ISO 11657:2014

Hydrometry — Suspended sediment in streams and canals — Determination of concentration by surrogate techniques

ISO 11657:2014 specifies methods for determination of the concentrations and particle-size distributions of suspended sediment in streams and canals by surrogate techniques. ISO 11657:2014 covers brief description of the operating principle of each method and details of some of the instruments available.

Hydrométrie — Sédiments en suspension dans les cours d'eau et dans les canaux — Détermination de la concentration par des techniques de substitution

General Information

• Status: Published
• Publication Date: 23-Jun-2014
• ICS: 17.120.20 - Flow in open channels
• Technical Committee: ISO/TC 113/SC 6 - Sediment transport
• Drafting Committee: ISO/TC 113/SC 6 - Sediment transport
• Current Stage: 9093 - International Standard confirmed
• Completion Date: 17-Jan-2020

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 11657
First edition
2014-07-01
Hydrometry — Suspended sediment in
streams and canals — Determination
of concentration by surrogate
techniques
Hydrométrie — Sédiments en suspension dans les cours d’eau et dans
les canaux — Détermination de la concentration par des techniques
de substitution
Reference number
ISO 11657:2014(E)
©
ISO 2014

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ISO 11657:2014(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO 2014
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form
or by any means, electronic or mechanical, including photocopying, or posting on the internet or an intranet, without prior
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Tel. + 41 22 749 01 11
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Published in Switzerland
ii © ISO 2014 – All rights reserved

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ISO 11657:2014(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Measuring principles . 2
4.1 Transmission . 2
4.2 Scattering . 2
4.3 Transmission — Scattering . 3
4.4 Diffraction . 3
5 Properties of sediment of importance for sediment surrogate techniques .3
5.1 General . 3
5.2 Particle size . 4
5.3 Particle colour . 4
6 Methods for determination of suspended sediment concentration by
surrogate techniques . 4
6.1 General . 4
6.2 Bulk optics . 4
6.3 Laser diffraction (LD) . 5
6.4 Acoustic back scatter (ABS) . 5
7 Calibration and validation . 6
Annex A (informative) Determination of the concentration of suspended sediment by
optical techniques . 9
Annex B (informative) Determination of the concentration of suspended sediment by laser
diffraction technique .12
Annex C (informative) Determination of the concentration of suspended sediment by acoustic
back scatter .14
Bibliography .18
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ISO 11657:2014(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.
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 on the meaning of ISO specific terms and expressions related to conformity
assessment, as well as information about ISO’s adherence to the WTO principles in the Technical Barriers
to Trade (TBT) see the following URL: Foreword - Supplementary information.
The committee responsible for this document is ISO/TC 113, Hydrometry, Subcommittee SC 6, Sediment
transport.
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ISO 11657:2014(E)

Introduction
Sedimentation and sediment transport in streams, rivers, reservoirs and estuaries are key parameters
in many scientific, environmental, engineering, and agricultural problems. Success in managing and
solving sedimentation problems requires comprehensive knowledge of sediment movement. This
requires reliable methods of estimation of sediment load with high-quality data. The amount of sediment
transport data being collected, however, has steadily declined in recent decades largely due to difficulty
and costs associated with field methods used for data collection. High temporal resolution data of high
quality are needed to better understand and more adequately describe many sedimentation processes.
The bed load and suspended load broadly constitute total sediment load. However, the scope of this
International Standard is confined to the measurement of suspended sediment. Conventional methods
for measurement of suspended sediment concentrations in streams rely on the principle of collecting
samples of water-sediment mixture at various points in time and space using suitable sampling
equipment and deployment methods and analysing the samples in laboratory for estimating the sediment
concentration. These methods are labour intensive, expensive and can be hazardous. Moreover, the
accuracy of these methods in estimating the sediment concentration of rivers and streams over a period
of time may not be dependable due to the large spatial and temporal variability associated with the
transport of suspended sediment.
Continuous and accurate estimation of suspended sediment concentration is essential in certain
situations such as:
a) in hydropower projects for the safety of the turbines and other machinery, reservoir silting and
flushing;
b) water-supply projects for monitoring water quality;
c) storm water run-off from urban areas;
d) silting of wetlands; and
e) long-term monitoring of sediment transport in rivers and streams, in order to obtain reliable base
lines that can be used for decision making.
In such situations, automatic and cost-effective techniques are essential to collect high-quality data on
suspended sediment concentrations and particle sizes.
Recent technological advances in the fields of optics and acoustics have provided new sediment-
surrogate technologies and methods to determine suspended sediment fluxes and characteristics.
Some of these methods can be used to measure suspended sediment concentration at higher resolution,
with greater automation and potentially lower cost than traditional methods. These methods involve
surrogate technologies that derive the suspended sediment concentration from measurements of optical
backscatter, laser diffraction and acoustic backscatter.
The measurement of suspended sediment concentration (SSC) in the water samples can be carried out
with the help of nephelometry, transmission, laser diffraction and acoustic back scatter techniques. The
working principles, applications, advantages and disadvantages, limitations and usable instruments of
the above techniques are elaborated in this International Standard. The optical backscatter technique
is readily available and relatively inexpensive. Optical backscatter sensor sensitivity depends on grain
size, colour and composition. The advantages are small size and small sample volume, linear and
high frequency response, insensitive to ambient light, large measuring range and low cost. The laser
diffraction (LD) technique is also readily available and cost effective. The acoustic backscatter is another
technique for measurement of SSC in the aquatic ecosystems. Measurements are possible for a range of
sediment sizes that is dependent on the acoustic frequency. The available maximum sampling depth will
be limited at high concentrations.
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INTERNATIONAL STANDARD ISO 11657:2014(E)
Hydrometry — Suspended sediment in streams and canals
— Determination of concentration by surrogate techniques
1 Scope
This International Standard specifies methods for determination of the concentrations and particle-size
distributions of suspended sediment in streams and canals by surrogate techniques. Methods based on
bulk-optical principle of water such as transmission and nephelometry are the most commonly used
surrogates for determining suspended sediment concentrations (SSC). Instruments and techniques
based on acoustic attenuation and/or acoustic backscatter principles are also in use for measurement of
suspended sediment concentration. Instrumentation based on the laser diffraction principle is also used
for the measurement of particle size distribution. This International Standard covers brief description
of the operating principle of each method and details of some of the instruments available.
The detailed method and principle of optical and acoustical transmission, nephelometry, and optical
back scatter (OBS), laser diffraction technique (LD) and acoustic back scatter technique (ABS) with their
limitations are described in Annex A, Annex B and Annex C respectively.
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 4363, Measurement of liquid flow in open channels — Methods for measurement of characteristics of
suspended sediment
ISO 13320, Particle size analysis — Laser diffraction methods
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 772, ISO 4363 and ISO 13320
and the following apply.
3.1
surrogate technique
indirect method in which a substitute object or property is used for measurement in place of the original
object or property
Note 1 to entry: Optical and acoustic properties of water-sediment mixture such as optical transmission, acoustic
scattering and laser diffraction are some of the surrogates for measurement of suspended sediment concentration.
3.2
nephelometry
any method for estimating the concentration of particles in suspension by measuring the intensity of
scattered light
Note 1 to entry: Light scattering depends upon number, size distribution, colour, composition (as manifested in
the complex index of refraction) and shape characteristics of the particles.
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ISO 11657:2014(E)

4 Measuring principles
Optical and acoustical methods can be used for continuous measurement of sediment concentration.
The measuring principles for the above surrogate techniques are similar and can be classified in three
categories as described in 4.1 to 4.4 (see Figure 1).
4.1 Transmission
The source and detector are placed opposite to each other at a distance l as shown in Figure 1 A. The
source emits a collimated light beam with intensity l . The sediment particles in the measuring volume
o
reduce the beam intensity by absorption and scattering resulting in a reduced detector signal. The
relationship between the detector signal (l ) and the sediment concentration (c) is described by Beer’s
t
[43]
Law and is given by Formula (1):
−kcl
l=le (1)
to
where
l is the transmitted light through a sample of length l in water of sediment concentration c;
t
l is the incident intensity of the emitter source;
o
k is a constant depending on the sediment, water, and instrument characteristics.
4.2 Scattering
The source and detector are placed at an angle φ relative to each other shown in Figure 1 B. The detector
receives a part of the radiation scattered by the sediment particles in the measuring volume. The
relationship between detector signal (l ) and sediment concentration (c) is given by Formula (2):
s
−kc
2
l=kl ce (2)
s3 o
where
l is the incident intensity of the emitter source;
o
k is a constant depending on the sediment, water, and instrument characteristics;
2
k is a calibration coefficient depending on instrument geometry, particle properties (size
3
distribution, shape, index of refraction or composition), optical /acoustic wave length
and travel distance (l).
NOTE Often, the distance l cannot be defined in optical backscatter type systems.
An important limitation of the scattering method is the strong nonlinearity of the relation between the
detector signal and sediment concentration for large concentrations. Even in low concentrations where
the response is linear, the output depends strongly on grain size and colour. For instance, colour alone
[37]
may change the calibration by a factor of 10 for higher concentration and the grain size may cause
an additional change in calibration. For example, the calibration is shown to change by a factor of 20
between a white 5 μm sediment and a grey 10 μm sediment. As such, changes in sediment properties
are not uncommon in nature, which are generally not known during the course of monitoring. Spot
calibration from samples is likely to be contaminated by unknown errors when sediment properties
change in space/time. The errors can reach several hundred percent and greater. However, the use of
laser sensors is able to overcome these errors to great extent.
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ISO 11657:2014(E)

4.3 Transmission — Scattering
This method is based on the combination of transmission and scattering, as shown in Figure 1 C.
Key
1 source 4 detector
2 detector 5 detector
3 measuring volume
Figure 1 — Basic principles of optical and acoustic methods
4.4 Diffraction
The phenomenon of bending of light from its straight line path around the corners of an obstacle or slit is
known as diffraction. Diffracted light can produce fringes of light, dark or coloured bands. This property
is used for measuring suspended sediment concentration in laser diffraction instruments.
Laser diffraction measures suspended sediment concentration by measuring the angular variation
in intensity of light scattered as a laser beam passes through a dispersed particulate sample. Large
particles scatter light at small angles relative to the laser beam and small particles scatter light at large
angles. The angular scattering intensity data are then analysed to calculate the size of the particles
responsible for creating the scattering pattern, using the Mie theory of light scattering.
5 Properties of sediment of importance for sediment surrogate techniques
5.1 General
The transport of sediment is based on hydraulic characteristics and physical properties of the sediment.
Some of these properties are of importance for evaluating the accuracy and precision with which the
sediment surrogate technologies described in this International Standard can determine SSC.
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ISO 11657:2014(E)

5.2 Particle size
Suspended sediment size is of importance for bulk optical and acoustical methods as these fundamentally
respond to the surface area of the particles. If the surface area changes but the concentration remains
constant these sensors will report a change in concentration that is proportional to the change in surface
[37]
area . The concentration output from LD sensors does not change with particle size.
5.3 Particle colour
The output of single-parameter bulk optical sensors depends strongly on particle colour. Sediment colour
[37]
changes alone may change the calibration by a factor of 10 for higher concentration. Combined with
changes in grain size this may cause an additional change in calibration. For example, the calibration can
[37]
be shown to change by a factor of 20 between a white 5 µm sediment and black 10 µm sediment . The
concentration output from LD or acoustical sensors is not influenced by particle colour.
6 Methods for determination of suspended sediment concentration by surrogate
techniques
6.1 General
The surrogate methods employ in situ measurement using sensors that measure either
a) the bulk optical properties of the water-sediment mixture, including transmission, nephelometry
and optical backscatter (OBS) sensors, or
b) laser diffraction (LD) sensors.
The methods also include sensors that measure the acoustical properties of the water-sediment mixture
such as acoustic backscatter (ABS).
6.2 Bulk optics
Measurements of the bulk-optical properties of water-sediment mixture are the most common means
for determining turbidity (water clarity) and estimating SSC in rivers. A number of optical instruments
are commercially available. Bulk-optic instruments can be categorized as follows.
a) Transmissometers, which employ a light source beamed directly at the sensor. The instrument
measures the light transmission, i.e. the part of the light not scattered by the suspended particles.
b) Nephelometers, which measure light scattered by suspended particles (rather than light
transmission). The light reaching the detector is directly proportional to the amount of sediment
particles scattering the source beam if their size, shape, colour and composition do not change.
Nephelometers can be divided into two general categories:
1) Turbidity meters generally measure 90° or forward scattering. Nephelometric measurements
typically are expressed in turbidity units defined by the light source, detection angle, and
whether the sensor has single or multiple detectors. The units of turbidity from a calibrated
nephelometer are called nephelometric turbidity units (NTU).
2) Optical backscatter (OBS) instruments measure backscattered infrared light, usually at 165°
from the emitter, in a small (concentration dependent) volume.
These instruments provide an estimate of the suspended sediment concentration from a single point.
Both transmission and scattering are functions of the number, size, colour, index of refraction, and
shape of suspended particles. Particles of all sizes can be measured in this way. However, the sensitivity
of these bulk optics methods depends on bulk particle area concentration, i.e. C/d, or Σ Ci/di where
I
C is volume concentration [when particles are smaller than the wavelength of light λ, the summation
includes a weight factor corresponding to the scattering efficiency of particles, which for such small
particles is other than 2 (the value for particles > λ)] and d is particle size. In other words, the method
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ISO 11657:2014(E)

is progressively less sensitive to increasing particle size. It also follows that the maximum working
concentration depends linearly on particle size. The details of the method are given in Annex A.
These bulk-optical instruments are generally inexpensive, do not have moving parts unless a wiper for
the optical window is used, and provide rapid sampling capability. The instruments rely on empirical
calibrations to convert measurements to estimates of SSC. No generic calibration that can be used to
calibrate the output from a transmissometer or nephelometer to SSC is possible.
There are several drawbacks associated with use of bulk-optic instruments that include:
a) lack of consistency in instrument measurement characteristics;
b) variable instrument response to grain size, composition, colour, shape, and coating;
c) biological fouling or damage to optical windows;
d) nonlinear and censored responses of sensors at high sediment concentration; and
e) variable response with dissolved constituents causing colour.
Maximum concentration limits for these instruments depend in part on particle-size distributions. An
optical backscatter (OBS) sensor has a generally linear response at concentrations less than about 2 g/l
for clay and silt, and 10 g/l for sand although the exact concentrations at which the response becomes
nonlinear is size dependent. The upper concentration limit for transmissometers additionally depends
on the optical path length [see Formula (1)].
Transmissometers are more sensitive at low concentrations but nephelometers and OBS sensors have
a broader operating range of concentrations. Because of the relation between calibration to particle
size and particle colour, nephelometers and OBS sensors are best suited for application at sites with
relatively stable particle-size distributions and colour.
6.3 Laser diffraction (LD)
The LD principle is described briefly in this subclause (for details see ISO 13320).
A laser beam is directed into the sample volume where particles in suspension scatter, absorb, and reflect
light. Scattered laser light is received by an array of detectors that allow measurement of the scattering
at multiple angles from the original direction of the beam. This yields a vector of light scattering
intensities with one numerical value for each detector. Using a suitable mathematical procedure and
optical model the scattering intensities are converted into a volumetric size distribution in discrete
size classes defined by the scattering angles covered by the detectors. By summing the individual
elements of the particle size distribution the total volume concentration for the size range covered by
the instrument is obtained.
The name laser diffraction derives from the original application of this method where light scattering
at multiple very small forward angles was measured. At these small forward angles (about < 10°), the
scattering is dominated by diffraction, rendering particle composition (i.e. refractive index) of only
secondary importance.
The LD method offers a fundamentally different basis for in situ measurement of the concentration (as
well as sizes) of suspended sediment particles at a point in the water column. Unlike bulk optical or
acoustic methods, the LD method does not suffer from a significant change in calibration with changing
sediment colour, composition or size for sediment sizes within the instrument measurement limits and
it does not require any calibration by the user. This property has led to the broad acceptance of the
method in applications ranging from measurements of biological specimens to ceramics and particles
of all types.
6.4 Acoustic back scatter (ABS)
Characterization of SSC using backscatter and attenuation of acoustic signals in water has been described
and developed for several decades. The basic principles are that acoustic waves passing through a
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ISO 11657:2014(E)

water-sediment mixture will scatter and attenuate as a function of sediment, fluid, and instrument
characteristics. The acoustic metrics of backscatter and attenuation relate functionally to sediment
characteristics (concentration, size, shape and density) within an ensonified volume after adjusting for
the influence of fluid and instrument characteristics. Specific formulae have been developed to correct
for the non-sediment (instrument and water) factors affecting acoustic metrics. A significant limitation
of single-frequency systems is that the metrics of acoustic attenuation and backscatter amplitude may
change due to changes in sediment concentration and/or sediment size. The amplitude of acoustic
backscatter from sediment may increase with increased concentration at a fixed size distribution or
with increased sediment size at a fixed concentration; and acoustic attenuation also varies with size and
particle density. Multi-frequency acoustic systems, however, have been successfully used to estimate both
sediment concentration and size characteristics. The optimal frequency(ies) for measuring sediment
characteristics will depend on the sediment sizes and the channel depth and/or width of measurement.
Acoustics have been successfully used at concentrations up to 30 g/l, with very short acoustical path
lengths. The relation of acoustics to sediment generally performs poorly at concentrations less than
about 20 mg/l. Measurements can be made from fixed acoustic instruments in side- or down-looking
configurations; or from mobile acoustic Doppler current profilers.
7 Calibration and validation
7.1 For the bulk optical and acoustic methods, in situ calibration between SSC, obtained from water
samples, and the signal measured by the sensor is necessary. The purpose of the calibration is to account
for the variability in the constants k, k , and k in Formulae (1) and (2). For LD, in situ calibration is not
2 3
necessary in order to obtain the volumetric concentration of the suspended sediment. However, in order
to convert the volume concentration from a LD measurement to SSC the bulk density of the particles must
be applied. The bulk density can be assumed, modelled, or derived from water samples.
The in situ calibration should be done under flow conditions that cover the entire range of velocities, SSC
and measuring positions (close to bed and water surface). For bulk optical or acoustic methods, regular
ca
...

DRAFT INTERNATIONAL STANDARD ISO/DIS 11657
ISO/TC 113/SC 6 Secretariat: BIS
Voting begins on Voting terminates on

2012-12-03 2013-03-03
INTERNATIONAL ORGANIZATION FOR STANDARDIZATION  •  МЕЖДУНАРОДНАЯ ОРГАНИЗАЦИЯ ПО СТАНДАРТИЗАЦИИ  •  ORGANISATION INTERNATIONALE DE NORMALISATION


Sediment in streams and canals — Determination of
concentration by surrogate techniques
Sédiments dans les cours d'eau et dans les canaux — Détermination de la concentration par des techniques
de substitution

ICS 17.120.20









To expedite distribution, this document is circulated as received from the committee
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Pour accélérer la distribution, le présent document est distribué tel qu'il est parvenu du
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Secrétariat central de l'ISO au stade de publication.



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REFERRED TO AS AN INTERNATIONAL STANDARD UNTIL PUBLISHED AS SUCH.
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©  International Organization for Standardization, 2012

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ISO/DIS 11657

Copyright notice
This ISO document is a Draft International Standard and is copyright-protected by ISO. Except as permitted
under the applicable laws of the user’s country, neither this ISO draft nor any extract from it may be
reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic,
photocopying, recording or otherwise, without prior written permission being secured.
Requests for permission to reproduce should be addressed to either ISO at the address below or ISO’s
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Reproduction may be subject to royalty payments or a licensing agreement.
Violators may be prosecuted.

ii © ISO 2012 – All rights reserved

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ISO/DIS 11657
Contents Page
Foreword .iv
Introduction.v
1 Scope.1
2 Normative references.1
3 Terms and definitions .1
4 Units of measurement.2
5 Measuring principles.2
5.1 Transmission .2
5.2 Scattering .2
5.3 Transmission – Scattering .3
6 Properties of sediment.4
6.1 General .4
6.2 Properties of individual particles.4
6.3 Bulk characteristics .4
7 Methods for determination of suspended-sediment concentration by surrogate
techniques.4
7.1 General .4
7.2 Bulk Optics.5
7.3 Laser Diffraction (LD).6
7.4 Acoustic Back Scatter (ABS) .7
8 Particle size analysis.7
8.1 Expression of particle-size distribution.7
9 Calibration and validation.7
10 Summary.10
Annex A Determination of the concentration of suspended sediment .11
Annex B Determination of the concentration of suspended sediment .15
Annex C Determination of the concentration of suspended sediment by acoustic back scatter .17
Bibliography.20







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ISO/DIS 11657
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 11657 was prepared by Technical Committee ISO/TC 113, Hydrometry, Subcommittee SC 6, Sediment
Transport.

iv © ISO 2012 – All rights reserved

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ISO/DIS 11657
Introduction
Sedimentation and sediment transport in rivers, streams and reservoirs is a world-wide
environmental, engineering, and agricultural issue. Success in managing and solving sedimentation
problems requires comprehensive knowledge of sediment movement, reliable methods of
estimation of sediment load, and improvement in data quality. The amount of sediment-transport
data being collected, however, has steadily declined in recent decades largely due to difficulty and
costs associated with field methods used for data collection. High temporal resolution data are
needed to better understand and more adequately describe many sedimentation processes.
The bed load and suspended load broadly constitute total sediment load. However, the scope of this
standard is confined to the measurement of suspended sediment. Conventional methods for
measurement of suspended-sediment concentrations and particle-size distributions in streams rely
on the principle of collecting samples of water-sediment mixture at various points in time and space
using suitable sampling equipment and deployment methods, and analyzing the samples in
laboratory for estimating the concentrations and particle-size distributions. These methods of
collecting sediment data are labour intensive and expensive and can be hazardous. Moreover, the
accuracy of these methods in estimating the sediment concentration of rivers and streams over a
period of time may not be dependable due to the large spatial and temporal variability associated
with the transport of suspended sediment.
Continuous and accurate estimation of suspended-sediment concentration is essential in certain
situations such as:
a) in hydropower projects for the safety of the turbines and other machinery, and
b) water-supply projects for monitoring water quality
c) storm water run-off from urban areas
d) long-term monitoring of sediment transport in rivers and streams, in order to obtain reliable base
lines that can be used for decision making
In such situations, automatic and cost-effective techniques are essential to collect high-quality data
on suspended-sediment concentrations and particle sizes.
Recent technological advances in the fields of optics and acoustics have provided new sediment-
surrogate technologies and methods to determine fluvial suspended-sediment fluxes and
characteristics. Some of these methods can be used to measure suspended sediment concentration
at higher resolution, with greater automation and potentially lower cost than traditional methods.
These methods involve calculating the concentration of suspended sediment from detectable optical
backscatter, laser diffraction and acoustic backscatter.

© ISO 2012 – All rights reserved v

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DRAFT INTERNATIONAL STANDARD ISO/DIS 11657

Hydrometry — Suspended Sediment in streams and canals -
Determination of concentration by surrogate techniques
1 Scope
1.1 This International Standard specifies methods for determination of the concentrations and
particle-size distributions of suspended sediment in streams and canals by surrogate techniques.
Methods based on bulk-optical principle of water such as transmission, nephelometry and optical
backscatter are the most commonly used surrogates for determining suspended-sediment
concentrations (SSC). Instruments and techniques based on acoustic attenuation and/or acoustic
backscatter principles are also in use for measurement of suspended-sediment concentration.
Instrumentation based on the laser diffraction principle is also used for the measurement of particle
size distribution. This Standard covers brief description of the operating principle of each method
and details of some of the instruments available.
1.2 The detailed method and principle of Nephelometry, Optical Back Scatter (OBS), Laser
Diffraction technique (LD) and Acoustic Back Scatter technique (ABS) with their limitations are
described in annexure A, B and C respectively.
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.
ISO 4363, Measurement of liquid flow in open channels – Methods for measurement of
characteristics of suspended sediment.
ISO 13320-2009, Particle size analysis – laser diffraction methods
3 Terms and definitions
For the purpose of this international standard, the definitions given in ISO 772, ISO 4363 and ISO
13320 apply, together with the following:
3.1
surrogate technique
An indirect method in which a surrogate/substitute object or property is used for measurement in
place of the original object or property.
NOTE – Optical and acoustic properties of water-sediment mixture such as optical transmission, acoustic
scattering, and laser diffraction are some of the surrogates for measurement of suspended sediment
concentration.
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3.2
nephelometry
Any method for estimating the concentration of particles in suspension by measuring the
intensity of scattered light (turbidity), often at right angles to the incident beam.
NOTE Light scattering depends upon number, size distribution, colour, composition (as manifested in the
complex index of refraction) and shape characteristics of the particles.
4 Units of measurement
The units of measurement used in this International Standard are SI units in accordance with
the appropriate parts of ISO 80000.
5 Measuring principles

Optical and acoustical methods are used for continuous measurement of sediment
concentration. The measuring principles for the above surrogate techniques are similar and
can be classified in three categories as follows (refer Figure 1):

5.1 Transmission

The source and detector are placed opposite to each other at a distance l as shown in figure
1 A. The sediment particles in the measuring volume reduce the beam intensity resulting in a
reduced detector signal. The relationship between the detector signal (l ) and the sediment
t
[23]
concentration ( c ) is following Beer’s Law :

−k cl
l = l e         (1)
t o

where

l is the transmitted light through a sample of length l in water of sediment concentration

t
c , and l is the incident intensity of the emitter source entering the water sample. The

o
variable k in the exponent depends on the sediment, water, and instrument characteristics.

5.2 Scattering
The source and detector are placed at an angle (φ) relative to each other shown in figure
1 B. The detector receives a part of the radiation scattered by the sediment particles in the
measuring volume. The relationship between detector signal (l ) and sediment concentration
s
( c ) is:

−k c
2
l = k l c e (2)
s 3 o

where

  l is the incident intensity of the emitter source,

o

  k in the exponent depends on the sediment, water, and instrument characteristics, and

2

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ISO/DIS 11657
  k is calibration coefficient depending on instrument geometry, particle properties (size
3
distribution, shape, index of refraction or composition), optical /acoustic wave length and
travel distance (l). Often, the distance l can not be defined in optical backscatter type
systems.

An important limitation of the scattering method is the strong nonlinearity of the relation between the
detector signal and sediment concentration for large concentrations. Even in low concentrations
where the response is linear, the output depends strongly on grain size and colour. For instance,
[37]
colour alone may change the calibration by a factor of 10 for higher concentration ; and the grain
size may cause an additional change in calibration. For example, the calibration is shown to change
by a factor of 20 between a white 5 micron sediment and a grey 10 micron sediment. As such,
changes in sediment properties are not uncommon in nature, which are generally not known during
the course of monitoring. Spot calibration from samples is likely to be contaminated by unknown
errors when sediment properties change in space/time. The errors can reach several hundred
percent and greater. However, the use of laser sensors is able to overcome these errors to great
extent.

5.3 Transmission – Scattering
This method is based on the combination of transmission and scattering, as shown in figure 1 C.



Key

1  Source                       4  Detector T
2  Detector                      5  Detector V
3  Measuring Volume
Figure 1  Basic principles of optical and acoustic methods
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ISO/DIS 11657
6 Properties of sediment
6.1 General
The transport of sediment is based on hydraulic characteristics and physical properties of the
sediment. The properties of sediment are classified by individual particle characteristics and
bulk characteristics.

6.2 Properties of individual particles
Suspended-sediment size is the most commonly used parameter to designate the properties
of individual particles. The particle size, particle settling velocity and the flow characteristics
govern the movement of the suspended sediment. Settling velocity in turn depends upon the
relative density, shape and the diameter of the suspended sediment particles.

Since natural sediments are shaped irregularly, a single length or diameter has to be chosen
to characterize the size. Four such diameters, i.e. nominal diameter, projected diameter,
sedimentation diameter and sieve diameter, are often used for different particle sizes or
purposes (for example, sieve diameter for coarse and medium particles, sedimentation
diameter for fine particles which are not usually separated by sieves). The nominal diameter
has significance in suspended-sediment transport, but is useful in the study of suspended-
sediment concentration in rivers, lakes and reservoirs. From the optical/acoustic scattering
perspective a property that represents composition is required. In the case of optical
scattering, this property is the complex index of refraction. The real part of this complex
number represents bending of light (cf. Snell’s Law) and is central to light scattering. The
imaginary part represents absorption by the particles. Particle colour is manifested as a
wavelength-dependent imaginary part of the refractive index. In contrast, for acoustic
scattering, the speed of sound in the particle material is important. This is equal to the
square root of the ratio of bulk modulus of elasticity, (often denoted by E), and the density,
denoted by ρ. Absorption in the particle may be represented by a similar imaginary factor.
Both scattering processes depend on the ratio of this property divided by the same for water,
i.e. it depends on a mismatch of the key property.

6.3 Bulk characteristics
As sediments typically consist of large numbers of particles differing in size, shape, relative
density and settling velocity, it is essential to find some parameters that represent the
characteristics of the group of particles as a whole. Therefore, a sample of sediment is
usually divided into classes according to their characteristics (size and settling velocity) and
the percentage by number, area, volume or mass of the total in each class is determined for
the particular characteristic. Frequency distribution curves may be drawn from these data
and their parameters (e.g. mean size, median size, standard deviation, median absolute
deviation) determined. It is important to realize that the frequency distribution curves for the
number, area, volume and mass distribution may be very different from each other for a
given sample of sediment. Consequently, the computed parameters, e.g. the mean size will
also be different.

7 Methods for determination of suspended-sediment concentration by
surrogate techniques

7.1 General
The surrogate methods employ in-situ measurement using sensors that measure the bulk
optical properties of the water-sediment mixture, including transmission, nephelometry and
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optical backscatter (OBS), and laser diffraction (LD). The methods also employ in-situ measurement
using sensors that measure the acoustical properties of the water-sediment mixture; acoustic
backscatter (ABS) and attenuation (transmission loss).
7.2 Bulk Optics
Measurements of the bulk-optical properties of water are the most common means for determining
water clarity and estimating suspended-sediment concentrations in rivers. A number of optical
instruments are commercially available. Bulk-optic instruments can be categorized as:

a) Transmissometers, which employ a light source beamed directly at the sensor. The
instrument measures the light transmission, i.e. the part of the light not scattered by the
suspended particles.

b) Nephelometer, which measure light scattered by suspended particles (rather than light
transmission). The light reaching the detector is directly proportional to the amount of
sediment particles scattering the source beam if their size, shape, colour and composition
does not change. Nephelometer can be divided into two general categories:

i) Turbid meters generally measure 90° or forward scattering. Nephelometric
measurements typically are expressed in turbidity units defined by the light source,
detection angle, and whether the sensor has single or multiple detectors. The units of
turbidity from a calibrated nephelometer are called Nephelometric Turbidity Units
(NTU).

ii) Optical backscatter (OBS) instruments measure backscattered infrared light in a small
(concentration dependent) volume.

These instruments provide measurements from a single point. Both transmittance and scattering are
functions of the number, size, colour, index of refraction, and shape of suspended particles.
Particles of all sizes can be measured in this way. However, the sensitivity of these bulk optics
methods depends on bulk particle area concentration, i.e. C/d, or Σ C /d where C is volume
I i i
concentration [when particles are smaller than the wavelength of light λ, the summation includes a
weight factor corresponding to the scattering efficiency of particles, which for such small particles is
other than 2 (the value for particles >λ)] and d is particle size. In other words, the method is
progressively less sensitive to increasing particle size. It also follows that the maximum working
concentration depends linearly on particle size. The details of the method are given in annexure A.

These bulk-optical instruments are generally inexpensive, do not have moving parts unless a wiper
for the optical window is used, and provide rapid sampling capability. The instruments rely on
empirical calibrations to convert measurements to estimates of SSC. No generic calibration that can
be used to calibrate the output from a transmissometer or nephelometer to SSC is possible.
There are several drawbacks associated with use of bulk-optic instruments that include:

i. Lack of consistency in instrument measurement characteristics
ii. Variable instrument response to grain size, composition, colour, shape, and coating
iii. Biological fouling or damage to optical windows
iv. Nonlinear and censored responses of sensors at high sediment concentration, and
v. Variable response with dissolved constituents causing colour.
Maximum concentration limits for these instruments depend in part on particle-size distributions.
The Optical Backscatter Sensor (OBS) has a generally linear response at concentrations less than
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about 2 g/l for clay and silt, and 10 g/l for sand although the exact concentrations at which
the response becomes non-linear is size dependent. The upper concentration limit for
transmissometers additionally depends on the optical path length (see equation 1).

Transmissometers are more sensitive at low concentrations but OBS has broader operating
range of concentrations. Because of the relation between OBS calibration to gain and
particle size and particle colour, OBS is best suited for application at sites with relatively
stable particle-size distributions and colour.

NOTE  However, for most OBS’s, the operating range depends on gain switching, which has to be different for
different particle sizes and concentrations.

7.3 Laser Diffraction (LD)
The name Laser Diffraction derives from the original application of this method where light
scattering at multiple very small forward angles was measured. At these small forward
angles (< about 10°), the scattering is dominated by diffraction, rendering particle
composition (i.e. refractive index) of only secondary importance. This property leads to the
broad acceptance of the method in applications ranging from biological specimens to
ceramics and particles of all types. Subsequent extension of the LD method included
scattering at higher angles, (to nearly 180-degrees) at which point knowledge of particle
refractive index becomes essential. There are many commercial laboratory LD systems that
limit measurements to small angles, and these cover the most common particle size ranges
of interest, typically from ~0,1 to ~3,000 microns).
A laser beam is directed into the sample volume where particles in suspension scatter,
absorb, and reflect light. Scattered laser light is received by an array of detectors that allow
measurement of the scattering at multiple angles from the original direction of the beam.
Median particle size is calculated from this multi-angle scattering. The optical path length in
commercial systems can range from a few millimeters to several centimeters. Some
manufacturers have developed instruments that cover upper concentration limits given by
d/L mg/L, where d is particle diameter in microns and L is path length of laser through the
suspension in meters (this relation is for mass density of 2,65). For example, for particles of
size 10 microns and a path-length of 5 cm, the upper limit is nearly 200mg/L. Such a limit
does not define a sudden cut-off of operation. Instead, it defines a boundary beyond which
multiple scattering of light (or re-scattering of once scattered light) increasingly reduces
accuracy. The corresponding lower limits of measurable concentrations approach 0,1mg/L.

The LD method offers a fundamentally superior basis for in-situ measurement of the sizes of
suspended particles at a point in the water column. Unlike simpler bulk optical or acoustic
methods, the diffraction method does not suffer from a significant change in calibration with
changing sediment colour, composition or size for sediment sizes within the instrument
measurement limits.

The laser diffraction method originally was developed to produce ‘equivalent sphere’ size
distributions; i.e. a size distribution of spheres that would produce the same multi-angle
scattering pattern as was observed. This was possible because light scattering by
[23]
homogeneous spheres is fully described by Mie theory , which computes scattering at
arbitrary angles and is applicable to all size particles, from molecular to planetary scale. So,
to determine the equivalent sphere size distribution, a size distribution of spherical particles
was estimated that would produce the multi-angle scattering observed from the sample.

Since the late 2000’s, improvement in LD inversion methods has removed the restriction of
constructing ‘equivalent spheres’ size distribution from the LD measurement and methods
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now exists that can recover the ‘sieve size’ distribution of samples using LD, with no assumption of
[4]
spherical particles .

7.4 Acoustic Back Scatter (ABS)
Characterization of SSC using backscatter and attenuation of acoustic signals in water has been
described and developed for several decades. The basic principles are that acoustic waves passing
through a water-sediment mixture will scatter and attenuate as a function of sediment, fluid, and
instrument characteristics. The acoustic metrics of backscatter and attenuation relate functionally to
sediment characteristics (concentration, size, shape and density) within an ensonified volume after
adjusting for the influence of fluid and instrument characteristics. Specific formulas have been
developed to correct for the non-sediment (instrument and water) factors affecting acoustic metrics.
A significant limitation of single-frequency systems is that the metrics of acoustic attenuation and
backscatter amplitude may change due to changes in sediment concentration and/or sediment size.
The amplitude of acoustic backscatter from sediment may increase with increased concentration at
a fixed size distributi
...