Constructing river basin sediment budgets for the

CSIROLANDandWATERConstructingRiverBasinSedimentBudgetsfortheNationalLandandWaterResourcesAudit.IanProsser,PaulRustomji,BillYoung,ChrisMoran,AndrewHughesCSIROLandandWater,CanberraTechnicalReport15/01,July2001ConstructingRiverBasinSedimentBudgetsfortheNationalLandandWaterResourcesAudit.IanProsser,PaulRustomji,BillYoung,ChrisMoran,AndrewHughesCSIROLandandWaterTechnicalReportNumber15/011ConstructingRiverBasinSedimentBudgetsfortheNationalLandandWaterResourcesAudit.0Introduction1DefinitionofRiverLinks3SedimentBudgetforBedload5TransientModel5InputsofBedload6SedimentTransportCapacity9ConstructionoftheBedloadBudget11SteadyStateBudgetforBedload13ComparisonofTransientandSteadyStateBedloadModels14SedimentBudgetforSuspendedLoad15ReservoirDeposition19ContributionofSedimenttotheCoast19SuspendedSedimentBudgetUnderNaturalConditions20Hydrology21MeanAnnualFlow21SedimentTransportCapacity22BankfullDischarge23MedianOverbankDischarge24ModelImplementationandValidation25Acknowledgements31References31IntroductionThisdocumentdescribesmethodsusedintheNLWRAriversandsustainableagricultureprojectstorepresenttheerosionofsedimentfromriverbanksandthepropagationofgully,hillslopeandriverbanksourcedsedimentthrougharivernetwork.Essentiallyitdescribesmethodsforconstructingsedimentbudgetsthroughrivernetworks.Thebudgetsincludestorageofsedimentonfloodplains,inthebedoftheriverandinreservoirs.Calculationsaremadeofthesedimentoutputfromeachriverlinkandthecontributionofsedimenttothecoast,oranyotherreceivingbody,fromallsub-catchments.Thebudgetstreattwotypesofsediment:suspendedsedimentandbedload.AsuitofArcInfoTMprogramsareusedtodefinerivernetworksandtheirsub-catchments;importrequireddata;implementthemodel;andcompiletheresults.ThesearereferredtocollectivelyastheSedNetmodel:theSedimentRiverNetworkModel.Forthisproject,suspendedsedimentischaracterisedasfinetexturedsedimentcarriedatrelativelyuniformconcentrationthroughthewatercolumnduringlargeflows.Itstransportisgenerallylimitedbythesupplyofsedimenttoriversratherthanbythesedimenttransportcapacity(OliveandWalker,1982;Williams,1989).Themainprocessfornetdepositionof2suspendedsedimentisoverbankdepositiononfloodplains(e.g.Wallingetal.,1992).Theamountofdepositiondependsupontheresidencetimeofwateronthefloodplainandthesedimentconcentrationoffloodflows.Thevelocitiesofsuspendedmaterialwithinchannelsarerelativelyhighsowecanassumethatitsresidencetimeislow,thatthereisnegligibletransientdepositionofsuspendedsediment,andthatsteadystateconditionshavebeenreachedsinceEuropeansettlementincreasedthesupplyofsediment.Suspendedsedimentissourcedfromsurfacewasherosionofhillslopes,gullyerosionandriverbankerosion.Thesedimentbudgetisreportedasmeanannualvaluesforeitherthecurrentlanduseorforpre-Europeannativevegetationcover.Bedloadissedimenttransportedingreatestproportionsnearthebedofariver.Itmaybetransportedbyrolling,saltation,orforshortperiodsoftime,bysuspension.Transportoccursduringperiodsofhighflow,overdistancesofhundredstothousandsofmetres(Nicholasetal.,1995).Residencetimesofcoarsesedimentinrivernetworksarerelativelylongsothereistransientdepositiononthebedasthesedimentworksitswaythroughtherivernetwork.Inadditiontothatdeposition,anincreaseinsedimentsupplyfromacceleratedpost-Europeanerosioncancausethetotalsupplyofsedimentinhistoricaltimestoexceedthecapacityofariverreachtotransportsedimentdownstream.Theexcesssedimentwillbestoredonthebedandtheriverwillhaveaggradedoverhistoricaltimes(Trimble,1981;Meade1982).Therehasbeenasignificantincreaseinsupplyofsandandfinegraveltoriversinhistoricaltimesanddepositionofthisbedloadhasformedsandslugs:extensive,flatsheetsofsanddepositedoverpreviouslydiversebenthichabitat(Nicholasetal.,1995;Rutherfurd,1996).Thebedloadbudgetaimstopredicttheformationofthesesandslugs.3Figure1.Arivernetworkshowinglinks,nodes,Shrevemagnitudeofeachlinkandinternalcatchmentareaofamagnitudeoneandamagnitudefourlink.Thebasicunitofcalculationforconstructingthesedimentbudgetsisalinkinarivernetwork.Alinkisthestretchofriverbetweenanytwostreamjunctions(ornodes;Figure1).Eachlinkhasaninternalsub-catchment,fromwhichsedimentisdeliveredtotherivernetworkbyhillslopeandgullyerosionprocesses.Theinternalcatchmentareaisthecatchmentarea.addedtothelinkbetweenitsupperandlowerlimits(Figure1).Forthepurposeofthemodel,theinternalcatchmentareaoffirstorderstreamsistheentirecatchmentareaoftheriverlink.AdditionalsedimentissuppliedfrombankerosionalongthelinkandfromanytributariestothelinkDefinitionofRiverLinksAbranchingnetworkofriverlinksjoinedbynodeswasdefinedfromtheAUSLIG9digitalelevationmodel(DEM)ofAustralia.Thealternativemethodofdefiningrivernetworksfrommappedstreamswasdismissedbecauseinplacesthepositionofthemappedstreamsdoesnot11114324coincideexactlywithvalleysintheDEM.TheDEMisrequiredtogeneratetopographicattributesofcatchmentareaandslopeofeachriverlinkanderrorsinpositioningproducemanyspuriousresults.TheArcInfoflowaccumulationalgorithmwasusedtodefinethecatchmentareaofallcellsintheDEM.Therivernetworkwasdefinedasbeginningatacatchmentareaof50km2.Thiswasanarbitra