initial watershed and reach review - us forest service · initial watershed and reach review ......

4Initial Watershed and Reach Review4.1 Review the Road Context

4.2 Review Resource Values

4.3 Evaluate Watershed Risk Factors

4.4 Conduct the Initial Site Reconnaissance

4.5 Assess Site Suitability

4.6 Defining Project Objectives and Initial Design Concept

4.7 Document Your Findings

4.8 Initial Review Example

Stream Simulation

Figure 4.1—Steps and considerations in initial watershed and reach review.

Review the road context l Access needs l Road location l Road management objectives l Landownership and partnership potential

Review watershed and site resource values l Aquatic species, habitats, and conditions l Terrestrial animal passage needs l Flood-plain values l Water uses

Evaluate watershed-scale risk factors l Geomorphic hazards l Event history l Past and projected land management l Crossing maintenance history l Channel stability

Evaluate site risk factors l Channel stability l Potential for blockage by debris, ice, and/or sediment l Flood-plain constriction l Large elevation change across existing structure l Channel sensitivity to change Evaluate site suitability

Establish project objectives lTrafficaccessrequirements l Degree of stream continuity lDegreeofflood-plaincontinuity l Aquatic and terrestrial animal passage requirements l Channel restoration

RESULTS

Site suitability evaluation l Type of crossing

Broad project objectives l Full aquatic organism passage l Terrestrial wildlife passage lFullflood-plaincontinuity l Channel restoration, etc.

Steps and Considerations in Initial Watershed and Reach Review

Chapter 4—Initial Watershed and Reach Review

Thefirstphaseofthecrossing-designprojectisthewatershed-scalereviewandsitereconnaissance(figure4.1).Itcanbecompletedquicklyatlow-risksiteswherestreamandwatershedconditionsarewellknown.Theprocessappliestoreplacements,removals,andnewinstallations,andmuchofitappliestoanycrossing,whetherornotitisastreamsimulation.

Thequestionstoanswerinthisphaseare:

lIsthesitesuitableasacrossinglocation?Determiningsitesuitabilityismostlyamatterofweighingrisksandconsequences.Theteamcanlearnagreatdealaboutrisksandenvironmentalconsequencesinthisphasebysynthesizinghistorical,management,andwatershedconditioninformation.Thatinformation,alongwithasitewalk-through,isusuallysufficientforidentifyingsitesthatareunsuitableforanyrigidstructureandunsuitableforstreamsimulation.

lWhatarewetryingtoachievewiththisproject?Settingrealisticprojectobjectivesrequiresknowledgeofwatershedandroadnetworkconditionsthatonlyabroad-scalereviewcanprovide.Settingrealisticobjectivesalsorequiressomeunderstandingofthestreamreach,whichyoucangetfromaquickreconnaissanceofthesite.Projectobjectivesmaylaterbevalidated,statedinmoredetail,orchangedinlightofnewinformation.

lDositecharacteristicsandprojectobjectiveslendthemselvestostreamsimulation?Thefeasibilityofusingstreamsimulationdependsonbothprojectobjectivesandsiteconditions.Inthisrapidinitialreview,youcanidentifysomeimportantsiteconditionsthatmightmakestreamsimulationinfeasibleorcomplicated,anddecidewhethertopursuestreamsimulationasanoption.Thebroadoverviewalsowillindicatehowcomplextheprojectislikelytobe.

4.1 REvIEW ThE Road ConTExT

Note:BecausemostForestServicecrossingprojectstodayareonalreadyexistingroads,thisguideusuallyassumesthecrossing-designprojectisforareplacement.Fornewcrossingsandcrossingremovals,thestepsandconsiderationsareessentiallythesame.

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Stream Simulation

Before planning a crossing replacement, always ask the questions: Is the road necessary? Is there a better location for the road and/

or crossing?

Consult existing planning documents, such as the area roads analysis and pertinent watershed analyses. Those analytical efforts should show:

l Location and type of the resources the road accesses.

l Long-term access needs in the area.

l Expected future development and its effects on road use and stability.

l Road standard needed.

l Stability and appropriateness of the current road location.

This information allows a reasonable evaluation of the long-term need for the road and whether it justifies expected maintenancerequirements.

Ifaroadanalysishasbeendone(section2.1),itwillindicatewhethertheroadshouldremainatitscurrentlocationorcouldberelocated.Ifnot,makethosedeterminationsbeforecontinuing.

Reviewroad management objectivestoidentifytrafficaccessrequirements—animportantcomponentofthecrossingprojectobjective.Whattransportationneedsaretobeserved,atwhatstandard,forhowlong,atwhatcost?Forsomeseasonallyclosedroadsonintermittent streams,a fordorotherlow-watercrossingmaysuffice.Ifaroadisbeingclosedorputintolong-termstorage,removingcrossingstructuresmightbeanoptionuntiltheroadreopens.Roadsthatmuststayopenduringallbutthelargestfloodswillrequireastructurethatreliablypassesnotonlylargefloodsbutalsothesedimentanddebristheycarry.Safetyisaprimaryconsideration.

Afterreviewinglandownershipinthearea,identifypotentialpartnersforpassageandhabitatrestorationamongdownstreamorupstreampropertyowners.Otherinterestedparties—suchaswatershedcouncils,countyroaddepartments,andwildlifeinterestgroups—mightbepossiblepartners.

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4.2 REvIEW RESoURCE vaLUES

Tobuildanunderstandingofthedegreeofpassagerequiredatasite,compileexistinginformationonwatershed-andsite-resourcevalues.Backgroundinformationmightcomefromstreamsurveys,watershedinventories,specialusesdatabases,andthepersonalknowledgeofforestspecialists,amongothersources.Wherethecrossingisapassagebarrier,habitatvalueforupstreamreachesisanespeciallycriticalpieceofinformation.Ithelpsestablishthecontextandpriorityofapossiblepassage-restorationproject.Ifexistinginformationisnotadequate,dothenecessaryfieldinvestigations.

Examplesofpotentialresourcesvaluesmightinclude:

lThreatenedorendangeredaquaticspecies.

lExcellentorrareaquatichabitats(bothup-anddownstreamofthecrossing)thatneedprotectionfromexcessivesedimentandotherpollutantsatallcosts.

lTerrestrialanimaltravelroutes(forexample,thevalleyisanimportantmigrationcorridorforlargemammals).

lSpecializedflood-plainhabitats(forexample,ground-water-fedchannelsprovidecrucialcool-waterrefugesforfish).

lFlood-plainwaterstorageforfloodattenuation,maintenanceofbaseflows,andmaintenanceofriparianhabitats.

lDomestic,municipal,orirrigationwatersupplies.

lCulturalorarcheologicalresources.

lRecreation.

lAesthetics.

Wherehigh-valueoruniqueresourcescouldbeaffected,theconsequencesofpartiallyblockingmovementofanimals,water,sediment,and/ordebrismaybeunacceptable.Wheresevereconsequencescombinewithahighriskofcrossingfailure,suchasinareassubjecttodebris torrents,considerrelocatingthecrossingtoamoresuitablelocation.Thevalueandsensitivityoftheresourcesatriskarealsotwoofthefactorsthatdictatethelevelofeffortthatshouldgointothedesignandthedegreeofstream continuitythecrossingshouldprovide(seealsosection4.6).

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Stream Simulation

4.3 EvaLUaTE WaTERShEd RISk FaCToRS

Takea“big-picture”lookatlarge-scalewatershedconditionsandprocessesthathaveorcaninfluencethecrossingreach.Someofthemare:

lGeologicorgeomorphichazards.

lHistoryoffloodingandgeologic/geomorphicevents.

lPast,current,andanticipatedlandmanagementinthecontributingwatershed.

lRegionalchannelinstability(forexample,downstreamchannel incision;seeappendixA.7.2)

Togetherwithafieldvisittothesite,thewatershedbackgroundinformationprovidesabasisforunderstandinghowthechannelhasrespondedtowatershedeventsinthepast.Thisknowledge,inturn,helpspredictthedirectionanddegreeoffuturechannelchange.Predictingfuturechangesiscriticalbecausestream-simulationstructuresmustaccommodatefuturestreambedchanges.Keyquestionsinclude:

lWhateventsandprocessesledtothecurrentchannelform?Isthechannelstable,orisitstilladjustingtopastevents?

lWhatwatershedchangesarelikelyduringthelifeofthestructure?Howmighttheyaffectrunoffandsediment loads?

lWhatchannelchangesarelikelyduringthelifeofthestructure?Howwillthestreamrespondtolargefloods?

Toanswerthesequestions,ithelpstoknowwhatthewatershedhasdeliveredintermsoffloods,debrisflows,droughts,etc.,andhowfuturelandusechangesmightchangeflowsandsedimentanddebrisloads.Onthesitescale,itisimportanttoknowwhatcurrentreachconditionsareandhowresponsive(sensitive)thereachistochangesinwater,sediment,anddebrisloads(seesection5.3).Dependingonthecomplexityofthesiteandthewatershed,theseinterpretationscanbehardtomake.Someoneknowledgeableinwatershedandchannelprocessesshouldguidetheteamininterpretingwatershedandchannelriskfactors.

Note: Appendix A describes geomorphic

concepts used in stream simulation.

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4.3.1 Geomorphic hazards

Researchthegeology,soil,vegetation,andhydrologyofthegeneralarea.Interpretthesecharacteristicsintermsoftheirlikelyeffectonwatershedprocessesandsitestability.Ifawatershedanalysishasalreadybeencompleted,thisinformationwillbeavailable.Ifnot,tailorthedetailoftheinvestigationtotheapparentrisksatthesite.Forexample,a3-foot-widestreamonaclosedroadmaynotrequirethesamelevelofeffortasa20-foot-wideriveronahighway.

Evaluateeachsiteforitsproximitytopotentiallyunstablelandformsthatcoulddramaticallychangesedimentanddebrisloadingtothecrossingreach(seesidebar“InformationSources”).Lookforfeaturessuchas:

lSlopestabilityproblemssuchaslandslidesandearthflows.

lSnow-avalanchechutes.

lDebristorrent-pronechannels.

Inaddition,thesiteitselfmaybelocatedonaninherentlyunstablelandformsusceptibletosedimentdepositionorerosion(forexample,alluvialfans,deltas,coastalbluffs).Geologicmaterialsmaybehighlypronetoerosion,suchasunconsolidatedglacialsands.Thesefeaturesraiseredflagsaboutsitestability.

Information Sources. Information sources commonly available on national forests are watershed analyses, access- and travel-management plans, aquatic-habitat inventories, geographic information systems layers, Infra (Forest Service database housing information about constructed features on national forests) and the Natural Resources Information Systems (NRIS) database. U.S. Geological Survey professional papers, water-supply papers, technical reports, and surface-geology maps are valuable resources for helping identify geologic hazards. In more populated areas, State and local agency maps and reports are often available. Land-type maps with descriptions of dominant geomorphic processes and hazards are available on some forests. Do not rely solely on published information. Field and aerial photo interpretations are essential in identifying geomorphic hazards.

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Stream Simulation

4.3.2 history and Location of Land Cover Changes and Watershed Events

Informationneededincludes:

lLocationofthereachinthewatershedandinrelationtolandformsoractivitiesthatcouldinfluencewater,sediment,andwoodinputtothechannelsuchas:geomorphichazards,in-channelgravelextractionoperations,large-scaleriparianforestharvest,roadandcrossingfailures,dams,etc.

lHistoryofwatershedlanduseandroadsystem.

lMaintenancehistoryatcrossingsite.

lHistoryofmajorhydrologiceventssuchasfires,floods,mass wasting,anddroughts.

lRecentfloodevents.

lTypeandintensityofchannelresponsestothoseevents.

lProjectedlanduseandroadsystemchangesinthewatershed.

Thishistoricalinformationisthebackgroundneededtodevelopanunderstandingofcurrentreachconditionasitrelatestopasteventsandcurrentwatershedconditions(seefigure4.2foranexample).Isthereachchanging?Howhavepastchangesaffectedtheexistingcrossing?Whatisthedirectionofchange?Forexcellentformalexamplesofthistypeofhistoricalwatershedanalysis,seeWissmaretal.(1994);McIntoshetal.(1994);andStillwaterSciences(2005).

Collectinformationoncrossingmaintenanceandfailurehistorytogetanideaofhowwelltheexistingstructurehasperformedatthesite.Thisinformationwillgiveanideaofchannelprocessesthataffectthecrossing,andhelpidentifychronicproblemsthatthenewstructureshouldsolve.

Inaddition,analyzehowrunofftimingandamountandsediment loads maychangeinthefutureasaresultofexpectedwatershedeventssuchasfires,landslides,ordevelopment.Projecthowthereachmayrespondtothosechanges.

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Figure 4.2—Flood-damage surveys can provide historical context for stream condition. (a) On Gap Creek in northeastern Washington, extensive erosion occurred on a riparian road in unconsolidated glacial sands during a 1993 flood. (b) Sediment filled the channel for several years but this transport channel remained stable and the sediment progressively cleared out during subsequent high flows.

4.3.3 offsite Channel Stability

Instabilityelsewhereinthewatershedcanaffectacrossingstructureovertime.Forexample,aheadcutcouldmigrateupstreamandundermineastructure.(RefertoappendixA,sectionA.7.2foradiscussionofheadcutsandchannelincision.)Alternatively,ifanupstreamreachisunstable,itcoulddramaticallyincreasesedimentanddebrisloadingtothesite.Sincethecrossingstructurewillhavetoaccommodateanylarge,enduringchangesinthechannel,itisimportanttopredictthemagnitude,direction,andtimingoflikelychannelchanges.

(a)

(b)

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Stream Simulation

Detectingsignificantchannelinstabilityinthewatershedisnotalwayspossiblewithoutfieldwork.Whereforestcoverisnottoodense,atimeseriesofaerialphotographscanshowchangesinchannelreachplanformandinstability.Photosmightshownoticeablechangeinchannelwidth,rapidgrowthandmovementofdepositionalbars,andgrowthofalluvialfansattributarymouths(Grant1988).Thesechangesfrequentlyareassociatedwithobservablelandusessuchasmining,agriculture,subdivisionandroaddevelopment,orforestharvest.Channelincisionisacommontypeofregionalinstabilitycausedbychannelstraightening,gravelmining,orlossofanimportantgrade controlfeature.Historicalaccountsofstreamandwatershedconditionssometimesareavailableinlocallibrariesorfromcommunityelders.

4.4 CondUCT ThE InITIaL SITE REConnaISSanCE

Withthisbackgroundknowledgeaboutthewatershedandtheroad,theprojectteamshouldtraversethechannelup-anddownstreamofthecrossingto(a)getageneraloverviewofchannelconditionsintheproject reach and(b)identifykeygeomorphicfeaturesandpotentialchannelstabilityconcerns.Theactuallengthofthereconnaissancedependsinpartonhowmuchinformationalreadyexistsaboutthestream.Ifgoodstreamsurveysarenotavailable,thereconnaissancemayneedtoextendwellupstreamfromthecrossingtoevaluatetheextent,accessibility,andqualityofhabitat.Iftheteamhasconfidenceintheaccuracyoftheexistingsurveyinformation,walkthechannelforatleast30-to50-channelwidthsup-anddownstreamofthecrossing.Thereconnaissanceshouldbelongerformoreresponsivechannels,suchaswherethestreambedismoremobile,orbanksaresensitivetodisturbance.Besuretogofarenoughtoconfidentlyassesschannelconditionsoutsidetheexistingstructure’sareaofinfluence.

“Read”thestreamforcluesaboutthemagnitudeofoverbankfloodsand

channel-forming flows,thefrequencyandtypeofsedimenttransportevents,andotherchannelprocesses,suchasdebristransport,beaverinfluences,bankerosion,streambedaggradationanddegradation,andgeneralchannelstability.(The sidebar provides a checklist of questions that might be a useful starting point.)

Identifyunstablefeaturesthatcouldaffectthecrossing,suchasasedimentwaveprogressingdownstream,anunstabledebrisjamthatcouldfail,apotentiallandslide,oranactiveheadcut.Considerhowthecrossingisalignedrelativetothestreamandwhetherthealignmentcouldbeimproved.Beawareofrecentlargefloodsorotheruniqueoccurrencesthatmightaffectinterpretationsofchannelconditions.Observinghowthestreamhasrespondedtotheexistingcrossingstructurecanhelpyoupredictstreamresponseswhenthestructureisreplaced.

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Initial Site Reconnaissance Tickler Checklist

Note: This checklist is not exhaustive. There are likely many other questions that should be answered in different environments. Modify it as needed.

4 What effects has the existing crossing had on the stream? How high is the perch, if any?

4 How prevalent is woody debris? What role does it play in channel structure and stability? How stable is it? Does the riparian area provide a future supply of wood?

4Isthereahigh-conveyancefloodplain?Isthereevidenceofscour,sediment,andwooddepositiononthefloodplain?Locateside channels and swales. Are there culverts or dips at these locations?

4Whatprocessesmodifythechannel(forexample,debrisflows,meander shift, ice or debris jamming, beaver, etc.)?

4 Are the banks stable?

4 What are the dominant streambed materials and how mobile are they?

4 Is culvert alignment creating stability problems (for example, with plugging, bank erosion)? Should alternative alignments be considered?

4 Is the channel a response or a transport reach? What channel type is it?

4 Are there natural or other barriers to aquatic species passage in the reach?

4 Are there solid grade controls (e.g., boulder weirs, bedrock outcrops, high-stability log weirs) in the reach? These locations canfunctionasendpointsforthelongitudinalprofilesurveyedinthe site assessment (chapter 5).

4 Is the downstream reach incised? If so, should the crossing be retained as a grade control?

4 Is there a reach similar to the project site nearby that might be a potential reference reach?

4 What features might constrain construction activities at the site?

4 Are there specialized habitats that require protection during construction?

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Stream Simulation

Duringthesitereconnaissance,thinkthroughtheelementsofstream-simulationdesign(describedinchapter6)toverifythatstreamsimulationisactuallyfeasibleatthesite.Sketchaplan-viewmapofthechannelandadjacentfloodplainorvalleysideslopes.Annotatethemapwithobservations,suchaslocationofhighflowmarks,severebankerosion,andbedrockoutcrops.(Seesection5.1.1formorediscussiononsketchmaps.)Nowisagoodtimetoestablishphotopoints.Ifmultiplesitevisitsbecomenecessary,theremaybeopportunitytophotographthesiteatdifferentflows.Locatethephotopointsonthesketchmap,andmarktheminthefield.

Mostimportantly,focusonthestabilityoftheexistingchannelanditsresponsivenesstowaterandsedimentinputsfromnaturalandanthropogenicdisturbances.Sinceastream-simulationdesignmustaccommodatethepotentialrangeofchannel adjustmentsduringtheservicelifeofthereplacementstructure,channel stabilityandresponsivenesstodisturbancesstronglyaffectthedesign.Ingeneral,responsereachesaremoresensitivethantransportreaches.AsdescribedinappendixA,sectionA.2,responsereachestendtohavefiner,moreerodiblematerials,andaremorepronetosedimentdeposition,channelwidening,channelscouring,andchannel migration.Knowledgeofchanneltypes(appendixA.6)canoftenhelpwithinterpretingchannelresponsiveness.

Duringthesiteassessment(chapter5),channelcharacteristicsaffectingresponsivenessandstabilitywillbefullydocumented,butsomechannelcharacteristicsandgeomorphicsettingsthatcancomplicatedesignareeasilyobservableduringtheinitialwalk-through(seesidebar“ReachConditions”).

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Reach Conditions Requiring Special Consideration

l Existing structures with large elevation drops (perched).

lHighfloodplain-conveyance.

l Active lateral channel migration.

l Depositional reaches: alluvial fans, braided streams, concave stream reaches.

l Channels with large amounts of woody debris, especially channelspronetodebrisflowsorwithinadebris-flowrunoutzone.

l Channels prone to icing.

lChannelswithunusualflowregimes,suchasestuarinechannelswithtidalinfluences,glacial-meltwaterchannels,palustrine(wetland)channelswheregroundwaterandareafloodingareimportantinfluences,tributarychannelsbackwateredbythemainstem.

l Channels with intermittently exposed bedrock.

l Unstable channels (laterally or vertically unstable).

These channel characteristics and geomorphic settings are not universally or equally hazardous. In most situations, designs that mitigate risks to acceptable levels are feasible. Usually, mitigating designs will affect project costs to some degree, so be aware from the outset that these conditions may entail additional costs.

Descriptionsofchannelcharacteristicsandgeomorphicsettingsrequiringspecialconsiderationalongwithsomeoftheirfieldindicatorsfollow:

Wheresubstantialaggradationaboveand/orincisionbelowtheexistingstructurehaveoccurred,thereplacementstructuredesignneedstoaddressthelargechangeinstreambedelevation.Suchsituationscancompromisethefeasibilityofstreamsimulation,andtheirimplicationsareanalyzedinfulldetailduringthesiteassessmentanddesignphases(chapters5and

Existing structures with large elevation drops

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Stream Simulation

6).Documentingthesituationnowalertsmanagersthatthedesignmayrequiremorethantheusualcareandeffort.Iftheexistingstructureisfunctioningasagradecontrolonanincisingchannel(seeappendixA.7.2),theteamwillneedtoconsiderwhethertopreservethegradecontrol.

High flood plain-conveyance Overbank flowsmaytransportlargequantitiesofsedimentanddebris

onhigh-conveyancefloodplains.Thesesitesrequirespecialdesignelementstoavoidputtingthesimulatedstreambedatriskbyconcentratingfloodwatersthroughthecrossingstructure(seesection6.5.1.1).Geomorphicevidenceofsubstantialflowonthefloodplainincludes:scouredchannelsorswales,slack-watersedimentdeposits,buriedvegetation,treesscarredbyfloatingdebris,andsmalldebrisaccumulationsupstreamofobstructions.

active lateral channel migration Rapidchannelshiftingacrossthevalleyfloormaycausealignment

problemsforthecrossingandstructuredesignwillneedtoaccountfortherateandextentoflateralmigration(figure6.4).

Estimatechannel-migrationratesfromhistoricalaerialphotographs,anecdotalinformation,and/orfieldobservations,althoughthefirsttwotechniquesmaybedifficulttouseinsmallchannelsobscuredbyvegetationorlocatedinremoteareas.Inmeanderingchannels,considerthefollowingcharacteristicswhenevaluatingtheriskofchannelmigrationinthefield:

lCondition,type,andsuccessionalstage(age)ofvegetationonchannelbanksandbars.(Thesecansometimesindicatetheratesofshiftingandheightsofflooding;forexample,ageofvegetationonexistingpointbarscanindicaterateofbargrowth.Therootstrengthofbankplantswithdenseand/ordeeprootinghabitscanlimitchannelshifting.)

lPresenceofacutoffchannel,abandoned channel,orswalealonganinnerchannelbend(onthepointbar).

lCompositionandstratigraphyofbankmaterials.(Arebanksedimentscohesiveornoncohesive?Arecertainlayersmoreresistantorsusceptibletoerosion?)

lEvidenceofactivebankscourontheoutsideofbends,suchaspiecesofbank,exposedrootmasses,orfallenwholetreesorshrubslyingatthebanktoeorinthestream.(Becarefulnottoconfusechannelmigrationwithbankerosionresultingfromsedimentaccumulationaboveanundersizedculvertthathasforcedflowagainstoneorbothbanks.)

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lRecentsedimentdepositiononpointbarsthathaspartiallyburiedvegetation.

lLargein-channeldebrisaccumulations,withevidenceofflowdiversionontotheadjacentfloodplainorterracesurface.

lExtremeanglesofstreamapproachtoaculvertinlet.(Thesemayindicate(1)thatthestreamhasmigratedsincetheexistingstructurewasbuilt,(2)thatsedimentdepositionupstreamfromanundersizedculvertinitiatedlocalbankerosion,changingthestream’sangleofapproach,or(3)thecrossingwaspoorlyalignedwiththestreamwheninstalled.)

Somechannelshiftingintheimmediatevicinityofacrossingmayhavebeencausedbytheoriginalcrossingalignment.Forexample,whereastraightculvertreplacedameanderbend,thestreammayhaverespondedbyerodingbanksanddevelopingnewmeanderstorestoretheoriginalchannellength.Theseverityofthisresponsedependsontheamountofchannelshorteningandthecompositionofstreambedandstreambankmaterial.

Channelmigrationislikelytobesloweronmoderatelyentrenchedandentrenched channelsbecausetheshiftingchannelmusterodehigherbanks.However,itcanhappen.Forexample,debrisjamsthatbackwater themainchannelcanforcewatertoovertoptheadjacentterraceandinciseintothesurface.Iftheprocesscontinues,itcanleadtochannel avulsion.

depositional reaches Braidedstreams,alluvialfans,andreacheswherestreamslopeflattenstendtoexperiencelateralchannelshiftingduetoaggradationorsedimentdepositiononbars(figure4.3).Reviewtheaerialphotosofthewatershedabovethereach,lookingforactivesedimentsources,areaspronetomasswasting,etc.Considerhowpastlandusesinthewatershedaffectederosionandsedimentationrates,andhowexpectedland-usechangesmayaffecttheminfuture.Keepinmindthatsedimentdepositionmaybechronic(forexample,landusemayincreaseupstreambankerosionandlong-termsedimentsupply)orepisodic(forexample,occasionallandslides).

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Stream Simulation

Figure 4.3—Depositional reach on Kiowa Creek, Colorado. The channel shifted location across the valley bottom during a flood several years before this photograph was taken, when aggradation put additional erosive pressure on banks.

Ingeneral,itisfarbettertoavoidlocatingaroadonanalluvialfan.Thepotentialforsedimentdepositionandchannelshiftonfansmakesforseveremaintenanceheadaches.Ifanalluvialfanlocationisunavoidable,observetheupper,middle,andlowersectionsofthefanforrecentsedimentdepositionactivityoractivechannelincision.Coarsesedimentfromthewatershedmaybeactivelydepositingduringfloodeventsneartheupperportionofthefan.Thechannelmaysplitintopoorlydefineddistributariesasitflowsdownthefan,andtheirlocationsmaychangeasdepositedsedimentand/ordebrisjamsblockthem.Onsomefans,thestreammayhaveincisedthroughthefandeposits,sothatdepositionisoccurringfurtherdownstream.Theseobservationshelpdeterminetheleastactivesectionofthefan—thebestplacetolocatetheroadcrossinginadifficultgeomorphicsetting.However,thisleastactivesectionofthefanmaystillhavethepotentialtobecomemoreactiveduringtheservicelifeofthestructure.

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Channels with large amounts of woody debris Observethepresence,stability,size,andaccumulationpotentialofwood

intheprojectreach,especiallyupstreamoftheroadcrossing.Iflargewoodisabundantinornearthechannel,woodmayplayanimportantroleinmaintainingchannelstabilityandcontrollinggrade.Itmayalsoposearisktothereplacementstructure.

Thefollowingquestionshelpinevaluatingwoodydebrisrisksandroles:

lArethereindividualwoodpiecesorlargewoodydebrisstructuresinthechannel?Isthewoodydebriswellanchored,oristhereevidenceofrecenttransport?Aremostofthewoodpiecesgenerallylongerthanchannelbankfullwidth?(Pieceslongerthanbankfullwidthtypicallyhavelimitedmobility.)

lIsthewoodmostlysolidandlikelytolast,orisitdecayingandsubjecttobeingwashedaway?

lIfthewatershedhasahistoryofwood-dominateddebrisflows,isthecrossingwithintheprojecteddebris-flowrunoutzone?

lIfstepsinthechannelaremaintainedbywoodydebris,howstablearethesteps?(seefigure4.4).

lAretherelow-gradientchannelsegmentswithunusuallyfinebedmaterial?(Checktoseeifthesechannelsegmentsarecontrolledbyembeddedpiecesofwood.Especiallyinfine-grainedchannels,evensmallpiecesofwoodcancontributetochannelbedstability.)

lDotreesborderthedownstreamchannelassuringcontinuedwoodinputstothechannel?Dodownstreamchannelconditionsandstabilitydependonupstreamwoodydebrisinputs?(Ifso,woodtransportthroughthecrossingstructuremaybecriticaltothelong-termstabilityofthewholereach.)

lHaswoodydebrisbeenpreviouslyremovedfromthisstreamforfishhabitatimprovement,floodhazardmitigation,etc.?

Table4.1showssimplecriteriaforassessingtheriskthatwoodydebrismayplugacrossingstructure.Reachesmayhaveanyorallofthecharacteristicsdescribedforaparticularclass.

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Stream Simulation

Table 4.1—Qualitative criteria for assessing the risk of plugging by woody debris at a road-stream crossing structure

Woody debris Risk description

LOW l Debris mostly absent or well anchored on banks and in channel.

l Debris dispersed uniformly along the reach (i.e., it has not moved).

lAvailable wood is much larger than the stream’s ability to move it (i.e., large trees in small streams).

l Little or no wood available for local recruitment.

l Bed material not anchored by debris.

l Woody debris likely to remain at or near source area.

MODERATE l Most wood pieces anchored in the channel bed or channel banks.

l Potential for local recruitment of wood.

l History of occasional maintenance to remove wood at the crossing.

l Small translational slides or undercut slopes adjacent to channel.

HIGH l Unstable accumulations of woody debris present along banks, gravel bars, and channel constrictions.

l Most wood pieces not anchored to bed or banks.

l Considerable wood available for local recruitment.

l History of frequent maintenance to remove wood at the crossing.

lUpstreamwatershedsusceptibletodebrisflows.

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Figure 4.4—(a) A wood-controlled step exhibiting high stability. Note the large-diameter logs embedded in the bank. (b) A wood-controlled step exhibiting moderate stability, Mitkof Island, Alaska. (c) A wood-controlled step exhibiting low stability, New Hampshire. Note the small-diameter pieces and lack of embedment in the bank.

Channels proneto icing Incoldregions,icecanplayhavocwithcrossingstructures,especially

onlow-gradientstreams.Duringspringbreakup,movingicecanhitanddamageastructure.Icejamscanalsodamthechannel,potentiallycausingfloodwaterstoovertoptheroad.Theseproblemsaremostcommononperennial streams andnearlakeoutlets.Inwetlands,groundwaterseepingfromstreambankscanbuildthicklayersoficethatsometimesreducethesizeofculvertopenings.

(a) (b)

(c)

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Stream Simulation

Fieldevidencethaticejamsandaccumulationsmayposeariskincludes:

lIce-impactscarsontheupstreamsideoftrees(onbanksoroverhangingthestream).Thesecanbeseveralfeetupthetreebecauseoficedambreak-outfloods.

lIsolatedpilesofgravelorcobblesonthebanksorfloodplainbeforespringrunoff.Sedimentsoverliesnow,ice,orlastyear’soldvegetation.

lBlocksoficepresentonbanksafterspringthaw,especiallynearmeanderbends,onpointbars,andabovenaturalchannelconstrictions.

lDiscontinuousscourholesorchannelsthatbeginonthefloodplainawayfromthestreambank,thenjointhemainchanneldownstream.

lWeepingcutbanksorwetlandsnexttocrossings.

Todeterminewinter-icethicknessinthearea,seeUSACE(1999).

Channels withunusual flow regimes Designingastream-simulationcrossing(astablechannelwithstreambed

characteristicssimilartothenaturalchannel)requirestheflowregimebewellunderstood,whateverthatregimemaybe.Someunusualflowconditionsmakedesignmoredifficultbecauseoftheirunpredictability(forexample,glacialmeltwater,backwateredtributary).Thefine-grainedbedmaterialscommoninpalustrineandestuarinechannelscanlimitthefeasibilityofconstructinganembeddedculvert.

Channels withintermittentlyexposed bedrock Manytimesintermittentbedrockisadesignadvantage,becauseitlimits

theextentofverticalchanneladjustmentafterplacementofthenewcrossing.However,italsocanbeaproblem.Forexample,ifundetecteduntilconstruction,bedrockcanbeasurpriseobstructiontoplacingaculvertatthecorrectelevation.Likewise,ifacrossinghappenstobelocatedjustdownstreamofanaturalbedrockbarrierthatisnowburiedunderthebackwatersedimentwedge,thenewinstallationwillexhumethebarrier.

Theimportantthingistonoticethepresenceofshalloworintermittentlyexposedbedrockduringthewalkthrough.Theteamcanthenplantodetermineitsextentanddesignforit.

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Unstable channels Stablechannelsvaryfromnearlystaticandunchangingtohighlydynamicandadjustable.Distinguishinghighlydynamicbutstablechannelsfromunstableonescanbedifficult(seeappendixA,sectionA.4).Trulyunstablechannelsareundesirablelocationsforstreamcrossings.Theyareparticularlyundesirableforstream-simulationcrossingsbecauseoftheneedtoprojectthechangesthatarelikelytooccuroverthecrossinglifetime,anddesignforthem.Theremaybenostablereferencereachforadesigntemplate.

Assessoverallchannelstabilityoutsidetheinfluenceoftheexistingcrossing.Asingleindicatorofinstabilityisnotnecessarilyconclusivebyitself.Lookforothergeomorphicevidencealongthelengthofthereachthatconfirmsorchallengesyourconclusionofchannelinstability.Indicatorsofstabilityorinstabilityshouldbeconsistentthroughoutthereach.Inaddition,usestablechannelsinnearbysimilarlandscapepositionsasbenchmarksforcomparison.

Recentsedimentdepositionmaysuggestachannelisunstableandundergoingaggradation(Pfankuch1978;Copelandetal.2001)(figure4.5).Fieldevidencecanincludethefollowing:

lLarge,mid-channelbardepositsthathavelittleornovegetation.

lLoosebedmaterialwithfreshsurfaces.

lUnusuallyhighpercentageoffinematerialonthestreambed.

lLittledifferencebetweensurfaceandsubsurfacestreambedmaterials;poorlyarmored streambed.

lFlood-plainvegetationburiedbydepositedsediment.

lUplanddry-sitevegetationlocatedlowonthebankordeadonthefloodplain(indicatesrecentchannelfilling).

Evaluatingbankstabilityisoftenkeytodeterminingwhetherachannelisstableorunstable.Fieldevidencecaninclude:

lSubstantialandconsistentbankcaving,toppling,orslumping.

lIrregularchannelwidthandscallopedbanks.

lUnstableundercuts.

lTensioncracksatelevationsabovebankfull.

lShallow-rooted,sparse,orweakbankvegetation.

lArtificialbankarmoring(riprap)mayindicatepastbankinstability.

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Stream Simulation

High,unstablebankscanalsobeassociatedwithchannelincisionorgullying(figure4.6).Ifaheadcuthasreachedtheexistingculvert,youmayfindadistinctdifferenceinbankheightandstabilitybetweentheup-anddownstreamchannels.(SeeappendixA.7.2andsection5.3.4fordescriptionsoftypicalchanneltypechangesassociatedwithincisingchannels.)

Figure 4.5—Massive gully erosion upstream (figure 4.6) caused channel filling and flood-plain sedimentation in this depositional reach, eastern Colorado.

Figure 4.6—Channel widening after recent incision, eastern Colorado.

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Oneusefulprocedureforrapidlyassessingchannelstabilityinthevicinityofroad-streamcrossingsisbyJohnsonetal.(1999).Theirprocedure,whichbuildsonseveralearliermethods(Pfankuch1978;SimonandDowns1995;Thorneetal.1996;Rosgen1996),isbasedon13qualitativeandquantitativeindicators,eachofwhichisratedwithapointsystem(table4.2).Theseratingsareweightedandadded,producinganoverallstabilityratingforthechannelatthecrossing.Someofthesitevariables(11through13)helpinevaluatingchannelresponsetotheexistingstructure.Johnsonetal.(1999)provideguidanceoninterpretingtheresultstoidentifythetypeofinstability(lateral,vertical,largetransport/depositionofdebrisorsediment)andstabilizationneedsatthesite.Anyreach-basedassessmentprocedurelikethisshouldbeinterpretedinthecontextoflarger-scalestabilityissues,suchasregionalincision.Theteamcanthenfocusitseffortsduringthedetailedsiteassessmentonthemajorrisksatthesite.

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Stream Simulation

Table 4.2—Stability indicators, descriptions, and ratings (Johnson et al. 1999, used with permission of the American Society of Civil Engineers)

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4.4.1 Construction Issues

Duringtheinitialreview,identifyfeaturesthatmightlimitconstructionaccess.Showthemonthesitesketch,andflagthemtoensurethatthesiteassessmentsurveywillincludethem.Suchfeaturesinclude:

lUtilitycorridors,buriedutilitylines.

lWetlands.

lSoftsoils.

lCriticalhabitats.

lSteepslopes.

lRights-of-way.

lPropertyboundaries.

lExistinglandings,opportunitiesforstorageandstagingareas.

lRoadwaylines-of-sight.

4.5 aSSESS SITE SUITaBILITy

Theteamcannowmakeafirstassessmentofsitesuitabilityforthecrossing.Again,ifpossible,avoidlocationswhererapidchannelchangecanbeanticipated(figures4.7and4.8).Crossingsindynamicreacheshaveahigherpotentialforfailurethanastablesite.Iftheconsequencesoffailurewouldalsobehigh,seriouslyconsiderrelocatingtoamorestablesite.Thecostofmovingtheroadmaybemorethanoffsetbythelowerriskofdamagetotheroadortohigh-valuehabitatsandbythelowermaintenancerequirements.

Althoughstreamsimulationispossibleatmanyriskysites,specialdesignconsiderationsarenecessary.Tomitigatesuchrisks,makeeveryefforttothoroughlyunderstandcurrentstreamconditionsandpotentialchangesduringthelifeoftheproject.Designingastructurethataccommodatesthosechangesandminimizesthepotentialforand/ortheconsequencesoffailureatsuchasitewilltakemoreeffortandcare.Boththedesignprocessandthestructureitselfmaybemoreexpensivethanatsimplersites.

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Stream Simulation

C

Longitudinal Profile View

Plan View

DBC to D

Transitional zone fromC channel type to

D channel type.Existing road

Optimal crossinglocation

Optim

al road Loca

tion

Brewster Creek Road Culvert Replacement, Lolo National Forest, Montana

Example provided by Traci Sylte

WhereBrewsterCreekexitsitsnarrowvalleyontoawider,flatterfloodplain,itdepositssedimentandformsanalluvialfan(figure4.7).TheBrewsterCreekroadcrossesneartheheadofthefanwheresedimentbeginstodepositasthegradeflattens.

Figure 4.7—Brewster Creek crossing plan-view sketch. Origi-nal drawing by Traci Sylte.

The previous culvert, ap-proximately half as wide as the bankfull channel, was full of sediment. As a result, the streamfrequentlyoverflowedthe road. The forest replaced the culvert with a new bottom-less box culvert in the same location. The new structure, which spans the bankfull width,wasdesignedforfishpassage. It was also designed topassthe100-yearflow,withsome free board under the deck.

Figure 4.8—Brewster Creek road replace-ment box culvert, filled to 85 percent of its rise after 1 year.

The year after construction, the new culvertalsofilledwithsedimenttoabout 85 percent of its rise. The stream stilloverflowstheroadfrequently.Asimple recognition that the crossing was located in a depositional zone, coupled with an easy road-location change to only 150 feet upstream (figure4.7),couldhaveavoidedthisproblem.

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4.6 dEFInInG PRojECT oBjECTIvES and InITIaL dESIGn ConCEPT

Togetherwithconsiderationsoftrafficaccessneeds,maintenancerequirements,safety,andfunding,thegeomorphichazardsandecologicalvaluesidentifiedduringtheinitialreviewprovidethebasisfordefiningpreliminaryprojectobjectives.Theseobjectivesarepreliminarybecausetheymaychangeastheteamlearnsmoreaboutthesiteconstraintsandopportunitiesduringthesiteassessment(chapter5).Throughoutthepredesignphasesoftheproject,theentireteam—aswellasthemanager—shouldbeinvolvedasobjectivesaresetorrevisedinlightofnewinformation.Incaseswhereobjectivesconflict,prioritiesmaybereshuffled.Tomakesuretheobjectivesandprioritiesareclearandthatallparticipantsunderstandtheminthesameway,writeobjectives,anddocumentanychangesastheyoccur.

Objectivesshouldresponddirectlytotherisksandresourcevaluesassociatedwiththeproject—byminimizingboththepotentialandconsequencesoffailure,inaccordancewiththeimportanceoftheresources.Forexample,ifconditionsforceacrossingtoremainnearhigh-qualityspawninghabitat,animportantobjectivewouldbetominimizetheriskofdegradingthathabitat;theprojectteammightthereforeconsideralower-riskstructure,suchasavalley-spanningbridge.Ifregionalchannelincisionisoccurring,oneobjectivemaybetopreservethecrossingasalocalbase-levelcontrol.Tominimizetherisktoaquaticpopulations,atleastpartialpassagecouldbeprovidedbyinstallingabypassfishwayorafishladder.

Someexamplesofecologicalprojectobjectivesfollow.Referbacktosection2.4foramoredetaileddiscussionoftheseobjectives.[Roadsafety,trafficinterruptibility,andothertransportationsystemobjectivesalsoenterintoafullobjectivesstatement.]

lProvidepassageforaquaticorganisms.

lMinimizetheriskofculvertplugging.Onchannelswheretheriskofpluggingbywood,sediment,oriceisveryhigh,objectivesmightbetominimizeboththeprobabilityofplugging(byprovidingalargeopening)andtheconsequences(bydesigningthestructuretosustainovertoppingflowsandpreventstreamdiversion).

l Maintainflood-plainfunctionsandcontinuity.Wherefloodplainshaveimportanthabitatsformedduringoverbankflows,maintainingthenaturalfloodingregimeandprovidingforflood-watercontinuitydownthevalleymaybeimportant.

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Stream Simulation

lAccommodatechannelshifting.Wheremeandersaremigratingrapidlyacrossthefloodplain,designthestructuretoaccommodatechannelmovementasmuchaspossible(seesection6.1.1.3).

lProvideterrestrialwildlifepassage.Accommodateanimalsthatuseriparianareasformovementwheretrafficvolumeand/orfillheightmakecrossingtheroadinfeasible.

lMaintaingradecontrol.Whereaheadcutisprogressingupstreamandtheexistingcrossingisprotectingupstreamhabitats,youmaydecidetomaintainthatprotection.Youmightmakethesamedecisionwhereanundersizedculvertbacksupwaterandsediment,creatinganunusuallyvaluablewetlandhabitat.Incaseslikethese,streamsimulationmaynotbefeasible,sotheinstallationmayrequirespecialmeasures,suchasafishladder,ramp,orsidechannel,toprovideforpassageofsomeorallaquaticspecies.

lRestoreadegradedchannel.Whereachannelhasinciseddownstreamoftheexistingculvertanddegradedimportanthabitat,anobjectivemightberestoringbothpassageandhabitat.Thisworkwouldinvolverestoringthechannelsuchthatthetransitionacrosstheroadcrossingisasnearlyseamlessaspossible.

lMaintainabarrieragainstinvasiveexoticspecies.Withthisobjective,streamsimulationisnotadesignoption.Undersizedculvertssometimesfunctionaspartialorfullbarriers.Culvertsnotspecificallydesignedforexclusion,however,maynotbe100-percenteffective,becausesomeindividualanimalsmaybeabletonegotiatethematsomeflows.

Identifyingpreliminaryobjectivesdoesnotimplythatthefinaldesignmustfullyachievethem.Newinformationmaycausetheteamtomodifythem,andmoredetailedprojectobjectiveswillbeformulatedafterthedetailedsiteassessment.Bythistime,though,someofthesiteconditionsorobjectivesthatprecludestreamsimulationasadesignoption(maintainingabarrier),orthatcallitsfeasibilityintoquestion(maintainingagradecontrol)areknown.Theteamprobablyhasaninitialideaofthetypeofstructure(culvertorbridge)necessaryforachievingtheobjectives.

Anotherresultoftheinitialassessmentisthattheproject’scomplexityisnowknown,andtheteamcanjudgetheappropriatelevelofdetailforthesiteassessmentanddesignefforts(seeboxbelow).Stableandstraightforwardsitesdonotrequiregreatdetailforensuringstructure

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stabilityandaquaticorganismpassage.However,wheretheriskfactorsorprojectobjectivesmaketheprojectmorecomplexorwheretrafficcanonlybebrieflyinterruptedduringconstruction,ahigherlevelofeffortisjustified.

Factors Determining Level of Site Analysis

1. Site history: Has the crossing structure failed before? Has it been a continual maintenance problem? What is the channel condition (historic and existing)?

2. Watershed history: Are there known active or historic geohazards (earthflow,landslides,etc.)inthewatershedorinadjacentwatersheds with similar characteristics (rock types, soils, vegetation, climate)?

3. Location: Where in the watershed is the site located, and on what type of landform) alluvial fan, glacial outwash plains, hillslope, etc.)?

4.Designlife,roadmanagementobjective,projectconstraints:Isthis a highway or a logging road? What is the desired design life of a the structure? Are options at the site constrained by power lines, rights-of-way, property boundaries, or other infrastructures?

5. Channel type: What is the channel type? Is it sensitive to changes or fairly stable?

6. Is the channel incised or incising?

7.Consequencesoffailure:Whatwilloccurifthestructurefails?Whatisthespatialrelationshiptosensitiveresources(fish,riparian, vegetation, property, etc.), and how would failure impact them? What are the consequences of failure in terms of resources, monetary costs, loss of access, public safety?

4.7 doCUmEnT yoUR FIndInGS

Summarizetheimportantfindingsfromthewatershedandreachreviewinaconvenientformat(narrative,map,form)fortheprojectfile.Thisdocumentationwillcontinuetoprovidelarge-scalecontextandremindersofimportantoffsiteconditionsthroughouttheprojectprocess,andwillhelpyouverifythelevelofdetailneededforassessment.Includeacompletesetofphotostakenfrompermanentlymarkedphotopoints.

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Stream Simulation

4.8 InITIaL REvIEW ExamPLE

ThefollowingMitkofIsland,Alaska,exampleshowshowaTongassNationalForestteamdocumentedtheinitialreviewanduseditforriskassessment,sitesuitabilitydetermination,validationofprojectobjectives,andpreliminarydecisionsonstructuretypeanddesignmethod.[TheexampleusestheRosgen(1994)channelclassificationsystem.]

Forthisexample,informationgatheredintheofficeincluded:

lLocation.

lExistingstructure.

lAccessandtravelmanagement.

lAreadescription.

lGeology.

lSoils.

lVegetation.

lSitehistory.

lSlopestability.

Theprojectteamperformedthefollowinglocal-reach-scaleassessmentsduringtheirreconnaissancefieldvisit:

lChanneltypes.

lChannelstability.

lLargewoodydebrisrisk.

lRiskofsedimentretention.

lStreambanksensitivity.

lSiteproximitytoimportantorsensitiveresources.

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Initial Geomorphicassessment forCrossing 6235-17.59 (Information provided by Bob Gubernick)

Location:MitkofIsland,SoutheastAlaska,Road6235,milepost17.59.

Existing Structure:Theexistingculvertdoesnotpassspawningadultsorjuvenilesalmonidsduetoa1.9-footperchattheoutlet.Beaveractivityoccursinthearea,withadamlocatedintheculvertinlet(figure4.9).Thisculvertisscheduledforreplacement.

Figure 4.9—Existing culvert on Road 6235, milepost 17.59 (Tongass National Forest). (a) Culvert inlet. (b) Culvert outlet.

Access and Travel Management: Road6235isapermanent,high-usemainlinearterialroad(maintenancelevel3),sotrafficinterruptionscannotbetolerated.Theroadmustbesafelypassablebylow-clearancevehiclesinallweatherconditions.

Area Description:Thesiteisinanarrowvalleybottombelowauniformhillslope.Descendingthehillslope,thechannelissteepandmoderatelyincised.Itentersthemainstemchannelsoonafterreachingthebroader,flatterfloodplain.Thecrossingsiteislocatedneartheslopetransitionbetweenthehillslopeandthewidefloodplain.

(a)

(b)

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Stream Simulation

Interpretation:Thesiteisaresponsereachthatmaybesubjecttosedimentdepositionatthetransitiontoaflatterslope.Largeverticaladjustmentscanoccur.

Geology:Theareaiscomposedofsedimentarydeposits(marinegreywacke,mudstone,andconglomerates),andesitic-to-basalticvolcanicrocks,andregionallymetamorphosedequivalentsofthesestrata(source:GerhelsandBerg1992).

Interpretation:Sedimentaryandmetasedimentarymaterialscanvarygreatlyindurabilityandareusuallyplatyinshape.

Soils:ThehillslopesoilisintheKupreanofseries(originisweatheredsedimentaryrock).Thevalleybottomsoilissiltyalluvium(source:forestGISlayer).

Interpretation:Kupreanofseriessoilshavehighsiltcontents.Onsteepslopes,theyaresusceptibletotranslationallandslides,whichcaninitiateadebrisflowortorrent.Checkslopestabilitycharacteristics.

Vegetation:Thehillslopeisdominatedbyamixedconiferseries(Sitkaspruce,westernandmountainhemlock,cedar).Thevalleybottomisasedgeandbogplantcommunityadjacenttothemainchannel.Amountainhemlock/blueberryseriesliesfurtherfromthechannel(source:forestGISlayer).Theareaisprimarilypristine(99+percent),withonlyasmallmanagedsection(source:airphotos1985and1998).Theforestanticipatesnonewmanagementactivities.

Interpretation:Allplantseriesarecomposedofdense,deeplyrootedvegetationthatstabilizesbanksandlimitslateralmigration.

Site History:Theoriginalculvertwasinstalledinthelate1960s.Periodicbeaveractivityhascausedcontinualmaintenanceproblems(source:maintenancerecordsandpersonalcommunicationfrommaintenanceforeman).

Interpretation: Beaveractivitywilllimitoptions.Tominimizelong-termmaintenanceneeds,considerstructureswithwideopeningssuchasbridgesorembeddedboxculvertswithremovablelids(ventedfords).Toavoidmakingthecrossingmoreattractivetobeavers,designwillhavetominimizeroadelevation.

Slope Stability:Airphotos(1963,1979,1985)shownoindicationsofslopeinstability(landslides,debrisflows).

Hillslopesabovethesiterangebetween18-to36-percentslope,decreasingto16percentonthelowerslopes.Themoderateslopes,availablelower-sloperun-outlengthof1,500feet,andlackofactivityin40yearsofthe

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photorecordindicatethatthesitehasextremelylowriskfromdebrisfloworlandslides(figure4.10).

Interpretation:Slopestabilityisnotaconcern.Verticalclearance(toaccommodatedebrisflows)isnotanissue.

Figure 4.10—Map of slope classes above crossing. Slopes are mostly moderate in the upper watershed, and the risk of slope instability is low. Tongass National Forest GIS layer.

crossing

4—32

Stream Simulation

ChannelTypes:

lHillslope:high-gradient,step-poolchannelscomposedofbedrock,boulders,and/orcobbles(RosgenA1atoA3).

lValleybottom(abovesite):riparianwetland;low-gradientpool-rifflechannelcomposedofsiltandclay,withbeaveractivity(E6).

lValleybottom(belowsite):moderatelyslopedpool-rifflechannelcomposedprimarilyofgravels(C4).

Figure 4.11—1985 aerial photo.

crossing

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Channel Stability: Thechannelabovethesiteisnotvisibleonthe1963,1979,or1985aerialphotos(figure4.11).Belowthesite,thechannelappearsstable,withnoobservablechangeinthephotos.Neitherthesequenceofaerialphotosnorthereconnaissancefieldvisitshowsanyevidenceofrapidchannelchangeineitherthetributaryorthemainstem.

Interpretation:Nosystem-widebase-leveladjustmentsarevisibleoranticipated.Nomajoradjustmentsindesignareneeded.

Large Woody Debris Hazard: Woodinthesteepsectionofthechannelislarge(greaterthan1-footdiameter)andisgenerallyeitherwell-embeddedorinstabledebrisjams.Littledebristransportisanticipated,andthesiteisfarenoughawayfromtheedgeofthevalleybottomthattheriskofpluggingbylargewoodtransportedfromupslopeislow.However,theriskofpluggingresultingfrombeaveractivityishigh.

Interpretation:Openingshouldbelarge,becauseofbeaveractivity.

Risk of Sediment Retention:Hillslope:low(transport channel).Valleybottom:high(responsechannel).

Interpretation:Thebeaverpondisanaggradationalzone.Ifthepondisremoved,thefinematerialalsomayneedtoberemovedforwater-qualityprotection.

Streambank Sensitivity:Sensitivityislowforbothuplandsandlowlands.Deep-rootedvegetationholdsbankstogetherbothonthehillslope(mixedconifers)andonthefloodplain(sedge,berrybrush,andoccasionalconifer).Sedgeandberrybrushareextremelydeeprootedanddenseintheimmediateup-anddownstreamreaches.

Interpretation: Bankscanadjusttominorchangeswithoutdestabilizing.Minoralignmentchangesshouldnotposeaproblem.

Site Proximity to Important or Sensitive Resources:Immediatelyadjacenttosite(30feetdownstream)ishighqualitysalmon-spawninghabitat.

Interpretation:Proximitytospawninghabitatmeansthatsitedesignshouldhaveahighsafetyfactor.Sedimentcontrolisamajorconcern,givencloseproximityoftheupstreampond.

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Stream Simulation

Overall Risk Assessment:Basedonthestabilityofhillslopes,thechanneltypesinthearea,andonthephotorecord,overallriskislow.

ProjectObjectives:

lProvidefreepassageforaquaticspecies,sediment,andwoodydebris(stream-simulationdesign).

lUseculvertorlow-profilebridgeifcosteffective.(Keepapproachfillslow.Ifselectingaculvert,designroadforovertoppingandminimizeriskofsedimentationfrombeavers’pluggingtheculvert.)

lMinimizetheinstallation’sattractivenesstobeaverbyusingaslargeanopeningaspossible.

lRemovebeaverdam,buttrytomaintainsomewaterdepthupstreamifpossible.

lMinimizesedimentreleasedtothedownstreamspawningareaduringconstructionandovertime.

lMaximizeflood-plainconnectivitybyinstallingadditionalculvertsinsidechannelsandfloodswales.

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