SIST EN ISO 748:2008

SLOVENSKI STANDARD
SIST EN ISO 748:2008
01-januar-2008
1DGRPHãþD
SIST EN ISO 748:2001
+LGURPHWULMD0HUMHQMHSUHWRNDWHNRþLQYRGSUWLKNDQDOLK]XSRUDERKLGURPHWULþQLK
NULODOLSODYDþHY,62
Hydrometry - Measurement of liquid flow in open channels using current-meters or floats
(ISO 748:2007)
Hydrometrie - Durchflussmessung in offenen Gerinnen mittels
Fließgeschwindigkeitsmessgeräten oder Schwimmern (ISO 748:2007)
Hydrométrie - Mesurage du débit des liquides dans les canaux découverts au moyen de
débitmetres ou de flotteurs (ISO 748:2007)
Ta slovenski standard je istoveten z: EN ISO 748:2007
ICS:
17.120.20 Pretok v odprtih kanalih Flow in open channels
SIST EN ISO 748:2008 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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

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EN ISO 748:2007 (E)
Contents Page
Foreword.3

2

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

3

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INTERNATIONAL ISO
STANDARD 748
Fourth edition
2007-10-15


Hydrometry — Measurement of liquid
flow in open channels using current-
meters or floats
Hydrométrie — Mesurage du débit des liquides dans les canaux
découverts au moyen de débitmètres ou de flotteurs




Reference number
ISO 748:2007(E)
©
ISO 2007

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ISO 748:2007(E)
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ii © ISO 2007 – All rights reserved

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

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ISO 748:2007(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 748 was prepared by Technical Committee ISO/TC 113, Hydrometry, Subcommittee SC 1, Velocity area
methods.
This fourth edition cancels and replaces the third edition (ISO 748:1997), which has been technically revised.

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INTERNATIONAL STANDARD ISO 748:2007(E)

Hydrometry — Measurement of liquid flow in open channels
using current-meters or floats
1 Scope
This International Standard specifies methods for determining the velocity and cross-sectional area of water
flowing in open channels without ice cover, and for computing the discharge therefrom.
It covers methods of employing current-meters or floats to measure the velocities. It should be noted that
although, in some cases, these measurements are intended to determine the stage-discharge relation of a
gauging station, this International Standard deals only with single measurements of the discharge; the
continuous recording of discharges over a period of time is covered in ISO 1100-1 and ISO 1100-2.
NOTE The methods for determining the velocity and cross-sectional area of water flowing in open channels with ice
cover are specified in ISO 9196.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO 772, Hydrometric determinations — Vocabulary and symbols
ISO 1088, Hydrometry — Velocity-area methods using current-meters — Collection and processing of data for
determination of uncertainties in flow measurement
ISO 2537, Hydrometry — Rotating-element current-meters
ISO 3455, Hydrometry — Calibration of current-meters in straight open tanks
ISO/TS 15768, Measurement of liquid velocity in open channels — Design, selection and use of
electromagnetic current meters
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 772 apply.
4 Principle of the methods of measurements
4.1 The principle of these methods consists of determining velocity and cross-sectional area. A measuring
site is chosen conforming to the specified requirements (see Clause 5); the width, depending on its magnitude,
is measured either by means of steel tape or by some other surveying method, and the depth is measured at
a number of points (known as verticals) across the width, sufficient to determine the shape and area of the
cross-section (see Clause 6).
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ISO 748:2007(E)
Velocity observations using current-meters are made at each vertical preferably at the same time as
measurement of depth, especially in the case of unstable beds (see 7.1.5).
Velocity observations can also be made using surface floats or velocity-rods (see 7.2).
4.2 The discharge is computed either arithmetically or graphically by summing the products of the velocity
and corresponding area for a series of observations in a cross-section. If unit width discharge is required, it is
generally computed from the individual observations at each measurement vertical.
5 Selection and demarcation of site
5.1 Selection of site
The site selected should comply as far as possible with the following requirements.
a) The channel at the measuring site should be straight and of uniform cross-section and slope in order to
minimize abnormal velocity distribution. When the length of the channel is restricted, it is recommended
for current-meter measurements that the straight length upstream should be at least twice that
downstream.
b) Flow directions for all points on any vertical across the width should be parallel to one another and at right
angles to the measurement section.
c) The bed and margins of the channels should be stable and well defined at all stages of flow in order to
facilitate accurate measurement of the cross-section and ensure uniformity of conditions during and
between discharge measurements.
d) The curves of the distribution of velocities should be regular in the vertical and horizontal planes of
measurement.
e) Conditions at the section and in its vicinity should also be such as to preclude changes taking place in the
velocity distribution during the period of measurement.
f) Sites displaying vortices, reverse flow or dead water should be avoided.
g) The measurement section should be clearly visible across its width and unobstructed by trees, aquatic
growth or other obstacles.
h) Measurement of flow from bridges can be a convenient and sometimes safer way of sampling width,
depth and velocity. When gauging from a bridge with divide piers, each section of the channel should be
measured separately. Particular care should be taken in determining the velocity distribution when bridge
apertures are surcharged or obstructed.
i) The depth of water at the section should be sufficient at all stages to provide for the effective immersion
of the current-meter or float, whichever is to be used.
j) If the site is to be established as a permanent station, it should be easily accessible at all times with all
necessary measurement equipment.
k) The section should be sited away from pumps, sluices and outfalls, if their operation during a
measurement is likely to create unsteady flow conditions.
l) Sites where there is converging or diverging flow should be avoided.
m) In those instances where it is necessary to make measurements in the vicinity of a bridge, it is preferable
that the measuring site be upstream of the bridge. However, in certain cases and where accumulation of
ice, logs or debris is liable to occur, it is acceptable that the measuring site be downstream of the bridge.
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ISO 748:2007(E)
n) The measurement of flow under ice cover is dealt with in ISO 9196. For streams subject to formation of
ice cover, the requirements of measurement specified in this International Standard can be used during
the free water season.
o) It may, under certain conditions of river flow or level, prove necessary to carry out current-meter
measurements on sections other than the original chosen location. This is quite acceptable if there are no
substantial unmeasured losses or gains to the river in the intervening reach and so long as all flow
measurements can be related to any stage value recorded at the principal reference section.
5.2 Demarcation of site
5.2.1 If the site is to be established as a permanent station or likely to be used frequently for future
measurement, it should be provided with means for demarcation of the cross-section and for determination of
stage. Where the site is used only once, or infrequently, and there are no means of determining stage values
on site, care should be taken to ensure that the water level and/or flow do not change significantly during the
measurement period.
5.2.2 The position of each cross-section, normal to the mean direction of flow, shall be defined on the two
banks by clearly visible and readily identifiable markers. Where a site is subject to considerable snow cover,
the section line-markers may be referenced to other objects such as rock cairns.
5.2.3 The stage shall be read from a gauge at intervals throughout the period of measurement and the
gauge datum shall be related by precise levelling to a standard datum.
5.2.4 An auxiliary gauge on the opposite bank shall be installed where there is likelihood of a difference in
the level of water surface between the two banks. This is particularly important in the case of very wide rivers.
The mean of the measurements taken from the two gauges shall be used as the mean level of the water
surface and as a base for the cross-sectional profile of the stream.
6 Measurement of cross-sectional area
6.1 General
The cross-sectional profile of the open channel at the gauging-site shall be determined at a sufficient number
of points to establish the shape of the bed.
The location of each point is determined by measuring its horizontal distance to a fixed reference point on one
bank of the channel, in line with the cross-section. This in turn allows calculation of the area of individual
segments separated by successive verticals where velocities are measured.
6.2 Measurement of width
6.2.1 Measurement of the width of the channel and the width of the individual segments may be obtained by
measuring the horizontal distance from or to a fixed reference point which shall be in the same plane as the
cross-section at the measuring site.
6.2.2 Where the width of the channel permits, these horizontal distances shall be measured by direct
means, for example a graduated tape or suitable marked wire, care being taken to apply the necessary
corrections given in Annex A. The intervals between the verticals, i.e. the widths of the segments, shall be
similarly measured.
6.2.3 Where the channel is too wide for the above methods of measurement, and a boat is used, the
horizontal distance may be determined by optical or electronic distance-meters, by the use of a differential
Global Positioning System, or by one of the surveying methods given in Annex B.
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ISO 748:2007(E)
6.3 Measurement of depth
6.3.1 Measurement of depth shall be made at intervals close enough to define the cross-sectional profile
accurately. The number of points at which depth shall be measured should be the same as the number of
points at which velocity is measured (see 7.1.3).
6.3.2 The depth shall be measured by employing either sounding-rods or sounding-lines or other suitable
devices. Where the channel is of sufficient depth, an echo-sounder may be used. If the velocity is high and the
channel is sufficiently deep, it is preferable to use an echo-sounder or other device which will not require large
corrections. Difficulty may be experienced when attempting to measure depth at times of high velocity.
Annex C of this document offers alternative methods.
6.3.3 When a sounding-rod or sounding-line is used, it is desirable that at least two readings be taken at
each point and the mean value adopted for calculations, unless the difference between the two values is more
than 5 %, in which case two further readings shall be taken. If these are within 5 %, they shall be accepted for
the measurement and the two earlier readings discarded. If they are again different by more than 5 %, no
further readings shall be taken but the average of all four readings shall be adopted for the measurement,
noting that the accuracy of this measurement is reduced.
When an echo-sounder is used, the average of several readings shall always be taken at each point. Regular
calibrations of the instrument shall be carried out under the same conditions of salinity and temperature as
those of the water to be measured.
Where it is impracticable to take more than one reading of the depth, the uncertainty in measurement may be
increased (see Clause 9).
6.3.4 Where measurements of the depths are made separately from the velocity measurements and the
water level is not steady, the water level shall be observed at the time of each measurement of the depth.
When this is not possible, the water level shall be observed at sufficient intervals for the value of the level at
the time of each determination of depth to be obtained by interpolation.
6.3.5 When, during the determination of discharge, the bed profile changes appreciably, depth
measurements should be carried out by taking one depth reading at each point at the beginning and one at
the end of the velocity measurement at each vertical, and the mean value of these two measurements shall be
taken as the effective depth. Care should be exercised when taking repeated soundings to avoid disturbance
of the bed.
6.3.6 Inaccuracies in soundings are most likely to occur owing to:
a) the departure from the vertical of the sounding rod or line, particularly in deep water, when the velocity is
high;
b) the penetration of the bed by the sounding weight or rod;
c) the nature of the bed when an echo-sounder is used.
Errors due to a) may be minimized by the use, where practicable, of an echo-sounder, or pressure-measuring
device. The effect of drag on a sounding line may be reduced by using a streamlined lead weight at the end of
a fine wire. A correction shall be
...