6.SCS Method

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1、1SCSMethodTheSCSMethodisbaseduponfactorsempiricallydeterminedbytheUSSoilConservationService(nowcalledtheNaturalResourcesConservationService,NRCS).Itisusedinthedesignofhydraulicstructuressuchasculverts,detentionbasins,drainageditchesetc.Forlargedrainageareas,theSCSmethodisnotrecommen

2、dedbecauseofthediversityofrainfallandcatchmentcharacteristics.TheparametersinvolvedintheSCSmethodareRainfall:Allrainfalleventsareassumedtobe24-hourduration.RainfallpatternshavebeenstatisticallycategorizedintofourtypesdependinguponthegeographicalregionsoftheUS.The4typesofregionsandth

3、evarious24-hourrainfalldepthsfordifferentreturnperiodsaregivenintheformofmapswithisohyets.Infiltration:Lossesduetoinfiltration,evapo-transpirationandothercauses(e.g.depressionstorage)aredescribedbyasetofcurvenumbers(CN).Eachdrainagebasinisassignedacurvenumberbaseduponthetypeofsoil,a

4、ntecedentsoilmoisturecontent,andsurfaceconditionssuchasvegetation.TheCNvaluesaregiveninTableformfordifferenthydrologicalsoilgroups.EachsoiltypeisassignedahydrologicsoilgroupA,B,C,orD.GrouptypeArepresentsthehighestrateofinfiltrationwhilegrouptypeDrepresentsthelowestrate.Ifthebasincon

5、sistsofdifferentCN’s,thenacompositeCNiscomputed.Initiallosses:InitiallossisdescribedbytheparameterIa,whichisempiricallyrelatedtoCN(seeTable).VeryoftenitistakenasIa=0.2S.TimeofConcentration:Itiscomputedasthesumoftimeoftravelandtimeofoverlandflow.Theformeriscalculatedinthetraditionalw

6、aywhereasthelatteriscalculatedusinginformulagiveninTR55:0.80.007(nL)T=t0.50.4(P)S20whereTt–overlandtraveltime(hr)n-roughnesscoefficient(asgivenintheTableinTR55)L-lengthofflow(ft),withamaximumvalueof300ft(about90m).P2-2-yearprecipitation(in)S0–slopeInSIunits,theaboveformulatranslates

7、into0.85.476(nL)T=t0.50.4(P)S20whenLisinmetres,PinmmandTtinminutes.TheslopeShasnodimension.1UrbanHydrologyforSmallWatersheds,TechnicalReleaseNo.55(commonlyreferredtoasTR55),USSoilConservationService,USDepartmentofAgriculture(1986)1Drainagearea:DrainageareaAminsquaremiles.Rainfallexc

8、ess:Thisistherainfa