rta supplement to austroads part 5 - 2008 drainage design

RTA Austroads Guide Supplements

Publication Number: RTA/Pub. 11.096 Supersedes : Nil Publication date: 3 March 2011

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AUSTROADSGUIDETOROADDESIGNPART5(2008):DRAINAGEDESIGN

GeneralAustroadshasreleasedtheGuidetoRoadDesignPart5:DrainageDesignandallroadagenciesacrossAustralasiahaveagreedtoadopttheAustroadsguidestoprovidealevelofconsistencyandharmonisationacrossalljurisdictions.ThisagreementmeansthatthenewAustroadsguidesandtheAustralianStandards,whicharereferencedinthem,willbecometheprimarytechnicalreferencesforusewithintheAuthority.

Thissupplementisissuedtoclarify,addto,ormodifytheAustroadsGuidetoRoadDesignPart5:DrainageDesign. TheRTANSWacceptstheprinciplesintheAustroadsGuidetoRoadDesignPart:DrainageDesignwithvariationsdocumentedinthissupplement.Thesear5v

iationsfallintotwocategories: RTAComplementaryMaterial:RTADesignReferenceDocumentsreference

materialthatcomplementstheAustroadsGuides.ThesedocumentsincludeRTAManuals,TechnicalDirectionsand/orotherreferencematerialandaretobereadinconjunctionwiththeAustroadsGuides.

RTADepartures:RTAroaddesignpracticesthatdepartfromtheAustroadsGuides.

eealsotheRTASupplementsfortheotherpartstotheAustroadsGuidetoRoadSDesignSeriesandtheRTASupplementstotheotherAustroadsSeries.AllReferenceDocumentsmentionedinthispartshouldbecheckedagainsttheRTADesignReferenceDocuments.

R TASupplementtoAustroadsGuidetoRoadDesignPart5DrainageDesign

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7.1INTRODUCTION7.1.1 AimoftheGuideThissectionoftheGuiderelatestothehydrologyandhydraulicsofroadrelatedwaterflows.Itisintendedtodealwithurbandrainageandtheestimationofpeakflows in small to medium sized rural catchments. A separate document willdescribe themethods that are appropriate in respect to bridge hydrology andhydraulics.So far as is possible, the Guide attempts to set firm guidelines for variousparameters, such as Average Recurrence Intervals (ARI's), widths of flowsetc, to assist in achieving a uniform approach to drainage mattersthroughouttheState.Although this sectionof theGuide isdirectly concernedwith hydrology andhydraulics, it should always be borne in mind that the design of drainagestructu dbyavarietyoffactors,including:reswillbeinfluence Constructioncosts ntsMaintenanceandoperatingcostsand/orrequireme A e s t h e t i c s Safetyandconvenienceofthepublic esoflocalcouncilsandstatutorybodiesPerformanceobjectiv LegalrequirementsDependinguponthecircumstancesineachcase,someofthesefactorsmaybemoreimportantthanhydrologicalorhydraulicrequirements.TheprimesourcesofdataandmethodologyforthisSectionare: "AustralianRainfallandRunoff",EditedbyD.H.Pilgrim, InstitutionofEngineersAustralia(1987). "StormDrainageDesignforSmallUrbanCatchments",by J.Argue,AustralianRoadResearchBoard(1986) "Flood Plain Development Manual", New South Wales Government(1986)thersourceshavebeenused,andarereferencedwhereappropriate.O

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7.1.2 ComputerAidedDrainageDesignConstantdevelopmentofmicrocomputershasallowedthecreationofabankofprograms that extend the ability of designers to examine designs quickly andeffectively.Theuseofofftheshelfsuites,suchasILSAX,RORB,HEC2,WBNMis

detail i tendorsed, but ed nstructions must remain in the par icular usermanuals.TheiruseisreferredtoratherfleetinglyinthisGuide.This may lead a casual reader to the erroneous presumption that manualmethodsarefavoredover suchprograms. It is the intentionof thisGuidetopointreaders towardssuchprograms,whileat thesametimedescribingmanyanual methods that are still of relevance and use, particularly for small esignswherethemathematicsaresimple.

md7.1.3 RationalMethodAdvanced hydrological models have been developed in recent years,culminating inthedemiseof theRationalMethodasadefinitivemodel,and itsreplacement by the Probabilistic Rational Method. This method predicts themagnitudeofafloodofaspecifiedAverageRecurrenceInterval(ARI)basedonthe known rainfall of the same ARI, without in any way reflecting the realprocesses of precipitation, infiltration and runoff that occur within a givencatchment. The policy of the Authority is to use the Probabilistic RationalMethod,wherebetterdataisnotavailable.

7.1.4 Minor/MajorConceptThegrowingrecognitionofthedamagethatmayresul t to the road and i t s surrounding development has led to the introduction of techniques ofaddressing overflows. The consideration of both minor and major flowsthrough/inasystemisadvocatedwheresignificantdamageisreasonablyforeseeable.7.1.5 InteractionwithOthersThe Authority is generally responsible for the drainage of the ribbon of landrunning through a catchment. Local councils are responsible for drainageon acatchmentwidebasis.Hence,theAuthorityseekstocooperatewithlocalgovernment to assist in meeting its overall objectives. These will also bediscussedinthisSection.Other government departments and statutory authorities may also need to beconsultedinsomecircumstances.ThesecanincludetheMaritimeServicesBoard(for discharge into tidal water bodies), the State Pollution Control Commission(regarding pollutant control), Soil Conservation Service, the Department ofWaterResourcesand theWaterBoard (particularly incatchmentareas).Thus theuseofspecialised devices such as retarding basins, pollutant traps etc may need to beconsidered in some locations. The Public Works Department can provide datarelating to tidal waters, harbours, rivers, lakes, storm surge, rainfall and floodhistograms



7.2RAINFALLINTENSITYREQUENCYDURATIONCURVESF

7.2.1 GeneralThisSectiondealswiththeproductionoftheIntensityFrequencyDuration(IFD)curvesforestimatingtherainfallintensitiesrequiredforthedesignofdrainagesystems.Themethodofproducing IFDcurves isbasedupontheprocedurespresented in the1987editionofAustralianRainfallandRunoff,Volumes1and2(publishedbytheInstitutionofEngineers,Australia).AcopyofVolume2ofA.R&R.willberequiredtoobtainthevariousfactorsinvolvedintheproductionoftheIFDcurves.TheinformationgiveninVolume2ofA.R&Rresultedfromaprojectcommencedin1982bythe Australian Bureau of Meteorology to provide a full coverage of IFD data for AverageRecurrenceIntervals(ARI)upto100yearsandfordurationsfrom5minutesto72hours.EarliereditionsofA.R&Rweredevelopedfromarestrictedsetofdataandwasinadequateformanyareasofthecountry.Iflocal,accuraterainfallinformation(recordedoveralongperiodoftime)isavailable,thenitshouldbeusedfordrainagedesign.Suchinformationisavailablefromanumberofsourcesincluding the Bureau of Meteorology (with the location specified by C.M.A. mappingcoordinates),thelocalgovernmentbodyinwhichtheprojectlies,ortheWaterBoardiftheprojectiswithintheSydneymetropolitanarea(thelocationisspecifiedbytheuseofUBDmap references). TheWater ResourcesDepartment and PublicWorksDepartmentManlyValeLaboratorycanalsoprovidethisinformation.7.2.2 AccuracyThemethodsusedinobtainingtheinputdataandproducingtheIFDcurveshaveacertaindegreeofinaccuracy(rangingfrom5%foranARIof2years,72hourduration,upto15%foran ARI of 50 years, 1 hour duration). Generally, because of the increased amount ofextrapolation required beyond the recorded rainfall data, the accuracy of design rainfallinformationreduceswithincreasingARIanddecreasingduration.The1987editionofA.R&Rrecommends theadoptionof the followingorderofaccuracywhencalculatingIFDrainfallintensities.



Table7.2.1RequiredAccuracyofIFDResults

CALCULATED VALUE OF IFD INTENSITY I

REQUIRED ACCURACY

9.99 (mm/h) 2 decimal places 10.0 to 49.9 (mm/h) 1 decimal place

50 (mm/h) no decimal place 7.2.3 Methods All methods involve fitting the basic annual maximum rainfall data fordurations ranging from5minutes to 72 hours to a LogPearson Type IIIdistribution,withasmallpositiveskewnessuptoavalueof0.8.Depending upon the particular requirements of a project the IFD curvescan edbyanyofthefollowingtechniques:beproduc Algebraic omBureauofMeteorologyRequestfr Graphical ComputeraidedOnecomputeraidedprocedureisbasedonanOpenAccessIIspreadsheetfor use on a personal computer. Copies of this spreadsheet are availablefromtheRoadDesignSectionoftheAuthority.Theothercomputeraidedprocedure is the program "IFDARR" produced by Mr. Y. Kuczera of theUniversity of Newcastle. This program is available from Road DesignSection at no charge. The Bureau of Meteorology will provide IFDinformationataparticularsiteforafeeofapproximately$130.00.Allprocedureshavesteps thatare interchangeableandthedesignermayuse a method in its entirety, or select the most convenient steps fromeachprocedure.The following sections dealing with the production of IFD curves havebeen presented in a format designed to ease the process. Each Sectionbeginswith a brief description, followed by a step by step guide. A proforma is included at the end of each Section (the steps of which matchthose in the text). It is intended that the proforma be photocopied andusedasadesignaid.



7.2.4 AlgebraicProcedureThismethodisincludedbecausesomedesignersprefertouseanalgebraicapproach.Generally, the graphical, or spreadsheetmethods arequicker,more convenient andsufficientlyaccurate,giventhederivationoftheoriginaldata.TheprocedureisgivenasAppendix1.7.2.5 GraphicalProcedureThismethodofdeterminingtheIFDcurvesforalocalityisinmanywayspreferabletothealgebraicprocedure.Firstly,itissimplertoperform.Secondly,theaccuracyofthegraphical method is well within the limits of accuracy of the IFD input data forstandardARI'sandalldurations. Indeed,anymistakesthatmayoccuraregenerallymore apparentwith the graphicalmethod, and thus are detected earlier thanwithothertechniques.7.2.6 ComputerAidedMethodThisdescriptionoutlinesthemethodofdeterminingdesignrainfallintensitiesusinganOpenAccess11spreadsheetentitledA:IFD.FMD.Alsoonthedisk isabackupcalledA:HEX.FMD.CopiesofthisspreadsheetareavailablefromtheRoadDesignSectionoftheAuthorityinSydney.Thespreadsheetperformsthecalculationsdescribedinthe1987A.R.&R.,Chapter2,forIFDrelationshipsatasite.TheoutputisatableofrainfallintensitiesIfordurationsof6min,1,12and72hours,forARI'sof1,2,5,10,20,50and100years.TheseintensitiesmaythenbeplottedonaDurationInterpolationDiagram(asshownonFigureA7.2.2 inAppendix2). Intensities for intermediatedurationsmaybereadfromthegraph.Anotherprogram"IFDARR"producesatableofintensities."I", forvariousdurationsand ARI"s. It also produces coefficients for polynomials which will predict "I" andproduceatableofvaluesfor

t x I 0-4

7.2.7 BureauofMeteorologyTheBureauofMeteorologywillproducearainfallintensitydiagramandanIFDtableforaparticularsite.Theirfeeis$130.00and`thelatitudeandlongitudeofthesiteisrequired.



7.3URBANHYDROLOGYANDPIPENETWORKDESIGN7.3.1GeneralThisSectionconcentratesonurbanhydrologyandpipenetworkdesign,whilstgivingabackgroundtothetheoreticalaspects.Wheremorecomplexproblemsareencountered,designersareencouragedtorefertoreferencebookswhichevotemorespacetorigorousdevelopmentofthevariousdesigntechniques,articularly,ARandR,(1987),Chapter14.

dp7.3.2ObjectivesheobjectivesofurbandrainagedesignwithintheAuthorityare:T

achievingabalancebetweenthedisruptionstotrafficflowcausedbyminor

storms,andthecostsofdrainagefacilities, promotingroadsafety,andpedestrianamenitybypreventinggutterflows

fromintrudingexcessivelyontotrafficpathsduringminorstorms, providingroaddrainagesystemswhicharecomplementaryto,andintegral

with,theobjectivesoflocalauthorities.Thisinvolvesconsiderationofexistinginstallations,landuse,townplanning,andthemultipleuseoflandfortransport,recreation,drainage,waterconservationandpollutioncontrol,

ensuringthatfloodingdepthandvelocityofflowarewithindefinedlimits, reducingthemassandvolumeofpollutantsdischargingfromthedrainage

systemintoopenwaterwaysbyadheringtotherequirementsoftheStatePollutionControlCommission,

havinganequitablebasisforsharingcost.These objectives arise out of the statutory responsibility of the Authority toprovideforvehiclesandpedestrians.



7.3.3Minor/MajorConceptIndrainagesystems,overflowsshouldbecarriedalongplannedroutestotheoutfallwithoutdivertingintounwantedlocations.Thisapproachisformalisedinheminor/majorapproachtodrainagedesign.ThisisdiscussedunderandhowninFigure7.3.1ts(a)MinorDrainageSystemsThis is the gutter and pipe network to carry runoff from minor (i.e. morefrequent)storms.ThepathofminorsystemsmaydifferforthenaturaldrainagepathbyfollowingstreetlayoutsTheminordrainagesystemminimisesnuisancefloodingofstreets.(b) Major Drainage Systems

Thisisthesystemwhichconveysrunofffrommajor(i.e.lessfrequent)stormstotrunk drains. Any flow that cannot be provided for by the minor system ishandledbytheroaditself,andotherchannels.Themajor drainage system generally follows existing drainage patterns. Theirdesign can be complex when they do not follow the same path as the minordrainage.Themajordrainageapproachshouldnotbe confusedwith trunkdrainage; theconceptrelatestodrainagesystemsoperatingduringstormsoflargemagnitude.The overall aim of the minor / major approach is to ensure that hazardoussituationsdonotariseonstreetsandfootpaths,andthatallstructuresinurbanreasareprotectedagainstfloodwaterstoasimilarstandardtothatapplyingtoonesadjacenttoriversormajorstreams.az



DRAINAGESYSTEMBEHAVIOURDURINGMINORANDMAJORSTORMS

MAJOR/MINORDESIGNSTANDARDS

THEMAJOR/MINORCONCEPTOFDRAINAGEDESIGN Figure7.3.1



7.3.4DesignProceduresTwomethodsofdeterminingthedesignflowsforurbandrainagenetworksthatarethe theAuthorityare:mostappropriatefor

TheRationalMethod

TheILSAXprogram(a) RationalMethodThismethodrepresentstherefinement,overmanyyears,ofamethodoriginallydevelopedbyMulvaney in1851 todescribe the runoff fromsmallagriculturalcatchments in Ireland.Themethodhasbeencomprehensivelydescribed in itsmost generally accepted form by Argue in the ARRB Special Report No 34"Storm Drainage Design in Small Urban Catchments". That report may beutilised as a source book for additional information to that supplied in thisSection of the Guide. "Australian Rainfall and Runoff 1987", published by theInstitutionofEngineersisalsorecommended.TheRationalMethodwasatone timealsoregardedbymanyasadeterministmodel, that is, it was regarded as describing the way a particular catchmentreceived andmodified rainfall and discharged it as runoff. It has been found(Frenchetal,CivilEngineeringTransactions,I.E.Aust,Vol16pp95102)thattheRationalMethod isapoormethodwhentreated thisway.Asaresult, it isnowregardedasapurelystatisticalmethodofderivingdesignfloods.ItisusedtoestimateapeakflowofselectedARIfromanaveragerainfallintensityofthesameARI.Becausethemethodisnowrecognisedasastatisticalmethod,itcannotbeusedto predict peak runoff from particular storm events. It cannot model acatchmentwithspatiallyvaryingrainfall.Thecalculationsareofarepetitiousnature,andmaybestbehandledusingaspreadsheet.(b) ILSAXProgramTheILSAXmethodhasbeendevelopedbyDrG.O'LoughlinoftheNSWUniversityofTechnologyasanenhancedandextendedversionoftheILLUDASprogram(IllinoisUrbanStormwaterAreaSimulator)andILLUDASSA(aSouthAfricanmodificationofILLUDAS).TheacronymILSAXreferstothe"ILLUDASModel,SouthAfricanvariation,withsomethingXtra".Inthisprogram,hydrographsaregeneratedusingthetimeareamethod.Rainfallmodelsareappliedtothecatchmentmodel,andrunoffsduringdifferenttimeintervalsarecalculated.TheprogramisdescribedfullyintheUniversityofTechnology's


publication"TheILSAXProgramforUrbanSformwaterDrainageDesignandAnalysis"byDrGO'Loughlin.(RTAlibraryreference)Inotherapplications,suchasurbanandruralstreamflow,othermodelssuchasRORBandHEC2arealsosuitable.(SeeA.R.&R.chapters9and14)


7.3.5S tantTerminologyomeImpor (a) HydrologyHydrologydealswiththecharacteristicsofwaterflowingfromacatchment.Itisaseduponstatisticalanalysisofobservedrainfalldataandcatchmentb

parameters.Byitsverynature,hydrologyisaninexactscience,andthedeterminationofrunoffcharacteristicsrequiresagreatdealofcareandexpertiseinhandlingandnalysinghydrologicaldata,whichoftenhavegreatervariabilitythanotherypesoat fdata.(b) HydraulicsHydraulicsisthestudyofwaterflowingwithindrainagesystems.Itdealswithdeterminingappropriatepipesizesandgullypitlocations;itmayincludeopenhannelflowsandspecialisedstructures,suchasretentionbasins,andenergyissipa

cd tors.(c) HydrographShapesAhydrographisagraphshowingtherelationshipbetweenthevolumetricflowrate(usuallyinM3/sec)andthetimeofoccurrenceoftheflowrate.Thustheshapeofthegraphgivesanindicationofthetypeofstorm,andrunoffcharacteristicsofacatchmentarea.Theunthinkingapplicationofprinciplesofhydraulicefficiencymayresultindesignswithfastflowsinsmooth,directchannels.Designersshouldbeawareofthistendency,asitmayleadtosudden,highflowsatthepeakofthehydrograph.Thiswillresultinthedrainagesystembeingstrainedduringthisshortpeak.Itisreferabletodesignwithaviewtoachievingalowerpeakdischargeoverap

longerduration.Thisprincipleistheonethatisappliedintheinstallationofretentionbasins.Inthesedevicesaproportionoftherunoffisstoredforashortperiodoftimebeforedischarge.Thisresultsinaflattenedhydrographwithalowerpeakflowrate.Theflatteningofthehydrographresultsinlowerconstructioncostsforthemallerdrainagefacilitiesrequiredcomparedtothosenecessarywhenprovidingorhighpeakflows.sf



Otherbenefitsmayalsoresultfromtheloweringofflowvelocities,includingincreasedpublicsafetyandtheabilitytoreducethestructuralstrength(andhencecost)ofdrainagestructures.ThisisillustratedinFigure7.3.3.



(d)AverageRecurrenceIntervaloftheTheaveragerecurrenceinterval(ARI)istheaverageorexpectedvalue

periodbetweenexceedancesofagivendischarge.TheARIhasbeenadoptedastheprobabilitytermusedinthisguideinaccordancewithcurrentAustralianusage.Theinclusionoftheword"average"mphasises,toboththepractitionerandlayman,theessentialrandomnessoferainfallandrunoffeventsovertime.TheARIisbasedontheanalysisofthepartialseriesoffloodsrecordedattheite.Iflocalinformationisnotavailable,datapresentedinthisGuidemaybesused.Nonetheless,thedevelopmentofalocaldatabaseisencouraged,especiallyforareaswherefloodingordrainagedesignhaveproventobedifficultinthepast.Localcouncils,WaterBoard,WaterResourcesDepartment,PublicWorksepartmentandtheBureauofMeteorologymaybegoodsourcesoflocalrainfallD

information.TheARIforaparticulardrainagesystemshouldalwaysbedeterminedinonsiderationofultimatedevelopment.Ingeneral,adoublingofARIincreasescthecostofthedrainagesystembyapproximately10%.ypicalfactorsthatneedtobeconsideredindeterminingtheARIincludesome,rallofthefollowing:

To Thelevelofhydraulicperformancerequired. fconstruction,operationandmaintenance.Costso Safety Aesthetics. Regionalplanninggoals. Legalandstatutoryplanningrequirements



Figure7.3.4Conceptually, longerARI's areuseduntil themarginal costs required tohandle the largerflowsareequaltothe.marginalbenefitsderivedfromthereductionofflooddamages,ThisisillustratedifFigure7.3.4.Generally,thereisanobligationtoensurethatdrainageprovisionsdonot causedanger to life or property greater than thatwhichwould occurwithout thestructure(s).ThemajordrainagesystemisnormallydesignedforanARIof100years.Asignificantpartofthis flow (in the order of 50%) flows above the ground, and the remainder through theundergroundpipenetwork.Inexceptionalcases,majordrainagesystemsshouldbedesignedformoreseverestormssuchas: Alocationwherefloodingmayresultinmajorpropertydamage.

Locationswhereaccessiscrucialfordefence,civildefenseorotheremergencyservices.

erConversely,thereareareaswhichmaybesubjecttoinundationfromariv

breakingitsbanksorr

.frombackwater.Insuchcases,itmaybeappropriatetodesignthemajodrainagesystemforlessseverestorms.EachcasemustbetreatedonitsmeritsTheminordrainagesystemisnormallydesignedforanARIof5years.ThevariabilityofrainfallwithinanyparticulartimeframecanbecalculatedbymeansoftheBinomialProbabilityDistribution.Thus,thereisa70%probabilitythatthethreeyearARIstormwilloccurinanythreeyearperiod,a67%probabilitythatthefiveyearARIstormwilloccurinanyfiveyearperiodanda65%probabilitythatatenyearARIstormwilloccurinanytenyearperiod.



7.3.6TheRationalMethodUsingtheRationalMethod,thedesignflowratefromacatchmentareaiscalculatedromthefollowingrelationship:f

Where: Q=CIA/360hedesignflowrate(m3/sec)Q=t

C= the dimens ionless runof f Coefficient1=the

hmentarea(ha)rainfallintensity(mm/hr),correspondingtoaparticularstormdurationandaveragerecurrenceinterval,and

A=thecatc(a)RunoffCoefficientsRunoff coefficient C can be interpreted in different ways: as a ratio betweenrunoffandrainfallvolumes;astheratiooftheirpeakrates;orastheratiooftherunofftorainfallfrequencyInthislast,"probabilistic"interpretation,thevalueofC does not relate to a particular storm. It covers the whole range of possibleevents,involvingdifferentcombinationsofrainfallsandantecedentconditions.Todeterminethecoefficientofrunoffofanurbancatchment,thedesignermustcommence with a map of the catchment area being studied. Existingdevelopmentandzoningisdeterminedfrominspectionandtownplanningmaps.Itisnotconsideredappropriatetodesignforpotentialchangesinlanduse,sinceitisuneconomictoinvestfundsindrainagestructureswhichmaynotbeneeded.Developersarenowrequiredtorestrictoutflowstothenaturaldischarge.Previousmethod of estimating C (eg A.R & R. 1977) involved relating rainfallintensity with surface type and land use. This approach is no longerrecommended.Figure 7.3.5 relates the coefficient for a 10 year ARI C10, to the pervious andimpervious fractions of the catchment, and to its rainfall climate, expressedthrough the 10 year ARI, 1 hour rainfall intensity, (1011). This relationshipreflectstheexperienceofdrainageauthoritiesandevidencefromthefewgaugedurbancatchmentswithreliablelengthsofrecord.The currentmethod involves using the relationships shownon Figure 7.3.5 toderiveavalueofthecoefficientofrunoffforanARIof10years(C10).ThisvalueisthenmultipliedbyaFrequencyfactorFytodeterminethecoefficientofrunofffortherequiredARI(Cy).Incertaincircumstances,theresultingvalueofCywillbegreaterthan1.00.ifthisisthecase,thenalimitingvalueofCy=1.00shouldbeadopted.Thereislittlefirmevidenceforanallowanceforeitherslopeorthesoiltype.Iftherearesignificantlocaleffects,andreliabledataisavailable,thenadjustmentsforsoiltypemaybeincorporatedwithinthecalculations.



The relationships shown with Figure 7.3.5 apply to areas that are essentiallyhomogeneous,orwheretheperviousandimperviousportionsaresointermixedthat an average is appropriate. In cases where portions of a catchment areignificantlydifferent,theyshouldbeseparatedanddifferentvaluesofCapplied.s(b)EquivalentImperviousAreasDesign flowrates for pit inlets are calculated for local contributing subcatchments,whilethoseforpipesarecalculatedonaccumulatedareasdrainingthrougheachpipesectionorreach.It is inappropriate to simply add the separate flows from each subcatchment(except for small property drainage systems) as this overestimates the totalflowrates(unlessallsubcatchmentshaveanidenticaltimeofconcentration).A more accurate approach is to sum the products of C and A values for subcatchments. The design flowrate is then calculated by multiplying the totalequivalent impervious area by the values of intensity I corresponding to thetimesofconcentrationatvariouspointsalongadrainageline.Useofequivalentimperviousareasallowsdifferentzoneswithinasubcatchmenttobecombined.For example, in an area consisting of three subcatchments with landusesdenotedbya,bandc,thecombinedequivalentimperviousareais:

(c)TravelTimesThe timerequired for runoff fromthe furthestpointof thecatchment to reachtheoutletisdefinedasthe"timeofconcentration".InRationalMethodcalculations,themaximumdischargefromthecatchmentistaken to occur (subject to some particular exceptions) when ally parts of thecatchmentarecontributingtotheflow.Thisassumptionisbaseduponamodelwherewaterflowsasdiscretedroplets,(clearlynotthecase).Nonetheless,thisconcept may still lead to a suitable solution for the calculation of maximumrainfallintensityandpeakrunoff.Flowmaytravel invarious forms fromthecatchmentextremitytothepointofdischar reemaincategories:ge.Theseformshaveth Overlandorsheetflow Rooftogutterflow(e.g.overroof,guttering,downpipeandpipe) *Openchannelflow(e.g.concretegutter,Swale,pipeorearthdrain)



Theminimumtraveltimeforanycomponentofacatchmentfromitsmostextremepointtothepointofdischarge,shouldbe5minutes.Although contoursmay indicate that overland flowmay cross boundary,orfencelines,itis

NOTE:Theupper linerepresentsconditions forareaswhere10I1 is70mm/horgreater,andthelowerlineisforareaswhere10I1is25mm/horless.ForreasinbetweenthesevaluesavalueofC10canbeinterpolatedusinga C10=0.

110(1f)9xf+CWhere:

C10=10yearARIrunoffcoefficient.run fficientC110=Perviousarea offcoe

=0.1+0.0133(10I125)f=Fractionimpervious(0.0to0.1)



ForARI;otherthan10years, theC10 al is ultipliedbya frequencyfactorFyfromthelistbelow: v ue m

Cy=FyxC10When:C10 =CoefficientofrunofffordesignyearARI

Fy=frequencyfactorC10=10yearARIrunoffcoefficient

FREQUEN CTORFyARI(years) CYFA1 0.802 0.855 0.9510 1.0020 1.0550 1.15100 1.20RATIONALMETHODRUNOFFCOEFFICIENTSFigure7.3.5

Frequently the case that cultivation, fences, dwarfwalls and other constructionsinducewatertoflowalongthoselines.When defining subareas of catchments, the actual line of flow is a matter forinspectionorjudgmentinparticularcases.(i)OverlandFlowinHomogenousCatchmentsThismethodofcalculatingtimeofconcentrationtcissuitableonlyforcatchmentswithhomogenousslopesandroughness.Itcannotbeappliedtolargecatchmentswithvaryingslopesorsurfaces.verlandfloworsheetflowiscalculatedbythekinematicwaveformula:O



Table7.3.1SurfaceRoughnessFactorsSURFACE TYPE ROUGHNESS COEFFICIENT

n

Concrete or asphalt 0.010- 0.013

Bare sand 0.010- 0.016

Gravelled surface 0.012-0.030

Bare clay-loam soil (eroded) 0.012- 0.033

Sparse vegetation 0.053- 0.130

Short Prairie Grass 0.100- 0.200

Lawns 0.170- 0.480

Ascanbeseen fromTable7.3.1, thecoefficientshowsawiderangeofvalues forlawns and grassed surfaces.These values aredependentupon thedepthof flowrelativetothelengthofthegrass(thedeepertheflow,thelesstheretardingeffecteofthegrass).Careshouldb takenwiththeselectionofn*,asthevalueofthetimeofconcentrationissensitivetothisvalue.Thebestmeansof solving thisequationwithout iteration isbyuseofa rangeofdifferentrainfallintensitiesforvariousdurationsandARKTheseareobtainedforalocation using Intensity FrequencyDuration (IFD) curves. These intensities Iareraisedtothepowerof0.4andmultipliedbyarangeofvaluesfordurationt.ThisgivesamatrixoftxI0.4valuesrelativetoARIanddurationt.Theknownvaluesforlengthoftravel l,surfaceroughnessn*andsurfaceslopearesubstituted intotheequation,andtheexpressionrearrangedintotheform:

txI0.4= Some value hevalueinthisexpressioniscomparedtothenearestvalueintheabovematrix,andthe

dingvaluefordurationtobtained.TcorresponxampleE

Thisexampleisgiveninthe1987editionofARMandillustratesthetechniquegivenabove.subcatchmentinPenrithN.S.W.,hasalengthoftravelof60m,aslopeof0.02m/m(2.0%)ndaroughnessof0.05.Thustimeofconcentrationis:Aa



Amatrixof rainfall intensitiesat the location forrangesofARIsanddurations iscalculatedusingIntensityFrequencyDuration(IFD)curves:able7.3.2 RainfallIntensitiesforCatchmentinPenrithAreaExampleT

Intensity (mm/hr)

DURATION t(min) ARI

1 ARI 2

ARI 5

ARI 10

ARI 20

ARI 50

ARI 100

5 75 97 122 140 168 195 2206 70 91 118 135 157 184 208

7 65 85 109 123 147 170 191

8 63 82 104. 118 140 163 1839 59 76 99 111 132 155 173

10 57 74 95 108 128 149 169

11 55 72 92 105 123 143 161

12 53 69 88 100 120 140 15513 51 66 85 96 114 133 14914 49 64 83 93 110 128 14315 47 61 80 90 106 123 14016 46 60 78 88 104 120 136

17 45 59 76 86 101 118 132

18 44 57 74 83 98 115 128

19 42 55 72 81 95 112 12320 41 54 70 78 93 108 121

These values of intensity l are raised to the power of 0.4 andmultiplied by thecorrespondingduration tc Thesevaluesarethenplacedinanothermatrix.



Table 7.3.3 t x I0.4 Values for Catchment in Penrith Area Example

txIO.4 DURATION

t(min) ARI 1

ARI 2

ARI 5

ARI 10

ARI 20

ARI 50

ARI 100

5 28 31 34 36 39 41 43 6 33 36 40 43 45 48 51

7 37 41 46 48 52 55 57

8 42 47 51 54 58 61 64 9 46 51 57 59 63 68 71

10 50 56 62 65 70 74 78 11 55 61 67 71 75 80 84 12 59 65 72 76 81 87 90 13 63 69 77 81 86 92 96 14 66 74 82 86 92 98 102 15 70 78 87 91 97 103 108 16 74 82 91 96 103 109 114 17 78 87 96 101 108 115 120

18 82 91 101 105 113 120 125

19 85 94 105 110 117 125 130 20 88 99 109 114 123 130 136

The nearest duration corresponding to tc,xIO.4=43.3 for an ARI of 5 years ispproximately6.5minutes.ForaoneyearARIitisabout8.4minutes,a100yearaARI,5minutesandsoon. heminimumoverlandflowtimetobeusedforanycatchmentelement(e.q.travelTtimefromcrownofroadtogutter)istwominutes.The calculation for sheet flow should be limited to a distance of approximately00metres.afterthisdistance,flowtendstobebetterrepresentedbyopenchannelt r t2formulae,as heflowcollectsintotiny ivule sratherthansheetflow. or similar reasons, overland flow time from the extremity of a residentialllotmenttogutterorchannelshouldbetakenasastandard15minutes.Fa(ii)OverlandFlowinHete ogenousCatchmentsCatchments with areas of varying slope and surface characteristics should beivided into smaller segments and the calculated time of concentration for each

r

dcombined.The kinematic wave formula assumes that there are no flows entering thecatchment fromupstream. It isnotcorrect tosimplyaddvaluesof txIO.4 foreachsegment.ThiscausescomplicationswiththeselectionoftheappropriatevalueforntensityItobeusedintheformulaasitshouldbetheaverageintensityovertheotaltimeinvolved.


it


Aniterativeproceduretocalculatethetotaldurationandintensity sgivenbythefollowingmethodofcombiningsegments: iExampleConsiderthepreviousexampleforthecatchmentinPenrith(iesegmentA),withalengthoftravelof60m,aslopeof2%andaroughnessof0.05.Tothiscatchmentaddanadditionalsegment(segmentB)witha lengthof travelof80m,aslopeof1.5%andaroughnessof.07.Findtheoverlandflowtime.0

The time of concentration for this heterogenous catchment is given by the followingexpression:



Thisislessthantheminimumof5minutes,soado tavalueof5minutes.Thiswilliveanewtotaltimeofconcentrationof: p14.4+5=19.4minuteg

(b) Roof-to-Gutter Flow

Roofto havethefollowingvalues:gutterorchannelflowtimesaretakentoResidentialroofs 5minCommercial/industrialroofs5tol5minIfterminalonsitewells(iewellswhichretainwaterpermanently),pitsorsumpsreused,runoffmaybetakenasnilandtheareascontributingtothoseterminalsamayberemovedfromconsideration. onterminal devices, such as retention wells and seepage beds are taken toncreasetraveltimethroughthatparticularelementby10minutes.Ni(iii)OpenChannelFlowWhereoverlandflowisconcentrated,eithernaturally,orbydesign,intoanearthorrass linedchannel,ManningsFormula foropenchannelcanbeusedtoestimategflowtimesandcharacteristics. lternatively,traveltimesforstreetkrebsandchannelsmaybedeterminedfromigure6.6.4a(fromtheVictorianRCA"RoadDesignManual").AF



T able7.3.4 Manning'sRoughnessCoefficients'

SURFACE TYPE TYPICAL nVALUES

SURFACE TYPE TYPICAL nVALUES

Channels Excavated in Earth Overbank Flow Areas

Straight uniform and clean 0.016-0.020 Short pasture grass, no brush 0.025-0.035

Winding, with grass and some weeds 0.025-0.033 Long pasture grass, no brush 0.030-0.050

Channels Excavated in Rock Light brush and trees 0.040-0.080

Smooth and uniform 0.025-0.040 Medium to dense brush 0.070-0.160

Jagged and irregular 0.035-0.050 Dense growth of willow trees 0.110-0.200 Small Natural Streams - (surface width at

bankfull stage < 30m)

Open Constructed Channels

Straight uniform and clean 0.025-0.033 Concrete pipes or box sections 0.011-0.012

Clean, winding, with some pools and shoals 0.033-0.045 Concrete (trowel finish) 0.012-0.015

Sluggish weedy reaches with deep pools 0.050-0.080 Concrete (formed without finishing) 0.013-0.018

Very weedy reaches with deep pools 0.075-0.150 Sprayed concrete (gunite) 0.016-0.020

Steep streams with gravel, cobbles, boulders

0.030-0.070 Bricks Pitchers or dressed stone in mortar

0.014-0.016 0.015-0.017

Major Natural Streams - (surface width at Random stones in mortar or rubble masonry 0.020-0.035 bankfull stage > 30m) Rock lining or riprap 0.025-0.030

Regular cross sect. no boulders or brush 0.025-0.060 Corrugated metal (depending on size) 0.020-0.033

Irregular and rough cross section 0.035-0.100 Earth (clear) 0.018-0.025

Short pasture grass, no brush 0.025-0.035 Earth (with weeds or gravel) 0.025-0.035

Long pasture grass, no brush 0.030-0.050 Rock cut 0.035-0.040

Light brush and trees 0.040-0.080 Short grass 0.030-0.035

Medium to dense brush 0.070-0.160 Long grass 0.035-0.050

Dense growth of willow trees 0.110-0.200



IntheareashowninFigure7.3.7,theflowsarrivingatPitsAandBarecalculatedy the method above. The pipe AB is designed to take the flow entering Pit A.bHowever,pipeBCmustbedesignedtotakeflowsfrombothPitAandPitB.Itisincorrecttosimplyaddthepeakflowsenteringeachpit,ascalculatedbytheational Method, since these are probably calculated using different times ofRconcentrationandthepeaksdonotcoincide,asshowninFigure7.3.8.Toestimatetheactualpeakmorecorrectly,oneprocedureistoapplytheRationalo o a m o (Methodt thec mbinedc tch entofPitsA andB,usingthe longer ftcBor tcA+timeoftravelinpipeAB)thisensuresthatthewholecatchmentiscontributing.Another is to consider the partialarea corresponding to the shorter time.alculations need to bemade for both assumptions, and the greatest calculatedlowrateisadopted.alculationsusually

CfCt beginatthetopofapipesystem.Astheyproceeddownwards,he equivalent impervious areas of the accumulated subcatchments are added.hesearemultipliedbytherainfallintensity,IcorrespondingtotheappropriatetcTforeachpipealongthesection.hisprocedurealsoapplieswheretwolinesofpipescombine.T



Assuming that the area shown in Figure 7.3.7 is located at Penrith, N.S.W., thefollowingcalculationsareusedtodesignpit inletsAandBandpipesABandBC.These are simplified to aid understanding. ARRB Report No 34 (Argue)ecommends an ARI of 13 years for playing fields and an ARI of 5 years forssuchascarparks.(Table2.1)rgroundInletAhecatchmentdrainingtothispointispervious.Foradiagonalflowpathfromtheorthwest e it,Tn cornerofth playingfieldtothep141=0.028m/m

L=141mandS=4/

(Length) (Slope) doptinga1yearARIfortheplayingfield,usingthekinematicwaveequationwithoughnessn=0.06andTable7.3.3ArtcA isestimatedtobe15.75minutes,andthecorrespondingintensity(fromtheIFDrelationshipinFigure7.3.2)is46mm/h.PortMacquariehasa1hour,10yearARIintensityof65mm/h.FromFigure7.3.4

C=0.63x0.ThenQA=0.50x46x1.0/360=0.064m3/s

8=0.50

(C)(I)(A)Pit inlet A should be sized to accept this flow unless it is acceptable to

ypassflowtoPitBorotherdownstreamareas.permitbPipeABThismustbesizedtocarryflowsupto0.064m3/s,assumingthatanARIof1

acceptable.yearisInletBThe catchment is completely impervious. For the paved car park, amaximumlowpath is traced diagonally from the northwest corner. Assume that aifferentARIof5yearsisusedforthisarea


fd


Usingn=0.013inthekinematicwaveequation,with

5yearARIintensityof122mm/hatItissetequalto5minutes,correspondingtoathePenrithsite..9x0.95=0.86Figure7.3.4givesaCvalueof0

ThusQ=0.86x122x1.0/360=0.291m3/s(291L/s)

tatBshouldbesizedtoadmitthisflow.TheinlePipeBC

ocalculateflowsgertc.ThispipemustcarryflowsfromPitsAandB.Onepossibilityistrombothcontributingcatchments,thefullareaflow,usingthelonortheflowsacrosstheplayingfieldandalongPipeABtopointB.

fFAssumingvelocityofflowinpipeABis2.5m/s,fromhydrauliccalculations.UsingthelargerARIof5years,thekinematicwaveequationappliedtotheplayingieldplusthepipetraveltime,givesatcof10.5+0.7=11.2minutes,correspondingftoanintensity,Iof128mm/h.

x128/360 husQ=(0.50x1.0+0.86x1.0)T=0.484m3/s(484L/s)In thepartialareacalculation,adopting tc=5minutes,means thatonly the flowfromtheplayingfieldinthefirst50.7=4.3minutescanbeconsidered(thepipelowtimestillbeingtakenas0.7minutes).Assumingthat40%ofthefieldareacanedrainedinthistime,thefollowingvaluescanbeestimated:fbI=172mm/hCA=0.50andCB=0.860.86x1.0)x172/360GivingQ=(0.50x0.40+=0.506m3/s(506L/s)



asdesignflowrateforpipeBC.Thepar ialareaestimateishigher.tNote that itwaspossible tovary the recurrence interval for variouspartsof thedesign.WhentheRationalMethodisusedindesigninthisway,itisnotwithasinglestormevent,butwithanumberofstormshavingdifferent timesofconcentration,eacheingthecrucialstormforaparticularpartofthesystem.Theproceduredoesnotsteminarealstorm.bsimulatethebehaviourofthesy7.3.8DesignofFlowinGuttersFlowinguttersshouldbeconsideredwithrespecttowidth,depth,velocityandARLMattersthatactasconstraintsonthesefactorsmayinclude: wheelpaths.Possibilityofvehicleshydroplaningifflowintrudesinto kerbsidelaneWhethertherearevehiclesparkedinthe riansbysplashingDisturbancetopedest Locationofbusstops eroadNeedforpedestrianstocrossth Accesstopedestriancrossings ionExistenceofshouldersontheroadNumberoftrafficlanesineachdirect Minimumnumberoflanesoftrafficwhichshouldremainopenduringa

severestorm Affectonadriverorpassengeremergingfromaparkedvehicle Possibilityofparkedcarsorevenpedestriansbeingcarriedawaybythe

flowofwater Theaffectofcrossfall/superelevationandlongitudinalgradeondepthand

velocityofflow yoralldisruptionsaretolerableThefrequencyatwhichan Trafficvolumeandspeed Driverexpectationofroadconditions

Probabilityofblockageofpitinletsbydebrisorrubbish



LOGPERSONTYPEIII(LPIII)INTERPOLATIONDIAGRAMFORSTANDARDAVERAGERECURRENCEINTERVALSFigureA7.2.1


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