5.Unit Hydrograph Methods

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1、UNITHYDROGRAPHMETHODS1.DEFINITIONS(i)UnithydrographAunithydrographisadischargehydrographresultingfromoneunitofdirectrunoff(1mmor1cm)generateduniformlyoverthecatchmentareaatauniformrateduringaspecifiedperiodoftimecalledtheunitduration.TheUnithydrographmethodisoneofth

2、ebasictoolsinhydrologicalcomputations.ItwasoriginallypresentedbyL.K.Shermanin1932.Itisfoundedupontheassumptionthattherainfallexcess-directrunoffprocessislinearwhichimpliesthattheprinciplesofsuperpositionandproportionalityhold.(ii)DirectrunoffDirectrunoffisthediffere

3、ncebetweenthetotalrunoffandbaseflow.Itisthatcomponentdirectlyresultingfromrainfall.(iii)RainfallexcessRainfallexcessisthatportionoftherainfallthatcontributestodirectrunoff.Theprinciplesofproportionalityandsuperpositionleadtoveryusefulpracticalapplicationsoftheunithy

4、drographconcept.Forexample,ahydrographofdischargeresultingfromaseriesofrainfallexcessesmaybeconstructedbysummingupthehydrographsduetoeachsingleunitofrainfallexcess.Theyalsoimplythatthetimebaseofdirectrunoffhydrographsresultingfromrainfallexcessesofsameunitdurationis

5、thesameregardlessoftheintensity.(iv)UnitdurationTheunitdurationistherateatwhichthecatchmentisfilleduptoarainfallexcessofunitdepth.e.g.1mmofrainfallexcesscouldbeappliedatdifferentrates.Thecatchmentcanbethoughtofasatankorreservoirwhichreceivestherainfallexcessandrelea

6、sesitthroughanoutlet.Thecatchmentcharacteristicsarereflectedinthedischargecoefficientofthetankorreservoiroutletwhichaffectstherecessionpartofthehydrograph.Therisingpartisaffectedbytherateofapplicationoftheunitrainfallexcess.2.DERIVATIONOFAUNITHYDROGRAPHFORAUNIFORMST

7、ORM1(i)Procedure•Selectastormwhichisisolated,intenseanduniformoverthecatchmentandtimebyscanningthroughrainfallandrunoffrecords.Thedurationofthestormshouldnotbegreaterthantheperiodofrise(timeofconcentration).•Plottheobserveddischargehydrographandseparatethebaseflow•D

8、eterminetheareaunderthedirectrunoffhydrograph.Theareamaybeexpressedasavolumeofrunoff(dischargextime=volume)orasadepthofrunoffbydividingbyt