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Technical Supplement 14J

(210–VI–NEH,August2007)

Use of Large Woody Material for Habitat and Bank Protection

Part 654 National Engineering Handbook



(210–VI–NEH,August2007)

Advisory Note

Techniquesandapproachescontainedinthishandbookarenotall-inclusive,noruniversallyapplicable.Designingstreamrestorationsrequiresappropriatetrainingandexperience,especiallytoidentifyconditionswherevariousapproaches,tools,andtechniquesaremostapplicable,aswellastheirlimitationsfordesign.Notealsothatprod-uctnamesareincludedonlytoshowtypeandavailabilityanddonotconstituteendorsementfortheirspecificuse.

Cover photos:Top—Logsandrootwadsmaybedesignedtoprotecterod-ingstreambanks.

Bottom—LargewoodymaterialisanimportantecologicalcomponentofmanystreamsintheUnitedStates.

IssuedAugust2007

(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

(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




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

TS14J–3(210–VI–NEH,August2007)




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





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)





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)





• 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.





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)





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





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)





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.





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


Younggrowthbaldcypress,Douglas-fir,westernlarch,longleafoldgrowthpine,oldgrowthslashpine,younggrowthredwood,tamarack,oldgrowtheasternwhitepine

Moderatelyresistanttoheartwooddecay


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


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.





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.





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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