ISO 11657:2014

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









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©  International Organization for Standardization, 2012

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ISO/DIS 11657

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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.

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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.

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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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ISO/DIS 11657
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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ISO/DIS 11657
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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ISO/DIS 11657
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
...