EN ISO 748:2007

Hydrometry - Measurement of liquid flow in open channels using current-meters or floats (ISO 748:2007)

ISO 748:2007 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.
ISO 748:2007 covers methods of employing current‑meters or floats to measure the velocities. It deals only with single measurements of the discharge; the continuous recording of discharges over a period of time is covered in ISO 1100-1 and ISO 1100-2.

Hydrometrie - Durchflussmessung in offenen Gerinnen mittels Fließgeschwindigkeitsmessgeräten oder Schwimmern (ISO 748:2007)

Diese Norm legt Verfahren zur Bestimmung von Geschwindigkeit und Querschnittsfläche von Wasser, das ohne Eisdecke durch offene Gerinne fließt, und für die Berechnung des daraus resultierenden Durchflusses fest.
Sie umfasst Verfahren, bei denen die Geschwindigkeiten mit FließgeschwindigkeitsmessgerätFließgeschwindigkeitsmessgeräten und Schwimmern gemessen werden. Obwohl diese Messungen in den meisten Fällen für die Bestimmung der Abflusstafel einer Pegelstation vorgesehen sind, befasst sich diese Norm nur mit Einzelmessungen des Durchflusses; die Daueraufzeichnung von Durchflüssen über einen Zeitraum wird in ISO 1100 1 und ISO 1100 2 behandelt.
ANMERKUNG   Die Verfahren zur Bestimmung von Geschwindigkeit und Querschnittsfläche von Wasser, das mit einer Eisdecke bedeckt durch offene Gerinne fließt, werden in ISO 9196 festgelegt.

Hydrométrie - Mesurage du débit des liquides dans les canaux découverts au moyen de débitmètres ou de flotteurs (ISO 748:2007)

L'ISO 748:2007 spécifie des méthodes permettant de déterminer la vitesse et l'aire de la section droite d'un écoulement d'eau en chenal non couvert par la glace et de calculer le débit à partir des résultats des mesurages.
Elle couvre les méthodes faisant usage de moulinets ou de flotteurs destinés au mesurage des vitesses. Il est à noter que, bien que dans certains cas ces mesurages soient entrepris pour déterminer la relation hauteur-débit d'une station hydrométrique, l'ISO 748:2007 ne traite que de simples mesurages du débit; l'enregistrement en continu des débits durant une certaine période fait l'objet de l'ISO 1100‑1 et de l'ISO 1100‑2.

Hidrometrija - Merjenje pretoka tekočin v odprtih kanalih z uporabo hidrometričnih kril ali plavačev (ISO 748:2007)

General Information

• Status: Withdrawn
• Publication Date: 14-Oct-2007
• Withdrawal Date: 13-Apr-2025
• ICS: 17.120.20 - Flow in open channels
• Technical Committee: CEN/TC 318 - Hydrometry
• Drafting Committee: CEN/TC 318 - Hydrometry
• Current Stage: 9960 - Withdrawal effective - Withdrawal
• Start Date: 01-Dec-2021
• Completion Date: 14-Apr-2025
• Ref Project: SIST EN ISO 748:2008 - Hydrometry - Measurement of liquid flow in open channels using current-meters or floats (ISO 748:2007)

Relations

• Replaces: EN ISO 748:2000 - Measurement of liquid flow in open channels - Velocity-area methods (ISO 748:1997)
• Effective Date: 22-Dec-2008
• Replaced By: EN ISO 748:2021 - Hydrometry - Measurement of liquid flow in open channels - Velocity area methods using point velocity measurements (ISO 748:2021)
• Effective Date: 08-Dec-2021

Standards Content (Sample)

SLOVENSKI STANDARD
01-januar-2008
1DGRPHãþD
SIST EN ISO 748:2001
+LGURPHWULMD0HUMHQMHSUHWRNDWHNRþLQYRGSUWLKNDQDOLK]XSRUDERKLGURPHWULþQLK
NULODOLSODYDþHY,62
Hydrometry - Measurement of liquid flow in open channels using current-meters or floats
(ISO 748:2007)
Hydrometrie - Durchflussmessung in offenen Gerinnen mittels
Fließgeschwindigkeitsmessgeräten oder Schwimmern (ISO 748:2007)
Hydrométrie - Mesurage du débit des liquides dans les canaux découverts au moyen de
débitmetres ou de flotteurs (ISO 748:2007)
Ta slovenski standard je istoveten z: EN ISO 748:2007
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.

EUROPEAN STANDARD
EN ISO 748
NORME EUROPÉENNE
EUROPÄISCHE NORM
October 2007
ICS 17.120.20 Supersedes EN ISO 748:2000
English Version
Hydrometry - Measurement of liquid flow in open channels using
current-meters or floats (ISO 748:2007)
Hydrométrie - Mesurage du débit des liquides dans les Hydrometrie - Durchflussmessung in offenen Gerinnen
canaux découverts au moyen de débitmètres ou de mittels Fließgeschwindigkeitsmessgeräten oder
flotteurs (ISO 748:2007) Schwimmern (ISO 748:2007)
This European Standard was approved by CEN on 21 September 2007.
CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European
Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national
standards may be obtained on application to the CEN Management Centre or to any CEN member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CEN member into its own language and notified to the CEN Management Centre has the same status as the
official versions.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland,
France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal,
Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36  B-1050 Brussels
© 2007 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 748:2007: E
worldwide for CEN national Members.

Contents Page
Foreword.3

Foreword
This document (EN ISO 748:2007) has been prepared by Technical Committee ISO/TC 113 "Hydrometric
determinations" in collaboration with Technical Committee CEN/TC 318 “Hydrometry” the secretariat of which
is held by BSI.
This European Standard shall be given the status of a national standard, either by publication of an identical
text or by endorsement, at the latest by April 2008, and conflicting national standards shall be withdrawn at the
latest by April 2008.
This document supersedes EN ISO 748:2000.
According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following
countries are bound to implement this European Standard: Austria, Belgium, Bulgaria, Cyprus, Czech
Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,
Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain,
Sweden, Switzerland and the United Kingdom.
Endorsement notice
The text of ISO 748:2007 has been approved by CEN as a EN ISO 748:2007 without any modification.

INTERNATIONAL ISO
STANDARD 748
Fourth edition
2007-10-15
Hydrometry — Measurement of liquid
flow in open channels using current-
meters or floats
Hydrométrie — Mesurage du débit des liquides dans les canaux
découverts au moyen de débitmètres ou de flotteurs

Reference number
ISO 748:2007(E)
©
ISO 2007
ISO 748:2007(E)
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but
shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In
downloading this file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat
accepts no liability in this area.
Adobe is a trademark of Adobe Systems Incorporated.
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation
parameters were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In
the unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below.

©  ISO 2007
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 either ISO at the address below or
ISO's member body in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail [e-mail]
Web www.iso.org
Published in Switzerland
ii © ISO 2007 – All rights reserved

ISO 748:2007(E)
Contents Page
Foreword. v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions. 1
4 Principle of the methods of measurements. 1
5 Selection and demarcation of site . 2
5.1 Selection of site . 2
5.2 Demarcation of site. 3
6 Measurement of cross-sectional area . 3
6.1 General. 3
6.2 Measurement of width. 3
6.3 Measurement of depth. 4
7 Measurement of velocity. 5
7.1 Measurement of velocity using current-meters. 5
7.1.1 Rotating-element current-meters . 5
7.1.2 Electromagnetic current-meters . 5
7.1.3 Measurement procedure . 5
7.1.4 Oblique flow . 6
7.1.5 Determination of the mean velocity in a vertical . 7
7.1.6 Errors and limitations. 10
7.2 Measurement of velocity using floats. 11
7.2.1 General. 11
7.2.2 Selection of site . 11
7.2.3 Measuring procedure . 11
7.2.4 Types of float. 11
7.2.5 Determination of velocity. 12
7.2.6 Main sources of error . 13
8 Computation of discharge . 13
8.1 General. 13
8.2 Graphical method . 13
8.2.1 Depth-velocity-integration . 13
8.2.2 Velocity-area integration method (velocity-contour method) . 14
8.3 Arithmetic methods . 16
8.3.1 Mean-section method. 16
8.3.2 Mid-section method. 16
8.4 Independent vertical method. 17
8.5 Mean-section method — Horizontal planes . 20
8.6 Determination of discharge from surface-float velocity measurements . 20
8.7 Determination of discharge for variations of water level . 22
8.7.1 General. 22
8.7.2 Computation of discharge . 22
8.7.3 Computation of mean water level . 22
9 Uncertainties in flow measurement . 23
9.1 General. 23
9.2 Definition of uncertainty. 23
9.3 Method of calculating the uncertainty in discharge by measurement of velocity by
current-meter. 24
9.3.1 General. 24
ISO 748:2007(E)
9.3.2 Contributory uncertainties. 24
9.3.3 Example. 26
9.3.4 Combined uncertainty . 26
9.4 Method of calculating the uncertainty in discharge by measurement of velocity using
floats. 27
9.4.1 General . 27
9.4.2 Contributory uncertainties. 27
9.4.3 Combined uncertainty in discharge. 28
9.4.4 Example. 28
Annex A (informative) Correction for sag, pull, slope and temperature in measurement of cross-
section width by tape or wire. 30
Annex B (informative) Distance measurement across the cross-section . 33
Annex C (informative) Corrections for wetted length of wire when measuring depths with wire not
normal to surface . 36
Annex D (informative) Correction for drift . 39
Annex E (informative) Uncertainties in the velocity-area measurement. 40
Annex F (informative) Determination of mean velocity from float measurements. 44
Bibliography . 46

iv © ISO 2007 – All rights reserved

ISO 748:2007(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
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. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. 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.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 748 was prepared by Technical Committee ISO/TC 113, Hydrometry, Subcommittee SC 1, Velocity area
methods.
This fourth edition cancels and replaces the third edition (ISO 748:1997), which has been technically revised.

INTERNATIONAL STANDARD ISO 748:2007(E)

Hydrometry — Measurement of liquid flow in open channels
using current-meters or floats
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 or floats to measure the velocities. It should be noted that
although, in some 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 ISO 1100-1 and ISO 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 ISO 9196.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO 772, Hydrometric determinations — Vocabulary and symbols
ISO 1088, Hydrometry — Velocity-area methods using current-meters — Collection and processing of data for
determination of uncertainties in flow measurement
ISO 2537, Hydrometry — Rotating-element current-meters
ISO 3455, Hydrometry — Calibration of current-meters in straight open tanks
ISO/TS 15768, Measurement of liquid velocity in open channels — Design, selection and use of
electromagnetic current meters
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 772 apply.
4 Principle of the methods of measurements
4.1 The principle of these methods consists of determining velocity and cross-sectional area. A measuring
site is chosen conforming to the specified requirements (see Clause 5); 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 (known as verticals) across the width, sufficient to determine the shape and area of the
cross-section (see Clause 6).
ISO 748:2007(E)
Velocity observations using current-meters are made at each vertical preferably at the same time as
measurement of depth, especially in the case of unstable beds (see 7.1.5).
Velocity observations can also be made using surface floats or velocity-rods (see 7.2).
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. If unit width discharge is required, it is
generally computed from the individual observations at each measurement vertical.
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. When the length of the channel is restricted, it is recommended
for current-meter measurements that the straight length upstream should be at least twice that
downstream.
b) Flow directions for all points on any vertical across the width should be parallel to one another and at right
angles to the measurement section.
c) The bed and margins of the channels should be stable and well defined at all stages of flow in order to
facilitate accurate measurement of the cross-section and ensure uniformity of conditions during and
between discharge measurements.
d) The curves of the distribution of velocities should be regular in the vertical and horizontal planes of
measurement.
e) Conditions at the section and in its vicinity should also be such as to preclude changes taking place in the
velocity distribution during the period of measurement.
f) Sites displaying vortices, reverse flow or dead water should be avoided.
g) The measurement section should be clearly visible across its width and unobstructed by trees, aquatic
growth or other obstacles.
h) Measurement of flow from bridges can be a convenient and sometimes safer way of sampling width,
depth and velocity. When gauging from a bridge with divide piers, each section of the channel should be
measured separately. Particular care should be taken in determining the velocity distribution when bridge
apertures are surcharged or obstructed.
i) The depth of water at the section should be sufficient at all stages to provide for the effective immersion
of the current-meter or float, whichever is to be used.
j) If the site is to be established as a permanent station, it should be easily accessible at all times with all
necessary measurement equipment.
k) The section should be sited away from pumps, sluices and outfalls, if their operation during a
measurement is likely to create unsteady flow conditions.
l) Sites where there is converging or diverging flow should be avoided.
m) In those instances where it is necessary to make measurements in the vicinity of a bridge, it is preferable
that the measuring site be upstream of the bridge. However, in certain 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.
2 © ISO 2007 – All rights reserved

ISO 748:2007(E)
n) The measurement of flow under ice cover is dealt with in ISO 9196. For streams subject to formation of
ice cover, the requirements of measurement specified in this International Standard can be used during
the free water season.
o) It may, under certain conditions of river flow or level, prove necessary to carry out current-meter
measurements on sections other than the original chosen location. This is quite acceptable if there are no
substantial unmeasured losses or gains to the river in the intervening reach and so long as all flow
measurements can be related to any stage value recorded at the principal reference section.
5.2 Demarcation of site
5.2.1 If the site is to be established as a permanent station or likely to be used frequently for future
measurement, it should be provided with means for demarcation of the cross-section and for determination of
stage. Where the site is used only once, or infrequently, and there are no means of determining stage values
on site, care should be taken to ensure that the water level and/or flow do not change significantly during the
measurement period.
5.2.2 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.
5.2.3 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.
5.2.4 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 separated by successive verticals where velocities are measured.
6.2 Measurement of width
6.2.1 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.
6.2.2 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.
6.2.3 Where the channel is too wide for the above methods of measurement, and a boat is used, the
horizontal distance may be determined by optical or electronic distance-meters, by the use of a differential
Global Positioning System, or by one of the surveying methods given in Annex B.
ISO 748:2007(E)
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. The number of points at which depth shall be measured should be the same as the number of
points at which velocity is measured (see 7.1.3).
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. Difficulty may be experienced when attempting to measure depth at times of high velocity.
Annex C of this document offers alternative methods.
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 as
those of the water to be measured.
Where it is impracticable 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 sufficient intervals for the value of the level at
the time of each determination of depth to be obtained by interpolation.
6.3.5 When, during the determination 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.
6.3.6 Inaccuracies in soundings are most likely to occur owing to:
a) the departure from the vertical of the sounding rod or line, particularly in deep water, when the velocity is
high;
b) the penetration of the bed by the sounding weight or rod;
c) 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 30° in view of the inaccuracies involved. Two alternative methods of applying the correction are
given in Annex C.
Errors due to b) may be reduced by fitting a base plate 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.
4 © ISO 2007 – All rights reserved

ISO 748:2007(E)
6.3.7 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 shall 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 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
7.1.1 Rotating-element current-meters
Rotating-element current-meters shall be manufactured, calibrated and maintained according to ISO 2537 and
ISO 3455. They should be used only within their calibrated range and fitted on suspension equipment similar
to that used during calibration.
In the vicinity of the minimum speed of response, the uncertainty in determining the velocity is high. Care
should be exercised when measuring velocities near the minimum 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 where the depth at the point of measurement is
less than four 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. An exception to this is the case where
the cross-section is very shallow at one side but is the best available.
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 uncertainty due to 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 may be capable of operation in shallower
depths than rotating element current-meters and of detecting and measuring flow reversal.
No electromagnetic current-meter should be selected for use where the depth at the point of measurement is
less than three times the vertical dimension of the probe (see ISO/TS 15768). An exception to this is the case
where the cross-section is very shallow at one side but is the best available.
7.1.3 Measurement procedure
Velocity observations are normally made at the same time as measurements of the depth. This method shall
be particularly 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 6.2.2 and 6.2.3.
In judging the specific number n of verticals in small channels (< 5 m) that are to be defined for the purpose of
determining flow at a particular location, the following criteria shall be applied. These criteria shall be the
minimum requirement and only practical constraints of time, costs, or on site conditions should result in a
reduction of these numbers.
⎯ Channel width < 0,5 m n = 5 to 6
⎯ Channel width > 0,5 m and < 1 m n = 6 to 7
ISO 748:2007(E)
⎯ Channel width > 1 m and < 3 m n = 7 to 12
⎯ Channel width > 3 m and < 5 m n = 13 to 16
⎯ Channel width > 5 m n W 22
For channel widths > 5 m, the number of verticals shall be chosen so that the discharge in each segment is
less than 5 % of the total, insofar as possible, and that in no case should exceed 10 %.
In all instances, measurements of depth made at the water's edge are additional to the above. The first and
last verticals should be as close as practically possible to the water’s edge.
It is further recommended that the location of the verticals be selected after a previous cross-section survey.
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. The current-
meter shall be held so that it is not affected by any disturbances of flow.
Current-meter counters or velocity indicators with a digital display of low resolution should not be used at low
velocities, e.g. less than 0,15 m/s.
When the orientation of the current-meter can be controlled, e.g. when wading gauging with rods, the meter
should be held at right angles to the measuring cross-section. Where oblique flow occurs, or the cross-section
is not at right angles to the direction of flow (see 7.1.4) and the meter is suspended, it will align itself with the
direction of flow. In such cases, the meter shall be allowed to adjust to the flow before readings are started.
Care should be taken to ensure that the current-meter observations are not affected by random surface-waves
and wind.
When a number of points in a vertical are to be measured, a number of current-meters fixed to the same rod
or cable 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 (see Annex C).
NOTE The selection and use of appropriate suspension equipment is described in ISO 3454 and ISO 4375.
The velocity at each selected point shall be observed by exposing the current-meter for a minimum of 30 s.
Where the velocity is subject to periodic pulsations in excess of 30 s, the exposure time should be increased
accordingly (see ISO 1088).
The current-meter shall be removed from the water or brought to the surface at intervals for visual
examination, usually when passing from one vertical to another.
A spin test, where appropriate, should be performed before and after each discharge measurement to ensure
that the mechanism of the current-meter operates freely (see ISO 2537).
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
6 © ISO 2007 – All rights reserved

ISO 748:2007(E)
there is insignificant wind, the angle of flow throughout the vertical can be assumed 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 between the flow direction and the perpendicular to the cross-section is θ the velocity
used for the computation of flow discharge shall be:
vv= cosθ (1)
corrected measured
NOTE Some current-meters are equipped to measure the normal component of velocity directly when held
perpendicular to the measurement cross-section in oblique flow. This correction would not be applied in such cases.
7.1.5 Determination of the mean velocity in a vertical
7.1.5.1 Choice and classification
The choice of the method for determining velocity depends on certain factors. These are: time available, width
and depth of the channel, bed conditions in the measuring section and the upstream reach, rate of variation of
level, degree of accuracy desired 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).
7.1.5.2 Velocity distribution method
Using this method, the values of the velocity are obtained from observations at a number of points in 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 in each vertical with a difference in
readings between two adjacent points of not more than 20 % with respect to the higher value. The location of
the top and the bottom readings should be chosen, taking into account the specification under 7.1.1 and 7.1.2
(see also ISO 1088).
The velocity observations at each position are then plotted graphically and the unit width discharge or mean
velocity determined by planimeter, digitizer or equivalent method. The mean velocity in the vertical may also
be obtained by dividing the unit width discharge by the total depth.
NOTE 1 This method may not be suitable for routine discharge measurements because the apparent gain in precision
may be offset by errors resulting from change of stage during the long period of time needed for making the measurement.
NOTE 2 Although this clause deals primarily with the determination of mean velocity in the vertical, it may be
necessary to apply the same principle to the determination of mean velocity close to the vertical side or wall of a channel.
The velocity curve can be extrapolated from the last measuring point to the bed or vertical side of the channel by
calculating v from Equation (2):
x
m
x
⎛⎞
vv= (2)
xa⎜⎟
a
⎝⎠
where
v is the open point velocity in the extrapolated zone at a distance x from the bed or vertical side;
x
v is the velocity at the last measuring point at a distance a from the bed or vertical side;
a
m is an exponent.
ISO 748:2007(E)
The mean velocity, v , between the bottom (or a vertical side) of the channel and the nearest point of measurement
(where the measured velocity is v ) can be calculated directly from Equation (3):
a
⎛⎞m
vv= (3)
⎜⎟ a
m+1
⎝⎠
Generally, m lies between 5 and 7 but it may vary over a wider range depending on the hydraulic resistance. The value
m = 4 applies to coarse beds or vertical sides while m = 10 is characteristic of smooth beds or vertical sides.
m is obtained as follows:
⎛⎞
C 2 g
ver
m=+⎜⎟0,3 (4)
⎜⎟
gg+C
⎝⎠ver
where
g is the acceleration due to gravity (m/s );
0,5
C is Chezy's coefficient on a vertical (m /s).
ver
NOTE 3 An alternative method of obtaining the velocity in the region below 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
7.1.5.3.1 General
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 theoretical
velocity profiles.
It is recommended that for a new gauging section the accuracy of the selected method be assessed by
comparing the results of preliminary gaugings with those obtained from the velocity distribution method.
7.1.5.3.2 One-point method
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.
7.1.5.3.3 Two-point method
Velocity observations shall be made on 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 shall be taken as the mean velocity in the vertical.
7.1.5.3.4 Three-point method
Velocity observations shall be made on each vertical by exposing the current-meter at 0,2, 0,6 and 0,8 of the
depth below the surface. The 0,6 measurement may be weighted and the mean velocity v obtained from
Equation (5):
vv=+0,25 2v+v (5)
( )
0,2 0,6 0,8
8 © ISO 2007 – All rights reserved

ISO 748:2007(E)
7.1.5.3.5 Five-point method
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 v may be
determined from a graphical plot of the velocity profile with a planimeter, or from Equation (6).
vv=+0,1 3v+ 3v+ 2v+v (6)
( )
surface0,2 0,60,8 bed
7.1.5.3.6 Six-point method
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 7.1.5.3.7.2). 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 v may be found algebraically from
Equation (7).
vv=0,1 +2v+222v+++v v v (7)
( )
surface0,2 0,40,6 0,8 bed
7.1.5.3.7 Surface one-point method
7.1.5.3.7.1 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
...