ASTM D4411-03(2008)
(Guide)Standard Guide for Sampling Fluvial Sediment in Motion
Standard Guide for Sampling Fluvial Sediment in Motion
SIGNIFICANCE AND USE
This guide is general and is intended as a planning guide. To satisfactorily sample a specific site, an investigator must sometimes design new sampling equipment or modify existing equipment. Because of the dynamic nature of the transport process, the extent to which characteristics such as mass concentration and particle-size distribution are accurately represented in samples depends upon the method of collection. Sediment discharge is highly variable both in time and space so numerous samples properly collected with correctly designed equipment are necessary to provide data for discharge calculations. General properties of both temporal and spatial variations are discussed.
SCOPE
1.1 This guide covers the equipment and basic procedures for sampling to determine discharge of sediment transported by moving liquids. Equipment and procedures were originally developed to sample mineral sediments transported by rivers but they are applicable to sampling a variety of sediments transported in open channels or closed conduits. Procedures do not apply to sediments transported by flotation.
1.2 This guide does not pertain directly to sampling to determine nondischarge-weighted concentrations, which in special instances are of interest. However, much of the descriptive information on sampler requirements and sediment transport phenomena is applicable in sampling for these concentrations, and 9.2.8 and 13.1.3 briefly specify suitable equipment. Additional information on this subject will be added in the future.
1.3 The cited references are not compiled as standards; however they do contain information that helps ensure standard design of equipment and procedures.
1.4 Information given in this guide on sampling to determine bedload discharge is solely descriptive because no specific sampling equipment or procedures are presently accepted as representative of the state-of-the-art. As this situation changes, details will be added to this guide.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. Specific precautionary statements are given in Section 12.
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Designation: D4411 − 03(Reapproved 2008)
Standard Guide for
Sampling Fluvial Sediment in Motion
This standard is issued under the fixed designation D4411; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope D3977Test Methods for Determining Sediment Concentra-
tion in Water Samples
1.1 This guide covers the equipment and basic procedures
forsamplingtodeterminedischargeofsedimenttransportedby
3. Terminology
moving liquids. Equipment and procedures were originally
developed to sample mineral sediments transported by rivers 3.1 Definitions:
but they are applicable to sampling a variety of sediments
3.1.1 isokinetic—a condition of sampling, whereby liquid
transportedinopenchannelsorclosedconduits.Proceduresdo
moves with no acceleration as it leaves the ambient flow and
not apply to sediments transported by flotation.
enters the sampler nozzle.
1.2 This guide does not pertain directly to sampling to 3.1.2 sampling vertical—an approximately vertical path
determine nondischarge-weighted concentrations, which in
from water surface to the streambed.Along this path, samples
specialinstancesareofinterest.However,muchofthedescrip- are taken to define various properties of the flow such as
tive information on sampler requirements and sediment trans-
sediment concentration or particle-size distribution.
port phenomena is applicable in sampling for these
3.1.3 sedimentdischarge—massofsedimenttransportedper
concentrations, and 9.2.8 and 13.1.3 briefly specify suitable
unit of time.
equipment. Additional information on this subject will be
3.1.4 suspended sediment—sediment that is carried in sus-
added in the future.
pension in the flow of a stream for appreciable lengths of time,
1.3 The cited references are not compiled as standards;
being kept in this state by the upward components of flow
howevertheydocontaininformationthathelpsensurestandard
turbulence or by Brownian motion.
design of equipment and procedures.
3.1.5 For definitions of other terms used in this guide, see
1.4 Information given in this guide on sampling to deter-
Terminology D1129.
mine bedload discharge is solely descriptive because no
3.2 Definitions of Terms Specific to This Standard:
specific sampling equipment or procedures are presently ac-
3.2.1 concentration, sediment—the ratio of the mass of dry
ceptedasrepresentativeofthestate-of-the-art.Asthissituation
sediment in a water-sediment mixture to the volume of the
changes, details will be added to this guide.
water-sediment mixture. Refer to Practice D3977.
1.5 This standard does not purport to address all of the
3.2.2 depth-integrating suspended sediment sampler—an
safety concerns, if any, associated with its use. It is the
instrument capable of collecting a water-sediment mixture
responsibility of the user of this standard to establish appro-
isokinetically as the instrument is traversed across the flow;
priate safety and health practices and determine the applica-
hence, a sampler suitable for performing depth integration.
bility of regulatory limitations prior to use. Specific precau-
3.2.3 depth-integration—a method of sampling at every
tionary statements are given in Section 12.
point throughout a sampled depth whereby the water-sediment
2. Referenced Documents
mixture is collected isokinetically to ensure the contribution
from each point is proportional to the stream velocity at the
2.1 ASTM Standards:
point. This method yields a sample that is discharge-weighted
D1129Terminology Relating to Water
over the sampled depth. Ordinarily, depth integration is per-
1 formed by traversing either a depth- or point-integrating
This guide is under the jurisdiction ofASTM Committee D19 on Water and is
the direct responsibility of Subcommittee D19.07 on Sediments, Geomorphology, sampler vertically at an acceptably slow and constant rate;
and Open-Channel Flow.
however, depth integration can also be accomplished with
Current edition approved Oct. 1, 2008. Published November 2008. Originally
vertical slot samplers.
approved in 1984. Last previous edition approved in 2003 as D4411–03. DOI:
10.1520/D4411-03R08.
3.2.4 point-integratingsuspended-sedimentsampler—anin-
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
strument capable of collecting water-sediment mixtures isoki-
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
netically. The sampling action can be turned on and off while
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. the sampler intake is submerged so as to permit sampling for a
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D4411 − 03 (2008)
specified period of time; hence, an instrument suitable for the sediment will require an excessive number of samples.
performing point or depth integration. Gaging and sampling stations should not be located at sites
where there is inflow or outflow. In rivers, sampling during
3.2.5 point-integration—a method of sampling at a fixed
floods is essential so access to the site must be considered.
point whereby a water-sediment mixture is withdrawn isoki-
Periods of high discharge may occur at night and during
netically for a specified period of time.
inclement weather when visibility is poor. In many instances,
3.2.6 stream discharge—the quantity of flow passing a
bridges afford the only practical sampling site.
given cross section in a given time. The flow includes the
mixture of liquid (usually water), dissolved solids, and sedi- 5.4 Sampling frequency can be optimized after a review of
ment.
the data collected during an initial period of intensive sam-
pling. Continuous records of water discharge and gauge height
4. Significance and Use
(stage)shouldbemaintainedinanefforttodiscoverparameters
4.1 This guide is general and is intended as a planning that correlate with sediment discharge, and, therefore, can be
guide. To satisfactorily sample a specific site, an investigator used to indirectly estimate sediment discharge. During periods
must sometimes design new sampling equipment or modify of low-water discharge in rivers, the sampling frequency can
existing equipment. Because of the dynamic nature of the usually be decreased without loss of essential data. If the
transport process, the extent to which characteristics such as sediment discharge originates with a periodic activity, such as
massconcentrationandparticle-sizedistributionareaccurately manufacturing, then periodic sampling may be very efficient.
represented in samples depends upon the method of collection.
5.5 The location and number of sampling verticals required
Sedimentdischargeishighlyvariablebothintimeandspaceso
at a sampling site is dependent primarily upon the degree of
numerous samples properly collected with correctly designed
mixing in the cross section. If mixing is nearly complete, that
equipment are necessary to provide data for discharge calcu-
is the sediment is evenly and uniformly distributed in the cross
lations. General properties of both temporal and spatial varia-
section, a single sample collected at one vertical and the water
tions are discussed.
discharge at the time of sampling will provide the necessary
data to compute instantaneous sediment-discharge. Complete
5. Design of the Sampling Program
mixing rarely occurs and only if all sediment particles in
5.1 The design of a sampling program requires an evalua-
motion have low fall velocities. Initially, poor mixing should
tion of several factors. The objectives of the program and the
be assumed and, as with sampling any heterogeneous popula-
tolerable degree of measurement accuracy must be stated in
tion, the number of sampling verticals should be large.
concise terms. To achieve the objectives with minimum cost,
care must be exercised in selecting the site, the sampling 5.6 If used properly, the equipment and procedures de-
frequency, the spatial distribution of sampling, the sampling scribed in the following sections will ensure samples with a
equipment, and the operating procedures. high degree of accuracy. The procedures are laborious but
manysamplesshouldbecollectedinitially.Ifacceptablystable
5.2 A suitable site must meet requirements for both stream
3 coefficients can be demonstrated for all anticipated flow
discharge measurements and sediment sampling (1). The
conditions, then a simplified sampling method, such as pump-
accuracy of sediment discharge measurements are directly
ing, may be adopted for some or all subsequent sampling.
dependent on the accuracy of stream discharge measurements.
Streamdischargeusuallyisobtainedfromcorrelationsbetween
6. Hydraulic Factors
stream discharge, computed from flow velocity measurements,
the stream cross-section geometry, and the water-surface el-
6.1 Modes of Sediment Movement :
evation (stage). The correlation must span the entire range of
6.1.1 Sediment particles are subject to several forces that
discharges which, for a river, includes flood and low flows.
determine their mode of movement. In most instances where
Therefore, it is advantageous to select a site that affords a
sediment is transported, flow is turbulent so each sediment
stable stage-discharge relationship. In small rivers and man-
particle is acted upon by both steady and fluctuating forces.
made channels, artificial controls as weirs can be installed.
The steady force of gravity and the downward component of
These will produce exceptionally stable and well defined
turbulent currents accelerate a particle toward the bed. The
stage-discharge relationships. In large rivers, only natural
force of buoyancy and the upward components of turbulent
controls ordinarily exist. Riffles and points where the bottom
currents accelerate a particle toward the surface. Relative
slope changes abruptly, such as immediately upstream from a
motionbetweentheliquidandtheparticleisopposedbyadrag
natural fall, serve as excellent controls. A straight uniform
force related to the fluid properties and the shape and size of
reach is satisfactory, but the reach must be removed from
the particle.
bridge piers and other obstructions that create backwater
6.1.2 Electrical charges on the surface of particles create
effects.
forces that may cause the particles to either disperse or
5.3 A sampling site should not be located immediately
flocculate. For particles in the submicron range, electrical
downstream from a confluence because poor lateral mixing of
forces may dominate over the forces of gravity and buoyancy.
6.1.3 Transport mode is determined by the character of a
particle’s movement. Clay and silt-size particles are relatively
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
this standard. unaffected by gravity and buoyant forces; hence, once the
D4411 − 03 (2008)
particles are entrained, they remain suspended within the body 6.2 Dispersion of Suspended Sediment:
of the flow for long periods of time and are transported in the
6.2.1 The various forces acting on suspended-sediment
suspended mode.
particles cause them to disperse vertically in the flow. A
6.1.4 Somewhat larger particles are affected more by grav-
particle’s upward velocity is essentially equal to the difference
ity.Theytravelinsuspensionbuttheirexcursionsintotheflow
between the mean velocity of the upward currents and the
arelessprotractedandtheyreadilyreturntothebedwherethey
particle’s fall velocity. A particle’s downward velocity is
become a part of the bed material until they are resuspended.
essentially equal to the sum of the mean velocity of the
6.1.5 Still larger particles remain in almost continuous
downward currents and the particle’s fall velocity.As a result,
contact with the bed.These particles, termed bedload, travel in
there is a tendency for the flux of sediment through any
aseriesofalternatingstepsinterruptedbyperiodsofnomotion
horizontal plane to be greater in the downward direction.
when the particles are part of the streambed.The movement of
However, this tendency is naturally counteracted by the estab-
bedload particles invariably deforms the bed and produces a
lishment of a vertical concentration gradient. Because of the
bed form (that is, ripples, dunes, plane bed, antidunes, etc.),
gradient, the sediment concentration in a parcel of water-
that in turn affects the flow and the bedload movement. A
sediment mixture moving upward through the plane is higher
bedload particle moves when lift and drag forces or impact of
than the sediment concentration in a parcel moving downward
another moving particle overcomes resisting forces and dis-
through the plane. This difference in concentration produces a
lodgestheparticlefromitsrestingplace.Themagnitudesofthe
netupwardfluxthatbalancesthenetdownwardfluxcausedby
forces vary according to the fluid properties, the mean motion
settling. Because of their high fall velocities, large particles
and the turbulence of the flow, the physical character of the
have a steeper gradient than smaller particles. Fig. 1 (2) shows
particle, and the degree of exposure of the particle.The degree
(for a particular flow condition) the gradients for several
of exposure depends largely on the size and shape of the
particle-size ranges. Usually, the concentration of particles
particle relative to other particles in the bed-material mixture
smaller than approximately 60 µm will be uniform throughout
and on the position of the particle relative to the bed form and
the entire depth.
other relief features on the bed. Because of these factors, even
6.2.2 Turbulent flow disperses particles laterally from one
in steady flow, the bedload discharge at a point fluctuates
bank to the other. Within a long straight channel of uniform
significantly with time.Also, the discharge varies substantially
cross section, lateral concentration gradients will be nearly
from one point to another.
symmetrical and vertical concentration gradients will be simi-
6.1.6 Within a river or channel, the sizes of the particles in
lar across the section. However, within a channel of irregular
transport span a wide range and the flow condition determines
cross section, lateral gradients will lack symmetry and vertical
themodebywhichindividualparticlestravel.Achangeinflow
gradients may differ significantly. Fig. 2 (3) illustrates the
conditions may cause particles to shift from one mode to the
variability within one cross section of the Rio Grande.
other.
6.2.3 Sediment entering from the side of a channel slowly
6.1.7
...
This document is not anASTM standard and is intended only to provide the user of anASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation:D 4411–98 Designation:D4411–03 (Reapproved 2008)
Standard Guide for
Sampling Fluvial Sediment in Motion
This standard is issued under the fixed designation D4411; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This guide covers the equipment and basic procedures for sampling to determine discharge of sediment transported by
moving liquids. Equipment and procedures were originally developed to sample mineral sediments transported by rivers but they
are applicable to sampling a variety of sediments transported in open channels or closed conduits. Procedures do not apply to
sediments transported by flotation.
1.2 This guide does not pertain directly to sampling to determine nondischarge-weighted concentrations, which in special
instances are of interest. However, much of the descriptive information on sampler requirements and sediment transport
phenomena is applicable in sampling for these concentrations, and 9.2.8 and 13.1.3 briefly specify suitable equipment.Additional
information on this subject will be added in the future.
1.3 The cited references are not compiled as standards; however they do contain information that helps ensure standard design
of equipment and procedures.
1.4 Information given in this guide on sampling to determine bedload discharge is solely descriptive because no specific
samplingequipmentorproceduresarepresentlyacceptedasrepresentativeofthestate-of-the-art.Asthissituationchanges,details
will be added to this guide.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory
limitations prior to use. Specific precautionary statements are given in Section 12.
2. Referenced Documents
2.1 ASTM Standards:
D1129 Terminology Relating to Water
D3977Practice for Determining Suspended-Sediment Concentration in Water Samples 3977 Test Methods for Determining
Sediment Concentration in Water Samples
3. Terminology
3.1 Definitions:
3.1.1 isokinetic—a condition of sampling, whereby liquid moves with no acceleration as it leaves the ambient flow and enters
the sampler nozzle.
3.1.2 sampling vertical—an approximately vertical path from water surface to the streambed. Along this path, samples are
taken to define various properties of the flow such as sediment concentration or particle-size distribution.
3.1.3 sediment discharge—mass of sediment transported per unit of time.
3.1.4 suspended sediment—sediment that is carried in suspension in the flow of a stream for appreciable lengths of time, being
kept in this state by the upward components of flow turbulence or by Brownian motion.
3.1.5 For definitions of other terms used in this guide, see Terminology D1129.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 concentration, sediment—the ratio of the mass of dry sediment in a water-sediment mixture to the volume of the
water-sediment mixture. Refer to Practice D3977.
3.2.2 depth-integrating suspended sediment sampler—an instrument capable of collecting a water-sediment mixture isokineti-
cally as the instrument is traversed across the flow; hence, a sampler suitable for performing depth integration.
This guide is under the jurisdiction of ASTM Committee D-19 on Water and is the direct responsibility of Subcommittee D19.07 on Sediments.
Current edition approved Dec. 10, 1998. Published March 1999. Originally published as D4411–84. Last previous edition D4411–93.
This guide is under the jurisdiction of ASTM Committee D19 on Water and is the direct responsibility of Subcommittee D19.07 on Sediments, Geomorphology, and
Open-Channel Flow .
Current edition approved Oct. 1, 2008. Published November 2008. Originally approved in 1984. Last previous edition approved in 2003 as D4411–03.
ForreferencedASTMstandards,visittheASTMwebsite,www.astm.org,orcontactASTMCustomerServiceatservice@astm.org.ForAnnualBookofASTMStandards
, Vol 11.01.volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D4411–03 (2008)
3.2.3 depth-integration—amethodofsamplingateverypointthroughoutasampleddepthwherebythewater-sedimentmixture
iscollectedisokineticallytoensurethecontributionfromeachpointisproportionaltothestreamvelocityatthepoint.Thismethod
yields a sample that is discharge-weighted over the sampled depth. Ordinarily, depth integration is performed by traversing either
a depth- or point-integrating sampler vertically at an acceptably slow and constant rate; however, depth integration can also be
accomplished with vertical slot samplers.
3.2.4 point-integrating suspended-sediment sampler—an instrument capable of collecting water-sediment mixtures isokineti-
cally.The sampling action can be turned on and off while the sampler intake is submerged so as to permit sampling for a specified
period of time; hence, an instrument suitable for performing point or depth integration.
3.2.5 point-integration—a method of sampling at a fixed point whereby a water-sediment mixture is withdrawn isokinetically
for a specified period of time.
3.2.6 stream discharge—the quantity of flow passing a given cross section in a given time. The flow includes the mixture of
liquid (usually water), dissolved solids, and sediment.
4. Significance and Use
4.1 This guide is general and is intended as a planning guide. To satisfactorily sample a specific site, an investigator must
sometimesdesignnewsamplingequipmentormodifyexistingequipment.Becauseofthedynamicnatureofthetransportprocess,
the extent to which characteristics such as mass concentration and particle-size distribution are accurately represented in samples
dependsuponthemethodofcollection.Sedimentdischargeishighlyvariablebothintimeandspacesonumeroussamplesproperly
collected with correctly designed equipment are necessary to provide data for discharge calculations. General properties of both
temporal and spatial variations are discussed.
5. Design of the Sampling Program
5.1 Thedesignofasamplingprogramrequiresanevaluationofseveralfactors.Theobjectivesoftheprogramandthetolerable
degree of measurement accuracy must be stated in concise terms. To achieve the objectives with minimum cost, care must be
exercised in selecting the site, the sampling frequency, the spatial distribution of sampling, the sampling equipment, and the
operating procedures.
5.2 Asuitable site must meet requirements for both stream discharge measurements and sediment sampling (1). The accuracy
of sediment discharge measurements are directly dependent on the accuracy of stream discharge measurements. Stream discharge
usually is obtained from correlations between stream discharge, computed from flow velocity measurements, the stream
cross-section geometry, and the water-surface elevation (stage). The correlation must span the entire range of discharges which,
for a river, includes flood and low flows. Therefore, it is advantageous to select a site that affords a stable stage-discharge
relationship. In small rivers and man-made channels, artificial controls as weirs can be installed.These will produce exceptionally
stableandwelldefinedstage-dischargerelationships.Inlargerivers,onlynaturalcontrolsordinarilyexist.Rifflesandpointswhere
thebottomslopechangesabruptly,suchasimmediatelyupstreamfromanaturalfall,serveasexcellentcontrols.Astraightuniform
reach is satisfactory, but the reach must be removed from bridge piers and other obstructions that create backwater effects.
5.3 A sampling site should not be located immediately downstream from a confluence because poor lateral mixing of the
sediment will require an excessive number of samples. Gaging and sampling stations should not be located at sites where there
isinfloworoutflow.Inrivers,samplingduringfloodsisessentialsoaccesstothesitemustbeconsidered.Periodsofhighdischarge
may occur at night and during inclement weather when visibility is poor. In many instances, bridges afford the only practical
sampling site.
5.4 Sampling frequency can be optimized after a review of the data collected during an initial period of intensive sampling.
Continuous records of water discharge and gauge height (stage) should be maintained in an effort to discover parameters that
correlate with sediment discharge, and, therefore, can be used to indirectly estimate sediment discharge. During periods of
low-water discharge in rivers, the sampling frequency can usually be decreased without loss of essential data. If the sediment
discharge originates with a periodic activity, such as manufacturing, then periodic sampling may be very efficient.
5.5 Thelocationandnumberofsamplingverticalsrequiredatasamplingsiteisdependentprimarilyuponthedegreeofmixing
in the cross section. If mixing is nearly complete, that is the sediment is evenly and uniformly distributed in the cross section, a
single sample collected at one vertical and the water discharge at the time of sampling will provide the necessary data to compute
instantaneous sediment-discharge. Complete mixing rarely occurs and only if all sediment particles in motion have low fall
velocities. Initially, poor mixing should be assumed and, as with sampling any heterogeneous population, the number of sampling
verticals should be large.
5.6 If used properly, the equipment and procedures described in the following sections will ensure samples with a high degree
of accuracy. The procedures are laborious but many samples should be collected initially. If acceptably stable coefficients can be
demonstrated for all anticipated flow conditions, then a simplified sampling method, such as pumping, may be adopted for some
or all subsequent sampling.
Discontinued; see 1994 Annual Book of ASTM Standards , Vol 11.02.
The boldface numbers in parentheses refer to the list of references at the end of this standard.
D4411–03 (2008)
6. Hydraulic Factors
6.1 Modes of Sediment Movement:
6.1.1 Sedimentparticlesaresubjecttoseveralforcesthatdeterminetheirmodeofmovement.Inmostinstanceswheresediment
is transported, flow is turbulent so each sediment particle is acted upon by both steady and fluctuating forces. The steady force of
gravity and the downward component of turbulent currents accelerate a particle toward the bed. The force of buoyancy and the
upward components of turbulent currents accelerate a particle toward the surface. Relative motion between the liquid and the
particle is opposed by a drag force related to the fluid properties and the shape and size of the particle.
6.1.2 Electrical charges on the surface of particles create forces that may cause the particles to either disperse or flocculate. For
particles in the submicron range, electrical forces may dominate over the forces of gravity and buoyancy.
6.1.3 Transport mode is determined by the character of a particle’s movement. Clay and silt-size particles are relatively
unaffected by gravity and buoyant forces; hence, once the particles are entrained, they remain suspended within the body of the
flow for long periods of time and are transported in the suspended mode.
6.1.4 Somewhat larger particles are affected more by gravity. They travel in suspension but their excursions into the flow are
less protracted and they readily return to the bed where they become a part of the bed material until they are resuspended.
6.1.5 Still larger particles remain in almost continuous contact with the bed. These particles, termed bedload, travel in a series
of alternating steps interrupted by periods of no motion when the particles are part of the streambed. The movement of bedload
particlesinvariablydeformsthebedandproducesabedform(thatis,ripples,dunes,planebed,antidunes,etc.),thatinturnaffects
the flow and the bedload movement. A bedload particle moves when lift and drag forces or impact of another moving particle
overcomes resisting forces and dislodges the particle from its resting place. The magnitudes of the forces vary according to the
fluid properties, the mean motion and the turbulence of the flow, the physical character of the particle, and the degree of exposure
of the particle. The degree of exposure depends largely on the size and shape of the particle relative to other particles in the
bed-material mixture and on the position of the particle relative to the bed form and other relief features on the bed. Because of
these factors, even in steady flow, the bedload discharge at a point fluctuates significantly with time. Also, the discharge varies
substantially from one point to another.
6.1.6 Within a river or channel, the sizes of the particles in transport span a wide range and the flow condition determines the
mode by which individual particles travel. A change in flow conditions may cause particles to shift from one mode to the other.
6.1.7 For transport purposes, the size of a particle is best characterized by its fall diameter because this describes the particle’s
response to the steady forces in the transport process.
6.2 Dispersion of Suspended Sediment:
6.2.1 The various forces acting on suspended-sediment particles cause them to disperse vertically in the flow. A particle’s
upward velocity is essentially equal to the difference between the mean velocity of the upward currents and the particle’s fall
velocity. A particle’s downward velocity is essentially equal to the sum of the mean velocity of the downward currents and the
particle’s fall velocity. As a result, there is a tendency for the flux of sediment through any horizontal plane to be greater in the
downward direction. However, this tendency is naturally counteracted by the establishment of a vertical concentration gradient.
Because of the gradient, the sediment concentration in a parcel of water-sediment mixture moving upward through the plane is
higher than the sediment concentration in a parcel moving downward through the plane.This difference in concentration produces
a net upward flux that balances the net downward flux caused by settling. Because of their high fall velocities, large particles have
a steeper gradient than smaller partic
...
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