ISO 4360:2020

INTERNATIONAL ISO
STANDARD 4360
Fourth edition
2020-06
Hydrometry — Open channel flow
measurement using triangular
profile weirs
Hydrométrie — Mesure de débit des liquides dans les canaux
découverts au moyen de déversoirs à profil triangulaire
Reference number
ISO 4360:2020(E)
©
ISO 2020

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ISO 4360:2020(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
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ISO 4360:2020(E)

Contents Page
Foreword .v
1 Scope . 1
2 Normative references . 1
3  Terms and definitions . 1
4 Symbols . 1
5 Principle . 2
6 Installation . 2
6.1 General . 2
6.2 Selection of site . 2
6.3 Installation conditions. 3
6.3.1 General. 3
6.3.2 Measuring structure . 4
6.3.3 Approach channel . 5
6.3.4 Downstream channel . 5
7 Maintenance . 6
8 Measurement of head(s) . 6
8.1 General . 6
8.2 Location of head measurement(s) . 6
8.2.1 Modular (free) flow . 6
8.2.2 Non-modular (drowned) flow . 6
8.3 Gauge wells . 7
8.4 Zero setting . 9
8.5 Dimensions .10
9 Discharge characteristics .10
9.1 Formulae of discharge .10
9.1.1 Modular (free) flow .10
9.1.2 Non-modular flow .11
9.2 Coefficients .11
9.2.1 Coefficient of discharge, C .11
d
9.2.2 Coefficient of velocity for modular flow, C .11
v
9.2.3 Non-modular flow reduction factor, f, with crest tappings .11
9.2.4 Non-modular flow reduction factor, f, with tailwater recorder .11
9.3 Limitations .12
10  Uncertainties of flow measurement .12
10.1 General .12
10.2 Combining measurement uncertainties .13
10.3 Uncertainty of discharge coefficient u(C ) for the triangular profile weir .14
d
10.4 Uncertainty budget.14
11 Example .15
11.1 General .15
11.2 Characteristics — Gauging structure .15
11.3 Characteristics — Gauged head instrumentation .15
11.4 Discharge coefficient .15
11.5 Discharge calculation .16
11.6 Uncertainty statement .16
Annex A (informative) Introduction to measurement uncertainty .18
Annex B (informative) Sample measurement performance for use in
hydrometric worked examples .26
Annex C (informative) Spreadsheet for use with this document .29
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ISO 4360:2020(E)

Bibliography .30
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ISO 4360:2020(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 on 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 the following
URL: www .iso .org/ iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 113, Hydrometry, Subcommittee SC 2,
Flow measurement structures.
This fourth edition cancels and replaces the third edition (ISO 4360:2008), which has been technically
revised.
The main changes compared to the previous edition are as follows.
— The calculations and examples have been updated to correct an error in the previous edition.
— A URN has been added containing a spreadsheet that has been developed to support the standard
and facilitate calculation of discharge and uncertainty (see Annex C).
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.
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INTERNATIONAL STANDARD ISO 4360:2020(E)
Hydrometry — Open channel flow measurement using
triangular profile weirs
1 Scope
This document specifies methods for the measurement of the flow of water in open channels under
steady flow conditions using triangular profile weirs. The flow conditions considered are steady flows
which are uniquely dependent on the upstream head and non-modular (drowned) flows which depend
on downstream as well as upstream levels.
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
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 http:// www .electropedia .org/
4 Symbols
Unit of
Symbol Quantity
measurement
α dimensionless Coriolis coefficient
2
A m area of approach channel
B m width of approach channel
b m breadth of weir crest perpendicular to flow direction
C dimensionless coefficient of discharge
d
C dimensionless coefficient of velocity
v
C f dimensionless combined coefficient of velocity for non-modular flow
v
f dimensionless non-modular (drowned) flow reduction factor
2
g m/s acceleration due to gravity
H m total head relative to crest level
gauged head relative to crest level (upstream head is inferred if no subscript
h m
is used)
N dimensionless number of measurements in a set
p m height of weir (difference between upstream mean bed level and crest level)
3
Q m /s volumetric rate of flow
as parameter standard uncertainty in parameter specified in parentheses
u ()
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ISO 4360:2020(E)

Unit of
Symbol Quantity
measurement
*
% percentage uncertainty in parameter specified in parentheses
u ()
v m/s mean velocity
U % expanded percentage uncertainty
Subscripts:
0 datum
1 upstream
2 downstream
c combined
p measured crest tapping head above crest level
max maximum
min minimum
5 Principle
The discharge over a triangular profile weir is a function of the upstream head on the weir (for modular
flow), upstream and downstream head (for non-modular flow), the geometrical properties of the weir
and approach channel and the dynamic properties of the water.
6 Installation
6.1 General
The required conditions regarding selection of site, installation conditions, the measuring structure,
the approach channel, the downstream channel, maintenance, measurement of head, and gauge wells
which are generally necessary for flow measurement are given in the following subclauses.
6.2 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 can be made to conform) to the requirements necessary for accurate measurement
by a weir.
Particular attention should be paid to the following features in selecting the site:
a) 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 any raised upstream water level due to the measuring structure;
e) conditions downstream including such influences as tides, confluences with other streams, sluice
gates, mill dams and other controlling features which might cause non-modular flow;
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;
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ISO 4360:2020(E)

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 clearance of rocks or boulders from the bed of the approach channel;
j) the effect of wind; wind can have a considerable effect on the flow in a river or over a weir, especially
when these are wide and the head is small and when the prevailing wind is in a transverse direction.
If the site does not possess the characteristics necessary for satisfactory measurement, the site shall be
rejected unless suitable improvements are practicable.
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 construction of a weir.
If the existing velocity distribution is irregular and no other site for a gauge is feasible, due consideration
shall be given to checking the distribution after the installation of the weir and to improving it if
necessary.
A complete and quantitative assessment of velocity distribution may be made by means of a current-
meter, other point velocity measurement technique or an acoustic Doppler profiler. Information about
[1] [2]
the use of current-meters is given in ISO 748 and information on Doppler profilers in ISO 24578 .
Figure 1 gives examples of satisfactory velocity distributions.
NOTE The contours refer to values of local flow velocity relative to the mean cross-sectional velocity.
Figure 1 — Examples of satisfactory velocity distributions
6.3 Installation conditions
6.3.1 General
The complete measuring installation consists of an approach channel, a measuring structure and a
downstream channel. The conditions of each of these three components affect the overall accuracy of
the measurements.
Installation requirements include features such as the surface finish of the weir, the cross-sectional
shape of the channel, the channel roughness and the influence of control devices upstream or
downstream of the gauging structure.
The distribution and direction of velocity have an important influence on the performance of the weir,
these factors being determined by the features mentioned above.
Once an installation has been installed, the user shall prevent any change which could affect the
discharge characteristics.
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ISO 4360:2020(E)

6.3.2 Measuring structure
The structure shall be rigid and watertight and capable of withstanding flood flow conditions without
distortion or fracture. It shall be at right angles to the direction of flow and shall conform to the
dimensions given in the relevant clauses.
The weir comprises an upstream slope of 1 (vertical) to 2 (horizontal) and a downstream slope of 1
(vertical) to 5 (horizontal). The intersection of these two surfaces forms a straight line crest, horizontal
and at right angles to the direction of flow in the approach channel. Particular attention shall be given
to the crest itself, which shall possess a well-defined corner of durable construction. The crest may be
made of pre-formed sections, carefully aligned and jointed, or may have a non-corrodible metal insert,
as an alternative to in situ construction throughout.
The dimensions of the weir and its abutments shall conform to the requirements indicated in Figure 2.
Weir blocks may be truncated but not so as to reduce their dimensions in plan to less than h for the
max
1:2 slope and 2 h for the 1:5 slope.
max
Figure 2 shows the general arrangement of the triangular profile weir.
Key
1 upstream head measurement
2 crest tapping head measurement
3 gauge wells
4 crest tappings
5 limit of truncated sections
6 downstream head measurement
7 direction of flow
Figure 2 — General arrangements of the triangular profile weir
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ISO 4360:2020(E)

6.3.3 Approach channel
On all installations, the flow in the approach channel shall be smooth, free from disturbance and shall
have a velocity distribution as satisfactory as possible over the cross-sectional area. This can usually
be verified by inspection or measurement. In the case of natural streams or rivers, this can only be
attained by having a long straight approach channel free from projections into the flow. Figure 1 gives
examples of satisfactory velocity distributions.
The following general requirements shall be complied with.
a) As the altered flow conditions due to the construction of the weir might cause a build-up of shoals
of debris upstream of the structure, which in time might affect the flow conditions, the likely
consequential changes in the water level shall be taken into account in the design of gauging
stations.
b) In an artificial channel, the cross-section shall be uniform and the approach channel shall be
straight for a length equal to at least 5 times its water-surface width.
c) In a natural stream or river, the cross-section shall be reasonably uniform and the approach
channel shall be straight for a sufficient length to ensure a satisfactory velocity distribution.
d) If the entry to the approach channel is through a bend, or if the flow is discharged into the channel
through a conduit or a channel of smaller cross-section, or at an angle, then a longer length of
straight approach channel is likely to be required to achieve a regular velocity distribution.
e) Flow conditioning devices such as baffles and flow straighteners shall not be installed closer to the
points of measurement than a distance 10 times the maximum head to be measured.
f) Under certain conditions, a standing wave can occur upstream of the gauging device, for example
if the approach channel is steep. Provided that this wave is at a distance of not less than 30 times
the maximum head upstream, flow measurement is feasible, subject to confirmation that a regular
velocity distribution exists at the gauging station and that the Froude number in this section is no
more than 0,6.
If a standing wave occurs within this distance, the approach conditions and/or the gauging device shall
be modified.
6.3.4  Downstream channel
The channel downstream from the structure is usually of no importance if the weir has been designed
so that the flow is modular (i.e. unaffected by tailwater level) under all operating conditions. A
downstream gauge may be provided to measure tailwater levels to determine if and when non-modular
flow occurs. The downstream gauge shall be installed sufficiently far downstream to avoid excessively
disturbed flow and be truly representative of downstream channel conditions. This shall be determined
on a site by site basis.
In the event of the possibility of scouring downstream, which phenomenon can also lead to the
instability of the structure, particular measures to prevent this happening should be adopted. The
design of such measures is outside the scope of this document.
A crest tapping and separate gauge well shall be fitted if the weir is designed to operate in a non-
modular condition or if there is a possibility that the weir could drown in the future.
The latter circumstance could arise if the altered flow conditions due to the construction of the weir
have the effect of building up shoals of debris immediately downstream of the structure or if river
works are carried out downstream at a later date.
Fish passage baffles can be installed on the downstream face of the weir to improve fish passage as set
[3]
out in ISO/TR 19234 .
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ISO 4360:2020(E)

7 Maintenance
Maintenance of the measuring structure and the approach channel is important to secure accurate
continuous measurements.
The approach channel shall be kept free of silt, vegetation and obstructions which might have
deleterious effects on flow conditions specified for the standard installation.
Where used, gauge wells and their entry from the channel shall also be kept clean and free from deposits.
The downstream channel shall be kept free of obstructions which could result in non-modular flow.
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.
Head-measurement piezometers, connecting conduits and gauge wells shall be cleaned and checked for
leakage. The device used to measure head shall be checked periodically to ensure accuracy.
If a flow straightener or similar is used in the approach channel, it shall be kept clean.
8 Measurement of head(s)
8.1 General
Where spot measurements are required, the heads can be measured by staff gauges, hooks, points,
wires or tape gauges.
Where continuous records are required, recording gauges shall be used.
[4]
Devices for the measurement of head are described in ISO 4373 .
Periodic checks on the measurement of the head in the approach channel shall be made.
The accuracy of the head measuring device shall be considered when considering the uncertainty of the
flow measurement (see Clause 10).
NOTE As the size of the weir and head reduces, small discrepancies in construction and in the zero setting
and reading of the head measuring device become of greater relative importance.
8.2 Location of head measurement(s)
8.2.1  Modular (free) flow
Flow is modular when it is independent of variations in tailwater level. This requirement is met when
the tailwater total head is equal to or less than 75 % of the upstream total head.
The location for the measurement of head on the weir should be at a sufficient distance upstream from
the weir to avoid the region of surface drawdown. On the other hand, it should be close enough to the
weir to ensure that the energy loss between the section of measurement and the control section on the
weir shall be negligible. Taking these considerations into account, the head-measurement section shall be
located at a distance between 2 and 4 times the maximum head (2 h to 4 h ) upstream of the crest.
max max
8.2.2  Non-modular (drowned) flow
A significant error in the calculated discharge will develop if the tailwater total head above crest
level exceeds 75 % of the upstream total head, unless a crest tapping or downstream (tailwater)
measurement is provided and two independent head measurements are made.
When a crest tapping is used, non-modular flow occurs when the head recorded by the crest tapping
exceeds 25 % of the upstream total head. Where the weir is designed to operate under non-modular
flow, a second measurement of head is required. For more accurate flow measurement, the head shall be
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ISO 4360:2020(E)

measured within the separation pocket immediately downstream of the crest. Alternatively, the head
can be measured in the channel downstream (tailwater) of the structure. However, the uncertainties in
the flow measurements made using tailwater data will generally be greater than those obtained from
a well-maintained crest tapping. The optimum position for the crest tapping is at the centre of the weir
crest. The tapping may be off-centre on weirs wider than 2,0 m provided that the distance from the
centreline of the crest tapping to the nearer side wall or pier is greater than 1,0 m.
8.3  Gauge wells
Where there are water surface irregularities, a gauge well (sometimes referred to as a stilling well)
may be used to improve the stability of the measurement and thus reduce the effect of short period
variations due to surface movements caused by wind or waves.
Gauge wells shall be vertical and of sufficient height and depth to cover the full range of water levels. In
field installations, they shall have a minimum height of 0,6 m above the highest water levels expected.
Gauge wells shall be connected to the appropriate head measurement positions by means of pipes, slots
or holes.
Both the well and the connecting pipe shall be watertight. Where the well is provided for the
accommodation of the float of a level recorder, it shall be of adequate size and depth. Care shall be taken
to ensure that there is sufficient cle
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