ISO 18481:2017

Hydrometry - Liquid flow measurement using end depth method in channels with a free overfall

ISO 18481:2017 specifies a method for the estimation of the sub-critical flow of clear water in a smooth, essentially horizontal channel (or a gently sloping channel), abruptly discontinued at bottom by a hydraulic structure, with a vertical drop and discharging freely. Such an overfall forms a control section and offers a means for the estimation of flow using the end depth measurement method. A wide variety of channel cross-sections with overfall have been studied, but only those which have received general acceptance after adequate research and testing, and therefore do not require in situ calibration, are considered. This document covers channels with the following types of cross-sections: a) rectangular with confined and unconfined nappe; b) trapezoidal; c) triangular; d) circular; e) parabolic. The flow at the brink is curvilinear; therefore, the measured depth at the drop is not equal to the critical depth as computed by the principle based on assumption of parallel flow. However, the end depth and the critical depth (as in the case of the assumption of parallel flow) have unique relation, which is used to estimate the flow through these structures.

Hydrométrie — Mesure du débit liquide dans les chenaux à déversoir sans pelle

General Information

• Status: Published
• Publication Date: 07-Dec-2017
• ICS: 17.120.20 - Flow in open channels
• Technical Committee: ISO/TC 113/SC 2 - Flow measurement structures
• Drafting Committee: ISO/TC 113/SC 2 - Flow measurement structures
• Current Stage: 9093 - International Standard confirmed
• Start Date: 31-Mar-2023
• Completion Date: 13-Dec-2025

Relations

• Consolidated By: ISO 14978:2018 - Geometrical product specifications (GPS) - General concepts and requirements for GPS measuring equipment
• Effective Date: 06-Jun-2022
• Revises: ISO 4371:1984 - 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)
• Effective Date: 28-Jul-2012
• Revises: ISO 3847:1977 - Liquid flow measurement in open channels by weirs and flumes - End-depth method for estimation of flow in rectangular channels with a free overfall
• Effective Date: 28-Jul-2012

Overview

ISO 18481:2017 - Hydrometry: Liquid flow measurement using end depth method in channels with a free overfall specifies a standardized procedure to estimate sub‑critical flow of clear water in smooth, essentially horizontal (or gently sloping) channels that terminate abruptly with a vertical drop (a free overfall). The method uses the measured end depth at the brink of the drop and the unique relationship between end depth and critical depth to estimate discharge. The standard applies to commonly used channel cross‑sections that do not require in‑situ calibration.

Key topics and technical requirements

ISO 18481:2017 addresses practical and technical elements needed to apply the end depth method reliably:

• Covered channel types: rectangular (confined and unconfined nappe), trapezoidal, triangular, circular and parabolic cross‑sections.
• Principle: relationship between end depth (measured at the drop) and critical depth for estimating discharge in free overfall control sections.
• Installation and site selection: guidance for selecting suitable sites and installing drop structures that form a valid control section.
• Measurement of end depth: requirements for head‑measuring devices, gauge datum determination, and techniques to measure the end depth accurately.
• Specifications for drop structures and installation: structural and geometric recommendations to ensure predictable hydraulic behavior.
• Discharge relationships: documented discharge formulas and coefficients of discharge appropriate to each accepted cross‑section type (confined/unconfined nappe distinctions).
• Maintenance and practical limitations: inspection and upkeep of gauges and structures, and constraints on applicability (e.g., flow conditions, surface roughness).
• Uncertainty and errors: identification of error sources, methods to evaluate uncertainty in the coefficient values and measurements, and procedures for combining uncertainties to produce total discharge uncertainty.

Practical applications and who uses this standard

ISO 18481:2017 is intended for professionals and organizations involved in open‑channel flow measurement and water resource monitoring:

• Hydrologists and hydraulic engineers measuring discharge in irrigation channels, drainage systems, flumes and small waterways.
• Water utilities and wastewater managers seeking non‑intrusive, low‑cost flow measurement options.
• Environmental agencies and researchers conducting streamflow monitoring and hydrometric studies.
• Consultants and contractors designing or modifying overfall structures for flow monitoring without in‑situ calibration.

Benefits include standardized measurement practices, repeatable discharge estimation for accepted channel geometries, and guidance on uncertainty assessment.

Related standards

• Normative reference: ISO 772 (Hydrometry - Vocabulary and symbols).
• ISO 18481:2017 supersedes and merges earlier documents ISO 3847:1977 and ISO 4371:1984.

Keywords: ISO 18481:2017, hydrometry, end depth method, free overfall, discharge measurement, open channel flow, critical depth, coefficient of discharge, channel cross‑sections, uncertainty.

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 18481
First edition
2017-12
Hydrometry — Liquid flow
measurement using end depth method
in channels with a free overfall
Hydrométrie — Mesure du débit liquide dans les chenaux à déversoir
sans pelle
Reference number
©
ISO 2017
© ISO 2017, Published in Switzerland
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form
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written permission. Permission can be requested from either ISO at the address below or ISO’s member body in the country of
the requester.
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ii © ISO 2017 – All rights reserved

Contents Page
Foreword .v
1 Scope . 1
2  Normative references . 1
3  Terms and definitions . 1
4  Symbols and abbreviated terms . 2
5  Principle . 2
6  Installation . 2
6.1 General . 2
6.2 Selection of site . 2
7  Measurement of end depth . 3
7.1 General . 3
7.2 Head measuring devices . 3
7.3 Gauge datum . 3
8 Maintenance . 3
8.1 General . 3
8.2 Types. 4
8.3 Specifications for the drop structure . 5
8.4 Specifications for installation . 6
8.5 Determination of gauge zero . 6
8.6 Discharge relationship . 6
8.7 Coefficient of discharge . 6
8.7.1 Confined nappe . 6
8.7.2 Unconfined nappe . 6
8.8 Practical limitations . 6
8.9 Uncertainty of measurement . 7
9  Triangular channel drop structure . 7
9.1 Specifications for the drop structure . 7
9.2 Specifications for installation . 7
9.3 Specifications for head measurement . 7
9.3.1 General. 7
9.3.2 Determination of channel angle . 7
9.3.3 Determination of gauge zero . 8
9.4 Discharge formula — Unconfined . 8
9.5 Practical limitations . 8
9.6 Uncertainty of measurement . 8
10  Trapezoidal channel drop structure . 9
10.1 Specifications for the drop structure . 9
10.2 Specifications for head measurement . 9
10.2.1 General. 9
10.2.2 Determination of gauge zero . 9
10.3 Discharge formula — Unconfined . 9
10.4 Practical limitations .10
10.5 Uncertainty of measurement .10
11  Circular channel drop structure .11
11.1 Specifications for the drop structure .11
11.2 Specifications for head measurement .11
11.2.1 General.11
11.2.2 Determination of gauge zero .11
11.3 Discharge formula — Unconfined .11
11.4 Practical limitations .13
11.5 Uncertainty of measurement .13
12  Parabolic channel drop structure .14
12.1 Specifications for the drop structure .14
12.2 Specifications for head measurement .14
12.2.1 General.14
12.2.2 Geometry .14
12.2.3 Determination of gauge zero .14
12.3 Discharge formula — Unconfined .15
12.4 Practical limitations .15
13  Uncertainties of flow measurement .15
13.1 General .15
13.2 Sources of error .15
13.3 Kinds of error .16
13.4 Uncertainties in coefficient values .16
13.5 Uncertainties in measurements made by the user .17
13.6 Combination of uncertainties to give total uncertainty on discharge .17
13.7 Example .17
Bibliography .20
iv © ISO 2017 – 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
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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
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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 on 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 the following
URL: www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 113, Hydrometry, Subcommittee SC 2,
Flow measurement structures.
This first edition of ISO 18481 cancels and replaces ISO 3847:1977 and ISO 4371:1984, which have been
merged and technically revised.
INTERNATIONAL STANDARD ISO 18481:2017(E)
Hydrometry — Liquid flow measurement using end depth
method in channels with a free overfall
1 Scope
This document specifies a method for the estimation of the sub-critical flow of clear water in a smooth,
essentially horizontal channel (or a gently sloping channel), abruptly discontinued at bottom by a
hydraulic structure, with a vertical drop and discharging freely. Such an overfall forms a control section
and offers a means for the estimation of flow using the end depth measurement method. A wide variety
of channel cross-sections with overfall have been studied, but only those which have received general
acceptance after adequate research and testing, and therefore do not require in situ calibration, are
considered. This document covers channels with the following types of cross-sections:
a) rectangular with confined and unconfined nappe;
b) trapezoidal;
c) triangular;
d) circular;
e) parabolic.
The flow at the brink is curvilinear; therefore, the measured depth at the drop is not equal to the critical
depth as computed by the principle based on assumption of parallel flow. However, the end depth and
the critical depth (as in the case of the assumption of parallel flow) have unique relation, which is used
to estimate the flow through these structures.
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 772, Hydrometry — Vocabulary and symbols
3  Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 772 apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at http://www.electropedia.org/
— ISO Online browsing platform: available at https://www.iso.org/obp
4  Symbols and abbreviated terms
Symbol Unit Definition
A m area of approach channel
2a m semi-latus rectum of parabola
A m area of flow at critical depth
c
b m width of rectangular channel
C — coefficient of discharge
d m diameter of the circle
h m end depth corresponding to the maximum anticipated discharge
e
z — side slope 1 vertical to z horizontal
Q m /s total discharge
m m top width of flow
t
D m critical depth
c
D m end depth
e
θ radian apex angles subtended by the top width of flow at the centre of the circle
radian semi-vertex angle of triangular channel
∝
5  Principle
The un-submerged flow at an abrupt end of a long channel can be referred to as free overfall. In many
cases, the measurement of flow depth at the free overfall is possible and could be used for discharge
estimation. Such a discharge measurement method does not generally require any obtrusive structure
to be built. Many available overfall structures constructed for other reasons could also be used for the
discharge measurement with minor modifications.
There is a unique relationship between the flow discharge and the critical depth in an open channel.
The ratio of end depth to the critical depth (EDR) established theoretically and verified experimentally
offers an easy method to measure the discharge using end depth method.
6  Installation
6.1  General
General requirements of overfall discharge measurement installation are described in the following
clauses. Special requirements of different types are described in clauses which deal with specific types.
6.2  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 may be made to conform) to the requirements necessary for measurement by
the end depth method. The potential application of this method of flow measurement is at proposed
or existing water and waste water treatment plants, where flumes and channels form part of such
installations. The discharge measurement using end depth can be installed on existing flumes and
channels after verification that they conform to the requirements necessary for measurement by the
end depth method or they can be modified to make them conform to the requirements. Particular
attention should be paid to the following features in selecting the site and ensuring the necessary flow
conditions.
a) An adequate straight length (at least 20h , where h is the end depth corresponding to the maximum
e e
discharge anticipated) of channel of regular cross-section should be available upstream of the drop.
2 © ISO 2017 – All rights reserved

b) The flow in the approach channel shall be uniform and steady, with the velocity distribution
approximating that in a channel of sufficient length to develop satisfactory flow in smooth, straight
channels. Baffles and flow straighteners can be used to simulate satisfactory velocity distribution,
but their location with respect to the measuring section shall be not less than the minimum length
prescribed for the approach channel.
c) The channel bottom should be horizontal. Gentle positive slopes not greater than 1 in 2 000 are
admissible; the flow shall be sub-critical, practically uniform upstream of the drop, and the water
surface shall be relatively stable and free from perturbations at even during low velocities.
d) The side walls, as well as the bottom, shall be smooth as far as possible (in this document, a
smooth surface shall correspond to a neat cement finish). The finish of the structure shall be well
maintained; changes in wall roughness due to various forms of deposition will change the discharge
relationship.
e) The end (face) of channel shall be normal to its longitudinal centre line and water shall be allowed
to fall freely beyond this point.
f) In the case of a confined nappe, the downstream side walls shall be extended to a distance not less
than six times the maximum end depth.
g) In the case of unconfined nappe, the side walls shall end at the drop and nappe should be completely
free at the sides to permit unrestricted spreading.
h) The nappe bottom shall be fully aerated in all the cases.
7  Measurement of end depth
7.1  General
The end depth is computed by deducting the bed level (gauge datum) from water surface level, both
measured at the end or at the fall. The depth shall be measured exactly at the end (drop) of the channel.
The flow at the drop is fully curvilinear and any small error in the location of the gauge will result in
large error in measurement of discharge.
7.2  Head measuring devices
The water surface at the fall or end may be measured using a point gauge or other suitable measuring
device. The pointer shall be at the centre of the channel width. The use of hook gauge or any other
measuring device requiring insertion inside water is not advised and is discouraged. The flow would
drag the pointer and displace it away from the point of measurement or the pointer will vibrate leading
to inaccurate measurement. The device selected should not disturb the flow conditions at the free fall.
Stilling well or float well cannot be used for the measurement of the end depth.
7.3 Gauge datum
Accuracy of end depth measurement is critically dependent upon the determination of the gauge datum
or gauge zero, which is defined as the gauge reading corresponding to the channel bed (bottom) at
the end (drop) in case of rectangular or trapezoidal channels, or the lowest point of the triangular or
circular channel at the end (drop).
8 Maintenance
8.1  General
Maintenance of the drop structure is necessary to achieve the accuracy in measurement. The approach
channel shall be kept free of silt, vegetation and obstructions which might have deleterious effects
on the flow conditions specified for the standard installation. The downstream channel shall be kept
free of obstructions which might cause submergence or inhibit full ventilation of the nappe under all
conditions of flow.
The drop structure shall be kept clean. In the process of cleaning, care shall be taken to avoid damage
to the surface of the drop structure, particularly brink edge and upstream bed and side surface. The
head measuring devices like point gauge shall be checked periodically to ensure accuracy.
Dimensions in millimetres
Key
1 downstream face
Figure 1 — Drop edge of overfall
The drop edge shall be sharp at its intersection without any burrs. To ensure that the brink edge and
sides are sharp, a machined metallic rim could be fitted at the end of the channel. The thickness of the
metallic rim should be uniform and it should be between 1 mm and 2 mm along the flow. The metallic
rim shall be fitted flush with the vertical face of the overfall structure to ensure that no gaps exist
between the rim and the channel. The downstream edges of the metallic rim shall be chamfered if the
rim plate is thicker than the maximum allowable width along the flow. The surface of the chamfer shall
make an angle of not less than π/4 radians (45°) with a line extending along the horizontal channel
bed or side surfaces of the fall (see detail, Figure 1). The metallic rim shall be made of corrosion-
resistant metal; but if it is not, all the smooth surfaces and sharp edges shall be kept coated with a thin,
protective film (for example, oil, wax, silicone) applied with a soft cloth. If a flow straightener is used in
the approach channel, perforated plates shall be kept clean so that the percentage open area remains
greater than 40 %.
8.2 Types
The free overfall structures in rectangular channels are further classified into two types: confined
nappe and unconfined nappe.
The confined nappe is the jet formed by the flow where the guide walls of the structure extend to at
least six times the end depth at maximum flow beyond the brink edge and where the bottom of nappe is
sufficiently ventilated to ensure atmospheric pressure below the nappe (see Figure 2).
4 © ISO 2017 – All rights reserved

Key
1 aeration hole in side wall
...