ISO 9825:2005

INTERNATIONAL ISO
STANDARD 9825
Second edition
2005-05-15

Hydrometry — Field measurement of
discharge in large rivers and rivers in
flood
Hydrométrie — Mesurage in situ du débit des grandes rivières et des
débits de crue




Reference number
ISO 9825:2005(E)
©
ISO 2005

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ISO 9825:2005(E)
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ISO 9825:2005(E)
Contents Page
Foreword. iv
1 Scope. 1
2 Normative references . 1
3 Terms and definitions. 2
4 Units of measurement . 2
5 Appropriate techniques. 2
6 Nature of difficulties likely to be encountered. 2
6.1 Measured parameters . 2
6.2 Logistical problems . 3
7 Measurement of discharge in large rivers. 3
7.1 Problems of scale . 3
7.2 Current-meter method . 3
7.3 Moving-boat technique. 4
7.4 Acoustic Doppler method . 4
7.5 Other methods. 4
8 Measurement of flood flows up to bankfull stage . 5
8.1 Problems of flood-flow measurement. 5
8.2 Use of floats. 5
8.3 Moving-boat technique. 6
8.4 Acoustic Doppler method . 6
8.5 Radar . 6
8.6 Weirs. 6
8.7 Tracer dilution methods . 6
8.8 Indirect methods . 7
9 Measurement of flood flows above bankfull stage. 7
Bibliography . 9

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ISO 9825:2005(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 9825 was prepared by Technical Committee ISO/TC 113, Hydrometry.
This second edition cancels and replaces the first edition (ISO 9825:1994), which has been technically revised.
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INTERNATIONAL STANDARD ISO 9825:2005(E)

Hydrometry — Field measurement of discharge in large rivers
and rivers in flood
1 Scope
This International Standard deals specifically with the measurement of discharge in large rivers and the
measurement of rivers in flood. It also describes the relevant field measurements when it becomes necessary
to use indirect methods of estimating discharge.
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 748, Measurement of liquid flow in open channels — Velocity-area methods
ISO 772, Hydrometric determinations — Vocabulary and symbols
ISO 772, Amendment 1:2002
ISO 1070, Liquid flow measurement in open channels — Slope-area method
ISO 1438-1, Water flow measurement in open channels using weirs and Venturi flumes — Part 1: Thin-plate
weirs
ISO 3846, Liquid flow measurement in open channels by weirs and flumes — Rectangular broad-crested
weirs
ISO 3847, Liquid flow measurement in open channels by weirs and flumes — End-depth method for
estimation of flow in rectangular channels with a free overfall
ISO 4359, Liquid flow measurement in open channels — Rectangular, trapezoidal and U-shaped flumes
ISO 4360, Liquid flow measurement in open channels by weirs and flumes — Triangular profile weirs
ISO 4369, Measurement of liquid flow in open channels — Moving-boat method
ISO 4371, Measurement of liquid flow in open channels by weirs and flumes — End depth method for
estimation of flow in non-rectangular channels with a free overfall (approximate method)
ISO 4374, Liquid flow measurement in open channels — Round-nose horizontal broad-crested weirs
ISO 4377, Hydrometric determinations — Flow measurement in open channels using structures — Flat-V
weirs
ISO 6416, Hydrometry — Measurement of discharge by the ultrasonic (acoustic) method
ISO 6420, Liquid flow measurement in open channels — Position fixing equipment for hydrometric boats
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ISO 9825:2005(E)
ISO 8333, Liquid flow measurement in open channels by weirs and flumes — V-shaped broad-crested weirs
ISO 9213, Measurement of total discharge in open channels — Electromagnetic method using a full-channel-
width coil
ISO 9555-1, Measurement of liquid flow in open channels — Tracer dilution methods for the measurement of
steady flow — Part 1: General
ISO 9555-2, Measurement of liquid flow in open channels — Tracer dilution methods for the measurement of
steady flow — Part 2: Radioactive tracers
ISO 9555-3, Measurement of liquid flow in open channels — Tracer dilution methods for the measurement of
steady flow — Part 3: Chemical tracers
ISO 9555-4, Measurement of liquid flow in open channels — Tracer dilution methods for the measurement of
steady flow — Part 4: Fluorescent tracer
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 772 and Amendment 1 apply.
4 Units of measurement
The units of measurement used in this International Standard are SI units.
5 Appropriate techniques
Due to the dimensions of large rivers and the hazards associated with measuring flood flows, some of the
techniques available for discharge measurement on smaller rivers under normal flow conditions may not be
appropriate, or may need modification if used.
River dimensions, stream conditions, feasibility of measurements, measuring instruments and equipment,
purpose and available funds will, in a general sense, dictate the choice of methodology. In many instances,
the choice of technique will be decided upon by the physical conditions at the site. Hazards discussed in this
International Standard are confined to those peculiar to the measurement of discharge of large rivers and
rivers in flood.
Those techniques that may be partially or entirely appropriate within certain limitations imposed by degree of
difficulty of operation are the following:
a) velocity-area methods in accordance with ISO 748, ISO 4369, ISO 6416, ISO 6420, ISO 9213.
b) Tracer dilution methods in accordance with ISO 9555, Parts 1 to 4.
c) Weirs and flumes in accordance with ISO 1438-1, ISO 3846, ISO 3847, ISO 4359, ISO 4360, ISO 4371,
ISO 4374, ISO 4377, ISO 8333.
d) Indirect methods in accordance with ISO 1070.
6 Nature of difficulties likely to be encountered
6.1 Measured parameters
When any of the three parameters used to determine discharge (width, depth and velocity) is abnormally large,
it may cause problems that are not usually encountered.
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ISO 9825:2005(E)
Great width may pose problems for position fixing in the horizontal, and for measurement of velocity on any
overbank spill portion.
Great depth may create difficulty in locating a measuring device at the desired depth.
High velocities cause problems with regard to maintaining station, position fixing and location of
measurements in both the horizontal and vertical. The current meter is required to be calibrated to cover a
high range of velocities, for which facilities are generally not readily available.
These difficulties are accentuated by problems caused by floating debris, high turbulence and vortices, and
movement of large bed forms.
6.2 Logistical problems
The basic problems in the measurement of discharge of large rivers and rivers in flood are logistic, associated
either with the time required for measurement or with the need for special resources to be employed. However,
flood measurements may be accompanied by significant additional hazards to personnel and equipment.
An additional logistical problem is access to the flood measurement site, both in being able to reach the site
and getting to the site in time to measure a high flow. Because of this, indirect measurements assume great
importance in the case of rivers in flood.
7 Measurement of discharge in large rivers
7.1 Problems of scale
The problems relating to measurement of discharge in large rivers are essentially those of the following scales.
7.1.1 Great width
For most methods, great width presents difficulties in ascertaining the location of the measurement with
respect to the cross-section, since the orthodox means using tag-lines and optical survey instruments,
depending upon the actual dimensions, may preclude accurate results. More sophisticated position-fixing
equipment may be needed to overcome this problem, in accordance with ISO 6420.
7.1.2 Great depth
Great depth may call for a greater number of points on the vertical to be measured to sample mean velocity,
particularly if the vertical velocity distribution is not uniform due to the section being non-uniform.
7.1.3 High velocity
In many cases, high velocity creates difficulties, both with regard to locating a measuring platform at the
desired position on the horizontal and in maintaining that position, as well as enabling the desired point on the
vertical to be sampled. The turbulence, which usually accompanies high velocity, both compounds the
problems of sampling and reduces the accuracy of the measurement. In addition, high velocities require the
use of heavy sounding weights, which are difficult to use and increase the risk of injury to field personnel.
When using current meters, wet-line distortions lead to time-consuming corrections, and even then the
measurement point may well deviate from the cross-sectional line with which it is meant to conform.
7.2 Current-meter method
The standard method for measuring the discharge of large rivers is the velocity-area method using a current
meter, from a bridge, cableway, or powerboat or motor launch fitted with an echo-sounder. During high
velocity, fixing of position and other parameters may be made by cableway with a trolley for rivers up to 500 m
wide. From river widths of 500 m up to 1 km, a boat may be moved across the river with the help of a
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ISO 9825:2005(E)
cableway. For river widths greater than 1 km, the use of cableways is excluded. Stationary boats anchored
with great care and using conventional survey methods can be located for rivers up to 2 km wide, but for
greater widths more sophisticated position-fixing equipment may become necessary.
The care required in such operations, coupled with the need for an adequate number of verticals to be
measured, inevitably results in a single measurement becoming a very lengthy procedure. This can be
partially reduced by the use of several teams with several boats and current meters. Similarly, advance notice
of likely river-stage behaviour enables measurements to be planned to take advantage of stable stage
conditions, to counter the problem of lengthy measuring periods. Significant river traffic may cause further
delay.
7.3 Moving-boat technique
The moving-boat technique is a modification of the current-meter method
...