SIST EN ISO 748:2022

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.

---------------------- Page: 1 ----------------------
oSIST prEN ISO 748:2020

---------------------- Page: 2 ----------------------
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
© ISO 2020 – All rights reserved iii

---------------------- Page: 5 ----------------------
oSIST prEN ISO 748:2020
ISO/DIS 748:2020(E)

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
iv © ISO 2020 – All rights reserved

---------------------- Page: 6 ----------------------
oSIST prEN ISO 748:2020
ISO/DIS 748:2020(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 ., title, Subcommittee SC ., title.
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 2020 – All rights reserved v

---------------------- Page: 7 ----------------------
oSIST prEN ISO 748:2020

---------------------- Page: 8 ----------------------
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.
© ISO 2020 – All rights reserved 1

---------------------- Page: 9 ----------------------
oSIST prEN ISO 748:2020
ISO/DIS 748:2020(E)

— 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
V
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.
2 © ISO 2020 – All rights reserved

---------------------- Page: 10 ----------------------
oSIST prEN ISO 748:2020
ISO/DIS 748:2020(E)

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.
© ISO 2020 – All rights reserved 3

---------------------- Page: 11 ----------------------
oSIST prEN ISO 748:2020
ISO/DIS 748:2020(E)

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.
4 © ISO 2020 – All rights reserved

---------------------- Page: 12 ----------------------
oSIST prEN ISO 748:2020
ISO/DIS 748:2020(E)

— 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
© ISO 2020 – All rights reserved 5

---------------------- Page: 13 ----------------------
oSIST prEN ISO 748:2020
ISO/DIS 748:2020(E)

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):
x
1
x
 m
vv= (3)
 
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.
The
...