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Pilchuck River Bridge 581 Channel Migration Zone Study Final

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Pilchuck River Bridge 581 Channel Migration Zone Study Final PilchuckRiverBridge581 ChannelMigrationZoneStudy Final Preparedfor: SnohomishCounty Preparedby: NorthwestHydraulicConsultants andWatershedGeoDynamics Seattle,WA February26,2013 Acknowledgements Thefollowingindividualsparticipatedinthisevaluation: ErikRowland‡,P.E. ProjectManager,technicalanalysis KathyDubé*P.G.,P.E.G. Fielddatacollectionandtechnicalanalysis AndrewNelson‡ DesktopGISanalysis,fielddatacollection,andtechnicalanalysis ‡NorthwestHydraulicConsultants *WatershedGeoDynamics WegratefullyacknowledgetheassistanceofthefollowingSnohomishCountystafffor providingprojectinformation: JimWeelborg,P.E. LisaGirven PilchuckRiverBridge581 ChannelMigrationZoneStudy i ExecutiveSummary SnohomishCountyhasretainedNorthwestHydraulicConsultantstoprepareaChannel MigrationZone(CMZ)studytosupportthereplacementofBridge581thatcrossesoverthe PilchuckRiverandislocatedinasectionoftheriverwhereCountycoderecognizesthat channelmigrationislikelytooccur.ThisreportprovidesdocumentationoftheCMZ delineationprocessandsupportforbridgedesignconsiderationsrelatedtochannelmigration. ChannelmigrationgenerallyoccursviatwoprocessesthatareconsideredinaCMZanalysis: bankerosionandavulsion.Bankerosionoccurswhenriverflowshaveenoughenergytoerode thebankmaterialandresultsinincrementalmovementofthechanneltowardtheoutsideof bends(extension)ordownͲvalley(translation).Thecompositionofbankmaterialandbank vegetationinfluencesthesusceptibilityofbankstoerosion.Avulsionistheprocesswherethe riversuddenlyshiftstoanewchannellocationandoftenresultsfromchannelblockage related tolargewoodaccumulation. AerialphotographsandLiDARtopographydatawereusedtounderstandhistoricalchannel migrationpatternsandrates.Duringasitevisit,theconditionandpositionoferodingbanks weremapped,largewoodydebrisandlogjamsinventoried,andsurfacegrainsizedistributions determined.Bedloadtransportwascomputedusinganempiricalsedimenttransportfunction appliedtotherecentflowhistory. TheprojectreachofthePilchuckRiverisverydynamic.LongͲtermhistoricalaerialphotograph records,observationofgeomorphicfeaturesrevealedinLiDARtopography,andfield observationsrevealbothhighratesofbankerosionandapropensityforfloodplainͲspanning avulsions.Atsitesofactiveerosion,meandermigrationratesrangefrom8to44feetperyear andbankheightsrangefrom5to25feet.Areasofunconsolidated,nonͲcohesivebankmaterial aregenerallyassociatedwiththehighestratesoferosion.Oneparticularlyimportantsiteof erosionoccursatRiverMile(RM)15.9,whereerosionoftherightbankresultsinthedeliveryof alargeamountofsedimenttotheriverchannel.Thissedimentiscausingbargrowthandbank erosiondownstreamthroughthestudyarea.Anothernoteworthysiteoferosionisjust downstreamofthebridgeatRM15.1,wheresignificantpropertylosshasbeenoccurringalong therightbankforthepast6years.Erosionatthissitehasbeencausedbyarecentinfluxof sedimentfromupstreamandareductioninbankstrengthresultingfromalossofriparian vegetation. TheCMZintheprojectvicinitywasdelineatedusingguidelinesfoundinSection2oftheForest PracticesBoardManual(DNR,2004)assumingadesignlifeof75years.TheDNRmanual definestheCMZastheinclusivesumofthreesubcomponents:theHistoricalMigrationZone (HMZ),AvulsionHazardZone(AHZ),andErosionHazardArea(EHA),lessareasdisconnectedby maintainedinfrastructure,termedtheDisconnectedMigrationArea(DMA).TheHMZwas delineatedastheinclusivesumofallchannelsandactivesidechannelsvisibleinaerial photographsandmapsfrom1941to2011.Thesizeandloadoflargewoodinthestudyarea indicatesthatlocalverticalfluctuationsinchannelelevationontheorderof6feetabovethe topofbankelevationarelikelyinresponsetolocalizedaggradationupstreamoflogjams.The avulsionhazardzonewasdelineatedastheentiregeomorphicfloodplainsinceitiswithin6feet oftherivertopofbankelevationbasedonLiDARmapping. PilchuckRiverBridge581 ChannelMigrationZoneStudy ii Therateofbankerosion,andthereforetheappropriatewidthoftheEHA,dependslargelyon thecompositionoftheriverbank.Theglacialhistoryofthestudyareahasresultedinmuch variabilityinthefloodplainandterracematerialsthatmaybepartoftheriverbank. Uncertaintiesregardingthe extentofdifferenttypesofmaterialsrepresentasignificant limitationofthisanalysis.Severaldifferentmethodswereusedtodefinetheerosionhazard bufferdependingonthekindofgeologicalmaterialthatwasconsideredtobemostlikely presentbasedonexistinggeologicalmapping,fieldobservations,andbestprofessional judgment.The reachaveragemigrationratewasdeterminedtobe3.1ft/yr,butlocalized erosionratesovershorttimeperiodscanbeveryhigh.Thereachaveragemigrationratewas usedforareasmappedasalluvialterraces(showninFigure8),resultinginanerosionhazard bufferinthatareaof230 feet.Becauseextremelyfasterosionratesoftheglacialoutwash terrace(showninFigure2)tothewestoftheriverareobserved,anerosionhazardbufferof oneͲhalfmeanderamplitude(600feet)wasappliedinareaswhereerosionoftheoutwash terracewasconsideredlikely.Thealternative,extrapolationofthehighobservedrecent erosionrates,wouldresultinanunreasonableerosionbufferwidthlargerthanthewhole extentofvalleyexpansionsincetheexistingdrainagenetworkwasestablishedpostͲglaciation. Inareaswherehighwallsofglacialtilllimitmigration,anerosionbufferwidthof38feetwas applied.SeeSection6.3foradditionaldetailregardingmethodsusedtodefinetheEHA. ExistingriprapandleveesareobservedtobeineffectivelongͲtermbarrierstomigration; therefore,therearenodisconnectedmigrationareaswithintheCMZdelineationextentexcept Highway92.RelativeCMZhazardareasweredelineatedbasedonbestprofessionaljudgment oflikelyfuturechannelmigrationscenarios. Duetositeconstraints,theproposedbridgewillbeconstructedparalleltoanddirectly upstreamoftheexistingbridge.Thepresentbridgelengthis179’andtheproposedbridgewill beapproximately200’.TheareaproposedforBridge581,includingapproachroadways, abutments,andanysupportpiers,lieswithinthePilchuckRiverCMZ.Therightbankabutment lieswithinthemoderateCMZarea,whiletheleftbankabutmentandapproachroadwaylie withinthehighhazardarea.Localprotectionoftherightbankabutmentismostlikely sufficient.Becauseextremelydynamicanddifficulttopredictchannelmigrationisoccurring upstream,itisnotreasonabletodesignprotectionagainstalllikelychannelmigration scenarios,butrathertoplanonfuturebankprotectiononcethespecificthreatisknown. Significantsedimentaccumulationisoccurringinthevicinityofthebridge,andsothelowchord ofthebridgeshouldbeplacedhighenoughtoaccountforthisaggradationandcontinueto passfloodflowsandthesignificantloadoffloatinglargewoodydebris. TheproposedbridgewillnothaveanimpactontheoutsideboundaryoftheCMZ.Changesto localhydraulicsareuncertainandthereforeerosioneffectsarenotclearlydiscernibleonthe rightbank,justdownstreamofthebridge. PilchuckRiverBridge581 ChannelMigrationZoneStudy iii TableofContents 1 Introduction....................................................................................................................1 1.1 ChannelMigration............................................................................................................1 2 Methods.........................................................................................................................3 2.1 AerialPhotograph,HistoricalMap,andTopographicAnalysis........................................3 2.2 SiteVisit............................................................................................................................4 2.3 BedLoadTransport..........................................................................................................4 3 ProjectSetting................................................................................................................5 3.1 GeomorphicHistory.........................................................................................................5 3.1.1 GlacialHistory...........................................................................................................5 3.1.2 PostͲGlacialRiverHistoryandValleyGeology..........................................................5 3.2 BasinHydrology................................................................................................................6 3.3 UpstreamSedimentSources............................................................................................7 3.4 LargeWoodyDebris.........................................................................................................7 4 SedimentCharacterizationandTransport.......................................................................9 4.1 BedSedimentCharacteristics..........................................................................................9 4.2 BedLoadTransportandHistoricalAggradation..............................................................9 5 CurrentBankConditionsandHistoricalChannelMigration...........................................11 5.1 TopographicIndicationofHistoricalMigration.............................................................11 5.2 CurrentBankConditions................................................................................................11 5.3 PatternofHistoricalMigration......................................................................................12 5.4 AverageRateofHistoricalMigration.............................................................................14 5.5 HistoricalMigrationZone...............................................................................................14 5.6 AvulsionHazardZone.....................................................................................................15 5.7 ErosionHazardArea.......................................................................................................15 5.8 DisconnectedMigrationArea........................................................................................16
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