EN ISO 748:2000

SLOVENSKI STANDARD
01-julij-2001
0HUMHQMHSUHWRNDWHNRþLQYRGSUWLKNDQDOLK0HWRGHKLWURVWSRYUãLQD,62

Measurement of liquid flow in open channels - Velocity-area methods (ISO 748:1997)
Durchflußmessung in offenen Gerinnen - Geschwindigkeitsflächen-Verfahren (ISO
748:1997)
Mesure de débit des liquides dans les canaux découverts - Méthodes d'exploration du
champ des vitesses (ISO 748:1997)
Ta slovenski standard je istoveten z: EN ISO 748:2000
ICS:
17.120.20 Pretok v odprtih kanalih Flow in open channels
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

INTERNATIONAL IS0
STANDARD 748
Third edition
1997-08-01
Measurement of liquid flow in open
channels - Velocity-area methods
Mesure de dkbit des liquides dans /es canaux d&ouverts - Mkthodes
d ’exploration du champ des vitesses
Reference number
IS0 748: 1997(E)
IS0 748: 1997(E)
Contents Page
1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2 Normative reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ‘.
3 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
4 Principle of the methods of measurements . . . . . . . . . . . . . . . . . . . . . . . . . . .*. 1
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
5 Selection and demarcation of site
6 Measurement of cross-sectional area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
7 Measurement of velocity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
a Computation of discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9 Uncertainties in flow measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Annexes
A Correction for sag, pull, slope and temperature in measurement
of cross-section width by tape or wire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
B Measurement across the cross-section
C Corrections for wetted length of wire when measuring depths
with wire not normal to surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
D Correction for drift
E Uncertainty of a velocity-area measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
F Determination of mean velocity from float measurements . . . . . . . . 39
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
G Bibliography
0 IS0 1997
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced
or utilized in any form or by any means, electronic or mechanical, including photocopying and
microfilm, without permission in writing from the publisher.
International Organization for Standardization
Case postale 56 l CH-1211 Geneve 20 l Switzerland
Internet central @ iso.ch
c=ch; a=400net; p=iso; o=isocs; s=central
x.400
Printed in Switzerland
0 IS0
IS0 7483 997(E)
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 technical
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, governmental and non-governmental, in
liaison with ISO, also take part in the work. IS0 collaborates closely with the International Electrotechnical
Commission (IEC) on all matters of electrotechnical standardization.
Draft International Standards adopted by the technical committees are circulated to the member bodies for voting.
Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote.
International Standard IS0 748 was prepared by Technical Committee lSO/TC 113, Hydrometric determinations,
Subcommittee SC 1, Velocity area methods.
This third edition cancels and replaces the second edition (IS0 748:1979), which has been technically revised.
Annexes A to G of this International Standard are for information only.

This page intentionally left blank

IS0 748: 1997(E)
INTERNATIONAL STANDARD o IS0
Measurement of liquid flow in open channels - Velocity-area
methods
1 Scope
This International Standard specifies methods for determining the velocity and cross-sectional area of water flowing
in open channels without ice cover, and for 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 relation of a gauging station, this International
Standard deals only with single measurements of the discharge; the continuous recording of discharges over a
period of time is covered in IS0 1100-I and IS0 1100-2.
NOTE The methods for determining the velocity and cross-sectional area of water flowing in open channels with ice cover are
specified in IS0 9196.
2 Normative reference
constitute provisions of this
The following standard contains provisions which, through reference in this text,
International Standard. At the time of publication, the edition indicated was valid. All standards are subject to
revision, and parties to agreements based on this International Standard are encouraged to investigate the
possibility of applying the most recent edition of the standard indicated below. Members of IEC and IS0 maintain
registers of currently valid International Standards.
IS0 7723 996, Hydrometric determinations - Vocabulary and symbols.
3 Definitions
For the purposes of this International Standard, the definitions given in IS0 772 and the following definition apply.
3.1 unit-width discharge
discharge through a unit width of a section at a given vertical
4 Principle of the methods of measurements
4.1 The principle of these methods consists of measuring velocity and cross-sectional area. A measuring site is
chosen conforming to the specified requirements; the width, depending on its magnitude, is measured either by
means of steel tape or by some other surveying method, and the depth is measured at a number of points across
the width, sufficient to determine the shape and area of the cross-section.
Velocity observations are made at each vertical preferably 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 using current-meters. If unit width
discharge is required, it is generally computed from the individual observations.
In the integration method, the mean velocity is obtained directly.

0 IS0
IS0 748:1997(E)
Under certain circumstances, velocity observations can also be made using surface floats or velocity-rods. Other
methods consist of measuring the velocity along one or several horizontal lines of the section (e.g. moving-boat and
ultrasonic methods.)
4.2 The discharge is computed either arithmetically or graphically by summing the products of the velocity and
corresponding area for a series of observations in a cross-section.
5 Selection and demarcation of site
5.1 Selection of site
The site selected should comply as far as possible with the following requirements:
a) The channel at the measuring site should be straight and of uniform cross-section and slope in order to
minimize abnormal velocity distribution.
NOTE When the length of the channel is restricted, it is recommended for current-meter measurements, or other velocity-
meter measurements, that the straight length upstream should be at least twice that downstream.
Flow directions for all points on any vertical across the width should be parallel to one another and at right
W
angles to the measurement section.
The bed and margins of the channels should be stable and well defined at all stages of flow in order to facilitate
C)
accurate measurement of the cross section and ensure uniformity of conditions during and between discharge
measurements.
The curves of the distribution of velocities should be regular in the vertical and horizontal planes of
d)
measurement.
Conditions at the section and in its vicinity should also be such as to preclude changes taking place in the
e)
velocity distribution during the period of measurement.
Sites displaying vortices, reverse flow or dead water should be avoided.
fl
The measurement section should be clearly visible across its width and unobstructed by trees, aquatic growth
9)
or other obstacles. When gauging from a bridge with divide piers, each section of the channel should be treated
accordingly.
The depth of water at the section should be sufficient at all stages to provide for the effective immersion of the
h)
current-meter or float, whichever is to be used.
The site should be easily accessible at all times with all necessary measurement equipment.
.
The section should be sited away from pumps, sluices and outfalls, if their operation during a measurement is
1)
likely to create flow conditions inconsistent with the natural stage-discharge relationship for the station.
Sites where there is converging or diverging flow should be avoided.
k)
In those instances where it is necessary to make measurements in the vicinity of a bridge, it is preferable that
I)
the measuring site be upstream of the bridge. However in special cases and where accumulation of ice, logs
or debris is liable to occur, it is acceptable that the measuring site be downstream of the bridge. Particular care
should be taken in determining the velocity distribution when bridge apertures are surcharged.
The measurement of flow under ice cover is dealt with in IS0 9196 but for streams subject to formation of ice
m)
cover, requirements of measurement specified in this International Standard can be used during the free water
season.
0 IS0
IS0 748: 1997(E)
n) It may, at certain states of river flow or level, prove necessary to carry out current-meter measurements on
sections other than that selected for the station. This is quite acceptable if there are no substantial ungauged
losses or gains to the river in the intervening reach and so long as all flow measurements are related to levels
recorded at the principal reference section.
5.2 Demarcation of site
NOTE If the site is to be established as a permanent station or likely to be used for future measurement, it should be provided
with means for demarcation of the cross-section and for determination of stage.
52.1 The position of each cross-section, normal to the mean direction of flow, shall be defined on the two banks
by clearly visible and readily identifiable markers. Where a site is subject to considerable snow cover, the section
line-markers may be referenced to other objects such as rock cairns.
5.2.2 The stage shall be read from a gauge at intervals throughout the period of measurement and the gauge
datum shall be related by precise levelling to a standard datum.
52.3 An auxiliary gauge on the opposite bank shall be installed 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 shall 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
6.1 General
The cross-sectional profile of the open channel at the gauging-site shall be determined at a sufficient number of
points to establish the shape of the bed.
The location of each point is determined by measuring its horizontal distance to a fixed reference point on one bank
of the channel, in line with the cross-section. This in turn allows calculation of the area of individual segments
separating successive verticals where velocities are measured.
6.2 Measurement of width
Measurement of the width of the channel and the width of the individual segments may be obtained by measuring
the horizontal distance from or to a fixed reference point which shall be in the same plane as the cross-section at
the measuring site.
6.2.1 Where the width of the channel permits, these horizontal distances shall be measured by direct means, for
example a graduated 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, shall be similarly measured.
6.2.2 Where the channel is too wide for the above methods of measurement, the horizontal distance shall be
determined by optical or electronic distance-meters, or by one of the surveying methods given in annex B.
6.3 Measurement of depth
6.3.1 Measurement of depth shall be made at intervals close enough to define the cross-sectional profile
accurately. In general, the intervals shall not be greater than l/20 of the width.
NOTE 1 For small channels with a regula r bed profile, the num ber of i ntervals may be reduced. This may, h
oweve
‘r,
the accu racy of the determinati on of the bed profile (see 7.1.3 and clause
9).
NOTE 2 Accuracy of measurement of discharge is increased by decreasing the spacing between verticals.

0 IS0
IS0 748:1997(E)
6.3.2 The depth shall be measured by 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 and the channel is
sufficiently deep, it is preferable to use an echo-sounder or other device which will not require large corrections.
6.3.3 When a sounding-rod or sounding-line is used, it is desirable that at least two readings 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 shall be taken. If these are within 5 %, they shall be accepted for the
measurement and the two earlier readings discarded. If they are again different by more than 5%, no further
readings shall be taken but the average of all four readings shall be adopted for the measurement, noting that the
accuracy of this measurement is reduced.
When an echo-sounder is used, the average of several readings shall always be taken at each point. Regular
calibrations of the instrument shall be carried out under the same conditions of salinity and temperature of the water
to be measured.
NOTE Where it is imp #racticable to take more than one reading of the depth, the uncertainty in measurement may be
increased (see clause 9).
6.3.4 Where measurements of the depths are made separately from the velocity measurements and the water
level is not steady, the water level shall be observed at the time of each measurement of the depth. When this is
not possible, the water level shall be observed at intervals of 15 min and the value of the level at the time of each
determination of depth shall be obtained by interpolation.
NOTE 1 When, during the measurement of discharge, the bed profile changes appreciably, depth measurements should be
carried out by taking one depth reading at each point at the beginning and one at the end of the velocity measurement at each
vertical, and the mean value of these two measurements shall be taken as the effective depth. Care should be exercised when
taking repeated soundings to avoid disturbance of the bed.
NOTE 2 Inaccuracies in soundings are most likely to occur owing to:
the departure from the vertical of the sounding-rod or line, particularly in deep water, when the velocity is high;
a)
the penetration of the bed by the sounding-weight or -rod;
the nature of the bed when an echo-sounder is used.
Errors due to a) may be minimized by the use, where practicable, of an echo-sounder, or pressure-measuring device. The
effect of drag on a sounding-line may be reduced by using a streamlined lead weight at the end of a fine wire. A correction
shall be applied to the wetted length of wire if the wire is not normal to the water-surface. It is recommended that the angle of
departure from the vertical of the sounding line should not be greater than 300 in view of the inaccuracies involved. Methods of
applying the correction are given in annex C.
Errors due to b) may be reduced by fitting a baseplate to the lower end of the sounding-rod, or by fastening a disk to the end of
the sounding-line, provided they will not cause additional scour of fine bed material due to high velocities.
Errors due to c) may be reduced by selecting an echo-sounder frequency that most adequately depicts the bed-water interface.
NOTE 3 In certain cases, for example floods, it may be impossible to determine an adequate profile of cross-section during
For those cases, the full profile shall be determined by surveying methods, either before or after the
the measurement.
measurement. However, it should be recognized that this method is subject to errors due to possible erosion or deposition in
the cross-section between the time the profile is determined and the time of discharge measurement.
7 Measurement of velocity
7.1 Measurement of velocity using current-meters
7.1 .I Rotating-element current-meters
Rotating-element current-meters should be constructed, calibrated and maintained according to IS0 2537 and
IS0 3455.They should be used only within their calibrated range and fitted on suspension equipment similar to that
used during calibration.
63 IS0 IS0 748: 1997(E)
In the vicinity of the mini mum speed of response , the uncertainty in determin ing the veloc ity is high. Care should be
exerci sed when measuri ng velocities near the mi nimu m speed of response.
For high velocities, the propeller, in the case of propeller-type current-meters, or the reduction ratio where available,
shall be chosen in order that the maximum speed of rotation can be correctly measured by the revolution counter.
No rotating-element current-meter shall be selected for use in water where the mean depth is less than 4 times the
diameter of the impeller that is to be used, or of the body of the meter itself, whichever is the greater. No part of the
meter shall break the surface of the water.
7.1.2 Electromagnetic current-meters
Electromagnetic current-meters are acceptable for making measurements of point velocity. These current-meters
have the advantage that they have no moving parts and thereby eliminate all friction and resistance. They should
be calibrated throughout the range of velocity for which they are to be used, and should meet accuracy
requirements similar to rotating-element current-meters. They should not be used outside the range of calibration.
Electromagnetic current-meters are capable of operation in shallow depths and of detecting and measuring flow
reversal. No electromagnetic current-meter shall be selected for use in water whose mean depth is less than 3
times the vertical dimension of the probe.
and provide a digital readout of velocity
The control box of the electromagnetic meter should be splashproof
instantaneously or averaged over preset time periods.
The sensor of the electromagnetic meter should have a moulded epoxy resin pod with no protrusions, containing an
It shall be relatively immune to fouling or damage,
electromagnetic sensor and solid-state encapsulated circuitry.
simple to clean and maintain and be readily interchangeable.
7.1.3 Measurement procedure
Velocity observations are normally made at the same time as measurements of the depth. This method shall be
used in the case of unstable beds. Where, however, the two measurements are made at different times, the
velocity observations shall be taken at a sufficient number of places, and the horizontal distance between
observations shall be measured as described in 6.2.1 and 6.2.2.
In judging the specific number n of verticals that are to be defined for the purpose of gauging flow at a particular
location, the following criteria shall be applied.
Channel width > 0 and < 0,5 m n=3to4
n = 4 to 5
Channel width > 0,5 m and < 1 m
Channel width > 1 m and < 3 m n=5to8
n=8tolO
Channel width > 3 m and < 5 m
Channelwidth>5mand Channel width >I0 m n 2 20
In all instances, measurements of depth or velocity made at the water ’s edge are additional to the above.
It is further recommended that the location of the verticals be selected after a previous cross-section survey. When
the channel is sufficiently uniform it may be possible to reduce the number of verticals and to allocate equal
distance spacing between the verticals without conflicting with the above requirement.
The verticals uld be chosen so that the discharge in each segment
sho is less than 5 % of the total, insofar as
possible, and in no case sho luld it exceed 10 %.
that
0 IS0
IS0 748: 1997(E)
The current-meter shall be held in the desired position in each vertical by means of a wading-rod in the case of
shallow channels, or by suspending it from a cable or rod in the case of deeper channels. When a boat is used, the
current-meter shall be held so that it is not affected by disturbances of flow caused by the boat.
The current-meter shall be placed at the selected point in the vertical so that the horizontal axis of the meter is
parallel to the direction of flow at that point. The meter shall be allowed to adjust to the flow before the readings are
started.
NOTE Care should be taken to ensure that the current-meter observations are not affected by random surface-waves and
wind.
NOTE 2 When a number of points in a vertical are to be measured, a battery of current-meters fixed to the same rod can be
used to measure corresponding velocities simultaneously whilst ensuring that there is no mutual interference.
If there is any appreciable deflection of the cable on which the meter is suspended, a correction shall be applied for
the depth of the measuring-point. No generally applicable correction factor can be given, but it shall be determined
by the user for the particular instrument and conditions of measurement. However, the values given in annex C
may serve as a guide.
NOTE 3 The selection and use of appropriate suspension equipment is described in IS0 3454 and IS0 4375.
The velocity at each selected point shall be observed by exposing
a) a rotating-element current-meter for a minimum of 30 s, or
b) an electromagnetic current-meter for a minimum of 10 s.
Where the velocity is subject to periodic pulsations, the exposure time should be increased accordingly. (See
ISO/TR 7178.)
current-meter shall be removed from the water or brought to the surface at intervals for exam ination, usually
The
one
whe n passing from vertical to another.
A spin test, where appropriate, should be performed after each discharge measurement to ensure that the
mechanism operates freely (see IS0 2537).
More than one current-meter may be used in determining velocities in the individual verticals, different current-
meters being used for consecutive verticals.
In channels where the flow is unsteady, it is possible 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 conveniently chosen point in the main current.
7.1.4 Oblique flow
If oblique flow is unavoidable, the angle of the direction of the flow to the perpendicular to the cross-section shall be
measured and the measured velocity adjusted. Special instruments have been developed for measuring the angle
and velocity at a point simultaneously. Where, however, these are not available and there is insignificant wind, the
angle of flow throughout the vertical can be taken to be the same as that observed on the surface. This angle can
be measured with appropriate equipment provided that the operator is located above the measurement vertical. If
the channel is very deep or if the local bed profile is changing rapidly, this assumption shall not be accepted without
confirmation.
If the measured angle to the perpendicular to the cross section is g ,the velocity used for computation of flow
discharge shall be:
v
corrected = ‘measured ‘OS r
NOTE Some current-meters are equipped to measure the normal component of ve ‘locity directly when held perpendicular
to the measurement cross-section. This correction should not be applied in such cases.

0 IS0 IS0 748:1997(E)
7.1.5 Method for mean velocity measurement in a vertical
7.1.5.1 Choice and classification
These are: time available, width
The choice of the method for velocity measurement depends on certain factors.
and depth of the channel, bed conditions in the measuring section and the upstream reach, rate of variation of level,
degree of accuracy wanted and equipment used.
These methods are classified as follows:
a) Velocity distribution method (see 7.1.5.2).
b) Reduced point methods (see 7.1.5.3).
c) Integration method (see 7.1.5.4).
d) Other methods (see 7.1.5.5).
7.1.5.2 Velocity distribution method
Using this method, the values of the velocity are obtained from observations at a number of points on each vertical
between the surface of the water and the bed of the channel. The number and spacing of the points should be so
chosen as to define accurately the velocity distribution on each vertical with a difference in readings between two
The location of the top and the bottom
adjacent points of not more than 20 % with respect to the higher value.
readings should be chosen, taking into account the specification under 7.1 .I (see IS0 1088).
The velocity observations at each position are then plotted and the unit width discharge or mean velocity
determined by planimeter, digitizer or equivalent method.
NOTE 1 This method may not be suitable for routine discharge measurements beta use the apparent gain in precision
may
be offset by errors resulting from change of stage during the long period of time ne eded for making the measurement.
NOTE 2 The velocity curve can be extrapolated from the last measuring point to the bed or wall by calculating vx from the
equation
xi
= vu -
Yx
a
vx is the open point velocity in the extrapolated zone at a distance Xfrom the bed or wall.
va is the velocity at the last measuring point at a distance a from the bed or wall.
The mean velocity V between the bottom r a ve rtical side) of the cha nnel and the nearest point of measurement (where the
(0
measured velocity is v,) can be ca lculated di rectly from the equation
m is an exponent
d is the total depth of flow
Generally m lies between 5 and 7 but it may vary over a wider range depending on the
hydraulic resistance. The value m = 2
applies to coarse beds or walls while m = 10 is characteristic of smoot
h beds or walls.
0 IS0
IS0 748: 1997(E)
m is obtained as follows:
2&
m = 2 h + Cv,r +Q3
L 1
where
g = acceleration due to gravity (m/s ’);
C vert = Chezy ’s coefficient on a vertical (m ”*“/s).
NOTE 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.
7.1.5.3 Reduced point methods
These methods, less strict than methods exploring the entire field of velocity, are used frequently because they
require less time than the velocity-distribution method (7.1.5.2). They are based, however, on assumed velocity
profiles.
.
:hat for a new gaug ing section the accuracy of the selected method be assessed comparing
It IS recommended t
by
those obtai ned from the velocity distribution method.
the results of prelimi nary gaugings with
a) Two-point method
each vertical by exposing the current-meter at 0,2 and 0,8 of the depth
Velocity observations shall be made at
two values shall be taken as the mean velocity in the ve rtical.
below the surface. The average of the
One-point method
b)
Velocity observations shall be made on each vertical by exposing the current-meter at 0,6 of the depth below
the surface. The value observed shall be taken as the mean velocity in the vertical.
7.1.5.4 Integration method
In this method, the current-meter is lowered and raised through the entire depth on 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 water velocity and
should not in any event exceed 0,04 m/s. Two complete cycles should be made on each vertical and if the results
differ by more than 10 %, the operation (two complete cycles) should be repeated until results within this limit are
obtained. This method is suitable for propeller-type current-meters and cup-type meters provided the vertical
movement is less than 5 % of the mean velocity and for electromagnetic current-meters.
The integration method gives good results if the time of measurement allowed is sufficiently long (60 s to 100 s).
The technique is not normally used in depths of less than 1 m.
With a propeller-type current-meter, the average velocity can then be read from the instrument calibration as
equivalent to the average number of revolutions (being derived as the total number of revolutions divided by the
total time taken for the measurement in that vertical). Uncertainties introduced by using meters with more than one
calibration coefficient should be avoided.
By using a current-meter which measures velocity directly, such as the electroma current-meter, the mean
gnetic
velocity 0 n
the vertical can be obtained by direct reading of the instrument.
When a sounding-rod or -weight is used, it will not be possible to measure the velocity throughout the entire vertical;
a relatively large zone may, for example, remain unmeasured near the channel bed. An estimate of the unit width
discharge of this zone can be obtained from:
0 IS0 IS0 748:1997(E)
2 Vm x hj
-
u -
qU = the unit width discharge below the measured zone;
= mean velocity for the measured part of the vertical;
“rn
hl= the depth of the unmeasured zone.
Similarly, the unit width discharge for any unmeasured zone near the surface is obtained from:
Vm x hs
4 -
s -
where
4, = the unit width discharge above the measured zone;
h, = the depth of the unmeasured zone.
measuring equipment should be selected to minimize the depth of the unmeasured
As far as possible, the type of
zones.
7.1.5.5 Other methods
Six-point method
a)
Velocity observations are made by exposing the current-meter on each vertical at 0,2 - 0,4 - 0,6 and 0,8 of the
depth below the surface and as near as possible to the surface and the bed [see Note in d)]. The velocity
observations at each point are plotted in graphical form and the mean velocity or unit width discharge determined
with the aid of a planimeter.
Alternatively, the mean velocity may be found algebraically from the equation
+ 2 ”,, + 2 ”,, + 2 ”,, + 25, + v,,)
” = 03 ( “surface
Five-point method
b)
Velocity measurements are made by exposing the current-meter on each vertical at 0,2, 0,6 and 0,8 of the depth
below the surface and as near as possible to the surface and the bed. The mean velocity may be determined from a
graphical plot of the velocity profile with a planimeter, or from the equation.
+ 3 ”,, + 3 ”,, + 2 ”,, + v,,)
” = 0,1 ( “svrface
Three-point method
C)
Velocity observations are made by exposing the current-meter at each vertical at 0,2, 0,6 and 0,8 of the depth
below the surface. The average of the three values may be taken as the mean velocity in the vertical.
Alternatively, the 0,6 measurement may be weighted and the mean velocity obtained from the equation
v=
0,2w4J* + -lo,, + v,,,)
Surface one-point method
d)
In flashy or other conditions where the above methods are not feasible, velocity shall be measured at one point just
below the surface. The depth of submergence of the current-meter shall be uniform over all the verticals; and care
0 IS0
IS0 748:1997(E)
shall be taken to ensure that the current-meter observations are not affected by random surface-waves and wind.
This ‘surface’ velocity may be converted to the mean velocity in the vertical by multiplying it by a predetermined
coefficient specific to the section and to the discharge.
The coefficient shall be computed for all stages by correlating the ‘surface’ velocity with the velocity at 0,6 depth or,
where greater accuracy is desired, with the mean velocity obtained by one of the other methods previously
described.
It may be noted for guidance that in general, the coefficient varies between 0,84 and 0,90 depending upon the
shape of the velocity profile; the higher values between 0,88 and 0,90 are usually obtained when the bed is smooth.
NOTE The use of current-meters near to the surface, or to the bed of the channel, shall be in accordance with the
manufacturer ’s instructions (see also 7.1 .I).
7.1.6 Errors and limitations
Estimates of the possible errors that may occur when using the various methods detailed in 7.1.5 are given in 9.3.3.
It should be noted that these estimates are of possible random errors which may occur even when all the
precautions noted earlier and below are observed. If the measurement is not made under these best conditions,
additional uncertainty should be included when estimating the overall uncertainty of the measurement.
Errors arise
may
if the flow is unsteady;
a)
if material in suspension interferes with the performance of the current-meter;
if the direction of flow is not parallel to the axis of the propeller-type current-meter, or is oblique to the plane of
the cup-type meter, and if the appropriate correction factors are not known accurately;
if the current-meter is used for measurement of velocity outside the range established by the calibration;
d)
if the set-up for measurement (such as rods or cable suspending the current-meter, the boat etc.) is different
e)
from that used during the calibration of the current-meter, in which case a systematic error may be introduced;
if there is significant disturbance of the water surface by wind;
f )
g) if the current-meter is not held steadily in the correct place during the measurement, which is the case when
the boat is drifting (see annex D), or when an oscillating transverse movement occurs. In the latter case, the
resultant of the flow, velocity and the transverse velocities gives rise to serious positive errors.
7.2 Measurement of velocity using floats
This method shall only be used when it is impossible to employ a current-meter because of excessive velocities and
depths, because of the presence of material in suspension, where velocities are too low for current-meter
measurement or in cases of reconnaissance.
7.2.1 Selection of site
Three cross-sections shall be selected along the reach of the channel as described in clause 5, at the beginning,
midway and at the end of the reach. The cross-sections shall be far enough apart for the time which the floats take
to pass from one cross-section to the next to be measured accurately. The midway cross-sections shall be used
only for the purposes of checking the velocity measurement between the cross-sections at the beginning and at the
end of the reach. A minimum duration of float movement of 20 s is recommended.
7.2.2 Measuring procedure
The float shall be released far enough above the upper cross-section to attain a constant velocity before reaching
the first cross-section. The time at which the float passes each of the three cross-sections is then noted. This
procedure shall be repeated with the floats at various distances from the bank of the river. The distances of the
0 IS0 IS0 748: 1997(E)
float from the bank as it passes each cross-section may be determined by suitable optical means, for example, a
theodolite.
It is also possible to use the double stopwatch method described in annex F. This allows the determination of the
velocity of the float and the position of its path in the section in a single operation and without the need for special or
surveying equipment.
Increasing the number of floats used to determine the velocity in each segment will improve the accuracy of the
measurement.
The width of the channel shall be divided into a certain number of segments of equal width. If, however, the
channel is very irregular, each segment shall have approximately the same discharge. The number of segments
shall not be less than three, but where possible a minimum of five shall be used, the actual number of segments
depending on the time available for these observations at the particular stage of the river.
7.2.3 Types of float
7.2.3.1 General
The velocity of the water in each segment can be determined by
surface floats;
a)
b) double floats;
other types of float.
C)
NOTE Separately flowing blocks of ice, provided they are small, can be used as surface floats during ice drifting.
The coefficients for obtaining the mean velocity from the measurements for the various types of floats are given in
7.2.4.
7.2.3.2 Surface floats
These may be used during floods when velocity measurements are to be made quickly. They shall not be used
when their movement is likely to be affected by winds.
7.2.3.3 Double floats
These may be used for measurements of velocities in deep rivers. The sub-surface body may be positioned at 0,6
of the depth below the surface, or at other depths to obtain direct velocity measurements at these depths.
7.2.3.4 Other types of float
Other methods of obtaining the mean velocity in each segment may be used if the bed profile is regu lar over the
.
measuring reac h .
a) Sub-surface floats
These may be used for measurement of velocities in very deep rivers.
The length of the sub-surface float,
sometimes called the ‘multiple float ’, which consists of separate elements suitably attached together to permit
flexibility and supported by a surface float, shall be approximately equal to the water depth, but the float shall in no
case touch the bottom.
b) Velocity-rods
These may be used for measurement of velocities in the case of artificial or other regular channels where the cross-
section is uniform, the bed is free from weeds, and the depth of the water is constant. The velocity-rod (sometimes
called a float-rod) shall be at least 0,95 of the depth of the channel but shall not touch the bottom.
0 IS0
IS0 748: 1997(E)
7.2.4 Evaluation of velocity
7.2.4.1 Method
The float velocity shall be determined by dividing the distance between the cross-sections by the time taken by the
float to travel this distance. Several measurements of the float velocities shall be taken and the mean of these
measurements shall be multiplied by the appropriate coefficient to obtain the mean velocity in the segment. The
coefficient derived from current-meter measurements at the site at a stage as near as possible to that during the
float measurement may be used for converting the float velocity to mean velocity. This method gives an
approximate value of the flowrate.
NOTE The distance travelled by a float may be, in some cases, significantly long er than the distance between cross-sections.
In such cases it is the distance travelled which is used to estimate the velocity.
7.2.4.2 Surface floats
Where it is not possible to check the coefficient directly, it may be assumed for guidance that in general the
coefficient of the surface float varies between 0,84 and 0,90 depending upon the shape of the velocity profile. The
higher values are usually obtained when the bed is smooth, but values outside this range may occur under special
circumstances.
7.2.4.3 Double floats
Where it is not possible to check the coefficient directly, it may be accepted for guidance that when the sub-surface
body is situated at 0,6 of the depth, the coefficient is approximately equal to I,0 and at 0,5 of the depth, the
coefficient is approximately equal to 0,96.
7.2.4.4 Other types of float
Where a direct check on the coefficient is not possible, it may be assumed that the coefficient of the sub-surface
floats and velocity-rods varies in general over the range 0,8 to 1 ,O.
7.2.5 Main sources of error
Errors may occur during the measurement of discharge by floats and the main sources are listed below. They shall
be taken into consideration when estimating the overall error as given in clause 9.
Errors may arise:
if the coefficient from which the mean velocity is obtained from the float velocity is not known accurately;
a)
b) if too few segments are used for the velocity distribution;
if a sub-surface float or velocity-rod is used and the depth of the channel is not uniform throughout the
C>
measuring-reach;
d) if the float does not travel in the centre of the panel due to oblique currents;
if there is wind, but it should be noted that this error is generally negligible in comparison with others listed
e)
above, unless a surface float is used.
8 Computation of discharge
8.1 General
The method of determination of the mean velocity or unit width discharge in each vertical has been dealt with in 7.1
and 7.2. In these subclauses the method of determination of discharge from current-meter measurements and float
measurements is presented. These methods have been classed as the graphical method (8.2) and the arithmetic
0 IS0 IS0 748:1997(E)
method (83, the latter being particularly useful for computations carried out in the field. The methods given in 8.4
to 8.7 are applicable for special circumstances.
8.2 Graphical method
8.2.1 Depth-velocity-integration
The velocity readings recorded for each vertical are plotted against depth as shown in figure 1. The area contained
by the velocity curve produced for each vertical gives the discharge for unit width of the corresponding section.
Where necessary, velocity curves can be extrapolated to the surface and bed using the methods described in notes
at the end of 7.152. The values of unit-width discharges (is d) are then plotted on the upper part of the diagram
and joined to form a continuous curve. The area enclosed between this curve and the line representing the water
surface gives the total discharge through the section.
In the case of velocity measurements by the integration or reduced point methods, the unit-width discharge at each
vertical is obtained directly as the product of the mean velocity i? and the corresponding depth d.
When velocity measurements are not carried out on the same verticals on which the depth measurements are
made, the v curve shall be plott
...