Little Otter Creek Watershed: Phase 2 Stream Geomorphic Assessment
Geomorphic Principles Applied in Stream Simulation · Appendix A—Geomorphic Principles Applied in...
Transcript of Geomorphic Principles Applied in Stream Simulation · Appendix A—Geomorphic Principles Applied in...
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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
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Stream Simulation
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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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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
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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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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.
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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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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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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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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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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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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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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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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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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).
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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.
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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
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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.
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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
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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.
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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).
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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.
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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.
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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.
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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)
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Appendix A—Geomorphic Principles Applied in Stream Simulation
FigureA.21—Step-poolreach:(a)schematicplanviewandprofile,(b)step-poolreachonBoulderCreek,Colorado,and(c)forcedstep-poolchannel,MitkofIsland,Alaska.
(a)
(c)
(b)
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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)
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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)
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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)
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Appendix A—Geomorphic Principles Applied in Stream Simulation
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.
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Str
eam
bed
mo
bili
ty
Term
ed “
live
bed”
; sig
nific
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edim
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tran
spor
t at m
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s us
ually
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nea
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at h
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on g
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ay a
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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).
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Appendix A—Geomorphic Principles Applied in Stream Simulation
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).
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FigureA.25—
ChanneltypesdefinedbyRosgen(1994).U
sedbypermission.
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Appendix A—Geomorphic Principles Applied in Stream Simulation
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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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.