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)

Diese Internationale Norm 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.
Sie umfasst 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 nicht im Einzelnen beschrieben, wie diese Systeme anzuwenden oder einzusetzen sind. Hinsichtlich detaillierter Verfahren sollte auf Leitlinien von Geräteherstellern und entsprechenden Behörden Bezug genommen werden.

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)

Hidrometrija - Merjenje pretoka tekočin v odprtih kanalih - Metode območja hitrosti z uporabo točkovnih meritev hitrosti (ISO/DIS 748:2020)

General Information

Status
Not Published
Technical Committee
Current Stage
4599 - Dispatch of FV draft to CMC - Finalization for Vote
Due Date
24-Jun-2021
Completion Date
24-Jun-2021

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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,
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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
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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

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ii © ISO 2020 – All rights reserved
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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

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

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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

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

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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
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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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