Predicting Riparian Habitat Quantity and Diversity Using ... · Non-erodible rock revetments were...
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Predicting Riparian Habitat Quantity and Diversity Using 2D Landscape Evolution Modeling Kaitlyn Hammond Environmental Studies Senior Synthesis Seattle University March 10, 2015
Transcript of Predicting Riparian Habitat Quantity and Diversity Using ... · Non-erodible rock revetments were...
PredictingRiparianHabitatQuantityandDiversityUsing2DLandscapeEvolutionModeling
KaitlynHammond
EnvironmentalStudiesSeniorSynthesis
SeattleUniversity
March10,2015
Abstract
Alterationofriversystemshasresultedinmorphologicallysimplerivers,and
poor-qualityriparianhabitat.Astherelationshipbetweenhabitatdiversityand
ecosystemfunctionbecomesmoreapparent,managementhasbeguntofocuson
restoringnaturalhabitat-creatinggeomorphicprocesses.Wideningtheerodible
rivercorridorisonerestorationstrategy,butdeterminingthecorridorwidth
neededtofacilitateanincreaseinhabitatcanbechallenging.Thisstudyexploresthe
useofCAESAR-Lisflood,a2Dcellularhydraulicmodel,asatoolforquantifyingsite-
specificdifferencesresultingfromdifferenterodiblecorridorwidths.TheNorth
ForkoftheSnoqualmieRivernearNorthBend,Washingtonwasusedformodeling.
Non-erodiblerockrevetmentsweremodeledonthebanksofthemainchanneland
setbackoneriverwidth.After100yearsofsimulatedflow,theresulting
morphologywascomparedtoanunconstrainedcondition.Theunconstrained
conditionhadmoreactivesidechannelsattheendof100yearsthanthe
constrainedconditions,andmoresidechannelactivationthroughoutthedurationof
modeling.Theseresultssuggestthatawidererodiblecorridordoesfacilitatehabitat
formation.Additionally,thesuccessfultranslationofmodeloutputtohabitatquality
providesastartingpointtheuseof2Dmodelingasatoolforrestoration.Further
studiesshouldfocusondevelopingappropriatehabitatassessmentindicestomake
thisanaccessibletoolformanagers.
Introduction
Habitatheterogeneity,definedasaspatiallydiversedistributionofphysical
andbiologicalhabitatcomponentsinanenvironment,hasbeenshowntoinfluence
thelevelofbiodiversitythatcanbesupportedbyariverecosystem(Wardand
Tockner2001).Therelationshipbetweenhabitatheterogeneityandbiological
diversityisparticularlyevidentinripariansystems,astheinterfacebetween
aquaticandterrestrialecosystemsisassociatedwithavarietyofmicrohabitatsthat
supportmorebiologicalniches,andthereforemorespecies(Ward,Tockner,and
Schiemer1999,Malmqvist2002).Studieshaveexpressedtheimportanceof
biodiversityasafacilitatorofecosystemproductivityandresilience(Isbell,Polley,
andWilsey2009,Mace,Norris,andFitter2012),andforthisreason,many
restorationeffortsaimtoincreaselocalbiodiversity.
Humanmodificationstoriversystems,includingalterationofflowregimes,
canalization,andbankstabilization,haveeliminatedmuchofthespatialand
temporaldiversityassociatedwithriparianhabitat(BunnandArthington2002,
Ward,Tockner,andSchiemer1999).Manyofthesemodifications,particularlyriver
channelizationthroughuseofbankstabilizationstructures,destroybothsmall-scale
habitatfeaturesandthegeomorphicprocessesthatpromotehabitatheterogeneity
atalargerscale(AbbeandMontgomery1996;SeguraandBooth2010).Because
erosionandfloodingcandamagepropertyandotherpublicandprivateassets,
managementtendstofocusheavilyonbankstabilization.Whetheraccomplished
usingtraditionalrockriprap,orusingsofterapproaches,likeengineeredwoodor
bankplanting,bankstabilizationcaneventuallynarrowtherivercorridor,
diminishingthepotentialfornaturalhabitatcreatingprocessesandecological
succession(Piégayetal.2005).Findingtherightbalancebetweenstabilityandthe
ecologicalbenefitsassociatedwithinstabilityisachallengefacedinmanagement.
Managersmayconsidertheremovalofbankstabilizationstructuresand
wideningoftheerodiblecorridorasmeanstoreactivategeomorphicprocessesand
enhancebiodiversity,butassessingthepotentialoutcomesofdifferentplansis
difficult.Studiessuggestthatwideningtheerodiblecorridor,themanagement-
definedareathatarivercanerode,isbeneficialintermsofhabitatformation.Itis,
however,unclearhowwidethecorridormustbetoallowformeasurablehabitat
improvement(Buijseetal.2002;Palmeretal.2007).Additionally,variation
betweensitesmakesitdifficulttoapplya“onesizefitsall”managementstrategy
(Piégay2005).
Hydraulicmodelinghasbeenproposedasawaytoevaluatepotential
morphologicformsresultingfromdifferentmanagementstrategiesataparticular
site(Richards,Brasington,andHughes2002).However,beingpredictiveisdifficult,
andmodelingiscomputationallydemanding;therefore,hydraulicmodelinghasnot
yetbeenusedforthispurpose.Recentchangesto2Dcellularhydraulicmodels,a
classofgrid-basedmodels,havemadethemmoreefficientandbetterableto
representinstreamhydraulicconditions(Coulthard,Hicks,andVanDeWiel2007).
Becauseoftheefficiencyofthesemodels,theyprovideafeasibleoptionfor
exploringpredictivemodeling.
CAESAR-Lisflood,anumeric2Dcellularmodel,waschosenforuseinthis
study.ThisprojectseekstoutilizetheCAESAR-Lisfloodmodelto:
1. DeterminewhetherCAESAR-Lisfloodcanreasonablymodelchangesin
morphologyover100simulatedyears,andwhethertheprogramcanmodel
theareaofatypicalrestorationsitequicklyenoughtobeapracticaltoolfor
management.
2. Evaluatepotentialhabitatdifferencesresultingfromtheplacementof
modeledburiedrockrevetments(acommonlyusedbankstabilization
structure)atvaryinglevelsofsetbackfromthemainriverchannel.
Alow-cost,predictivetoolthatcanoperatequickly,andquantifydifferencesin
potentialoutcomes,wouldgiverestorationprojectmanagers,engineers,andthose
involvedinconservationmanagementabetterframeworkformakingdecisions
aboutprojectimplementation.
LiteratureReview
Determiningamethodforimprovingecologicalintegrityisacritical
challengefacedbyrestorationmanagers.Thisreviewwillexplorethebodyof
literaturesurroundingrestorationplanning,andpossibilitiesforinformingthe
decision-makingprocess.First,Iwillsummarizetheliteraturepertainingtoriver
restorationstrategies,focusingonthebenefitsassociatedwiththerestoration
geomorphicprocesses.Then,Iwilldiscussthepotentialuseofmodelingasatoolfor
restoration.Finally,Iwillevaluateseveralmetricsforhabitatassessment,and
suggesthowtheycouldbeincorporatedwithamodeltoprovideaneffectivetoolfor
enhancingdiversity.
Riverrestorationgoalsandmethods
Riverandstreamrestorationhasbecomeanincreasinglycriticalundertaking
aspeoplebegintounderstandriversystemservices(Palmeretal.2005).Rivers
provideavarietyofecosystemservices,includingwaterfiltration,nutrientcycling,
andclimateregulation,andmorebiodiversesystemsarebetterabletoprovide
thesefunctions(Isbell,Polley,andWilsey2009;Mace,Norris,andFitter2012).As
thisrelationshipbecomesclearer,restorationprojectshaveemphasizedthe
improvementofecologicalintegrityasaprinciplefocus.
Restorativeeffortsaremovingfroma“state-based”approachthatfocuseson
achievingaparticularsetofcharacteristicstoa“process-based”approachthat
emphasizestheimportanceoflarge-scaledriversofriparianfunction(Beechieetal.
2010).Ratherthaninstallinginstreamhabitatfeatures,likespawninggravelor
logjams,contemporaryprojectsseektobeself-sustainingbyremoving“barriers”
(bothliteralandfigurative)tonaturalriverevolution.Beechieetal.(2010)argue
thatripariandynamicsareinfluencedby“hierarchicallynested”physical,chemical
andbiologicalprocessesactingatbroadspatialandtemporalscales.Theydescribe
theimportanceofdesigningrestorationeffortstoaddresslarge-scaleprocesses,like
sedimenttransportandseasonalflowregimes,ratherthantheindividualsymptoms
oflargerissues.Restorationatanecosystemscalemaynotbepossibleatallsites
duetoeconomicconstraints,buteffortsbasedontheseprinciplescanleadto
successevenatasmallerscale(Beechieetal.2010).
Oneofthemainissueswith“state-based”strategiesisthedifficultyof
definingareferencestate(DufourandPiégay2009).Inthepast,mostrestoration
effortsfocusedonrestoringarivertoahistoricalstate,butidentifyingthedesired
stateproveddifficult.DufourandPiégay(2009)argueforanapproachsimilarto
thatofBeechieetal.(2010)thatfocusesonfunctionoverstate.However,Dufour
andPiégaydonotbelievethataprocess-basedapproachwillalwaysleadtodesired
outcomes,andsuggestintegratinghumanandsocietalneedsintodesigntoensure
success.
Restorationofgeomorphicprocessesiscomplicatedbysocietaldesiresfora
stableriverchannel.Whilebeneficialforpropertyandothersocietalassets,river
regulationandbankstabilizationcaneliminateprocessesthatcreatemorphologic
diversity(SeguraandBooth2010;Collins,Montgomery,andHaas2002).Processes
likeerosionhavebeenshowntoincreasemorphologicaldiversity,andbydoingso,
providehabitatbenefitstoriparianspecies.Forexample,sidechannelshavebeen
showntoprovidevaluablehabitatforlivingandbreedingformanyspecies(Stellaet
al.2011).Riverswithbraidedflowpatterns,orvariouschannelsofactiveflow
weavingaroundislands,areassociatedwithgreaterpopulationsoffishspecies
(Montgomeryetal.1999).Ingravel-beddedstreams,thepool-rifflesequencewith
areasoflowvelocitiesandhighdepthsalternatingwithhighvelocitiesandlow
depthshavebeenshowntobeespeciallyimportantforfish,bothforbreeding
spawning(Beechieetal.2005).Otherphysicalelementsplayseasonalroles.For
instance,overhangingbanksprovideshadeandcoolerwaterforfish,andoxbow
lakesprovideslow-waterrefugeinhigh-flowevents(Beechieetal.2005,Ishidaet
al.2010).Thesebenefitsmustbeweightedagainstthecostsassociatedwithan
activeerodiblecorridor,particularlythethreatofpropertydamage,andprojects
mustberesponsivetosocietalneedstoensuresuccess.
Modelingasatoolforrestoration
Currently,mostdecisionsregardingriverrestorationstrategiesarebasedon
informationgainedfromotherrestorationprojects,andresearchaboutrestorative
tools(Buijseetal.2002).Thisinformationisvaluableinguidingdecisions,butitis
notsite-specific,andtendstobe“fragmented,”onlyevaluatingtheeffectsofa
strategyononeparticularcomponentoftheriparianzone(Buijseetal.2002,889).
Itcanbechallengingtocompareproposeddesigns,particularlywhenthe
approachesutilizeavarietyoftoolsandareintendedtoaffectmultiplecomponents
oftheriparianzone.Thisbecomesevenmorechallengingwhenevaluatingsimilar
designs.Ateammaybedecidingbetweenimplementationofburiedrevetmentsset
backoneortwochannelwidthsfromthemainchannel.Awaytocomparethe
habitatqualityandquantitythatwouldresultfromthesedifferentrestoration
strategieswouldbeusefulforhelpingdistinguishbetweentwoapproaches.
Therearemanystrategiesforevaluatingexistinghabitatdiversityand
complexity,includingstreamcategorization,hydraulicmodeling,andspatial
variabilityindicies(Yarnell,Mount,andLarsen2006;Benjanker,Koenig,andTonina
2013),butevaluatingfutureoutcomesofspecificmanagementplansislimitedto
evidencebasedonthesuccessesandfailuresofotherrestorationprojects(Buijseet
al.2002).Thereisneedforasite-specificassessmentstrategy,particularlyonethat
canquantifythedifferencesinhabitatresultingfromdifferentstrategies.
Richards,Brasington,andHughes(2002)proposethatitcouldbepossibleto
usehydraulicmodelstoevaluatefuturesituations,notjustcategorizecurrent
conditions.Nosubstantialworkhasbeendoneonthetopicsince,asmodelingis
computationallydemanding,andchallengingtovalidate(VandeWieletal.2011).
However,recentupdatestocellularhydraulicmodelshaveimprovedtheir
computationalefficiency,andasthebodyofknowledgeaboutmodelcalibration
expands,theyhavebecomeanincreasinglyviableoptionforuseinpredicting
habitat(Coulthard,Hicks,andVandeWiel2007).
2Dcellularhydraulicmodelsuseagrid-basedmapofelevationandsediment
sizesaccompaniedwithwaterandsedimenttransportequationstosimulateriver
flowanderosionprocesses(Coulthard,Hicks,andVandeWiel2007).Coulthard,
Hicks,andVandeWiel(2007)identifytwomajorcategoriesofcellularmodels:
LandscapeEvolutionModels(LEMs),whichusesteadyflowequations,andnon-
steadyflowhydraulicmodels.LEMsareusefuloverlongspatialandtemporalscales,
butneglectimportanthydraulicinformation.Non-steadyflowmodelsaremuch
morecomputationallydemanding,butareclosertorealityintheirrepresentationof
in-channelflow.
Coulthardetal.(2013)combinedbothtypesofmodelsintoanewmodel
calledCAESAR-Lisflood.Thismodelinparticularisagoodoptionforpredicting
futurehabitatconditions,asmanagersmustevaluatebothlong-termchangesthat
resultfromamanagementchoice,aswellastheimmediatehydraulicconditions,as
theyarerelevanttohabitat.Additionally,becauseitismorecomputationally
efficientthanother2Dcellularmodels,itispossibletosimulate50-100yearsof
flow(animportanttime-scaleformanagers)inashortenoughtimeperiodtobe
usefulformanagers.
Themodelisrunusinganelevationmapfortheareatobemodeled,daily
flowvolumesandvelocities,andasedimentsizedistributionfromthesite.Foreach
cellinthemodel,sedimentandwaterareaddedfromupstream.Ifthecapacityfor
sedimenttransport(whichiscalculatedbasedonflowvelocity)exceedsthesupply,
sedimentistakenfromthebed,andthebedislowered.Ifthesedimentsupplyis
greaterthanthetransportcapacity,thebedraises.Themodelalsoallows
specificationofcellsasbedrockthatcannotbeeroded.Thisfunctionprovidesan
easywaytomodelbankstabilizationstructures.Thesourcecodeforthemodelis
alsofreelyavailable,whichallowsuserstointegratemodifications,andpresentsno
financialburdentomanagersseekingtousethemodel.
Intheirreviewof2Dhydraulicmodels,Coulthard,Hicks,andVandeWiel
(2007)notedthatwhilecellularmodelsareausefultypeofmodel,theydohave
certaintechnicalproblemsthatmeantheyshouldnotbeusedforexactprediction,
butinsteadtopredictsetsofpossiblefutures,ormorphologicalforms.Oneofthe
mainissuestheyidentifyisthedifficultyassociatedwithmodelinglateralerosion.
However,theequationsusedinthesemodelsmakesthemmoreeffectiveat
representingbraidedriversystemsthanothermorphologicalpatterns(Coulthard,
Hicks,andVandeWiel2007).Zilianietal.(2013)supplementedthesefindingsby
comparingtheCAESARmodeltoarealsystem,andfoundthatthemodeleffectively
simulatedgeneralmorphologicchangesandsedimentoutput,butwaslesseffective
atrepresentingfinerscalefeatures,likebraidingintensity.Thesestudiescollectively
indicateCAESAR-Lisfloodisappropriateoptionformodelingfuturehabiat
conditions.
Frommodeloutputtohabitatcomparison
Whilemethodologiesexistforevaluatingspatialvariationandhabitat
heterogeneityinrivermorphology(Yarnell,Mount,andLarsen2006),thereis
currentlynometricincorporatingbothbioticpreferencesandspatialvariation.
Creatinganindexforassessmentbasedonspeciespreferenceswouldallow
managerstobetterevaluatehowamanagementplanaffectsimportantfocalspecies
inanarea.
InthePacificNorthwest,salmonidscanbeausefulindicatorofbiological
integrity.Becauseofthedirectuseandculturalvaluesattachedtosalmonids,they
areoftenthedrivingforcebehindrestorationefforts.Thesespecieshavecomplex
lifehistorystrategiesthatrequireavarietyofmicrohabitats(Beechieetal.2014;
SaraevaandHardy2009),makingthemagoodindicatorofhabitatdiversity.They
arealsoanimportantgroupofspecieseconomically.Salmonidssitatthe
intersectionbetweenecologicalandsocialrestorationgoals,makingthema
particularlyusefulfocalspeciesforthisproject.
Conclusions
Habitatmodelingcouldprovideextremelyusefulinformationtomanagers,
andisnowpossiblewithupdatedmodels.Thesemodelsshouldnot,however,be
usedtopredictexactfuturesduetothestochasticnatureofriversystems.Agood
habitatindextoassessmodeloutputwilltakeintoaccountmanyofthediverse
featuresoftheriparianlandscape,includingoffchannelhabitat,geomorphological
features,likeoff-channelpoolsandgravelbars,aswellashydraulicinformation,
includingwaterdepthandvelocity.Theimportanceofthesefeatureshasbeen
evaluatedindividually,butcombiningthemprovidesanewwayofassessinghabitat
quality.Thepredictionsbasedonmodelassessmentcouldbetterallowmanagersto
worktowardrestoringnaturalprocessesthatcouldenhancehabitatinthelongrun,
ratherthanartificiallycreatinghabitatfeatures.
Methodology
StudySite
Iperformedmodelingona2.5kmstretchoftheNorthForkofthe
SnoqualmieRivernearNorthBend,Washington.Thisisagravel-bedded,braided
riversystem.Thesitecontainsabridgethatmaybeatriskoffailureiferosionon
thesouthbankcontinues(Ruebel,personalcommunication).KingCountyis
developingaplantoprotectthebridge,butintendstousesofterengineering
approachesinordertoactivategeomorphicchangeattheprojectsite(Ruebel,
personalcommunication).Whilemodelingcannotbeusedtopredictexactlywhat
theriverwilllooklikeinthefuture,theexplorationofdifferentmorphologicforms
andtheresultinghabitatconditionsfromdifferentmanagementdesignstrategies
maybeofuseasthecountydeliberatesbetweenplans.Because2Dmodelsaremost
accuratewhenrepresentingbraidedriversystems(Coulthard,Hicks,andVanDe
Wiel2007),thisisanappropriatechoiceforaprojectsite.
ExperimentalDesign
Isimulatedthreedifferentconditionsrepresentingincreasingerodible
corridorwidthusingthebedrockfunctionintheCAESARmodel:buriedrevetments
alongthebanksofthemainchannel,buriedrevetmentssetbackoneriverwidth
fromthemainchannel,andanunconstrainedcondition(Figure1).ADigital
ElevationMap(DEM)obtainedfrom2009KingCountyLIDARwasusedastheinitial
conditionformodeling.Iobtainedaveragedailyflowvaluesforthepast50years
fromUSGSstreamgauge12142000,agaugelocatedupstreamfromtheprojectsite,
andduplicatedthedatatorepresent100yearsofflow.Tominimizeruntime,only
thetop2%offlowvolumeswereused,ascalculationsusingtheBedload
AssessmentinGravel-beddedStreams(BAGS)calculatorindicatedthat98%of
sedimenttransportoccurredwiththetop2%offlows(WilcockandCrowe2003).A
sedimentdistributionforthesiteincluding9sedimentsizeswasinputtedtothe
model,andsedimentwasrecirculatedthroughthemodel.Twoadditionaltestruns
includingthelowflowswereperformedtoevaluatethedifferencebetweenthe
narrowrevetmentsandtheunconstrainedcondition.Theserunsusedinputted
sedimentcalculatedbasedonflowvelocitiesusingBAGS.4sedimentsizeswere
usedinthesesimulations.Theserunsincorporateduseofthevegetationand
sedimentdepositionparameters.Allothermodelparametersaredescribedin
appendix1.
Aftersimulating100yearsofflow,theresultingDEMsweremodeledfortwo
dayswithanaveragesummerlowflow(seeappendix1)todeterminewhich
channelswereactiveduringnon-floodconditions.Modeloutputwasanalyzedin
ArcGIS.
Ateverymomentofsimulatedtime,themodelrecordsavalueforeachcell
forwaterdepth,velocity,andaveragesedimentsize.Aftersimulating100yearsof
flowandrunningasummerlowflowthroughtheresultingDEM,IusedArcGISto
findcellsthatmetdepthandvelocityconditionscorrespondingwithhabitat
preferencesforCohoandChinookpresmoltasdescribedbyGoodmanetal.(2014).
WhileSnoqualmieFallsblocksthepassageofanadromousfish,definedasthosefish
thatmigratefromtheoceantofreshwatertospawn,studiesindicaterainbowtrout,
aspeciesthatispresentatthesite,havethesamehabitatrequirementsasCohoand
Chinooksalmon(Quinn2005,202-203).Thisagegrouptendstobelimitedby
habitatavailability,makingitanappropriate,albeitnotentirelyinclusive,indicator
ofhabitatquality(Goodmanetal.2014).
Results
Thesimulationof100yearsofhighflowstookapproximately11hours(see
appendix1forcomputerspecifics).After100simulatedyears,theunconstrained
conditionshowedsmallsidechannelsactivatedevenduringasummerlowflow
(Figure2).ComparisonwiththeoriginalDEMsuggeststhattheriverisreoccupying
abandonedchannels.Theconstrainedconditionsdidnothaveanyactiveside
channelssummerflow,withtheexceptionofonechannelsplitpresentinthe
narrowconditionthatwaspresentintheinitialcondition(Figures3and4).
Across-sectionalanalysisoftheunconstrainedconditionshowedarelatively
widechannelwithgentlyslopedbanks.D50,theaveragesedimentdiameter,was
loweronthefloodplainandhigherinthemiddleofthechannel(Figure5).Thisis
consistentwithnormalriverconditions.Thenarrowconditionhadasharper
channelslopeandanirregularsedimentsizepatternthroughthecross-sectionthat
wascoarserthanseenintheunconstrainedposition(Figure6).Thesetback
conditionresultedinawiderchannelthanboththeunconstrainedandnarrow
conditions,butthesedimentsizeprofilewassimilartothatoftheunconstrained
run(Figure7).
Thecumulativetimeinundatedforeachcellovertheperiodof100yearscan
beseeninFigures8,9,and10.Thesefiguresshowthatthesidechannelactivation
intheunconstrainedrunwasmoreconsistentoverthedurationofthesimulation
thantheconstrainedruns.Ontherightrideofthemapoftheunconstrained
condition(Figure8),thereareseveralredtoorangecoloredchannelsbraidedina
waythatistypicalforabraidedriversystem.Thereisalsooneredtoorange
coloredchannelnearthetopofthemap.Thiswasahistoricchannel,anditappears
asthoughtheriverhasreoccupiedthischannelmanytimesduringthesimulation.
Infigures9and10,thesidechannelsvisibleonthebottomofthemaps,closertothe
leftside,arelocatedclosertogether,andwouldnotbeconsistentwithapatternof
multiplechannelsactivatedsimultaneously.
Figure11showstheriverprofilethroughthecenterofthemainchannelof
theunconstrainedrunduringthelowflowsimulation.Thepatternofhighvelocities
associatedwithlowdepthsandlargeraveragesedimentsize(D50)isconsistent
withwhatwouldbeexpectedforapool-rifflegravel-beddedriver.Sedimentsizes
foreachoftheselocationsareappropriate,giventhesedimentsizeinputs.
Thecumulativeamountofsedimentfluxwasplottedovertimeforallthree
scenarios(Figure12).Allthreeconditionsproducedfairlylinearoutputs,butthe
mostsedimentfluxoccurredinthesetbackconditionandtheleastinthenarrow
condition.ComparisontoUSGShistoricalrecordsstatinghistoricalsedimentflux
revealsthevaluesfortheunconstrainedandsetbackconditionsarewithinan
appropriatesedimentfluxrange,butthevalueforthenarrowconditionistoosmall.
Forthetrialrunsincludingthelowflows,modeltimewasapproximately15
hours.Cross-channelelevationandsedimentprofileswereconsistentwithexpected
results.Thenarrowconditionhadasharpinclineandcoarsesedimentinthemiddle
ofthechannel.Theunconstrainedconditionhadawiderchannel,slightlywider
thantheinitialchannel,andsedimentsizeinthemiddleofthechannelwaslarger
thanonthefloodplain,butstillsmallerthanintheconstrainedrun.
Thevelocityoutputofthelowflowtrialrunswasmapped,andvelocities
werecharacterizedbasedontypicaltransitionsbetweenmorphologicunitsas
identifiedbyPasternakandSenter(2011).Planebedisdefinedasavelocityless
than0.3m/s,insetchannelsasbetween0.3and0.6m/s,steepinsetchannelsas
greaterthan0.6m/s.Thefinalcategory,theareaswithvelocitiesmorethan0.9m/s,
aredefinedastherunmorphologicunit.Inthenarrowrun(Figure13),thechannel
isverynarrowandthererangeofmorphologicunitsisdifficulttodistinguish.Inthe
unconstrainedrun(Figure14),thechanneliswider,andthespatialpatternis
recognizable,andconsistentwithwhatwouldbeexpectedforagravel-beddedriver.
Thefastareas(blue)areontheinsideofbends,andtheslowerparts(greenand
yellow)areinthesidechannelareas.
Salmonidhabitatarea(definedbyGoodmanetal.2014)wasmappedforthe
highflowruns(Figures15and16).Thehabitatareas(Table1)foreachrunwere
fairlyconsistent.Therewaslessareaintheunconstrainedrunthaninthenarrow
run.Themosthabitatareawasinthesetbackrun.However,muchofthehabitat
areaintheunconstrainedrunwasinthesidechannels.
Insummary,physicaldiversitywasmoreintheunconstrainedrunsthanin
theconstrainedruns.Whilehabitatmeasurementssuggestmorehabitatinthe
constrainedrun,muchofthehabitatintheunconstrainedrunwasintheside
channels.
Discussion
Theunconstrainedconditionhadthemostsidechannelactivityoutofthe
threetrials,bothattheendofthetrialandcumulativelyduringsimulation.Thisis
consistentwiththeliteraturesuggestingawidererodiblecorridorcanenhance
morphologicdiversity.Asexpected,inthenarrowcorridor,themainchannelwas
stationarythroughoutthedurationoftherun.Thecross-channelprofileofthe
narrowcorridorconditionsrevealedthatthechannelbecamenarroweranddeeper,
whichisconsistentwithobservationsinrealsystems(Beagleetal.2015).However,
thesedimentprofilesuggeststhattoomuchfinesedimentmaybeerodingfromthe
system.Addinganappropriateamountofsedimenttothesystem,ratherthan
recirculatingsediment,mayresultinamorerealisticsedimentprofile.
Inthesetbackcondition,thechannelwidened,andcumulativesedimentflux
washigherthanintheunconstrainedcondition.Judgingbythecross-sectional
profileandcomparisonofthefinalelevationmaptotheinitialcondition,itappears
thatthechannelerodeduntilitcametotherevetments,and“bouncedoff,”running
intotherevetmentsontheotherside.Becausetheerodiblecorridorwasrelatively
narrow,thisprocesscontinued,resultinginawideractivechannelandmore
sedimentfluxthanfortheunconstrainedcondition(Figure12).Timeconstraints
preventedmodelingofthesetbackconditionwithallflows,butthisresultwould
likelybeimprovedbysupplyingsediment.
Thehabitatareaintheunconstrainedrunwaslessthanthesetbackrun,and
allthreerunswerefairlysimilarincalculatedhabitatarea.However,theside
channelareadidappeartobeanimportantcontributortohabitatarea(Figure14).
Themodeloutputsvelocityanddepthateachtimestep,meaningthatthisissimply
asnapshotofwhathabitatexistsatasinglepointintime.Amodificationtothe
modelallowingthetrackingofcumulativetimeacellmeetsthespecifiedhabitat
conditionwouldlikelybemorerepresentativeofthelong-termhabitatchanges
associatedwithdifferentmanagementstrategies.
Theallflowmodelresultssuggestthatthisstrategymaybeabetterindicator
offuturehabitatconditionsthanthesolelyhighflowcondition.Originally,Itrieda
fewtrialswithallflows,butbecauseIwasusing9grainsizes,itwasextraordinarily
slow.However,reducingthenumberofgrainsizesdecreasedruntimesignificantly.
Whileitispossibletorunthemodelwithalltheflowsandgrainsizes,itwouldtakea
verylongtime,andmaynotbesuitableformanagement.However,thiswouldbea
necessarytestforthetheoreticalexperimentsabouttheimportanceofriver
corridorwidth.
Thehabitatparametersusedinthisstudywereverylimited.Developingan
indexforassessingthephysicaloutputofthemodelspatially,similartotheuseof
Shannon’sdiversityindexperformedbyYarnelletal.in2006wouldbethefirststep
inturningtheoutputofthemodelintoausefultoolforrestoration.Thiscouldbe
appliedtothemapsforallflowsclassifyingcertainhabitatunits.Theuseofanindex
likeShannon’sdiversityindexwouldbeanotherusefultoolforexploringhabitat
quantity.Incorporatingotheravailableparameters,likesedimentsize,wouldalso
increaseitsutility.Connectingcurrentlyexistingrivermorphologiestolocal
biologicalintegrityestimateswouldbeanimportantwayoffieldvalidatingthe
results.
ThesepreliminarytestsindicatethattheCAESAR-Lisfloodmodelhasthe
potentialtoquantifydifferencesbetweenpossiblerestorationstrategies,andina
reasonableamountoftime.Thechannelprofilefordepth,averagesedimentsize,
andvelocityindicatedthatthemodeliscapableofrepresentingthepool-riffle
sequenceexpectedinagravel-beddedriversystem.Whilethevalidityofthe
constrainedconditionswouldbeimprovedbyaddingsedimenttothesystem,the
modeliscapableofproducingreasonablerivermorphologyandappropriately
distributingsediment.
Futurestudiesshouldexploreothermodelparameters,andincorporate
moresedimentsizes,anduseofacommonbiologicalindex.Furtherexplorationof
themodel’soperationcouldresultinausefulandapproachabletoolforrestoration.
Figure1.Positionsofnon-erodiblerockrevetmentsusedforCAESARmodelruns.Revetmentsareplacedalongthebanksoftheexistingchannel.
Figure2.Unconstrainedconditionafter100yearsofsimulatedflow.ResultingDEMismodeledwithasummerlowflow(4cms).Colorsrepresentvelocitywithdarkerblueindicatinggreatervelocity.Redlineindicatespositionofcross-section(Figure5).
TablesandFigures
Figure3.Narrowconditionafter100yearsofsimulatedflow.ResultingDEMismodeledwithasummerlowflow(4cms).Colorsrepresentvelocitywithdarkerblueindicatinggreatervelocity.Redlineindicatespositionofcross-section(Figure6).
Figure4.Setbackconditionafter100yearsofsimulatedflow.ResultingDEMismodeledwithasummerlowflow(4cms).Colorsrepresentvelocitywithdarkerblueindicatinggreatervelocity.Redlineindicatespositionofcross-section(Figure5).
Figure5.Cross-sectionofunconstrainedconditionafter100years.Averagesedimentdiameterislargerinthemiddleofthechannelthanthefloodplain.
Figure6.Cross-sectionofnarrowconditionafter100years.Averagesedimentdiametershowsnopatternacrossthechannelprofile.
Figure7.Cross-sectionofsetbackconditionafter100years.Averagesedimentdiameterislargerinthemiddleofthechannelthanthefloodplain,andchanneliswiderthanintheunconstrainedcondition(Figure5).
Figure8.Cumulativetimeinundatedduring100-yearsimulationforunconstrainedcondition.Colorsrangefrompurpletoredforincreasingtimeinundated.Sidechannels(orange)visibleonthebothsidesofthemainchannel(red)wereinundatedforlongerthanthesurroundingfloodplain.
Figure9.Cumulativetimeinundatedduring100-yearsimulationfornarrowcondition.Colorsrangefrompurpletoredforincreasingtimeinundated.Sidechannels(orange)canbeseenprimarilyonthesouthsideofthemainchannel(red).
Figure10.Cumulativetimeinundatedduring100-yearsimulationforsetbackcondition.Colorsrangefrompurpletoredforincreasingtimeinundated.Veryfewareasoutsidethemainchannel(red)wereinundatedforasignificantportionofthesimulation.
Figure11.Depth,velocity,andaveragesedimentdiameter(D50)profileforthelengthofthemainchanneloftheunconstrainedconditionafter100yearssimulation.Sedimentsizeandvelocityareinverselycorrelatedwithdepth,asexpectedforapool-rifflesequence.
Figure12.Cumulativesedimentfluxovertimefortheunconstrained,narrow,andsetbackconditions.
Figure15.HabitatareainunconstrainedconditionforCoho/Chinookpresmoltafter100yearsofflow(Depth<1m,velocity<1m/s).
TestConditions HabitatArea
Unconstrained 2970m2
Narrow 2410m2
SetBack 3140m2
Table1.Habitatareacalculatedduringsummerlowflowafter100simulatedyears.
Figure16.HabitatareainconstrainedconditionforCoho/Chinookpresmoltafter100yearsofflow(Depth<1m,velocity<1m/s).
Figure13.Velocityduringlowflowinconstrainedrunafter100simulatedyearswithallflows.Lowvelocitiescanbeseenalongtheedgesofthechannel,andthereislittleareaofhighvelocities(blue).
Figure14.Velocityduringlowflowinunconstrainedrunafter100simulatedyearswithallflows.Velocityisproperlysortedalongedges,andthereisaconsistentdistributionofmorphologicunits.
Appendix1ModelInformation
CAESAR-Lisfloodisa2DcellularmodelthatintegratestheCAESARlandscapeevolutionmodel(LEM)withtheLISFLOOD-FPhydraulicmodel(Coulthardetal.2013).Combiningthemodelsprovidesgreaterefficiencyforlargerspatialandtemporalscaleswithoutthelossofprecisionprovidedbynon-steadyflowhydraulicequations,makingthismodelanappropriatechoiceforthisstudy.
Inadditiontothemodelefficiency,themodelwasselectedbecause:• Itusessize-specificsedimenttransportequations(WilcockandCrowe,2003)
torepresentfluvialerosionanddeposition,therebyallowingsimulationofchangesinbedtexture
• Outputprovideswaterdepth,velocity,andaveragesedimentparticlesizevaluesforeverycellonthegrid,whichcanbeusedtocalculatehabitatareaatanypointinthesimulation
• Sourcecodeisavailableforcustomization,allowingustotrackcellsmeetingconditionsforhabitatranges
ComputerUsed4core3.5ghzXeonPCDEM:18,012cellgrid(114x158),10mcellsModelInputHydrology AveragedailyflowvalueswereobtainedfromUSGSstream
gage12142000.Flowentersthemodelatthreearbitrarilychosenpointsclosetothenorthedgeofthemap.Lowflowsusedaflowof9cms.
DigitalElevationMap(DEM)
Themapusedwasobtainedfrom2009KingCountyLIDAR.Itwascorrectedforwaterdepthbasedoncolorinanaerialimage,resampledat10mresolution,andmodifiedinArcGIStopreventflowfromleavingthenorthernboundary.
Sediment Distributionwascalculatedfromabar-toppebblecount.Thesupplyisspecifiedbyrecirculatingsedimentleavingthelowerboundaryofthemap.
Revetments ThesearerepresentedbyabedrockfilecreatedbymodifyingtheoriginalDEM
Lateralerosion Thisisaparameterthatmustbecalibratedforaparticularstudysitebyexperimentation.Itestedarangeofvaluesfrom0.01to0.00001,andfoundthatforthissite,avalueof0.0002resultedinanappropriatemorphologywithcurvatureexpectedgiventhecurrentandhistoricalconditionsofthesite.
HydraulicRoughness
Thisparameteraffectsthevelocityofthein-channelflow.Manning’snwasfoundbycalibrationtohighwatermarks.
Allothermodelparameterswereleftunaltered.
