Watershed Analysis with the Hydrologic Engineering

ERDC/TNSMART-04-2September20041WatershedAnalysiswiththeHydrologicEngineeringCenter’sRiverAnalysisSystem(HEC-RAS)byChristopherR.GoodellandGaryW.BrunnerPURPOSE:TheobjectivesofthisdocumentaretoprovideageneraldescriptionoftheHEC-RASmodel,itscapabilitiesandlimitations,datarequirements,traditionalandinnovativemethodsforHEC-RAShydraulicmodelcalibration,andtopresentacasestudyHEC-RASmodelapplicationwherebythecapabilitiesofHEC-RASaredemonstratedasameanstosupportsystem-widemodelingandassessmentinausabledecision-supportframework.MODELDESCRIPTIONOverview.HEC-RASisanintegratedsystemofsoftware,designedforinteractiveuseinamulti-taskingenvironment.Thesystemiscomprisedofagraphicaluserinterface(GUI),separatehydraulicanalysiscomponents,datastorageandmanagementcapabilities,graphicsandreportingfacilities.TheHEC-RASsystemwillultimatelycontainthreeone-dimensionalhydraulicanalysiscomponentsfor:(1)steadyflowwatersurfaceprofilecomputations;(2)unsteadyflowsimulation;and(3)movableboundarysedimenttransportcomputations.Akeyelementisthatallthreecomponentswilluseacommongeometricdatarepresentationandcommongeometricandhydrauliccomputationroutines.Inadditiontothethreehydraulicanalysiscomponents,thesystemcontainsseveralhydraulicdesignfeaturesthatcanbeinvokedoncethebasicwatersurfaceprofilesarecomputed.ThecurrentversionofHEC-RASsupportssteadyandunsteadyflowwatersurfaceprofilecalculations.Newfeaturesandadditionalcapabilitieswillbeaddedinfuturereleases.On-linehelpisavailablefromwithintheHEC-RASsoftware.HelpcanbeaccessedbyselectingtheHelpmenuoptionatthetopofeachwindow,orbypressingtheF1functionkey.CapabilitiesandLimitations.HEC-RASisdesignedtoperformone-dimensionalhydrauliccalculationsforafullnetworkofnaturalandconstructedchannels.ThefollowingisadescriptionofthemajorcapabilitiesofHEC-RAS.TheuserinteractswithHEC-RASthroughagraphicaluserinterface(GUI).Themainfocusinthedesignoftheinterfacewastomakeiteasytousethesoftware,whilestillmaintainingahighlevelofefficiencyfortheuser.Theinterfaceprovidesforthefollowingfunctions:•Filemanagement.•Dataentryandediting.•Hydraulicanalyses.•Tabulationandgraphicaldisplaysofinputandoutputdata.ERDC/TNSMART-04-2September20042•Reportingfacilities.•On-linehelp.Steadyflowwatersurfaceprofiles.TheSteadyFlowWaterSurfaceProfilescomponentofthemodelingsystemisintendedforcalculatingwatersurfaceprofilesforsteadygraduallyvariedflow.Thesystemcanhandleafullnetworkofchannels,adendriticsystem,orasingleriverreach.Thesteadyflowcomponentiscapableofmodelingsubcritical,supercritical,andmixedflowregimewatersurfaceprofiles.Thebasiccomputationalprocedureisbasedonthesolutionoftheone-dimensionalenergyequation.Energylossesareevaluatedbyfriction(Manning'sequation)andcontraction/expansion(coefficientmultipliedbythechangeinvelocityhead).Themomentumequationisutilizedinsituationswherethewatersurfaceprofileisrapidlyvaried.Thesesituationsincludemixedflowregimecalculations(i.e.,hydraulicjumps),hydraulicsofbridges,andevaluatingprofilesatriverconfluences(streamjunctions).Theeffectsofvariousobstructionssuchasbridges,culverts,weirs,andstructuresinthefloodplainmaybeconsideredinthecomputations.Thesteadyflowsystemisdesignedforapplicationinfloodplainmanagementandfloodinsurancestudiestoevaluatefloodwayencroachments.Also,capabilitiesareavailableforassessingthechangeinwatersurfaceprofilesduetochannelimprovements,andlevees.Unsteadyflowsimulation.ThiscomponentoftheHEC-RASmodelingsystemiscapableofsimulatingone-dimensionalunsteadyflowthroughafullnetworkofopenchannels.TheunsteadyflowequationsolverwasadaptedfromDr.RobertL.Barkau'sUNETmodel(Barkau1992,HEC1997).Thisunsteadyflowcomponentwasdevelopedprimarilyforsubcriticalflowregimecalculations.However,withthereleaseofVersion3.1,themodelcannowperformmixedflowregime(subcritical,supercritical,hydraulicjumps,anddrawdowns)calculationsintheunsteadyflowcomputationsmodule.Thehydrauliccalculationsforcrosssections,bridges,culverts,andotherhydraulicstructuresthatweredevelopedforthesteadyflowcomponentwereincorporatedintotheunsteadyflowmodule.Sedimenttransport/movableboundarycomputations.Thiscomponentofthemodelingsystemisintendedforthesimulationofone-dimensionalsedimenttransport/movableboundarycalculationsresultingfromscouranddepositionovermoderatetimeperiods(typicallyyears,althoughapplicationstosinglefloodeventsarepossible).Thesedimenttransportpotentialiscomputedbygrainsizefraction,therebyallowingthesimulationofhydraulicsortingandarmoring.Majorfeatureswillincludetheabilitytomodelafullnetworkofstreams,channeldredging,variousleveeandencroachmentalternatives,andtheuseofseveraldifferentequationsforthecomputationofsedimenttransport.Themodelwillbedesignedtosimulatelong-termtrendsofscouranddepositioninastreamchannelthatmightresultfrommodifyingthefrequencyanddurationofthewaterdischargeandstage,ormodifyingthechannelgeometry.Thissystemcanbeusedtoevaluatedepositioninreservoirs,designchannelcontractionsrequiredtomaintainnavigationdepths,predicttheERDC/TNSMART-04-2September20043influenceofdredgingontherateofdeposition,estimatemaximumpossiblescourduring