0 I R 748 - 1968 (E)
7.1.4.1 Velocity-distribution method
The measurement of the mean velocity by this method is obtained from velocity
observations made at a number of points along each vertical between the surface
of the water and the bed of the channel. The spacing of the points should be so
chosen that the difference of velocity between two adjacent points is not more than
20 % with respect to the higher value of the two. The top and bottom points should
be located respectively near to the water surface and bed of the channel taking into
account the specification of clause 7.1.4. The velocity observations at each position
should be plotted in graphical form and the mean velocity determined with the aid
of a planimeter.
NOTES
1. This method may not be suitable for routine discharge measurements because the apparent
gain in precision may be more than offset by errors resulting from change of stage during the
long period of time needed for making the measurement.
2. The velocity curve can be extrapolated from the last measuring point to the bed or wall by
calculating v, from the equation
1
vx = va (z) n;
where
U, is the point velocity in the extrapolated zone at a distance x from the bed,
va is the velocity at the last measuring point at a distance a from the bed.
The mean velocity Ex between the bottom (or a vertical side) of the channel and the nearest
point of measurement (where the measured velocity is Va) can be calculated directly from the
equation
- rn
v, = -
m+iVa
Generally m varies between 5 (for coarse sides) and 7 (for smooth sides).
3. An alternative method of obtaining the velocity in the region beyond the last measuring point
is based on the assumption that the velocity for some distance up from the bed of the channel
is proportional to the logarithm of the distance x from that boundary. If the observed velocities
at points approaching the bed are plotted against log x, then the best-fitting straight line through
these points can be extended to the boundary. The velocities close to the boundary can then
be read from the graph.
7.1.4.2 Reduced-point methods
(a) Two-point method
Velocity observations should be made at each vertical by exposing the current
meter at 0.2 and 0.8 of the depth below the surface. The average of the two
values should be taken as the mean velocity in the vertical.
NOTE. - This method is applicable without correction for depths greater than 1 m (3 ft)
where measurements are made under ice cover.
(b) One-point method
Velocity observations should be made at each vertical by exposing the current
meter at 0.6 of the depth below the surface. The value observed should be
taken as the mean velocity in the vertical.
NOTE. - This method is applicable with correction for depths shallower than 1 m (3 ft)
where measurements are made under ice cover, the correction factor being taken as 0.92.
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IS0 / R 748 - 1968 (E)
7.1.4.3 Integration method
In this method the current meter is lowered and raised through the entire depth at
each vertical at a uniform rate. The speed at which the meter is lowered or raised
should not be more than 5 % of the mean velocity of flow in the cross-section, and
in any case it should not be greater than 0.04 m/s (approximately 0.1 ft/s).
Two complete cycles are made in each vertical and, if the results differ by more
than 10 %, the measurement is repeated.
For calculating the mean velocity in the vertical, the average number of revolutions
per second is determined and this is incorporated in the formula for the current
meter calibration coefficient. The error introduced by using the normal meter
calibration coefficient for measurement near the bed of the channel is not significant,
provided that the current meter is not allowed to remain in its lowest position for
any appreciable length of time. This method is only used in water having a depth
greater than 1 m (3 ft).
7.1.4.4 Oth
...