SIST EN ISO 748:2022
(Main)Hydrometry - Measurement of liquid flow in open channels - Velocity area methods using point velocity measurements (ISO 748:2021)
Hydrometry - Measurement of liquid flow in open channels - Velocity area methods using point velocity measurements (ISO 748:2021)
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.
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 Strömungsmessgeräten mit rotierenden Elementen, akustischen Doppler-Strömungsmessgeräten (ADV, en: Acoustic Doppler Velocimeter), akustischen Doppler-Geräten zum Messen von Geschwindigkeitsprofilen (ADCP, en: Acoustic Doppler Current Profiler) im stationären 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 wird auf Leitlinien von Geräteherstellern und entsprechenden Behörden Bezug genommen.
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)
Le présent document spécifie des méthodes permettant de déterminer la vitesse et l’aire de la section transversale d’un écoulement d’eau à surface libre et de calculer le débit à l’aide de dispositifs de mesurage de la vitesse par point.
Il est applicable aux méthodes utilisant les courantomètres à élément rotatif, les vélocimètres à effet Doppler acoustique (ADV), à la méthode stationnaire avec profileur acoustique de vitesse à effet Doppler (ADCP), et aux mesurages de la vitesse superficielle, y compris les flotteurs et autres systèmes de vitesse superficielle.
Bien que certaines procédures générales soient abordées, le présent document ne décrit pas en détail comment utiliser ou déployer ces systèmes.
NOTE Pour les procédures détaillées, se référer aux lignes directrices des fabricants d’instruments et des organismes publics appropriés.
Hidrometrija - Merjenje pretoka tekočin v odprtih kanalih - Metode za določanje območja hitrosti z uporabo točkovnih meritev hitrosti (ISO 748:2021)
Ta dokument določa metode za določanje hitrosti in prečnega prereza območja vode, ki teče v odprtih kanalih, ter za izračun izpusta z uporabo naprav za točkovno merjenje hitrosti. Uporablja se za metode, pri katerih se uporabljajo merilniki pretoka z vrtljivim elementom, akustični Dopplerjevi merilniki hitrosti (ADV), stacionarna metoda akustičnega Dopplerjevega merilnika pretoka (ADCP), merjenje površinske hitrosti, vključno s plovci in drugimi sistemi površinske hitrosti. Obravnavani so nekateri splošni postopki, vendar ta dokument ne zagotavlja podrobnega opisa, kako uporabljati ali uvajati te sisteme. OPOMBA: Za podroben opis postopkov glej smernice proizvajalcev instrumentov in ustreznih javnih agencij.
General Information
Relations
Standards Content (Sample)
SLOVENSKI STANDARD
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
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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.
Contents Page
European foreword . 3
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.
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 748:2021(E)
© 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
ii
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 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
iv
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.
v
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);
V
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 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 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 as practically possible to the water’s edge.
The device used for point velocity measurement shall be held in position for a minimum of 30 s to obtain
a good representation of mean velocity. It shall be held so movement of the instrument is minimized
during the measurement period.
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 document, the following criteria can be used but the level
of uncertainty of the overall measurement will be much greater.
— 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
ISO 748:2021(E)
See Table D.6 for guidance on percentage uncertainty in measurement of mean velocity due to a limited
number of verticals.
7.1.3 Oblique flow
If oblique 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 as shown by Formula (2):
v =v × cosθ (2)
c m
where
v is the velocity corrected;
c
v is the velocity measured.
m
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 is not applied in such cases.
7.1.4 Determination of the mean velocity in a vertical
7.1.4.1 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 7.1.4.2);
b) reduced point methods (see 7.1.4.3);
c) integration method (see 7.1.5).
7.1.4.2 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 7.1.2 and 7.1.3.
This subclause deals primarily with the determination of mean velocity in the vertical. It can be
necessary to apply the same principle to the determination of mean velocity close to the vertical side
ISO 748:2021(E)
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
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 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 Formula (4):
a
m
v = v (4)
a
m+1
Generally, m lies between 5 and 7 but it can vary over a wider range depending on the hydraulic
resistance. The value m = 4 applies to coarse beds or coarse vertical sides while m = 10 is characteristic
of smooth beds or smooth vertical sides.
m is obtained as shown by Formula (5):
C
2 g
ver
m= +03, (5)
g gC+
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 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.
7.1.4.2.1 ADCP stationary method
In the ADCP stationary method, the ADCP is held in a specific location for a specified time and then
averaging the data at that vertical to obtain a mean velocity profile or a depth-integrated mean velocity
at that location.
It should be noted that ADCP instrumentation cannot measure velocity near the ADCP transducers,
above the transducers or near the bed. Current manufacturer software allows extrapolation in these
areas based upon the measured velocities to compute a mean velocity for the vertical.
7.1.4.3 Reduced point methods
7.1.4.3.1 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 (see 7.1.4.2).
ISO 748:2021(E)
It is recommended that for a new gauging section the accuracy of the selected method be assessed by
the velocity distribution method.
7.1.4.3.2 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.
7.1.4.3.3 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. See Formula (6):
vv=+05, v (6)
()
02,,08
An alternative method of determining the mean velocity of a vertical is the Kreps method which uses
velocity observations at the surface and at 0,62 of the depth below the surface.
When using the Kreps method, velocity observations shall be made as near as possible to the surface
and 0,62 of the depth below the surface. See Formula (7):
vv=×03,,10+×634 v (7)
00,62
NOTE The Kreps method, which was developed by the Austrian hydrologist Harald Kreps, is also a two-
[21]
point method .
7.1.4.3.4 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 Formula (8):
vv=+02, 52vv+ (8)
()
02,,06 08,
7.1.4.3.5 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 is
obtained from Formula (9):
vv=+01, 33vv++2vv+ (9)
()
00,,20 60,b8 ed
7.1.4.3.6 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. The mean velocity v can
be found from Formula (10):
vv=+01, ()22vv ++22vv + v (10)
00,,20 40,,60 8 bed
7.1.4.3.7 Alternate sampling methods
Alternative sampling methods for determining the mean velocity in the vertical may be utilized under
exceptional circumstances, e.g. high velocity, rapidly changing stage or floating debris, provided the
ISO 748:2021(E)
method applied can be demonstrated by experiment to give results of a similar accuracy to those listed
above.
7.1.5 Integration method
In the integration method, the velocity throughout each vertical is measured by raising and lowering a
current meter through the entire depth on each vertical at a uniform rate. The speed at which the meter
is lowered or raised should not be more than 5 % of the mean water velocity and should not in any event
exceed 0,04 m/s. Two complete cycles should be made on each vertical and, if the results differ by more
than 10 %, the operation (two complete cycles) should be repeated until results within this limit are
obtained.
The integration method gives good results if the time of measurement allowed is sufficiently long (60 s
to 100 s). The technique can be, but is not normally, used in depths of less than 1 m.
The average number of revolutions is the total number of revolutions divided by the total time taken for
the measurement in that vertical. The average velocity can then be read from the instrument calibration
corresponding to the average number of revolutions. Uncertainties introduced by using meters with
more than one calibration equation should be avoided.
7.1.6 Errors and limitations
Estimates of the possible errors that can occur when using the various methods detailed in 7.2 are given
in Clause 9. It should be noted that these estimates are of possible random errors which can occur even
when all the precautions noted earlier and below are observed. If the measurement is not made under
these best conditions, additional uncertainty shall be included when estimating the overall uncertainty
of the measurement.
Errors can arise:
a) if the flow is unsteady;
b) if material in suspension interferes with the performance of the current meter;
c) if oblique flow occurs, and the appropriate correction factors are not known accurately;
d) if the instrument used for measurement of velocity is outside the range established by the
calibration;
e) if the set-up for measurement (such as rods or cables suspending the current meter, the boat, etc.) is
different from that used during the calibration of the instrument, in which case it is possible that a
systematic error is introduced;
f) if there is significant disturbance of the water surface by wind;
g) if the device is not held steadily in the correct place during the measurement or when an oscillating
movement occurs; in the latter case, the resultant of the flow velocity and the transverse velocities
gives rise to serious positive errors.
7.2 Determination of mean velocity from surface velocity
7.2.1 General
Traditionally, determination of mean velocity from surface velocity was not encouraged as uncertainties
are high. As technologies have developed, there are a greater range of techniques and instruments that
are able to calculate mean velocity more accurately using measurements from the water surface.
Instruments that are designed to measure discharge by measuring surface velocity only shall conform
to the relevant parts of this document.
ISO 748:2021(E)
7.2.2 Non-contact systems
A range of instruments are available to measure surface velocity. Some are described in Annex C.
Particular attention shall be paid to Clause 5.
Measurement of the cross-sectional area shall be in accordance with Clause 6.
The velocity coefficient at a site shall be derived using a proven technique. If the site is to be used
regularly, an index rating shall be calculated. This shall be applied to the surface velocity measured to
ensure the mean velocity is used in the calculation of the discharge.
Calculation of uncertainties shall be with reference to 9.3 and ISO 25377:2020.
7.2.3 Surface one-point method by current meter
The depth of submergence of the current meter shall be uniform over all the verticals; care shall be
taken to ensure that the current meter observations are not affected by random surface-waves and
wind. This “surface” velocity may be converted to the mean velocity in the vertical by multiplying it by
a predetermined coefficient specific to the section and to the discharge.
The coefficient shall be computed for all stages by correlating the velocity at the surface with the
velocity at 0,6 of the depth or, where greater accuracy is desired, with the mean velocity obtained by
one of the other methods described previously.
7.2.4 Measurement of velocity using floats
A full description of this method is described in Annex B.
This method shall only be used when it is impossible to employ other point measurement devices,
however, it is a useful technique in cases of reconnaissance or because of access difficulties, excessive
velocities and depths or the presence of material in suspension.
7.2.5 Exceptions
Where it is not possible to check the coefficient directly, it may be assumed for guidance that, in general,
the coefficient of the surface velocity varies between 0,84 and 0,90 depending on the shape and velocity
profile of the channel.
7.2.6 Main sources of error
Errors that can occur during the measurement of surface velocity are listed below. They shall be taken
into consideration when estimating the overall error as given in Clause 9.
Errors can arise:
a) if the coefficient from which the mean velocity is obtained from the surface velocity is not known
accurately;
b) if the cross-section has been measured incorrectly;
c) if the cross-section is unstable, i.e. has a moving bed;
d) if the measured velocity does not reflect the true velocity due to unstable flow or oblique currents;
e) if floats are used and their motion is biased relative to the water surface motion due to wind.
ISO 748:2021(E)
8 Computation of discharge
8.1 Arithmetic methods
8.1.1 General
The methods shown below can be enhanced by adding additional bathymetric verticals with no velocity.
This is especially us
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