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Contents listsavailable atScienceDirect

Ecological Engineering

journal homepage:www.elsevier.com/locate/ecoleng

Short communication

Utilising theboundary layerto helprestore theconnectivity offish habitats and populations.An engineeringdiscussion

Hubert Chanson

The Universityof Queensland,School ofCivil Engineering,Brisbane, Australia

ARTICLEINF O

Keywords:

Boundary layer

Culverts

Turbulence typology

Experimental techniques

ABSTRACTWhile leadingscholars emphasisedthe roleof turbulencein waterwaysand thecomplex fish-turbulence in-

teractions, whatdo wereally knowabout turbulence?A recentpaper developeda comparisonbetween different

boundary treatmentto improveupstream passageof smallfish inbox culverts.The limitationsof thework are

discussed. Itis arguedthat thepractical engineeringdesign implicationscannot beignored, whilea solidun-

derstanding ofturbulence typologyis abasic requirementto anysuccessful boundarytreatment conduciveof upstreamfish passage.

1. Presentation

During thelast decades,concerns aboutthe ecologicalimpact of culverts onstream connectivityhave ledto someevolution indesign (Chorda etal., 1995;Warren Jr.and Pardew,1998 ;Hotchkiss andFrei,

2007). Theimpact interms offish passagemay adverselya ffect the

upstream anddownstream eco-systems( Briggs andGalarowicz, 2013). Common culvertfish passagebarriers encompassperched outletwith excessive verticaldrop atthe culvertoutlet, highvelocities andtur- bulence inthe barrel,debris accumulationat theculvert inlet,and standing wavesin inletand outlet( Behlke etal., 1991;Olsen andTullis,

2013;Wang etal., 2018).Watson etal. (2018)presented acomparison

between different boundarytreatment toimprove upstreampassage of smallfish inbox culvertbarrel. Implicitlytheir workwas conductedfor small waterflow ratesand nottested forlarge flood eventscorre- sponding toculvert designdischarges. Thewriter hastaught thehy- draulic designof culvertsto over5000 Australiancivil engineersfrom

1990 to2019 atthe Universityof Queensland,Australia, andhe wrotea

number ofbook chapterson hydraulicdesign ofculverts (Chanson,

1999;Chanson, 2004;Chanson andFelder, 2017) andseveral review

articles (Chanson, 2000,2001, 2007). Basedupon thisexperience and expertise, thepaper arguesthat thetesting procedureof thelong- itudinal beamdesign wasbiased andthe practicalengineering design implications cannotbe ignored.It isshown thatbiological andhydro- dynamic testingsshould beconsistent withthe engineeringdesign ap- proach ofculverts. Inparticular, anunderstanding ofturbulence ty- pology isuppermost criticalto asuccessful boundarytreatment to

restore connectivityof fish habitatsand populationat roadcrossings. 2. Longitudinalbeam designsin acontext

Channels withlongitudinal beamshave beenstudied fordecades in chemical engineering,environmental andsanitary engineering,aero- nautics, astronautics,biology andgeology. Designshave beenused for close toa centuryin watertreatment plants( Randtke andHorsley,

2012). Longitudinalbeams alongchannel wallshas beensuccessfully

tested forthe enhancedrate ofheat transfer( Naik etal., 1999;Chang et al.,2008 ), masstransfer (Stamou, 2008), andbiological filtration (Roo, 1965). Longitudinalribs andbeams areused ina numberof stages ofwater treatmentplants, e.g.maze flocculator, high-rateclar- ification tubesettler, sedimentationbasin withplate settlers,sludge clari fier (Degremont, 1979). Similardesigns areincorporated into stormwater treatmentsystems andcombined sewers( FNDAE, 1988). In alluvial channels,longitudinal troughsand ridgesmay developalong the mobile bedwith preferentialsediment transportmode (Nezu and

Nakagawa, 1984

;Shvidchenko andPender, 2001). Small-sizelong- itudinal beamscan producenet dragreduction, withappropriate groove spacings( Bushnell andMcginley, 1989;Choi etal., 1993). The scales offast swimmingsharks havefine longitudinalridges, enabling faster swimming( Nitschke, 1983). Arelated applicationis theflow past seal fur,due tothe streamwisefur pattern( Itoh etal., 2006).

3. Testingprotocol: incompatibilitywith culvertdesign methods

The dataof Watson etal. (2018)were presentedfor aconstant bulk velocity irrespectiveof theboundary treatment,e.g. smooth(control), ledge, beam,ba ffles.Fig. 1presents photographsof threeboundary

treatments: smooth(control), squarebeam, smallcorner baffles. Thehttps://doi.org/10.1016/j.ecoleng.2019.105613

Received 28February 2019;Received inrevised form7 August2019; Accepted28 September2019

E-mail address:h.chanson@uq.edu.au.

‹(OVHYLHU%9$OOULJKWVUHVHUYHG

experimental approachis questionablebecause (a)the bulkvelocity is not constantalong thechannel inpresence ofa free-surface,in response to energylosses, gravitye ffect andtailwater conditions,and (b)the bulk velocityis notan engineeringdesign parameterfor culvertsand road crossings. In ahorizontal rectangularchannel, thewater depthand velocity vary withlongitudinal distanceas functionsof theboundary treatment andflow resistance( Henderson, 1966). Thebackwater profile would typically bea H2pro file (Chow, 1959), andthe bulkvelocity mayvary by morethan 20%depending uponthe boundaryconditions (e.g.

Cabonce etal., 2017,2019

). Experimentalobservations in12 mlong

0.5 widehorizontal channelare presentedin Fig. 2, andthe facilities

were similarto thoseused byWatson etal. (2018). Fora unitdischarge q=0.111 m 2 /s, thedata ofshowed abulk velocityincreasing from

0.64 m/sto 0.71m/s alongthe smooth(control) boundaryflume, and

from 0.54m/s to0.72 m/swith smalltriangular baffles (Fig. 2b).Furthermore, fora givenbulk velocityat afixed location,the water

discharge changesin responseto theboundary treatmentand asso- ciated energylosses (Rouse, 1938;Chow, 1959;Sturm, 2001). Basic hydraulic engineeringcalculations demonstratethat, inthe same12 m long 0.5mwidechannel, thewater flow rateincreased by10% to25% from asmooth (control)condition toa triangularba ffle treatment,to achieve thesame bulkvelocity. Therate indischarge increaseis a function ofthe referencebulk velocity,sampling measurementlocation and tailwaterconditions. Basically,the testingprocedure ofWatson et al.(2018) is stronglybiased against,and wouldprovide meaningless results for,high- flow resistanceboundary treatment(s),e.g. triangular baffles, cross-bars,full-height sidewallba ffles. In practice,the hydraulicdesign parametersof culvertare thewater discharge anda fflux (Herr andBossy, 1965;Concrete PipeAssociation of Australasia,2012 ;Chanson, 1999). Thedesign offish-friendly cul- vert designrequires biologicaldata compatibleto engineeringdesign procedures anduseable byprofessional engineers( Katopodis and Gervais, 2016;Leng etal., 2019). Amore appropriatemethodology is the comparisonof fish swimmingperformances betweendi fferent boundary treatmentstested withidentical waterdischarge, aspre- viously undertaken( Wang etal., 2016;Cabonce etal., 2017). Forex- ample,Cabonce etal. (2019)demonstrated conclusivelysome im- proved upstreamtraversability andendurance ofjuvenile silverperch (Bidyanus bidyanus) withsmall triangularcorner baffles (Fig. 1c), compared toa smooth(control) channelgeometry, fora relativelylarge discharge (q= 0.111m 2 /s). Whilea smallfraction ofsmall fish could be disorientedin thenegative wakebehind theba ffle(Cabonce etal.,

2018), theproportion offish negotiatingsuccessfully theba ffles were

substantially larger,by nearly50%, thanin thesmooth-wall control flume. There areclear evidencesthat someboundary treatmentcan assist with upstreamfish passage.At thesame time,a numberof boundary treatments havenegative impacton theengineering design.in turnon the totalcost, andpossibly onthe structuralintegrity ofthe structure with associatedsafety concernsfor thehuman population.The design offish-friendly roadcrossings andculverts cannotdissociate ecology, engineering andpractical considerations.

4. Practicalconsiderations oflongitudinal beams- Design,

manufacturing, installation,operation, blockage The longitudinalbeam designmight providesome strikingresult in terms offish passageand behaviourfor very-smallwater dischargesin idealised laboratorysituation withPVC andglass surfaces.Its im- plementation tohydraulic structuredesigns musthowever becarefully considered withinthe engineeringdesign ofa culvert,because thebeam does impacton theculvert operationand performancesat small, medium andlarge waterdischarges, aswell ason theupstream passage of largerfish. The hydrodynamicmotion inthe longitudinalsquare beamchannel leads toa complicatedfluid dynamics.The strongestturbulence is generated inthe cornerregions, i.e.external andinternal cornersas- sociated withthe regionsof sharpestcurvature (Prandtl, 1952)(Fig. 3), and theire ffects areseen inmost partsof thechannel (Sanchez etal.,

2018). Secondarycurrents developas aresult ofthe hydrodynamic

singularities generatedby thesharp corneredges, assketched inFig. 3. The sharpedges andcorners ofthe squarebeam constitutewell-known hydrodynamic discontinuity,conducive ofstrong secondarycurrents (Kennard, 1967;Gessner, 1973). Thecomplex turbulentflow motion has furthera markede ffect onthe flow resistanceof thechannel andin turn onthe dischargecapacity, aspreviously reported( Kennedy and

Fulton, 1961).

A numberof technicalchallenges encompassthe manufacturingand installation ofthe beam( Fig. 1b), aswell asoperational considerations. The preferredmanufacturing processof longitudinalbeam channel would bein factory,to ensurethat thebeam positionand alignmentare (A) (B) (C) Fig. 1.Boundary treatmentin a0.5 mwide boxculvert barrelchannels -Flow direction fromright toleft.

H. Chanson

within strictspeci fications. Thisis particularcritical toensure sharp edges, asany roundingwould bemost detrimentalto thelow-velocity zone sizeand culvertperformance (Sanchez etal., 2018). Anin-situ fitting wouldnot meetthe samestandards, leadingpossibly toa sub- stantially differentflowfield, withadverse impacton theculvert op- eration andfunction. In-situinstallation, e.g.for retrofitting, would

only befeasible inrelatively largeculvert cells:i.e. greaterthan 1.5m to 1.8m,andthe installationtolerances areunlikely tobe betterthan ±10mm.

The studyconsidered 0.05× 0.05m

2 square beam,positioned

0.05 mabove thefloor. Duringoperation, thecavity underneaththe

beam isat riskof siltationand sedimentation,as wellas blockage.The accumulation ofsolid particlescould leadto apartial orcomplete blockage ofthe lowvelocity regions,because thecavity flow istoo slow and belowcurrent guidelinesfor self-cleaning( QUDM, 2013). Larger debris, includingrocks, branches,trees, couldalso becomejammed beneath thebeam andledge, obstructingthe cavityand reducingfur- ther theculvert dischargecapacity, thusimpacting adverselyon the upstream passageof bothsmall-bodied andlarger fish. Simply, theusage oflongitudinal beamin culvertsmust becon- sidered carefullyin aholistic fashionas partof theculvert design process. Anumber ofpractical considerationsshow majortechnical challenges duringdesign, manufacturing,installation andoperation. In many instances,alternative designsshould bepreferred andim- plemented, includingasymmetrical largeroughness andpossibly small corner baffles.

5. Summary- Andwhat aboutthe boundarylayer?

The titlestated some"utilisation ofthe boundarylayer ". How?A boundary layeris aflow regionwhere thehydrodynamic propertiesare affected byboundary friction( Schlichting, 1979;Bailly andComte-

Bellot, 2015

). Ina culvertbarrel channel,detailed hydrodynamic measurements showedthat theflow isfully-developed andthe boundary layeroccupies thewhole flow area( Cabonce etal., 2017,

2019;Wang etal., 2018). Whileall configurations testedby theauthors

corresponded tosome formof turbulentboundary layers,there were fundamental differences inthe turbulencetypology andkey hydro- dynamic processes,that cannotbe ignored.With asmooth channel (control) (Fig. 1a), thedominant mechanismof energydissipation isthe boundary skinfriction, withsmall secondarycurrent ofPrandtl's second kind inthe bottomcorners (Rodriguez andGarcia, 2008). Inpresence of longitudinal ledgeand beam( Fig. 1b), strongsecondary circulationof x (m) )m(dhtpedretaW

0246810

0.14 0.16 0.18 0.2

0.220.24

Control (smooth)

Baffles (L

b =0.33 m)

Baffles (L

b =2.0 m) Beam

Smooth flume

L b = 0.33 m L b = 2.0 m

Square beam

(A) x (m)

VyticolevkluB

naem )s/m(

0246810

0.45 0.5 0.55 0.6 0.65

0.70.75

Control (smooth)

Baffles (L

b =0.33 m)

Baffles (L

b =2.0 m) Beam

Smooth flume

L b = 0.33 m L b = 2.0 m

Square beam

(B)

Fig. 2.Longitudinal profile ofwater depthd and

bulk velocityV mean in the12 mlong 0.5m wide horizontalflume forQ =0.0556 m 3 /s. Comparison between control(smooth boundary)channel, channel withsquare beam,and channelwith small triangular baffles (h b =0.133m). Legendincludes the longitudinalba ffle spacingL b . Dataset: Cabonce et al.(2017, 2019),Sanchez etal. (2018). Notethat the latterdata setwas adjustedto matchthe tail- water conditions. Fig. 3.Sketch ofsecondary currentof Prandtl'ssecond kindin aturbulent flow parallel toan outercorner, lookingdownstream.

H. Chanson

Prandtl's secondkind occurs,linked tothe developmentof large streamwise vortices( Tamburrino andGulliver, 2007;Sanchez etal.,

2018), aswell assurface longitudinalstreaks (Levi, 1965). Withsmall

triangular baffles (Fig. 1c), theflowfield isdominated byfluid streamline separationat theedge ofeach baffle(Cabonce etal., 2019), with anegative wakebehind andboil ofthe first kind( Schlichting,

1979).

The interpretationof theturbulence typologyis uppermostcritical to asuccessful boundarytreatment conduciveto upstreampassage of small-bodied weak-swimmingfish. Aprecise knowledgeof theentire three-dimensional velocityfield isessential, becausethe rateof workquotesdbs_dbs44.pdfusesText_44

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