ISO 2425:2010

INTERNATIONAL ISO
STANDARD 2425
Third edition
2010-12-01
Hydrometry — Measurement of liquid
flow in open channels under tidal
conditions
Hydrométrie — Mesurage du débit des liquides dans les canaux
découverts dans des conditions de marée

Reference number
©
ISO 2010
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but
shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In
downloading this file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat
accepts no liability in this area.
Adobe is a trademark of Adobe Systems Incorporated.
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation
parameters were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In
the unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below.

©  ISO 2010
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or
ISO's member body in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.org
Web www.iso.org
Published in Switzerland
ii © ISO 2010 – All rights reserved

Contents Page
Foreword .iv
1 Scope.1
2 Normative references.1
3 Terms and definitions .1
4 Abbreviated terms.2
5 Principles of methods of measurement.2
5.1 General .2
5.2 Single measurement methods .2
5.3 Continuous measurement methods.2
6 Special considerations and choice of method.3
6.1 Special considerations .3
6.2 Choice of method .4
7 Measurement of tidal flow .6
7.1 Techniques for single measurements of tidal flow.6
7.2 Techniques appropriate for continuous measurement of tidal flow.10
8 Uncertainties in tidal flow measurement .12
8.1 General .12
8.2 Uncertainties in measurement by velocity area method.12
Annex A (informative) Measurement of tidal flow by cubature method.16
Annex B (informative) Measurement methods suitable for tidal flow conditions.20
Annex C (informative) Record of velocity measurement of a tidal river (see 7.1).22
Annex D (informative) Measurement of tidal flow using an acoustic Doppler velocity meter
(see 7.1) .24
Bibliography.27

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 2425 was prepared by Technical Committee ISO/TC 113, Hydrometry, Subcommittee SC 1, Velocity area
methods.
This third edition cancels and replaces the second edition (ISO 2425:1999), which has been technically
revised. It also incorporates the Amendment ISO 2425:1999/Amd.1:2003. Annex D on measurement of tidal
flow using an acoustic Doppler velocity meter has been added.

iv © ISO 2010 – All rights reserved

INTERNATIONAL STANDARD ISO 2425:2010(E)

Hydrometry — Measurement of liquid flow in open channels
under tidal conditions
1 Scope
This International Standard provides a summary of recommended methods for the determination of liquid flow
in tidal channels, special consideration being given to those techniques that are either unique to or particularly
appropriate for application under tidal conditions, including treatment of uncertainties.
Reference is also made, where appropriate, to methods for the determination of flow in non-tidal channels, but
attention is drawn to their limitations with respect to practicality and/or uncertainty.
This International Standard does not describe alternative methods, such as the use of weirs, flumes, dilution
gauging, salt velocity and floats, although they might be suitable under certain conditions, especially where
the effect of tides only impedes and does not stop or reverse the passage of stream flow. These methods are
described in detail in other International Standards.
This International Standard specifies two types of technique:
a) techniques for single measurements of tidal flow;
b) techniques for continuous measurement of tidal flow.
Annex A specifies the cubature method of measurement. Annex B specifies methods for the determination of
flow under tidal conditions, and Annex C gives an example of the computation for a single vertical. Similar
computations are possible for other verticals. Annex D describes the determination of tidal flow using an
acoustic Doppler velocity meter.
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 748:2007, Hydrometry — Measurement of liquid flow in open channels using current-meters or floats
ISO 772, Hydrometry — Vocabulary and symbols
ISO 1100-1, Measurement of liquid flow in open channels — Part 1: Establishment and operation of a gauging
station
ISO 6416, Hydrometry — Measurement of discharge by the ultrasonic (acoustic) method
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 772 apply.
4 Abbreviated terms
ADCP acoustic Doppler current profiler
ADP acoustic Doppler profiler
ADV acoustic Doppler velocimeter
ADVM acoustic Doppler velocity meter
5 Principles of methods of measurement
5.1 General
Tidal flow measurement can be an instantaneous rate of flow or a total volume of flow during a flood or ebb
tide. The unsteady nature and change in direction of flow under tidal conditions create problems of
measurement additional to those associated with the measurement of the discharge of unidirectional streams.
The methods specified in ISO 748, ISO 1100-1, ISO 4369, ISO 9123, ISO/TR 9823 and ISO 9825 cannot
therefore always be applied to tidal channels. Any change in water quality brought about by tidal conditions
can affect the methods specified in ISO 6416 and ISO 9213.
For various reasons, direct measurements of velocity in tidal channels are more liable to greater uncertainty
than those made under conditions of unidirectional flow.
The methods of measurement in this International Standard can be grouped into either single or continuous
measurements.
5.2 Single measurement methods
5.2.1 Velocity area method
At a chosen gauging site, the velocity of flow and the area of cross-section of the channel are measured. The
product of these measurements at any instant is the rate of flow or discharge past the gauging site at that
instant. It is referred to as the velocity area method and includes the following techniques.
a) current meter from a fixed station;
b) acoustic Doppler profiler or acoustic Doppler velocity meter from a fixed station;
c) current meter from a moving station (moving boat);
d) acoustic Doppler current profiler from a moving station (moving boat).
5.2.2 Cubature method
In an area that includes a stretch of river channel and its flood plain, surface areas and rise in water level of
stored water are measured at known time intervals. Volumes of stored water are computed, and the flow into
the upstream stretch of river is estimated, from which the average rate of flow is determined (see Annex A).
5.3 Continuous measurement methods
5.3.1 Ultrasonic method (ISO 6416)
Transducers are positioned on each bank of the river channel, such that the acoustic path is at an oblique
angle to the direction of flow. The time taken for a pulse of sound to travel in both directions is measured and
2 © ISO 2010 – All rights reserved

compared. From these two times, the velocity of the water can be computed. Knowledge of the cross-
sectional area allows computation of discharge.
5.3.2 Electromagnetic method (ISO 9213)
A horizontal coil is constructed above or below a river channel. A magnetic field is generated by an alternating
current and voltages are induced in the flowing water, which acts as an electrical conductor. After calibration,
measurements of electrical parameters and water depth provide a means of determining the discharge.
5.3.3 Acoustic Doppler velocity method from a fixed station
Acoustic Doppler velocity meters (ADVMs) may be horizontally or vertically oriented and shall be fixed to a
bridge pier or abutment, or other stable mountable structure for horizontal mountings, or to the channel bed for
vertical mountings. The ADVMs measure an index velocity that is related to the measured average velocity of
the channel (mean velocity) determined from current meter measurements and channel cross-sectional area.
A separate water level-to-area relation is developed from regularly measured cross-sectional geometry at or
near the location of the ADVM. Discharge is computed as a product of the mean velocity and cross-sectional
area. The acoustic Doppler velocity method can be implemented using the following techniques:
a) horizontal measurement from a fixed station or stations;
b) vertical measurement from a fixed station or stations;
c) a combination of the horizontal and vertical methods at a fixed station.
5.3.4 Unsteady flow models
Unsteady flow models may be used for computing continuous records of discharge in open channels in both
tidal and non-tidal conditions. These models, however, are not applicable where a longitudinal density
gradient, such as a salt-water wedge, is present.
Unsteady flow models are based on the numerical solution of non-linear partial differential equations that
describe gradually varied unsteady flow in open channels. The available models employ one or more of
several numerical computation techniques. Data requirements, which can be substantial, depend on the
numerical techniques employed by the model selected. It is necessary that techniques for the application of
unsteady flow models and the data requirements be clearly defined and understood for successfully
computing discharges.
6 Special considerations and choice of method
6.1 Special considerations
Changes in water level at the mouth of a river due to tidal action cause backwater effects in the channel.
These changes can alter water level and flow magnitude only, or water level, flow magnitude, and direction of
flow. The entire flow might be reversed in direction, or only some of the flow might be reversed due to
variations in the density gradient.
Most flow-gauging techniques are generally best suited to conditions closely approximati
...