Hydrometry - Measurement of liquid flow in open channels - Velocity area methods using point velocity measurements (ISO 748:2021)

Hydrometrie - Durchflussmessung in offenen Gerinnen mittels Fließgeschwindigkeitsmessgeräten (ISO 748:2021)

Dieses Dokument legt Verfahren zur Bestimmung von Geschwindigkeit und Querschnittsfläche von Wasser, das durch offene Gerinne fließt, und für die Berechnung des Durchflusses unter Einsatz von Geräten zur punktuellen Geschwindigkeitsmessung fest.
Es ist anwendbar auf Verfahren unter Anwendung von Fließgeschwindigkeitsmessgeräten mit rotierenden Elementen, akustischen Doppler-Geschwindigkeitsmessgeräten (ADV, en: Acoustic Doppler Velocimeter), akustischen Doppler-Geräten zum Messen von Geschwindigkeitsprofilen (ADVP, en: Acoustic Doppler Velocity Profiler) – Stationäres Verfahren, Messung der Oberflächengeschwindigkeit unter Einbeziehung von Schwimmern und anderen Systemen zur Bestimmung der Oberflächengeschwindigkeit.
Obwohl einige allgemeine Verfahren erörtert werden, wird in diesem Dokument nicht im Einzelnen beschrieben, wie diese Systeme anzuwenden oder einzusetzen sind.
ANMERKUNG Hinsichtlich detaillierter Verfahren sollte auf Leitlinien von Geräteherstellern und entsprechenden Behörden Bezug genommen werden.

Hydrométrie - Mesurage du débit des écoulements à surface libre - Méthodes d'exploration du champ des vitesses utilisant le mesurage de la vitesse par point (ISO 748:2021)

Hidrometrija - Merjenje pretoka tekočin v odprtih kanalih - Metode za določanje območja hitrosti z uporabo točkovnih meritev hitrosti (ISO 748:2021)

General Information

Status
Published
Public Enquiry End Date
29-Sep-2020
Publication Date
12-Dec-2021
Technical Committee
Current Stage
6060 - National Implementation/Publication (Adopted Project)
Start Date
08-Dec-2021
Due Date
12-Feb-2022
Completion Date
13-Dec-2021

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Standards Content (sample)

SLOVENSKI STANDARD
SIST EN ISO 748:2022
01-februar-2022
Nadomešča:
SIST EN ISO 748:2008
Hidrometrija - Merjenje pretoka tekočin v odprtih kanalih - Metode za določanje
območja hitrosti z uporabo točkovnih meritev hitrosti (ISO 748:2021)

Hydrometry - Measurement of liquid flow in open channels - Velocity area methods using

point velocity measurements (ISO 748:2021)
Hydrometrie - Durchflussmessung in offenen Gerinnen mittels
Fließgeschwindigkeitsmessgeräten (ISO 748:2021)
Hydrométrie - Mesurage du débit des écoulements à surface libre - Méthodes

d'exploration du champ des vitesses utilisant le mesurage de la vitesse par point (ISO

748:2021)
Ta slovenski standard je istoveten z: EN ISO 748:2021
ICS:
17.120.20 Pretok v odprtih kanalih Flow in open channels
SIST EN ISO 748:2022 en,fr,de

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN ISO 748:2022
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SIST EN ISO 748:2022
EN ISO 748
EUROPEAN STANDARD
NORME EUROPÉENNE
December 2021
EUROPÄISCHE NORM
ICS 17.120.20 Supersedes EN ISO 748:2007
English Version
Hydrometry - Measurement of liquid flow in open
channels - Velocity area methods using point velocity
measurements (ISO 748:2021)

Hydrométrie - Mesurage du débit des écoulements à Hydrometrie - Durchflussmessung in offenen Gerinnen

surface libre - Méthodes d'exploration du champ des mittels Fließgeschwindigkeitsmessgeräten (ISO

vitesses utilisant le mesurage de la vitesse par point 748:2021)
(ISO 748:2021)
This European Standard was approved by CEN on 24 October 2021.

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-CENELEC 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-CENELEC Management

Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,

Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,

Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and

United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels

© 2021 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 748:2021 E

worldwide for CEN national Members.
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SIST EN ISO 748:2022
EN ISO 748:2021 (E)
Contents Page

European foreword ....................................................................................................................................................... 3

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SIST EN ISO 748:2022
EN ISO 748:2021 (E)
European foreword

This document (EN ISO 748:2021) has been prepared by Technical Committee ISO/TC 113

"Hydrometry" 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 June 2022, and conflicting national standards shall be

withdrawn at the latest by June 2022.

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. CEN shall not be held responsible for identifying any or all such patent rights.

This document supersedes EN ISO 748:2007.

Any feedback and questions on this document should be directed to the users’ national standards

body/national committee. A complete listing of these bodies can be found on the CEN website.

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,

Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland,

Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of

North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the

United Kingdom.
Endorsement notice

The text of ISO 748:2021 has been approved by CEN as EN ISO 748:2021 without any modification.

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SIST EN ISO 748:2022
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SIST EN ISO 748:2022
INTERNATIONAL ISO
STANDARD 748
Fifth edition
2021-11
Hydrometry — Measurement of liquid
flow in open channels — Velocity
area methods using point velocity
measurements
Hydrométrie — Mesurage du débit des écoulements à surface libre —
Méthodes d'exploration du champ des vitesses utilisant le mesurage
de la vitesse par point
Reference number
ISO 748:2021(E)
© ISO 2021
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SIST EN ISO 748:2022
ISO 748:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2021

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on

the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below

or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
© ISO 2021 – All rights reserved
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SIST EN ISO 748:2022
ISO 748:2021(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 Site selection ............................................................................................................................................................................................................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 ..................................................................................................................................................................... 3

7 Measurement of mean velocity ............................................................................................................................................................. 4

7.1 Determination of mean velocity using point velocity measurements .................................................. 4

7.1.1 General ........................................................................................................................................................................................ 4

7.1.2 Measurement procedure ............................................................................................................................................. 4

7.1.3 Oblique flow ............................................................................................................................................................................ 5

7.1.4 Determination of the mean velocity in a vertical .................................................................................. 5

7.1.5 Integration method .......................................................................................................................................................... 8

7.1.6 Errors and limitations ................................................................................................................................................... 8

7.2 Determination of mean velocity from surface velocity ...................................................................................... 8

7.2.1 General ........................................................................................................................................................................................ 8

7.2.2 Non-contact systems ...................................................................................................................................................... 9

7.2.3 Surface one-point method by current meter ............................................................................................. 9

7.2.4 Measurement of velocity using floats .............................................................................................................. 9

7.2.5 Exceptions ................................................................................................................................................................................ 9

7.2.6 Main sources of error ..................................................................................................................................................... 9

8 Computation of discharge .......................................................................................................................................................................10

8.1 Arithmetic methods ........................................................................................................................................................................ 10

8.1.1 General ..................................................................................................................................................................................... 10

8.1.2 Mean-section method.................................................................................................................................................. 10

8.1.3 Mid-section method ...................................................................................................................................................... 10

8.1.4 Bathymetric verticals ................................................................................................................................................. 11

8.2 Independent vertical method ................................................................................................................................................. 11

8.3 Mean-section method — Horizontal planes .............................................................................................................. 13

9 Uncertainties in flow measurement ............................................................................................................................................13

9.1 General ........................................................................................................................................................................................................13

9.2 Method of calculating the uncertainty in discharge by measurement of velocity by

current meter ...................................................................... .................................................................................................................. 14

9.2.1 General ..................................................................................................................................................................................... 14

9.2.2 Contributory uncertainties .................................................................................................................................... 14

9.3 Method of calculating the uncertainty in discharge by measurement of velocity

using floats .............................................................................................................................................................................................. 16

9.3.1 General ..................................................................................................................................................................................... 16

9.3.2 Contributory uncertainties .................................................................................................................................... 16

9.3.3 Combined uncertainty in discharge ............................................................................................................... 17

9.4 Limitations .............................................................................................................................................................................................. 18

9.5 Interpolated variance estimator (IVE) .......................................................................................................................... 19

9.6 Q+ .................................................................................................................................................................................................................... 19

9.7 Flaure ........................................................................................................................................................................................................... 19

Annex A (informative) Use of point velocity current meters ..................................................................................................20

iii
© ISO 2021 – All rights reserved
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SIST EN ISO 748:2022
ISO 748:2021(E)

Annex B (informative) Surface velocity measurement using floats ...............................................................................23

Annex C (informative) Example surface velocity systems ........................................................................................................27

Annex D (informative) Uncertainties in the velocity-area measurement ................................................................29

Annex E (informative) Velocity measurement under ice conditions ..............................................................................32

Annex F (informative) Corrections for wetted length of wire when measuring depths with

wire not normal to surface ....................................................................................................................................................................38

Bibliography .............................................................................................................................................................................................................................41

© ISO 2021 – All rights reserved
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SIST EN ISO 748:2022
ISO 748:2021(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.

The procedures used to develop this document and those intended for its further maintenance are

described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the

different types of ISO documents should be noted. This document was drafted in accordance with the

editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).

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. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www.iso.org/patents).

Any trade name used in this document is information given for the convenience of users and does not

constitute an endorsement.

For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and

expressions related to conformity assessment, as well as information about ISO’s adherence to

the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see

www.iso.org/iso/foreword.html.

This document was prepared by Technical Committee ISO/TC 113, Hydrometry, Subcommittee SC 1,

Velocity area methods, in collaboration with the European Committee for Standardization (CEN)

Technical Committee CEN/TC 318, Hydrometry, in accordance with the Agreement on technical

cooperation between ISO and CEN (Vienna Agreement).

This fifth edition cancels and replaces the fourth edition (ISO 748:2007), which has been technically

revised. The main changes compared with the previous edition are as follows:
— the document has been updated to take account of technological developments;
— Clause 7 has been revised to reduce uncertainties in measurements;
— ISO 9196 regarding measurement under ice conditions has been incorporated.

Any feedback or questions on this document should be directed to the user’s national standards body. A

complete listing of these bodies can be found at www.iso.org/members.html.
© ISO 2021 – All rights reserved
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SIST EN ISO 748:2022
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SIST EN ISO 748:2022
INTERNATIONAL STANDARD ISO 748:2021(E)
Hydrometry — Measurement of liquid flow in open
channels — Velocity area methods using point velocity
measurements
1 Scope

This document specifies methods for determining the velocity and cross-sectional area of water flowing

in open channels and for calculating the discharge employing point velocity measurement devices.

It is applicable to methods using rotating-element current meters, acoustic doppler velocimeters (ADVs),

acoustic doppler current profiler (ADCP) stationary method, surface velocity measurement including

floats and other surface velocity systems.

Although some general procedures are discussed, this document does not describe in detail how to use

or deploy these systems.

NOTE For detailed procedures, refer to guidelines from instrument manufacturers and the appropriate

public agencies.
2 Normative references

The following documents are referred to in the text in such a way that some or all of their content

constitutes requirements 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, Hydrometry — Vocabulary and symbols
ISO 25377:2020, Hydrometric uncertainty guidance (HUG)
3 Terms and definitions

For the purposes of this document, the terms and definitions given in ISO 772 apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
4 Principle of the methods of measurements
The principle depends upon determining velocity and cross-sectional area.
This is characterized as shown by Formula (1):
QV= A (1)
where
Q is the flow (m /s);
is the mean velocity (m/s) (averaged over the cross-section);
© ISO 2021 – All rights reserved
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SIST EN ISO 748:2022
ISO 748:2021(E)
A is the cross-sectional area (m ).
A measuring site shall be chosen conforming to the specified requirements.

The cross-sectional area shall be measured by a method specified in this document, appropriate to the

dimensions.
Velocity observations shall be made by a method specified in this document.
The discharge shall be calculated by a method specified in this document.
5 Site selection
5.1 Selection of site
The site selected should conform to the following requirements.

a) The channel at the measuring site shall be straight and of uniform cross-section and slope in order

to minimize abnormal velocity distribution. The straight length should be at least six times the

width of the channel upstream, and at least three times the downstream width.

b) Flow directions for all points on any vertical across the width shall be parallel to one another and

at right angles to the measurement section.

c) The bed and margins of the channels shall 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 shall be regular in the vertical and horizontal planes of

measurement.

e) Conditions at the section and in its vicinity shall 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 shall be avoided.

g) The measurement section (including approach and exit) shall be clearly visible across its width and

unobstructed by trees, aquatic growth or other obstacles.

h) When gauging from a bridge with divide piers, each section of the channel shall be measured

separately. Particular care shall be taken in determining the velocity distribution when bridge

apertures are surcharged or obstructed.

i) The depth of water at the section shall be sufficient at all stages to ensure whichever device is

deployed it conforms to the manufactures minimum criteria for use.

j) If the site is to be established as a permanent station, it shall be easily accessible at all times with all

necessary measurement equipment appropriate to the flow conditions.

k) The section shall 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 shall be avoided.

m) If a suitable straight section includes a bridge, wading and boat measurements shall be made away

from the effects of the bridge.

n) The measurement of flow under ice cover is dealt with in Annex E. For streams that are subject

to formation of ice cover, the main part of this document shall be used when the stream is free

flowing.
© ISO 2021 – All rights reserved
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SIST EN ISO 748:2022
ISO 748:2021(E)

o) It may, under certain conditions of river flow or level, prove necessary to carry out measurements

on sections upstream or downstream of 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.

NOTE Ideal measurement conditions can be found when all requirements are satisfied. If ideal conditions

are not available, it is still possible to make a measurement, but uncertainty will be increased.

5.2 Demarcation of site

5.2.1 A permanent station, or one likely to be used frequently for future measurement, shall be

provided with means for demarcation of the cross-section and for determination of stage.

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 natural objects and, if possible, the

position noted using a global navigation satellite system (GNSS).

5.2.3 The stage shall be read from a gauge at the start and end of the measurement period. If the

water level changes rapidly, a level measurement is recommended to be taken at least every 30 min.

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. 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 and to minimize the uncertainty in the calculation of

the cross-sectional area.
6.2 Measurement of width

Measurement of the width of the channel and the width of the individual segments shall be obtained by

measuring the horizontal distance from or to a fixed reference point which shall be in the same vertical

plane as the cross-section at the measuring site.
6.3 Measurement of depth

Measurement of depth shall be made at intervals close enough to define the cross-sectional profile

accurately. The number of points at which depth is to be measured shall be at each vertical where

velocity is measured.

The number of sampling verticals depends on the variability of the water depth in the cross-section.

This number is adequate when the number of points does not significantly change the value of the

cross-section obtained.

Where it is impracticable to take more than one reading of the depth, the uncertainty in measurement

may be increased (see Clause 9).
When measuring depths with a wire not normal to the surface, see Annex F.
© ISO 2021 – All rights reserved
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SIST EN ISO 748:2022
ISO 748:2021(E)
7 Measurement of mean velocity
7.1 Determination of mean velocity using point velocity measurements
7.1.1 General

A range of instruments are available to measure point velocity. These are described in Annex A.

7.1.2 Measurement procedure

Velocity observations are normally made at the same time as measurements of the depth. Where,

however, the two measurements are made at different times, such as at a pre-surveyed station, 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 and 6.3

For all measurements, the best professional judgement of an experienced hydrographer should be used,

and detailed notes regarding the measurement and assumptions made should be included in the record.

In judging the recommended minimum number of verticals in small channels that are to be defined for

the purpose of determining flow at a particular location, the following criteria shall be applied.

— Channel width < 0,5 m n ≥ 15
— Channel width > 0,5 m and < 5m n ≥ 20
— Channel width > 5 m n ≥ 22

As far as possible, verticals should be chosen so that the discharge of each segment is less than 5 % of

the total and shall not exceed 10 % of the total.

For very small channels, practical considerations do not always allow the recommended minimum

number of verticals.

The distance between two verticals shall be greater than the width of the sensor and should not be less

than the minimum recommendations of the specific instrument used.

In all instances, measurements of depth made at the water’s edge are additional to the above. The first

and last verticals shall be as close a
...

SLOVENSKI STANDARD
oSIST prEN ISO 748:2020
01-september-2020

Hidrometrija - Merjenje pretoka tekočin v odprtih kanalih - Metode območja hitrosti

z uporabo točkovnih meritev hitrosti (ISO/DIS 748:2020)

Hydrometry - Measurement of liquid flow in open channels - Velocity area methods using

point velocity measurements (ISO/DIS 748:2020)
Hydrometrie - Durchflussmessung in offenen Gerinnen mittels
Fließgeschwindigkeitsmessgeräten (ISO/DIS 748:2020)

Hydrométrie - Mesurage du débit des cours d'eau - Méthodes d'exploration du champ

des vitesses utilisant le mesurage de la vitesse par point (ISO/DIS 748:2020)
Ta slovenski standard je istoveten z: prEN ISO 748
ICS:
17.120.20 Pretok v odprtih kanalih Flow in open channels
oSIST prEN ISO 748:2020 en,fr,de

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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oSIST prEN ISO 748:2020
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oSIST prEN ISO 748:2020
DRAFT INTERNATIONAL STANDARD
ISO/DIS 748
ISO/TC 113/SC 1 Secretariat: BIS
Voting begins on: Voting terminates on:
2020-06-18 2020-09-10
Hydrometry — Measurement of liquid flow in open
channels — Velocity area methods using point velocity
measurements
ICS: 17.120.20
THIS DOCUMENT IS A DRAFT CIRCULATED
This document is circulated as received from the committee secretariat.
FOR COMMENT AND APPROVAL. IT IS
THEREFORE SUBJECT TO CHANGE AND MAY
NOT BE REFERRED TO AS AN INTERNATIONAL
STANDARD UNTIL PUBLISHED AS SUCH.
IN ADDITION TO THEIR EVALUATION AS
ISO/CEN PARALLEL PROCESSING
BEING ACCEPTABLE FOR INDUSTRIAL,
TECHNOLOGICAL, COMMERCIAL AND
USER PURPOSES, DRAFT INTERNATIONAL
STANDARDS MAY ON OCCASION HAVE TO
BE CONSIDERED IN THE LIGHT OF THEIR
POTENTIAL TO BECOME STANDARDS TO
WHICH REFERENCE MAY BE MADE IN
Reference number
NATIONAL REGULATIONS.
ISO/DIS 748:2020(E)
RECIPIENTS OF THIS DRAFT ARE INVITED
TO SUBMIT, WITH THEIR COMMENTS,
NOTIFICATION OF ANY RELEVANT PATENT
RIGHTS OF WHICH THEY ARE AWARE AND TO
PROVIDE SUPPORTING DOCUMENTATION. ISO 2020
---------------------- Page: 3 ----------------------
oSIST prEN ISO 748:2020
ISO/DIS 748:2020(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2020

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting

on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address

below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Fax: +41 22 749 09 47
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2020 – All rights reserved
---------------------- Page: 4 ----------------------
oSIST prEN ISO 748:2020
ISO/DIS 748:2020(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 Site selection ............................................................................................................................................................................................................. 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 ..................................................................................................................................................................... 3

7  Measurement of velocity .............................................................................................................................................................................. 4

7.1 Determination of velocity using point velocity measurements .................................................................... 4

7.1.1 Measurement procedure ........................................................................................................................................... 4

7.1.2 Skew flow ............................................................................................................................................................................... 5

7.1.3 Determination of the mean velocity in a vertical ................................................................................. 5

7.1.4 Integration method ........................................................................................................................................................ 8

7.1.5 Errors and limitations ................................................................................................................................................. 8

7.2 Determination of mean velocity from surface velocity ........................................................................................ 9

7.2.1 General...................................................................................................................................................................................... 9

7.2.2 Non-contact systems .................................................................................................................................................... 9

7.2.3 Surface one-point method by current-meter .......................................................................................... 9

7.2.4 Measurement of velocity using floats ............................................................................................................ 9

8  Computation of discharge ........................................................................................................................................................................10

8.1 Arithmetic methods — General .............................................................................................................................................10

8.1.1 Mean-section method ...............................................................................................................................................10

8.1.2 Mid-section method ...................................................................................................................................................11

8.1.3 Bathymetric verticals ................................................................................................................................................11

8.2 Independent vertical method ..................................................................................................................................................12

8.3 Mean-section method — Horizontal planes ...............................................................................................................14

8.4 Index velocity method ...................................................................................................................................................................14

9  Uncertainties in flow measurement ..............................................................................................................................................15

9.1 General ........................................................................................................................................................................................................15

9.2 Limitations ...............................................................................................................................................................................................15

9.3 Method of calculating the uncertainty in discharge by measurement of velocity by

current-meter........................................................................................................................................................................................16

9.3.1 General...................................................................................................................................................................................16

9.3.2 Contributory uncertainties ..................................................................................................................................17

9.3.3 Example ................................................................................................................................................................................18

9.3.4 Combined uncertainty .............................................................................................................................................18

9.4 Method of calculating the uncertainty in discharge by measurement of velocity

using floats...............................................................................................................................................................................................19

9.4.1 General...................................................................................................................................................................................19

9.4.2 Contributory uncertainties ..................................................................................................................................20

9.4.3 Combined uncertainty in discharge .............................................................................................................20

9.4.4 Example ................................................................................................................................................................................20

Annex A (informative) Use of point velocity current meters ....................................................................................................22

Annex B (informative) Surface Velocity measurement using floats ..................................................................................24

Annex C (informative) Example surface velocity systems ...........................................................................................................29

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Annex D (informative) Uncertainties in the velocity-area measurement ...................................................................31

Annex E (informative) Velocity measurement under ice conditions ...............................................................................34

Annex F (informative) Corrections for wetted length of wire when measuring depths with

wire not normal to surface ......................................................................................................................................................................40

Bibliography .............................................................................................................................................................................................................................44

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Foreword

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bodies (ISO member bodies). The work of preparing International Standards is normally carried out

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This document was prepared by Technical Committee ISO/TC ..., title, Subcommittee SC ..., title.

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complete listing of these bodies can be found at www .iso .org/ members .html.
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oSIST prEN ISO 748:2020
DRAFT INTERNATIONAL STANDARD ISO/DIS 748:2020(E)
Hydrometry — Measurement of liquid flow in open
channels — Velocity area methods using point velocity
measurements
1  Scope

This International Standard specifies methods for determining the velocity and cross-sectional area of

water flowing in open channels and for calculating the discharge employing point velocity measurement

devices.

It covers methods using rotating element Current Meters, Acoustic Doppler Velocimeters (ADV), Acoustic

Doppler Velocity Profiler (ADVP) – Stationary method, Surface Velocity measurement including floats

and other surface velocity systems.

Although some general procedures are discussed, it does not describe in detail how to use or deploy

these systems. For detailed procedures, reference should be made to guidelines from instrument

manufacturers and appropriate public agencies.
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 772:2011, Hydrometry — Vocabulary and symbols

ISO 1088:2007, Hydrometry — Velocity-area methods using current-meters — Collection and processing of

data for determination of uncertainties in flow measurement
ISO 2537:2007, Hydrometry — Rotating-element current-meters
ISO 3455:2007, Hydrometry — Calibration of current-meters in straight open tanks

ISO/TR 24578:2012, Hydrometry — Acoustic Doppler profiler — Method and application for measurement

of flow in open channels
ISO 25377:2007, Hydrometry — Hydrometric uncertainty guidance (HUG)

ISO 5168:2005, Measurement of fluid flow — Procedures for the evaluation of uncertainties

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
The principle depends upon determining velocity and cross-sectional area.
This is characterized by
— A measuring site shall be chosen conforming to the specified requirements.

— Cross sectional area shall be measured by a method specified in this standard, appropriate to the

dimensions.
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— Velocity observations shall be made by a method specified in this standard.
— The discharge shall be calculated by a method specified in this standard.
QV= A (1)
where
Q Flow
Mean Velocity
A Cross sectional Area
5 Site selection
5.1 Selection of site
The site selected shall comply with the following requirements.

a) The channel at the measuring site shall be straight and of uniform cross-section and slope in order

to minimize abnormal velocity distribution. The straight length upstream shall be as at least twice

that downstream.

b) Flow directions for all points on any vertical across the width shall be parallel to one another and

at right angles to the measurement section.

c) The bed and margins of the channels shall 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 shall be regular in the vertical and horizontal planes of

measurement.

e) Conditions at the section and in its vicinity shall 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 shall be avoided.

g) The measurement section shall be clearly visible across its width and unobstructed by trees,

aquatic growth or other obstacles.

h) When gauging from a bridge with divide piers, each section of the channel shall be measured

separately. Particular care shall be taken in determining the velocity distribution when bridge

apertures are surcharged or obstructed.

i) The depth of water at the section shall be sufficient at all stages to ensure whichever device is

deployed, it conforms to the manufactures minimum criteria for use.

j) If the site is to be established as a permanent station, it shall be easily accessible at all times with all

necessary measurement equipment appropriate to the flow conditions.

k) Measurements made directly from a bridge should usually be made on the downstream side, taking

care to avoid turbulence and eddies produced by bridge piers and other structures.

l) The section shall be sited away from pumps, sluices and outfalls, if their operation during a

measurement is likely to create unsteady flow conditions.
m) Sites where there is converging or diverging flow shall be avoided.
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n) If a suitable straight section includes a bridge, wading and boat measurements shall be made

upstream of the bridge.

o) The measurement of flow under ice cover is dealt with in Annex E. For streams that are subject to

formation of ice cover, the main part of this standard shall be used when the stream is free flowing.

p) It may, under certain conditions of river flow or level, prove necessary to carry out measurements

on sections upstream or downstream of 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.

Note Ideal measurement conditions will be found when all requirements are satisfied. If ideal conditions are not

available, it may still be possible to make a measurement, but uncertainty will be increased.

5.2 Demarcation of site

A permanent station or one likely to be used frequently for future measurement, shall be provided with

means for demarcation of the cross-section and for determination of stage.

5.2.1 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 the start and end of the measurement period. If the water

level changes rapidly, a level measurement is recommended to be taken every 30 min.

5.2.3 If there is a possibility of a difference in the level of the water surface between two banks then

an auxiliary gauge shall be considered. 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. 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 and to minimize the uncertainty in the calculation of

the cross sectional area.
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 vertical plane as the cross-section at the measuring site.
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 is to be measured shall be at least the same as

the points at which velocity is measured.

The number of sampling points depends on the variability of the water depth in the cross section. This

number is adequate when the number of points does not significantly change the value obtained.

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Where it is impracticable to take more than one reading of the depth, the uncertainty in measurement

may be increased (see Clause 9).
7  Measurement of velocity
7.1  Determination of velocity using point velocity measurements

A range of instruments are available to measure point velocity. These are described in Annex A

7.1.1  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, such as at a pre surveyed station, 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 and 6.3

For all measurements, the best professional judgement of an experienced hydrographer should be used,

and detailed notes regarding the measurement and assumptions made should be included in the record.

In judging the recommended minimum number 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.
— Channel width < 0,5 m n ≥ 15
— Channel width > 0,5 m and < 5m n ≥ 20
— Channel width > 5 m n ≥ 22

— The closeness of adjacent verticals should not be less than the minimum recommendations of equip-

ment providers for specific instrument.

NOTE 1 For very small channels, practical considerations may not allow the recommended minimum number

of verticals.

NOTE 2 insofar as possible, verticals should be chosen so that the discharge of each segment is less than 5 % of

the total.

In all instances, measurements of depth made at the water's edge are additional to the above. The first

and last verticals shall 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 device used for point velocity measurement shall be held in position to ensure movement is

minimised during the measurement period. It shall be held so that it is not affected by any disturbances

of flow.

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.

For continuity with previous versions of this standard the following criteria can be used but the level of

uncertainty of the overall measurement will be much greater.
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— Channel width < 0,5 m n = 5 to 6
— Channel width > 0,5 m and < 1 m n = 6 to 7
— 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 ≥ 22

— See Annex D.4.4 Table D6 for a guide to percentage uncertainty in measurement of mean velocity

due to limited number of verticals
7.1.2  Skew flow

If skew flow is unavoidable, either the velocity component perpendicular to the cross section should

be measured directly or the velocity magnitude measured and corrected based upon the angle from

perpendicular. 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 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, subjected to tides or 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:
V V
corrected = measured x cosθ (2)

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.

7.1.3  Determination of the mean velocity in a vertical
7.1.3.1  Choice and classification

The choice of the method for determining mean velocity depends on certain factors. These are: safety,

time available, width and depth of the channel, bed conditions in the measuring section and the

upstream reach, rate of change of stage, degree of accuracy desired and equipment used.

These methods are classified as follows:
a) velocity distribution method (see 7.1.3.2);
b) reduced point methods (see 7.1.3.3);
c) integration method (see 7.1.3.4).
7.1.3.2 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 shall 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 shall be chosen, taking into account the

specification under 7.1.1 and 7.1.2.

NOTE 1 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

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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 Formula (3):
 m
vv= (3)
 
 
where

v is the open point velocity in the extrapolated zone at a distance x from the bed or vertical side;

v is the velocity at the last measuring point at a distance a from the bed or vertical side;

m is an exponent.
The
...

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