ISO/TR 9212:2015

Hydrometry — Methods of measurement of bedload discharge

ISO/TR 9212:2015 reviews the current status of direct and indirect bedload-measurement techniques. The methods are mainly based on grain size distribution of the bedload, channel width, depth, and velocity of flow. This Technical Report outlines and explains several methods for direct and indirect measurement of bedload in streams, including various types of sampling devices. The purposes of measuring bedload-transport rates are to a) increase the accuracy of estimating total sediment load in rivers and deposition in reservoirs, b) gain knowledge of bedload transport that cannot be completely measured by conventional suspended-sediment collection methods, c) provide data to calibrate or verify theoretical transport models, and d) provide information needed in the design of river diversion and entrainment structures. NOTE The units of measurement used in this Technical Report are SI units.

Hydrométrie — Méthodes de mesurage du débit des matériaux charriés sur le fond

General Information

• Status: Published
• Publication Date: 03-Jun-2015
• 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: 6060 - International Standard published
• Start Date: 04-Jun-2015
• Due Date: 16-May-2016
• Completion Date: 16-May-2016

Relations

• Revises: ISO/TR 9212:2006 - Hydrometry — Measurement of liquid flow in open channels — Methods of measurement of bedload discharge
• Effective Date: 26-May-2012

Overview

ISO/TR 9212:2015 - "Hydrometry - Methods of measurement of bedload discharge" is a Technical Report that reviews current direct and indirect bedload measurement techniques. It summarizes methods based on grain-size distribution, channel geometry (width, depth) and flow velocity, and explains sampling devices and strategies for determining bedload transport in rivers and streams. Units used in the report are SI units.

Key Topics

• Direct measurement methods
  • Bedload samplers (basket/box, frame-and-net, pressure-difference types)
  • Bedload traps (pit, trough, vortex-tube) for near-complete capture of moving bed material
  • Requirements and characteristics of an ideal sampler; sampling procedures, identification, calculations and error considerations
• Indirect measurement methods
  • Differential and volumetric measurements (deposition surveys)
  • Dune-tracking techniques (moving-boat and in-situ echo sounder approaches)
  • Tracers and bedload-surrogate monitoring technologies
  • Remote sensing (LiDAR) and acoustic instruments including Acoustic Doppler Current Profiler (ADCP)
• Design and strategy
  • Site selection guidance (reach vs cross-section), accounting for spatial and temporal variability
  • Sampling strategy development tailored to local flow regimes and logistics
• Supporting content
  • Procedures for sample processing and particle-size distribution measurement
  • Annex on bedload-surrogate monitoring technologies and bibliography for further reading

Applications

ISO/TR 9212:2015 provides practical guidance for tasks where reliable bedload data are required:

• Improving estimates of total sediment load and reservoir deposition forecasting
• Supplementing suspended-sediment collection where bedload is not captured
• Providing calibration and verification data for sediment transport models
• Informing design of river engineering works (diversions, entrainment structures, channels, canals) to avoid unwanted bedload transfer
• Monitoring river-bed change, navigation channel maintenance and habitat assessments

Who should use this standard

• Hydrologists and hydrometrists
• River and coastal engineers
• Reservoir and dam designers
• Sediment transport researchers and modelers
• Environmental agencies and water-resource managers
• Consultants performing field sampling and instrument-based monitoring

Related Standards

• ISO 772 (Hydrometry - Vocabulary and symbols) is normatively referenced for terms and definitions.
• Document developed by ISO/TC 113 (Hydrometry), Subcommittee SC 6 (Sediment transport).

Keywords: ISO/TR 9212, hydrometry, bedload discharge, bedload measurement, sediment transport, bedload samplers, bedload traps, dune-tracking, LiDAR, acoustic instruments, ADCP, grain size distribution.

Standards Content (Sample)

TECHNICAL ISO/TR
REPORT 9212
Third edition
2015-06-01
Hydrometry — Methods of
measurement of bedload discharge
Hydrométrie — Méthodes de mesurage du débit des matériaux
charriés sur le fond
Reference number
©
ISO 2015
© ISO 2015, Published in Switzerland
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
written permission. Permission can be requested from either ISO at the address below or ISO’s member body in the country of
the requester.
ISO copyright office
Ch. de Blandonnet 8 • CP 401
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Tel. [phone]
Fax [phone]
[e-mail]
www.iso.org
ii © ISO 2015 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Measurement of bedload . 1
4.1 General . 1
4.2 Direct measurement methods . 1
4.3 Indirect measurement methods . 2
5 Design and strategy of measurement of bedload discharge . 2
6 Site selection . 2
7 Bedload samplers and traps . 3
7.1 Bedload samplers . 3
7.1.1 Requirements of an ideal bedload sampler . 3
7.1.2 Basket or box type sampler . 3
7.1.3 Frame and net sampler. 4
7.1.4 Pressure-difference sampler . 4
7.1.5 Advantages and disadvantages . 5
7.1.6 Characteristics of bedload samplers . 9
7.2 Measurement using bedload trap .10
7.2.1 Vortex tube bedload trap .10
7.2.2 Pit and Trough trap .11
7.2.3 Advantages and disadvantages .12
8 Procedures for measurement of bedload discharge using bedload samplers .12
8.1 General .12
8.2 Sample identification .13
8.3 Calculations .14
8.4 Errors .15
9 Indirect measurement of bedload .16
9.1 General .16
9.2 Differential measurement .16
9.3 Volumetric measurement .16
9.4 Dune-tracking .17
9.4.1 Moving boat .17
9.4.2 In situ echo sounder . .17
9.4.3 Accuracy of the dune-tracking methods .18
9.5 Tracers.18
9.6 Remote sensing LiDAR .18
9.7 Acoustic instruments .19
9.8 Acoustic Doppler current profiler .19
[8]
Annex A (informative) Bedload-surrogate monitoring technologies .20
Bibliography .24
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
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ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
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.
This third edition cancels and replaces the second edition (ISO/TR 9212:2006), which has been
technically revised.
iv © ISO 2015 – All rights reserved

Introduction
The knowledge of the rate of sediment transport in a stream is essential in the solution of practically all
problems associated with the flow in alluvial channels. The problems include river management, such
as design and operation of flood control works, navigation channels and harbours, irrigation reservoirs
and canals, and hydroelectric installations. The bedload and suspended load broadly constitute total
sediment load. The bedload is the material transported on or near the bed by rolling or sliding (contact
load) and the material bouncing along the bed, or moving directly or indirectly by the impact of bouncing
particles (saltation load). Knowledge of the bedload-transport rate is necessary in designing reservoir
capacity because virtually 100 % of all bedload entering a reservoir accumulates there. Bedload should
not enter canals and distributaries and diversion structures should be designed to minimize the transfer
of bedload from rivers to canals.
The bedload-transport rate can be measured either as mass per unit time or volume per unit time. Volume
measurements should be converted to a mass rate. Measurements of mass rate of movement are made
during short time periods (seconds, minutes), whereas measurements of volume rates of movement are
measured over longer periods of time (hours, days). Regardless of whether the mass or volume rate is
measured, the average particle-size distribution of moving material should be determined. Knowledge
of particle-size distribution is needed to estimate the volume that the bedload material will occupy after
it has been deposited. Knowledge of particle-size distribution also assists in the estimation of bedload-
transport rates in other rivers transporting sediment.
The movement of bedload material is seldom uniform across the bed of a river. Depending upon the
river, hydraulic, and sediment properties (size and gradation), the bedload may move in various forms,
such as ripples, dunes, or narrow ribbons. Its downstream rate of movement is also extremely variable.
It is difficult to actually sample the rate of movement in a river cross-section or to determine and verify
theoretical methods of estimation.
TECHNICAL REPORT ISO/TR 9212:2015(E)
Hydrometry — Methods of measurement of bedload
discharge
1 Scope
This Technical Report reviews the current status of direct and indirect bedload-measurement
techniques. The methods are mainly based on grain size distribution of the bedload, channel width,
depth, and velocity of flow. This Technical Report outlines and explains several methods for direct and
indirect measurement of bedload in streams, including various types of sampling devices.
The purposes of measuring bedload-transport rates are to
a) increase the accuracy of estimating total sediment load in rivers and deposition in reservoirs,
b) gain knowledge of bedload transport that cannot be completely measured by conventional
suspended-sediment collection methods,
c) provide data to calibrate or verify theoretical transport models, and
d) provide information needed in the design of river diversion and entrainment structures.
NOTE The units of measurement used in this Technical Report are SI units.
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
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 772 apply.
4 Measurement of bedload
4.1 General
Bedload can be measured by direct measuring bedload samplers or by indirect methods.
4.2 Direct measurement methods
a) Bedload samplers
In this method, a mechanical device or sampler is required for measuring the bedload-transport
rate. The bedload sampler is designed so that it can be placed directly on the channel bed in the flow,
to collect a sample of the bedload over a specific time interval. A sample thus obtained represents a
time-averaged mass per unit width per unit time.
b) Bedload trap
The best measurement of bedload would occur when all of the bedload moving through the river
cross was measured. Slot or pit samplers or traps meet this goal with near 100 % efficiencies.
4.3 Indirect measurement methods
All other methods of bedload measurement in which no mechanical device or bedload sampler is used,
are indirect methods. These include differential measurements of total and suspended-sediment loads,
periodic volumetric measurements of accumulated sediment depositions, dune tracking, tracers, remote
sensing, and acoustic measurements of moving sediment.
5 Design and strategy of measurement of bedload discharge
Measurement of bedload is difficult because it is highly variable in both space and time. Bedload generally
varies greatly both longitudinally along the channel and transversely across a cross section. These
variations are caused by several factors and are difficult to predict. The design of bedload sampling
needs to account for the spatial and temporal variability inherent in the processes of bedload transport.
Pit, vortex-tube, or other samplers that sample for long periods of time and encompass a significant
portion of the width of a stream cross section integrate the fluctuations in bedload-transport rate in
a cross section. In many instances, time, monetary constraints, or logistics precludes the use of these
types of samplers.
The use of portable samplers that essentially only collect samples at a point for short periods of time
is often the only practical way to collect samples of bedload. To effectively use portable samplers, the
number and location of the samples collected shall be carefully designed. Sufficient information about
the temporal and spatial variability is collected. To accomplish this task, information on the scales of
spatial and temporal variability is needed. To design an adequate sampling strategy, these time and
length scales shall be known at least approximately before the sampling procedure is defined.
Flow in many streams and rivers are not steady for periods of hours to days. For streams in which
variable flow is the norm, portable samplers will not be practical unless many flow events can be
sampled. No single sampling design can be used at all stations. A sampling design should be derived for
each site where bedload is to be sampled. Initial samples collected can provide information to serve as
a basis for developing the sampling plan.
6 Site selection
a) Depending upon the method of measurement, the site for conducting bedload measurements can
be either a river reach or a cross-section. The site should be relatively close to the geographical
location where bedload-transport rate information is needed. There should be no inflow or outflow
from the river between the measuring site and the site where bedload transport estimates will be
used.
b) When using a method such as dune-tracking, a straight reach where the channel width and depth
are fairly uniform throughout the reach is desirable. Flow through the reach should be uniform and
steady during the bedload-measurement period (see 9.4).
c) A single cross-section site should be selected if the method of measurement is by bedload sampler.
The channel width and mean depth of the cross-section site should be representative of the
average channel width and depth upstream and downstream. Ideally, a cross-section used for
bedload measurement by bedload sampler should be at the centre of a straight reach selected for
measurement of bedload by the dune-tracking method.
d) If it is not possible to place the cross-section site in the centre of an ideal straight, uniform reach,
then the cross-section should be located at least 10 to 20 channel widths downstream from any
2 © ISO 2015 – All rights reserved

bend in the channel. It should not be located at an excessively narrow section, such as might be
present at a bridge site, or at an excessively wide section.
7 Bedload samplers and traps
7.1 Bedload samplers
7.1.1 Requirements of an ideal bedload sampler
In order that the samples taken are truly representative of the bedload material of a river at the point of
sampling, the ideal bedload sampler should fulfil the following technical requirements.
a) It should be calibrated for bedload-sampler efficiency of specific sediment particle sizes.
b) It should be designed to minimize disturbances to normal bedload movement. In particular, local
erosion near the sampler mouth should be avoided so as to not form scour holes.
c) The lower edge of the sampler and nozzle should be in contact with the river bed.
d) The velocity of inflow at the mouth of the sampler should be as close as possible to the ambient
velocity of the stream at the sampling point, irrespective of what this velocity may be. This aspect
is very important if large sampling errors are to be avoided.
e) The mouth of the sampler should always face into the current and the sample should be taken
parallel to flow direction at the sampling point, into a specially designed chamber.
f) The mouth of the sampler should be outside the zone of the disturbances of the flow set up by the
body of the sampler and its operating gear and the flow lines should be as little disturbed as possible,
especially near the mouth.
g) The sampler should be able to collect only those particles moving as bedload, without contamination
by suspended sediment.
h) The sampler should be portable, yet sufficiently heavy to minimize deflection of the supporting
cable from the vertical due to current drag. A separate anchor is recommended for the sampler,
wherever possible.
i) The sampler should be simple in design and robust in construction and should require minimum
maintenance and care in operation.
j) It should be capable of collecting representative bedload samples under varying bed configurations.
k) The sampler should be designed for easy removal of the sampled material into a container for
transfer to a laboratory.
l) The volume of the sample collected should be sufficient for the determination of mass and particle-
size distribution.
m) The efficiency of the sampler should be independent of length of sampling over a reasonable time.
n) The efficiency of the sampler should be independent of the size of bedload particles and flow velocity.
7.1.2 Basket or box type sampler
This type of sampler consists of a basket or box, usually made of mesh material on all sides except the
front and bottom. The bottom may be solid or of loosely woven iron rings or steel mesh, to enable it to
conform to the irregular shape of the stream bed. The sampler is placed on the channel bed with the help
of a supporting frame and cables. A steering fin or vane(s) attached to the basket ensures positioning of
the instrument in the direction of the flow. The sediment is collected in the basket by causing a reduction
of the flow velocity and/or screening the sediment from flow for a measured time period.
Since a part of the bedload is dropped in front of the sampler, the efficiency of basket type samplers is
only about 45 %, for average sediment sizes varying from 10 mm to 50 mm. However, due to their large

capacity, basket type samplers are well suited for measuring of transport rate of large-sized sediment
[7]
.
7.1.3 Frame and net sampler
These are portable samplers consisting of a steel or aluminium frame and a trailing net for collecting
the sediment. The samplers can be used in small wadable streams. The samplers are anchored to the
streambed with steel rods driven through the frames. These samplers can be deployed for 1 h or more,
depending on the transport rate, so they can average out short-term temporal variations in transport
rates.
The sampler shown in Figure 1 was designed for use in small mountain streams. The frame, which was
fabricated from aluminium, 0,3 m wide, 0,2 m high, and 0,1 m deep. The netting, which extends about
1 m downstream from the frame, is sturdy nylon mesh with 3,5 mm openings. The sampler is able to
trap gravel particles as small as 4 mm and cobbles particles as large as 128 mm.
SCALE
APPROX. 0,3 m
Key
1 aluminium frame
2 bottom piece, bevelled
3 aluminium ground plate, inclined in front, with holes
4 adjustable nylon straps
5 slits at top and bottom on each side of the frame
6 smooth stakes, rolled steel
7 nylon netting
[2]
Figure 1 — Schematic diagram of a portable frame and net sampler
7.1.4 Pressure-difference sampler
This type of sampler is designed so that the velocity of water entering the sampler and the stream
velocity is approximately equal. Equalization of velocity is accomplished through creation of a pressure
drop at the exit due to a diverging configuration between the entrance and the exit. These are flow-
through samplers that trap coarse material behind baffles or in a mesh bag attached to the exit side or
in a specially designed chamber. The Scientific Research Institute of Hydrotechnics (SRIH) and Sphinx
samplers (see Figure 2 and Figure 5) are examples of samplers with internal baffles. The Arnhem, Helley-
Smith, US BLH-84, and US BL-84 are examples of mesh bag samplers (see Figure 3, Figure 4, Figure 6,
and Figure 7)
4 © ISO 2015 – All rights reserved

Key
1 transverse partitions
2 entrance
NOTE This is a pressure-difference bedload sampler. The SRIH sampler was the first of this type to be
developed. Such samplers can sample particles as small as fine sand to as large as 200 mm. Efficiencies are
extremely variable.
[10]
Figure 2 — Scientific Research Institute of Hydrotechnics (SRIH) sampler
7.1.5 Advantages and disadvantages
Portable samplers are generally inexpensive to acquire, but can be expensive to operate and suffer from
uncertain calibrations.
Dimensions in metres
Key
1 steering fin
2 entrance
3 rubber connection
4 mesh bag
NOTE This is a pressure-difference bedload sampler. The Arnhem or Dutch sampler comprises a rigid
rectangular entrance connected by a diverging rubber-neck to a basket of 0,2 mm to 0,3 mm mesh. Efficiencies
[13]
are variable, but generally about 70 % . It is suitable for collection of fine bedload material. The fine net of the
sampler can get clogged leading to a drop in efficiency of the sampler.
[14]
Figure 3 — Arnhem sampler
6 © ISO 2015 – All rights reserved

Dimensions in millimetres
Key
1 bag to tail attachment spring 6 rail attachment bolt
2 mesh polyester monofilament, 0,2 mm 7 hole for bag attachment spring
3 dot fastener 8 slot top rail to fit tail
4 aluminium alloy weld tail pieces except where side 9 aluminium tubing filled with lead after farming
rails join tail
5 sliding collar 10 tubing spacers, where necessary
NOTE This is a pressure-difference bedload sampler with a 76-mm square entrance nozzle and an area
[9]
expansion ratio of 3,22 . Field experiments indicate a nearly 100 % sampling efficiency for sizes from about
[6]
0,5 mm to 16 mm . Laboratory studies indicate that sampling efficiencies vary widely with particle size and
[11]
transport rate, ranging from 150 % for sand and small gravel and close to 100 % for coarse gravel .
[9]
Figure 4 — Helley-Smith bedload sampler
Dimensions in metres
NOTE This is a direct measurement sampler developed by Vinckers, Bijker and Schijft (see Reference [22]).
The hydraulic efficiency varies from about 1,09 for clear flow to about 1,0 for extreme conditions. Sampling
efficiency varies from about 93 % for particle sizes finer than 0,2 mm to about 85 % for sizes finer than about
0,09 mm.
[22]
Figure 5 — Sphinx sampler
NOTE The US BLH-84 is a hand-held 4,5 kg, wading type sampler used to collect bedload samples from a
stream of wading depth. The sampler consists of an expanding nozzle, a sampler bag, and a wading rod assembly.
Particle sizes less than 38 mm at mean velocities up to 3 m/s can be measured with this sampler. It was developed
by Reference [21]. Size of sampler: length: 711 mm, width: 140 mm, mass: 4,5 kg.
[5]
Figure 6 — US BLH-84 Wading type bedload sampler
8 © ISO 2015 – All rights reserved

NOTE The US BL-84 is a cable suspended 14,4 kg, sampler to collect bedload samples from a stream of any
depth. The sampler consists of an expanding nozzle mated to a frame, and a sampler bag. Particle sizes less than
38 mm at mean velocities up to 3 m/s can be measured with this sampler. It was developed by Reference [21]. Size
of sampler: length: 921 mm, width: 381 mm, mass: 14,4 kg.
[21]
Figure 7 — US BL-84 Cable suspended bedload sampler
7.1.6 Characteristics of bedload samplers
Since the sampling conditions encountered in streams vary widely, a single sampler for all conditions
cannot be recommended. Factors such as cost, availability, and specific requirements of the sampling
also influence the choice of the sampler to a great extent. Table 1, which summarizes the characteristics
of some commonly used samplers, can assist in the selection of a sampler in given conditions.
As the data obtained is affected by the sampling action and the mechanism of the sampler, any change
in the sampler would itself introduce a variable. Therefore, the results obtained from different samplers
might not be comparable.
Table 1 — Samplers commonly used for bedload measurement
Disturbance Acceptability to
Hydraulic Sampler
Type Description of flow various field
stability efficiency
characteristics conditions
Frame and Portable bedload trap with 0,3 m Near-bottom flow Anchored to Variable from −50 Suitable for sam-
net by 0,2 m opening and trailing velocity increases streambed. to +20 % pling coarse parti-
nylon net with 3,5 mm openings. by about 30 %. cles (greater than
4 mm) in streams
that are wadable at
high flow.
SRIH This is a pressure difference Efficiencies The sampler design
bedload sampler. extremely to measure parti-
variable cles as small as fine
sand or as large as
200 mm.
Table 1 (continued)
Disturbance Acceptability to
Hydraulic Sampler
Type Description of flow various field
stability efficiency
characteristics conditions
Arnhem Consists of a rigid rectangular Variable Variable About 70 % Generally
entrance connected by a diverg- restricted to
ing rubber neck to a basket of collection of fine
0,2 mm to 0,3 mm mesh fixed to bedload material
a large framework by springs in (2 mm); portable.
such a way that the entrance is
in contact with the bottom when
the sampler is lowered onto the
bed.
Helley - Tear-drop shaped, aluminium Intake Stable in Variable from Varying sizes, from
Smith tubing frame connecting expand- velocities are velocities about 100 % for hand-held wading
ing brass entrance to aluminium consistently higher up to gravel to more sampler to heavy
tailfins; aluminium tubing filled than ambient 3 m/s. than 150 % for sampler suspended
with lead for mass (weight); velocities. sand from cables; fairly
bedload particles are trapped in streamlined; port-
a polyester mesh bag attached able.
to exit.
Sphinx In this sampler the flow enters The hydraulic Sampling effi-
through a rectangular nozzle efficiency varies ciency varies
that gradually becomes circular, from about 1,09 for from about 93 %
then through a series of settling clear flow, to about for particle sizes
chambers, and then out a wide 1,0 for extreme finer than 0,2 mm
exit at the rear. conditions to about 85 % for
sizes finer than
about 0,09 mm
US BLH-84 The sampler is constructed of Mean veloc- Variable from The sampler design
wading type aluminium and is 711 mm long. ities up to near 100 % for enables collection
sampler Consists of an expanding nozzle, 3 m/s (this 11 mm to 32 mm of particle sizes
a sampler bag, and a wading rod velocity is particles to 125 % less than 38 mm at
assembly. The sampler has a higher than to 160% for finer mean velocities up
[10][11]
76-mm square entrance nozzle, safe wading particles to 3 m/s.
and nozzle and an area expan- velocities).
sion ratio (ratio of nozzle exit
area to entrance area) of 1:40. A
polyester mesh bag with mesh
openings of 0,25 mm is attached
to the rear of the nozzle assem-
bly with a rubber “O” ring.
US BL-84 The sampler consists of an Mean veloc- Variable from The sampler design
cable expanding nozzle mated to a ities up to near 100 % for enables collection
suspended frame, and a sampler bag. The 3 m/s. 11 mm to 32 mm of particle sizes up
sampler sampler has a 76-mm square particles to 125 % to 38 mm at mean
entrance nozzle and an area to 160% for finer velocities up to
[10][11]
expansion ratio of 1,40. The particles 3 m/s.
US BL-84 is constructed of
stainless steel and aluminium,
is equipped with tail fins, and is
921 mm long by 381 mm wide.
The sampler should be supported
by a steel cable and reel to be
lowered into a river or stream for
taking a bedload sample.
7.2 Measurement using bedload trap
7.2.1 Vortex tube bedload trap
The samplers consist of a 45 % diagonal slot in a concrete broad crested weir constructed across the
channel at the measurement site. A vortex is generated in the diagonal slot and from 5 % to 15 % of the
flow carries the bedload sediment to a trap on the side of the channel. The sediment is then weighed and
10 © ISO 2015 – All rights reserved

sampled and returned to the stream downstream of the weir (Robinson, 1962[18]; Milhous, 1973[15];
Tacconi and Billi, 1987[20]).
NOTE 1 This is a vortex tube bedload sampler designed by the Swiss Federal Institute of Technology Zurich.
The hydraulic tests showed that the principle of vortex tubes is suited for the extraction of transported sediment.
The results demonstrated extracting rates over 95 % under appropriate hydraulic conditions. The tube geometry
is dependent on sediment size, channel width, and economical aspects.
NOTE 2 Left: headrace channel (right) and residual flow reach (left), middle: types of the investigated vortex
tubes, right side: vortex tube cross-sections, arrows indicate the direction of flow.
Figure 8 — Vortex tube bedload trap
7.2.2 Pit and Trough trap
These samplers are used on small flashy streams where the bedload moves during a flood event. The
samplers are installed in the bed of the channel by burying the sampler so that the top is flush with
the surface of the bed. They consist of small containers that catch and retain all bedload sediment that
[10]
is transported to the sampler . The bedload is either removed and weighed after a flood event or
[13][16]
weighed continuously by a pressure pillow in the bottom of the trap . Another pit-type trap uses a
[6]
continuous conveyor belt, which carries the bedload to a weighing station on the stream bank .
Key
1 reinforced concrete outer box
2 steel inner box with slotted covers
3 pressure pillow
4 shelter for bubbler system
5 tubes from bubbler system to pillow
Figure 9 — Example of a pit and trough sediment trap that captures and weighs the sediment
transported as bedload during the measurement period (adapted from Reference [13] and
Reference [16])
7.2.3 Advantages and disadvantages
The bedload trap operates reliably on relatively small gravel-bed stream, but they are not portable and
the initial construction cost is high.
8 Procedures for measurement of bedload discharge using bedload samplers
8.1 General
Many problems in determining bedload discharge over the wide range of sediment and hydraulic
conditions found in nature have yet to be resolved. Among these problems, it should be noted that
a) quantification of physical relations is not complete enough to estimate the bedload discharge,
b) quantitative measurements are applicable only to specific site studies at the time of measurement,
and
c) direct measurement devices are useful for only a very limited range of sediment size and hydraulic
conditions.
As a result, no single apparatus or procedure has been universally accepted as completely adequate for
the determination of the bedload discharge over the wide range of sediment and hydraulic conditions
found in nature.
The type of sampler and the technique of sampling used will depend on a large number of factors namely,
stream velocity, depth, width, particle size, transport rate, channel stability, and bed configuration. The
transport rate of bedload not only changes from point to point in a cross-section but also exhibits widely
variable short-term and long-term fluctuations at a fixed point. These variations in the measurement of
12 © ISO 2015 – All rights reserved

bedload discharge mean that short-term measurements at a point are very likely to be non-representative
of the mean bedload discharge at that point. Therefore, each sampling point should be sampled many
times over an adequately long period in order to achieve any reasonable accuracy. The number of
sampling points in a cross-section is usually dependent on funding and manpower available. However, it
should be noted that the more points are sampled, the greater is the degree of accuracy.
The sampling time interval will be determined by the volume of bedload material in transport and the
capacity of the sampler used. Generally, the quantity of material collected should not exceed two-thirds
of the sampler capacity.
Among the potential problems inherent in the manual deployment of bedload samplers is the orientation
of the deployed bedload sampler with respect to flow direction, deployed sampler movement, and
inadvertent collection of bed material. A bedload sampler orientation other than directly upstream
may collect bedload from a stream section that is less than the nozzle’s width resulting in a systematic
negative bias in the capture of bedload.
Additionally, a bedload sampler that swings upstream as it is lowered to the bed can gouge into a bedform
and collect bed material that may be spuriously included in bedload. These problems tend to be most
[4]
prevalent in cable deployments. Use of a stayline and tetherline assembly minimizes or eliminates the
above-mentioned problems. This assembly enables the bedload sampler to be lowered vertically to the
bed and to be restrained from further movement.
Regardless of the method for deploying manual bedload samplers, without observing the bedload
sampler as it is deployed, one cannot be certain of the orientation or movement of the sampler once
on the bed, nor could one confirm or refute that the sampler collected bed material by gouging the
bed. When the deployed bedload sampler cannot be directly observed, affixing a video camera and
light source above and behind the bedload-sampler nozzle to provide video of bedload approaching the
nozzle can enable the operator to qualitatively assess of the reliability of the sample bedload collected.
8.2 Sample identification
In order to properly evaluate the bedload samples, the following items should be recorded on the
individual sampler container:
a) river name and location;
b) date of collection;
c) start time of collection;
d) cross-section location;
e) stationing on the cross section;
f) length of sampling time;
g) depth of water;
h) water temperature;
i) water discharge;
j) type of sampler used.
8.3 Calculations
The computation of bedload discharge from measurements made by direct methods employs the
Formula (1) which is applicable for all conditions for determining the total sediment discharge of a given
particle-size range:
TD= //eQ++QF−+QE1− eQ (1)
() ()
sM usM1 sM ts2
where
T is the total sediment discharge of the size range considered;
D is the discharge of the size range as measured with the bedload sampler; if the sampler
measures more than the bedload discharge, D includes some of the suspended-sediment
discharge; if the sampler measures only the bedload discharge, D = B (B being the bedload-
transport rate);
e is the efficiency of the bedload sampler in measuring the bedload discharge of the size
range;
Q is the measured suspended-sediment discharge of the size range. It equals the product of
sM
the total water discharge, a units-conversion constant, and the velocity-weighted mean
concentration in the sampled zone;
Q is the unmeasured suspended-sediment discharge of the size range at the depth between
usM1
the lowest point measured by the suspended-sediment sampler and the highest point meas-
ured by the bedload sampler. It equals the product of the water discharge at this depth, a
units-conversion constant, and the difference between the velocity-weighted mean concen-
trations in the sampled zone and at this depth;
F is the fraction of flow at the depth measured by the bedload sampler with respect to total
flow;
E is the efficiency of the bedload sampler in measuring the suspended-sediment discharge of
the size range that passes at the depth measured by the sampler;
Q is the total suspended-sediment discharge of the size range that passes at the depth meas-
ts2
ured by the bedload sampler.
Simplifications of Formula (1) can be made for different combinations of particle-size ranges (expressed
as bedload or suspended load), vertical distribution of the suspended-sediment concentration, and type
of bedload measuring apparatus. Table 2 shows the simplified formula for each combination of relevant
parameters.
14 © ISO 2015 – All rights reserved

Table 2 — Formulae for computing the total sediment discharge of a size range
Particle Type of Equivalent
size bedload
range measur-
Simplified formula
trans- ing
D/e Q Q F Q
sM usM1 ts2
ported appara-
a b
as tus
s W 0 Q 0 0 0 T = Q
sM sM
s Y (E/e) Q Q 0 F FQ T = Q
ts2 sM sM sM
s Z (E/e) Q Q 0 F FQ T = Q
ts2 sM sM sM
σ W 0 Q Q 0 0 T = Q + Q
sM usM1 sM usM1
c
σ Y (E/e) Q Q 0 F Q T = (D/e) + Q − FQ + (1 − E/e) Q
ts2 sM ts2 sM sM ts2
σ Z (E/e) Q Q Q F Q T = (D/e) + Q + Q − FQ + (1 − E/e)
ts2 sM usM1 ts2 sM usM1 sM
c
Q
ts2
β W B/e 0 0 0 0 T = (D/e)
β Y B/e 0 0 F 0 T = (D/e)
β Z B/e 0 0 F 0 T = (D/e)
β, s W B/e Q 0 0 0 T = (D/e) + Q
sM sM
β, s Y (B/e) + (E/e) Q 0 F FQ T = (D/e) + Q − (E/e) Q
sM sM sM ts2
Q
ts2
β, s Z (B/e) + (E/e) Q 0 F FQ T = (D/e) + Q − (E/e) Q
sM sM sM ts2
Q
ts2
β, σ W B/e Q Q 0 0 T = (D/e) + Q + Q
sM usM1 sM usM1
β, σ Y (B/e) + (E/e) Q 0 F Q T = (D/e) + Q − FQ + (1 − E/e) Q
sM ts2 sM sM ts2
Q
ts2
β, σ Z (B/e) + (E/e) Q Q F Q T = (D/e) + Q + Q − FQ + (1 − E/e)
sM usM1 ts2 sM usM1 sM
Q Q
ts2 ts2
a
β: bedload; s: suspended sediment having a uniform vertical distribution; σ: suspended sediment having a non-uniform
vertical distribution.
b
W: measures only bedload; Y: measures bedload plus suspended sediment in all of unsampled depth; Z: measures
bedload plus suspended sediment in part of unsampled depth.
c
Or Q + Q where Q is the unmeasured suspended-sediment discharge in unsampled depth.
sM usM usM
8.4 Errors
Bedload discharge is especially important during periods of extremely high discharge and in landscapes
of large topographical relief, where the river gradient is steep (such as in mountains). Measurement of
bedload is extremely difficult. Most bedload movement occurs during periods of high discharge on steep
gradients when the water level is high and the flow is extremely turbulent. Such conditions also cause
problems when making field measurements.
Despite many years of experimentation, sediment-monitoring agencies have so far been unable to devise
a standard sampler that can be used without elaborate field calibration or that can be used under a wide
range of bedload conditions.
Even with calibration, the measurement error can be very large because of the inherent hydraulic
characteristics of the samplers and the immense difficulty with representative sampling of the range of
sizes of particles in transit as bedload in many rivers.
Unless bedload is likely to be a major engineering concern (as in the filling of reservoirs), agencies should
not attempt to measure it as part of a routine sediment-monitoring programme. Where engineering
works demand knowledge of bedload, agencies shall acquire the sp
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