ISO 11657:2014

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 2014
© ISO 2014
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ii © ISO 2014 – All rights reserved

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
Foreword
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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.
iv © ISO 2014 – All rights reserved

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.
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.
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
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;
k is a calibration coefficient depending on instrument geometry, particle properties (size
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.
2 © ISO 2014 – All rights reserved

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.
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
4 © ISO 2014 – All rights reserved

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
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
calibrations over a range of flows and sedimentary conditions are required because the constants k,
k , and k may change in time due to temporal variations in grain size, shape, colour or composition.
2 3
In practice, the bulk optical and acoustical sampling methods can only be used in combination with a
mechanical sampling method to collect water-sediment samples for calibration. LD instruments measure
particles between a minimum and maximum size that is a function of the instrument scattering angle
measurement. If the size of the suspended sediment is within the size range of the LD instrument, then
LD does not require recalibration except for changes in particle bulk density, which is necessary in
order to convert the volume concentration to mass concentration. If some sediment is outside of the size
range of the LD instrument, consult the manufacturers’ manual for information about how the overall
concentration, accuracy and precision are affected.
In rivers most of the suspended sediments are transported during floods. Therefore it is recommended
to take water samples for calibration especially during floods.
7.2 The accuracy and precision of data produce
...