WATER RESOURCES RESEARCH, VOL. 36, NO. 9, PAGES 2711-2720, SEPTEMBER 2000
Flow and sediment transport measurements in a simulated
ice-covered
channel
M. Muste Iowa Institute of Hydraulic Research,Universityof Iowa, Iowa City
F. Braileanu Primatech, Phoenix, Arizona
R. Ettema Iowa Instituteof HydraulicResearchand Departmentof Civil and EnvironmentalEngineering Universityof Iowa, Iowa City
Abstract. Laboratoryflume experimentsconductedto illuminate flow field and sediment transportcharacteristics in ice-coveredsand-bedchannelsare discussed. The overall purposeof the experimentswas to examinehow level ice coversaffect flow distribution, flow turbulencecharacteristics, and sedimenttransportrates.The experimentswere conductedwith a nonrefrigeratedflume fitted with a sandbed and plywoodpanelsusedto simulatecover-floatingice covers.A discriminatorlaser-dopplervelocimeterwasusedto measurewater and sedimentparticlevelocitiessimultaneously. The additionof a cover and an increasein its roughness increasesflow depth and decreases bulk flow velocity, therebyreducingsedimenttransportratescomparedto thosein openwater flow. The Reynoldsshearstress,turbulenceintensities,and distributionof sedimentsize over the depth also change. river channel and leads to the following consequences: increasedflow depth, altered flow distribution,decreasedbulk Flow in an ice-coveredchanneltypicallyis fully developed, flow velocity,and reducedbed load transportratesrelativeto asymmetric,turbulentchannelflow.Flow asymmetryoccursby homologous parameters in equivalentopenwaterflows.Ice cover virtue of differenthydraulicroughnessof the top and bottom alsosignificantly changes the turbulencecharacteristics of channel boundariesof the channel. Turbulence generated at each flow and the local-scaledynamicsof sedimenttransport. boundarydispersesover the flow depth,with substantialmixing of turbulencefrom both boundariesoccurringover a central core of flow and with significantconsequences for sedi1.
Introduction
2. Experiments
ment transport.
The primary purposeof the flume experimentspresented herein was to obtain turbulence measurements and fundamen-
tal insightsinto flow and sedimenttransportbehaviorin channel flow as a surfacecoveris addedand roughened.The novel feature of the presentstudyis the simultaneousbut separate measurementof water and suspended-sediment velocities.A laser-basedinstrument,a discriminatorlaser-dopplervelocimeter (DLDV), previouslydevelopedat Iowa Institute of Hydraulic Research[Musteet al., 1996],was employedfor this purpose.The presentstudyis oneof few studieson the turbulencecharacteristics of asymmetricchannelflows[Hanjalicand Launder, 1972;Parthasarathy and Muste, 1994;Patel and Yoon, 1995;Smithand Ettema, 1995]. The experimentswere conductedwithin the context of a broader investigationof ice-coveredeffectson flow and sediment transport[Ettemaet al., 1999a].Sincemanyriversin the Northern Hemisphereare ice-coveredfor substantialperiods during winter, it is important to gain insightsinto ice-cover effectson flow distributionand sedimenttransportdynamics. An ice covernearlydoublesthe wetted perimeterof a typical Copyright2000 by the AmericanGeophysicalUnion. Paper number2000WR900168.
The geometryandmainvariablesfor a flowundera floating ice cover are shownin Figure 1. Throughoutthe paper the subscripts O and I denotebulk conditionsof open water and coveredflow,respectively. The subscripts b andi denotevalues pertaining to the bed and cover, respectively.Appendix A definesthe remainingnotation. The experiments were performed using a sedimentrecirculatingflume,30.0m long,0.91m wide,and0.45m deep. The flume hasglass-sided wallsto facilitateviewingand useof laser-basedtechniques.The flume flow rate was determined from orificeplatesin the return-flowpipes.The uncertaintyof
theflowratemeasurement was5 x 10-5 m3/s.Foursynchronizedscrew-driven jackslocatedat the endsand quarterpoints of the flume allow the flume to tilt about its midsection.
The
resolutionof the slopesettingwasabout5 x 10-4. Water surface elevationswere measuredusing eight piezometers spacedat 3.048-mintervalsalongthe flume and 0.065 m above the flume base. A vernier with resolution of 0.3 mm was used
to measurewater levelsin the piezometers.Linear regression of the water surfaceelevationprovidedan estimateof average water surfaceslopesfor whichthe total uncertaintywasabout 0.2%. A digitalthermometermeasuredthe water temperature with 0.1øC resolution.
0043-1397/00/2000WR900168509.00
Ice coverswere simulatedusingfree-floating,1.22-m-long, 2711
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Figure 1. Variablesinfluencingflow and sedimenttransport in an ice-coveredalluvial channel. Variables % and •i are shearstressesalongbed and ice cover,respectively;v is kinematicviscosityof water; 7s and 7 are specificweightsof sediment andwater,respectively; hi is coverthickness; andD5o is medianparticlesizeof bed sediment.
TRANSPORT
MEASUREMENTS
tensityfrom an auxiliaryphotodetector(the discriminator). Differentiation of the signalscoming from fine dust in the water and from sedimentparticleswasmadein postprocessing basedon the amplitudeof the incomingDoppler burst.Several nonintrusiveacoustically and opticallybasedinstrumentshave been recentlyused to capture separatelythe hydrodynamic characteristics of phasesin flowswith suspensions [e.g.,Bennett and Best,1995;Gillandtet al., 1997;Kafiori et al., 1995b;Modarress et al., 1984;Parthasarathy andFaeth,1990;Taniereet al., 1997],but the presentexperimentsappearto be the firstmeasurementsfor channel-covered flowscarryingalluvial sand. Directional ambiguityand directionalbiaswere eliminated by frequencyshifting.Velocitybiaswas eliminatedby operatingthe systemat highsignalratesandpropertime averaging of the processoroutput,as discussed by Durstet al. [1976].The gradientbiasdueto the finite sizeof the measurement volume was estimatedto be lessthan 1%. Experimentaluncertainties were calculatedusingthe analysisof Moj•ht [1982] and were found to be less than 3% for the streamwisemean-velocity measurements, 6% for root meansquare(RMS) velocityfluctuation measurements,and lessthan 10% for the Reynolds
0.9-m-wide,13-mm-thickplywoodpanels.Given the relatively smalllengthof the channelsimulated,it is reasonableto treat the coversas rigid. The bottom surfaceof the panelswas paintedfor the smoothcover.For the roughcovercondition shear stress measurements. the undersidesurfaceof the panelswaslinedwith rectangular2.2. Flow Conditions shapedwoodenstrips,12.5mm wide, 8.5 mm high,and 0.9 m Table 1 summarizes flow conditions used for one series of long, glued at 50.8-mmintervals. The sedimentbed compriseda uniform fine sand,with a experiments.A secondseries,run at steeperenergygradient, median diameterDs0 = 0.25 mm, geometricstandarddevi- producedessentiallythe sameresultsas did the seriespreationrra = 1.4 (sizerange0.075-0.45mm), and specific sentedherein.Ettema et al. [1999a]report both seriesof re-
gravitySg = 2.64.
sults. The flow conditions
2.1.
controlledsuchthat unit dischargeand sloperemainedconstant(operatorcontrol),but the meandepthandvelocityvaried (systemcontrol).Each seriesinvolvedfour runs,onefree-
Discriminator Laser-Doppler Velocimetry
The discriminatorlaser-dopplervelocimeter(DLDV), its arrangement,and operatingconditionsare fully describedby Musteetal. [1996].The mainfeaturesof the DLDV systemand its operating conditionsare briefly describedherein. The DLDV was basedon a three-beamlaser-dopplervelocimeter (LDV) set up to measuretwo of the three velocitycomponents. The size of the DLDV
measurement
volume was 0.650
mm in diameterand 4 mm in length.Positivedirectionsfor the measured horizontal and vertical componentsof velocities were downstreamand upward,respectively.Correctionswere not needed for the velocitycomponentsmeasuredwith the DLDV, becausethe instrumentwas set on a carriage that movedon rails parallel to the flume bottom. Dust and sedimentparticlesconveyedby the flow scattered the incominglaserlight. The light wasconvertedby two photomultipliersand sentto a pair of signalprocessors (IFA 550, TSI) signalprocessors, each for a velocitycomponent.The signalscatteredby dustwas characterizedby lower Doppler signalamplitudethan that producedby sedimentparticles.A datalinkmultichannelinterface(TSI Model DL100) facilitated simultaneous recordingof velocitiesfrom LDV and signalin-
for each series were selected and
surface flow and three covered flows with covers of different
roughness. The slopedifferenceamongthe equivalentflowsof a seriesdid not exceed2%, whereasthe water dischargedifference did not exceed1.8%. The relative roughnesses (k•,/
Yo,• and ki/Y•) and the prevailingvalue of the DarcyWeisbach
resistance
coefficient
for the flow conditions
are
givenin Table 2. Particleroughnesswas calculatedas k•, = 2Ds0 and was constantfor all experiments.Estimationof the sand-grainequivalentroughness for the coverswasmadeusing load-cell measurements of the shear force on the underside of
the covers[Smithand Ettema, 1995]. Table 2 also provides additionalparametersfor the testedflows. Each seriesof experimentsbeganwith the openwater flow over a level bed of sand.As the open water experimentprogressed,bed forms developed,increasingthe bed resistance andrequiringincreases in the flumeslopeto maintainuniform flow in the flume. Flume slopeadjustmentsoccurredat intervals of approximately2 hours,when and if measurements of the water surfaceslopeindicateda departurefrom the uniform flow.This cycleof flume slopeadjustmentcontinued,typically
Table 1. Summaryof ExperimentalConditions Bed
Run
Aspect Ratio
Slope S0,
Energy
Bulk
Reynolds Number
Froude
Discharge
Average Depth
SlopeS,
Velocity
UYo,•/v,
Number
Q, m3/s
YoJ,m
b/Yo.•
X10-4
XI0-4
U, m/s
XI04
$/(gYo,i)0'5
1.49 1.48 1.49
Open
0.0318
0.107
8.50
1.5
Smooth
0.0318
0.122
7.46
1.5
Rough
0.0318
0.143
6.36
1.5
0.327 0.286 0.244
3.06 3.05 3.05
0.32 0.26 0.20
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2713
Table 2. Additional Flow Characteristicsfor the Seriesof Experiments Bed Relative Roughness Run
kt,/Yo,•
Cover Relative Roughness
Nominal Friction Factor
Bed Shear Velocity
Cover Shear Velocity
Zero Reynolds Stress Elevation
Maximum Velocity Elevation
ki/YI
fo orfl
u.b, m/s
u. i, m/s
Y./Yo,i
Yv/Yo.1
---
Open
0.0047
0.096
0.0360
Smooth
0.0041
0.0001
0.077
0.0259
0.0131
Rough
0.0035
0.1382
0.122
0.0268
0.0239
for 2 or 3 days,until the boundaryresistanceand gravityforces reached a dynamic equilibrium, whereby the water surface sloperemainedequal to the flume slopewithin 2%. The experimentswith the smoothand rough coverswere conducted similarly,followingthe open water experiment. The bed forms for all the tests were in the ripple-dune regime,suchthat the bed surfacecomprisedan irregulararray of three-dimensional crests and troughs with fluctuations around the mean bed height of +2 cm (see Figure 2). As reported by Smith and Ettema [1995], a dune-bed channel respondsto cover presenceby increasingthe dune length, while dune celerity and steepnessreduce. Ripples superimposedon dunesremainedconstantin size. The water temperaturein the flume was kept at 20øC,with smallfluctuations,from experimentto experiment,of _+1.3øC. Temperaturecontrolwasmaintainedusinga water-chillersys-
..0.80 0.60
0.75 0.59
microscaleof turbulence)t, estimatedassuggested byNezuand Nakagawa [1993], varied from 0.47 to 0.64 mm. The ratio Dso/)t varied from 0.4 to 0.5. Gore and Crowe[1989] suggest that values greater than 0.1 for this ratio are attributable to turbulenceenhancementof the underlyingflow, whereasvalues less than 0.1 are associated with turbulence
attenuation.
The presentexperiments do not includemeasurements with dear water flowsto testthe validityof the abovementionedcriterion. Another important parameter in turbulencemodulation is
the Stokesnumber,St = 7p/71,where ?p = Vg/g is the particle relaxation time, ?l = )•/tt' is a representativetime
scaleof the flow[Bestet al., 1997],Vg is the particlesettling velocity,and u' is the RMS of the streamwisevelocitycomponent. The Stokesnumbersfor the flows ranged from 0.47 to 0.63, with valuesdecreasingawayfrom the bed. This rangefor the Stokes numbers is dose to the critical value, St = 1, used
tem, which recirculated water from the flume, and heater coils
by Gore and Crowe [1989] to delineate enhancementversus that were used only during wintertime. During each experi- attenuation of turbulenceproduced by sediment presence. ment, water temperaturerose mildly becauseof energydissi- However, severalerrorsmay be associatedwith calculationof pation in the system(flow, pumps,and piping);the rise was the flow scales,the most probable one being the use of anaabout + IøC by the end of experiment.Water temperaturewas lytical estimatesfor clearwater flowswithout cover. recordedevery5 min during the experiments. Sediment concentrationsfor all the experimentsdiscussed 2.3. Data Acquisition herein are small; volumetric concentrations are less than 4 x Velocity measurementswere made over a vertical transect
10-4;andparticlesizeswerelargerthanthedissipative length located at the flume's centerline, 18 m downstream from scaleof the turbulencefor eachflow investigated.The Taylor
flume'sinlet. DLDV measurements of flowvelocitywere made at elevationsabovethe relative elevationY/Yo or y/Y• = 0.2, which coincidedwith the averageelevationof bed form crests along the flume. Flow elevationsbelow this level were not accessiblefor velocity measurementbecausethe dunes and ripples obstructedthe laser beamsused for the DLDV. Bed form celerities along the flume were measuredprior to the DLDV measurements;the celeritieswere measuredalong the flumecenterlinefor representative (meanheightabout4-5 cm and wavelengthabout 75 cm) bed forms. DLDV measurementsalonga verticalprofilewere taken 3 timeswhile a representativebed form was passingunderneaththe probe. The resultsof the three velocityprofileswere averagedin postprocessing.For suchconditions,flow velocitiesabovethe traveling bed form are not directlyrelated by the relativepositionof the bed forms with respectto the DLDV measurementlocation. The measurementsare akin to those made to quantify the mean and turbulence
characteristics
of flow above the bound-
ary havinga uniform array of large-scaleroughnesselements (e.g., arraysof transverse ribs). DLDV processorswere operated in the coincidencemode with a time window of 100/xs to ensure that measurementsof both velocity componentswere made on the same particle. Velocity measurements were recordedin randommode over at Figure 2. View of discriminator laser-doppler velocimetry least 10,000 data points to yield significantstatistics.Postpro(DLDV) test sectionshowingthe siphonlocation and the cessingof the recordeddata wasconductedusinga filter of +2 samplestandarddeviations. DLDV measuringvolume.
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Suspended-sediment sampleswere withdrawnover a range of preestablished elevationsacrossthe flow depth.Ten successive samplesof 1 min each were made at each elevationto obtain the final data for the sedimentconcentrationprofile. During the samplingthe bed formsmoved,and the local flow conditionsaltered. Therefore the samplingprocedureproduced temporally and spatiallyaveragedconcentrationprofiles. Suspended-sediment concentrations were measuredby meansof a suspension-sampling tube,which consistsof a 4.5mm-ID siphonwithdrawalpipe. The samplerwasset about20 mm downstreamof the DLDV measuringvolume to not interfere with the DLDV measurements, as shownin Figure 2. The withdrawalpipewasfittedwith tubingand an intermediate valve.The valvewasusedto regulateflow into the nozzleto be at the local ambientvelocityof the flow at the nozzleelevation (isokineticsampling).The samplesof suspendedsediment were collected in glass containers.Sampleswere then decanted,oven dried, and weighted. Particle-diameterdistributionsof suspended-sediment samplescollectedwith the siphonwere determinedusinga microscope.The microscopewas fitted with a referencegrid of known spacing.Particlesin each samplewere counted, and
TRANSPORT
MEASUREMENTS
between the mean-water and sedimentvelocity) is larger nearer the bed. Figures 3-5 also present a velocityprofile obtained without discriminatingbetweenwater motion and sedimentmotion. The combinedprofile is labeled as "mixture." It is the profile of flow velocitythat wouldbe obtained usingconventionalLDV instrumentation. The mixtureprofile is closerto the profileof suspended-sediment velocityoverthe lowerregionof the flowbut almostcoincides with the profileof water velocityover the upper region of flow. This finding is explainedby the fact that the velocitymeasurements closerto the bed contain substantiallylarger concentrationsof suspendedbed sediment.It is obviousthat velocitiesinferredby the mixtureprofilesdo not entirelyrepresentthe velocitiesof water or sedimentin regionsnear a loosebed. This findingis importantbecausemostpublishedvelocitydatarepresentmixture velocityreportedaswater velocity. Velocityprofilesfor the flowconditionswith the smoothand rough covers(Figures4 and 5) reveal that the velocitylag increasesascoverroughness increases. As for openwater flow case,velocitylag increases towardthe bed,thoughthereseems to be no clear trend for it at elevations
above the elevation
of
maximumvelocity. The findingspresentedhere augment similar resultsreBed load measurementswere carried out usinga sediment ported in prior studiesconductedwith LDV-based systems trap set in a false-bedapron,locatedfour flumewidthsdown- employingphasediscrimination.Notably,Rashidiet al. [1990], streamof the suspended-sediment measurementlocation.The Kafioriet al. [1995b],MusteandPatel [1997],and Taniereet al. trap extendedacrossthe full width of the flume and captured [1997] showthat the mean velocitiesof suspended particles all of the bed material in transport.After the dynamicequi- and fluid differ, whether the fluid is air or water. Data from librium wasreached,three samplings were made. To obtain a thosestudiesshowthat in an outer regionof flow, the average measurement,the trap was coveredwith a steel plate and fluid velocitiesare higherthan thosefor suspended-sediment placedinto the falsebed. The coverwasremoved,a timer was particles,thatvelocitylagislargernearthe bed (wherea larger started, and bed material was collecteduntil the tray was al- velocitygradientexists),andthat thevelocitylagincreases with most full. The collection time varied from 10 to 30 min funcsedimentconcentrationand particlesize.However, data pretion of the sedimenttransportrate magnitude.The differences sentedby Kafiori et al. [1995b]and Taniereet al. [1997] also between the amounts of sediment collected for each test for a showthat closeto a flat bed (i.e., Y/Yo < 0.1), sediment given flow condition were within 2%. The quotient of the particlestravel fasterthan the surroundingfluid. Bed formsin collectedvolume of sedimentand time of collectiongivesthe the presentexperimentsprecludedmeasurements in the nearaveragebed load rate. wall region, and they probablyradicallyalter the flow field their dimensions
were estimated.
there too.
3.
Results
Before presentingthe results,it is usefulto reiteratethat the meanvelocitiesand turbulencevaluespresentedfor eachflow elevationare temporal-meanvaluesof the flow overa multiple bedform patch(through,back,andcrest)of loose-bedchannel beneath
a surface cover. The measurements
The reasonfor the mean velocitydifferencebetweenfluid and sedimentis not obvious.Kafiori et al. [1995b] speculate that the lower particlevelocitycomparedwith the flow canbe
reflect variations
attributableboth to bed form passageat the measurement location and flow turbulence.A similar experimentalprocedure was usedby Bennettet al. [1998].
0.8
3.1.
0.6
Mean Velocity Profiles
The impositionof a free-floatingcoverand the roughening of the coverundersidedecreasedthe bulk velocityand Froude numberof the flow. The plotsof the streamwisevelocityprofiles, normalizedwith the maximumwater velocityfor each flow caseand elevationsy and normalizedwith Yo or Y• for openor coveredflows,respectively, are presentedin Figures3, 4, and 5. It is evidentthat the relativeelevationof the velocity maximum
shifted downward
from the cover as the cover was
roughened. The common
characteristic
for all tests is that mean veloc-
©
rmxture
O
water
ß
sediment
oo •:•o ß) ß
0.2
0.0 0.6
o
O
• • I I I • • • • I • I I I I I I I • I 0.7
0.8
0.9
1.0
U/Umax
ities of suspended-sediment movementlag water velocitiesat Figure 3. Normalized mean velocityprofilesfor the open all flow elevations.In addition,the velocitylag (the difference water flow.
MUSTE
ET AL.: FLOW AND SEDIMENT
0.8
mixture
0
water
ß
sediment
ß
e¸
o
e©o
0.6
Yo,i; hereY, is the elevationof zeroReynolds stress. It is
•)o
ß ) •
0
evidentin Figure6 that for openwater flow,the straight-line profilefits the Reynoldsstressmeasurements reasonably well. Thisfindingindicatesthat fullyturbulentdevelopedflowpre-
o
o
o
ß
ß
2715
3.2.1. Reynoldsstresses. Profiles of Reynoldsstressfor the three casesare givenin Figure6. Straightlineswere fitted to the measuredprofileusingthe shearvelocityat the wall for 0 < y < Y, and the shearvelocityat the coverfor Y, < y
sediment
ß
water
[]
water
0
water
,,i,Ii•,•l,,,•l,•,,I,,,, 0.2
i
0.4
0.6
0.8
.0 0.0
0.2
0.4
y/To
Figure 9.
0.6
Y/Y• Streamwise
turbulence
intensities
0.8
.0
0.0
i
i
i I
0.2
i
i
i
i
I
0.5
i
i
0.4
• i i
0.6
y/Y/ for water and sediment.
i
i
i
,
i
0.8
i
i
i
I
0.0 1.0
2718
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,
,
Open
1.5
i
,
,
i
,
,
•
,
i
•
,
,
Smooth Cover
o
,
i
,
•
i
i
i
,
,
,
,
Rough Cover
1.5
DDDD D []
DD•
©
1.0
1.0
0.5
0.5
o
sediment
ß
water
[] sediment i
0.0
0.4
0.2
0.6
0.8
i
[] water i
1.0 0.0
i
I
i
i
i
i
0.2
I
0.4
Y/Yo
i
i
J i
I
i
i
i i
0.6
I
i
i
i
0.8
•> sediment ]
i i•i i water i ii] i ii iIiiii Iii ii
I
1.0
0.0
0.2
0.4
0.6
0.8
0.0
1.0
Y/Y/
Figure 10. Vertical turbulenceintensitiesfor water and sediment.
sediment were low, and therefore the amount of sediment in the size distributionof suspendedsedimentover flow depth with each samplewas small. Someusefulqualitativeobservations wasalsoobservedby Kaftoriet al. [1995b]in experiments can be ventured, however. monosizeparticlesof specificgravity1.03.They notedthat the The size distributionof suspendedsedimentwas measured concentration distribution becomes more uniform as sediment As largerparticles at four elevationsfor openwater and smooth-cover flowsand sizereducesand asshearvelocityincreases. three elevationsfor rough-coverflow. Practicalconstraintslim- settle faster and are less affected by turbulencefluctuations, ited sampling above the bed forms (below the elevation fewer large particlesappear in the upper elevationsof flow. Y/Yo,i • 0.2) for all flowcases. For the coveredflowscases, Smallerparticles,however,settle more slowly,and their moparticleconcentrations weretoo low to be analyzedforY/Yi > tion is much more influencedby turbulencerandomfluctua0.6. tions.Consequently, their presenceis sustainedin the upper Variations in sediment-sizedistributionacrossthe depth region of flow. were consideredin termsof the specificdiameterD i% , which showsthe i% by weight of sedimentfiner than a givensedi4. Conclusions ment size,notably,i = 15, 50, and 84. Figure 11 showsthis The addition of a cover and increasein its roughnessindistributionfor the open-channelflow experiments.It can be notedthat the smallersedimentis more likely to reachhigher creasesflow depth and decreasesbulk velocityof flow. These elevations in the flow. For the covered-flow conditions the effects cause a reduction in sedimenttransport rates and sediment-size distribution shifted farther toward the finer fractherebyin concentrations of suspended sedimentcomparedto tion of the bed sedimentwith increasingelevationof flow.The thosein open water flow. Increasingcoverroughnessfurther shiftwasmoreobviousfor the coarser-size fractions(i.e., Dso, reducedoverall concentrationsof suspendedsediment.The D84) thanfor the finer fraction(Dis). Figure13 illustratesthe DLDV measurementsshow the same general trends in the variationof concentrationin the sedimentfractionsD •s, Dso, meanvelocityprofilesfor water and sediment. The combinedeffects of fluid forces actingon sediment, and D84 throughoutthe depth. The trendsfollow the exponentialdistributionshownin Figure11. As expected,sediment sedimentconcentration,and water velocitygradientsall contributed to a lag betweenthe water and sedimentvelocities. near the bed is coarser(seeFigure 13, roughcover). The dependenceof the sedimentconcentrationprofile on Only a few other studiesgive similar insightsinto flow with suspendedsediment[e.g.,Kaftori et al., 1995a, 1995b;Muste andPatel,1997;Bestet al., 1997].However,the datapresented 1.0 in thosestudiesare for flat-bedopen water flows.
y/Yo= 0.63
ß
0.8
ß
d5o=0.208 mm []
[] []
ß d84 =0.297 mm
[]
ß
/
ß
[]
Smooth Cover
RoughCover
1.0
y/Yo= 0.29 d5o = 0.250 mm c/84 = 0.350 mm
[]
0.6
OpenChannel
[] ß
'
'
' ' ' ' ''[
'
'
' ' '2 '.:!
_
_
_
0.2
-
' '..'2 ,T
...L..Bed Load
-
0.4
_
_
0.2_
_
_
_
_
0.0 0.01
I I I llllll 0.1
I I I IIllll 1
I I I llllll
I I fillIll
10
100
] I IIIIIII
_
0.6
/
_
:
--o- Suspended Load
0.8
Dß
0.4--
'
Total Load[
I I I II1•'
1000
10000
_
0.0 0.0001
i
i
i
i i Illl
0.001
I
I
I
I Illll
I
0.01
i
I
! I ll,I
............
0.1
C (mg/1)
Figure 11. Sedimentconcentrationprofiles.
Figure 12. Sedimenttransportrates.
I
I
I I III
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i
i •1111
i
i
i
i iiiii
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i
i
i
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i l-
i
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i
i
• iiiIi
i
TRANSPORT
i
i
i illll
Flow
i
i
MEASUREMENTS
i
i
2719
i
i
i
i
i ill
i
i
i
i
i-
Rough Cover - 0.8
Smooth Cover
_
0.6
0.6
0.4 '-
0.4
_
0.1
-,-d•5•••------••• 0.2
+ d15
' + d15 ] • d84
0.2: -o-dSO
--o--d50
•
1
o. 1
I
I
--o--dS0
d84 t I Illll
I
•
1
C (mg/])
I •
I t IIIII
lO
d84 1
10
C (mg/])
C (mg/1)
Figure 13. Concentrationprofilesfor selectedsediment-size fractions.
The turbulencebehaviorinferredby the presentstudyis the result of the combined effects of a cover on a loose-bed flow,
the presenceof the sedimentin suspension,and sediment concentrationgradient.The trendsin the turbulencefluctuations determined
Appendix A Reynoldsstressis definedas
-u•, =
from the data are as follows:
1. NormalizedReynoldsshear stressesdecreasedin the near-bedregionand increasednear the top of the flow (i.e., near the cover)comparedto the openwater flow.The elevation of zero Reynoldsshearstresslies closerto the smoother surfacethan doesthe elevationof maximumvelocity.In a small central region the productionof turbulent kinetic energyis negative,and diffusioneffectsbecomesignificant.This lossof turbulenceenergyin the regionwhere the smoother-and the rougher-wallturbulencestructuresmix is attributableto the net interactionof Reynoldsstresses of oppositesigns.As the top surfacebecame rougher, the productionof turbulence closeto it becomeslarger.Thus significanttransportof turbulent kinetic energy and shear stress took place from the rougherto the smoothersurface. 2. Turbulenceintensitiesfor the streamwisecomponentof water flow increasedslightlynear the bed and near the cover undersidecomparedto open water flow. Increase in cover roughnessdecreasedturbulenceintensitiesnear the bed but had the reverseconsequence nearthe cover.The sametrends
n
• (5 - U)(3- V)
(A1)
i=1
where u and •, are the streamwise
and vertical velocities fluc-
tuationsat a point,respectively; U and V are the time-averaged streamwiseand vertical fluid velocities,respectively,over n measurements;and 5 and 3 are the instantaneousstreamwise
and verticalvelocitycomponents,respectively. Turbulence
intensities
for water and sediment
are the root-
mean-square(RMS) of the instantaneousvelocity components.They are calculatedas 1/2
u'=
[ln(a-u) ] ]1/2
(A2a)
i=1
1 n
1]! •
(A2b)
i--1
The shearvelocityu, can be calculatedby severalmethods (Clauser method, momentumbalance, and Reynoldsstress profile).It shouldbe notedthatthesemethodsarestrictlyvalid tical velocitycomponent. 3. Turbulenceintensitiesfor sediment(both streamwise for flowswithout sediment.Small deviationsare expectedfor dilute sedimentconcentrations.The followingmethodswere andverticalcomponents) were slightlyhigherthanfor waterin used in the presentpaper: all the cases measured.
were found for the turbulence
intensities associated with ver-
Sedimenttransportratesdecreasedwith coverpresenceand increasing coverroughness. However,the proportionof sedimenttransportin suspension increaseddespitethe decreasein total rate of sedimenttransport.This unforeseenfindinglikely relatesto changesin the behaviorof the large vortical structures in the outer layer. Variation of sedimentsizewith the elevationis yet a further complexity. The changesin the sizedistributionwith elevation have direct consequences on the modulationof turbulence. These observationsunderscore the current inadequacy of knowledgeon sedimententrainmentand suspension, evenfor simpleflowssuchasopenwaterflowwith suspended sediment only in dilute concentrations.
1.
From momentum balance in a uniform flow,
u, = x/#RS,
(A3)
whereSlstheenergy slope(equaltothebedslopeforuniform flow),# is the gravitationalacceleration, andR is the hydraulic radius. For a two-dimensionalfully developedopen channel
flowin a widechannelthe depthYo,/, canbe usedin placeof the hydraulicradius. 2. Assuminglinearvariationfor the total stress(molecular and turbulent)and considering the contributionof the molecular stressnegligible,the measureddistributionof Reynolds stressis extrapolatedto the bed to find the shearvelocity. For a fullydevelopedasymmetric channelflow(x axisin the
2720
MUSTE
ET AL.: FLOW
AND
SEDIMENT
streamwisedirectionandy axisin the verticaldirection)the mean streamwisemomentumequationreducesto
+
=u,2 -
- u*2 ro'
where•, is the kinematicviscosity of the fluid,Yo,• is the flow
depth,andu*bandu,• aretheshearvelocities atthebed(y = 0) and the top (y = Yo,D surfaces, respectively. The value u,• = 0 for thecaseof anopen-channel flow.In thecaseof a •o-dimensional flow with infinite aspectratio,
u + u = u = aSro.
(AS)
DefiningY, asthe heightof the planeof zerototal stressfrom the bed and settingthe right-handsideof (A4) equal to zero gives
Y,
u,2 b
Yo-• = u,2b + u,2, '
MEASUREMENTS
Ettema, R., F. Braileanu,and M. Muste, Flume experimentson flow and sedimenttransportin ice-coveredchannels,Tech. Rep. 404, Iowa Inst. of Hydraul.Res.,Univ. of Iowa, Iowa City, 1999a. Ettema, R., M. Muste, and S. Coleman, Flume notes on sediment transportand boil formationin ice-coveredchannels,in Proceedings of the IIHR XXVIII Biennial Congress[CD-ROM], Iowa Inst. of Hydraul. Res., Iowa City, 1999b. Gillandt,I., T. Schulze,U. Fritsching,andK. Bauchage,Simultaneous measurementof continuousand suspended phasein a two-phasejet flow,Flow Meas.Instrum.,8(3/4), 199-206, 1997. Gore, R., and C. T. Crowe, Effect of particle size on modulating turbulentintensity,Int. J. MultiphaseFlow, 15, 279-285, 1989. Hanjalic,K., and B. E. Launder,Fully developedasymmetricflow in a planechannel,J. Fluid Mech.,51, 301-335, 1972. Kaftori, D., G. Hetsroni, and S. Banerjee,Particle behaviorin the turbulentboundarylayer, I, Motion, deposition,and entrainment, Phys.Fluids,7(5), 1095-1106,1995a. Kaftori, D., G. Hetsroni, and S. Banerjee, Particle behavior in the turbulentboundarylayer,II, Velocityanddistributionprofiles,Phys. Fluids,7(5), 1107-1121,1995b. Modarress,D., H. Tan, and S. Elgobashi,Two-component LDA measurementin a two-phaseturbulentjet, A/AA J., 22(5), 624-630,
1984. (A6) Moffat, R. J., Contributionsto the theoryof single-sample uncertainty
Using (A6), (A4) canbe rewrittenas
=u,2
TRANSPORT
,
(A7)
whichis similarto the streamwisemomentumequationfor the caseof an open-channel flow,with the flow depthreplacedby Y,.
analysis, J. FluidsEng., 104, 250-258, 1982. Muste,M., andV. C. Patel,Velocityprofilesfor particleandliquidin open-channel flow with suspendedsediment,J. Hydraul. Eng., 123(9), 742-751, 1997. Muste, M., R. N. Parthasarathy,and V. C. Patel, Discriminatorlaserdopplervelocimetryfor measurementof liquid and particlevelocities in sediment-ladenflows,Exp. Fluids,22, 45-56, 1996. Nezu, I., and H. Nakagawa,Turbulence in Open-Channel Flows,A. A. Balkema, Brookfield, Vt., 1993.
Parthasarathy,R. N., and G. M. Faeth, Turbulencemodulationin Previousandpresentmeasurements showedthat the viscous homogeneousdilute particle-ladenflows,J. Fluid Mech., 220, 485-
514, 1990. term in (A7) is negligible;hencethe heightof the planeof zero R. N., and M. Muste,Velocitymeasurements in asymtotal shearstressis the same as that of zero Reynoldsstress. Parthasarathy, metricturbulentflows,J. Hydraul.Eng.,120(9), 1000-1020,1994. Using this last assumptionin combinationwith the measured Patel, V. C., and J. Y. Yoon, Applicationof turbulencemodelsto Reynoldsstressdistributionand Y, determinedfrom measureseparatedflow over rough surfaces,J. Fluids Eng., 117, 234-241, 1995. ments(estimatedfrom the least squaresfits to the measureinterments), the value of the shear velocity at the bed can be Rashidi,M., G. Hetsroni, and S. Banerjee,Particle-turbulence determined.
actionin a boundarylayer,Int. J. MultiphaseFlow,16, 935-949, 1990. Reynolds,A. J., TurbulentFlowsin Engineering, JohnWiley,New York, 1974.
Seal, C. V., and C. R. Smith, Visualization of mechanism for three-
Acknowledgments.The authorsthank the U.S. Army Corps of dimensionalinteractionand near-walleruption,J. Fluid Mech.,394, Engineers Cold Regions Research and Engineering Laboratory 193-203, 1999. (CRREL) and the National Sedimentation Laboratory(NSL) of the Smith, B. T., and R. Ettema, Ice-cover influence on flow and bedload Agriculture ResearchService,U.S. Department of Agriculture, for transportin dune-bedchannels,Rep. 374, Iowa Inst. of Hydraul. fundingprovidedin supportof this study.The constructiveobservaRes., Univ. of Iowa, Iowa City, 1995. tionsof the reviewersare greatlyappreciated. Taniere, A., B. Oesterle, and J. C. Monnier, On the behavior of solid particlesin a horizontalboundarylayer with turbulenceand saltation effects,Exp.Fluids,23, 463-471, 1997. References Tatinclaux,J.-C., and M. Gogus,Asymmetricplaneflowwith application to ice jams,J. Hydraul.Eng.,109(11),1540-1554,1983. Bennett, S. J., and J. L. Best, Mean flow and turbulence structure over fixed, two-dimensionaldunes:Implicationsfor sedimenttransport F. Braileanu, Primatech, 2929 North 44th Street, Phoenix, AZ and bedformstability,Sedimentology, 42, 491-513, 1995. Bennett,S. J., J. S. Bridge,andJ. L. Best,Fluid and sedimentdynamics 85018.(florin_braileanu.email.msn.com) R. Ettema and M. Muste, Iowa Institute of Hydraulic Research, of upper stageplane beds,J. Geophys.Res.,103(C1), 1239-1274, 1998. Universityof Iowa, Iowa City, IA 55242.(
[email protected]; Best, J. L., S. J. Bennett, J. S. Bridge, and M. Leeder, Turbulence
[email protected]) modulationand particlevelocitiesover flat sandbedsat low transport rates,J. Hydraul.Eng., 123(12), 1118-1129,1997. Colombini,M., and G. Parker, Longitudinalstreaks,J. Fluid Mech., 304, 161-183, 1995.
Durst, F., A. Melling, and J. H. Whitelaw,Principlesand Practiceof Laser-Doppler Anemometry, Academic,SanDiego, Calif., 1976.
(ReceivedAugust2, 1999;revisedMay 8, 2000; acceptedMay 19, 2000.)
