ISO 3846:2008

INTERNATIONAL ISO
STANDARD 3846
Third edition
2008-02-15

Hydrometry — Open channel flow
measurement using rectangular
broad-crested weirs
Hydrométrie — Mesure de débit des liquides dans les canaux
découverts au moyen de déversoirs rectangulaires à seuil épais



Reference number
ISO 3846:2008(E)
©
ISO 2008

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ISO 3846:2008(E)
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ISO 3846:2008(E)
Contents Page
Foreword. iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions. 1
4 Symbols . 1
5 Installation . 2
5.1 General. 2
5.2 Selection of site . 2
5.3 Installation conditions. 3
6 General maintenance requirements. 4
7 Measurement of head(s) . 5
7.1 General. 5
7.2 Stilling or float well. 5
7.3 Zero setting . 5
8 Rectangular broad-crested weirs. 6
8.1 Specification for the standard weir. 6
8.2 Location of the head gauge section . 7
8.3 Provision for modular flow . 7
9 Discharge relationships . 7
9.1 Modular flow discharge equation. 7
9.2 Modular coefficient of discharge . 7
9.3 Limitations for operation in the modular flow range . 10
9.4 Drowned flow discharge equation . 10
9.5 Limitations for operation in the drowned flow range. 10
10 Uncertainties in flow measurement . 11
10.1 General. 11
10.2 Statement of the uncertainty of a flow measurement in a channel . 11
10.3 Combining measurement uncertainties . 12
10.4 Uncertainty of discharge coefficient u(C) for the broad-crested weir . 13
10.5 Uncertainty budget . 13
11 Example . 13
11.1 General. 13
11.2 Characteristics — Gauging structure. 14
11.3 Characteristics — Gauged head instrumentation . 14
11.4 The discharge coefficient . 14
11.5 The discharge estimate. 14
11.6 Uncertainty statement . 15
Annex A (informative) Modular limit. 17
Annex B (informative) Gauged head discharge coefficient and total head discharge coefficient . 18
Annex C (informative) Introduction to measurement uncertainty. 19
Annex D (informative) Sample measurement performance for use in hydrometric worked
examples. 26
Bibliography . 28

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ISO 3846:2008(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 3846 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 3846:1989), of which it constitutes a technical
revision.

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INTERNATIONAL STANDARD ISO 3846:2008(E)

Hydrometry — Open channel flow measurement using
rectangular broad-crested weirs
1 Scope
This International Standard lays down requirements for the use of rectangular broad-crested weirs for the
accurate measurement of flow of clear water in open channels under free flow conditions.
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, Hydrometry — 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 weir crest perpendicular to flow direction
C — discharge coefficient (gauged head)
f — drowned flow reduction factor
fC — combined coefficient of discharge
C — discharge coefficient (total head)

d
C — coefficient of velocity

v
E m encoder height from datum
e m random uncertainty in the width measurement

b

2
g m/s acceleration due to gravity
H m total head above crest level

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

L m length of weir in the direction of flow
n — number of measurements in a set
p m height of weir (difference between mean bed level and crest level)

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ISO 3846:2008(E)
3
Q m /s volumetric rate of flow
S — submergence ratio, h /h

2 1
S — modular limit

1
v m/s mean velocity in the approach channel

1
U % expanded percentage uncertainty

*
u (b) % percentage uncertainty in b

*
u (C) % percentage uncertainty in C

*
u (h ) % percentage uncertainty in h

1 1
*
u (Q) % percentage uncertainty in Q

Subscripts
1 upstream
2 downstream
c combined
E encoder
t distance between the (upstream) gauged head and the encoder
5 Installation
5.1 General
The conditions regarding the preliminary survey, selection of site, installation, approach channel, maintenance,
measurement of the head, and stilling or float wells, which are generally necessary for flow measurement, are
given in 5.2, 5.3, 6 and 7. The particular requirements for the rectangular broad-crested weir are given
separately in Clause 8.
5.2 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 flow measurement by the
weir.
Particular attention shall be paid to the following features in selecting the site for the weir:
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 (see 5.3.2);
d) the effects of any increased upstream water level due to the measuring structure;
e) the conditions downstream, including influences such as tides, confluences with other streams, sluice
gates, mill dams and other controlling features, which might cause drowning;
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;
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ISO 3846:2008(E)
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 effects of wind, which can have a considerable effect on the flow in a river, or over a weir, especially
when the river or weir is 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 measurements, 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 the 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.
Several methods are available for obtaining a more precise indication of irregular velocity distribution. These
include velocity rods, floats or concentrations of dye, which can be used in small channels; the last is useful to
check the conditions at the bottom of the channel. A complete and quantitative assessment of the velocity
distribution may be made by means of a current-meter or other point velocity instruments. More information
[1]
about the use of current-meters is given in ISO 748 . Further information on measuring river velocities using
[3]
acoustic Doppler profilers can be found in ISO/TS 24154 .
5.3 Installation conditions
5.3.1 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 from the
gauging structure.
The distribution and direction of velocity have an important influence on the performance of a weir, these
factors being determined by the features mentioned above.
Once a weir has been installed, the user shall prevent any changes which could affect the discharge
characteristics.
5.3.2 The approach channel
On all installations, the flow in the approach channel shall be smooth, free from disturbance and 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. The following general requirements shall be
complied with.
a) The altered flow conditions owing 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.
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ISO 3846:2008(E)
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.
5.3.3 The 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.
5.3.4 Downstream of the structure
If the downstream channel is rectangular and of the same width as the weir for a distance equal to twice the
maximum head downstream from the downstream face of the weir, then it is not necessary to ventilate the
nappe, particularly for high values of h /L.
1
The channel further downstream from the structure is usually of no importance as such, provided that the weir
has been designed to ensure that the flow is modular (i.e. unaffected by tailwater level) under all operating
conditions.
However, the water level may be raised sufficiently to drown the weir if the altered flow conditions due to the
construction of the weir cause the build-up of shoals of debris immediately downstream of the structure or if
river works are carried out at a later date.
Any accumulation of debris downstream of the structure shall therefore be removed.
6 General maintenance requirements
Maintenance of the measuring structure and the approach channel is important to secure continued accuracy
of the measurements.
In the event of the possibility of scouring downstream, which may lead to instability of the structure, particular
measures to prevent this happening may be required.
It is essential that the approach channel to weirs be kept clean and free from silt and vegetation as far as
practicable for at least the distance specified in 5.3.2. The float well and the entry from the approach channel
shall also be kept clean and free from deposits.
The weir 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.
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ISO 3846:2008(E)
7 Measurement of head(s)
7.1 General
The head upstream of the measuring structure may be measured by a hook gauge, point gauge or staff gauge
where spot measurements are required or by a recording gauge where a continuous record is required. In
many cases, it is preferable to measure heads in a separate stilling well to reduce the effects of surface
irregularities.
The discharges calculated using the working equation are volumetric figures, and the liquid density does not
affect the volumetric discharge for a given head provided that the operative head is gauged using a liquid of
identical density. If the gauging is carried out in a separate well, 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.
It shall, however, be ensured that the gauge is not located in a pocket or still pool, but that it measures the
piezometric head.
7.2 Stilling or float well
Where provided, the stilling well shall be vertical and, for field installations, shall extend at least 0,6 m above
the maximum estimated water level.
Stilling wells shall be connected to the channel by an inlet pipe, or slot, large enough to permit the water in the
well to follow the rise and fall of the head without significant delay. For field installations, the level of the inlet
pipe shall be at least 0,1 m below minimum water level.
The connecting pipe or slot shall, however, be as small as possible with regard to ease of maintenance.
Alternatively, the connecting pipe or slot shall be fitted with a constriction to damp out oscillations due to short
amplitude waves.
The well and the connecting pipe or slot shall be watertight. The well shall be of adequate diameter and depth
to accommodate the float of a level recorder, if used.
The well shall also be deep enough to accommodate any sediment which may enter, without the float
grounding. The float well arrangement may include an intermediate chamber between the stilling well and the
approach channel, of similar proportions to those of the stilling well to enable sediment to settle out. For ease
of maintenance, the pipework may be fitted with valves.
[2]
More detailed information on the stilling well may be obtained from ISO 1100-1 .
7.3 Zero setting
A means of checking the zero setting of the head measuring device shall be provided, consisting of a datum
related to the level of the weir crest.
A zero check based on the level of the water when the flow ceases is liable to incur serious errors from
surface tension effects and shall not be used.
With decreasing size of the weir and the head, small errors in construction and in the zero setting and reading
of the head measuring device become of greater importance.
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ISO 3846:2008(E)
8 Rectangular broad-crested weirs
8.1 Specification for the standard weir
The crest of the standard weir shall be a smooth, horizontal, rectangular plane surface (in these specifications
a “smooth” surface shall have a surface finish equivalent to that of rolled sheet metal). The width of the crest
perpendicular to the direction of flow shall be equal to the width of the channel in which the weir is located.
The upstream and downstream end faces of the weir shall be smooth, plane surfaces and they shall be
perpendicular to the sides and the bottom of the channel in which the weir is located. The upstream face, in
particular, shall form a sharp right-angle corner at its intersection with the plane of the crest.
If the upstream corner of the weir is slightly rounded, the discharge coefficient can increase significantly.
A typical sketch of the weir is shown in Figure 1.

Key
1 head gauging station
2 upstream stilling well
3 horizontal crest
4 downstream stilling well
5 3h to 4h
1,max 1,max
6 flow
7 10h
1,max
Figure 1 — Rectangular broad-crested weir
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ISO 3846:2008(E)
8.2 Location of the head gauge section
8.2.1 Upstream head measurement
Piezometers or a point-gauge station for the measurement of the head on the weir shall be located at a
sufficient distance upstream from the weir to avoid the region of surface drawdown. They (or it) shall, however,
be close enough to the weir for the energy loss between the section of the measurement and the control
section on the weir to be negligible. It is recommended that the head measurement section be located at a
distance equal to three to four times the maximum head (i.e. 3 h to 4 h ) upstream from the upstream
1,max 1,max
face of the weir.
8.2.2 Downstream head measurement
If the weir is to be operated in the drowned flow range, a measurement of downstream head is required. The
downstream head measurement position shall be 10h downstream from the face of the weir. At this
1,max
location, the turbulence associated with energy dissipation near to the weir has subsided to an acceptable
level. The downstream head measurement position should be located within the parallel sidewalls of the weir
structure.
8.3 Provision for modular flow
Flow over a rectangular broad-crested weir is not affected by tailwater levels if the crest level is chosen such
h
2
that the submergence ratio, S= , does not exceed the modular limit. The modular limit is given in Annex A.
h
1
9 Discharge relationships
9.1 Modular flow discharge equation
The equation of discharge is based on the use of a gauged head:
3/2
⎛⎞2
1/ 2 3 / 2
Qg= bCh (1)
⎜⎟ 1
3
⎝⎠
where
Q is the volumetric rate of flow;
g is the acceleration due to gravity;
b is the width of the weir perpendicular to the direction of flow;
C is the gauged head discharge coefficient;
h is the upstream gauged head related to the crest elevation.
1
9.2 Modular coefficient of discharge
The gauged head discharge coefficient, C, is given in Figure 2 and Table 1 as a function of h /L and h /p,
1 1
where L is the length of the weir in the direction of flow and p is the height of the weir with respect to the
bottom of the approach channel.
Intermediate values of C may be obtained by linear interpolation.
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ISO 3846:2008(E)

NOTE For the meaning of the dashed lines, see 9.3.
Key
X h /L
1
Y h /p
1
Figure 2 — Coefficient of discharge, C, in terms of h /p and h /L
1 1
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ISO 3846:2008(E)
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Table 1 — Gauged head discharge coefficients
h /p C for the following values of h /L
1 1
0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1 1,2 1,3 1,4 1,5 1,6 1,7 1,8
0,1 0,850 0,850 0,850 0,861 0,870 0,885 0,893 0,925 0,948 0,971 0,993 1,016 1,039 1,062 1,085 1,106 1,130 1,148
0,2 0,855 0,855 0,855 0,864 0,874 0,888 0,907 0,930 0,954 0,977 1,001 1,026 1,050 1,074 1,096 1,120 1,142 1,159
0,3 0,864 0,864 0,864 0,868 0,879 0,894 0,913 0,936 0,961 0,986 1,011 1,037 1,061 1,085 1,110 1,132 1,152 1,169
0,4 0,873 0,873 0,873 0,874 0,885 0,901 0,920 0,945 0,969 0,995 1,021 1,047 1,072 1,097 1,122 1,144 1,163 1,180
0,5 0,882 0,882 0,882 0,883 0,894 0,909 0,929 0,954 0,978 1,005 1,032 1,057 1,083 1,109 1,133 1,154 1,173 1,188
0,6 0,892 0,892 0,892 0,894 0,904 0,920 0,941 0,964 0,990 1,016 1,043 1,067 1,094 1,120 1,143 1,164 1,182 1,196
0,7 0,901 0,901 0,901 0,906 0,916 0,932 0,952 0,975 1,000 1,026 1,052 1,077 1,104 1,129 1,152 1,171 1,188 1,203
0,8 0,911 0,911 0,912 0,916 0,926 0,942 0,962 0,985 1,010 1,036 1,062 1,086 1,112 1,136 1,158 1,176 1,194 1,209
0,9 0,921 0,921 0,922 0,926 0,936 0,952 0,972 0,996 1,020 1,046 1,072 1,096 1,120 1,143 1,163 1,181 1,199 1,214
1,0 0,929 0,929 0,931 0,936 0,946 0,962 0,982 1,006 1,031 1,056 1,081 1,106 1,128 1,150 1,169 1,187 1,204 1,220
1,1 0,935 0,937 0,940 0,946 0,956 0,972 0,993 1,017 1,042 1,066 1,092 1,115 1,138 1,159 1,117 1,195 1,212 1,228
1,2 0,941 0,944 0,949 0,956 0,966 0,982 1,004 1,028 1,053 1,077 1,103 1,126 1,148 1,168 1,186 1,204 1,222 1,237
1,3 0,946 0,951 0,957 0,966 0,977 0,993 1,016 1,040 1,063 1,089 1,114 1,136 1,158 1,178 1,196 1,214 1,232 1,250
1,4 0,953 0,959 0,967 0,975 0,986 1,005 1,028 1,050 1,075 1,101 1,124 1,147 1,168 1,187 1,206 1,224 1,244 1,266
1,5 0,961 0,968 0,975 0,984 0,997 1,018 1,040 1,061 1,086 1,111 1,134 1,156 1,176 1,196 1,215 1,235 1,258 1,277
1,6 0,972 0,978 0,985 0,994 1,010 1,030 1,050 1,073 1,096 1,119 1,142 1,164 1,184 1,204 1,224 1,245 1,268 1,289
NOTE The recommended limits of application are those values which appear within the unshaded area.

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ISO 3846:2008(E)
The coefficient of discharge, C, has a constant value of 0,85 in the range 0,1 u h /L u 0,3 and for h /p < 0,15.
1 1
On the basis of the variation in C with h /L, a distinction can be made between the following types of flow (see
1
Figure 3):
a) broad-crested flow, 0,1 u h /L u 0,4: the flow across the weir is parallel to the crest for a certain portion;
1
b) short-crested flow, 0,4 u h /L u 1,6: the flow is totally curvilinear.
1

a)  Broad-crested weir 0,1 u h /L u 0,4 b)  Short-crested weir 0,4 u h /L u 1,6
1 1
Figure 3 — Flow patterns over rectangular broad- and short-crested weirs
9.3 Limitations for operation in the modular flow range
The following general limitations are recommended.
To avoid surface tension and viscous effects, h W 0,06 m, b W 0,30 m and p W 0,15 m.
1
There ar
...