geomorphic principles applied in stream simulation · appendix a—geomorphic principles applied in...

AGeomorphic Principles Applied in Stream SimulationA.1 Why Consider Fluvial Processes in Crossing

Design?

A.2 The Watershed Context

A.3 Channel Characteristics

A.4 Channel Stability and Equilibrium

A.5 Fluvial Processes

A.6 Channel Classification Systems

A.7 Unstable Channels

Stream Simulation

Appendix A—Geomorphic Principles Applied in Stream Simulation

Thisappendixverybrieflyreviewsfluvialprocesses(i.e.,processespertainingtoriverorstreamaction)andchannelcharacteristicsthatprojectteamsconsiderwhenevaluatingsiteconditionsatroad-streamcrossingsanddesigningstream-simulationstructures.Chapters4,5,and6describehowteamsapplytheseconceptsinstream-simulationsiteassessmentanddesign.

Trainingandexperienceingeomorphologyareessentialforassessingchannelconditions,interpretingchannelresponsesandfluvialprocesses,anddesigningasimulatedstreambed.Mosthydrologists,geomorphologists,geotechnicalengineers,andhydraulicengineersalreadywillbefamiliarwithmanyoftheconceptswearepresentinghere.Ifyouareareaderforwhomthematerialisnew,theinformationinthisappendixisnotadequatefordevelopingjourney-levelgeomorphologyskills.Youmaywanttoreviewthereferencescitedhereandattendtrainingcoursestoexpandyourknowledge.Projectteammembersareresponsibleforrecognizingwhenadditionalexpertisemustbebroughtin—especiallywhenchannelconditionsarecomplexanddifficulttointerpret(seesidebarinsection3.3).

A.1 Why ConSider FluviAl ProCeSSeS in CroSSinG deSiGn?

Streamsaredynamicsystemsthatcanreadilychangeinresponsetohumanornaturaldisturbances.Streamscontinuallyerodesedimentand woodfromtheirboundariesandredepositthatmaterialatotherlocationsinthechannel.Manystreamsalsoshiftlocationlaterallyacrossthevalleybottom.Streambedelevationschangeasthestreamtransports,deposits,andstores woody debrisandsediment.Duringfloods,streamsoverflowtheflood-plainsurface,erodinganddepositingsedimentanddebris,andconstructingriparianhabitats.

Road-streamcrossingsarerigidstructuresthatlockthestreaminplaceandelevation,preventingthesenormaldynamicprocesses.Inthepast,crossingshavetypicallybeennarrowerthanthestream,causingbackwateringandsedimentdepositionattheinlet[figureA.1(a)].Narrowculvertsalsoincreasewatervelocitycausingchannelscourinordownstreamofthecrossing[figureA.1(b)].

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Stream Simulation

FigureA.1—(a)Aggraded(filled)channelupstreamofnarrowculvert;(b)incised(scoured)channeldownstreamofculvert,SaveCreek,OlympicNationalForest,Washington.

Aschapter1explains,suchchannelresponsestoculvertscanultimatelyinhibitorpreventaquaticspeciespassage.Theseresponsesalsocancausemassiveproblems—bothfortheroadandthestream—duringlargefloods.Pluggingwithdebrisandsedimentiscommonatculverts.Fillfailureorstreamdiversioncanfollow,asthewaterovertopstheroadorrunsalongtheroaduntilitpoursoffontoahillslopeorintoanotherdrainage(figure1.17).Scouringatnarrowbridgesoropen-bottomarchescanalsocausethesestructurestofail.

Stream-simulationdesignprovidesforbothaquaticspeciespassageandlong-termstabilityofthestructureandtheconstructedstreambed.Withinthelimitsofanecessarilyrigidstructure,streamsimulationaimstoprovideenoughspaceforthestreamchanneltoadjusttochangingflowsandsediment loads,justasthenaturalchanneldoes.Toachievethisobjective,theprojectteammustunderstandhowfluvialprocesses

(a)

(b)

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shapethecurrentchannelatasite.Theteammustbeabletopredictfuturechannelresponsestochangesinwatershedandclimaticconditions,andtheymustalsobeabletopredicthowthechannelwillrespondtothenewcrossingstructure.

A.2 The WATerShed ConTexT

Thesite’slocationinthewatershedisimportant.Dependinginpartontheirpositioninthewatershed,channelreaches(streamsegmentswithrelativelyhomogenouscharacteristics)canbedividedintothreegeneraltypes(MontgomeryandBuffington1993,1997):

(1) Sourcereachesareheadwaterchannelswithfewifanyfluvialcharacteristics.Hill-slopeprocessessuchassurfaceerosionandsoilcreepdeliversedimenttothesechannels,whichstoreituntillargefloweventsordebrisflowsscouritout.

(2) Transportreachesaretypicallysteepstreamsthattendtoresisterosion,becausetheyhavepersistentbedandbankstructuresdominatedbylargeparticlesizes(boulders,cobbles,gravels,andwood).Althoughthesereachesstoresomesediment(e.g.,behindpiecesofwoodydebris),ingeneraltheyhavehightransportcapacities.Whensedimentsupplyincreases,theytendtopasstheincreasequicklytolower-gradientreaches.Channelmorphologydoesnotchangeverymuchinresponsetochangesinwaterorsedimentinputs.

(3) Responsereachesarelower-gradientreacheswheresedimenttransportislimitedbyrelativelylowtransportcapacity.Thatis,whensedimentsupplyfromupstreamincreases,itislikelytodepositinaresponsereach.Thereachwilloftenrespondtochangesinsedimentsupplyordischargebymakinglargeadjustmentsinchannelsize,shape,slope,orpattern.AsMontgomeryandBuffington(1993)pointout,thefirstresponsereachdownstreamofaseriesoftransportreachislikelytobeanextremelysensitivesitewhenwaterorsediment regimeschangeintheupstreamwatershed.

Thisappendixreferstothesereachtypesthroughout.Theyarehelpfulasshorthanddescriptorsoflikelychannelresponsivenesstoenvironmentalchange.Understandingthedifferencesbetweenstreamsintheirresponsivenesstoenvironmentalchangesisveryimportantinstream-simulationdesign.

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Stream Simulation

Whilesomewatershedshaveamoreorlessregularsequenceofsource,transport,andresponsereachesfromheadwaterstomouth(figureA.2),reachtypesareoftendistributedinamorecomplexway.Localgeologiccontrolscancreatemeanderingmountainmeadowstreams(responsereaches)neartheheadwaters,andverysteeptransportreachesmaybenearthedownstreamendoftributariesonriverbreaks.

FigureA.2—Idealizeddistributionofreachtypesinawatershed.DrawnbyL’TangaWatson.

Asintegralpartsofthewatershedecosystem,streamsreflecttheeffectsofclimate,geology,soils,vegetation,basinshape,andlanduseinthewatershed.Thesefactorscontrolwaterandsedimentinputstothestream.Inturn,waterandsediment,interactingwithriparian vegetation and channelboundarymaterials,controlfluvialprocessesanddeterminechannelcharacteristics.

Muchcanhappentochangethesecontrollingfactorsoverthelifetimeofacrossingstructure.Landuseischangingrapidlyinmanyareas,particularlynearnationalforestboundarieswherepeoplecanbuildhomesandinterfacedirectlywith“nature.”Roadbuildingiscontinuinginsomelocations,androadsarebeingimprovedforrecreationaccess.Off-roadvehicleusecanaffectthehydrologicsystem,ascangrazingandfire.Inmanylocations,streamsareexperiencingorrecoveringfromlarge-scalemining,logging,andremovingofwoodydebris.Allthesechangescanhavelargeindividualandcumulativeeffectsonthehydrologicregime.Evenasingleunusualfloodcancreatelarge,long-lastingchangesinastreamsystem,requiringdecadesforrecovery.

Response reaches

Mostly transport reaches

Smallest headwaterchannels are sourcereaches.Larger streams are transport reaches.

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Obviously,whathappensupstreaminawatershedaffectsdownstreamchannelreaches.However,downstream-landuseorriverchangesalsocanaffectupstreamareasiftheyinduce channel incision(i.e.,downcutting).Forexample,channelizationforurbanoragriculturaldevelopmentspeedsupwaterflow,increasesitserosivepowerandcauseschannelstoincise.Removalofwoodydebrisfromachannel,e.g.,toreducetheriskoffloodingcanhavethesameeffect.Gravel-miningoperationsthatdigin-channelpitscanlowerthebase levelforallupstreamreaches.Theseactionsoftenproduce headcutting,inwhichanoversteepenednickpointmigratesupstream(figureA.3),causingthebedtoinciseuntilitequilibratesatalower,lesserodibleslope.Manyexistingculvertsarefunctioningasgrade controls,protectingupstreamreachesfromchannelincisioncausedbymigratingheadcuts.

FigureA.3—Activenickpointmigratingupstream,MeadowCreek,NezPerceNationalForest,Idaho.(a)Lookingdownstreamacrossnickpoint;(b)lookingupstreamatnickpoint.Brightstreambedindicatesrecentlymobilizedmaterial.

(a)

(b)

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Stream Simulation

Causeandeffectcanbedifficulttodetermine,notonlybecauseunseenoffsitechangesmaybeaffectingasitebutalsobecauseasignificantlagtimemayexistbetweencauseandeffect.Forexample,headcutsrelatedtochannelstraighteninginthe1960swerestillactivelymigratingupstreaminnorthernMississippiinthe1980s(Harveyetal.1983).Therecanalsobecascadingeffects.Ifbankvegetationisremoved(e.g.,byagriculture,logging,grazing,orconstruction)fromaparticularlysensitivereach,thechannelmayresponddramatically.Bankerosioncouldcausetheaffectedreachtowidensignificantly,releasinglargevolumesofsediment.Thatsedimentmaybedepositedinadownstreamreach,potentiallydestabilizingstreambanksthere.

Existingchannelconditionsmaydependonfactorsoreventsfarremovedspatiallyandtemporallyfromthesite.Tounderstandthepastandpredictfuturechannelresponses,analyzethetemporalsequenceandspatialdistributionofwatershedactivities.Thisinformationiscriticaltomakinginformedandaccurateinterpretationsofchannelconditionsattheroad-streamcrossing.Thisanalysisispartofphase1ofastream-simulationproject—theinitialwatershedreview(seechapter4).

A.3 ChAnnel ChArACTeriSTiCS

A.3.1 Streambed Material

Achannelreachcanbedescribedasbedrock,colluvial,oralluvialaccordingtothecompositionofitsbedandbanks(MontgomeryandBuffington1997;Knighton1998).Bedrockchannelshaveconsiderablesegmentsofresistantbedrock(inexcessof50percent)exposedalongthe flow boundaryorthebedrockmaybeoverlaidbyathinveneerofalluvium,i.e.,materialtransportedbythestream(TinklerandWohl1998)(figureA.4).Bedrockchannelstendtobequitestable.Manyaresituatedinnarrowvalleysandlackfloodplains.Thelackofsedimentinbedrockchannelsindicatesthatsedimentisefficientlytransportedthroughthereach(MontgomeryandBuffington1997).Eveninthesetransportreaches,however,thereareusuallylocalized,transientsedimentaccumulationsbehindwoodydebrisorotherchannelfeatures,andtheseaccumulationsmayformveryimportanthabitatsforaquaticspecies(McBainandTrush2004).

Channelscomposedofmaterialdepositedbygravity-drivenprocessessuchascreep,surfaceerosion,debrisflows,landslides,androckfallsarereferredtoascolluvialchannels(atypeofsourcereach,figureA.2).Typically,theyarelocatedinthesteepheadwaterareasofthewatershed,

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wheremass wastingisthedominantgeomorphicprocess(MontgomeryandBuffington1993,1997).Colluvialchannelsarecomposedofangularboulders,cobbles,gravels,andsands.Normal(shallow)streamflowisinsufficientformobilizingmostofthematerial;intermittentdebrisflowsaretheprimaryprocessformobilizinganddeliveringthecoarsecolluvialmaterialdownstream(MontgomeryandBuffington1997).

FigureA.4—Bedrockchannelsaretransportreaches.

Alluvialchannelsarecomposedofalluvium;thatis,theirbankandbedmaterialsweretransportedanddepositedbythestream.Theyareabletoadjusttheirformbyerodinganddepositingsedimentinresponsetochangesinflowandsedimenttransportconditions.Thefrequencyanddegreeof channel adjustmentisstronglyrelatedtoparticlesize;channelscomposedofgravelandsmallcobbles(figureA.5)aremoreresponsivetoflowandsedimentsupplychanges,whereaschannelscomposedoflargecobblesandbouldersarerelativelystableatmostflowsandmayonly

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Stream Simulation

changeformduringinfrequent,exceptionalfloodswithlargesedimentinputs.Sand-bedchannelsarehighlyresponsive,andtheirbedsareusuallycontinuouslyinmotionatmostflows.

FigureA.5—Alluvialresponsereach.

Channelsincohesive materials(withsignificantclaycontent)mayormaynotbealluvial.Manyareincisedinto residual soils.Althoughtheircharacteristicsvarygreatlydependingonslope,ingeneraltheydonottransportverymuchbedload.Mostsedimentistransportedinsuspension.

Inchannelscomposedofgravels,cobbles,andboulders,bedmaterialisoftensegregatedintotwolayers(figureA.6).Thebedsurfaceconsistsofaone-ortwo-grain-thicklayerofcoarserparticlesoverlyingsmallergravelsorsandsbeneaththesurface.Thisoverlyingcoarselayerisreferredtoasthearmorlayer.Themedianparticlesizeofthearmorlayerisusually1.5-to3.0-timescoarserthanthemedianparticlesizeofthesubarmorlayer(Reidetal.1998;BunteandAbt2001),althoughratiosashighas6and7havebeenreported(e.g.,AndrewsandParker1987;Kingetal.2004;Barryetal.2004).Thepresenceofanarmorlayerindicatesthatthechannelcantransportmoresedimentthanisavailablefromupstreamareas,whereasthelackofanarmorlayerindicatesabalancebetweensedimentsupplyandtransportcapacity(MontgomeryandBuffington1997).Thearmorlayerincreasesthestreambed’sresistancetoerosion.Oncethearmorbreaches,however,thewholestreambedcanmobilize,andgeneralscouroccurs.Ingeneral,unarmored streambedsaremoremobilethanarmoredones;thatis,bedsedimentmovesatlowerflowsandmorefrequentlyinanunarmoredstreambedthanitwouldinanarmoredone.

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Note:TheparticlesizeterminologyweuseinthisdocumentisfromtheWentworthclassificationsystem,inwhichparticlediameterdoublesforeachsuccessivecategory(tableA.1).

TableA.1—Definitionsofparticlesizecategoriesusedinthisguide:Wentworthclassificationsystem

Particle Description mm inches

Bedrock >2,048 80

Large–verylargeboulders 1,024–2,048 40–80

Mediumboulders 512–1,024 20–40

Smallboulders 256–512 10–20

Largecobbles 128–256 5–10

Smallcobbles 64–128 2.5–5

Verycoarsegravels 32–64 1.26–2.5

Coarsegravels 16–32 0.63–1.26

Mediumgravels 8–16 0.31–0.63

Finegravels 4–8 0.16–0.31

Veryfinegravels 2–4 0.08–0.16

Verycoarsesands 1.0–2.0 0.04–0.08

Coarsesands 0.50–1.0 0.02–0.04

Mediumsands 0.25–0.50 0.01–0.02

Finesands 0.125–0.25 0.005–0.01

Veryfinesands 0.062–0.125 0.002–0.005

Silts/clays <0.062 <0.002

FigureA.6—Thearmorlayercanbeseenonthiserodedgravelbar,FlatheadRiver,Montana.

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Stream Simulation

A.3.2 Channel Slope

Slopeisanimportantvariabledeterminingtheoverallenergyofthestreamfortransportingwaterandsediment.Slopeisalsooneofthechannelcharacteristicsmostfrequentlyalteredbycrossingstructuresthatareundersizedorinstalledatslopesdifferentfromthatofthenaturalchannel.

Asageneralrule,channelslopedecreasesgoingdownstreaminthewatershedfromtheheadwaterstothelowersedimentdepositionzone(figureA.2).Locally,thechannelslopemaysteepenorflattenbecauseoffactorssuchasbedrock,coarsermaterial,tectonicactivity,andbase-levelchanges(Knighton1998).Thegeneraldecreaseinchannelslopeacrossthewatershedcorrespondstoanincreaseinflood-plainwidth,channelsinuosity(seeA.3.3),andaverageflowdepth;adecreaseinbedmaterialsize;andadecreaseintheinteractionsbetweenvalleyslopesandthestream.Steepchannelsusuallyhavecoarsersediments,discontinuousnarrowfloodplainsorno flood plains,narrowvalleybottoms,andrelativelystraightplanforms whencomparedtolow-gradientchannels.

Abase-levelcontrolisanystructurethatfixesthelowestelevationtowhichastreamreachcandowncut.Commonexamplesofbase-levelcontrolsareverystabledebrisjamsorconcreteweirs.Foratributary,theultimatebaselevelistheelevationofthemasterstreamatatributaryjunction.Whenabase-levelcontrolisremovedoraltered,upstreamchannelslopechangesconcomitantly.Base-levelcontrolisanimportantconceptinstreamsimulation.Ifthebase-levelcontrolchangesoverthelifeofthestructure,thealteredslopemaydestabilizethesimulatedstreambed.

Atthereachscale,channelslopecanbemeasuredastheslopeofthechannelbedorastheslopeofthewatersurface.Italsocanbemeasuredalongthethalweg(representinglowflow)oralongthemidpointofthechannel(representinghighflow).Instream-simulationdesign,thechannelbedalongboththethalwegandthe bankfullwatersurfaceslopecanbeimportant(seesection5.2.2.2bankfullsidebar).

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The thalweg is a line running along the channel bed (i.e., longitudinally), connecting the lowest points. In figure A.7, the thalweg meanders along the bottom of the otherwise straight channel. The thalweg in figure A.7 is longer than the channel as a whole, because the thalweg bends back and forth along the channel bottom. The thalweg’s longer length makes its slope lower than the average channel slope. As the water surface rises in this channel during a high-flow event, flow straightens out and slope increases.

Figure A.7—This straight reach of the San Pedro River, Arizona, has ameanderingthalweg.

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Stream Simulation

Localchannelslopesvary,reflectingthepresenceofmultiplebedforms suchassteps,riffles,pools,andobstructions(figureA.8).Athigherflows,watersurfaceslopeevensoutsomewhatbecausebedformsaresubmerged.

FigureA.8—Pool-riffleandstep-poolchannelprofilesshowingvariablelocalslopes.FromKnighton(1998).permissiontouserequested.

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A.3.3 Channel Pattern

Channelpatterns—alsoreferredtoasplanformcharacteristics—areusuallyclassifiedasstraight,meandering,braided,oranastomosing(figureA.9).Patternisdeterminedbyfactorslikeslope,confinement,sedimentsupply,channelandvalleymaterials,andriparianvegetation(Knighton1998).

Straightalluvialchannelsarerelativelyrareinnature.Moststreamstendtomeander,unlesstheyaretightlyconfinedinanarrowvalleyorgully.Channelsinuosity—theratioofstreamlengthtovalleylength—describesthedegreeofmeandering(seefigureA.10).Meanderingstreamsareinherentlymoredynamic,andtheirtendencytoshiftlocationacrossthevalleybottomincreaseswithsinuosity,bedload,andslope.Themoreerodiblethebanks,themorechangeablethestream.

FigureA.9—Channelpatterns.FromThorneetal.(1997),reproducedwithpermissionfromJohnWileyandSons,Ltd.

Meanderwavelength(L),amplitude(A),andradius of curvature(Rc)

describethegeometryofindividualmeanders(figureA.11).Theradiusofcurvatureisofparticularinterestinstream-simulationdesign,becauseitaffectsthedistributionofwatervelocitiesacrossthechannel.Atabend,watervelocityishigherneartheoutsidebankthanneartheinsidebank.Thiscross-sectionaldifferenceinvelocitycauseserosionontheouterbankanddepositionontheinsidebank,oftenresultinginmeandershift.Atroad-streamcrossings,radiusofcurvaturecanaffecttheriskofalignmentchangesoverthelifeofthecrossing(seesection6.1.1).

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Stream Simulation

FigureA.10—Channelsinuosityischannellengthdividedbyvalleylength.

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FigureA.11—Commonmeandergeometrymeasurements.

Braidedchannelsconsistofmultiplewideandshallowchannelsseparatedbypoorlyvegetatedbardeposits.Individualchannelsandbarsfrequentlyshiftposition[figureA.12(b)].Abraidedpatternindicatesthatsedimentsupplyishighandthatthechannelbedandbanksarereadilyeroded.Despitethefactthatchannelsandbarscontinuallyshift,thesizeandslopeofthechannelwithinthelimitsofthebraidedareamayremainthesame.Abraidedchannellikethisisindynamic equilibriumwithexistinggeomorphicconditions(Knighton1998).

Anastomosingchannelsarealsomultithreaded.However,theindividualchannelsareseparatedbyhighlystablevegetatedbarsorislands[figureA.12(c)].Anastomosingchannelstypicallyforminenvironmentswherethevalleybottomiswide,floodingishighlyvariable,floodplainsarefrequentlyinundated,andbanksarerelativelyresistanttoerosion(Knighton1998).

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Stream Simulation

(a)

(c)

(b)

FigureA.12—Streampatterns(a)meanderingreachontheDosewallipsRiver,OlympicNationalForest,Washington;(b)braidedriverintheArcticNationalWildlifeRefuge.(USFWSAlaskaphotogallery); (c)anastomosingreachonMedicineBowNationalForest,Wyoming.

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A.3.4 Channel Dimensions, Confinement, and Entrenchment

Width-to-depthratiosareoftenusedtocharacterizechanneldimensions(usuallybankfullchanneldimensions—seesectionA.4.1).Lowwidth-to-depthratiosindicatethechannelisnarrowanddeep,whereashighwidth-to-depthratiosindicatethatthechanneliswideandshallow.Width-depthratios,however,donotdescribeacross-section’ssymmetry.Bothsymmetryandwidth-to-depthrelationsvarylongitudinallyalongagivenchannel,and,inmeanderingchannels,theyarestronglyinfluencedbythecross-section’slocationrelativetobends.Crosssectionslocatedatchannelbendstypicallyhaveasymmetricshapesreflectingthepoolandpointbar(channeltypeC,figureA.13),whereascrosssectionsinstraightchannelsegmentshavesymmetrical,morerectangularshapes(channeltypeB,figureA.13).

Vegetationstronglyinfluenceschannelshape.Banksdenselyvegetatedwithdeep-rootedspecieshavenarroweranddeeperchannelsthanthosewiththinlyvegetated,grassybanks(HeyandThorne1986).Thecohesivenessofthebankmaterialalsoinfluenceschannelshape.Channelswithcohesivebanks(siltsandclays)havenarroweranddeeperchannelsthanchannelswithnoncohesive(sand,gravel)banks(Knighton1998).

Theterm“channelentrenchment”describesthedegreetowhichflowisverticallycontained(figureA.13).Thatis,asdischargeincreases,flowinanentrenchedstreamisconfinedeitherbythevalleywallsorbysteep,highstreambanks.ThisguideusesRosgen’s(1994)definitionofchannelentrenchment:theratiobetweenflood-pronewidthandchannelbankfullwidth.Flood-pronewidthisthewidthofthefloodplainorvalleybottomatanelevationtwotimesthe maximum bankfull depth.Generally,theflood-pronewidthisconsideredtocorrespondwithfloodshavingrecurrence intervalsoflessthan50years(Rosgen1994).

Channelswithentrenchmentratiovalueslessthan1.4are“entrenched,”indicatingeitherthatthevalleybottomisnarroworthattheadjacentvalleysurfaceisnotfrequentlyflooded(e.g.,itisaterrace).Channelswithentrenchment-ratiovaluesgreaterthan2.2are“slightlyentrenched,”indicatingthattheflood-pronevalleybottomsurfaceiswiderelativetothechannel.Channelswithentrenchmentratiovaluesbetween1.4and2.2areconsideredmoderatelyentrenched.

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Stream Simulation

FigureA.13—Channelentrenchment(fromRosgen1994).

Instreamsimulationweusetheentrenchmentratioasanindicatorofpotentialsiterisksassociatedwithfuturealignmentchanges;thatis,slightlyentrenchedchannelstendtoundergoalignmentchangesastheyshiftacrossthefloodplain.Slightlyentrenchedchannelsalsoaremorelikelytohaveroadfillsthatobstructfloodplains.Flood-plainobstructioncancauseproblemsforacrossingstructurebyconcentratingfloodflowsthroughit.

A.3.5 Channel Bedforms

Naturalstreamchannelshaveavarietyof bed structuresknownasbedforms,whichreflectlocalvariationsinhydraulics,particlesize,andsedimenttransport.Incoarse-grainedchannels,structuressuchaspebble clusters,transverseribs,andcobble-boulderstepscausecomplexflowpatternsofconvergenceanddivergence.Thesepatternsinturninfluencebedload transport ratesandpatterns(Brayshawetal.1983;Koster1978;WhitakerandJaeggi1982).Insand-bedchannels(figureA.14),thechannelbediseasilymobilizedintodifferentbedforms(ripples,dunes,antidunes)thatcorrespondtovariationsinflowintensity(Knighton1998).

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FigureA.14—Dependingonflowintensity,bedstructuressuchasripples,dunes,andantidunescanforminsandbedchannels,dramaticallychangingchannelroughness.RedrawnafterSimons,Li&Associates1982.

Ingravel-bedchannels,thedominantformofbedtopographytendstobealternatingpoolsandrifflesinlow-gradientchannels,andpoolsandstepsinhigh-gradientchannels.Inpool-rifflechannels,poolsarescouredalongtheoutermarginsofchannelbendsanddownstreamfromobstructionssuchasbedrockoutcropsorlargewoodydebrisstructuresthatlocallyconstrictthechannel.Poolsandpointbarsarelocatedatbends,andrifflesarelocatedinstraightchannelsegmentsbetweensuccessivemeanders.Atlowflows,flowisdeepandslowinpools,whereasflowintheadjacent,steeperrifflesisshallowandfast(figureA.15).Theaveragespacingbetweenpoolsinapool-rifflechannelisgenerallybetween5-to7-channelwidths,butspacingisvariablealongagivenchannelandcanrangefrom1.5-to23.3-channelwidths(KellerandMelhorn1978).Thespacingofpool-rifflesequencescanbeinfluencedbylargewoodydebris,largeobstructions,orbedrockoutcrops(Lisle1986;Montgomeryetal.1995).

FigureA.15—Apool-rifflereachontheFlatheadRiver,Montana.

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Stream Simulation

Step-poolsequencesarecommonbedformsinhigh-gradient,coarse-bedalluvialchannels.Stepsarecomposedofcobbles,boulders,bedrock,and/orlargewoodydebristhatextendacrosstheentirechannelperpendicularorobliquetoflow(figureA.16).Plungepoolsformatthebaseofeachstepandoftencontainfinermaterial.Instep-poolchannels,thespacingbetweenstepsrangesbetween1-and4-channelwidthsandisprimarilyafunctionofgradient,withlessdistancebetweenstepsasgradientincreases(Whitaker1987;Chin1989;MontgomeryandBuffington1997).Theheightandlengthofstepsarealsoafunctionofgradient,withstepheightsincreasingandsteplengthsdecreasingasgradientincreases(Whitaker1987;Grantetal.1990).

FigureA.16—Step-poolchannelinnorthernIdaho.

A.3.6 Flow resistance or Channel roughness

Watervelocityinastreamdependsonchannelresistance(roughness),aswellaswaterdepthandchannelslope.Astreamsimulationmimicsnatural-channelroughnesstokeepvelocitiessimilarandtorecreatethevelocitydiversitythatallowsforawidevarietyofspeciestopassthecrossing.

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Totalflow resistanceisinfluencedbythecombinedinteractionsofchannel-bedmaterial,bedforms,water-surfaceandbed-surfaceslopevariability,channelalignment,bankirregularities,andvegetation.Totalflowresistancecanbedividedintothefollowingthreecategories(Bathurst1997;Knighton1998):

l Free-surface resistancerepresentsenergylossesassociatedwithsurfacewavesandhydraulic jumps(e.g.,flowplungingoverastep).

l Channel resistancerepresentsenergylossescausedbywater-surfaceandbed-surfaceslopevariability(e.g.,slopevariabilityassociatedwithpool-riffleandstep-poolsequences),bankirregularities(e.g.,bedrockoutcrops,largewoodydebriscomplexes),andvariabilityinchannelalignment(e.g.,channelbends).

l Boundary resistancerepresentsenergylossescausedbyanumberoffactors,includinggrainroughness,formroughness,andvegetationroughness.

Channelresistancecanbeverysignificantinchannelswithmanypiecesofdebris,rockoutcropsorlargeboulders,and/orsharpbends.However,boundaryresistanceistheprimaryfactorinfluencingtotalflowresistanceofmostchannels(Limerinos1970;Hey1979;Bathurst1985;Jarrett1985).Boundaryresistanceincludesthefollowingcomponents:

lGrainroughnessrepresentsenergylossescausedbythesizeoftheparticlesandtheheighttowhichtheyprojectintotheflow:Largerparticleshavegreaterflowresistancethansmallparticles.

lFormroughnessrepresentsenergylossescausedbybedforms.

lVegetationroughnessrepresentsenergylossesassociatedwithtypeanddensityofvegetationalongchannelbanks.Taller,morerigid,andmoredenselypackedstemsincreasevegetationresistancetoflowandreduceshearstressesonbankandflood-plainsurfaces(ArcementandSchneider1989).

Boundaryresistancevarieswithdischarge,becausethedepthofwaterinfluencesthedegreetowhichthechannel-bedsediments,bedforms,andbankvegetationinteractwiththeflowingwater.Aswaterdepthincreases,theinfluenceofgrainandformroughnessdecreaseswhilevegetationroughnessincreases,becausemorewaterisincontactwiththebankvegetation.Boundaryresistanceonthefloodplain,causedbymicrotopography,vegetation,etc.,alsocontrolstheamountofwaterflowingoverthefloodplain(i.e., flood-plain conveyance).

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Stream Simulation

Ingravel-andcobble-bedchannels,grainroughnessistheprimarycomponentofboundaryresistance.Inboulder-bedchannelswithsteptopography,thecombinationofindividualparticles(grainroughness)andsteps(formroughness)determinesboundaryresistance.Insand-bedchannels,formroughnessismoreimportantthangrainroughness,becausecontinualbedformchanges(ripples,dunes,antidunes)causevariationsinboundaryresistance(figureA.14).

A.4 ChAnnel STABiliTy And equiliBriuM

Stablechannelsarechannelsthatarenotexperiencingrapid,lastingchangeindimensionsorslope.Whilestablechannelsadjusttoawiderangeofflowsandsedimentinputs,theiraveragedimensionsremainthesameoverlongperiods(decadestocenturies).

Intheshortterm,astablechannelreachmayadjustwidth,depth,and/orslopeinresponsetoafloworsedimentinputeventsuchasafloodorlandslide.However,withtime,channeldimensionsreturntotheequilibriumstate.Onaverage,astablereachisneitheraggradingnorincising,neitherwideningnornarrowing,andtheamountofsedimentcominginisthesameastheamountleavingit.Recognizingthatsuchchannelsarestablebutnotstatic,wedescribethemasbeinginquasi-equilibrium(figureA.17).

FigureA.17—Inquasi-equilibriumchannels,widthanddepthvaryaroundlong-termaveragevalues.AfterSchumm(1977).

A—23


Forachanneltobeinquasi-equilibrium,environmentalconditions,suchastheamountandtimingofrunoffandsedimentinput,alsomustbeapproximatelyconstant(orchangingveryslowly)overthedecade-to-centurytimescale.Baselevelalsomustremainthesame.Ifthesecontrolschangeenoughtocrossa“responsethreshold,”thedestabilizedchannelcanchangedramaticallyandrapidly,goingthroughaseriesofadjustmentsbeforereachinganewquasi-equilibriumstate(Schumm1977).

As we gain more understanding of climatic variability, and as human uses of land and rivers intensify, geomorphologists are increasingly skeptical about whether modern streams actually achieve quasi-equilibrium over “engineering time” (Macklin and Lewin 1997). El Niño and the North Atlantic Oscillation cause changes in rainfall regimes large enough to cause river adjustments (Lewin et al. 1988) on decade and longer time scales. In many forested environments, changing land management may be expected to progressively alter runoff and sediment-load regimes. Crossing designers should recognize the possibility that the conditions controlling stream morphology may not be stable over a structure’s lifetime. Watershed-scale investigations that deal with past, present, and future conditions, such as those outlined in chapter 4, are critical for providing the context needed for prudent design.

Mostchannelsimmediatelyadjacenttoanarrowroad-streamcrossingstructureadjusttheirformtoestablisha“new”quasi-equilibriumwiththeconditionsimposedbytheundersizedstructure(culvert).Typicalresponsesincludeaggradationandchannelwideningimmediatelyupstreamfromtheculvertinlet,andchannelwideningandincisionimmediatelydownstreamfromtheculvertoutlet.Theseadjustmentsmakethechannelmoreefficientintransportingsedimentanddissipatingflowenergy,andcreateamorestablechannelform.However,thesesameadjustmentsmaypreventaquaticorganismsfrommigratingfreelyalongthe stream corridor.Astream-simulationstructurewillrestorestreamandecologicalconnectivityattheroad-streamcrossing.Duringandafterconstructionofthestream-simulationstructure,thechannelwilladjustitsformtoestablishanewquasi-equilibrium.

A—24

Stream Simulation

A.4.1 equilibrium and Bankfull Flow

Observablechannelcharacteristicsaretheresultofbotharangeofpastdischargesandthetemporalsequenceoffloods.Nonetheless,asingledischargevalueiscommonlyusedtorepresentthe“channel-forming flow”(Knighton1998).Bankfulldischarge—themaximumdischargethechannelcancontainbeforewaterovertopsitsbanksontothefloodplain—isgenerallytakentorepresentthechannel-formingdischargeinresponsechannelsandmoderate-gradienttransportchannels.Inmanyenvironments,bankfullisapeakthatisequaledorexceededfrequently—aboutevery12 to2years.Becausethispeakisfrequentandbecauseitusuallytransportsasignificantamountofsediment,itisgenerallyfoundtotransportmoresedimentcumulativelythananyotherflowoveralongperiodoftime(Hey1997).

Sincewaterandsedimentinputscontinuallyfluctuate,thechannelcontinuallyadjusts.However,unlessitistrulyunstable,itsdimensionswillvaryaroundequilibriumvaluesthatcanoftenbeconsistentlyrelatedtobankfulldischarge(EmmettandWolman2000)(seefigureA.18).Basedontheserelationships,bankfulldischargeisoftenusedasthereferencedischargefordesigningchannels(Hey1997).Weusebankfullinstreamsimulationforthesamereason.

FigureA.18—Relationshipofbankfullchanneldimensions(determinedinthefieldusinggeomorphicindicators)tobankfulldischarge(determinedfromgaugerecordsatobservedbankfullelevation).DatafromCastroandJackson(2001).

0

1

10

100

1,000

1 10 100 1,000 10,000 100,000

bankfull discharge (cfs)

bankfull width and depth (ft)

widthdepth

data from Castro and Jackson 2001

A—25


Bankfullisnotthechannel-formingflowinallstreams.Insteeptransportstreamswithlargebedmaterial,theflowthatmovesthelarge,structuralbedformscanbemuchhigher(i.e.,lessfrequent)thaninlow-gradientalluvialchannels.Thechannel-formingflowmaybethe25-yearfloworhigherinaboulder-bedchannel,dependingonsedimentinputsfromthewatershed(MontgomeryandBuffington1996;Grantetal.1990).

A.5 FluviAl ProCeSSeS

Thissectiondescribeskeyprocessesthatbotharecreatedandaffectedbychannelmorphologiccharacteristicssuchaspattern,channelshape,slope,andbedstructure.Understandingtheseprocessesiscentraltodesigningastream-simulationstructurethatcansustainitselfinthechangingstreamenvironmentoverthelongterm.

A.5.1 Sediment dynamics

Themorphologyofachannelreflectstheinteractionbetweenhydrodynamicforcesactingonthechannelbedandtheresistingforcesofthematerialsthatmakeupthechannelbed.Whenthehydrodynamic(liftanddrag)forcesexceedtheresistingforces(particleweightandfriction),sedimentisentrained(mobilized),transported,andlaterdeposited,causingthechanneltochangeitsformorgrain-sizedistribution.

Generally,sedimentisentrainedandtransportedaswaterrisesandpeaksinarunoffevent,anditisdepositedagainashighflowrecedes.Stabilityofaconstructedstreambed—likeallstreambeds—dependsonthebalancebetweenentrainmentandtransportofbedmaterialandresupplybydepositionofmaterialtransportedfromupstream.

Entrainmentofnoncohesivesedimentsbyflowingwaterdependson:

lSedimentproperties:size,shape,density,pivot angle.

s Larger,heavierparticlesrequirefasterdeeperflowtomove.Angularrockstendtolocktogetherbetterthanroundedrocks,andtheyresistrolling.Elongatedrockstendto‘shingle’orimbricate(overlap)alongthedirectionofflow,andtheycanformveryresistantbedsurfaces.

A—26

Stream Simulation

lChannel-bedcomposition:particlepackingandorientation,sorting,distributionofbedforms,anddegreeofparticleexposuretoflow.

s Inpoorlysortedchannelbeds,thestabilityofaparticleisinfluencedbytheparticlesadjacenttoit(Andrews1983;WibergandSmith1987;Komar1987)(figureE.1).Smallerparticlesareshieldedbehindlargerparticlesinpoorlysortedbeds,andstrongerflowsarenecessaryforentrainingthemthaninawell-sortedbed.Largerparticles,incontrast,areentrainedatweakerflowsthaninawell-sortedbed,becausetheyprojectintotheflow.Particlesthatprojecthigheraremoreexposedtotheforceofthewater,andthisincreasedexposureenhancestheirentrainmentdespitetheirgreaterweight.

lFlowhydraulics:velocity,slope,waterdepth,andturbulence.

Shear stress is a measure of the hydrodynamic force exerted by flow on the channel bed and banks. Critical shear stress for a particle is the force that entrains it, that is, that initiates its motion by lifting it off or dragging it along the bed.

Watervelocityandshearstressvarywithlocalchangesinchannelslopecontrolledbysuchthingsaswoodydebris,rockweirs,steps,orgravelbars.Thesebedstructuresflattenlocalslopesothattheupstreambedretainssmallerparticlesthanabedofuniformslope.Evensmallembeddedpiecesofwoodcancontrolslope.Instreamsimulation,averageslopeisanimportantparameter,buttheteammustalsopayattentiontothebedstructuresthatcontrolslopeandcreateboth‘sedimentstoragesites’anddiversepathwaysforanimalmovement.

Understandingtherelativemobilityofdifferentbedmaterialsandstructuresisalsocritical.Forexample,sand-bedchannelsarehighlymobile,andtheirbedsarecontinuouslyinmotionatmostflows.Insomegravel-andcobble-bedchannels,thesurfaceofcoarsegravelsandcobblesisrelativelystableduringfrequent,moderatefloods,althoughlargequantitiesofsandsandgravelsmoveoverthecoarsesurfacelayer(JacksonandBeschta1982).Manygravel-bedstreamsarearmored,andtheirtightlypackedsurfacelayershavebeenwinnowedoffinermaterials.Theseintermediate-mobilitystreamsmaytransportverylittlesedimentuntilflowisabletobreachthearmorlayer.Cobble-andboulder-bedchannelsarequiteresistanttoerosion,andtheselargerocksmoveonly

A—27


duringinfrequent,exceptionalfloods(MontgomeryandBuffington1993;Knighton1998).Duringfrequent,moderatefloods,however,largequantitiesofsandandgravelcanbetransportedoverandaroundtherelativelyimmobilecobbleandboulderstructures.

A.5.2 vertical Channel Adjustment

Ashighflowentrainssediment,partsofthestreambedmaylowerorrisebyinchesorevenfeet.Then,asflowrecedesandsediment transport capacitydrops,thescouredorfilledsectionsmayreturntotheirprefloodelevation(Andrews1979).Aftertheevent,thatscourandfilloccurredmaynotbeatallevident,becausethestreambedoftenequilibratesatthesameelevationasbefore.Stream-simulationculvertsneedenoughheadroomandbeddepthtopermittheseprocessestooccur.Highflowscourandfillislessimportantinstreambedsthatareresistanttoerosion(e.g.,wherebedmaterialislarge,well-packed,orimbricated).

Longer-lastingverticalchangesoccurwhensedimentorwaterregimeschange,orwhenchannelsarestraightenedorclearedofdebris.Channelsaggrade(fill)whensedimentsuppliedfromupstreamexceedsthelocaltransportcapacity,andtheydegradeorincise(cut)whenthereverseistrue.Aggradationistheverticalriseinthebedelevation,ariseresultingfromsedimentdeposition,whichcanoccurupstreamofabackwaterstructuresuchasabeaverdamoranundersizedculvert.Aggradationisacommonriskatconcaveslopetransitions(figure5.12).Italsocanoccurifflowisremovedfromachannelbydiversionorifsedimentloadsincreaseasaresultoflandusechanges.

Channelincision(ordegradation)isaloweringofchannelelevationthatoccurswhenlocalerosionexceedsdepositionofsedimenttransportedfromupstream.Followingaresomefamiliarlocationswherechannelincisioncommonlyoccurs:

lStreamreachesbelowdams,whichcutoffthesupplyofsedimentandaltertheflow regime.

lForeststreamswherewoodthatcontrolledgradehasbeenremoved.

lWatershedswherethefrequencyormagnitudeofpeakflowshasincreasedduetolandcoverorclimaticchanges.

Channelincisioncancreateaself-reinforcingfeedbackloop.Asthechanneldeepens,largerandlargerfloodsarecontainedwithinitsbanks.Thestreambedexperiencesincreasingshearstress,andcontinuestoinciseuntilitencounterserosion-resistantmaterial.

A—28

Stream Simulation

Alltheseprocessescanseverelyaffectsimulatedstreambeds.Projectteamsshouldunderstandthedirectionandmagnitudeofprobableverticalchannelchangeoverthelifetimeoftheplannedstructure,andtheyshoulddesignthestructuretoaccommodatethosechanges.

A.5.3 lateral Channel Adjustment

Manystylesoflateralchanneladjustmentexist,andsomeofthemoccurinresponsetoverticaladjustments.Aggradingchannelstendtowidenbecause,asthechannelfills,flowsapplymoreerosivepressuretothebanks(figure4-3).Ontheotherhand,sedimentdepositionalsocanresultinchannelnarrowingifvegetationisabletocolonizenewbardepositsalongthebanks.Althoughincisingchannelsareinitiallynarrow,theytendtowidenastheirbanksbecometallerandmorepronetosloughing(figures4.6andA.28).

Anotherfluvialprocessimportantinstream-simulationdesignislateral-channelmigration.Asdescribedinchapter1,lateralshiftingcanchangethestream’salignmenttoacrossing,andaffectthecrossing’sabilitytopasswater,sediment,anddebris.Acrossinglocatedonachannelbendmayneedtobepositionedasymmetricallyoverthechanneltoaccommodatefuturechannelshifting.Ifthebendissharportherateofchannelmigrationishigh,alternativesolutionssuchasabridgespanningthezoneofpotentiallateralmigrationmaybenecessary.

Innarrowvalleyswherethevalleywallsareclosetothechannel,thepotentialforlateral-channelmigrationislimited.However,streamsinwidealluvialvalleysshiftpositionlaterallyacrossthevalleybottom,andtheprocessmaybeeithergradualorrapid.Low-gradientsandandgravelchannelsgraduallyshiftbymeandermigration;duringfrequent,moderatefloods,thestreamerodestheouterbanksofbendsandbuildsthepointbarontheinsidebank.Suddenandpronouncedlateralshiftingcanoccurduringinfrequent,large-magnitudefloodsorwhenwaterscoursaroundobstructionssuchassedimentorwoodaccumulations.

Therateofmeandermigrationdependson:

lBendgeometry(tighterbendstendtomigratefaster).

lTheresistanceoftheouterbanktoerosion(bankheight,materials,vegetation,moisture,etc.).

lThemagnitudeanddurationofthehydraulicforcesactingonthebank(Knighton1998).

A—29


Certaintypesofsinuousplanformpatternsindicateasystematicdownstream,down-valleymeandermigration,whileothersindicateaprocessofperiodicbendcut-offs(Thorne1997;Knighton1998)(figureA.19).

FigureA.19—Typesoflateral-channeladjustment.FromThorne(1997).ReproducedwithpermissionfromJohnWiley&Sons,Ltd.

Regardlessofvalleywidth,standingtreesandlargewoodydebrisinandalongthestreamcansubstantiallyaffectchannelprocessesbyincreasingflowresistance,affectingbankerodibility,andprovidingobstructionstoflow(Hickin1984;Thorne1990).Largewoodydebrisdepositedinandalongchannel/flood-plainmarginscanalter channel patterns by diverting flowaroundtheobstructionorcreatinglow-velocityzoneswheresedimentandorganicmatterdeposit(Fetherstonetal.1995;AbbeandMontgomery1996).Thisdepositioninturnprovidesfreshsurfacesfortheestablishmentofnewvegetation.Dependingonthevegetationtype,rootingstrengthcanstabilizethosesurfacesandinfluencethedegreeoflaterchannelmigration.

Bankvegetationhasastronginfluenceonlateraladjustability.Deep-rootednativespeciesoftenprovideverystrongbankreinforcement.Ifnativespeciesarereplacedbyshallower-rootedexoticplants,bankerosioncanaccelerate,causingthechanneltowidenorincreasingtherateofmeandermigration.

A—30

Stream Simulation

A.5.4 Flood-plain inundation and dynamics

Afloodplainisavalleysurfacebeingconstructedasthecurrentstreamdepositssediment.Itisatemporarysedimentstorageareaalongthevalleybottom,composedofsedimentsdepositedduringoverbankfloods.Inmeandering,low-gradientchannelswithrelativelylarge,well-developedfloodplains,lateral accretionisthedominantflood-plainformationprocess.Inotherwords,theflood-plainsurfaceisformedasthestreambuildspointbarsduringmeandermigration(NansonandCroke1992).Insteepchannelswithnarrow,discontinuous flood plains,vertical accretion(sedimentdepositionontopofthefloodplain)isthedominantflood-plainformingprocess,becausecoarsechannelsedimentsinhibitchannellateralmigration(NansonandCroke1992).

Flowoccursfrequentlyoveratruefloodplain(wheneverbankfulldischargeisexceeded).Other,higherflatvalleysurfaces(terraces)arefloodedatlessfrequentintervals.Terracesurfacesarenotbeingconstructedbythecurrentstream,althoughitmaybeerodingthem.Bothlowterracesandfloodplainscanhaveerosionanddepositionfeatures,andthe“flood-prone zone”(figureA.13)mayencompassboth.

Floodplainsarekeyelementsaffectingchannelstabilityinmanyresponsereaches.Thestream’sabilitytooverflowthefloodplainlimitschannelerosionduringhighflowsbylimitingflowdepthinsidethemainchannel.Duringaflood,flowinthemainchannelisfastanddeep,whileflowovertheflood-plainsurfaceisslowerandshallower.Thereisgrowingrecognitionthatriparianforestsplayasignificantroleinthedevelopmentofchannelandflood-plainmorphology.Theseforestsstabilizefloodplainsduringhighflowsandcontributelargewoodydebrisinandalongchannelsthatmodifiesflowhydraulicsandsedimenttransport(e.g.,Thorne1990;AbbeandMontgomery1996).

Thedensityandtypeofvegetationonthefloodplaininfluencethevelocityanddepthofflowoveritssurface,therebyinfluencingflood-plainconveyance,whichisthewaterdischarge(volumeperunittime)acrossthefloodplainorflood-pronezone.Flood-plainconveyanceisaveryimportantvariableatastream-simulationcrossing,becauseduringafloodthevolumeofflowonahigh-conveyancefloodplainmaybesolargethatitrequiresspecialhandlingtoavoidconcentratingflowthroughthecrossing.

A—31


A.6 ChAnnel ClASSiFiCATion SySTeMS

Toprovideaframeworkforassessingchannelconditions,interpretingfluvialprocesses,predictingchannelresponses,andmakingdesignrecommendations,thisguideusesthechannel-typeclassificationsthatMontgomeryandBuffington(1993,1997)andRosgen(1994,1996)developed.Bothclassificationsareusefulinstreamsimulationforsomewhatdifferentpurposes.

Astheinformationinthisappendixonlysummarizestheseclassificationsbriefly,westronglyencourageyoutoreadtheoriginalpapers.

A.6.1 Montgomery and Buffington Channel Classification

TheMontgomeryandBuffingtonchannel-classificationsystemisbasedprimarilyonstreambedstructure(bedforms).Theclassification,whichappliestomountainstreams,identifiessixdistinctalluvialchanneltypesandtwononalluvialchanneltypes(bedrockandcolluvial,sectionA.3.1).Theclassificationofthealluvialtypesisbasedonbedstructureandtheresultingchannelroughnessandenergydissipationcharacteristics.MontgomeryandBuffington(1993,1997)alsodistinguish“forcedmorphologies,”inwhichflowobstructions(suchaswood)“force”achannelmorphologythatisdifferentfromwhatwouldexistiftheobstructionswerenotpresent.

A—32

Stream Simulation

Cascadechannels(figureA.20)generallyoccuronsteepslopes(i.e.,about10-to30-percentslope),andarefrequentlyconfinedbyvalleywalls.Theirbedsare‘disorganized,’withcobblesandbouldersscatteredorclusteredthroughout.Smallpoolsthatdonotspantheentirechannelwidth—andtumbling,turbulentflowovertheindividualrocks—characterizethistype.Thelargeparticlesthatformthebedmobilizeonlyduringverylargefloods(50-to100-yearflows),andtheymayincludehillslope-derivedmaterials(e.g.,colluviumfromdebrisflows,rockfalls)aswellasfluviallyplacedsediments.

Step-poolreaches(figureA.21)havelargerocksorpiecesofwoodthatformchannel-spanningsteps,usuallyspacedataboutonetofourchannelwidths.Beloweachstepisapoolcontainingfinersediment.Becauseenergyisefficientlydissipatedasflowfallsintothepools,thisbedstructureismorestablethanwouldbeexpectedforalessorganizedstreambed.Thestepsmobilizeandreformduringlargefloods,butfinersedimentmovesoverthestepsduringmoderatehighflows.Typicalaveragechannelslopesrangefrom3-to10-percentslope.

FigureA.20—Cascadereach:(a)schematicplanviewandprofile,and(b)cascadereachonSelwayRiver,Idaho.

(a)

(b)

A—33


FigureA.21—Step-poolreach:(a)schematicplanviewandprofile,(b)step-poolreachonBoulderCreek,Colorado,and(c)forcedstep-poolchannel,MitkofIsland,Alaska.

(a)

(c)

(b)

A—34

Stream Simulation

Plane-bedreaches(figureA.22)“havelongstretchesofrelativelyfeaturelessbed”(MontgomeryandBuffington1993,1997)withoutorganizedbedforms.Theyareon“moderatetohighslopesinrelativelystraightchannels,”usuallywitharmoredgravel-cobblebeds.Bedmobilizationoccursatflowsnearbankfull.InRosgen’ssystem,aplane-bedreachmightbeeitheraB-orG-channeltype,andcouldhavebedmaterialasfineassand.

FigureA.22—Plane-bedreach:(a)schematicplanviewandprofile,and(b)plane-bedreachontheSitkumRiver,Washington.

Pool-rifflereaches(figureA.23)havelongitudinallyundulatingbeds,witharepeatingsequenceofbars,pools,andrifflesregularlyspacedatabout5-to7-channelwidthsapart.Largewoodydebriscanalterthespacing.These

(a)

(b)

A—35


channels,whichusuallyhavefloodplains,maybesand-tocobble-beddedstreams.Dependingontheirdegreeofarmoring,bedmobilizationmayoccuratorbelowbankfull.ThesemaybeRosgenC,E,orFstreams(seesectionA.6.2forRosgenclassifications).

FigureA.23—Pool-rifflereach:(a)schematicplanviewandprofile,(b)pool-rifflereachonLibbyCreek,Washington.

(a)

(b)

A—36

Stream Simulation

Dune-ripplereaches(figureA.24)havelowgradientswithsandandfine-gravelbeds.Thesestreambedstransportsedimentatvirtuallyallflows,andthebedformschangedependingonwaterdepthandvelocity(figureA.14).Ifthechannelissinuous,thesestreamsalsocanhavepointbars.

FigureA.24—Dune-ripplereach:(a)schematicplanviewandprofile,and(b)dune-ripplereachonCoalCreek,Washington.Photo:KozmoKenBates.

(a)

(b)

A—37


BecausetheMontgomeryandBuffingtonchanneltypesarebasedonstreambedmorphology,theyarehighlyusefulforstream-simulationdesign,wherewemimicbedstructureandchannelroughnesstocreateasimulated channelthatwilladjustsimilarlytoitssurroundingreaches.Eachtypeisuniquelyadjustedtotherelativemagnitudesofsedimentsupplyandtransportcapacity.Thisrelationshipdetermineshowsensitivethechannelistochangesinwaterandsedimentinputs.

MontgomeryandBuffington(1997)wereabletodetermineforeachchanneltypethetypicalfrequencywithwhichthestreambedismobilized(tableA.2).Knowingthetypicalfrequencyisimportantforstreamsimulation,becausethesimulatedbedshouldmobilizeatthesameflowsasthesurroundingreaches.Transportreachessuchascascadeandstep-poolchannels,forexample,arerelativelystable.Thecoarsebedmaterialthatcontrolschannelforminthesechanneltypesmobilizesonlyininfrequentfloods,althoughfinersedimentsanddebrisareefficientlyconveyedoverthelargerocksduringnormalhighflows.Responsereachessuchaspool-riffleanddune-ripplechannelscanexperiencesignificantandpersistentchangesinchanneldimension,slope,andplanformwhenhydrologicconditionsandsedimentsupplychange.Thesechannelsoffermorechallengetocrossingdesignersthandothemorestabletransportreachtypes.Chapter6outlinesdesignoptionsforstreamsimulationsinvariouschanneltypes.SeeMontgomeryandBuffington(1993,1997)foracompleteexplanationoftheirclassificationsystem.

A—38

Stream Simulation

Str

eam

bed

mo

bili

ty

Term

ed “

live

bed”

; sig

nific

ant s

edim

ent

tran

spor

t at m

ost fl

ows.

Arm

ored

bed

s us

ually

mob

ilize

nea

r ba

nkfu

ll.

Nea

r ba

nkfu

ll or

at h

ighe

r flo

ws,

de

pend

ing

on g

rain

siz

e.

Fin

e m

ater

ial m

oves

ove

r la

rger

gra

ins

at fr

eque

nt fl

ows.

Bed

-for

min

g ro

cks

mov

e at

hig

her

flow

s de

pend

ing

on

size

; ofte

n >

Q30

.

Sm

alle

r be

d m

ater

ial m

oves

at

mod

erat

e fr

eque

ncie

s (fl

oods

hig

her

than

ban

kful

l). L

arge

r ro

cks

are

imm

obile

in fl

ows

smal

ler

than

~Q

50.

Bed

load

mov

es o

ver

bedr

ock

at

vario

us fl

ows

depe

ndin

g on

its

size

. M

ay b

e th

in la

yer

of a

lluvi

um o

ver

bedr

ock.

Woo

d ca

n st

rong

ly a

ffect

se

dim

ent m

obili

ty.

Fin

e se

dim

ent m

oves

ove

r im

mob

ile

bed

at m

oder

ate

flow

s de

pend

ing

on

its s

ize.

May

be

thin

laye

r of

allu

vium

overim

mobilebed.

En

tren

chm

ent

Slig

ht

Slig

ht

Slig

ht to

en

tren

ched

Mod

erat

ely

entr

ench

ed to

en

tren

ched

Ent

renc

hed

Any

Any

Slo

pe2

<0.

1%

0.1-

2%

1-3%

3-6%

6-30

%

Any

Any

Do

min

ant

rou

gh

nes

s an

d s

tru

ctu

ral

elem

ents

1

Sin

uosi

ty, b

edfo

rms,

ba

nks.

Sm

all d

ebris

may

pr

ovid

e st

ruct

ure.

Bar

s, p

ools

, gra

ins,

si

nuos

ity, b

anks

.

Gra

ins,

ban

ks.

Ste

ps, p

ools

, ban

ks.

Deb

ris m

ay a

dd

sign

ifica

nt s

truc

ture

.

Gra

ins,

ban

ks.

Bed

and

ban

ks.

Sin

uosi

ty, b

anks

, bed

irr

egul

ariti

es.

Bed

mat

eria

l

San

d to

m

ediu

m g

rave

l

Gra

vel,

ofte

n ar

mor

ed

Gra

vel t

o co

bble

, usu

ally

ar

mor

ed

Cob

ble

to

boul

der

Bou

lder

Roc

k w

ith

sedi

men

t of

vario

us s

izes

in

tran

spor

t ov

er r

ock

surf

ace

Silt

to c

lay

Dun

e-rip

ple

Poo

l-riffl

e

Pla

ne-b

ed

Ste

p-po

ol

Cas

cade

Bed

rock

Cha

nnel

s in

coh

esiv

e m

ater

ials

(s

i/cl)

Nonalluvial Alluvial reaches reaches

RE

AC

H T

YP

E

TY

PIC

AL

CO

ND

ITIO

NS

Transport reaches Response reaches

TableA.2—Characteristicsofchanneltypes(adaptedfrom

Montgom

eryandBuffington,(1993,1997)

1 Anychanneltypecanbe‘forced.’Inforcedchannels,woodydebrisisanimportantstructuralelement.

2 Slopeisnotadiagnosticcriterion,andsloperangesoverlapmorethanthe‘typical’valuesinthistablereflect.S

loperangesshow

nhereare

from

figures16and19inMontgom

eryandBuffington(1993).Seealsofigure6andrelatedtextinMontgom

eryandBuffington(1997).

A—39


A.6.2 Rosgen Channel Classification

Rosgen’s(1994)majorchanneltypesarebasedonthefollowingchannelvariables:entrenchment,width-depth ratio,pattern,andgradient.Rosgen’smajorchanneltypeclassesareparticularlyusefulinstreamsimulationbecausetheyreflectthedegreeofchannelentrenchment—animportantvariableforassessingrisksassociatedwithstreamsimulation.Streamswithhighentrenchmentratios(unentrenchedchannels,RosgentypesC,DA,andE)haverelativelywidefloodplainsthatmaybefloodedfrequently.Toavoidconcentratingoverbankflood-plainflowsthroughthepipe,teamsmustincorporatespecialdesignfeaturesinstream-simulationinstallationsonthesechanneltypes.Streamswithlow-entrenchmentratios(entrenched channels,RosgentypesA,B,andG)havefewerrisksassociatedwithflood-plaininundationandlateraladjustmentpotential.

EachofRosgen’sninemajorchanneltypes(seefigureA.25)hastypicalsloperangesthatcanbequitebroad.Subgroupswithineachofthemajortypesaredividedbybedmaterialtypeanddesignatedwithnumbers.Rosgen’ssystemdoesnotspecificallyconsiderchannelswherewoodydebrisisadominantinfluenceonmorphology.

Rosgen(1994)developedinterpretationsofeachchanneltype’ssensitivitytoadisturbance,itsrecoverypotential,susceptibilitytobankerosion,andrelianceonvegetationforformandstability.Hisinterpretationsaboutchannelresponsestodisturbanceareveryusefulforpredictinghowthechannelmightchangewhensomechangeoccursinwaterorsedimentinput,whenlocalconditions(suchasriparianvegetation)change,orduringandafterchannelincision(seealsosectionA.7).Projectteamsneedtoconsiderthesepotentialchangeswhenassessingsiteandwatershedrisksandpotentialchannelresponsestothecrossing(chapter4).

A—40

Stream Simulation

FigureA.25—

ChanneltypesdefinedbyRosgen(1994).U

sedbypermission.

A—41


A.7 unSTABle ChAnnelS

A.7.1 inherently unstable landforms and Channel Types

Somechanneltypesareinherentlyunstable;thatis,theyarenaturallysubjecttorapidchangesinchannellocation,dimension,orslope.Certainlandformsalsoarenaturallyunstable,andthechannelsthatdrainthemaresubjecttoepisodic(andsometimesunpredictable)changes,whichmaydestabilizethemforaperiodoftime.Likestreamsaffectedbyunusuallylargefloodsorotherevents,recoverycantakeyearsordecades,dependingonchannelresilienceafterdisturbance.

Braided streams[figureA.12(b)]aredifficultsitesforroad-crossingstructures,becausetheyhavehighsedimentloadsthatcanplugstructuresandbecauseindividualchannelscanchangelocationduringfloods.Thesestreamsarebestavoidedascrossingsites.(However,wherethebraidedchannelasawholeisconfinedandunabletoshiftlocation,ateammightconsideranopenstructurethatcrossestheentirechannel.)

Active alluvial fansarelocatedwhereaconfined channelemergesintoawidervalley,spreadsout,anddepositssediment(figureA.26).Duringhighdebris-ladenflows,somuchsedimentmaybedepositedthatitblocksthemajorchannel;consequently,flowjumpstoanewlocationandformsanewchannel.Severalchannelsmaybeactiveatonce.Crossingstructurescanbeisolatedwhenthechannelchangeslocation,andstructurescanalsoexacerbatethelikelihoodofchannelshiftiftheyplugfrequently.Evenwhereafandoesnotappeartobeactive,itstillconstitutesariskylocationforstructuresofanykind,becausearareflood/debrisfloweventcanresultincatastrophicsedimentdeposition.

FigureA.26—AlluvialfanborderingtheNoatakRiver,Alaska.Photo:USFWSAlaskaImageLibrary.

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Stream Simulation

Forallofthesereasons,avoidplacingnewcrossingsonfansandbraidedchannels.

Arroyosareincisedorincisingchannels,usuallywithephemeralflowregimes.Theyarefoundinsemiaridandaridenvironmentswherehighflowsareoftenextremelyflashy.Littleornoriparianvegetationmayborderanarroyochannel,andthebankscanbehighlyerodible.Duringhighflows,thechannelmaycarrylargeamountsofsedimentanddebris,andmaybepronetoshiftinglocation.Someofthesechannelsarebraided,andtheproblemstheyposeforcrossingsofanykindarethesameasthoseforbraidedstreams.

Onornearslopes prone to mass wasting,largeerosionaleventscanbeexpectedtocausesignificantchangesinthedownstreamchannel(figureA.27).Evenstabletransportreaches,iftheyareimmediatelydownstreamofaslopepronetolandslides,earthflow,gullying,orseverebankerosion,canbeexpectedtoundergofloweventswheresedimentloadsarehighenoughtocauseaculverttoplug.Insteepterrain,wheremanycrossingsexistonasinglechannel,thedominoeffectofasinglecrossingfailurecancascadedownstreamandactuallycauseadebrisflow.Unconsolidatedfine-grainedglacialdepositsareespeciallysubjecttorapidsurfaceerosionandslumping,andwecanexpectchannelsdrainingthemtoexperiencelargebed-elevationchangesfrombothheadcuttingandepisodicsedimentinputsfromsurroundingslopes.Siteslocatedatthetransitionpointbetweenatransportandresponsereachareparticularlyvulnerabletosedimentdepositionduringlargeerosionalevents.

FigureA.27—Streamerodingthetoeofaslumpislikelytotransportlargevolumesofsedimentthatmayplugdownstreamculverts.

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Unconfined meandering streamsonwidefloodplainsarepronetochannelshiftbymeandermigration,asdescribedearlier.Suchstreamsarenonethelessconsideredtobeinequilibriumaslongastheymaintainconsistentchanneldimensionsandslope.Inmanycases,theirrateofmeandermigrationmaybeslowrelativetothelifeofthestructure.However,landdevelopmentandmanagementfrequentlyacceleratethisnaturalprocessofchannelmigration,aconsiderationtobearinmindbeforeinvestinginacrossingstructure.Ashiftingchannelcanmovesothatitnolongerapproachesthecrossingperpendicularly—andasharpangleofapproachtendstoincreasesedimentdepositionabovetheinletbyforcingthewatertoturn.Likewise,asharpangleincreasesthepotentialfordebrisblockageandthereforeovertoppingfailure.

Anadditionaleffectofcrossingsonsuchchannelsisthattheirapproachesareoftenonroadfillraisedaboveseasonallywetorinundatedfloodplains.Blockingthefloodplainobstructstosomedegreetheerosionalanddepositionalprocessesthatconstructandmaintainfloodplainsandthediversehabitatstheyoffer.Theroadfillmayobstructside channels that are essentialhabitatsandmigrationcorridorsforfish.Forcingtheoverbank flowstoconcentrateinthestructurecanalsocausescourthroughordownstreamofthecrossing.

A.7.2 Channels responding to disturbances

Streamsthathavebeendestabilizedbychangesinvegetativecover,base level control,climaticevents,earthquake,etc.,canundergomajorchangesinelevation,channelwidthanddepth,and/orothercharacteristicsbeforereturningtoaquasi-equilibriumstate.Thechangesoftenoccurinapredictablesequence,representedconceptuallyaschannel-evolutionmodels.

Oneclassicchannel-evolutionmodelisespeciallyimportanttounderstandduringworkonstreamcrossings.Thismodel(Schumm,Harvey,andWatson1984)describeschannelincisionthatcouldbedueeithertochannelization(channelstraighteningand/orconstriction),base-levellowering,orincreasesinrunoff.Inthismodel(figureA.28),anunentrenchedstreamdowncuts,banksbecomeunstableanderode,andthechannelwidensuntilanewfloodplainand/orunentrenchedstreamsystemestablishesatthelowerelevation.

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Stream Simulation

FigureA.28—Channelevolutionmodelshowshowachannelevolvesfromactiveincisiontostabilization(Castro2003).

Channelincisionprogressesupstreamunlesstheheadcutischeckedbyanatural-orengineered-gradecontrol,suchasaroad-streamcrossingstructure.Downstreamreachesareatalaterstageintheevolutionarysequencethanupstreamones,andcanthereforebeusefulforpredictingthemagnitudeofchangestobeexpectedupstream.Thisevolutioncantakeyears,decades,orcenturies,dependingontheresistanceofthematerialsbeingeroded,andcanaffectentiredrainagebasins.Tributariesfarremovedfromtheoriginalcauseofincisioncanbeaffectedasheadcutsmoveupthemainchannelandlowerthebaselevelfortributaries.Thestagesaremoreclearlydistinguishableinstreamswithcohesivebedandbankswhereactivelyerodingfeatures(erodingbanks,nickpoints)holdsteepslopes.Ingranular materials(figureA.3),thefeaturesarelesseasilydistinguishedbecausetheyarelessabrupt(FederalInteragencyStreamRestorationWorkingGroup1998).Wherechannelsegmentsupstreamanddownstreamofacrossinghaveverydifferentcharacteristics,understandingwhetherthosedifferencesareduetochannelevolutionorsomeothercauseiscriticaltoastream-simulationdesign.

Ifitisnotpossibletoavoidanunstablechannelbyrelocatingthecrossing,predictthedirectionoffuturechange,anddesignthestructuretoaccommodateit.Doingallofthiswellrequiresabackgroundandexperienceinfluvialgeomorphologyand river dynamics.

geomorphic principles applied in stream simulation · appendix a—geomorphic principles applied in...

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