ISO 4360:2008

INTERNATIONAL ISO
STANDARD 4360
Third edition
2008-03-01

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 4360:2008(E)
©
ISO 2008

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ISO 4360:2008(E)
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ISO 4360:2008(E)
Contents Page
Foreword. iv
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
7 Maintenance . 6
8 Measurement of head(s) . 6
8.1 General. 6
8.2 Location of head measurement(s) . 6
8.3 Gauge wells . 6
8.4 Zero setting . 9
9 Discharge characteristics . 9
9.1 Equations of discharge . 9
9.2 Coefficients . 10
9.3 Limitations. 10
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. 16
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
Bibliography . 28

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ISO 4360:2008(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 4360 was prepared by Technical Committee ISO/TC 113, Hydrometry, Subcommittee SC 2, Flow
measurement structures.
This third edition cancels and replaces the second edition (ISO 4360:1984), of which it constitutes a technical
revision.

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INTERNATIONAL STANDARD ISO 4360:2008(E)

Hydrometry — Open channel flow measurement using
triangular profile weirs
1 Scope
This International Standard 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 drowned flows which depend on downstream as
well as upstream levels.
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
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 772 apply.
4 Symbols

2
A m area of approach channel
B m width of approach channel
b m breadth of weir crest perpendicular to flow direction
C discharge coefficient
C coefficient of discharge

d
C coefficient of velocity

v
C f combined coefficient of velocity
v
E m elevation of ultrasonic air range sensor above hydraulic datum

f drowned flow reduction factor
2
g m /s acceleration due to gravity
H m total head relative to crest level

h m gauged head relative to crest level (upstream head is inferred if no subscript is used)

N number of measurements in a set
p m height of weir (difference between mean bed level and crest level)
3
Q m /s volumetric rate of flow
*
 percentage uncertainty in parameter
u()
v m/s mean velocity
U % expanded percentage uncertainty
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ISO 4360:2008(E)
Subscripts:
1 upstream
2 downstream
c combined
p measured crest tapping head above crest level
5 Principle
The discharge over a triangular profile weir is a function of the upstream head on the weir (for free flow),
upstream and downstream head (for drowned flow), experimentally determined coefficients, 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 stilling or float wells
which are generally necessary for flow measurement are given in the following sub-clauses.
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 increased 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 submerged 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;
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.
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ISO 4360:2008(E)
Several methods are available for obtaining a more precise indication of irregular velocity distribution: velocity
rods, floats or concentrations of dye can be used in small channels, the latter being useful in checking
conditions at the bottom of the channel. A complete and quantitative assessment of velocity distribution may
be made by means of a current-meter or other point velocity measurements. Information about the use of
[1]
current-meters is given in ISO 748 . Further information on measuring river velocities using acoustic Doppler
[5]
profilers can be found in ISO/TS 24154 .
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.
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.
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ISO 4360:2008(E)
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 1:2 slope
max
and 2 h for the 1:5 slope.
max
Figure 2 shows the general arrangement of the triangular profile weir.
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 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 channel shall be
straight for a sufficient length to ensure a regular 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 may be required to achieve a regular velocity distribution.
e) Baffles 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 may occur upstream of the gauging device, e.g. 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.
Ideally, high Froude numbers should be avoided for accurate flow measurement.
If a standing wave occurs within this distance, the approach conditions and/or the gauging device shall be
modified.
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ISO 4360:2008(E)

Key
1 upstream head measurement
2 crest tapping head measurement
3 gauge wells
4 crest tappings
5 limit of truncated sections
Figure 2 — General arrangements of the triangular profile weir
6.3.4 Downstream channel
The channel downstream from the structure is usually of no importance as such 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 shall be provided to measure tailwater levels to determine if and when drowned flow occurs.
In the event of the possibility of scouring downstream, which phenomenon may also lead to the instability of
the structure, particular measures to prevent this happening may be necessary.
A crest tapping and separate stilling well shall be fitted if the weir is designed to operate in a drowned
condition or if there is a possibility that the weir may drown in the future.
The latter circumstance may 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.
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ISO 4360:2008(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. The float well and the entry from the approach
channel shall also be kept clean and free from deposits. The downstream channel shall be kept free of
obstructions which might cause the weir to drown.
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 stilling wells shall be cleaned and checked for
leakage. The hook or point gauge, manometer, float or other instrument used to measure head shall be
checked periodically to ensure accuracy.
If a flow straightener is used in the approach channel, perforated plates shall be kept clean so that the
percentage open area remains greater than 40 %.
8 Measurement of head(s)
8.1 General
Where spot measurements are required, the heads can be measured by vertical gauges, hooks, points, wires
or tape gauges. Where continuous records are required, recording gauges shall be used.
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.
Piezometers or point-gauge stations for the measurement of head on the weir shall be located at a sufficient
distance upstream from the weir to avoid the region of surface drawdown. On the other hand, they shall 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. In this International Standard, it is recommended that the head-
measurement section shall be located at a distance equal to twice the maximum head (2h ) upstream of the
max
crest.
8.2.2 Drowned flow
A significant error in the calculated discharge will develop if the tailwater total head above crest level exceeds
75 %, unless a crest tapping is provided and two independent head measurements are made.
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 nearest side
wall or pier is greater than 1,0 m.
8.3 Gauge wells
It is usual to measure the upstream head in a gauge well to reduce the effects of water surface irregularities.
[2]
NOTE 1 Devices for the measurement of head are described in ISO 4373 .
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ISO 4360:2008(E)
Periodic checks on the measurement of the head in the approach channel shall be made.
Where the weir is designed to operate under drowned flow, a second measurement of head is required. For
accurate flow measurement, the head shall be measured within the separation pocket immediately
downstream of the crest. Alternatively, but with less precision, the head may be measured in the channel
downstream of the structure.
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 maximum water levels expected. Gauge
wells shall be connected to the appropriate head measurement positions by means of pipes.
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.
The pipe shall have its invert not less than 0,10 m below the lowest level to be gauged.
Pipe connections to the upstream and downstream head measurement positions shall terminate either flush
with, or at right angles to, the boundary of the approach and downstream channels. The channel boundary
shall be plain and smooth (equivalent to carefully finished concrete) within a distance 10 times the diameter of
the pipes from the centre line of the connection. The pipes may be oblique to the wall only if they are fitted
with a removable cap or plate, set flush with the wall, through which a number of holes are drilled. The edges
of these holes shall not be rounded or burred. Perforated cover plates are not recommended where weed or
silt are likely to be present.
The static head at the separation pocket behind the crest of the weir shall be transmitted to its gauge well as
follows.
a) An array of tapping holes shall be set into a plate covering a cavity in the crest of the weir block.
b) The underside of the plate shall be supported on a manifold into which the static head is communicated
via an array of feed tubes.
NOTE 2 No firm rule can be laid down for determining the size of the connecting pipe to the upstream well, because
this is dependent on a particular installation, e.g. whether the site is exposed and thus subject to waves, and whether a
larger diameter well is required to house the floats of recorders.
c) A horizontal conduit shall lead from the cavity through the weir block beneath the crest and terminating at
the gauge well.
d) A flexible transmission tube shall communicate static head within the manifold to the gauge well.
e) A watertight seal around the transmission tube shall prevent static head within the cavity from influencing
the static head transmitted from within the manifold.
NOTE 3 This may be at a different pressure because of leakage around the perimeter of the cover plate.
These arrangements minimize the occurrence of silting within the communication path between the separation
pocket and the gauge well and allow effective purging of the pipework by the occasional backflushing of the
system. For this purpose, a volume of water shall periodically be introduced to the gauge well.
Figure 3 shows the general arrangement for the crest tapping installation.
The crest tapping shall consist of five to ten holes of 10 mm diameter drilled in the weir block with centres
75 mm apart, 20 mm down from the weir crest on the 1:5 slope. The edges of the holes shall not be rounded
or burred. The number of holes shall be sufficient to ensure that the water level in the stilling well follows
variations in crest separation pocket pressure without significant delay.
Adequate additional depth shall be provided in wells to avoid the danger of floats (if used) grounding either on
the bottom or on any accumulation of silt or debris.
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ISO 4360:2008(E)
The gauge well arrangement may include an intermediate chamber of similar size and proportions to the
approach channel, to enable silt and other debris to settle out where it may be readily seen and removed.
The diameter of the connecting pipe or width of slot to the upstream well shall be sufficient to permit the water
level in the well to follow the rise and fall of head without appreciable delay. Care should be taken however not
to oversize the pipe, in order to ensure ease of maintenance and to damp out any oscillations due to short
period waves.

a)  Side view (in direction of flow) b)  Side view (normal to flow)

c)  View of tappings from underneath the weir
Key
1 crest tappings
2 feed tubes communicating crest head to the manifold (some shown as single lines only)
3 manifold [section in view b)]
4 cavity in the crest of the weir block
5 conduit leading to a gauge well
6 transmission tube (other end sealed within the conduit but communicating head in the manifold to the gauge well)
7 holes for screw-mounting the crest plate onto the weir block
Figure 3 — General arrangement for the crest tapping installation
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ISO 4360:2008(E)
8.4 Zero setting
Accurate initial setting of the zeros of the head measuring devices with reference to the level of the crest, and
subsequent regular checking of these settings, is essential.
An accurate means of checking the zero at frequent intervals shall be provided. Benchmarks, in the form of
horizontal metal plates, shall be set up on the top of the vertical side walls and in the gauge wells. These shall
be accurately levelled to ensure their elevation relative to crest level is known.
NOTE Instrument zeros can be checked relative to these benchmarks without the necessity of resurveying the crest
each time. Any settlement of the structure may, however, affect the relationships between crest and benchmark levels and
it is advisable to make occasional checks on these relationships.
A zero check based on the water level (either when the flow ceases or just begins) is susceptible to serious
errors due to surface tension effects and shall not be used.
The crest elevation shall be measured with respect to benchmarks at regular intervals across the breadth of
the weir with not less than ten measurements in total. The mean of these crest elevation measurements shall
be used to define the gauge zero.
9 Discharge characteristics
9.1 Equations of discharge
9.1.1 Modular (free) flow
In terms of total head, the basic discharge equation for a triangular p
...