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Technical Supplement 14J
(210–VI–NEH,August2007)
Use of Large Woody Material for Habitat and Bank Protection
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Part 654 National Engineering Handbook
Use of Large Woody Material for Habitat and Bank Protection
Technical Supplement 14J
(210–VI–NEH,August2007)
Advisory Note
Techniquesandapproachescontainedinthishandbookarenotall-inclusive,noruniversallyapplicable.Designingstreamrestorationsrequiresappropriatetrainingandexperience,especiallytoidentifyconditionswherevariousapproaches,tools,andtechniquesaremostapplicable,aswellastheirlimitationsfordesign.Notealsothatprod-uctnamesareincludedonlytoshowtypeandavailabilityanddonotconstituteendorsementfortheirspecificuse.
Cover photos:Top—Logsandrootwadsmaybedesignedtoprotecterod-ingstreambanks.
Bottom—LargewoodymaterialisanimportantecologicalcomponentofmanystreamsintheUnitedStates.
IssuedAugust2007
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(210–VI–NEH,August2007) TS14J–i
Contents Introduction TS14J–1
Area of applicability TS14J–1
Environmental and habitat considerations TS14J–1
Design TS14J–4
TypesofLWMstructures................................................................................TS14J–4
Selectingatypeofstructure..........................................................................TS14J–4
DimensionsforintermittentLWMstructures..............................................TS14J–6
Forceandmomentanalysis...........................................................................TS14J–6
Ballastandanchoring.....................................................................................TS14J–9
Materials.........................................................................................................TS14J–10
Cost.................................................................................................................TS14J–12
Maintenance..................................................................................................TS14J–12
Tables Table TS14J–1 Limitationsonapplicabilityoflargewood TS14J–2 woodstructures
Table TS14J–2 PublishedvaluesfordesignvariablesforLWM TS14J–3 structures
Table TS14J–3 Classificationoflargewoodinstreamstruc- TS14J–5 tures
Table TS14J–4 Comparisonofdesirabilityofvarioustree TS14J–11 speciesforstreamstructures
Table TS14J–5 Reportedcostsforstreamstabilizationand TS14J–13 habitatenhancementstructures
Figures Figure TS14J–1 Largehistoricallogjamsoflargewoodymater- TS14J–3 ial,GreatRaft,RedRiver,LA
Figure TS14J–2 WhiteRiver,IN,withlargewoodydebris TS14J–4
Figure TS14J–3 Definitionsketchforgeotechnicalforceson TS14J–8 buriedlog
TechnicalSupplement 14J
The Use of Large Woody Material for Habitat and Bank Protection
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(210–VI–NEH,August2007) TS14J–1
Introduction
Largewoodymaterials(LWM)havebeenusedforrivertrainingandstabilizationforcenturies.ManyoftheearliestrivertrainingstructuresbuiltonlargeriversintheUnitedStatesincludedwillowmattresses,brushmattresses,orwoodenpilingsdrivenintothebed.MorerecenteffortsincludetreerevetmentsandotherstructuresfeaturinglargewoodthatwereplacedintheWinooskiRiver,Vermont,inthe1930s,aspartofasuccessfulcomprehensivewatershedstabilizationproject(Edminster,Atkinson,andMcIntyre1949;U.S.DepartmentofAgriculture(USDA)NaturalResourcesConservationService(NRCS)1999a).Awide-rang-ingfederallyfundedstreambankprotectionresearchanddemonstrationprograminthe1970sincludedseveralfieldtrialsofLWM-basedprotectionschemes(U.S.ArmyCorpsofEngineers(USACE)1981).Mostoftheseinstallationsproducedfavorableshort-termresultsforerosioncontrolandintermsofcosts,althoughsomeprojectsweredamagedbyice(Hender-son1986).
Inthe1970s,GeorgePalmiterdevelopedasuiteoftechniquesinvolvingrepositioningLWMforcontrol-lingerosionandhigh-frequencyfloodingalonglow-gradient,medium-sizedriverscloggedwithdebrisandsediment.Hisapproachfeatureduseofhandtoolsandsmallpowerequipment(InstituteofEnvironmentalSciences,MiamiUniversity,1982;NationalResearchCouncil1992).A1986evaluationof137loghabitatstructuresintheNorthwestrevealedhighratesofdamageandfailure(FrissellandNawa1992).
Duringthe1990s,increasingappreciationoftheim-portanceoflargewoodinnaturalriverineecosystemstriggeredeffortstodesignstructuresthatemulatedtheformandfunctionofnaturallyoccurring,stableaccu-mulationsofwood,particularlyinriversofthePacificNorthwest(Abbe,Montgomery,andPetroff1997;Hil-derbrandetal.1998).However,rootwadcomposites,whicharecurrentlyamongthemostpopulartypesoflargewoodstructures,donotresembleanycommonlyobservedlargewoodformations.
Area of applicability
LWMstructuresareintendedtoprovidehabitatandstabilizationuntilwoodyriparianvegetationandstablebankslopescanbeestablished.LWMdecayswithinafewyearsunlessitiscontinuouslysubmerged.There-fore,structuresmadeentirelyorpartiallyofwoodymaterialsarenotsuitedforlong-termstabilizationunlesswoodispreservedbycontinuouswettingorwithchemicals.Woodystructuresarebestappliedtochannelsthatareatleastmoderatelystable,havegravelorwithfinerbedmaterial,andneedwoodforhabitat.MoredetailedcriteriaaresummarizedintableTS14J–1(adaptedfromFischenichandMorrow2000).
Woodymaterialstructures,likemostbankprotection,arenotsuitedforreacheswithactivebeddegradation.Streamsnottransportingsedimentsorsteep,high-ener-gysystemstransportinglargecobblesandbouldersareusuallynotgoodcandidatesforwoodymaterialstruc-tures.Althoughtherearemanyexamplesofwoodymaterialprojects,thebasisfordesignissomewhatlimitedbyalackofquantitativedatafordesign,perfor-mance,andenvironmentaleffects.Furthermore,manyofthemostimportantdesignvariablesareregionalorsitespecific.Anoverviewofpublishedvaluescom-putedorassumedforkeydesignvariablesisprovidedintableTS14J–2.Thistableisintendedtoprovideanimpressionofthelimitationsofcurrentdesigncriteria,andsuggesteddesignvaluesarepresented.Long-termperformanceinformationislimited(Thompson2002;USDANRCS1999a).Accordingly,woodstructuresarenotwellsuitedforhigh-hazard,high-riskprojects.
Environmental and habitat considerations
Althoughearlyinterestintheuseofwoodstructuresforstreamstabilizationwasdrivenbytheneedforlow-costapproaches,currentunderstandingincludesconsider-ationoftheimportantrolethatwoodymaterialsplayincreatingandprovidingthediverseconditionstypicalofaquatichabitats(Gurnelletal.2002).Knowledgeregard-inggeomorphicandecologicalfunctionsofwoodinriv-ersisrapidlyincreasing.ConsiderableevidencesuggeststhatstreamsacrossNorthAmericaweredominatedbyinputsandlargeaccumulationsofwoodymaterialspriortoEuropeansettlement(fig.TS14J–1).
TechnicalSupplement 14J
The Use of Large Woody Material for Habitat and Bank Protection
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Part 654 National Engineering Handbook
Use of Large Woody Material for Habitat and Bank Protection
Technical Supplement 14J
TS14J–2 (210–VI–NEH,August2007)
Table TS14J–1 Limitationsonapplicabilityoflargewoodstructures
Variable Considerations
Habitatrequirements Providesphysicaldiversity,cover,velocityshelter,substratesorting,pooldevelopment,under-cutbanks,andsitesforterrestrialplantcolonizationusingnaturalmaterials
ExistingLWMdensity Absentordepressedrelativetosimilarnearbyreachesthatarelightlydegraded
Sedimentload Generallynotsuitableforhigh-energystreamsactivelytransportingmateriallargerthangravel.LWMstructuresmayberapidlyburiedinhighsedimentloadreaches,diminishingtheiraquatichabitatvalue,butacceleratingrecoveryofterrestrialriparianhabitats
Bedmaterial Anchoringwillbedifficultinhardbedssuchascobble,boulder,orbedrock
Bedstability Notsuitableforavulsing,degrading,orincisingchannels.Thebestsituationsincludeareasofgeneralorlocalsedimentdepositionalongreachesthatarestableorgraduallyaggrading.De-positioninducedbyLWMstructuresmaybestabilizedbyplantedorvolunteerwoodyvegeta-tion,fullyrehabilitatinganaturallystablebankbythetimetheplacedwoodymaterialsdecay(Shields,Morin,andCooper2004).Unlikesomeoftheotherstructures,rootwadsoftencreatescourzones,notdeposition
Bankmaterial LWMstructuresplacedinbankswith>85%sandaresubjecttoflanking
Bankerosionprocesses Notrecommendedwherethemechanismoffailureismassfailure,subsurfaceentrainment,orchannelavulsion.Bestwhentoeerosionistheprimaryprocess
Flowvelocity Well-anchoredstructureshavebeensuccessfullyappliedtosituationswithestimatedveloci-ties—2.5m/s(D’AoustandMillar2000).Rootwadinstallationshavewithstoodvelocitiesof2.7to3.7m/s(AllenandLeech1997).Engineeredlogjam(ELJ)-typestructureswithstood1.2m/sinasand-bedstream(Shields,Morin,andCooper2004)
Siteaccess Heavyequipmentaccessusuallyisneededtobringinandplacelargetreeswithrootwads
Conveyance LWMstructurescanincreaseflowresistanceiftheyoccupysignificantpartsofthechannelprism(ShieldsandGippel1995;Fischenich1996)
Navigationandrecreation LWMshouldnotbelocatedwheretheywillposeahazardorpotentialhazardtocommercialorrecreationalnavigation.Potentialhazardsaregreatestforstructuresthatspanthechannel
Rawmaterials Suitablesourcesoftreesneedednearby
Risk Notsuitedforsituationswherefailurewouldendangerhumanlifeorcriticalinfrastructure
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TS14J–3(210–VI–NEH,August2007)
Part 654 National Engineering Handbook
Use of Large Woody Material for Habitat and Bank Protection
Technical Supplement 14J
Table TS14J–2 PublishedvaluesfordesignvariablesforLWMstructures
Quantity Used for Typical values Source
Densityofwooding/cm3(lowest,orworst-casecondition1/)
Buoyantforcecomputation
0.4to0.50.50.4to0.5
Shields,Morin,andCooper(2004)D’AoustandMillar(2000)D’AoustandMillar(1999)
Dragcoefficient Dragforcecomputation
0.7to0.9Upto1.50.4to1.21.01.2to0.3(tree)1.2(rootwad)
ShieldsandGippel(1995)Alonso(2004)Gippeletal.(1996)FischenichandMorrow(2000)D’AoustandMillar(2000)D’AoustandMillar(1999)D’AoustandMillar(1999)
Designlifeforwood,yr Planning 5to15 FischenichandMorrow(2000)
Soilstrength Analysisofloads/anchoringprovidedbyburiedmembers
Soilforcesonburiedmembersneglectedinordertobeconserva-tive.Rangeofvaluesbasedonsoiltypes
Shields,Morin,andCooper(2004)
1/ Worstcaseconditionspresumewell-driedwood.Drywoodrapidlyabsorbswaterandmayincreaseitsdensityby100%afteronly24-hrsubmergence(Thevenet,Citterio,andPiegay1998).However,criticalconditions,especiallyalongsmallerstreams,arelikelytooccurbeforewoodhashadtimetofullyabsorbwater.
Figure TS14J–1 LargehistoricallogjamsofLWM,GreatRaft,RedRiver,LA
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Part 654 National Engineering Handbook
Use of Large Woody Material for Habitat and Bank Protection
Technical Supplement 14J
TS14J–4 (210–VI–NEH,August2007)
Nativecommunitiesofplantsandanimalsdependonhabitatsprovidedbywood.Largewoodhasbeenob-servedtosupportstep-poolmorphology,generatelo-calscouranddeposition,andeventocreatedamsandtriggeravulsionsonstreamsofallsizes.Naturalwoodaccumulationsreduceflow-throughvelocityatbase-flow(ShieldsandSmith1992),facilitatingretentionoforganicmaterialsforprocessingbylowerlevelsofthefoodweb.Woodymaterialisanimportantsubstrateforbenthicmacroinvertebrates(WallaceandBenke1984)andprovidesdiversepoolhabitat,cover,andvelocityrefugiaforfishandotheranimals.Visualcoverfrompredatorsisimportantforfishinmanystreamecosystems.Terrestrialandamphibiousanimalsuseinstreamwoodforbaskingandperching.Riparianplantsoftenrapidlyestablishondepositionassociatedwithwoodymaterial.Habitatrehabilitationprojectsoftenfeatureadditionofwoodymaterialstostreams,primarilyforhabitatreasonsandonlysecondarilyforerosioncontrolorchannelstabilization(FischenichandMorrow2000).Localeffectsofwoodstructures(whethertheyinducescourordeposition)dependonstructuredesignandsitevariables.
Design
Designofwoodymaterialstructuresshouldfollowageomorphicandecologicalassessmentofthewater-shedandasimilar,moredetailedassessmentofthereachorreachestobetreatedincludingananalysisofexistingconditionsandanticipatedresponsesrelatedtostability,aswellashabitatdiversity.Siteassess-mentsaredescribedinmoredetailinNEH654.03.
Types of LWM structures
Existingdesignsforlargewoodstructuresmaybegroupedintoafewbasicconfigurations,asshownintableTS14J–3.Onlygeneralconceptsarepresented,asnumerousvariationsarefound.Combinationsofwoodymaterialswithstoneandlivingplantmateri-alsarecommon.ThefirstthreetypesshownintableTS14J–3areintermittentstructures,whilethelastthreeprovidecontinuousprotectionalonganerodingbank.Rootwadsmaybeplacedatspacedintervalsorinaninterlockingfashionsotheymaybeconsideredeitherintermittentorcontinuoustypes.Thedesignandconstructionofrootwadsandtreerevetmentsare
alsoaddressedinNEH654TSTS14I.Intermittentstruc-turesprovidegreateraquatichabitatdiversitythancontinuousprotection.Existingdesigncriteriaforengineeredlogjams(ELJ)weredevelopedbasedonexperienceinwide,shallow,coarse-bedstreamsinthePacificNorthwest.Applicationoftheseconceptstostreamswithrelativelydeepchannels,sandbeds,andflashyhydrologyrequiresconsiderablemodification(Shields,Morin,andCooper2004).FigureTS14J–2de-pictsLWM(alsoknownaslargewoodydebris)whereitisanimpedimenttoflowornavigation,asillustratedinfigureTS14J–2.Woodymaterialshavebeenshowntobeanintegralpartofstreamecosystems.However,LWMsuchasthiscanalsobeusedforrestorationpurposes.
Selecting a type of structure
ConfigurationofaLWMstructureshouldbeselectedusingsimilarcriteriathatareemployedforselectinganyapproachforstreamstabilizationorhabitatreha-bilitation:
• Theconfigurationshouldaddressthedomi-nanterosionprocessesoperatingonthesite(ShieldsandAziz1992).
• Keyhabitatdeficiencies(lackofpools,cover,woodysubstrate)shouldbeaddressed.
Figure TS14J–2 WhiteRiver,IN,withlargewoodyde-bris(PhotocourtesyofUSGS)
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TS14J–5(210–VI–NEH,August2007)
Part 654 National Engineering Handbook
Use of Large Woody Material for Habitat and Bank Protection
Technical Supplement 14J
Table TS14J–3 Classificationoflargewoodinstreamstructures
Configuration Sketch Description Strengths References
Engineeredlogjams
Intermittentstructuresbuiltbystackingwholetreesandlogsincrisscrossarrange-ments
Emulatesnaturalforma-tions.Createsdiversephysicalconditions,trapsadditionaldebris
Abbe,Montgom-ery,andPetroff(1997);Shields,Morin,andCooper(2004)
Logvanes Singlelogssecuredtobedprotrudingfrombankandangledupstream.Alsocalledlogbendwayweir
Low-cost,minimallyintrusive
Derrick(1997);D’AoustandMillar(2000)
Logweirs Weirsspanningsmallstreamscomprisedofoneormorelargelogs
Createspoolhabitat Hilderbrandetal.1998;Flosietal.(1998)
Rootwads Logsburiedinbankwithroot-wadsprotrudingintochannel
Protectslowbanks,providesscourpoolswithwoodycover
Treerevetmentsorroughnesslogs
Wholetreesplacedalongbankparalleltocurrent.Treesareoverlapped(shingled)andsecurelyanchored
Deflectshighflowsandshearfromouterbanks;mayinducesedimentdepositionandhalterosion
Crameretal.(2002)
Toelogs Oneortworowsoflogsrun-ningparalleltocurrentandsecuredtobanktoe.Gravelfillmaybeplacedimmediatelybehindlogs
Temporarytoeprotec-tion
Crameretal.(2002)
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Part 654 National Engineering Handbook
Use of Large Woody Material for Habitat and Bank Protection
Technical Supplement 14J
TS14J–6 (210–VI–NEH,August2007)
• Thefinishedprojectshouldfunctioninhar-monywiththeanticipatedfuturegeomorphicresponseofthereach.
• Economic,political,institutional,andconstruc-tionaccessissuesshouldbeconsidered.
• Suitablematerialsmustbeavailableforreason-ablecost.
• Safetyissuesforrecreationaluseofthecom-pletedprojectreachshouldbeaddressed,ifappropriate.
• Structureslikeweirsorspursthatprotrudeintotheflowtendtocreategreaterhabitatdiversitythanthosethatparallelbanks,likerevetments,withattendanteffectsonfish(Shields,Cooper,andTesta1995).
Dimensions for intermittent LWM structures
Thegeometryofintermittent(spur-type)LWMstruc-turesmaybespecifiedbycrestangle,length,eleva-tion,andspacing.Spur-typestructuresareaddressedinmoredetailinNEH654TS14H.
Thecrestangle(anglebetweenalinenormaltotheap-proachflowvectorandtheweircrest)maybesetat15degreesupstreamfromalinedrawnperpendiculartoflowtopromotedeflectionofovertoppingflowawayfromerodingbanks.Basedonresultsofstraightchan-nelflumetests,Johnson,Hey,etal.(2001)suggestedthatstonespur-typestructuresbeangledupstreamsothattheanglebetweenthebankandthecrestisbe-tween25degreesand30degrees.However,theanglescanapproach90degreesonstraighterchannels.Woodmembersembeddedinthebankwiththeirbuttsorrootwadspointingupstreammaygainstabilityasdragforcestendtopushthemintothebank.
Crestlengthforstructuresthatdonotspanthechan-nelmaybebasedonaprojectedvaluefortheequilib-riumwidthofthechannel.Alternatively,crestlengthmaybebasedonatargetflowconveyanceforthede-signcrosssection.Astep-by-stepprocedureforspac-ingthesestructuresisprovidedinNEH654TS14H.
Inincisedchannels,crestelevationsforELJ-typestructuresmustbehighenoughsothatthesedimentbermsthatformoverthestructuresstabilizeexisting
near-verticalbanks.Stablebankheightsandanglesmaybebasedongeotechnicalanalysesorempiricalcriteriabasedonregionaldatasets.CastroandSamp-son(2001)suggestcrestelevationbesetequaltothatofthechannel-formingflowstage.Conversely,Derrick(1997)suggeststhatevenverylowstructurescanex-ertimportantinfluenceonflowpatterns.Allotherfac-torsbeingequal,localscourdepthstendtobegreaterforhigherstructures.
Spacingbetweenintermittentwoodstructuresshouldbegreatenoughtoprovidesegmentsofunprotectedbanklinebetweenstructurestoreducecostandtocreatephysicalhabitatdiversity(Shields,Cooper,andKnight1995),butalsopreventflankingandstructuralfailure.Spacingforintermittentstructuresisnormallyexpressedasamultipleofthelengthofthestructurefrombanktoriverwardtip,measuredperpendiculartotheapproachflow(projectedcrestlengthoreffectivelength).SylteandFischenich(2000)suggestthatspac-ingbethreetofourtimestheprojectedcrestlengthforbendswithR
c/W>3(radiusofcurvature/bankfull
width),decreasingto0forRc/W<2.5.Tortuouschan-
nelscanbeproblematic.Shields,Morin,andCooper(2004)suggestedthatELJ-typestructuresshouldbespacedoneandahalftotwotimesthecrestlengthapart,followingcriteriafortraditionaltrainingstruc-turespresentedbyPetersen(1986).
Theembedmentlengthordimensionforbankkey-inforstructuresthatarepartiallyburiedinthebankvarieswithbankheight,soiltype,andstreamsize.Thekey-inshouldbesufficienttomaintainthepositionoftherestofthestructurethroughoutitsdesignlifeandshouldbegreaterforfrequentlyovertoppedandhighlyerodiblebanks(SylteandFischenich2000).
Force and moment analysis
Someworkershavedevelopedengineeringdesignproceduresforwoodstructuresthatconsideredalloftheimportantforcesactingduringdesignevents,thusallowingdesignofanchoringsystemsthatproducedgivenfactorsofsafety(Abbe,Montgomery,andPetroff1997;D’AoustandMillar2000;Shields,Morin,andCooper2004).Forcesthatmaybeconsideredinsuchananalysisincludebuoyancy,frictionbetweenthewoodystructureandthebed,fluiddragandlift,andgeotechnicalforcesonburiedmembers.Simplifiedapproacheswithinherentassumptionsareavailable,includingoneinNEH654TS14E.
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TS14J–7(210–VI–NEH,August2007)
Part 654 National Engineering Handbook
Use of Large Woody Material for Habitat and Bank Protection
Technical Supplement 14J
Buoyantforce—Thebuoyantforceisequaltotheweightofthedisplacedwatervolume.Thenetbuoy-antforce,
Fb ,isequaltothedifferencebetweentheweightofthestructureandtheweightofdisplacedwater:
F V V gb wood wood water water= − ρ ρ (eq.TS14J–1)
where:ρ =densityV =volume
g =thegravitationalaccelerationvectorintheverticaldirection
Forafullysubmergedstructure,
V V Vwood water= = and
F Vgb wood water= −( )ρ ρ
(eq.TS14J–2)
Woodstructuresmayhavecomplexgeometries,whichmakesdeterminationofvolumedifficult,particularlyforpartiallysubmergedstructures.Computationsmaybesimplifiedbyassumingthatlogsarecylindersorcones,adoptingadvantageouscoordinatesystems,andtreatingrootwadsandbolesasseparateelements(BraudrickandGrant2000;Shields,Morin,andCoo-per2004).Alternatively,avolumecomputedfromtheoutsidedimensionsofthestructuremaybemultipliedbyaporosityfactortoallowforairspaces.Thevenet,Citterio,andPiegay(1998)suggestedthatthisfactoris10percentforwoodjamsand7percentforshrubs.
Ifthewoodstructuremaybeapproximatedbyatri-angularprismofheight,h,andwithauniformspecificweightγ
structure,asimplesolutionforthedepth,d
wn,at
whichthestructurebecomesneutrallybuoyant(buoy-antforces=gravitationalforces)maybecomputedusing:
γ
γstructure
w
wn wnd
h
d
h= −
2 (eq.TS14J–3)
where:γ
w =specificweightofwater
Friction—Themovementoflargewoodstructuresbyslidingalongthebedwillberesistedbyafrictionalforce,
Ff ,withmagnitudeequaltothenormalforce,
Fn ,timesthecoefficientoffrictionbetweenthewoodymaterialandthebed.
F Ff bed n= µ (eq.TS14J–4)
Intheabsenceofmeasureddata,CastroandSampson(2001)assumedthatμ
bed=tanθ,whereθisthefriction
angleforthebedsediments.However,itshouldbenotedthatthenormalforce,
Fn ,approacheszeroasdepthincreasesandthestructureapproachesneutralbuoyancy.Therefore,
Ff maybeeffectivelyzerofordesignconditions.
Drag—ThedragforceonanLWMstructuremaybecomputedusingtheequation
F
C A U U
gcd
D w o o
=× γ
2
(eq.TS14J–5)
where:F d
=dragforceC
D =dragcoefficient
A =areaofstructureprojectedintheplaneperpen-diculartoflow
Uo=approachflowvelocityintheabsenceofthe
structure
c =unitvectorintheapproachflowdirection
Awoodymaterialstructuremaybetreatedasasinglebody,ratherthanasacollectionofindividualcylin-ders(Gippeletal.1996).Forstructureslocatedontheoutsideofbends,thecross-sectionalmeanvelocityshouldbeincreasedbyafactorof1.5toallowforhigh-ervelocitiesontheoutsideofbends(USACE1991b).Dragcoefficientsmaybecomputedusinganempiricalformula(ShieldsandGippel1995),andtypicallyrangefrom~0.7to0.9(tableTS14J–2).Dragcoefficientsforcylindersplacedperpendiculartotheflowreachvaluesashighas1.5forcylindersthatarebarelysub-mergedduetoforcesassociatedwiththeformationofstandingwaves(Alonso2004).DragcoefficientsforgeometricallycomplexobjectslikeLWMstructuresvarylesswithangleoforientationtotheflowthanforsimplecylindersandtendtofallintherangeof0.6to0.7(Gippeletal.1996).Alonso(2004)fitthefollowingregressionformulastolaboratorydataandsuggestedthatitmightbeusedtocomputethedragcoefficient,C
D:
C WG
d
R R
D
e e
= × −−
×
+ × − × +− −
1 0 354
1 062 2 10 3 10 26 12 2
. exp
. ×× −10 18 3Re
(eq.TS14J–6)
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Part 654 National Engineering Handbook
Use of Large Woody Material for Habitat and Bank Protection
Technical Supplement 14J
TS14J–8 (210–VI–NEH,August2007)
where:G =distancefromthebottomofthelogtothebedR
e =cylinderReynoldsnumber, Ud
vwhere:
U =magnitudeoftheapproachflowvelocityd =diameterofthelog
v =kinematicviscosityofthewaterW=factortoaccountfortheincreaseindragdue
tosurfacewaves,andmaybegivenby
Wz
d= −
+0 28 1 4. ln . (eq.TS14J–7)
whenz/d<4,andW=1whenz/d>4,
where:z =distancefromthelogcenterlinetothewater
surface
Dragforcesareexpectedtorapidlydiminishwithtimeduringthefirstfewhigh-floweventsaspatternsofscouranddepositionreshapethelocaltopography(Wallersteinetal.2001).
Lift—Theliftforce, F L
,onanLWMstructuremaybecomputedusingtheequation
FC A U U
geL
L w o o
=× γ
2 (eq.TS14J–8)
where:C
L =liftcoefficient
e =unitvectornormaltotheplanecontainingpri-maryflowdirection,
c ,andthetransverseaxisofthestructure
Theliftcoefficientonasinglecylinderplacedperpen-diculartotheflowisgreatest(~0.45)whenthecylin-derisincontactwiththebedanddeclinestonearzerowhenthegapbetweenthebottomofthecylinderandthebedexceedsonehalftimesthecylinderdiameter(Alonso2004).Aswithdrag,liftforceslikelyrapidlydiminishaspatternsofscouranddepositionreshapethelocaltopography(Wallersteinetal.2001).Exceptforraresituations,liftmaybeneglectedindesignofLWMstructures.
Geotechnicalforces—Theresistiveforcesduetopas-sivesoilpressureactingonburiedportionsoflogsaredirectreactionstofluidforces.Asimplifiedanalysisispresentedhere.Amoredetailedtreatmentthatin-
cludesslopingbanksandanonhorizontalwatertableispresentedbyWoodandJarrett(2004)andprovidesthebasisforanassociatedExcel®worksheet.Thefol-lowingequations(Gray2003)assumethatthe:
• logisembeddedhorizontallyinthestreambank
• topofthebankishorizontal
• bankiscomposedofhomogeneous,isotropicsoilwithspecificweightγ
soil,frictionangleφ
andcohesionc
• groundwatertableelevationinthebankisap-proximatelyequaltothestreamsurfaceeleva-tion,whichishighenoughtofullysubmergethelog(fig.TS14J–3)
• bankslopeisassumedtobenearvertical
• thelogisassumedtobefrictionless
Theloghasalength=L,diameterd,andisburiedadistanceDbelowthetopbankandahorizontaldepthL
em(embedmentlength).Thepassivesoilresistance
distributionisassumedtobetriangularwithitsmaxi-mumvalueatthebankfaceanddecreasinglinearlytozeroattheembeddedtipofthelog.Thisimpliesthattheresultantpassiveresistanceforceactsonthelogadistanceof2/3L
emfromtheembeddedtip.Theactive
earthpressureforceisassumedtobesmall,relativetothepassiveforce.
Lex
Lem
d
D
L c
e
Dw
Figure TS14J–3 Definitionsketchforgeotechnicalforcesonburiedlog
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TS14J–9(210–VI–NEH,August2007)
Part 654 National Engineering Handbook
Use of Large Woody Material for Habitat and Bank Protection
Technical Supplement 14J
Theverticalloadingonthelogduetotheweightofthesoilaboveitwillbegivenby:
F L dsoil em= ′σν (eq.TS14J–9)
where:
′ = −( ) −( ) +σ γ γ γν D D Dw soil water w soil (eq.TS14J–10)
where:γ
soil=moistortotalunitweightofthesoilabovethe
log
Alternatively,Fsoil
maybecomputedusingequationsdevelopedtocomputesoilloadingonconduitsburiedinditches.Whentheditchwidthisnogreaterthanthreetimesthelogdiameter,
F C BL
Dsoil d v d= ′σ 2 (eq.TS14J–11)
where:B
d =widthoftheditch
Cd =acoefficientthatcapturestheinteractionbe-
tweentheditchwallsandthefill
C
e
d
DBd
=
−
−
1
0 38
0 38.
.
(eq.TS14J–12)
forD
Bd
< 2 and (eq.TS14J–13)
C
D
Bdd
= (eq.TS14J–14)
forD
Bd
≥ 2 (eq.TS14J–15)
ThetwoapproachesforcomputingFsoil
convergeforditcheswithwidthsjustslightlygreaterthanthelogdiameter.
Assumingfrictionbetweenthesoilandlogisnegli-gible,thepassivesoilpressureforce,
Fp ,isgivenby
F L dp p em= 0 5. σ (eq.TS14J–16)
where:σ
p =passivesoilpressure
isgivenby
σ σνp p pK c K= ′ + ( )20 5.
(eq.TS14J–17)
where:
Kp =coefficientofpassiveearthpressure
isgivenby
Kp = +
tan2 452
φ (eq.TS14J–18)
Ifunknown,soilcohesion,c,mayconservativelybeas-sumedtoequal0.Ripariansoilsareoftennoncohesive,andcohesionincohesivesoilsiseffectively0whensoilsaresaturated.
Moments—Thedrivingmoment,
Md ,abouttheburiedtipoftheembeddedlogisgivenbythevectorsum
M F F LL
FL
ld d L emex
b= +( ) +
+
×
2 2
(eq.TS14J–19)
where
l istheunitvectoralongtheaxisoftheburiedlogandpositiveinthedirectionawayfromtheburiedtipandL
ex=L–L
em.Theresistingmoment,
Md ,willactoppositethedrivingmomentandisgivenbythevectorsum
M F L F L F L lr soil em p em c c=
+
+
×
1
2
2
3
(eq.TS14J–20)
where
Fc istherestrainingforceduetoanchorcablesorballast,andL
cistheappropriatemomentarmabout
theburiedtipoftheembeddedlog.
Ballast and anchoring
ForcesandmomentsduetoanchorsmaybeaddedtotheotherforcesactingontheLWstructuretocomputefactorsofsafety.Thefactorofsafetywithrespecttoforces,F
sf,istheratioofthemagnitudeoftheresul-
tantoftheresistingforcestothemagnitudeoftheresultantofthedrivingforceswithseparatefactorsofsafetycomputedforthevertical(y)andhorizontal(x,streamwise)directions.
FF F F
F Fsf
soil py cy
b Ly
y=+ +
+ (eq.TS14J–21)
FF F F
Fsf
soil px cx
Dy
x=+ + (eq.TS14J–22)
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Part 654 National Engineering Handbook
Use of Large Woody Material for Habitat and Bank Protection
Technical Supplement 14J
TS14J–10 (210–VI–NEH,August2007)
MractsoppositeM
d,andbothvectorsactalongahori-
zontalaxisthroughtheembeddedtipofthelog.There-fore,thefactorofsafetywithrespecttomoments,F
sm,
issimplytheratiooftheirmagnitudes:
FM
Msmr
d
= (eq.TS14J–23)
Anchoringsystemsshouldbedesignedtoachievefactorsofsafetygreaterthan2duetothehighlevelofuncertaintyincomputationsforimposedforces.Anchoringapproachesincludeplacingballast(soil,cobbles,boulders)onorwithinthestructure,embed-dingpartorallofthelargewoodinthebankorinastonestructure,andusingcable,marinerope,orchaintosecurethestructuretoboulders,soilanchors(NEH654TS14E),stumps,trees,deadmen,orpilings(Crameretal.2002;FischenichandMorrow2000).Whenlogsorwoodyelementsareusedasballast,itisimportantforthedesignertoconsidertheimplica-tionsofthewoodrottingandbecominglighter.Whenbouldersorbedmaterialareusedforballast,buoyant,drag,andliftforcesontheballastrockmustbecon-sideredintheforcebalance(D’AoustandMillar2000).Anelectronicspreadsheetmayfacilitatethiscalcula-tion.
Logsincomplexstructuresmaybeattachedtooneanotherortobouldersbydrillingholesthroughthelogsandpinningthemtogetherwithsteelrebar.Epoxyadhesivehasalsobeenusedforattachinglogs.Abbe,Montgomery,andPetroff(1997)favoranapproachthatmaybetermedpassiveanchoring(Crameretal.2002),inwhichtheshape,weight,ballast,andplace-mentofastructureareadequatetoresistmovementineventsuptothedesignflow.Passivelyanchoredstruc-turesmaybecomprisedofwoodmembersthatareattachedtooneanother,butnottoexternalanchors.Passiveanchoringisnotrecommendedforhighhazardsituations,siteswithvulnerableinfrastructuredown-stream,orsiteswherestructureswillbefrequentlyovertopped.
Materials
Minimumdimensions,species,andsourcesforwoodymaterialsshouldbespecifiedduringdesign.Crameretal.(2002)suggestthefollowingguidelinesforsizeoftreesandrootwads:
Dimension Minimum size
Rootwaddiameter Bankfulldischargedepth
Trunkdiameter 0.5×bankfulldischargedepth
Treelength 0.25×bankfulldischargewidth
Clearly,woodmaterialsthislargearenotalwaysavailable.Onsitesourcesarealwaysmosteconomi-cal;importinglargematerialscanbeextremelycostly.However,benefitstothestreamecosystemmustbeweighedagainsttheimpactsofclearingandgrubbingonexistingterrestrialhabitat.Complexwoodymateri-alstructuresthatfeaturenumerousbranchesandhighstemdensitylocallydecreaseflowvelocity,inducingsedimentdeposition.Accordingly,materialsshouldbeselectedthathavenumerousbranches,beingcarefulnottobreakorremovebranchesduringconstruction.Clearingwithinthestreamcorridorshouldbeavoided,butbarscalpingmaybeadvisableincertaincasestoprovidetemporaryreliefofouterbankerosioninasharpbend.Resultingwoodymaterials(willowroot-wadsandstems)maybeusedinstructurestotriggerrapidrevegetation.
Speciesthataredecayresistantarepreferred,suchaseasternredcedar(Juniperousvirginiana),westernredcedar(Thujaplicata),coastalredwood(Sequioasempervirens),Douglas-fir(Pseudotsugaspp.),orbaldcypress(Taxodiumdistichum).Rapidlydecayingspe-cies,suchascottonwood(Populusspp.),pinesnativetotheSoutheast(PinusechinataandPinustaeda),andalder(Alnusspp.),shouldbeavoided.However,asnoted,useoffreshlycutorgrubbedwilloworcot-tonwoodtreesmaybedesirableforquickrevegetationinstructuresthatarepartiallyburied.CommentsondecayratesareprovidedintableTS14J–4.
Decayratesareclimatedependent,duetotherequire-mentsofthefungiresponsibleforaerobicdecomposi-tionofwood.Ratesincreasewithincreasingtempera-tureandprecipitation.Scheffer(1971)developedthefollowingindexforcomparingpotentialdecayratesofabovegroundwoodstructuresindifferentclimaticregionsoftheUnitedStates.
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TS14J–11(210–VI–NEH,August2007)
Part 654 National Engineering Handbook
Use of Large Woody Material for Habitat and Bank Protection
Technical Supplement 14J
Table TS14J–4 Comparisonofdesirabilityofvarioustreespeciesforstreamstructures
SpeciesDurability (assuming wetting and drying)
Source of information1/
Cottonwood(Populusspp.) Poor JohnsonandStypula(1993)
Alder(Alnusspp.) Poor JohnsonandStypula(1993)
Maple(Acerspp.) Fair(willsurvive5to10yr) JohnsonandStypula(1993)
Hemlock(Tsugaspp.) Leastdurableofconifers JohnsonandStypula(1993)
Sitkaspruce(Piceasitchensis) Excellent JohnsonandStypula(1993)
Douglas-fir(Pseudotsugaspp.) Excellent(willsurvive25to60yr)32–56yr
JohnsonandStypula1993);Harmonetal.(1986)
Westernredcedar(Thujaplicata) Mostdesirable(willsurvive50to100yr)
JohnsonandStypula(1993)
Yellow-poplar(Liriodendrontulipifera) 0.4yr Harmonetal.(1986)
Aspen(P.tremuloides) 5yr Harmonetal.(1986)
Whitefir(A.concolor) 4yr Harmonetal.(1986)
Norwayspruce(Piceaabies) ~30yr Kruys,Jonsson,andStahl(2002)
Conifers(P.sitchensis,T.heterophylla,P.menziesii,T.plicata)
Half-lifeof~20yr HyattandNaiman(2001)
Blacklocust,redmulberry,Osageorange,Pacificyew
Exceptionallyhighheartwooddecayresistance
SimpsonandTenWolde(1999)
Oldgrowthbaldcypress,catalpa,cedars,blackcherry,chestnut,Arizonacypress,junipers,honeylocust,mesquite,oldgrowthredwood,sassafras,blackwalnut
Resistantorveryresistanttoheart-wooddecay
SimpsonandTenWolde(1999)
Younggrowthbaldcypress,Douglas-fir,westernlarch,longleafoldgrowthpine,oldgrowthslashpine,younggrowthredwood,tamarack,oldgrowtheasternwhitepine
Moderatelyresistanttoheartwooddecay
SimpsonandTenWolde(1999)
Redalder,ashes,aspens,beech,birches,buckeye,butternut,cottonwood,elms,basswood,truefirs,hackberry,hemlocks,hickories,magnolia,maples,pines,spruces,sweetgum,sycamore,tanoak,wil-lows,yellow-poplar
Slightlyornonresistanttoheartwooddecay
SimpsonandTenWolde(1999)
1/ InformationfromJohnsonandStypula(1993)isqualitativeandunsubstantiated.Evidently,thesecommentspertaintotheregionofKingCounty,Washington.Harmonetal.(1986)provideareviewofscientificliteraturedealingwithdecompositionratesofsnagsandlogsinforestecosystems.ThetimesfromHarmonetal.(1986)representthetimerequiredfor20percentdecomposition(mineralization)ofalogbasedonexponentialdecayconstantsobtainedfromtheliterature.Fragmentationoflogsinstreamsduetomechanicalabrasionwouldac-celeratethedecayprocess,aswouldmorefrequentwettinganddrying.Kruys,Jonsson,andStahl(2002)providedataondecayoffallenandstandingdeadtreesinaforestinmid-northernSweden.HyattandNaiman(2001)providedataonresidencetimeoflargewoodinQueetsRiver,Washington.SimpsonandTenWolde(1999)providedataforevaluatingwoodproducts,notwholetrees.
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Part 654 National Engineering Handbook
Use of Large Woody Material for Habitat and Bank Protection
Technical Supplement 14J
TS14J–12 (210–VI–NEH,August2007)
Climateindex= T DJan
Dec
−( ) −( ) ∑ 35 3
30 (eq.TS14J–23)
where:T =meanmonthlytemperature(ºF)D =meannumberofdaysinthemonthwith0.01
inchormoreofprecipitation
Thesummationrepresentsthesumofproductsforallofthemonthsoftheyear.Thesumisdividedby30tomaketheindexfallbetween0and100formostoftheUnitedStates.Forexample,Scheffercomputedvaluesof82.5,44.8,and22.0forAtlanta,Georgia;DesMoines,Iowa;andCasper,Wyoming,respectively.ThisimpliesthatawoodstructurewouldlastaboutfourtimeslongerinaclimatetypicalofWyomingthanonetypicalofGeorgia,allotherfactorsbeingequal.
SyntheticLWMforstreamworkisavailablecommer-cially(Boltonetal.1998).Theseproductsareengi-neeredtocomparefavorablywithnaturalmaterialsintermsofdurabilityorhabitatvalue.However,theymaybelesseffectiveintermsofhabitatcreationormorecostlythannaturalmaterials.Costcomparisonsshouldconsiderfullprojectlifecycles.
Cost
CostsforLWMstructuresareheavilyinfluencedbysitevariablesandmaterialsources.Crameretal.(2002)providetypicalcostrangesforlargewoodof$500to$750pertreewithrootwadand$200to$300pertreewithoutrootwad.Thesefiguresincludema-terial,haulingtothesite,excavation,spoilage,andinstallation.AdditionalcostinformationissummarizedintableTS14J–5.
Maintenance
LWMstructuresshouldbeviewedastemporarymea-surestotriggerdesirablenaturalchangesinchannelsandbanks.Accordingly,structuresgraduallydegradeandbreakdown.However,structuresshouldbemain-taineduntilplantedorinvadingwoodyplantshavesucceededinestablishinginthetreatedarea.Arela-tivelyhighlevelofmaintenanceisnecessaryifinitialconfigurationsaretobemaintainedformorethanafewyears.Annuallow-waterinspectionsareadvisable,
withparticularattentiontoanchoringsystems,decaystatusofwoodymaterials,hazardstodownstreaminfrastructure,anderosionpatterns.Habitatmonitor-ingmaybequalitative,butfieldmeasurementofwaterdepth,width,andvelocity(Shields,Knight,Morin,andBlank2003)ispreferable.Photodocumentationandcross-sectionalandthalwegsurveysaremosthelpfulindetectingchanges.Crameretal.(2002)recommendadditionalinspectionsfollowinganyeventthatequalsorexceedsthe1-yearflowduringthefirst3yearsfol-lowingconstruction.
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TS14J–13(210–VI–NEH,August2007)
Part 654 National Engineering Handbook
Use of Large Woody Material for Habitat and Bank Protection
Technical Supplement 14J
Table TS14J–5 Reportedcostsforstreamstabilizationandhabitatenhancementstructures
Year LocationProtected bank length,m
Unit cost1/, $/m
Comments Source
1987 NestuccaRiverandElkCreek,OR
1,960 24 119woodydebrisstructuresusing99matureconifersplacedforhabitatobjectives,notstabilization
HouseandCrispin(1990)
1990–91 NorthForkPorterCreek,WA
500 165 Fivedifferentlogconfigurationsanchoredwithcablesandbouldersforhabitatpurposesonly
Cederholmetal.(1997)
1990–91 NorthForkPorterCreek,WA
500 13 60trees>30cmdiametercutfelledintostreamfrombanksandtetheredtostumpswithcableforhabitatpur-posesonly
Cederholmetal.(1997)
1994 BuffaloRiver,AR 66 Cedartreerevetmentsandwillowrootwadsplantedinditches.Twoof13siteshavenotperformedwell
Personalcommunica-tion,DavidMott,Na-tionalParkService
1996 CowlitzRiver,WA 430 47 Engineeredlogjams.Includesestimateforvalueofdonatedmaterials
Abbe,Montgomery,andPetroff(1997)
1996 BayouPierre,MS 240 117 Eighttree-trunkbendwayweirsspaced30mapart.Weirsconsistedoftwotofourtreesperweircabledto0.15-msteelpipesdrivenintobed.Riprap-protectedkeys.Twostructuresfailed,othershaveperformedwell
Personalcommunica-tion,LarryMarcy,U.S.FishandWildlifeService
1988–97 Sixurbangravelbedstreams,PugetSound,WA
2,960 493 AnchoredandunanchoredLWMaddedforfloodcontrol,sediment/ero-sioncontrolandhabitatenhancement
Larson,Booth,andMor-ley(2001)
1998 Various,MO 722/ Doublerowtreerevetmentinstalledusingheavyequipment
Personalcommunica-tion,BrianTodd,StateofMissouri
1999 BitterrootRiver,MT 80 Rootwads BrownandGray(1999)
2000 LittleTopashawCreek,MS
1,500 80 72LWMstructuresinsmall,sand-bedstream.Unitcost=$95/mwhenwil-lowplantingisincluded
Shields,Morin,andCoo-per(2004)
2000 Various 40–200 Rootwads SylteandFischenich(2000)
2002 Various 40–80 Roughnesstrees Crameretal.(2002)
2002 Various,WA 70–200 Logtoe Crameretal.(2002)
1995–2002 Various,PA 79–2133/ Rootwads Wood(2003)
1/ Costsarefortheconstructioncontractanddonotincludedesignandcontractadministration.Constructionmaterials,mobilization,andprofitareincluded.
2/ Upperendofrangeprovidedbyoriginalsource3/ Anemergencyprojectthatincludedimportingfilltoreplacea10mhighbankcost$591/m
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