ISO 4362:1999

INTERNATIONAL ISO
STANDARD 4362
Second edition
1999-09-01
Hydrometric determinations — Flow
measurement in open channels using
structures — Trapezoidal broad-crested weirs
Déterminations hydrométriques — Mesure de débit dans les canaux
découverts au moyen de structures — Déversoirs trapézoïdaux à seuil
épais
A
Reference number
Contents
1 Scope .1
2 Normative reference .1
3 Terms and definitions .1
4 Installation — General considerations .1
5 Maintenance .3
6 Measurement of water levels.3
7 Trapezoidal broad-crested weirs in rectangular channels .4
8 Trapezoidal broad-crested weirs in trapezoidal channels.15
9 Uncertainties in flow measurement .24
10 Example .27
Bibliography.31
©  ISO 1999
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 the publisher.
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Printed in Switzerland
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© ISO
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 3.
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.
International Standard ISO 4362 was prepared by Technical Committee ISO/TC 113, Hydrometric determinations,
Subcommittee SC 2, Notches, weirs and flumes.
This second edition cancels and replaces the first edition (ISO 4362:1992), which has been extended to include the
use of the weir in trapezoidal channels in addition to its use in rectangular channels.
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INTERNATIONAL STANDARD  © ISO ISO 4362:1999(E)
Hydrometric determinations — Flow measurement in open
channels using structures — Trapezoidal broad-crested weirs
1 Scope
This International Standard specifies a method of steady-flow measurement in open channels using a trapezoidal
broad-crested weir under modular and non-modular conditions. Consideration is given to the use of the weir in both
rectangular and trapezoidal channels.
Limitations to the use of the weir are given in 7.6 and 8.6.
2 Normative reference
The following normative document contains provisions which, through reference in this text, constitute provisions of
this International Standard. For dated references, subsequent amendments to, or revisions of, the document do not
apply. However, parties to agreements based on this International Standard are encouraged to investigate the
possibility of applying the most recent edition of the normative document indicated below. For undated references,
the latest edition of the normative document referred to applies. Members of IEC and ISO maintain registers of
currently valid International Standards.
ISO 772:1996, Hydrometric determinations — Vocabulary and symbols.
3 Terms and definitions
For the purposes of this International Standard, the terms and definitions given in ISO 772 apply.
4 Installation — General considerations
NOTE Particular requirements for trapezoidal broad-crested weirs are given in clause 7 for trapezoidal broad-crested weirs in
rectangular channels; in clause 8 for trapezoidal broad-crested weirs in trapezoidal channels.
4.1 Selection of site
A preliminary survey shall be made of the physical and hydraulic features of the proposed site, to check that it
conforms (or may be made to conform) with the requirements necessary for discharge measurement by a weir.
Particular attention shall be paid to the following features in selecting the site:
a) the availability of an adequate length of channel of regular cross-section;
b) the existing velocity distribution;
c) the avoidance of a steep channel, if possible;
d) the effects of an excessive increase in upstream water level owing to installation of the measuring structure;
e) the sediment content of the stream and whether heavy deposition just upstream of the weir is likely to occur;
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f) the impermeability of the ground on which the structure is to be founded and the necessity for piling, grouting or
other means of controlling seepage;
g) the necessity for flood banks to confine the maximum discharge to the channel;
h) the stability of the banks and the necessity for trimming and/or revetment in natural channels;
i) the need for clearance of rocks or boulders from the bed of the approach channel;
j) the effect of wind on the flow over the weir, especially when the weir is wide and the head is small and when
the prevailing wind is in a direction transverse to the direction of flow.
If the site does not possess the characteristics necessary for satisfactory measurements, it shall not be used unless
suitable improvements are practicable.
The existing velocity distribution in the approach channel shall be checked by inspection and measurement using,
for example, current-meters, velocity rods and floats.
NOTE 1 Concentrations of dye are also useful to check the conditions at the bottom of the channel.
NOTE 2 A complete and quantitative assessment of the velocity distribution may be made by using a current-meter. More
information on the use of current-meters is given in ISO 748.
If an inspection of the stream shows that the existing velocity distribution is regular, then it may be assumed that the
velocity distribution will remain satisfactory after the weir has been constructed.
If the existing velocity distribution is irregular and no other site for a weir is feasible, due consideration shall be given
to checking the distribution after the installation of the weir and to improving it if necessary.
4.2 Installation conditions
4.2.1 General
The complete measuring installation consists of an approach channel, a measuring structure and a downstream
channel. The condition of each of these components affects the overall accuracy of the measurements.
In addition, features such as the surface finish of the weir, the cross-sectional shape of the channel and its
roughness, and the influences of the control section and devices upstream or downstream of the gauging structure
shall be taken into consideration.
These features together determine the distribution and direction of velocity, which have an important influence on
the performance of a weir.
Once an installation has been designed and constructed, the user shall prevent or rectify any physical changes in
the installation which could affect the discharge characteristics.
4.2.2 Approach channel
The flow in the approach channel shall be smooth, free from disturbances and shall have a velocity distribution as
symmetrical as possible over the cross-sectional area.
NOTE This can usually be verified by inspection or measurement.
In the case of natural streams or rivers, these flow conditions can only be attained by having a long straight
approach channel of uniform cross-section, free from projections at the side or on the bottom.
Unless otherwise specified, the following general requirements shall be met.
a) After construction of the weir, the flow conditions in the approach channel can alter owing to the build-up of
shoals of debris upstream of the structure. The likely consequential changes in the water level shall be taken
into account in the design of the structure.
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b) In an artificial channel, the cross-section shall be uniform and the channel shall be straight for a length equal to
at least 10 times its width.
c) If the entry of the approach channel is through a bend or if the flow is discharged into the channel either
through a conduit of smaller cross-section or at an angle, then a greater length of straight approach channel is
required to achieve a regular velocity distribution. There shall be no baffle nearer to the points of measurement
than 10 times the maximum head to be measured.
d) Under certain conditions, a standing wave may occur upstream of the gauging device, for example if the
approach channel is steep. Provided that this wave is at a distance upstream of not less than 30 times the
maximum head, flow measurement is feasible, subject to confirmation that a regular velocity distribution exists
at the gauging station. If a standing wave occurs within this distance, the approach conditions and/or the
gauging device shall be modified.
4.2.3 Weir structure
The structure shall be rigid and watertight, and capable of withstanding flood-flow conditions without displacement,
distortion or fracture. It shall be at right angles to the direction of flow and the geometry of the weir shall conform
with the dimensions given in this International Standard.
4.2.4 Downstream channel
The channel downstream of the structure is usually of no importance if the weir has been designed to operate under
free-flow conditions. If the weir is designed to operate under drowned conditions also, the downstream channel shall
be straight for a length of at least eight times the maximum head to be measured.
A downstream gauge shall be provided to determine the submergence ratio.
5 Maintenance
Maintenance of the measuring structure and the approach channel is important to secure accurate continuous
measurements.
It is essential that, as far as practicable, the approach channel to the weir be kept clean and free from silt and
vegetation for the minimum distance specified in 4.2.2. The float well and the entry from the approach channel shall
also be kept clean and free from deposits. The weir structure shall be kept clean and free from clinging debris and
care shall be taken in the process of cleaning to avoid damage to the weir crest.
6 Measurement of water levels
6.1 General
Where spot measurements are required, water levels (heads) upstream and downstream of the measuring structure
may be measured by using a hook gauge, a point gauge or a staff gauge. Where continuous records are required, a
float-operated recording gauge may be used; however, to reduce the effects of water surface irregularities, it is
preferable to measure water levels in a separate stilling well. Other head-measuring methods may be used provided
that sufficient accuracy is obtainable.
The discharges calculated using the working equations given in this International Standard are volumetric figures.
The liquid density does not affect the volumetric discharge for a given water level provided that the operative level is
gauged in liquid of identical density. If the gauging is carried out in a separate well, a correction for the difference in
density may be necessary if the temperature of the liquid in the well is significantly different from that of the flowing
liquid. However, it is assumed herein that the densities are equal.
6.2 Stilling or float well
Where provided, the stilling well shall be vertical and shall be 0,6 m higher than the maximum water level to be
recorded in the well. The bottom of the well shall be lower than the elevation of the weir crest.
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The well shall be connected to the channel by an inlet pipe or slot, which is large enough to permit the water in the
well to follow the rise and fall of head without significant delay.
The connecting pipe or slot shall, however, be as small as possible consistent with ease of maintenance, or shall
alternatively be fitted with a constriction to damp out oscillations due to short-period waves.
The well and the connecting pipe or slot shall be watertight. Where provided for the accommodation of the float of a
level recorder, the well shall be of adequate diameter and depth to give clearance around and beneath the float at
all stages. Adequate additional depth shall be provided in wells to avoid the danger of floats grounding on any
accumulation of silt or debris. The float well arrangement may include an intermediate chamber of similar size and
proportions between the stilling well and the approach channel to enable silt and other debris to settle out where
they may be readily removed.
6.3 Zero setting
An accurate means of checking the zero setting of the water-level measuring device shall be provided. For this
purpose, a pointer, set exactly level with the crest of the weir and fixed permanently in the approach channel, or
alternatively in the stilling well or float well where provided, may be used.
A zero check based on the level of the water when the flow either ceases or just begins is liable to serious errors
due to surface tension
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