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 2008
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ii © ISO 2008 – All rights reserved

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

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
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established has the right to be represented on that committee. International organizations, governmental and
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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.
iv © ISO 2008 – All rights reserved

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
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
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)

Q m /s volumetric rate of flow
S — submergence ratio, h /h
2 1
S — modular limit
v m/s mean velocity in the approach channel

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;
2 © ISO 2008 – All rights reserved

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
...