ISO 4377:2012

INTERNATIONAL ISO
STANDARD 4377
Fourth edition
2012-08-01
Hydrometric determinations — Flow
measurement in open channels using
structures — Flat-V weirs
Déterminations hydrométriques — Mesure de débit dans les canaux
découverts au moyen de structures — Déversoirs en V ouvert

Reference number
©
ISO 2012
©  ISO 2012
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ii © ISO 2012 – All rights reserved

Contents Page
Foreword . v
1  Scope . 1
2  Normative references . 1
3  Terms and definitions . 2
4  Symbols . 3
5  Characteristics of flat-V weirs . 4
6  Installation . 4
6.1  Selection of site . 4
6.2  Installation conditions . 7
6.3  Weir structure . 8
6.4  Downstream conditions . 8
7  Maintenance . 8
8  Measurement of head(s) . 9
8.1  General . 9
8.2  Stilling (gauge) wells . 9
8.3  Zero setting . 11
8.4  Location of head measurement sections . 13
9  Discharge relationships . 14
9.1  Equations of discharge . 14
9.2  Effective heads . 15
9.3  Shape factors . 16
9.4  Coefficient of velocity . 16
9.5  Conditions for modular/drowned flow . 18
9.6  Drowned flow reduction factor . 21
9.7  Limits of application . 28
10  Computation of discharge . 29
10.1  General . 29
10.2  Successive approximation method . 29
10.3  Coefficient of velocity method . 31
10.4  Accuracy . 32
11  Uncertainties in flow determination . 32
11.1  General . 32
11.2  Combining uncertainties . 33
*
11.3  Uncertainty in the discharge coefficient u (C ) for the flat-V weir . 34
De 68
*
11.4  Uncertainty in the drowned flow reduction factor u (C ) . 34
dr
11.5  Uncertainty in the effective head . 35
11.6  Uncertainty budget . 35
11.7  Variation of uncertainty with flow and uncertainty in mean daily flow and the daily flow
volume . 36
12  Examples . 37
12.1  Example 1 — Computation of modular flow at low discharge. 37
12.2  Example 1 — Uncertainty in computed discharge . 39
12.3  Example 2 — Computation of drowned flow at high discharge . 41
12.4  Example 2 — Uncertainty in computed discharge . 43
Annex A (normative) Velocity distribution . 46
Annex B (informative) Introduction to measurement uncertainty .47
Annex C (informative) Performance guide for hydrometric equipment for use in technical
standards .56
Bibliography .59

iv © ISO 2012 – All rights reserved

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 4377 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 4377:2002), which has been technically revised
to update the treatment of uncertainty to be consistent with the other standards relating to flow measurement
structures.
INTERNATIONAL STANDARD ISO 4377:2012(E)

Hydrometric determinations — Flow measurement in open
channels using structures — Flat-V weirs
1 Scope
This International Standard describes the methods of measurement of flow in rivers and artificial channels
under steady or slowly varying conditions using flat-V weirs (see Figure 1).
Annex A gives guidance on acceptable velocity distribution.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. 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
ISO/TS 25377, Hydrometric uncertainty guidance (HUG)

1:5
> H > 2H
1max. 1max.
-
-
> H
1max.
-
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 772 apply.
Dimensions in millimetres
R > 2H
- 1max.
10 H' but not ≤ 3H
1max.
h
25 H' but not ≤ 3H
1max.
p
1:2
1 7
p
Key
1 head gauging section b crest width
2 upstream tapping H' difference between the invert (apex) of the V and the
top of the V
H
3 stilling wells maximum upstream total head above crest elevation
1max
4 crest tapping h gauged head above lowest crest elevation
5 flow p difference between mean bed level and lowest crest
elevation
6 downstream head measuring point
7 minimum 100 mm above stilling basin level
8 limits of permissible upstream and downstream
truncations
Figure 1 — Triangular profile flat-V weir

2 © ISO 2012 – All rights reserved

0,1 b
0,4 b
b
m
4 Symbols
The following is a list of symbols used, with the corresponding units of measurement.
a
Meaning Units
Symbol
A Area of cross-section of flow m
B Width of approach channel m
b Crest width m
C Coefficient of discharge Non-dimensional
D
C Effective coefficient of discharge Non-dimensional
De
C Drowned flow reduction factor Non-dimensional
dr
C Coefficient of approach velocity Non-dimensional

2
g Gravitational acceleration (standard value) ms
H Total head above lowest crest elevation m
H Total effective upstream head m
1e
H Total effective downstream head m
2e
H Maximum upstream total head above crest elevation m
1max
h Gauged head above lowest crest elevation m
h Upstream gauged head m
h Effective upstream gauged head m
1e
h Downstream gauged head m
h Effective downstream gauged head m
2e
h Separation pocket head m
p
h Effective separation pocket head relative to lowest crest elevation m
pe
h′, H′ Difference between lowest and highest crest elevations m
K ,K Constants Non-dimensional
1 2
k Head correction factor m
h
L Distance of upstream head measurement position from crest line m
m Crest cross-slope (1 vertical: m horizontal) Non-dimensional
n Number of measurements in a set Non-dimensional
p Difference between mean bed level and lowest crest elevation m
3 1
Q Discharge m s
3 1
Q Total daily flow volume m d
dfv
t Measurement observation frequency time minutes
v Mean velocity at cross-section m/s
 Mean velocity in approach channel m/s
a
u Absolute uncertainty in head measurement m
h
u(E) Absolute uncertainty in gauge zero m
*
u ( Percentage uncertainty in discharge coefficient Non-dimensional
CD)
*
u (C ) Percentage uncertainty in coefficient of velocity Non-dimensional

*
u (C ) Percentage uncertainty in drowned flow reduction factor Non-dimensional
dr
*
u (h) Percentage uncertainty in head measurement Non-dimensional

*
u (H ) Percentage uncertainty in total effective head Non-dimensional
e
*
U (Q) Percentage uncertainty in discharge determination Non-dimensional
*
U (Q ) Percentage uncertainty in the daily mean flow Non-dimensional
dmf
*
U (Q ) Percentage uncertainty in the total daily flow volume Non-dimensional
dfv
Z , Z Shape factors Non-dimensional
h H
α Coriolis energy coefficient Non-dimensional
Subscript
1 denotes upstream value
2 denotes downstream value
e denotes “effective” and implies that corrections for fluid effects
have been made to the quantity
a denotes approach channel
5 Characteristics of flat-V weirs
The standard flat-V weir is a control structure, the crest of which takes the form of a shallow V when viewed in
the direction of flow.
The s
...