Journal of Sedimentary Research, 2007, v. 77, 197212Research ArticleDOI: 10.2110/jsr.2007.017MEANDER-BENDEVOLUTION,ALLUVIALARCHITECTURE,ANDTHEROLEOFCOHESIONINSINUOUSRIVERCHANNELS:AFLUMESTUDYJEFFPEAKALL,1PHILIPJ.ASHWORTH,2ANDJAMESL.BEST11EarthandBiosphereInstitute,SchoolofEarthandEnvironment,UniversityofLeeds,WestYorkshire,LS29JT,U.K.2DivisionofGeography,SchooloftheEnvironment,UniversityofBrighton,Sussex,BN24GJ,U.K.e-mail:j.peakall@earth.leeds.ac.ukABSTRACT: Realistic physical models of meandering rivers have proven extremely difficult to produce, particularly incomparisontotheformationofbraidedriversinthelaboratory. Hereweaddressthequestionof whysuchrealisticmodelmeanders are so difficult to reproduce, through the most realistic physical modeling of meandering channels yet achieved. Thispaper demonstrates that cohesion is a key variable in the development and maintenance of single-thread channels. In particular,cohesion must be sufficient to force the planform away from a braided state but low enough for active migration to continue andfor the avoidance of a gradual reduction and eventual cessation of planform movement (ossification).The enhanced realism ofthe experiments alsoenables the processes of meander evolution, andcriticallythe resultant alluvial architecture, tobeexaminedinaphysicalmodelforthefirsttime.Planformhistorycanbelinkedtodeposits,andthisprocessproductlinkageenablesthedepositional development of theexperimental depositstobecomparedagainst, andtotest, existingmodelsofbeddinggeometrieswithinpointbars.Herewedocumentthreemechanismsforbendcutoff,providenewprocessexplanationsforcertainmodesofbendevolutionincoarse-grainedmeanderingrivers, examinethegeometriesandspatial distributionofalluvial architecture, anddemonstrate that existingmodels of point-bar geometrysuccessfullyreproduce the larger-scaleaspectsofpoint-baraccretioninriversdominatedbyepisodicunit-baraccretion.INTRODUCTIONSignificant advances inthemodelingof river meanderinghavebeenmade in recent years. Theoretical models have been developed thatsimulatesuccessfullytheevolutionof meander planforms (e.g., Parker1976; Fredse1978; Ikedaet al. 1981; JohannessonandParker 1989),flowstructure withinmeander bends (e.g., SmithandMcLean1984;Nelson and Smith 1989a, 1989b; Hodskinson and Ferguson 1998),sediment sorting around meander bends (e.g., Parker and Andrews 1985;Bridge 1992), long-terminteractions of the channel planformwithfloodplaindeposits (e.g., Howard1984, 1996; Sunet al. 1996, 2001a,2001b), and the sedimentary architecture of meandering rivers (e.g.,Bridge 1978; Willis 1993; Gross and Small 1998). However, thesetheoretical advances have not been matched by laboratory studies, whereself-formed meandering channels with significant sinuosity have beenremarkablydifficulttoproduce(Paola2001).Althoughlow-sinuosity experimental channels withsubmergedbarshave beenreproduced(e.g., Friedkin1945; WolmanandBrush1961;Ackers1964), themeanderswidenandeventuallyevolveintoabraidedstate(Zimpfer1975; Schummetal. 1987, p.150; Parker1998). Indeed,Paola(2001) has arguedthat intheabsenceof vegetationall physicalmodeling experiments of river channels using noncohesive materialseventuallyresult inabraidedplanform. Several studieshaveproducedmorestablemeandering-channelplanformsbyutilizingclay-richorveryfine-grained (410 mm) sediment (Schummand Khan 1972; Jin andSchumm 1987; Smith 1998), with two studies forming moderate- to high-amplitude meanders (Jin and Schumm1987; Smith 1998) and oneshowing evidence of bend regeneration (Smith 1998). However, when runfor asufficient periodof time, thesestudies reveal atendencyfor themeanders to reach a point where further channel migration ceases(ossification of the planform) (Schumm and Khan 1972; Jin and Schumm1987; Smith 1998). The limitations in the existing studies of meander bendformation raise important questions as to why realistic physical models ofmeanderingriversaresohardtoproduce,andthereforewhatismissinginsuchlaboratoryexperiments.Asaconsequenceoftheselimitations,physicalmodelsofmeanderingchannels have not been used to study the detailed process of migration ofmeander bends andevolutionofpoint bars.Similarly,despitethewealthof information on the deposits of modern and ancient meanderingchannels (e.g., Campbell andHendry 1987; Willis 1993; Bridge et al.1995), physical models havenot beenusedtoexaminethedeposits ofmeandering rivers and their formative processes. This is in markedcontrast to physical modeling studies of braided river planform evolution(e.g., Ashmore 1982, 1991a, 1991b; Ashworth1996; Warburton1996;Shvidchenko and Kopaliani 1998) and depositional architecture andprocesses (Ashworthet al. 1994; Ashworthet al. 1999; Peakall 1995;Peakalletal.1996;Moretonetal.2002).This paper reports on a series of physical model experiments ofareduced-scale analogue of agravel-bedmeanderingriver, whichuseascaledsediment distributionfromanatural prototype. The physicalmodel allows point-bar growth and meander-bend evolution to bedescribedinunprecedenteddetail, andisthefirstexperimental studytopermit examinationof the preservedalluvial architecture that canberelatedtothechannelevolution.CopyrightE2007,SEPM(SocietyforSedimentaryGeology) 1527-1404/07/077-197/$03.00EXPERIMENTALPROCEDUREAgeneric modeling approach is adopted herein with the aimofrecreatingareasonableprocesssimilaritytoaprototyperiverorclassofrivers (Eaton and Church 2004). Key dimensionless hydraulic parametersinthe model (see Table1), suchas the flowReynolds number (Re),Froudenumber(Fr),grainReynoldsnumber(Re*),andWebernumber(We), were kept within a range of values that would ensure similarity withnatural rivers. The grain-size distributionof the model sediment wasbased on the River Rheidol, Wales (Lewin 1978; Lewin, personalcommunication), whose bulk grain size was estimated by combining datafromthepoint-bar andbank-topdepositsataratioof90:10,inordertoreflect the dominance of within-channel deposition. Because of the limitednumberofcommerciallyavailablesedimentsizepopulationswithwhichto prepare the experimental grain-size mix, the final model grain size hada close, but not exact, grain-size distribution compared to the fieldprototype (Fig.1). The model: prototype D50 ratio was 1:17 (Fig.1), andsilica flour, with a grain size between 1 and 20 mm, was used for the fine-grainedtail of thedistributiontorepresent silt tomediumsandintheprototype.Thesilicaflourusedisaffectedbysignificantcapillaryforcesand weak van der Waals forces, which mimic prototype cohesionproduced by clays and vegetation (Peakall et al. 1996), and its white colorwas a useful indicator of deposition of fine-grained sediment in the model.For coarse-grainedmeanderingrivers, Froudenumbers arepredom-inantly subcritical, but localized supercritical flow does periodically occur(Lewin1976;Teisseyre1977;Gustavson1978).FlowReynoldsnumbersareinthefullyturbulentregime(.2000),andgrainReynoldsnumbersshowminimal viscousforces(correspondingtomodel Re*intherange1570; Yalin 1971; Parker 1979; Ashworth et al. 1994; Peakall et al. 1996).Weber numbers, whichexpress the influence of surface tensionforcesrelative to inertial forces, should be within or above the probable range ofcritical values (10120; Peakall andWarburton1996). Values of thesedimensionless variables in the experiments, along with other modelparameters (Table1), illustrate that the model satisfies these broadcriteria.In terms of how closely a model replicates a natural system, this genericmodelingapproach, withitsemphasisonmaintainingkeydimensionlessvariableswithinplausibleboundariesandscalingthegrain-sizedistribu-tion, ismorerealisticthananalogueexperiments(e.g., Hooke1968; vanHeijst et al. 2001) or unscaled experiments (e.g., Heller et al. 2001; Sheets etal. 2002; Hicksonetal. 2005)andisclosertothegeneric-Froudescalemodeling that has beenusedwidely instudying braidedrivers (e.g.,Ashmore1982, 1991a, 1991b; Ashworth1996; Warburton1996; Shvid-chenkoandKopaliani1998;Moretonetal.2002).However,parameterssuchas the valley slope, waterdischarge, and channel dimensions arenotscaled from a prototype in the approach adopted herein.Theexperimentswereundertakenina3.7-m-wide, 5.5-m-longtiltingflume that recirculated water and the finest-grained sediment, but not thecoarser-grainedsediment (Fig.2; see Ashworthet al. 1994for furtherdetails). The flume was set at aninitial bedslope of 0.01, andfourseparate runs were undertaken at a constant water discharge(0.5160.04Ls21). Although time-varying discharge may be importantfortheplanformevolutionofnaturalrivers,previousphysicalmodelinghas satisfactorily reproduced many aspects of river dynamics andmorphology using a constant discharge (e.g., Leopold and WolmanFIG. 1.Grain-sizedistributionsforthephysicalmodelexperimentsandforthemeanderinggravel-bedRiverRheidol(Lewin1978).Grain-sizedistributionsfortheRheidolpoint-bargravelsandbank-topdeposits(takenfromfig.5,curvesAandE,ofLewin1978)arecombinedtogiveabulkdistribution,withtheassumptionthatthebank-topdepositsaccountforapproximately10%ofthedistribution(seetextfordetails).TABLE 1.Experimentalconditionsandhydraulicdataformeanderingchannels.Variable Mean MinimumMaximumNumberofReadingsWatertemperature,T,(uC) 21.1 18.2 23.4 203Dynamicviscosity, m,(mkgm21s21) 976 924 1048 203Meanflowvelocity,U,(ms21)a0.30 0.07 0.46 159Channelbedslope,Sb,b0.008 0.005 0.010 61Waterdepth,d,(mm)c14.6 4.0 28.0 32FlowReynoldsnumber,Re,d4488 - - -GrainReynoldsnumber,Re*,de35.7 - - -Froudenumber,Fr,dQ0.79 - - -Webernumber,We,dg18.0 - - -acalculatedfromsurfacevelocitiesmultipliedby0.8.bBasedonfloodplainslopedividedbychannelsinuosity.cAllvaluesrepresentmaximumchanneldepths.dMeanvaluesarecalculatedfromthecorrespondingmeaninputparameters.Note: becausewaterdepthistakenasthemaximumvaluemeasuredinacrosssection, ReisslightlyoverestimatedandFrisslightlyunderestimated,relativetousingmeanchanneldepth.eRe* is calculated using the D90 of the grain size distribution following standardprocedure,seePeakalletal.(1996).QLocalizedsupercriticality was observedthroughthe periodic formationofstandingwavesgWe 5rU2d/s,wheres5thesurfacetensioncoefficient.198 J.PEAKALLETAL. JSR1957; Ashmore 1982; Smith 1998; Paola 2001). Sediment was fedcontinuouslyintoa2.4-m-long, 0.2-m-wideinlet channel, positionedatthe center of the upstream edge of the stream table, and the input rate wasadjusted to prevent aggradation or degradation of the channel. Thedepositionanderosiondescribedhereinwas thus purelythe result oflateralmigrationofthemeanderbends.Monitoringofthewater-surfaceheight at the input andoutput points confirmedthat changes inbedheight were limited to less than 3mm (i.e., much smaller than the scale ofthechanneldepthandassociatedbarforms).All fourexperimentscommencedwithalevel floodplainsurfaceandastraightchanneldredgeddownthecenteroftheflume.Experiments1and2hadarectangularinitial channel crosssection, 0.15mwideand20mm deep, giving a width:depth (w:d) ratio of 7.5, whereas runs 3 and 4startedwithatrapezoidalcrosssection,0.15and0.10mwideatthetopand bottom respectively, with 45u edges and a maximum depth of 25mm,givingaw:dratioof6.Sedimentandwaterwereintroducedviaafixedcurved, feeder channel that was set at anangle of 30u tothe initialstraight, dredged channel at the flume entrance. The angled feederchannelwasusedtoaccelerateinitialmeander-benddevelopment,ashasbeensuccessfullyachievedinotherexperimental studies(e.g., SchummandKhan1972; Zimpfer 1975; JinandSchumm1987). The imposeddischarge filled the channels to within a few millimeters of the floodplainsurface,andtherewasnooverbankflowontothefloodplain.Measurements of channel cross-sectional morphology, and boththalweg and floodplain elevation, were made using a point gauge(precision60.5mm) fixedtoamovablebridgespanningthewidthofthe flume. Changes inchannel planformwere tracedfromrecordingsusing a video camera operating at 3 frames per second, positioned directlyabovethecenteroftheflume.Measurementsofthepositionsofchannelbanks and bar morphology (e.g., bar edges and scroll bars) wereconcentratedtowardsthecenterofthevideoframe,wherephotographicdistortionwas minimal. Flowvelocitywas measuredbydigitizingthepathways of surfacefloats onthevideorecords andmultiplyingthesevelocities by0.8inorder toconvert tomeandepth-averagedvelocity(Matthes1956; Leopoldetal. 1964; Graf1998). Wheneachexperimentwas halted, after 15 to 34hours, the flume was drained and the sedimentwas allowed todry over several days.Sections werethen cutthrough themeander-benddepositsat100mmintervals, perpendiculartothemeandownstreamflowdirection, andcolor photographs were takenof thepreservedsedimentsusingalarge-format(50mm340mm)camera.MEANDERINITIATIONANDPOINT-BARDEVELOPMENTAsinglebendandassociatedsubmergedbank-attachedbardevelopedinitially at theupstream end oftheflume, asflow from theangledfeederchannel interacted with the straight main channel. Sediment movement inFIG. 2.Cross-sectional and planform views of the experimental setup. Water and fine-grained suspended sediment were recirculated, whilst coarse-grained sedimentwastrappedinasettlingtank.BENDEVOLUTION,ARCHITECTURE,ANDTHEROLEOFCOHESIONINRIVERS 199 JSRthe rest of the straight channel was limited to migration of very low-relief,straight-crestedbedforms (cf. Fujita1989; Ashmore1982, 1991b), andthere was no development of a sequence of alternate submerged bars thathas beennotedinother studies of meander-bendevolutioninstraightchannels (e.g., Ackers andCharlton1970; Ashmore 1982; Lisle et al.1991; Bridge 1993a). The absence of an initial series of migrating alternatebars can be attributed to the channel width:depth ratios (,10), which arebelowthe critical limit for bar initiationandmigration(WhitingandDietrich 1993; Tubino et al. 1999; Pittaluga et al. 2001) and which inhibitinitialbedloadtransportandalternate-barformationinthedownstreampartoftheflume. Thesubmergedbank-attachedbarintheinitial bendincreasedinlengthandwidthas thebendincreasedinamplitudeandunderwent downstream translation. During this bend growth the channelwidened,thereby increasing thewidth:depth ratio. This isa forced barin the terminology of Kinoshita and Miwa (1974), since the bar formed asadirectresultofchannelcurvature. Developmentofthesubmergedbarinto a point bar occurred through aggradation of relatively coarse-grained sediment at the bar head (cf. Lisle et al. 1991) and the addition ofsuccessive unit bars bylateral accretion(Fig.3). As point-bar growthcontinued,achutechanneleventuallyformedbetweenthepointbarandthe inner bank (labeled a in Fig.3). Fine-grained silica flour,transported predominantly in suspension, settled both on the barplatformandalong the inner chute channel, withdepositioncausingelevationof the uppermost part of the point bar almost tothe watersurface. At this stage water movement over the upper point bar wasminimal. Subsequent meander migrationandchannel widening, whilstcausing minimal changes in elevation, was sufficient to cause exposure ofpartsoftheupperpointbar.Duringtheprocessofinitial point-bargrowthintheupstreambend,a second bend began to develop downstream through growth ofa submerged bar. Further bends developed successively in the samemanner until five or six bends were present along the entire flume length.Once a point bar was formed, subsequent development occurred throughaccretionofpartsofdownstream-migratingunitbars(e.g., Smith1978;Ashmore1982;Bridge1985,2003;termedlinguoidbarsbyLewin1978).Thesemigratingbarsincorporatedsediment erodedfromtheupstreamconcave bank into the downstream convex bar (i.e., erosion anddepositionfromthesamesideofthechannel), indicatingthatpoint-bargrowth was controlled dominantly by along-stream transport anddeposition(cf. Friedkin1945; Smith1998). Prominent scroll bars andassociated ridge-and-swale topography formed at the downstream end ofthepointbars,andrepresentedthehigherpartsoftheunitbars(Fig.4;cf.Ashmore1982,hisfig.18;Bridge1993a,hisfig.1a).Themarginsofthese unit bars were commonly at oblique angles to the mean flowdirectionandassociatedpoint-baredge(Fig.4). Intheinitial stagesofpoint-bardevelopment,andafterchutecutoffwhenthebendshadalowamplitude,unitbarsmigratedaroundthewholepointbar.However,asbend amplitude increased, the unit bars repeatedly stalled at or justupstreamof the bend apex (labeled b in Fig.3), leading to theformationofbar-headlobesthroughaccretionofunitbars.Thereisastrongsimilaritybetweentheprocessof point-bargrowthobservedinthepresentphysicalmodelandthemovementandaccretionof linguoid bars in the River Rheidol (Lewin 1978, his figs. 2 and 3), theupper parts of which were incorporated into the point-bar, forming scrollbars. Inthe River Rheidol, fine-grainedsandaccretes ontopof theFIG. 3.Photographofpointbarandchannel inthephysical model. Flowistowards the viewer, and the channel width at the top of the photo is approximately0.15m. The point bar is composed of several scroll bars (labeled c), with the higherparts of the bar beingupto, or above, the local water-surface level. Achutechannel ispresent ontheinnerpoint baradjacenttothechannel bank(a). Themostrecentlyaccretedunitbaronthepointbar(b)hasa darkercolorbecauseoftheabsenceofdepositionofwhitesilicaflourfromsuspension.FIG. 4.Photographofscrollbarsandassociatedridge-and-swaletopographyintheabsenceofflow.Viewlookingupstream.Themeasuringruleisapproximately23inchesinlength.Thepoint-bar(a)isundergoingchutecutoff (b), and scroll bars are recognized by theircurvilinear ridges. The older scroll-bar swales arepartiallyinfilledbyfine-grainedsilicaflour.Aunitbarwithsteepslipfaces(c)isbeginningtoaccreteontothedownstreampointbar.Thisimagewastakenattheendofanexperiment.Notehowasinglelargebendhasdevelopedbecause the input point is fixed and consequentlycannottranslatedownstream.200 J.PEAKALLETAL. JSRgravelsandwithinchutechannels,therebyelevatingthepointbartothelevel of the main floodplain(Lewin 1978; Blacknell 1982). Point-bargrowthbyaccretionofunitbarshasalsobeennotedinotherfieldandexperimentalstudies(e.g.,Ashmore1982;FergusonandWerritty1983).EVOLUTIONOFMEANDERBENDSTheinitial point-bar formationandmeander development describedabove were associated with increasing bend amplitude and sinuosity(Figs.5A,6A),andsuchbendexpansionisacommonformofmeanderdevelopment inbothsand-bedandgravel-bedrivers(e.g., Daniel 1971;Lewin1976; Hooke1977; LewinandBrindle1977; HookeandHarvey1983). After thisphase ofexpansion, meander development proceeded inanumberofdifferentways.ShoalingoftheUpstreamMeanderLimbandChuteCutoffUnit bars were observed to stall just upstreamof the bend apex,producingabar-headdepositandassociatedshallowingoftheupstreammeanderlimb(Fig.5B). Asaresult of theshoaling, flowwasdirectedacross the point bar, therebyproducingerosionandthe transport ofcoarse-grainedbedloadacross thetopof thepoint bar(Fig.5C). Thiscoarse-grainedbedloadproducedbar-headlobes throughaccretionofunitbars, whichweresubsequentlydissectedbyfurthererosion,leadingtoeventualchutecutoff(Fig.5D).Boththeprocessesofshoalingofthebendentryzone,andtherapiddepositionofcoarse-grainedsedimentonpointbars,havebeenidentifiedingravel-bedriverspriortochutecutoff(e.g.,Carson1986;Ashmore1991b;LuntandBridge2004).Analysisofthe predominantly gravel-bed rivers of Wales and the Welsh Borderlands(Lewis andLewin1983), includingthe prototype River Rheidol, alsodemonstratesthatsuchchutecutoffsarecommon.ChannelBlockingandChuteCutoffduetoBar-HeadDepositionatBendApicesIn a number of experimental meanders, bend expansion continued untilthe channel became partially blocked at the bend apex by bar-headdeposits fromthe front of a unit bar, producing local upstreamshallowing of the channel thalweg andcorresponding flowexpansion(Fig.6B). This bar-head deposition resulted in localized increased erosionof the outer bank and produced a distinct notch in the bank, andacorrespondingchangeinchannel alignment. Suchchangesinchannelalignmentresultedinupstreammigrationofthechanneldownstreamofthe apex, leading to increased bend asymmetry (Fig.6C). In all cases, thebends eroded into the homogeneous original floodplain sediments, whichwereseveralmillimetershigherthanthemeanflowdepth,andthereforethebendsmayhavebeenaffectedbyalimiteddegreeof confinement.Shallowing of the upstreammeander limb, together with increasingtortuosityof flowaroundthebend, directedflowontothepoint bar,subsequentlyleadingtoerosion, transport of pulses of coarse-grainedsedimentontothepoint-bar,andeventualchutecutoff(Fig.6D).Many of these processes have also been observed in gravel-bedmeanderingrivers, includinglocalizederosionoftheouterbank(Lewin1972; Hooke and Harvey 1983; Carson 1986), increasing bend asymmetrythroughtheupstreammigrationofthedownstreammeanderlimb(e.g.,Lewin1972, 1983; LewinandBrindle 1977; HookeandHarvey1983;Carson1986), andpartial channel blockingatbendapicesbybar-headdeposition (Teisseyre 1977). Furthermore, the physical model is similar tothe prototype River Rheidol, which displays upstreammovement ofdownstreammeander limbs associated with bends that are partiallyconfined, andareasofrapidlocalizedouter-bankerosion(Lewin1972,1978,1983;LewinandBrindle1977).Bend asymmetry is a widely recognized feature of river channels, and hasbeenlinkedtotheinertiaoftheflowdelayingbendinflection, althoughsuchasymmetryisnotlinkedtoupstreammovementofthedownstreammeander limbs (e.g., Kondratyev 1968; Hickin 1974; Carson and Lapointe1983; Parker et al. 1983, Furbish 1991). Two mechanisms have beensuggested for this upstreammeander-limb movement in gravel-bedchannels: (i)impingementagainstvalleysidesinconfinedchannels, and(ii) anincrease inmeander pathlengthleadingtoplanformdistortion(Lewin1972; LewinandBrindle 1977; Hooke andHarvey1983). Thestalling of unit bars at bend apices observed herein offers a newexplanationfor increased bend asymmetry in some gravel-bed meandering rivers.ChangesinUpstreamFlowOrientationLeadingtoChuteCutoffChute cutoff was observed to occur through changes in the orientation ofthe upstream flow, predominantly due to upstream chute cutoff (Fig.7A,B). Changes in upstream bend orientation due to channel erosion also ledtoflowbeingfocusedintopreexistingsloughchannels andacross theassociatedpointbar,withsubsequentlateralinfilling andpluggingoftheoriginal channel (Fig.7B). Chutecutoff developedthrougherosionandredistributionof the deposits,firstly asachute bar into the main channeland subsequently in the form of a bar-head lobe on the downstream pointbar, followed by further erosion and eventual cutoff (Fig.7B). Multi-loopchute cutoffs were produced where the change in upstream flow orientationwas initiated by upstreamchute cutoff, and such cutoffs have beenobserved in gravel-bed meandering rivers (Bridge et al. 1986) including theprototype River Rheidol (Lewis and Lewin 1983).PositionofChuteCutoffandSubsequentBendDevelopmentForallthreemechanismsdetailedabove,chutecut-offcouldoccuratany point across the point bar, althoughthe topographic lowof theinnermost slough was the most frequent location for cutoff. The course ofthenewchannelcouldalsobeassociatedwithlowareasadjacenttoanybar-headlobe or scroll bar. Inall cases, the abrupt increase inbendwavelength, and the corresponding decrease in bend sinuosity after chutecutoff,wasfollowedbyarapidexpansionofthebendandrepetitionofthe sequence of bend development. Due to the limited length of the flumeandthefixedinletpoint,whichpreventeddownstreamtranslationoftheinputbend, meanderwavelengthandamplitudeincreaseduntil asinglebend occupied the entire length of the flume. The experiments werestoppedat,orpriorto,thisstage.ALLUVIALARCHITECTURE:DEPOSITIONALELEMENTSSectioning of the preserved sediments allows the alluvial architecture ofthese model rivers tobe studiedandlinkeddirectlytothe formativeprocesses.Thislinkagebetweenprocessandproductisoftendifficultinfieldstudies, althoughcombinationsof trenches, cores, airphotos, andgeophysicaltechniquessuchasground-penetratingradarcanbeutilized,albeit withlesstemporal, andinsomecasesspatial, resolutionthanintheseexperiments.Fourdepositionalelementswererecognizedfromthetwo-dimensional cross sections on the basis of stratal structure, grain-sizevariations, and contacts with underlying and overlying sediments (Fig.8).Thesedepositional elements bear manysimilarities tothosedefinedinpast studies of gravel-bed meandering alluvium (e.g., Arche 1983; Forbes1983) and in past flume work investigating braided alluvium (Ashworth etal.1994; Ashworthetal. 1999;Peakalletal. 1996;Moretonetal. 2002;Sheetsetal.2002).Large-ScaleInclinedStrataandLateral-AccretionSurfacesSets of large-scale inclined strata, with lateral-accretion surfacesseparatingtheindividualstrata,areidentifiedbyvariations ingrainsize.Each stratumhas relatively coarse-grained material at its base withamuchthinnerfiner-grainedunitatitstop,withtheuppermostsurfaceBENDEVOLUTION,ARCHITECTURE,ANDTHEROLEOFCOHESIONINRIVERS 201 JSR202 J.PEAKALLETAL. JSRrepresentingthe lateral accretionsurface (Fig.8A). The angles of thelarge-scale inclinedstrata variedbetween3u and30u, withindividualstratashowingtheirhighestdipangleinthecenterofthestrataandanasymptoticdeclineininclinationtowardstheupperboundaryoftheset(e.g., Fig.8A, B). The lateral accretionsurfaces betweenstratacouldalways be identified at thetop of a setbut could not always be identifiedatthebaseoftheset(cf.Arche1983).Setsoflarge-scaleinclinedstratacontaineduptotensofindividual strata, withbetween3and15stratalaterally betweenmajor erosional discordances, andbetween1 and7strataat-a-pointinthevertical. Discontinuitiesindipanglearepresentbetweenadjacentinclinedstrata. Thebasal erosionsurfacedecreasesinelevationtowards the cut bankandis characterizedby a number ofdiscretetopographicsteps(e.g.,Fig.8A,cande)thatappearrelatedtothe accretionof individual large-scale inclinedstrata. The presence ofdiscretetopographicstepsandtheundulosenatureofthebasal erosionsurface therefore results inlateral variations inset thickness, withsetthickness increasing towards the cut bank (Fig.8A). The point-barsequences produced in these experiments showa range of grain-sizetrends, fromdistinct fining-upsequences(e.g., Fig.8A, f) tosequencesthat possess no discernible vertical variation (e.g., Fig. 8A, b).Comparison ofthecrosssections withtheplanform depositsalso revealsthatmajorlateral-accretionsurfacescanhaveasurfaceexpressionintheformofridge-and-swaletopographyonthescrollbar.Theselaboratoryobservationsareincloseagreementwithfieldstudiesof large-scale inclined strata in gravel-bed and some sand-bed meanderingrivers. Inclined strata have been observed in sand-bed rivers to fineupwards(BridgeandTye2000;Bridgeetal.2000).Inclinationsoflarge-scaleinclinedstrataof520uhavebeenrecordedincoarse-grainedrivers(McGowen and Garner 1970; Ori 1982; Arche 1983; Campbell andHendry1987) withdipsof upto25uinfiner-grainedpoint bars(Allen1984; Miall 1996). Setsoflarge-scaleinclinedstratahavebeenobservedfrom studies of sand-bed rivers to contain between 1 and 10 inclined stratainthevertical (BridgeandTye2000; Bridgeet al. 2000), andthebasalsurfacesofthesesetshave also beenobserved to bestepped(Bridgeet al.2000). Discontinuities anddiscordances betweenstratahave alsobeenrecognized and linked to the occurrence of unit bars, and shifts in channelposition, respectively (Bridge et al. 1995; Bridge 2003). The surfaceexpressionoflateral-accretionsurfacesintheformofbar-tailscrollbarshas been observed in coarse-grained channels (D az-Molina 1993) and hasbeenincorporatedintomodelsofscroll-bardevelopmentinfine-grainedandcoarse-grainedrivers (e.g., Puigdefabregas 1973; GiblingandRust1993; Lunt et al. 2004). The vertical grain-size sequences in coarse-grainedpoint barshavealsobeenfoundtobevariable, withexamplesof bothfining-upsequences (e.g., Arche1983) andsequences withverylimitedvertical variation(e.g., Lewin 1978 andBlacknell 1982 for the RiverRheidol; and Arche 1983). These grain-size trends, as well as the thicknessof point-bar deposits and the angle of large-scale inclined strata, have beenrelatedtothethree-dimensionaldistributionofflowdepthandgrainsizeinbends,andmodeofchannelmigration(BridgeandJarvis1976,1982;Jackson1976;Willis1989;Bridge1993a;seediscussionbelow).ChannelFillsThefillsofabandonedchannelsexhibitconsiderablevariability,fromlargelyhomogeneousdepositsoffinetomediumsand(e.g., Fig.8A)tofills which are composed of fine-grained clay and silt-size sediment(pickedoutbywhitesilicaflour), intercalatedwithcoarse-grainedsanddeposits (e.g., Fig.8C). Channel fills commonly have arcuate basalerosionsurfaces that maybeoverlainbycoarse-graineddeposits (e.g.,Fig.8A). Some fills exhibit fining-upward sequences (e.g., Fig.8B, D,E)whereas othersaremuchmoreuniformin theirgrainsize (e.g.,Fig.8A).Althoughthegeometries of channel fills canbescoop-shapedincrosssectioniflargelyunmodified,theyarefrequentlystronglydissected,andtheirformisdeterminedlargelybythedetailednatureof chutecutoff.Depositionof finetomediumsands is associatedwithcontinuedflowthrough the cutoff, whilst fine-grained fills of clays and silts are formed bydepositionfromslow-movingorstagnantwater. Depositionofcoarser-grainedsediments (e.g., coarse sand) above clays andsilts canoccurthrougheither partial or full channel reoccupation, or byprogressivemovementofcoarse-grainedmaterialfromtheupstream endofacutoff.ScourFillsScourfillsmayhavesteeplateral margins, arepredominantlyinfilledbycoarse-grainedsediment (e.g., Fig.8B, D), andhavetwomodes offormation:(i)theinfillofconfluencescours formedat junctionsbetweenchutechannels andthemainchannel, and(ii) pools that formontheoutsideofchannel bends. Confluencescours, presentatthemajorityofchannel junctions, can possess depths of up to 56 times the mean channeldepth, whereas outer-bankscoursmaybe23times themeanchanneldepth(AshmoreandParker1983;BestandAshworth1997).UpperBarDepositsUpper bar deposits are thin, predominantly fine-grained (mud, silt andvery fine to fine sand), and laterally extensive if preserved intact, althoughthey are frequently preservedas isolatedremnants (Fig.8A, E). Thisdefinitiondiffers fromthe upper-point-bar terminology (Bridge et al.1995; Bridge 2003), which is not applicable to the present modelingexperiments. The lower bounding surface of these deposits may beundulose or planar, dependent on the nature of the preexistingtopography. Upper bar deposits overlie the large-scale inclinedstrataandrepresent depositionfromflows traversing the point-bar surface.Deposition takes place through accretion of bar-top lobes, andsuspension fallout of the finest-grained material, which in the fieldrepresents silt andsanddeposits restingonthickgravel units that arecharacteristic of many coarse-grainedpoint bars (e.g., Blacknell 1982[RiverRheidol]; Forbes1983). Bar-toplobes(unitbars)havealsobeenrecognizedfrommeanderingandbraidedrivers(LuntandBridge2004;Luntetal.2004;SambrookSmithetal.2006).ALLUVIALARCHITECTURE:LINKINGPROCESSWITHPRODUCTTheprocessesresponsiblefortheformationof thefourdepositionalunits described above can be determined by matching the video history ofchannel change at a givencross-sectional positionwiththe preserveddepositsrevealedinexcavationofthesamecrosssection(Figs.8,9).Large-ScaleInclinedStrataandLateral-AccretionSurfacesThegeometryandgrain-sizedistributionsoflarge-scaleinclinedstratawithin point bars have been modeled quantitatively by Willis (1989,1993), on the basis of simulation of flow and sediment transport in curvedchannels(Bridge1982),andusedextensivelytoinferthepaleohydraulicsofancientriversfromtwo-dimensionaloutcrops(e.g.,Khanetal.1997;rFIG. 5.Four-phase model of meander-bend evolution through shoaling of the upstream meander limb: A) the channel bend undergoes bend expansion; B) shoaling ofthe upstream meander limb occurs through stalling of a unit bar just upstream of the bend apex; C) shoaling directs flow across the point bar, forming a bar-head lobe;andD)erosionacrossthepointbarleadstochutecutoff.BENDEVOLUTION,ARCHITECTURE,ANDTHEROLEOFCOHESIONINRIVERS 203 JSR204 J.PEAKALLETAL. JSRZaleha1997; Bridgeet al. 2000; Zalehaet al. 2001). Theunderpinningmodel of flow (Bridge 1982, 1992) has also been tested against data fromsand-bedrivers, but it hasnot beentestedwithdatafrommeanderinggravel-bed rivers (see Bridge 1984, 1992; Willis 1989). Furthermore, therehave been few studies with independent records of both three-dimensionalpoint-bar geometryandthetemporal historyof meander-bendgrowththat generatedthepoint-bar deposits (Bridgeet al. 1986; Bridgeet al.1995; Bridge et al. 1998) that are a prerequisite to test the model of Willis(1989,1993). Thelimitationsofthemodel ofWillis(1989,1993)canbeaddressedinpartusingtheexperimentsreportedherein, whereboththetemporal development of channel bends and the resultant point-bardepositsarequantifiedindependently.AccordingtoWillis (1989, 1993), thethickness of sets of large-scaleinclinedstrata(point-bar deposits) andanylateral variationinstratalinclinationaredueto:(1)changingorientationofthechannelrelativetotheorientationofthecrosssection;(2)changingpositionofthechannelbendrelative tothe cross section, and/or (3) temporal changes inthegeometryofthechannel andthebend. Keypredictionsofthemodel ofWillis (1989) are that: (i) as channel sinuosity increases, the dip of inclinedlarge-scale strata increases; (ii) the inclination of strata increasestowardsthe bend axis; (iii) an increasing deviation from orthogonal cross sectionsproduces an apparent elongation of the cross-channel width, andtherefore the average inclination of strata decreases; (iv) point-barsequences become thicker away from the meander-belt axis (i.e., from therFIG. 6.Four-phasemodelofmeander-bendevolutionthroughbar-headdepositionatbendapices:A)thechannelbendundergoesrapidexpansion;B)stallingofa unit bar at the bend apex produces local upstream shallowing of the channel thalweg and corresponding flow expansion; C) scour around the stalled unit bar promoteslocalizedenhancederosionoftheouterbank,andincreasedbendasymmetrythroughupstreammigrationofthechannel,downstreamoftheapex;andD)progressiveshoalingofthebendentrancezoneleadstochutecutoffacrossthepointbar.FIG. 7.Model of meander-bend evolution through changes in upstream flow orientation: A) upstream chute cutoff as shown on the right-hand side leads to a rapidchangeintheorientationofthelocalflow;B)thenewfloworientationleadstolocalizedouter-bankerosionandflowbeingdirectedontothedownstreampointbar,formingabar-headlobeandsubsequentlybendcutoff.Thechannelthenreturnstoastageofbendexpansion(seeFigs.5A,6A).BENDEVOLUTION,ARCHITECTURE,ANDTHEROLEOFCOHESIONINRIVERS 205 JSR206 J.PEAKALLETAL. JSRinnermost to the outer parts of a point-bar); (v) coarsening-upwardsequences are produced in the most upstream parts of bends, with fining-upward sequences being characteristic of the downstream parts of bends;and (vi) large lateral variations in grain size may occur, with the coarsest-graineddeposits being farthest away fromthe meander-belt axis andtowardsthebendapex.Twospecificexamplesarediscussedhere:Example 1. The cross section in Figure8A has three areas of large-scaleinclined strata that together forma single set with two erosionaldiscordances, representingacompositepoint-bardeposit. Several stagesinthedevelopmentofthepoint-bardepositcanbeidentified:(1) Thelow-anglesurfaces(35u)totherighthandsideofFigure8A(f)weredepositedbetweenhours1821ofexperiment1inalow-amplitude, long-wavelength bend (Fig.9A, B). Initially, thechannel orientationwasstronglyobliquetothesection, withthebendapexupstreamof thelineof section(Fig.9B). Withtime,bendtranslationandexpansionledtothebendapexmigratingdownstreamtowards thecross section, andthechannel becameorientedprogressivelylessobliquelytothesection(Fig.9B).(2) A slight change in channel alignment led to small-scale erosion byhour 22, which left a remnant bank edge (Fig.8Alabeled 2;Fig.9B).(3) Further bend translation and expansion between hours 2225.5 ledtodepositionofinclinedstratadippingatapproximately10u,andduring this time period the channel became oriented perpendiculartothesection(Fig.9C).(4) Erosion of the point bar occurred around 27hours into theexperiment as a result of an avulsion downstream, producingamarkeddeepeningofthebasalerosionsurface(Fig.8A,e).(5) Betweenhours27and28.5, steeplydippinginclinedstrataweredeposited(Fig.8A,d)duringaperiodwhenthebendwashighlysinuous and the section was just upstreamof the bend apex(Fig.9D).(6) Afurther phase of point-bar erosion occurred after 30hours,erodingbacktopoint a onFigure8A, andwas followedbya period of channel filling that was partly caused by a unit bar thatstalledatthesharpbendapex.Subsequenterosionandmigrationof theresultingshallowchannel erodedtheinner channel bank(Fig.8A, labeled1)andtruncatedtheupperpartsofthesteeplydippinglarge-scale inclinedstrata(Fig.8A, labeledd; Fig.9E).Thecrosssectionthereforeshowsdepositsthatreflectallthreeofthe controlling variables identified by Willis (1989), namelyvariationsinchannel geometry, channel orientation,andchannelpositionwithtime.The increase in dip of the large-scale inclined strata from right to left inFigure8Ais a function of the increase in bend sinuosity and lowerobliquityof the left-handpart of the sectionwithrespect tothe flowdirection, andalsodownstreambendtranslationsuchthat thesectionmovedfromthedownstreamlimbtowardsthebendapex(Figs.8A, 9).Thepoint-barsequenceincreasesinthicknesswithdistanceawayfromthe meander-belt axis, with minor variations reflecting the topography ofthebasal channel erosionsurface. Asignificant lateral variationinthegrainsizeofthelarge-scaleinclinedsurfacesisalsopresent,fromfinetomediumsandintercalatedwithseveralcoarsegrainedhorizons(Fig.8A,f), to coarse to very coarse sand deposits (Fig.8A, d), this being caused bythegrowthinbendamplitudeandsinuosity(cf. figure4sectionAofWillis 1989). Point-bar deposits displaybothfining-upsequences (e.g.,Fig.8A,f),andprofileswithverylimitedverticalgrain-sizetrends(e.g.,Fig.8A, b). However, the thicker strata are typically characterized by thelimitedtrends invertical grainsize incontrast tothe model of Willis(1989, 1993),wherethickerstratawereassociatedwithagreatertrendingrain size. With the exception of the spatial distribution of verticalvariations ingrainsize, these physical model results are inverycloseagreement with the model of Willis (1989, 1993) and with fieldobservations of point-bar deposits utilizing cores and ground-penetratingradar (Bridgeet al.1986;Bridgeet al.1995;Bridgeet al.1998;LuntandBridge2004).Example2.Figure8Billustratestwosetsoflarge-scaleinclinedstratadipping in opposite directions. Figure9F demonstrates that the left-handsideof section8B(h) wasformedinthefirst 6hoursof experiment 2,whereas the right-hand side of section 8B (j) was formed after chute cutoffacross the upstreambend had led to bank erosion and point-barconstructionontheothersideof thechannel. Therefore, althoughthealluvialarchitectureresemblesamid-channelbar,itwasactuallyformedbysuperimpositionof depositsfromtwo, temporallydistinct, meanderpointbars. Previousworkershavealsorecognizedtheanalogybetweenthedepositional processes of meanderingchannels andthecreationofmid-channel barsbetweentwosinuousanabranches(e.g., Bridge1993a;Ashworth1996). The two sets of inclined strata dipping in oppositedirections(Fig.8B)bearastrikingsimilaritytosketchesoftheinternalarchitectureof braidbars (e.g., Bridge1993a, his figs. 2123; Karpeta1993, hisfig. 6; Bridgeetal. 1998, theirfig. 8)andprovideanexcellentillustrationof thedifficultyof correctlyinterpretingchannel planformfrom subsurface alluvial deposits(cf. Bridge 1985, 1993b). Critically,oneset (j) crosscutsanearlierset (h), thusdemonstratingthat thetwosetscouldnot have formed at the same time around a single braidbar.However, suchinterrelationships couldoccur inbraided-river depositsfrom sedimentation produced arounddifferent braid bars. The point-barsequenceontheright-handsideofsection8B(j)showsalateralincreasein: (i) the thickness of sets of large-scale inclined strata; (ii) the inclinationof the strata, and (iii)the average grain size (Fig.8B; see variations fromleft to right). These lateral variations largely reflect the increase in channelsinuosity, and downstreamtranslation of the bend (thus moving therFIG. 8.Cross sections of alluvial architecture from the physical model experiments. The location of each cross section is shown in Figure9. The deposits have a sharp,erosionalbasewiththelightercolored,unsortedoriginalsedimentbeneath.Flowdirectioninallcases isawayfromtheviewer(i.e.,lookingdownstream;see Fig.9 fordetailedorientations).A)Crosssectionshowingapartiallyfilledchannelandapointbarexhibitinglarge-scaleinclinedstrataandassociatedlateral-accretionsurfaces(labeleddandfandbymultiplearrows),pickedoutbygrain-sizevariations.Threeareasoflarge-scaleinclinedstrataarepresent,withsteeplydippingstrata(2030u)adjacent to the channel fill (d) and lower-angle surfaces (35u) on the right hand side (f). These inclined strata represent a single set separated by prominent discordances(1 and 2, separating 8a, 8a1, and 8a2 in Fig.9) between increments of point-bar accretion, recording growth of the point bar (see Fig.9). Discrete topographic steps (c ande)arepresentinthebasalerosionsurface.Fine-grained,upperbardeposits(identifiedatposition(g)bywhitesilicaflour)conformablyoverliethelarge-scaleinclinedstrata. The point bar varies from fining up (f) to showing no discernible vertical variation (b). Position a marks a phase of erosion; see text for details. B) A channel fill,withscourfills atitsbase, ispresent(i)andwasformedfromanearly chutechannel.Thechannel fillisoverlainbypoint-bardepositscomposedoflarge-scaleinclinedstratadippinginoppositedirections(handj).C)Channelfillcomposedofalternatelayersoffine-andcoarse-grainedmaterial(k).Thearcuateerosionsurface(n)isoverlain by large-scale inclined strata (m). Finer-grained upper bar deposits (l) overlie, and are in erosional contact with, the abandoned channel and lower bar deposits.D) Channel fill with two prominent scour fills at its base (p), infilled by alternate layers of coarse- and fine-grained sediment. The channel fill then fines upwards into silts(o). Variationsin sediment sizebelowthe scour fillsare a resultofincompletehomogenizationof thesediment priorto theexperiment.Thissectioniscontinuous with,and at the same scale as, Section A (Fig.9). E) Filled chute channel (q), overlain by large-scale inclined strata of the lower point bar (r), which fine upwards to upper bardeposits(s).ThiscrosssectionisimmediatelydownstreamofsectionB(seeFig.9F).BENDEVOLUTION,ARCHITECTURE,ANDTHEROLEOFCOHESIONINRIVERS 207 JSR208 J.PEAKALLETAL. JSRsectionfromdownstreamof thebendapextothebendapex) andareagaininclose agreement withthe fieldobservations of Bridge et al.(1986),Bridgeetal.(1995),andBridgeetal.(1998)andmodelofWillis(1989,1993).Thequantitativemodel ofpoint-bardevelopmentproposedbyWillis(1989, 1993) is basedonthe concept of the force balance of bedloadgrains moving over a bar surface, with individual inclined strata related toepisodicdepositionduringfloods(Bridge1982, 1992, 2003; BridgeandTye2000; Bridgeetal.2000),anddoesnotaccountfortheinfluenceofaccretionfromunit bars. However, individual increments of point-baraccretioninboththephysicalmodelandmanynaturalrivers,includingthe prototype River Rheidol (Lewin 1978), are associated with thepassage of unit bars. The close agreement between the experimentsreportedhereinandthemodel of Willis (1989, 1993) suggests that theWillis model captures the larger-scale aspects of point-bar accretion,irrespective of the mechanismof accretion, and is therefore broadlyapplicabletotheepisodicaccretionofunitbarsingravel-bedrivers.Thepresentexperimentsdo,however, differfromthemodelofWillis(1989,1993)inonekeyrespectthespatialdistributionofverticalgrainsize. The physicalmodel shows littlevertical variation inthe thicker sets,whereas the model of Willis (1989, 1993) shows strongly fining-upsequences. The thicker parts of the deposits inthe experiments reveala series of alternations of coarse and fine strata, most probably the resultof depositionof bedload sheets withinthe experiments (cf. Ashmore1991b), although this cannot be confirmed. These strata are similar to thealternations of open-frameworkgravel andsands observedincoarse-grainedpoint bars(Lunt andBridge2004; Lunt et al. 2004) that havebeenlinkedtothemigrationofsuperimposedbedforms(bedloadsheetsand dunes) down bedform and bar fronts (Lunt and Bridge, in press). Theinfluence of bedforms and bars on vertical grain-size variations dominatestheforce-balanceeffect ongrainsizethat isemployedinthemodel ofWillis (1989). Consequently, the model of Willis (1989, 1993) mayoverestimate the magnitude of upward fining in thick, gravel-rich, steeplydipping large-scale inclined strata, and underrepresent the localizedvariations in grain size created by juxtaposition of coarse and fine strata.Thepresent experimentsdemonstratethat large-scaleinclinedstrataincoarse-grained rivers can formunder uniformflow conditions, andtherefore an individual stratum isnot necessarily theproduct of seasonalchanges (cf. Bridge2003; Lunt et al. 2004). This is inagreement withexperimental work by Lunt and Bridge (in press) that shows formation ofmultiple inclined strata under uniformflowconditions in gravel-richsediments.DISCUSSIONTheexperimentsreportedhereinhavesucceededinreproducingself-forming meandering channels that exhibit a significant sinuosity andundergo continuous active migration, but do not exhibit any tendency tobraid. A comparison is made below with previous experimental studies, inorder to explore the mechanisms that restrict braiding and maintainsignificant channel sinuosity. Experiments that produce low-sinuosity,self-formedchannelswithsubmergedbars(e.g.,Friedkin1945;Wolmanand Brush 1961; Ackers 1964; Zimpfer 1975; Schumm et al. 1987) all tendtowardsabraidedplanformasthechannelmigrates,sincesomeportionof the flow is maintained above the increasingly extensive submerged bardeposits. This results in a progressive increase in channel width anda decrease in average flow depth. In order to maintain long-termmeandering,point-bardepositsmustaccretetothelevel ofthebankfullwater height, andinnatural rivers this occurs throughpredominantlyfine-grainedbar-topdeposition, andoverbankdepositioninthecaseofbends and/or channel segments that have been abandoned. In the presentexperiments, accretionoccurreduntil theupper bar was almost at thewater surface. Further accretion could not occur under the imposedconstantdischarge.Three previous sets of experiments have reproduced self-formingmeanderingchannelswithpointbarsthataggradedtothewatersurfaceand did not braid (Schumm and Khan 1972; Jin and Schumm 1987; Smith1998). SchummandKhan(1972, p.1766) producedtrulymeanderingchannels byinitiallyproducingsubmergedbars, withsubsequent baremergenceoccurringinresponsetoadditionof a3%concentrationofkaolinite, coupled with a decrease in sand supply. Clay depositionoccurredinitiallyonthechannel banks, leadingtoanincreaseinbankstabilityandthereforecausingthalwegscouring,channeldeepening,andbar emergence, before the whole channel was coated with clay anderosionceased.However, atnostageintheseexperimentsdidpointbarsactivelyaccrete andmigrate. Atwo-layer model was usedbyJinandSchumm (1987), withalowersand layercapped bya thicklayer(80% ofchannel depth) consisting of over 50% kaolinite, which produced sinuouschannels withactive point bars,although theclaystopped erosionoftheupperpartofthebank(JinandSchumm1987).AsPaola(2001)noted,these meandering channels do not qualify as dynamic because they couldnot reform the ad hoc substrata themselves. Indeed, Paola (2001) suggeststhat had these experiments been run long enough, lateral migration wouldhave eliminated the clay cap and thestream would have braided, so longas the channel was not coatedinclay(cf. SchummandKhan1972).Thirdly, Smith(1998) usedvarious granular materials (fromdiatoma-ceousearthtocornstarch)mixedwithkaolinitetoproduceactivepointbars,channelswithahighsinuosity,andchutechannels.However,afterdevelopingahigh-sinuositychannel, migrationvirtuallyceasedandtheexperimentsreachedastateofstaticplanformequilibrium(Smith1998).Claywasanintegralpartofalloftheexperimentsdiscussedabove,incontrasttopreviousflumestudiesofself-formingsand-bedmeanderingchannels that did not use clay and were characterized by submerged bars.However, thepresenceof clayinmodelstudiescreates ascalingproblembecausetheelectrostaticandvanderWaalsforcesthatcontrolcohesionare scale-independent andare therefore more influential inthe modelrelative to natural rivers. The influence of this unscaled cohesion is seen inall three studies, where clay either rapidly stops channel migration(SchummandKhan1972), stopserosionoftheupperpartofthebank(Jin and Schumm1987), or prevents point-bar erosion and furtherchannel development after initial channel growth (Smith 1998). Incontrast, the experiments described herein used silica flour instead of clayforthefinestmodel grainsizes. Althoughsilicaflourisnotelectrostat-icallycharged, it isaffectedbybothcapillaryandweakvanderWaalsforces, both of which are weaker than the electrostatic forces in clays andtherefore mimic to some extent prototype cohesion (Peakall et al. 1996). Itis concluded here that the absence of clay and use of silica flour thereforeenables the formation of active point bars but does not lead to thecessation of erosion and ossification of the planform. The presentrFIG. 9.Tracings of the channel planformat differenttime increments, with positions ofthe sedimentary cross sectionsshown in Figure8. The snapshots are tracedfrom time-lapse video frames, and flow direction (arrows) is from right to left in all cases. Distances on the horizontal axis of each plot refer to distances downstream fromthe entry point, whereas distances on the vertical axis start from the left-hand side of the flume when viewed looking downstream. Panels (a) and (cf) show the position ofthe channel banks and point-bar deposits for various time increments. Panel (b) shows the positions of the outer edge of an accreting point bar as a function of time. In allcases, the time since initiation of the experiment is marked by t 5the number of hours. The positions of the sedimentary cross sections are labeled with the numbers ofcrosssectionsshowninFigure8.Labels8a,8a1,and8a2refertospecificpositionsinFigure8A(seeFigure8fordetails).BENDEVOLUTION,ARCHITECTURE,ANDTHEROLEOFCOHESIONINRIVERS 209 JSRexperiments therefore lend support to the contention that physical modelsofriverchannels,andtheirnaturalcounterparts,requiresomecohesionin order to maintain a meandering pattern (Murray and Paola 1994, 1997,2003; Paola 2001), and restrict the development of braiding. Paola (2001)suggested that this cohesion couldbe provided by fine-grained sedimentsand/or vegetation. Gran and Paola (2001) demonstrated that theincorporation of vegetation into a river physical model utilizingnoncohesive material was not, in itself, sufficient to promote meandering,although the vegetation did increase bank stability and moved thechannelpatterntowardsameanderingplanformfromaninitialbraidedstate. Subsequent field, numerical, and experimental studies haveindicatedthatvegetationcanplayakeyroleinreducingthenumberofactivechannels towards asingle channel,but theresultantchannels werenotentirelysingle-thread(e.g.,MurrayandPaola2003;Taletal.2004).Thepresent resultssuggest that fine-grainedsedimentsaresufficient topromote stable meandering, at least at the scale of these experiments. Theimportanceofcohesioninmaintainingameanderingplanform, andtherequirement that the imposedcohesionis of approximately the sameorder as the imposed shear stresses to enable erosion as well as deposition,thus perhaps explains why active meandering channels have been sodifficulttorecreateinthelaboratory.Theexperiments describedherein, andthethreesets of experimentsreviewedabove, areeachapplicabletoadistinctsubsetofmeandering-channel types. The study described herein represents coarse-grainedmeandering rivers, whilst the study of Schummand Khan (1972)simulatesarapidchangetoaclay-richsedimentsupply,asoccurredforsome rivers in the Quaternary (Schumm 1968) and some rivers affected byclay-rich mine tailings (Richards 1979). The study of Smith (1998)appears tobe highlyanalogous toclay-richmeanders suchas certaintypes of tidal channels (Fenies and Fauge`res 1998) and submarinechannels(Peakall et al. 2000a; Peakall et al. 2000b), whichreachstaticplanformequilibriumafter initial bendgrowth. The studyof JinandSchumm(1987) remains perhaps the best analogue for a sand-bedmeanderingriver.CONCLUSIONSA series of physical modeling experiments of meandering channels wasundertakenin order to examine planformevolution andto link thisevolution to the resultant depositional architecture. In these experiments,point bars grew by both accretion of parts of downstream-migrating unitbarsanddepositionofsuspendedsedimentontothebarplatform.Afterinitial bendexpansion, meanderdevelopmentandeventual chutecutoffproceededinthreedifferentways: (i)shoalingoftheupstreammeanderlimb, (ii) bar-head deposition at bend apices, leading to channel blocking,and(iii) viachangesinupstreamfloworientation. Thestallingof unitbars at bend apices is proposed as a mechanism for formation of the bendasymmetry observed in many gravel-bed rivers, whereby the downstreammeander limbs migrate upstream. Collectively, the processes of point-bargrowth and bend development in these experiments show a closesimilaritytothoseobservedinnaturalgravel-bedrivers.Sectioningofthepreservedsedimentshasallowed, forthefirst time,studyofthealluvialarchitectureofamodelmeanderingriver.Fourkeydepositionalelementswererecognized:(i)large-scaleinclinedstrataandassociatedlateralaccretionsurfaces;(ii)channelfills;(iii)scourfills,and(iv) upper bar deposits. Theexperiments reproducemanysedimentaryfeatures such as sets of large-scale inclined strata, with discontinuities anddiscordances of inclined strata, and stepped basal erosional surfaces. Theprocessesresponsiblefortheformationoflarge-scaleinclinedstratathatdominate these point bars were studied by matching the history ofchannel changetothepreserveddeposits. Thisprocessproductlinkagehas enabled many aspects of the point-bar model of Willis (1989, 1993) tobe tested, including consideration of the lateral variations in thethickness, orientation, andgrainsizeof large-scaleinclinedstrata. Theclose agreement between the experimental results and the theoreticalmodel of Willis (1989, 1993) suggests that the theoretical modelsuccessfullyreproduces the larger-scale aspects of point-bar accretion,irrespectiveof whetheraccretionis dominatedbyprogressivesedimen-tationduringfloodeventsortheepisodicaccretionofunitbars.These experiments represent the most realistic physical modelingofmeanderingchannelsyetproduced.Critically,thekeytosuccessintheseexperiments is the incorporationof arealistic magnitude of cohesion.Previousworkhasfocusedontheimportanceofvegetationinprovidingthis cohesion (e.g., Gran and Paola 2001; Tal et al. 2004), but the presentwork shows that fine-grained sediment alone can provide sufficientcohesion for sinuous river channels to develop and evolve. Theimportanceofsedimentcohesionandtherequirementtoscalecohesionmore appropriately within physical models probably explains whyactivelymeanderingchannels havebeensodifficult torecreateinpastlaboratorystudiesofmeanderingrivers.ACKNOWLEDGMENTSJPis grateful toBPExplorationfor theawardof astudentshipduringwhich this work was initiated, and to Mike Bowman (BP) for his enthusiasticsupport of the studentship. We are very grateful to John Lewin fordiscussionsandadviceontheRiverRheidol, andinparticularitsgrain-sizecharacterization. Ian Lunt is thanked for discussions on bar dynamics and forcomments on the final manuscript, while Chris Paola provided manystimulatingdiscussions onmeander dynamics. JBis grateful for awardofaLeverhulmeTrustResearchFellowshipthatassistedincompletionofthispaper. 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