oSIST prEN ISO 24577:2026

Hydrometry - Use of noncontact methods for measuring water surface velocity and determining discharge (ISO/DIS 24577:2026)

Scope of the proposed deliverable
To determine liquid flow, the following steps are necessary:
1) Measure water surface (or near surface) velocity with techniques using radar, laser or video images;
2) Adjust wind effects to the water surface velocity;
3) Translate the adjusted velocity to an averaged velocity by applying the velocity index or numerical computation;
4) Determine the area of the wetted cross section from the stage area relationship; and
5) Obtain water discharge by multiplying the averaged velocity by the wetted cross sectional area.
This procedure is applicable to different kinds of channel and river section.
Applications include:
•Rivers and streams;
•Artificial channels such as drainage ditches and irrigation channels;
•Wastewater flows discharging to sewer or the environment through channels or partially filled pipes;
•In sewer measurements;
•Process flows on wastewater treatment plants.
For any individual site the method to measure water surface velocity should be selected appropriately, based on the site conditions, nature of the application and uncertainty required. Take a special note
that non-contact methods should NOT be used where a tidal phenomenon is present.

Hydrometrie - Einsatz berührungsloser Verfahren zur Messung der Wasseroberflächengeschwindigkeit und zur Bestimmung der Abflussmenge (ISO/DIS 24577:2026)

Hydrométrie - Emploi de méthodes sans contact pour mesurer la vitesse de surface de l'eau et pour déterminer le débit (ISO/DIS 24577:2026)

Hidrometrija - Uporaba brezdotičnih metod za merjenje hitrosti toka na vodni površini in ugotavljanje pretoka (ISO/DIS 24577:2026)

Obseg predlaganega dokumenta
Za določitev pretoka tekočine so potrebni naslednji koraki:
1) Merjenje hitrosti vodne površine (ali hitrosti blizu površine) s tehnikami, ki uporabljajo radar, laser ali video slike;
2) Prilagoditev vplivov vetra na hitrost vodne površine;
3) Pretvorba prilagojene hitrosti v povprečno hitrost z uporabo indeksa hitrosti ali numeričnega izračuna;
4) Določitev površine mokrega prečnega prereza iz razmerja med višino in površino; in
5) Pridobitev vodnega pretoka z množenjem povprečne hitrosti s površino mokrega prečnega prereza.
Ta postopek je uporaben za različne vrste kanalov in rečnih odsekov.
Uporabe vključujejo:
- Reke in potoke;
- Umetne kanale, kot so odvodni jarki in namakalni kanali;
- Odtoki odpadne vode, ki se izlivajo v kanalizacijo ali okolje skozi kanale ali delno napolnjene cevi;
- Meritve v kanalizaciji;
- Procesni tokovi na čistilnih napravah za odpadne vode.
Za vsako posamezno lokacijo je treba metodo za merjenje hitrosti vodne površine izbrati ustrezno glede na pogoje na lokaciji, naravo uporabe in zahtevano negotovost. Posebno pozornost je treba nameniti dejstvu, da se metode brez stika NE smejo uporabljati tam, kjer je prisoten plimni pojav.

General Information

• Status: Not Published
• Public Enquiry End Date: 14-Jul-2026
• ICS: 07.060 - Geology. Meteorology. Hydrology
• ICS: 17.120.20 - Flow in open channels
• Technical Committee: IOVO - Water supply and waste water engineering
• Current Stage: 4020 - Public enquire (PE) (Adopted Project)
• Start Date: 21-May-2026
• Due Date: 08-Oct-2026
• Ref Project: prEN ISO 24577 - Hydrometry – Use of noncontact methods for measuring water surface velocity and determining discharge (ISO/DIS 24577:2026)

Standards Content (Sample)

SLOVENSKI STANDARD
01-julij-2026
Hidrometrija - Uporaba brezdotičnih metod za merjenje hitrosti toka na vodni
površini in ugotavljanje pretoka (ISO/DIS 24577:2026)
Hydrometry - Use of noncontact methods for measuring water surface velocity and
determining discharge (ISO/DIS 24577:2026)
Hydrometrie - Einsatz berührungsloser Verfahren zur Messung der
Wasseroberflächengeschwindigkeit und zur Bestimmung der Abflussmenge (ISO/DIS
24577:2026)
Hydrométrie - Emploi de méthodes sans contact pour mesurer la vitesse de surface de
l'eau et pour déterminer le débit (ISO/DIS 24577:2026)
Ta slovenski standard je istoveten z: prEN ISO 24577
ICS:
07.060 Geologija. Meteorologija. Geology. Meteorology.
Hidrologija Hydrology
17.120.20 Pretok v odprtih kanalih Flow in open channels
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

DRAFT
International
Standard
ISO/DIS 24577
ISO/TC 113/SC 1
Hydrometry — Use of noncontact
Secretariat: BIS
methods for measuring water
Voting begins on:
surface velocity and determining
2026-04-30
discharge
Voting terminates on:
ICS: 17.120.20; 07.060
2026-07-23
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENTS AND APPROVAL. IT
IS THEREFORE SUBJECT TO CHANGE
AND MAY NOT BE REFERRED TO AS AN
INTERNATIONAL STANDARD UNTIL
PUBLISHED AS SUCH.
This document is circulated as received from the committee secretariat.
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Reference number
ISO/DIS 24577:2026(en)
DRAFT
ISO/DIS 24577:2026(en)
International
Standard
ISO/DIS 24577
ISO/TC 113/SC 1
Hydrometry — Use of noncontact
Secretariat: BIS
methods for measuring water
Voting begins on:
surface velocity and determining
discharge
Voting terminates on:
ICS: 17.120.20; 07.060
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENTS AND APPROVAL. IT
IS THEREFORE SUBJECT TO CHANGE
AND MAY NOT BE REFERRED TO AS AN
INTERNATIONAL STANDARD UNTIL
PUBLISHED AS SUCH.
This document is circulated as received from the committee secretariat.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL,
© ISO 2026
TECHNOLOGICAL, COMMERCIAL AND
USER PURPOSES, DRAFT INTERNATIONAL
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
STANDARDS MAY ON OCCASION HAVE TO
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BE CONSIDERED IN THE LIGHT OF THEIR
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Published in Switzerland Reference number
ISO/DIS 24577:2026(en)
ii
ISO/DIS 24577:2026(en)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
4 Non-contact measuring methods . 2
4.1 General .2
4.1.1 Advantages and benefits .3
4.1.2 Limitations .3
4.2 Surface velocity radar .3
4.3 Laser Doppler radar .4
4.4 Image velocimetry .4
4.4.1 Cross correlation techniques LSPIV and SSIV .4
4.4.2 PTV .5
4.4.3 STIV .5
5 Fixed installations and temporary measurements . 5
5.1 General .5
5.2 Bridge . .6
5.3 River or channel bank .6
5.4 Aircraft, drones and satellites .6
6 Measurement strategies . 6
6.1 Sampling periods .6
6.2 Wind measurements .7
6.3 Cross-section and cross-sectional profile measurements .7
6.4 Water-level measurement.7
7 Computational methods . 8
7.1 General .8
7.2 Arithmetic methods (or velocity area methods) .9
7.2.1 n, Number of segments .9
7.2.2 Determination of α .10
7.3 Wind correction.11
7.4 Stage-area determinations .11
7.5 Index velocity method . 12
8 Uncertainty .12
8.1 Application of uncertainty methods for arithmetic and index-velocity methods . 12
8.2 Uncertainty of the arithmetic method . 13
8.3 Uncertainty of the index velocity method . 15
8.4 Other uncertainty estimation methods .16
Bibliography . 17

iii
ISO/DIS 24577:2026(en)
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.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO documents should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
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. Details of any patent
rights identified during the development of the document will be in the Introduction and/or on the ISO list of
patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical CommitteeISO/TC 113, Hydrometry, Subcommittee SC 1, Velocity
area methods.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
ISO/DIS 24577:2026(en)
Introduction
This document presents non-contact techniques for measuring water surface velocity and liquid flow in
open channels. Water surface velocity can be measured with different technologies, such as radar or video
images and then to be converted into discharge. These methods can be used either in the form of permanent
installations at a gauge or as episodic measurements requiring human labor in actual field measurements.
Especially for the episodic case the advantage of all non-contact types consists in including a safe distance
from the water and thus serves to increase the safety of staff. Because of the availability of different
measurement techniques, the user needs to select the most appropriate system in terms of site conditions,
such as type of flow field, width of the channel or river, and installation conditions. To obtain discharge,
the cross-sectional area of the flow field is also required. To estimate the cross-sectional area, the channel
cross-sectional profile as well as water level should be measured. Whilst not covered explicitly in this
Standard, care should also be taken in selection of the most appropriate bottom topography and water level
measurement for the site conditions. For river flows, water surface and river flow conditions during target
flood stages are key factors in selecting systems.
The structure of the following chapters reflects the typical order of any installation in practice, first
acquiring the data which means choosing the method and installing the instruments. Gathering the first
data will perhaps refine or redefine the measurement strategy. Then computational methods will be applied
or improved. Finally regarding and reducing the uncertainties is an important consideration in order to
provide quality-assured data from that measuring site.
The non-contact techniques for measuring water surface velocity are undergoing constant development.
Therefore, this document can only aim to show the basic principles and their possibilities and limitations; a
complete overview of all methods is not possible due to the dynamics in this area.

v
DRAFT International Standard ISO/DIS 24577:2026(en)
Hydrometry — Use of noncontact methods for measuring
water surface velocity and determining discharge
1 Scope
To determine liquid flow, the following steps are necessary:
1) Measure water surface velocity with techniques using radar, laser or video images;
2) Correct the water surface velocity due to wind effects if necessary;
3) Option a: Transform the corrected velocity to a depth-averaged velocity in one segment using the
arithmetic methods referring to chapter 7.2, secondly calculate each segment and then create the sum of
all segments to obtain the cross-sectional averaged velocity distribution;
3) Option b: Transform the corrected velocity to a cross sectional velocity using the index methods
referring to chapter 7.3;
4) Determine the area of the wetted cross section from the stage-area relationship;
5) Obtain discharge of each segment by multiplying the depth-averaged velocity in each segment by the
wetted cross-sectional area of each segment. And then create the sum of all segments to obtain whole
discharge in cross section.
This procedure is applicable to different kinds of channel and river sections.
Applications include:
— Rivers and streams;
— Artificial channels such as drainage ditches and irrigation channels;
— Process flows on wastewater treatment plants.
For any individual site the method to measure water surface velocity should be selected appropriately, based
on the site conditions, nature of the application and uncertainty required. Take a special note that non-
contact methods should not be used where a unique relation between surface velocity and depth averaged
velocity cannot be established, e.g. where tidal phenomena are present. This is caused by the variations
of flow magnitude and direction over depth being highly variable over time under these circumstances.
Regarding backwater zones or in the vicinity of obstacles the relation between surface velocity and depth
averaged velocity may be more complicated, but even here optical methods may be helpful to at least learn
the situation at the surface.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements 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 748:2021, Hydrometry — Measurement of liquid flow in open channels — Velocity area methods using point
velocity measurements
ISO 772:2022, Hydrometry — Vocabulary and symbols
ISO 4373, Hydrometry — Water level measuring devices

ISO/DIS 24577:2026(en)
ISO 15769:2010, Hydrometry — Guidelines for the application of acoustic velocity meters using the Doppler and
echo correlation methods
ISO 25377, Hydrometric uncertainty guidance (HUG)
National Industry Guidelines for hydrometric monitoring, Part 11: Application of surface velocity methods
for velocity and open channel discharge measurements, NI GL 100.11-2021; Australian Government, Bureau
of Meteorology
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 772:2022 and ISO 748:2021 apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
4 Non-contact measuring methods
4.1 General
Obtaining liquid flow requires the determination of the following components: measured flow, the cross-
sectional averaged velocity derived using different methods and the flow area. Non-contact measuring
equipment is used to determine water surface velocity. For non-contact flow measurement, different types of
equipment with different principles can be employed, including radar, laser and video images. In particular,
image velocimetry measures the travel velocity of tracer materials or visible water surface structures on
the water surface and surface waves. We generally assume that the travel velocity of water surface tracers
or visible water surface structures is equivalent to water surface velocity, which is not always true. In that
case we should try to correct for the mismatch or at least be aware that our assumption might not be true.
Flow area is deduced from measurement of the water level, or stage, and knowledge of the channel profile.
Non-contact techniques such as radar, ultrasonic level measurement or other water level sensors including
the optical devices themselves are used to measure the water level. In some systems, the water level sensor
is integrated within the body which also houses the velocity sensor.
The velocity and level sensors will be connected to an electronics unit, which may be separate or
integrated within the same housing. This will perform some or all of the following functions:
— Provide power to the system;
— Provide temperature and humidity controlled system;
— Record measurements;
— Interpret the signal returned by the sensors to determine measured surface velocity and water level;
— Compute the mean velocity from the measured velocity from a pre-established relationship;
— Compute the volumetric flow;
— Generate an output to a data logger or telemetry system;
— Provide a visual display of the measured and or computed values;
— Provide a user interface to programme and commission the system;
— Provide security devices.
ISO/DIS 24577:2026(en)
With some experimental or survey systems, these functions may be incorporated in software loaded onto a
measuring device.
4.1.1 Advantages and benefits
In most situations installation and maintenance can be completed without personnel needing to enter the
water or channel. This is beneficial in terms of health and safety by reducing the risk to staff.
Non-contact measurement methods have these benefits:
— In case of a video-based method you record a lot of the measured environment.
— As the sensor does not come in contact with the water, there is less risk of damaging, contamination and
fouling of the sensors, particularly in wastewater and other applications where contaminants may be
present.
— Number of visiting personnel to measurement site is nothing or less than ordinary method.
— Using non-contact type which can obtain discharge value in real time, the flow discharge can be
constantly monitored without creating a regular rating curve with stage/discharge.
— Creating a regular rating curve with stage/discharge has many benefits, such as being able to determine
the flow rate from the water level if the non-contact type breaks (a fail-safe function) and being able to
understand the hydraulic characteristics of the observation station and use them in analysis. Although
the non-contact method can be a good complement if the rating curve is unreliable.
4.1.2 Limitations
— Depending on the method you may only record a small part of the surface.
— A relationship between the surface velocity and the depth averaged or cross-sectionally averaged
velocity is needed.
— Surface velocity methods are sensitive to wind and vegetation effects.
— Image conditions can be affected by a number of environmental factors, these are heavy rainfall, heavy
snowfall and most of all fog. However, measurements in these situations can reliably be filtered out, e.g.
through comparison with learned surface velocity profiles, Peña-Haro et al. Also glare and reflection can
influence the measurements, but typically this is not significant.
— In the case of thermal infrared cameras, the uniformity of surface temperature can also pose a challenge.
— The bottom topography is not measured simultaneously, referring to erosion and or sedimentation.
4.2 Surface velocity radar
Throughout the document the device called radar is a surface velocity radar or it is explicitly stated that it is
about a radar only measuring the water level or stage.
Radar stands for "RAdio Detection And Ranging". Radar techniques use the Doppler effect to measure
water surface velocity by observing the frequency difference between the transmitted and received
electromagnetic beams. Radar measurement can be classified into two types: surface measurement, in
which electromagnetic beams are emitted over a wide area to obtain a two-dimensional velocity field, and
point measurement, in which beams are emitted to a limited illuminated area to obtain velocity values at
the point. Both of them basically observe velocity component along the beam direction. To obtain velocity on
the water surface, geometrical conversion, such as the depression angles of the device, should be carried out.
With a surface velocity radar you measure the surface velocity at a certain area. In case of homogenous flow,
this, a single point measurement, is sufficient to calculate the flow discharge. Even mobile or portable radar
solutions produce precise results.

ISO/DIS 24577:2026(en)
4.3 Laser Doppler radar
Laser Doppler radar instruments focus a beam of light from a laser which is mounted above the flow at a
specific point. The frequency difference of the transmitted and reflected light gives the velocity at the point
of measurement. The laser can be focussed at a precise point in the flow field; it can also penetrate the liquid
surface. In this way, a single laser can be used to scan both across the flow field horizontally and, to a degree
which depends on the turbidity of the water, through the depth. This enables the mean fluid velocity to be
estimated over a range of flow conditions.
4.4 Image velocimetry
Image velocimetry is a technique using images continuously filmed with video or a camera.
Three methods are practiced at present and more evolving such as SSIV. The cross-correlation techniques
Large Scale Particle Image Velocimetry (LSPIV) and Surface Structure Image Velocimetry (SSIV) determine
the velocity vector by matching water surface patterns of particle constellations or surface structures
between two images obtained at different time points.
The Particle Tracking Velocimetry (PTV) tracks a tracer and the Space Time Image Velocimetry (STIV)
monitors the movement of natural tracers, structures or water surface waves to determine the velocity
component.
Whichever velocimetry algorithm is used, the image filtering is crucial to separate surface velocity signal
from artefacts such as shapes, standing waves or other objects not representing actual surface velocity. That
is the case for prefiltering processes but also for postprocessing velocity results.
Infrared cameras and high sensitivity cameras or a proper illumination are available for 24-hour operation.
Aside from experimental studies, image conversion may be necessary when these techniques are applied
to actual flows. For example, in the case of the video images obtained from the river or channel bank with
small angles to horizontal and from a bridge or from the air with large angle to horizontal, images should be
converted with the explicit orthorectification method.
Installation / location-survey of rating points are necessary to conduct successful measurements with good
accuracy. Ground Control Points (GCP) need to be installed or specified, measured in X, Y, Z and defined in
the camera view. GCPs should be widely distributed in X, Y, and Z directions, the more GCPs are used the
more accurate. If also the camera position is measured, two GCPs and a visible shoreline are the minimum
conditions for the image transformation method. On the other hand, image conversion is possible even
without GCPs if the camera position and angle, lens constant and water surface height are known.
4.4.1 Cross correlation techniques LSPIV and SSIV
The Large Scale Particle Image Velocimetry (LSPIV) and Surface Structure Image Velocimetry (SSIV) are
velocity measurement techniques designed to detect and track persistent and congruent patterns and
surface structures from one image to the next in a time series of images, on the assumption that the similar
visual patterns in different locations are the results of the movement of water surface tracers or surface
waves. It can determine the velocity vector in stream and spanwise directions. Pattern detection can be
conducted by searching the filmed area with changing the size of the target pattern or structure as well as
a searching area. It is recommended to use a high-resolution video camera capable of taking either 24 fps
(PAL) or 30 fps (NTSC) or more. Both methods can be conducted without adding or injecting tracers under
good conditions. It can be used to measure water discharge and flow field. In general, the size of the targeted
pattern has similar size in entire section, though ratio of the resolution of camera to the pattern is not
constant in one image.
Therefore, the measurement accuracy of this technique depends on the distance from the camera to
the target point and angle from camera to the flow direction, i.e. on the resolution of pixels per pattern
or structure size. The feasibility and accuracy of the analysis also depend on the clarity of the structure
(pattern) on the water surface, which is determined by factors such as the way the light hits it.

ISO/DIS 24577:2026(en)
4.4.2 PTV
The Particle Tracking Velocimetry (PTV) is a velocity measurement technique designed to determine
the velocity vector by tracking identical objects between subsequent images. Usually, tracers that are
purposefully distributed in the area of measurement are used as trackable objects, but visible material, e.g.,
debris, driftwood, or anything that moves by water flow, can also work as tracers, when present on the
water surface. It is not generally true however, that the movement of water surface and the movement of
the tracer are identical because the latter is affected by how much portion of the tracer is under the water.
Similar to LSPIV and SSIV, it is capable of both water discharge measurement and flow field measurement.
In principle the measurement accuracy of this technique depends on the distance from the camera to the
target point and angle from the camera to the flow direction.
4.4.3 STIV
The Space Time Image Velocimetry (STIV) is a velocity measurement technique designed to track the
movement of water surface patterns or visible structures on the water surface along a search line. Search
lines are set in the flow direction to determine flow discharge and must be perpendicular to the reference
cross-section. The image along each search line is converted to a space and time image of which vertical
and horizontal axes represent time and space, respectively. If the flow is steady a space and time image
usually shows several straight lines (unless they are accelerating), each of which indicates the movement of
an object or structure along the line with respect to time.
The angle of each line indicates the speed of an object or structure. It is recommended to use a high-resolution
video camera capable of taking 30 frames per second. as standard, at least 7,5 frames per second. Space and
time image is applicable to anything visible on the water surface whose movement can be translated into
a space and time image. Unlike to the LSPIV and SSIV, which correlate spatial patterns in time, the STIV
method depends on the visibility of objects and structures as they move downstream. For long distances
with poor pixel resolution or very shallow angles (down to a few degrees to the horizontal), the correlation
of a spatial pattern is more likely to be lost, as compared to the line of a visible object or structure traveling
downstream along an inspection line. In this sense STIV is more suited for long distances and shallow
inclination angles from the camera to the water surface. The STIV method has a small computational load
and is easy to perform real-time observations. Even mobile or portable STIV solutions are also available.
5 Fixed installations and temporary measurements
5.1 General
One of the great advantages of non-contact current meters is flexible installation. Unlike contact methods,
they can be installed in a variety of ways, depending on the operators' intention as well as the conditions of
the measurement section. The description of this chapter includes both, fixed and temporary measurement.
Although several patterns of installation are possible, the options should be carefully considered in terms of
the performance of each device. For all kind of installations, the following should be considered:
— Mountings should be secure and rigid, to avoid misreading due to the sensor being moved, e.g. by wind;
— Mounting points for optical sensors should cover the whole cross section or as much as possible of the
river width;
— Mounting points for optical sensors should allow a suitable inclination to the water surface: ideally 15-60
degree, possible for (5)10-90 degree; angle is referred to the horizontal. There are no general constraints
for the viewing angle with respect to the streamwise direction. This is a rough overview, for specific
information look at the method you’re using.
— For permanent installations, both the laser Doppler velocimetry and the surface velocity radar, should be
mounted above the water surface.
— As far as possible, sites should be protected from vandalism and not present hazards to the public or
operators;
ISO/DIS 24577:2026(en)
— In either artificial channels or natural rivers and streams, sections should be chosen where the bed will
not change, for example due to bed mobilisation or sedimentation. If either of these processes is likely
to occur, operators should ensure that maintenance regimes are in place to keep the channel bed at the
same depth, otherwise there will be errors in the measurement;
— Installation in long, straight reaches is generally preferable as this will help ensure a stable velocity
profile for given stage across the channel and a regular profile through the depth of the water.
5.2 Bridge
Bridges are one of the most ideal locations for installing radar-based velocity sensors as the sensor can be
aligned with the flow direction and able to cover the cross-section. For optical methods the foundation or
one of the bridge ends is the ideal mounting point for the camera, as typically, it is more practical to get the
entire cross section into the cameras view and often it is simpler to reach these mounting points. Multiple
sensors may be installed across the river or channel, particularly where the channel is wide or the velocity
distribution across the section is irregular, e. g. skewed. The accuracy of measurement is maintained in each
measurement, since the distance between each device and the targeted area is constant, though care needs
to be taken when processing the readings to obtain an average velocity.
It is generally desirable to have the radar-based velocity sensors facing upstream, since water surface waves
will not be disturbed by bridge piers or abutments. Natural wave patterns are preferable for better quality
data processing. In some cases, however, facing the sensors downstream is recommended, for instance,
when additional measurements for verification purposes are conducted with the devices in a particular
manner, e.g., using an ADCP tied to the bridge. In the case of temporary measurement, stamping permanent
marks to indicate the location of the radar is also recommended to ensure reproducibility in subsequent
measurements.
5.3 River or channel bank
River or channel banks are often a suitable location for the installation of image velocimetry equipment as
well as for the installation of radar for surface measurement, since the entire cross section can be captured
without any obstructions as long as the size of the channel is less than limitation usages of each device.
5.4 Aircraft, drones and satellites
Aircraft can also be used to install image velocimetry. It is necessary to take images with both riverbanks
to obtain points for georeferencing, e.g., houses, bridges or whatever is appropriate as landmark. Moreover,
taking the video from a standing aircraft is easier to analyse than from a moving one. If the aircraft is
moving when taking the video, the video sequence should be stabilized at the post process. Aircraft can
be selected from several options including helicopters and drones, depending on the size of budget and the
condition of weather, as well as on the operators’ intention of how they should fly, i.e., flying over or hovering
above the river. Note that wind turbulences may disturb the control of the aircraft, and that drones are more
vulnerable than helicopters.
Satellite images are also a suitable source, although only large rivers promise any prospect of success due
to the resolution, spatial and temporal. In addition, weather conditions are crucial for this method; clouds
and fog, i.e. anything that obscures the river from the satellite's view, prevents it from being analysed.
Furthermore, the use of satellites is an expensive method, but this field is evolving quickly and this aspect
may no longer be relevant in the future.
6 Measurement strategies
6.1 Sampling periods
Choosing the adequate sampling periods to eliminate several existing sources of errors in practice:
— the turbulence of the flow,
ISO/DIS 24577:2026(en)
— the noise of the instrumentation, and
— the vibration of the installations.
Generally, there are two approaches for determining the sampling period:
In the first way you extend your sampling period to such a length, that oscillations are averaged out. In terms
of turbulence, it is then necessary to capture the longest event period, since turbulences occurs on different
scales, which result in different wave periods. In terms of the noise and vibration of the installations, it is
necessary to understand the event period as well. This can lead to very long sampling periods significantly
larger than a minute. Processing these amounts of data especially in the case of optical methods can cause
some delays because real time processing (processing time equals recording time) is up to now only available
on some system.
For the second approach you choose a rather short sampling period, e.g. 5 seconds, while at the same time
increasing the number of samples, e.g. 5 or 10, and thereby ensemble average the result over a longer period,
e.g. five or even 10 minutes average values.
6.2 Wind measurements
If you notice significant wind influence on the water surface velocity field either during on-site visits or
through analysed results, then real-time wind measurements become necessary to either compensate the
wind influence or to reject measurements during wind that is exceeding a certain threshold. To do so, both
wind speeds as well as wind direction need to be measured. Theoretically, you need to measure the wind
field that is present near the water surface, but this can lead to flooding of the instruments during floods.
It is therefore recommended to place the instruments at heights above the 20- year or 50-year return level.
6.3 Cross-section and cross-sectional profile measurements
A cross-section should be considered as a vertical plane perpendicular to the mean flow direction. If it is
not, geometric conversion is necessary to obtain appropriate discharge values. The combination of the dry
topography and the wet bottom topography, can be called cross-sectional profile. The optical sensors must
measure in the vicinity of the cross-section plane. For changing waterlevels, the top of the cross-section, i.e.
the water-surface line, can change its position relative to the field of view. Therefore it must be guaranteed
that the field of view of the optical sensor covers the entire range of the cross-section line. In the case when
the velocity measurement cannot be conducted at the same section with the water-level-measurement (see
6.4), water level at the cross-section can be calculated using the neighbouring upstream or downstream
water level gauge with accounting the water surface slope appropriately.
At locations where active riverbed evolution is recognized and the bed evolution causes large source of
uncertainty for discharge measurement, it is necessary to monitor the cross-sectional profile, in particular
the bottom topography during flooding, for example, by conducting measurement by acoustic type
instrumentation, such as echo sounder or ADCP. If it is not possible, monitoring before and after flooding
shall be acceptable.
In addition, we recommend monitoring surface velocity profiles as a function of their respective water-
levels. This provides the possibility to compare surface velocity profiles e.g. before and after a flooding
event. If the measured surface velocity profile exhibits significant changes, then this is likely due to a change
of the cross-sectional profile. The change of a surface velocity profile for a given water level serves as an
indication to re-measure the bottom topography.
6.4 Water-level measurement
Gauges are needed to measure the water surface elevation at a cross-section. As indicated before the water
level needs to be measured at the same location as the velocity measurement is being taken, depending on
the water surface slope this distance can differ. In some case of the optical velocity measurement the water
level can be determined with the same imaging sensor as it is used for the velocity measurements. In case
of the radar and laser Doppler velocity sensors the system has often already an integrated additional sensor
for the water level. In any case it is useful to install additional water level measurements in accordance with

ISO/DIS 24577:2026(en)
ISO 4373. In wide channels and rivers, more than one gauge may be installed on either side of the channel
and/or the middle of the channel on a cross-section line, as well as at nearby points as needed.
7 Computational methods
7.1 General
Velocity in cross-section is a 2D 3-component field that is distributed across the channel width and along
the vertical channel depth. It has turbulent fluctuations of 5 % - 10 % around the local mean velocities and,
in addition, it can have secondary flows which are stationary or fluctuate with longer time scales, minutes
rather than a few seconds, typically. Following ISO 748 two main approaches are used in practice to obtain
discharge from the measured surface velocity: the class of ‘arithmetic methods’, often referred to as
velocity area methods (e.g. AUZ guidelines NI GL 100.11-2021) and the class of ‘index velocity methods’,
which in fact also makes use of the area (e.g. Computing Discharge Using the Index Velocity Method, USGS
2012).
In the class of arithmetic methods, the cross-section is regarded as being made of segments and the
n
discharge is the result of QvA over the i=1 to n segments.

ii
i1
The arithmetic methods are facing the challenge of accurately estimating the mean velocity v for each
i
segment, based on the input of surface v
...