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7.4ChannelLiningsThetypeofchannelliningwhichmaybebestsuitedforaparticularpurposewilldependonavarietyoffactors,includinghydraulicconditions,economicfactors,soilconditions,materialavailability,aestheticsandcompatibilitywithexistingimprovements.Thefollowingdiscus—sionprovidesinformationregardingtheapplicabilityanddesignparametersforacceptableliningtypes.7.4.1ConcreteLinedChannelsReinforcedconcreteandshotcretearealternativeliningmaterialsforchannelswithlimitedright-of-wayand/orhighvelocities.Themostcommonproblemsofconcretelinedchannelsareduetobeddingandliningfailures.Typicalfailuresare:l)crackingduetosettlementofthesubgrade;2)crackingduetoremovalofbedandbankmaterialbyseepageforce;and3)crackingand?oatingduetohydrostaticbackpressurefromhighgroundwater.FlowRegimeConsiderations:Concretelinedchannelsareusuallydesignedfor?owconditionswheretheFroudenumberexceeds1.13and/orwhenvelocitiesexceed?vefeetpersecondforearthlinedchannels.Froudenumbersforsupercritical?owshallbegreaterthan1.13andlessthan2.0.LiningCriteria:Generally,ifslopessteeperthan2:1areused,thensafetyandstructuralrequirementsbecomeaprimaryconcern.Thethicknessoftheliningshouldbeaminimum6-inchesforconcreteand8-inchesforshotcrete.Thisthicknessisdependentonvelocityandmayincreaseasvelocityincreases.Additionally,designoftheliningshouldaccountforanyanticipatedvehicularloadsfrommaintenanceorotherequipment.Tables7.2and7.3giverequiredliningthicknessesforbothconcreteandshotcreteasa?inctionofvelocity.Table7.2RequiredConcreteLiningThickness LeveeThickness-TVelocities ReinforcingStraightCurvedReachReach0-10fps 6-inch8-inch#4@18"Bothways10-20fps 8-inch10-inch#4@18"BothwavsGuniteLevees(1-1/2:lmax.sideslopes) OpenChannelHydraulics Page7-16
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Table7.3RequiredShotcreteLiningThicknessLeveeThickness-T.. R.f.VelocitiesStraightCurvedemorcingReachReach0-10fps 8-inch10—inch#4(a),18"BothwaysRoughnessCoe?icient:Theroughnesscoe?icientforaconcrete?ningcanvaryfrom0.013foratroweled?nishto0.020foraveryroughorun?nishedsurface.Forshotcrete,roughnesscoef?cientscanvaryfrom0.016to0.025.Bedding:Long-termstabilityofconcretelinedchannelsdependsinpartonproperbedding.Undisturbedsoilsoftenaresatisfactoryasafoundationforconcreteliningwithout?irthertreatment.Expansiveclaysareusuallyanextremehazardtoconcreteliningandshouldbeavoided.Underdrainage:Theprobabilityofdamagingtheconcreteliningduetohydrostaticbackpressureandsubgradeerosioncanbegreatlyreducedbyprovidingunderdrains.Therearetwotypesofarti?cialdrainageinstallations.Onetypeconsistsof4-or6-inchdiameterperforatedpipelinesplacedingravel-?lledtrenchesalongoneorbothtoesoftheinsideslopes.Theselongitudinaldrainsareeitherconnectedtotransversecrossdrainswhichdischargethewateratoutletboxesoratgradecontrolstructures.Wherealesserdegreeofseepagecontroliswarranted,weepholesorcut-offwallsspacedatappropriateintervalsmaybeused.Shotcrete:Asachannellining,shotcreteisanacceptablemethodofapplyingconcretewithageneralimprovementindensity,bonding,anddecreasedpermeability.Thedesignconsiderationsdiscussedaboveapplyinthedesignofshotcretechannels.7.4.2RiprapLinedChannelsGradedriprapcanbeaneffectiveliningmaterialifproperlydesignedandconstructed.Riprapdesigninvolvestheevaluationof?veperformanceareas.Theseareasincludetheevaluationof:
O riprapquality;0 ripraplayercharacteristics;
0 hydraulicrequirements;
0 siteconditions;and
0 riverconditions.OpenChannelHydraulics Page7-17
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Speci?ccriteriaoutliningtheperformanceofthese?veconsiderationsareprovidedinAppendix1.7.4.3GroutedRockGroutedrockisastructuralliningcomprisedofrockthatisinterlockedandboundtogetherbymeansofconcretegroutinjectedintothevoidspaces.Groutedrockprovidesastableliningsimilartoconcretewiththeaddedadvantageofahigherroughnessfactorduetotherocksurfacesprojectingabovetherocklayer.Rock:RockforgroutingshouldconformtothepropertyrequirementsdescribedinSection7.4.2.Grout:Thegroutmixshouldbespeci?edtoprovidethestrengthanddurabilityrequiredtomeetthespeci?capplication.Theminimum28-daycompressivestrengthshallbe2,000psi,andtheslumpshallbewithintherangeof4to7inches.ThestoneaggregateshouldconformtotheNo.4ConcreteAggregateGradationRequirementandASTMC-33.Amaximumof30percentofthecementousmaterialmaybe?yash(ASTMC-618,TypeCorF).DesignConsiderations:Sincegroutedrockisastructuralliningsimilartoreinforcedconcrete,itissubjecttothesamedesignconsiderations.Rockmustbesizedfortheanticipatedhydraulicdesignconditions.Foundationconsiderationsmustbeevaluatedandprovisionsmadeforunderdrainageandseepagecontrol.Ifonlybankprotectionistobeprovided,thegroutedrockprotectionmustbeextendedbelowthechannelinverttoadepthbelowtheestimateddepthofscour.7.4.4ToeProtectionScouratapoorlydesignedtoeofachannelbankisoneofthemajorcausesoffailureofriprapbankprotection.Properdesignofprotectionagainsttoescourinvolvestwoparameters.First,anestimatemustbemadeofthemaximumscourexpectedtooccuroverthedesignlifeofthestructure.Second,ameansofprotectionmustbeprovidedforthemaximumdepthofscour.DesignGuidelines:Thetwomethodsofprovidingtoeprotectioninalluvialchannelsare:1 Toextendprotectionbelowtheestimateddepthofscour;and2 Toprovideprotectionthatadjuststothescourasitoccurs.The?rstmethodisthepreferredtechniquebecausetheprotectionisinitiallyplacedtoaknowndepth,andthedesignerdoesnothavetodependontheuncertaintiesassociatedwiththemethodthatadjuststothescour.Becauseoftheuncertainties OpenChannelHydraulics Page7-
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associatedwithself-adjustingtoeprotection,itisnotrecommendedforuseintheCityofHemet.TheFirstmethodoftoeprotectionrequirestheextensionofthebankprotectionintotheexcavatedchannelbed,seeFigure7.3.Themaximumdepthofextensionisafunctionofthevelocityof?owinthechannel.Thedepthofextensionshouldbeestablishedatthemaximumdepthofscour,orcut-01fdepth,associatedwiththemaximumvelocityinthechannel.Careshouldbetakenwhenestimatingcut-o?‘depthsalongcurvedchannelsreachesbecauseofthevelocitydistributioncausedbythesuper-elevatedwatersurface.Table7.7providescut-o?'depthsasa?mctionofmaximumchannelvelocityforbothstraightandcurvedchannelreaches.Table7.7ToeProtectionCut-OffDepthsVelocitiesStraightReaches*CurvedReach0-6fps. 6-foot 9-foot6-10fps 8-foot12-foot10-15fps. 10-foot15-foot18-20f.p.s. 14-foot21-foot*Checkthecut-o?‘depthforcurvedreachonTable7.10inSection7.6.Usethatdepthifgreaterthangivenhereon.7.4.5GrasslinedChannelsGrasslinedchannelsareconsideredaviablealternativeundercertaincircumstances.Situationsunderwhichgrasslinedchannelsshouldbeconsideredincludeapplicationswherea?at,shallowchannelisthebestalternative,inlargeplannedresidentialdevelopmentswhereopenspaceareaswillbeintegratedintothe?oodcontrolsystemorwhenanopendrainagestructurewillfrontamajorthoroughfareinanurbanarea.Inanycasewhereagrasslinedchannelisbeingconsidered,thealternativesshouldbethoroughlyinvestigatedandthese?ndingspresentedtostaffpriortopreparationofthePhaseIDrainageReport.Shouldstaffagreethatagrasslinedchannelisthebestalternative,thePhaseIDrainageReportwillberequiredtobeexpandedtocoverproposeddesigncriteria,methodsofirrigationandmaintenancemethods.
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DesignProceduresThissectionprovidestheuserofthismanualwithchecklists(procedures)tobefollowedduringdesignandexamplesofdesigncomputationsthatcanbefollowedinordertoclarifytheprocedures.7.5.1ArtificialChannelsTable7.8isthechecklisttobeemployedinthedesignofarti?cialchannels.MostapplicationsintheCityofHemetwillbeabletofollowtheSimpli?edDesignProcedure.ShouldconditionsfalloutsideoftheSimpli?edDesignProcedure,themorerigorousprocedure,asshownintheremainingsteps,mustbecompleted.ConditionsforUsingtheSimpli?edDesignProcedure:ThestepsrequiredfortheSimpli?edDesignProcedurearemarkedwithanasteriskinTable7.8andcanbeusedtodesignopenchannelsthatmeetthefollowingconditions:
0 Gradecontrolestablished;and
0 DesignparameterswithinthoselistedinTable7.1
OpenChannelHydraulics Page7-20
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Table7.8Simpli?edDesignProcedureDesignChecklistforArti?cialChannels mm
lnmnlDill-a 5mmstructurasExlslingchannelCharacterislicsEads?ngGradeCommExds?ngFloodPW Chamta?sllcsSeoulObsetvmlonsmmstreamDevelopmentLandUseChangesRainfall/RunoffRemimshipsPonlhleWm. andstutzglu ‘ Elchannels
' ClAlignment
' D GradeConlml‘ D ErmaComma-asconudemlnnlo!ngmorwuyDMigration ‘ U WaterLevelEconnmlcandNlum?veAnalysis
‘ a magnumorsbni?can?yDl?ummcancepls-:1Typeofmm ,
-E)TypeofCrossSecllon
“ DChantalAllmvmnl-u[m mandamus)-nHydrologlcandHydmucDaumngolAllemames ‘ nLeusl'l’mlswamEvaluaxlonDEnvionmemalledelalinm-aSafetyRequlmmenlsHydraulicMilly-it ' D Damna?onnl‘CentralElmemrypeof FlawPro?leU ChannelUnlngD NORM!DamcalcwmlonsDWaterSurfacePro?leCalculationsa BridgeHydraulicsD SupercdllmlChime!HydraullneDDDD -.
n u
.
DUUUUDD
supemmallonDropsham-Hydrau?eemmHydraulicModelserFKMCW1
ElNomalInspedlon(Rafaenws)
sedlouRamona:
OpenChannelHydraulics Page7-21
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Table7.9DesignChecklistforNaturalChannels
InitialDill
DUDUUUUD
mam;structureschannelCharacteristicsExis?ruFloodPomrmanoeCharacteristicsExistingGradeoomrol:500wOWN; EmmaStreamDevelopmam
- barns.Divemons
— FloodComm
-- waningFloodHlsuxyRalnfallIRunD?RemnanlpaPam:Gunponemsandmusic:
UDUUUU
channelsBudgeCompmenlsRiverAlbnmemCommSwiss0'M?galionAugnmControlStructuresGradecontrolswamNm-smmrnlMonum?nsemsmsAoqulsmon.nga?m)EconomlcandAlta-mm:Analysi-
DEIUDDD
Design-dimofSigm?tan?yDi?ersmConceptsHydrologimlandHydraulicMamantematlvmLuauTOMW C61EvalualkmEwen-aFloodEmuofComponentandAltema?vosmmmmrcmnmDowmemallonand00mm”!!!EvaluauonMy?uuuucmlylh UUUDDUUUDUDUDD:
Datemina?ma!Contvolmm a!TypeofFloodPm?leNormalDWI?!CalculationswasSurfacePm?laCakmlamwsuallyHEC-2}BiidwwHydraulicsSIMBedFumnWham)"sandBodRoughnesstubule,boulder.ofRum;RumnessmammalianVegefa?onorConmlna?onLlnlngRoughnessDuneandAn?duneHeightSupamn?ualChums]Hydm?lesDvwHydm?lwAverag-(Hm-elmPhysicalHymllcMadelsSeam-mTran-pomMayanMdlllonllCornwallDDD PermanentRoom!Pod-0mmmNormallmpeollms(Reform)
5W Rafemnce
OpenChannelHydraulics Page7-
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7.5.2DesignExampleExample:Improveasmallearth-linedchannelwithincisedlow?ow,bankandedgeerosionS0=0.006ft/ft,partiallyvegetatedsandysiltmaterialtoconvey100-yeardesign?ow,Q100=650cfs.Given:>>Availablechannelwidthof50feet,withapproximately3feetdepth>>Providegrass-linedmainchannelwithconcrete-linedlow?ow>>Use4:1sideslopesandprovideminimumfreeboardallowance.Step1:Checkchannelcapacity.A.SolveManningEquation(Equation7.1)1 Findcross-sectionalareaof?ow[Totalarea(AT)=areaoflow?owchannel(Ag)+areaofmainchannel(Amc)]AT=Alf+Amc =(1.5)(5)+3((l8+42)/2)
=7.5+90AT=97.5sf2.Findwettedperimeterandindicatea4inchthicknessforlow?owwall.PT=P1f+Pmo =(2(0.33)+2(1.5)+5)+((18—5.67)+2(3)(12+4+42)°-5)=8.67+(12.33+24.7)PT=45.7a3 Findhydraulicradius OpenChannelHydraulics Page7-23
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10.67 OpenChannelHydraulics
R=:24"T
_97.5 (7.13)45.7=2.13ftDetermineManning's"n"fromTable5.11Findcomposite"11"value:Concrete-linedlow?ow:11=0.015 n=[2.Pi711-15)/PT10.6711(P1)(N,}'5)+37.0(0.025)1-545.7(0016+0.15)]01,745.7n=0.023 [ (7.14)=[
Grass-linedmainchannel:11=0.025SubstitutevaluestosolveforslopeMultiplyEquation5.1byATandrearrange:Q=(1.49/11)ATR0-67so0-5 so=[Qn/(1.49ATR05012
=l650(0.023)/(1.49)(97.5)(2.13)“7l2
S0=0.0038?/ftSincethechannelbottomslopeof0.0038ft/ftissu?icienttoconveythedesign?owof650cfs,thesteeperexistingS0of0.00611/11willconveythe?owwithasmallercrosssectionalarea.Equation7.1canbesolvedwithATR0'67todeterminetheactualcrosssectionof?ow:ATR“67=Qn/1.49So0.5=650(0_023)1.49(0.006)°-5=129.5112-67Bytrialanderror,Yn=2.65e, Page7-
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AT=7.5+2.65((18+39.2)/2)=83.3sf,
T=8.67+((18-5.67)+2(2.65)(12+42)“)=42.9ft,andR=AT/PT=83.3/42.9=1.94it.FlowalongthechannelatS0=0.006?/?hasreducedthewaterdepthby0.35it.Notethatthecomposite"11"valuehasnotchangedwiththenewvaluesofPR,Pm,andPT.B.CheckvelocityandFroudeNumber1.V=Q/A =650/83.3V=7.8fps<8fpsallowableforBermudagrass-linedchannelswitherosionresistantsoilonly.2.CheckFroudeF=V/(gD)°-5(7.15)
7.8?0-5 (7.16)((32-2)(39'2))F=0.94>0.86(7.17)Thechannelisundernearcritical?owconditionsandwillnotbestableatabottomchannelslopeof0.006?/?forthedesign?ow.OnesolutionistoprovidegradecontrolstructurestomainS0=0.0038?/ft,therebyhavingV=QA=650/975=6.7fpsandF=V/(gD)°-5=6.7/((32.2)(97.5/42))°~5=
0.77,whichiswithintheacceptablerangeofsubcritical?ow.SeeChapter6forgradecontrolstructures.3.Checkchanneltransitions(seeChapter6)C.Con?rmfreeboardrequirementFB=0.25(Y+v2/2g) OpenChannelHydraulics Page7-25
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HHH HI...-HI-Iahlhub;inL-‘ll-h-I-I
Ir.- ?r-qqil-w
OpenChannelHydraulics
=0.25(3.7)FB=0.92ft
=1ftminimumSummary:>>Grass-linedchannelwith4:1sideslopes>>Velocity=6.7fps;F=0.77,subcritical?ow.SeeTable5.2forallowablegrassandsoiltypes.>>Channelslope=0.0038?/?<0.006?/?(existing)>>Providegradecontrol>>Provide1footminimumfreeboardallowance>>Check?owvelocitiesandhydraulicproprtiesforother?owsanticipated Page7-26
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[5’
wuom:t:#new:a”:«2mmhuWosndbin. aM:“an”..aka”5;swamN
bod.
TYPICALROCKRIP-RAPLEVEESECTIONS
Accessrd. cad:c
Exist.streambed1' 3LaRE.
CASEA
1Access Fu'1; Max.leadstage
Nate: Exist.streambedCase5istobeused :0inlieuofCaseA ~wherehighground0 k.waterisaproblem. ‘0" ".Cut-off3/ CASE8
OpenChannelHydraulicsFigure7.3 Page7-27
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?. w_ :_____._y_+_i_:i:__
_, *:_;_if: AT_:_____:____:___é__‘4_.__EZ:
7.6DESIGNADSTables7.10and7.11andFigures7.4and7.5areprovidedheretohelpindesigningopenchannels.Table7-10ScourDepthsonOuterCurves ll/ll////'//!’1//HX///’///49,0iW/e9/VI/‘i\V/§?V_1245
0.01 0.] 0.5RlVERWIDTH/OUTERCURVERADIUS
2.4 2.2
l.2
SCOURDEPTH/RIVERDEPTHOFFLOW
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Table7.11Manning'sRoughnessCoefficientsRoughnessCoef?cient(n)ChannelMaterialMinimumNormalMaximumCorrugatedmetal 0.0210.0250.030ConcreteTrowel?nish 0.0110.0130.015Float?nish 0.0130.0150.016Un?nished0.0140.0170.020Shotcrete,goodsection0.0160.0190.023Shotcrete.wavvsection0.0180.0220.025Asphalt(usemaximumvalue0.0130.0160.020whencarsarepresent)Constructedchannelswithearthorsandbottomandsidesof:Cleanearth;straight0.0180.0220.025Earthwithgrassandweeds0.0200.0250.030Earthwithtreesandshrubs0.0240.0320.040Shotcrete 0.0180.0220.025Concrete 0.0170.0200.024Dryrubbleorrip-rap 0.0230.0320.036Naturalchannelswithsandbot-tomandsidesof:Treesandshrubs 0.0250.0350.045Rock 0.0240.0320.040Naturalchannelwithrockbottom0.0400.0600.090OverbankFloodplainsDesertbrush,normaldensity0.0400.0600.080Densevegetation0.0700.1000.160 OpenChannelHydraulics Page7-29
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Good
mqnqqJNNnvIIII II
man0amValues0!Aliza/b!”3andARM/d00m
10V -_dd oOp/Apunq/Aioserum
Figure7.4CurvesforDeterminingtheNormalDepthOpenChannelHydraulicsFlowinOpenChannelsPage7-30
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p0‘.0.b.0
Om
0.0!0.0001
0.0!
0.00!
OpenChannelHydraulics
ValuuofF:/b"'torDropoxolomuctlonl0.. | ‘0
0.015 OJValue:attilde,"forcircularsections
Figure7.5CurvesDeterminingtheCriticalDepthinOpenChannels-~IIIIIIIIIIPPNN?OOU‘OU!Page7-31 100
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lIi1
mall-MI“ II"
7.7 7.8
LISTOFV ARIABLESA= Crosssectionalareab = BasewidthD= Depthof?owF = Froudenumber8 = Accelerationofgravity,32.2FT/S2n = Manning'sroughnesscoef?cientRadiusatchannelcenterline "1II
R = HydraulicradiusQ= Flowrateincubicfeetpersecond
5 = LongitudinalslopeinfeetperfootS0= SuperelevationT = TopwidthV= VelocityinfeetpersecondREFERENCESBrater,ErnestF.andHoraceWilliamsKing,1976,HandbookofHydraulicsfortheSolutionofHydraulicEngineeringProblems,SixthEdition.Chow,VenTe,1959OpenChannelHydraulics,McGrawHillDavidian,J.,1984,ComputationofWaterSurfacePro?lesinOpenChannels,U.S.GeologicSurvey,TechniquesofWaterResourcesInvestigations,Book3,Chapter415LosAngelesCountyFloodControlDistrict,HydraulicDesignManualUS.ArmyCorpsofEngineers,July1991,HydraulicDesignofFloodControlChannels,EngineerManualEM1110-2-1601.US.DepartmentofTransportation,FederalHighwayAdministration,1990,HighwaysintheRiverEnvironment,HydraulicandEnvironmentalDesignConsideration.
OpenChannelHydraulics Page7—2
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----,April1988,DesignofRoadsideChannelswithFlexibleLinings,HydraulicEngineeringCircularNo.15PublicationNo.FHW A—IP-87-7.
OpenChannelHydraulics Page7-
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TURESHYDRAULICSTRUC
Hydraulicstructuresareusedtocontrolwater?owcharacteristicssuchasvelocity,directionanddepth.Structuresmayalsobeusedtocontroltheelevationandslopeofthechannelbed,aswellasthegeneralcon?gurationandstabilityofthewaterway.Hydraulicstructuresincludechanneldropstructures,low?owcheckstructures,energydissipators,bridges,transitions,chutes,bendsandmanyotherspeci?cdrainagestructures.8.1CHANNELDROPSTRUCTURESTheterm"dropstructure"isbroadlyde?ned.Includedarestructuresbuilttorestorepreviouslydamagedchannels,structuresconstructedduringnewurbandevelopmenttopreventacceleratederosioncausedbyincreasedrunoff,andapplicationsinwhichotherspecializedhydraulicconditionsarecreatedinthe?owchannel.Thefocusofthefollowingdiscussionandcriteriaisondropstructureswithdesign?owsupto10,000cfs.8.1.1BasicComponentsofaDropStructureFigure8.1showsatypicalchanneldropstructurewithitsvariouscomponents.Onceaparticularstructuretypeisselectedfordesign,analysesareconductedtodeterminetheoptimalsizingandextentofthevariouscomponents.8.1.2DropStructureTypesDesignguidanceispresentedinthissectionforthefollowingdropstructures:Ba?ledChuteDropsVerticalHardBasinDropsVerticalRiprapBasinDropsSlopingConcreteDropsLowFlowCheckStructuresDuetoahighfailurerateandexcessivemaintenancecosts,dropstructureshavinglooseripraponaslopingfacearenotpermitted.Figure8.2showsschematicpro?lesforeachtypeofstructurelistedabove.
HydraulicStructures Page8-1
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CHANNDrcwdownZone
L1Deer‘. MannChanne\LowFlow/TrickleChannel
HowAcceleratesCriticalDepthAtCrestHydraulicJumpUpstreamorApproachDepth Tailwate_____ 7 DepthChannelLOWFlow/:rickleChannelInvert_Invert(SlopingorVertical)StillingBasinDropFace BGQnSM cmewmCHANNELPROHLE
Figure8.]
'
HydraulicStructuresTypicalDropStructureComponentsPage8-2
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8.2GENERALDESIGNPROCEDURESThesedesignproceduresaregeneralized.Usethemtoidentifythemostsuitableapproach,withtheunderstandingthatdetailedanalyticalmethodsanddesignspeci?ca-tionsmayvaryasafunctionofsiteconditionsandhydraulicperformance.Astandarddropstructuredesignapproachwouldincludeatleastthefollowingsteps:1. De?nethemaximumdesigndischarge(uwallythe100-year)andotherdischargesappropriateforanalysis(selecteddischargesexpectedtooccuronamorefrequentbasis,whichmaybehavedifferentlyatthedrop).Selectpossibledropstructurealternativestobeconsidered(Section8.2.3).Determinetherequiredlongitudinalchannelslopeandthetotaldropheightrequiredtoproducethedesiredhydraulicconditions.Establishthechannelhydraulicparameters,reviewingdropstructureandchannelcombinationsthatmaybemosteffective.Conducthydraulicanalysesforthestructure.Whereappropriate,applyseparatehydraulicanalysestothemainchannelandthelow?owzonesofthedroptodeterminetheextentofprotectionrequired,aswell.asthepotentialproblems/solutionsforeach.Performsoilsandseepageanalysestoobtainfoundationandstructuraldesigninformation.Combineseepageandhydraulicanalysisdatatodetermineforcesonthestructure.Evaluateuplift,overturningandsliding.Usespeci?cdesigncriteriatodeterminethedropstructuredimensions,materialrequirementsandconstructionmethodsnecessarytocompletethedesignfortheselectedstructures.
HydraulicStructures Page8-3
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I.BAFFLECHUTE2.VERTICALHARDBASIN
3.VERTICALRIPRAPBASIN4.SLOPINGCONCRETE
5.LOWFLOWCHECKSTRUCTURES
Figure8.2HydraulicStructuresDropStructureTypes Page8-4
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8.2.1CrestandUpstreamHydraulicsUsually,thestartingpointofdropanalysisanddesignoccursonthecrestsectionatthetopofthedrop.As?owpassesthroughcriticaldepthnearthecrest,upstreamhydraulicsareseparatedfromdownstream.TheCritical?owstatemustbecalculatedandcomparedwiththedownstreamtailwatere?‘ectwhichmaysubmergethecrestande?‘ectivelycontrolthehydraulicsatthecrest.Withcontrolatthedropcrest,upstreamwatersurfacepro?lecomputationsareusedtoestimatethedistancethatprotectionshouldbemaintainedupstream,thatis,thedistancetowherelocalizedvelocitiesarereducedtoacceptablevalues.Thewatersurfacepro?lecomputationsshouldincludeatransition/expansionheadloss,whichshouldtypicallyrangefrom0.3(modesttransitions)to0.5(moreabrupttransitions)timesthechangeinvelocityhead.Foragivendischarge,thereisabalancebetweenthecrestbasewidth,upstreamanddownstream?owvelocities,theFroudenumberinthedropbasinandthelocationofthehydraulicjump.Theseparametersmustbeselectedforeachspeci?capplication.VerticalorNearVerticalAbutmentsatDropCrest:Figure8.3presentsalternativedropcrestsataverticaldropstructure.Ingeneral,theobjectivesofupstreamhydraulicsandcrestdesignare:1 Tomaintainfreeboardintheapproachchannel,2 Tooptimizecrestandbasindimensionstoachievethemostcost-e?‘ectivestructure,and3 Topreventerosioninthetransitionzone,where?owacceleratesap-proachingthecrest.SlopingAbutmentsatDropCrest:Figures8.3and8.4showaschematiclayoutforthedropcrestandupstreamchannelataslopingdropstructure.Thedesignobjectivesdiscussedpreviouslyalsoapplyhere.8.2.2WaterSurfacePro?leAnalysisBackwatercomputationsshouldbecompletedforthechannelreachesupstreamanddownstreamoftheproposeddropstructuretoestablishapproach?owconditionsandtailwaterconditionsfortherangeofdesign?ows.Nextdeterminethelocationofthehydraulicjumpsothatthestillingbasincanbesizedtoadequatelycontainedthezoneofturbulence.
HydraulicStructures Page8-5
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comma-E TRANSlTAg?cm
LEGENNOTES: LIu—UPSTREAMTRANSITIONLENGTHLIu2.(b.—b2) Ltd—ooszmEANITRANSITIONLENGTHV2< < LuUPSTREAMROCKAPPROACHLENGTHLb-Ltd-Lb Lb—BASINLENGTHb.CHANNELBASEWIDTHbz—CRESTBASEWIDTH BASINb.I:
—. b:AREA DOWNSTREAM
RIPRAP BB CONCRETEACRESTW ALLPLANVERTICALDROPWITHCRESTEXPANSIONr BEGIN BASIN/SILLbIDOWNSTREANI
Lb
VERTICALDROPWITHCRESTCONICTION
\\ IV5”b:CRESTSECTION3 SECTIONALAYOUTOFDROPSWITHNEARLYRECTANGULARCRESTSECTION Figure8.3TypicalVerticalDropCrestHydraulicStructures Page8-6
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|{_ ’
m
'L:j
4 a.ansrECTION
LAYOUTFORDROP§MTHTRAEEZQIDALCREST§ECTION
LEGENDN0TES= Lyn—UPSTREAMTRANSITIONLENGTHLtu:2Ibi-bzI Lid—DOWNSTREAMTRANSITIONLENGTH[/4L.5mal-b LII—UPSTREAMnocxAPPROACHLENGTHLb—BASINLENGTHLf—5LOPEFACELENGTHbl—CHANNELBASEWIDTHbr—CRESTBASEWIDTH rA BBH*
\L" BASINSILLblonop5PE was?”BEGIN L° 5“'TRANSITION
ansrRIPRAP5 LA16EPLANSLOPINGDROPWITHCRESTCONSTRICTION
CHANNELSECTIONB
Page8-7HydraulicStructuresFigure8.4TypicalSlopingDropCrestCon?guration
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DropHeight,feetSoilConditions2 4 8 12Sandandgraveloverbedrockwithsuf?cientdepthofS' S' S/SwB'S/SwB'materialtoprovidesupport—groundwaterCTcCTcJSTST STprevalent CTfCTf/CTISandandgravelwithshallowdepthtobedrock—CTcCTc/STST STgroundwaterprevalentCWCWCWCWS" St. 8" SwBliSandandgravel,greatdepthstobedrock—S S S S/SwBgroundwaterprevalentCTcCTc/STSTSTSandandgravel,nogroundwater,orwatertableS S S S/SwBnormallybelowrequirement(forvariationcausedCTf/CTI011C11011bydepthtobedrocksee?rstcase) CWCWClay(andsilt)—-mediumtohard CTcCTcCTcCTcCWreducelengthfordif?cultback?llconditionsCTf/CTIonlyforlocalseepagezones/slitsSTexpensive—forspecialemblemsClay(andsilt)——softtomediumwithlensesof S S S S/SwBpermeablematerial—groundwaterpresentCTcCTcCTc/STSTClay(andsilt)—-softtomediumwithlensesof S S S S/SwBpermeablematerial—maybemoistbutnot CTcCTcCTc/STSTsigni?cantgroundwatersourceCTF011011011CWCWCWCW ' (considerscourinsheetpilesupport)
" (excavateintobedrockandsetintoconcrete)Legend8 SheetpileSwBSheetpilewithbracingandextrameasuresCTcCutoffTrenchback?lledwithconcreteSTSlurryTrench;similartoCTc;buttrenchwallsaresupportedwithslurryandthenlaterreplacedwithconcreteoradditivesthateffectcutoffCWCutoffWall;conventionalwall,possiblywithfooter,backfilled;notethattheeffectiveseepagelengthshouldgenerallybedecreasedbecauseofback?ilCTICutoffTrenchwithsyntheticlinerand?llCTfCutoffTrenchwithclay?ll Table8-1HydraulicStructuresGeneralSeepageCutoffTechniqueSuitabilityPage8-8
18.B
Forslopingdropstructures,watersurfacesmustbedeterminedforthesupercriticalpro?lesdownthefaceofthedrop.Thelocationofthehydraulicjumpisdeterminedbycomparisonofthespeci?cforce,F5,aboveandbelowthetoeofthedrop,usingthefollowingequation:
FS 2 z.1.+y— (8.1)g)2Thedepthy,fordownstreamspeci?cforcedetermination,isthetailwaterelevationminusthegroundelevationatthepointofinterest;typicallythemainbasinelevationorthetricklechannelinvert(ifthejumpistooccurinthebasin).Thedepthfortheupstreamspeci?cforce(supercritical?ow)isthesupercritical?owdepthatthepointofinterest.Whereamajorchannelincorporatesalow?owchannel,separateanalysesshouldbecompletedforthelow?owzoneandthemajorchanneloverbankzone.Thisisbecausethedeeper?owpro?leinthelow?owchannelhasahigherenergygradelinepro?le.Speci?cforceanalysisinthiszoneshowsthatthehydraulicjumpwillnotoccurinthesamelocationastherestofthe?owoverthedrop,andinmostcasesthejumpwilloccur?irtherdownstream.Toavoidthiscondition,thelow?owchannelshoulddaylightatthedownstreamfaceofthestructure.AnadditionaldiscussionofthehydraulicjumpphenomenonispresentedinSection8.6.8.2.3DropSelectionTherearefourmajorconsiderationsfortheselectionofthetypeofdropstructureforaparticularapplication:1)surface?owhydraulicperformance;2)foundationandseepagecontrol;3)economicconsiderations;and4)constructioncon-siderations.Otherfactorswhichcanaffectselectionarelanduses,aesthetics,safety,maintenanceandanticipateddownstreamchanneldegradation.SurfaceFlowHydraulicSystem:Theprimaryconsiderationfortheselectionofadropstructureshouldbefunctionalhydraulicperformance.Thesurface?owhydraulicsystemcombineschannelapproachandcresthydraulics,slopingorverticaldrophydraulicsanddownstreamtailwaterconditions.FoundationandSeepageControlSystems:Table8.]presentssometypicalfoundationconditionsandcontrolsystemstypicallyusedforvariousdropheights.Table8.1ispresentedonlyasaguide.Thehydraulicengineermustcalculatehydraulicloadingswhichcanoccurforavarietyofconditionssuchasinterimconstructionconditions,low?owand?ood?ow.
HydraulicStructures Page8-9
18.B
8.3DESIGNGUIDELINESFORDROPSTRUCTURES8.3.1Baf?eChuteDropsTheUSBRhasdevelopeddesignstandardsforareinforcedconcretechutewithba?leblocksontheslopingfaceofthedrop,whicharecommonlyreferredtoasba?ledapronorbaf?echutedrops.Thereareexcellentreferences,suchasDesignofSmallCanalStructures(USBR1974),thatshouldbeusedforthedesignofthesestructures.Theoptimalperformanceoccursforaunit?ow(q)atthechutewidthof35to60cfs/?.TheUSBRstatesthattherecommendeddesign?owof60cfs/?forba?lechutedropshasbeenexceededatseverallocationswithoutcausingsigni?cantproblems.Typicaldesignsconsistofupstreamtransitionwalls,arectangularapproachchute,aslopingapronof2:1,or?atter,slopewithmultiplerowsofbaf?epiers(seeFigure8.5).Thetoeofthechuteextendsbelowgradeandisback?lledwithlooserocktopreventunderminingthestructurebyeddycurrentsorminordegradationofthedownstreamchannel.Thepotentialfordebris?owmustbeconsidered.Usecautionwhenconditionsincludestreamswithheavydebris?ow,becausethebaf?escanbecomecloggedbetweentheinterstices,resultinginover?ow,lowenergydissipation,anddirectimpingementoftheerosivestreamjetonthedownstreamchannel.Basicdesigncriteriaandmodi?cationdetailsaregiveninFigures8.6and8.7.Remainingstructuraldesignparametersmustbedeterminedforspeci?csiteconditions.Recommendeddesignproceduresarediscussedbelow.GeneralHydraulicDesignProcedure.‘ 1 Determinethemaximumin?owrateandthedesigndischargeperunitwidth:
=2(IdW (8.2)Thechutewidth,W,maydependontheupstreamordownstreamchannelwidth,theupstreamhydrauliccontrolorlocalsitetopography.Generally,aunitdischargebetween35to60cfs/?ismosteconomical.
HydraulicStructures Page8-10
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18.B
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18.B
2 Anupstreamchanneltransitionsectionwithverticalwingwalls,constructed45degreestothe?owdirection,causes?owapproachingtherectangularchutesectiontoconstrict.Itisalsofeasibletousewallsconstructedat90degreestothe?owdirection.Ineithercon?guration,itisimportanttoanalyzetheapproachhydraulicsandwatersurfacepro?le.Often,theeffective?owwidthatthecriticalcrosssectionisnarrowerthanthewidthofthechuteopeningdueto?owseparationatthecornersoftheabutment.Tocompensatefor?owseparation,itisrecommendedthattheactualwidthconstructedbeonefootwiderthanthedesignanalysiswidthiftheconstrictedcrestwidthislessthan90percentoftheupstreamchannel?owwidth.Inanycase,thedesignshouldcarefullyconsidertheapproachhydraulicsandcontraction/separationeffects.Depthandapproachvelocitiesshouldbeevaluatedthroughthetransitiontodeterminefreeboard,scourandsedimentationzones.3 Theentrancetransitionisfollowedbyarectangular?owalignmentapron,typically5feetinlength.Theupstreamapproachchannelvelocity,V ,shouldbeaslowaspracticalandlessthancriticalvelocityatthecontrolsectionofthecrest.Figure8.6bgivestheUSBRrecommendedentrancevelocity.Inatypicalchannel,theentrancetransitiontotherectangularchutesectionwillproducethedesiredupstreamchannelvelocityreduc-tion.Theelevatedchutecrestabovethechannelelevation,asshowninFigure8.6a,shouldonlybeusedwhenapproachvelocitiescannotbecontrolledbythetransition.Specialmeasurestopreventaggradationupstreamwouldbenecessarywiththeraisedcrestcon?guration.EntranceModi?cation:1 Thetrickle?owchannelshouldbemaintainedthroughtheapron,approachandcrestsections.Itmayberoutedbetweenthe?rstrowofbaf?epiers.Thetricklechannelshouldstartagainatthebasinrockzonewhichshouldbeslightlydepressedandthengradeduptotransitiontothedownstreamchannel.Figure8.7cillustratesonemethodofdesigningthelow?owchannelthroughthecrest.2 TheconventionaldesignshowninFigure8.6bresultsinthetopofthebaf?esbeinghigherthanthecrest,whichcausesahigherbackwatereffectupstream.Figure8.7amaybeusedtoestimateextenttheoftheeffectandtodeterminecorrectivemeasures,suchasincreasingtheupstreamfreeboardorwideningthechute.AnothermeansofalleviatingtheseproblemsistheFujimotoentrance,developedbytheUSBRandillustratedinFigure8.7b.Theupperrowsofbaf?esaremovedonerowincrementdownstream.Theimportantadvantageofthisentranceisthatthereisnobackwatereffectoftheba?ies.Theserratedtreatmentofthemodi?edHydraulicStructures Page8—
18.B
crestbeginsdisrupting?owenteringthechutewithoutincreasingtheheadwater.Moreimportantly,thiscon?gurationprovidesalevelcrestcontrol.Therefore,theinvertoftheupstreamlow?owchannelmaybeadjustedtothecrestelevation,wideningthelow?owchannelasitapproachesthecrest,orthelow?owchannelmaybelowerandbroughtthroughtheserratedcrestasdescribedabove.StructuralDesignDimensions:1 HydraulicStructures
Assumecritical?owatthecrestanddeterminecriticaldepthforboththepeak?owandfor2/3ofpeak?ow.Forunitdischargesexceeding60cfs/?,Figure8.7bmaybeextrapolated.TheunitcriticaldepthassociatedwithadischargeinarectangularchutemaybedeterminedbyEquation8.3:
_cI20.33— — 8.3yc(g) ()
Thechutesection(baf?edApron)isconcretewithbaf?esofheight,Hb,equalto0.8timesthecriticaldepth.Thechutefaceslopeis2:1formostcases,butmaybereducedforlowdropsorwherea?atter-slopeisdesirable.Forunitdischargesgreaterthan60cfs/?,thebaf?eheightmaybebasedon2/3ofthepeak?ow;however,thechutesidewallsshouldbedesignedforpeak?ow(see4below).Baf?epierwidthsandspacesshouldbeequal,preferablyabout1.5HbbutnotlessthanHb.Thespacingbetweenrowsofba?leblocksshouldbeHbtimestheslope.Forexample,a2:1slopemakestherowspacingequalto2H,,paralleltothechute?oor.Theba?lepiersareusuallyconstructedwiththeupstreamfacenormaltothechute?oor.Fourrowsofba?lepiersarerequiredtoestablishfullcontrolofthe?ow,althoughfewerrowshaveoperatedsuccessfully.Atleastonerowofba?lesareburiedinriprapwherethechuteextendsbelowthedownstreamchannelgrade.Riprapprotectioncontinuesfromthechuteoutlettoadistanceofapproximately41-1,”orasnecessarytopreventeddycurrentsfromunderminingthewalls.Additionalrowsofbaf?esmaybeburiedbelowchannelgradetoallowfordownstreamchanneldegradation.Thebaf?edchutesidewallheight(measurednormaltothe?oorslope)shouldbe2.4timesthecriticaldepthbasedonpeakdischarge.Thewallheightwillcontainthemain?owandmostofthesplash.Thedesignoftheareabehindthewallshouldconsiderthatsomesplashmayoccur,butextensiveprotectionmeasuresarenotrequired.
Page8-15
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5 Determineupstreamtransitionandapronsidewallheightasrequiredbybackwateranalysis.Lowerbasinwingwallsaregenerallyconstructednormaltothechutesidewallsatthechuteoutlettopreventeddycurrenterosionatthedroptoe.Thesetransitionwallsareofaheightequaltothechannelnormaldepthplus1foot,andlengthsuf?cienttoinhibiteddycurrenterosion.6 Allconcretewallsandfootingdimensionsaredeterminedbyconventionalstructuralmethods.7 Themosttroublesomeaspectofthedesignisthedeterminationofthehydraulicimpactforcesinthebaf?estoallowthestructuralengineertosizeadequatesteelreinforcing.Figure8.7dmaybeusedasaguideline.Thestructuralengineershouldapplyaconservativefactorofsafety,asthecurveisbasedonrelativelysparseinformation.8.3.2VerticalHardBasinDropsTheverticalhardbasinisageneralizedcategorywhichcanincludeawidevarietyofstructuredesignmodi?cationsandadaptions.Avarietyofcomponentscanbeusedforboththehardbasinandthewall,variouscontractioneffectscanbeimplementedtoreduceapproachvelocitiesanddifferenttricklechanneloptionscanbeselected.Themaximumverticaldropheightfromcresttobasinforaverticalhardbasindropislimitedto3feet.Withsuf?cienttailwater,ahydraulicjumpisinitiated.Otherwise,the?owcontinueshorizontallyinthesupercriticalregimeuntilthespeci?cforceofthetailwaterissu?icienttoforcethejump.Energyisdissipatedintheturbulencethroughthehydraulicjump;thereforethebasinissizedtocontainthesupercritical?owandtheerosiveturbulentzone.Generally,aroughbasinisadvantageoussinceincreasedroughnesswillresultinashorterbasin.Figure8.8showsaverticaldropwithagroutedboulderbasin,andillustratesseveralmoreimportantdesignconsiderations.GeneralHydraulicDesignProcedure:1 Thedesignapproachusestheunitdischargeinthemainchannelandthetricklechanneltodeterminetheseparatewatersurfacepro?lesandjumplocationsinthesezones.Thebasinissizedtoadequatelycontainthehydraulicjumpandassociatedturbulent?ows.
HydraulicStructures Page8-16
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18.B
HydraulicStructures
Therocklinedapproachlengthendsabruptlyatthestructuralretainingcrestwallwhichhasanearlyrectangularcross-sectionandtricklechannelsection.Crestwallandfootingdimensionsaredeterminedbyconventionalstructuralmethods.OpenChannelHydraulics(Chow1959),makesabriefpresentationforthe"StraightDropSpillway",whichapplieshere.SeparateanalysiswouldneedtobeundertakenforthetricklechannelareaandthemainchannelareaaspreviouslydiscussedAddsubscripttforthetricklechannelarea,andsubscriptmforthemainchannelareainthefollowingequations.RefertoFigure8.9toidentifythefollowingparameters.LbisthedesignlengthwhichincludesLdandthedistancetothejump,D],whichismeasuredfromthedownstreamendofLd.Thejumplength,Lj,isapproximatedassixtimesthesequentdepth,Y2.Asafactorofsafety,toassureasuf?cientlengthforLb,0.6LjisaddedinthedesignofLb,suchthat Lszd+Dj+9.6Y2(3.4)Whenahydraulicjumpoccursimmediatelywherethenapphitsthebasin?oor,thefollowingvariablesarede?ned:L3,1=4-3Dn0‘”(8.5a)fWhere (8.5b)and Y7p=1.0Dno'22(8.6)fY1_ 0.4257f—0.54Dn(8.7) Page8-18
18.B
Y—2=1.66Dn0'27(8.8)Yf5 InthecasewherethetailwaterdoesnotprovideadepthequivalenttoorgreaterthanY2,thejetwillwashdownstreamassupercritical?owuntilitsspeci?cforceissuf?cientlyreducedtoallowthejumptooccur.Deter-minationofthedistancetothehydraulicjump,Dj,requiresaseparatewatersurfacepro?leanalysisforthemainandlow?owzones.Anychangeintailwatera?‘ectsthestabilityofthejumpinbothlocations.6 Thebasin?oorelevationisdepressedatdepthB,variablewithdropheightandpracticalfortrickle?owdrainage.Notethebasindepthaddstotheeffectivetailwaterdepth.Thebasinisconstructedofconcreteorgroutedrock.7 Thereisasillatthebasinendtobringtheinvertelevationtothatofthedownstreamchannelandsidewallsextendingfromthecrestwalltothesill.Thesillisimportantincausingthehydraulicjumptoforminthebasin.Buriedriprapshouldbeuseddownstreamofthesilltominimizeanylocalscourcausedbytheliftoverthesill.8.3.3VerticalRiprapBasinDropsAsshowninFigure8.11,thisstructureisessentiallyaplungepooldropthatincorporatesareinforcedconcretecrestwallwitharipraplineddissipationpoolbelow.Anearlyrectangularcrestsectionisrecommendedtoreducethewidthoftheplungepool.Maximumdropdepthislimitedtothree(3)feetduetosafetyconsiderationsandthepracticalityofobtaininglargebasinriprapforhigherdrops.Inthisstructure,?owpassingovertheverticalcrestwallplungesintoariprapbasinarea.Energyisdissipatedbyturbulenceintheplungepool.Submergencebyhightailwatercanlimittheabilityofthestructuretodissipateenergy.Looseriprapisplacedaccordingtotheinitialdesignspeci?cations.Therockissuccessivelyrearrangedbyin?owsuntilamorestabilizedplungepoolisformed.Thedepthofthescourhole,d,,andthenominalrocksizeareinverselyrelated.
HydraulicStructures Page8-19
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GeneralHyaiaulicDesignProcedure:ThehydraulicanalysisofthistypeofdropisgenerallysimilartothatpresentedinSection8.3.2forcresthydraulics.Thedesignofthe?exibleplungepoolisdescribedbelow.Thedesireddropacrossthestructureisthedifferenceinthebedelevationsoftheapproachchannelattheweirandthedownstreamchannelattheendofthestructure.LetthisdifferencebeHd.ItfollowsfromFigure8.10that:Hd=D—0.67d5(3.9)Thedesignermust?ndthecombinationofrocksizeandplungeheightDthatgivesadepthofscourwhichbalancesEquation8.9.Therelationshipbetweenrocksized50,plungeheight,H,(H=1.5Yc)anddepthofscourd,isgivenbyFigure8.11.Toobtainanadequatecuto?',thedepthoftheverticalwallthatformstheweircrestmustbeextendedbelowthebottomoftheexcavationfortheriprap.Thus,itusuallybecomesuneconomicaltodesignascourdepthd,,anygreaterthan0.3D.Tomeetthislimitationinthe?elditisnecessaryto:increasetherocksizedso;decreasetheplungeheightD(byusingmoredrops);decreaseH(byusingawiderstructure);or,touseanothertypeofdropstructure.' Thesideslopesinthebasinmustberiprappedalsoastherearestrongbackcurrentsinthebasin.Granular?ltermaterialisrequiredundertheriprap.Thesideslopesinthebasinshouldbethesameslopeasforthedownstreamchannel.8.3.4SlopingConcreteDropsThehydraulicconceptofthesestructuresistodissipateenergybyformationofaconventionalhydraulicjump,usuallyassociatedwithareversecurrentsurface?owasthesupercritical?owdownthefaceconvertstosubcritical?owdownstream.Numerousconceptshavebeeninvestigated.AmongthemaretheSaintAnthonyFalls(SAF)StillingBasin,andtheUSBRBasinsI,II,III,andIV .Thesedropsandassociatedbasinsaresuitedfordifferentkindsofsituations.TheSaintAnthonyFallsStillingBasinandtheUSBRBasins(exceptTypeI)allworkattechniquestoshortenthebasinlength.Figure8.12illustratesthevarioustypesofstillingbasinsforusewithslopingconcretedrops.GeneralHydraulicDesignProcedure:DesignproceduresforUSBRBasinsII,III,andIVandtheSAPStillingBasinarepresentedinHydraulicDesignofEnergyDissipatorsforCulvertsandChannels(FHW A,HEC-14,1983)andHydraulicDesignofStillingBasinsandEnergyDissipators(Peterka,1984).HydraulicStructures Page8-22
18.B
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18.B
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18.B
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AnalysisofchannelapproachandcresthydraulicsgenerallyfollowstheguidelinespresentedinSection8.2.8.3.5LowFlowCheckStructuresLow?owcheckstructuresandassociatederosioncontroltechniquescanbee?‘ectiveinstabilizingnaturalchannelsandotherunlinedchannels.Thesestructuresaredesignedtoprovidecontrolpointsandestablish/maintainstablebedslopeswithinthebase?owchannel.Checkstructuresarefrequentlysubmergedduringhigher?oodevents.Theapplicationandsizingiscomplexbecauseoftheneedtoaddressawiderangeof?ows.8.4CONDUITOUTLETSTRUCTURESConcreteenergydissipationorstillingbasinstructuresarerequiredtopreventscourdamagecausedbyhighexitvelocitiesand?owexpansionturbulenceatconduitoutlets.Outletstructurescanprovideahighdegreeofenergydissipationandaregenerallyeffectiveevenwithrelativelylowtailwatercontrol.Concreteoutletstructurescanbedesignedeasilyandaresuitableforawidevarietyofsiteconditions.Insomecases,theyaremoreeconomicalthanlargerockbasins,particularlywherelongtermcostsareconsidered.8.4.1RiprapProtectionatConduitOutletsAstillingbasinconstructedofloose,gradedriprapcanbeaneffectiveandeconomicalenergydissipationmeasureforaconduitoutlet.Longtermanalysisandobservationofoperatingcharacteristicshaveleadtothefollowingconclu-srons.0 Thedepth(hs),length(L)andwidth(W,)ofthescourholewererelatedtothecharacteristicsizeofriprap(dso),discharge(Q),brinkdepth(Y0)andtailwaterdepth(TW). 0 Thedimensionsofthescourholeinabasinconstructedwithangularrockwereapproximatelythesameasthedimensionsofascourholeinabasinconstructedofroundedmaterialwhenrocksizeandothervariablesweresimilar. 0 Whentheratiooftailwaterdepthtobrinkdepth(TW/Yo)waslessthan0.75andtheratioofscourdepthtosizeofriprap(hs/dso)wasgreaterthan2.0,thescourhole?mctionedveryef?cientlyasanenergydissipator.Theconcentrated?owattheculvertbrinkplungedintothehole,ajumpformedagainstthedownstreamextremityofthescourhole,and?owwasgenerallywelldispersedasitle?thebasin. 0 Themoundofmaterialwhichformedonthebeddownstreamofthescourholecontributedtothedissipationofenergyandreducedthesizeofthescourhole.
HydraulicStructures Page8—
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Ifthechannelprotectionis ed,computetheFrbrinkconditionsJ-E(%)7(for«angularsections).propn'ateforlocallyavailable(usually0.25isless Forhightailwaterbasins(TW/Yogreaterthan0.75)thehighvelocitycoreofwateremergingfromtheculvertretaineditsjet-likecharacterasitpassedthroughthebasin,anddiffusedinamannerverysimilartothatofaconcentratedjetdif?lsinginalargebodyofwater.Asaresult,thescourholewasmuchshallowerandgenerallylonger.Consequently,riprapmayberequiredforthechanneldownstreamoftherock-linedbasin.GeneraldetailsofthebasinareshowninFigure8.13.Principalfeaturesofthebasinare:Thebasinispre-shapedandlinedwithriprapofmediansized50.Thesurfaceoftheriprapped?ooroftheenergydissipatingpoolisconstructedatanelevation,ha,belowtheculvertinvert.Elevationhsistheapproximatedepthofscourthatwouldoccurinathickpadofriprapofsized50constructedattheoutfalloftheculvertifsubjectedtodesigndischarge.Theratioof11sto(150ofthematerialshouldbegreaterthan2.0andlessthan4.0.Thelengthoftheenergydissipatingpool,L5,is10hsor.3Wowhicheverislarger.Theoveralllengthofthebasin,Lb,is15h,or4W0whicheverislarger.GeneralHydraulicDesignProcedure:1 HydraulicStructures
Estimatethe?owpropertiesatthebrinkoftheculvert.EstablishtheblinkinvertelevationsuchthatTW/Y0islessthanorequalto0.75forthedesigndischarge.Forsubcritical?owconditions,useFigure8.14or8.15toobtainYo/D,thenobtainV0bydividingQbytheareaof?owassociatedwithY0.Distheheightordiameterdependingontheculvertscross-section.Iftheculvertisonasteepslope,V owillbethenormalvelocityobtainedfromtheofManning'sEquationfortheappropriateslope,sectionanddischarge.Fromsiteinspectionor?eldexperienceinthearea,determinewhetherornotchannelprotectionisrequiredattheculvertoutlet.requiroudeNumberfornon-reSelect(150/ycap-riprapthan(150/ycislessthan0.45).Obtainh,,yefromFigure8.16,andchecktoseethat2.0isless Page8-26
18.B
56 thanth50islessthan4.0.Thisprocessisiterative,thecomputationsshouldbeadjusteduntilthisvaluefallsbetween2.0and4.0.SizethebasinasshowninFigure8.13Designprocedureswhereallowabledissipatorexitvelocityisspeci?ed:Determinetheaveragenormal?owdepthinthenaturalchannelforthedesigndischarge.Extendthelengthofthebasinsothatthewidthofthebasin(atSectionA—A,Figure8.13),timestheaveragenormal?owdepthinthenaturalchannelisapproximatelyequaltothedesigndischargedividedbythespeci?edexitvelocity.Intheexitregionofthebasin,thewallsandapronofthebasinshouldbewarpedsothatthecrosssectionofthebasinattheexitconformstothecross-sectionofthenaturalchannel.Abrupttransitionsshouldbeavoidedtominimizeseparationzonesandresultanteddies.Ifhightailwaterisapossibilityanderosionprotectionisnecessaryforthedownstreamchannel,thefollowingdesignprocedureissuggested:Designaconventionalbasinforlowtailwaterconditionsinaccordancewiththeinstructionsabove.Estimatecenterlinevelocityataseriesofdownstreamcross-sectionsusingtheinfor-mationshowninFigure8.17.ShapethedownstreamchannelandsizeriprapusingguidelinespresentedinSection7andthestreamvelocityobtainedabove.8.4.2ConcreteOutletStructuresImpactstillingbasinsandadaptationofabaf?edaprontoconduitoutletsareviablealternativesforthedissipationofenergy.Initialdesignselectionshouldincludeatleastthefollowingaspectsconcerningconduitoutletstructures.
HydraulicStructures Page8-27
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1 Highenergydissipationisrequired—hydraulicconditionsexceedthelimitsforalternativedesigns,suchasriprapoutletprotection.2 Lowtailwatercontrolisanticipated.Forexample,atoutfallstodeten-tion/retentionfacilitiesthatareemptyoratlowwaterlevels.3 Useofconcreteismoreeconomicalduetostructuresizeoflocalavailabilityofmaterials.4 Siteconditionsdirecttheuseofanoutletstructuresuchaspublicuseareaswhereplungepoolsandstandingwaterareunacceptableorlocationswithseverespacelimitations.8.4.3ImpactStillingBasinDesignstandardsforthisstructurearebasedontheUSBRTypeVIBasin,commonlyreferredtoasanimpactdissipatororconduitoutletstillingbasin.TheTypeVIBasinisrelativelysmallstructurewhichprovideshighlyef?cientenergydissipationcharacteristicswithouttailwatercontrol.Thestructureisdesignedtooperatecontinuouslyatthedesign?owrate.Maximumentranceconditionsareupto50feetpersecondforvelocityandaFroudenumberlessthan9.0.Conditionsexceedingthis“criteriawouldbeextremelyrareintypicalurbandrainageapplications.Asaresult,theuseofthisoutletbasinislimitedonlybystructuralandeconomicconsiderations.Ageneralizeddesigncon?gurationforthisoutletbasinisillustratedinFigure8.18.ThestandardUSBRdesignhasbeenmodi?edforurbanapplicationstoallowdrainageofthebasinbottomduringdryperiods.Theimpactbasincanalsobeadaptedtomultiplepipeinstallations.Thesemodi?cationsarediscussedfollowingthebasiccriteria.Itshouldbenotedthatmodi?cationstothedesignmayeffectthehydraulicperformanceofthestructure.GeneralHydraulicDesignProcedureforStillingBasins(seeFigure8.17):1 Determinethedesignpipe?owrate,Q,andtheeffective?owarea,A,attheoutlet.Forpartial?owconditions,refertothepartial?owdiagraminSection8.6.6.Usingthecontinuityequationdeterminethe?owvelocityatthepipeoutlet.Assumethedepthisequaltothesquarerootofthearea,A,andcomputetheFroudeusingEquation7.3.2 Theentrancepipeshouldbeturnedhorizontalatonepipediameterequivalentlengthupstreamfromtheoutlet.Forpipeslopesgreaterthan15degrees,thehorizontallengthshouldbeaminimumof2pipediameters.
HydraulicStructures Page8-33
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Page8-34
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3 Donotusethistypeofoutletenergydissipatorwhenvelocitiesexceed50feetpersecondortheFroudeNumberexceeds9.0.Theseconditionswouldbeextremeandmustbeconsideredaspecialcase.Performanceisachievedwithatailwaterdepthequaltohalffull?owlevelinthepipeoutlet.4 Determinethebasinwidth(W)byenteringtheappropriateFroudeNumberandeffective?owdepthonFigure8.19.TheremainingdimensionsareproportionaltothebasinwidthaccordingtothelegendonFigure8.18.Notethatthebaf?ethickness,t,isasuggestedminimum.Itisnotahydraulicparameterandisnotasubstituteforstructuralanalysis.Thebasinwidthshouldnotbeincreasedsincethebasinisinherentlyoversizedforlessthandesign?ows.Largerbasinsbecomelesseffectiveasthein?owcanpassunderthebaf?e.5 Structurewallthickness,steelreinforcementandanchorwallsshouldbedesignedusingacceptedstructuralengineeringmethods.Hydraulicforcesontheoverhangingbaf?emaybeapproximatedbydeterminationofthejetmomentumforce:Fm=pVQ=1.94VQ (8.10)RiprapwithaminimumD50of18inchesshouldbeplacedinthereceivingchannelfromtheendofthesilltoaminimumdistanceequaltothebasinwidth.Thedepthofrockshouldbeequaltothesillheightoratleast2.5feet.LowFlowModi?cations:Thestandarddesignwillretainastandingpoolofwaterinthebasinbottomwhichisgenerallyundesirablefromasafetyandmaintenancestandpoint.Alow?owgapshouldbeextendedthroughthebasinendsillwall.Thegapinthesillshouldbeasnarrowaspossibletominimizee?‘ectsonthesillhydraulics.Thisimpliesthatanarrowanddeeper(1.5to2.0feet)low?owchannelwillworkbetterthanawidergapsection.Thelow?owwidthshouldnotexceed60percentofthepipediametertopreventthejetfromshort-circuitingthroughthecleanoutnotches.Theoptimalcon?gurationresultsincontinuousdrainageofthebasinareaandhelpstoreducetheamountofsedimententrapment.
HydraulicStructures Page8-35
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1 Forlargebasinswherethesillheightisgreaterthan2.0feet,thedepthdimension,d,(seeFigure8.18)maybereducedtoavoidasecondarydropfromthesilltothemainchannel.Thelow?owinverttherebymatchesthe?oorinvertatthebasinendandthemainchannelelevationisequaltothesill.Dimension(1shouldnotbereducedbymorethanone-thirdandnotlessthan2.0feet.Thisimpliesthatadeeperlow?owchannel(1.5to2.0feet)willbeadvantageousfortheseinstallations.Notethatdimensiondisalsoreducedattheminimumpipeinvertheightandatthebottomofthebaf?ewall.2 Asillsectionshouldbeconstructeddirectlyinfrontofthelow?ownotchtobreakupbottom?owvelocities.Thelengthofthissillsectionshouldoverlapthewidthofthelow?owbyaboutonefoot.Thegenerallayoutforthelow?owmodi?cationsisshowninFigure8.20.MultipleConduitInstallations:Wheremorethanoneconduitofdifferentsizeshaveoutletsincloseproximity,acompositestructurecanbeconstructedtotakeadvantageofcommonwalls.Thiscanbesomewhatawkwardsinceeachbasin"cell"mustbedesignedasanindividualbasinwithdifferentdimensions.Wheretwoconduitsofthesamesizehavecloseoutlets,thestructuresmaybecombinedintoasinglebasinasshowninFigure8.20. "
Thetotalwidthofadualinletbasincanbereducedtothree-fourthsofthetotalwidthforseparatebasins.Forexample,ifthedesignwidthforeachpipeisw,thecombinedbasinwidthwouldbe1.5w.Theremainingstructuredimensionsarebasedonthedesignwidthofaseparatebasin.Ifthetwopipehavedi?erent?ows,thecombinedstructureshouldbebasedonthehigherFroudeNumber?ows.8.4.4BaffleChuteEnergyDissipatorThebaf?echutehasalsobeenadaptedforuseatpipeoutlets.Thisstructureisparticularlysuitedtosituationswithvarylargeconduitoutfallsandatoutfallsinwhichsome?xturedegradationisanticipated.Generally,thistypeofstructureisonlycosteffectiveifagradedropisnecessarybelowtheoutfallelevationandahydraulicbackwatercanbetoleratedintheculvertdesign.Figure8.21illustratesageneralcon?gurationforabaf?edoutletforadoubleboxculvert.Inthiscase,anexpansionzoneoccursjustupstreamoftheapproachdepression.Thedepressiondepthisdesignedasrequiredtoachievethe?owvelocityatthechuteentranceasdescribedinSection8.3.1.Theremaininghydraulicdesignisthesameasforastandardba?lechute.Thesamecrestmodi?cationsareapplicabletoallowdrainageoftheapproachdepression,toreducetheupstreambackwaterelfectsofthebaf?es,andtoreducetheproblemsofdebrisaccumulationattheupstreamrowofbaf?es.
HydraulicStructures Page8-3
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8.5SPECIALCHANNELSTRUCTURES8.5.1ChannelTransitionsA?owtransitionisachangeofopenchannel?owcrosssectiondesignatedtobeaccomplishedinashortdistancewithaminimumamountof?owdisturbance.Speciallydesignedopenchannel?owtransitionsarenormallynotrequiredforhighwayculverts.Aculvertisnormallydesignedtooperatewithanupstreamheadwaterpoolwhichdissipatestheapproachvelocityand,therefore,negatestheneedforanapproach?owtransitions.Outlettransitionsmustbeconsideredinthedesignofallculverts,channelprotectionandenergydissipators.Designconsiderationsforsubcritical?owcharmeltransitionsarepresentedinHydraulicDesignofEnergyDissipatorsforCulvertsandChannels(FHW A,HEC-l4,1983).8.5.2SupercriticalFlowStructuresAccelerationChutes:Accelerationchutes,whetherleadingintoboxculverts,pipesorhighvelocitychannels,areoftenusedtoreducedownstreamcrosssections,hence,reducingcosts.Accelerationchutesarepotentiallyhazardousifinadequatelydesigned(seeUSBR1974).Highvelocity?owcanwashoutchannelsandstructuresdownstreaminshortorder,resultinginpropertydamageanduncontrolled?ow.Thereferencecitedaboveaddresseschutesingreaterdetailthancanbediscussedinthismanual.Refertothispublicationfordetailedanalysisanddesignprocedures.Bends:Structuresaregenerallyunnecessaryinsubcritical?owchannelsunlessthebendisofsmallradius.Structuresforsupercritical?owsarecomplexandrequirecare?ilhydraulicdesigntocontrol?ow.Bendsarenormallynotusedinsupercritical?owchannelsbecauseofthecostsinvolvedandthehazardsintroduced.Whenabendisnecessary,anditisnotpracticaltotakethe?owintothesubcriticalregime,thedesignerwillgenerallyconcludethatthechannelshouldbeplacedinaclosedconduitfortheentirereachofthebend,anddownstreamfarenoughtoeliminatethemainoscillations.Inbends,forcesareusuallylargerthanwhatisintuitivelyassumed.Themomentumequationpermitssolutionfortheforceactinguponthe?owboundaryatabend.
HydraulicStructures Page8-40
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F=MA V(8.11)where"deltav"representsthechangeindirectionand/ormagnitudeofthevelocitythroughthebend.Theforceduetopressureonthebendshouldalsobecalculatedwhenconduits?owunderpressure.AP=§(A(V)2)(8.12)where"deltaP"representsthepressurechangecausedbythedifferenceinthesquaresofthevelocitiesthroughthebend.Thetotalexertedforceonthebend,thetotalofmomentumandpressureforces,mustbecounteractedbyexternalforces.Allowablesoilbearingshouldbedeterminedusingsoiltestsifnecessary.Forceswhichcannotbehandlebyconduitbearinginthesoilmustbecompen-satedforbyadditionalthrustblocksorotherstructures.8.6CHARACTERISTICSOFTHEHYDRAULICJUMPWiththeexceptionofthebaf?echutedrop,allofthedropstructuresdescribedintheprevioussectionsusetheformationofahydraulicjumptodissipateenergy.Ahydraulicjumpoccurswhen?owchangesrapidlyfromlowstagesupercriticaltohighstagesubcritical.Hydraulicjumpscanoccur:1)whentheslopeofachannelchangesfromsteeptomild;2)atsuddenexpansionsorcontractionsinthechannelsection;3)atlocationswhereabarrier,suchasaculvertorbridge,occursinachannelofsteepslope;4)atthedownstreamsideofdipcrossingsorculverts;and5)whereachannelofsteepslopedischargesintoanotherchannel.Thetypeofhydraulicjumpthatforms,andtheamountofenergythatitdissipates,isdependentontheFroudeNumber(F).ThevarioustypeofhydraulicjumpsthatcanoccurarelistedinTable8.2 Table8.2TypesofHydraulicJumps UpstreamFroudeNumberTypeofJump EnergyLoss,%1<Fs1.7 UndularJump 0.51.7<Fs2.5 WeakJump 5to18OscillatingJump 18to44SteadyJump 44to709<F StrongJump 70to85 HydraulicStructures Page8-41
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8.6.5 JumpHeightThedepthof?owimmediatelydownstreamofahydraulicjumpisreferredtoasthesequentdepth.Thesequentdepthinrectangularchannelscanbecomputedusingthefollowingequation:1Y2;3mm+8F.2>“2—11(mThesolutionforthesequentdepthintrapezoidalchannelscanbeobtainedfromatrial-and-errorsolutionofEquation8.14,whichisderivedfrommomentumequations.Itisalsoacceptable,fordesignpurposes,todeterminethesequentdepthintrapezoidalchannelsfromEquation8.13.Equation8.13ismucheasiertosolveandproducesonlyslightlygreatervaluesforsequentdepththandoesEquation8.14.2Y3,“+b172,,+_Q_2173,,+21/22,,+_Q3 2 qA3 2gA (8.14) Figures8.22and8.23providegraphsofhydraulicjumpsforahorizontalrectangularchannelandahorizontaltrapezoidalchannel,respectively.JumpLengthThelengthofahydraulicjumpisde?nedasthedistance?'omthefrontfaceofthejumptoapointimmediatelydownstreamoftheroller.JumplengthcanbedeterminedfromFigures824and8.25.SurfacePro?leThesurfacepro?lesofahydraulicjumpmaybeneededtodesignthepro?leofextrabankprotection,ortrainingwallsforcontainmentofthejump.Thesurfacepro?lecanbedeterminedfromFigure8.26.JumpLocationInmostcases,ahydraulicjumpwilloccuratthelocationinachannelwheretheinitialandsequentdepthsandinitialFroudeNumbersatisfyEquation8.13.Thislocationcanbefoundbyperformingdirectstepcalculationsineitherdirectiontowardthesuspectedjumplocation,untilthetermsofthearesatis?ed.Speci?cforceanalysiscanthenbeusedbyemployingEquation8.1toestablishwhereajumpwilloccur.Thehydraulicjumpwillbegintoformwheretheunitspeci?cforceofthedownstreamtailwaterisgreaterthantheforceofthesupercriticalapproach?ow.UndularJumpAnundularhydraulicjumpisthetypeofjumpwhichoccurswheretheupstreamFroudeNumberinbetween1.0and1.7.Thistypeofjumpischaracterizedbyaseriesofundularwaveswhichformonthedownstreamsideofthejump.Experimentshaveshownthatthe?rstwaveofanundularjumpishigherthantheheightgivenbyEquation8.13.Therefore,theheightofthiswaveshouldbedeterminedasfollows:HydraulicStructures Page8-42
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(Y2D—Ylu)/Ylu=F21—1 (8.15)wherealltermsarepreviouslyde?ned8.6.6DesignAidsInadditiontotheTablesandFigurespreviouslyreferenced,Table6.3hasbeenincludedasanadditionalaidtotheuserofthismanual.
HydraulicStructures Page8-43
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00W001320019700262003250.0389004510.0513005750.0?50085007550081300871009290.0%50.104201097O11520120601259013120136401416014660151601566016140166201709017560160201847018910193501978020200.20620.210202142021820.22200225802295023310236602401024350246802500 UniformFlowinCircularSectionsFlowingPartlyF1111
OnDBIJSHZ0W70W1OM740w13-80002220003280W550W6040W7750N70011810014170.016740.019520.02250.025700291003270036500406O0448004920053700585006340068600739007930084900907009660102701089011530.12180.128401351014700.14%01561016300.17050177901854019290.2010208021602240232 615671619610Cublc1661persecond01/Mnnnmgslotmul.Mam-1mg50041110611111004131mechannet001mmand011h:water1111114:-0ns 0" °_'d6'7351/2015040.511057 052656 053736 054655055595o56547 057509056476 059449 060425 061404062366063369 064354 0653.410663.260673170.663.060.692.960702.87o712.79072271o732.63074256 0752.49o76242 o77236 076230 079225 0602.20061214062209 0632050.642.0006519560.661.91506716750.661.635069179709017600911724092166909316550.94162209515900.96155909715300961.5000991471100 Table8.3
04027041270422704327044260452604625O472404822049200501805115052120.53m05404054990559405687057800.587205964060540614306231063190640506489065730665506736068150689306969070430711507186072540.732007.1840.744507504075600.76120.7662077070.774907785078170.7841 02531025620259202621026490.2676027030272802753027760.279902821028420.28620288202900029170.2933029480.296202975O2987029980.13080301703024030310$3603039030420114303043030410.3038030330302603018030070.29950.29800.29630.29440.29210.28950.286502829O27870273502666 0239U24702550263027102790287029503030.311031903270335034303500.3580.3660.3730.3800.3880.39504020.409041604220429043504410447045304580.46304680.4730.4770.481048504880.4910.49404960.4970.498049804980496049404890.4830463 Ond8/351/2442415318836233631128626223-82151921701418126105 2%?
.024
0.9850.9650.9470.9280.9100.891087308560.838082108040.7870.7700.7530.736072007030.68706700.6540.6370.62106040.5880.5710.5530.5350.51704900.463 Page8-49
DBIJSHZ
0.78540.2500
18.B
8.7DESIGNEXAMPLES8.7.1OpenChannelDropStructuresDesignExampleforVerticalDropwithRiprapBasin(Stevens1981)Arockriprapverticaldropstructureistobeusedtodropthe?owinadrainagechannelaverticaldistanceof4feet.SeeFigure8.10.ApproachConditions:Theapproachchannelistrapezoidalincross-sectionwith:
» bedwithisB0=10ft
» sideslopesare2horizontalto1vertical
» designdischargeisQ=360cfs
» thedepthof?owisy0=4.00ft
» theaveragevelocityisV0=5.00fps
» thespeci?cenergyintheapproachchannelis
E0=yo+V02/2g=4-00+0-39=4-39?(8.16)WeirCrest:MakethelengthoftheweirB=10feet.Theheadontheweirrequiredtopassthedesign?owisasfollows:H=[Q/(5.67B)]°-67H=[360/(5.67x10)]0-67=3,43f, (8.17) Theheight0ftheweircrestis:P=E—Ha (8.18)P=4.39—3.43=0.96ftIfdrawdowncanbetoleratedintheapproachchannel,theweircrestshouldbeplacedatabedleveloftheapproachchannel;thatisP=0.Ifnot,makeP=1.0foot.HereuseP=1.0foot.Theheightofthewingwallsish=H=3.5ft(8.19) HydraulicStructures Page8-50
18.B
Basin:Theproblemisto?ndthedropheightDandrocksizedmsuchthatzis4.0feet.Thedepthof?owleavingthebasinis:D/d50=6.0 (8.21)Startbyusingthelargestrocksizethathasbeentestedinthemodel.ThatiscFirst,tryD=6.0feet,sothat(150=1.0foot,andH/D=0.57.Nowatrialanderrorsolutionisrequiredto?ndthedepth(12thatsatis?esboththecurvesinFigure8.12(page8-41)andtheequation:d2=Y2+0.67ds(8.22)Herec1sisthedepthofscour,infeet.ThecalculationsaresummarizedintheTable8.4.Theprocedureistoselectatrialvalueof(12computedZ/D,obtaind,/DfromFigure8.11,andthencomputed2fromtheaboveequation.Whenthetrialvalueandthecomputedvalueof(12agree,thatisonesolution.Forthe?rst,try:D= 6.0feetd2= 5.5feetandz = 6.0-1.0-1.5=3.5feet,whichislessthantherequireddrop(4feet).Therefore,alargerDisneeded.Inthiscase,avalueof8feetisassumedwhiled,=1.3feettomaintaintheD/d,ratioof6.Thisresultsind2=5.5feetandz=5.5feet,whichistoolarge.ThesetwotrialresultsforDandzaregraphicallyplottedinFigure8.28sothatanew(andhope?llly)?nalvalueofDcanbeselectedtoyieldthedesireddrop,2=4.0feet.TheinterpretationisthatD=6.5feetwhich,asshowninTable8.4,turnsouttobetherequiredresult.Thedesignvaluesare:D 6.5feetP 1.0feet(1501.1feetd 2.2feet HydraulicStructures Page8—
18.B
D,ft
8.0 6.5
FromFigure8.10,thelengthofthebasiniscalculatedasfollows1.)=4H+0.25DLb=4x3.43+0.25x65 =15.5feetTheminimumdepthofrockintheplungepoolis1.5ds=3.3feet(Figure8.10),andtheminimumthicknessoftheripraponthesideslopesist=2.0d50=2.20feet(Figure8.14).
Table8.4CalculationsofBasinDimensions
dwftH/DTrial(1led,/Dd,,ft0.67d,,ftResultingd2,ft d2,ft1.00.575.00.830.462.761.855.856.01.000.301.801.215.215.50.920.382.281.535.530k z=6.0—1.0—1.5=3.5ft.$4.011.
1.3 7.00.880.181.440.964.966.00.750.252.001.335.335.50.690.282.241.495.490k z=8.0—1.0—1.55.5ftat4.0ft.
1.10.53 0.920.281.821.215.215.50.85 2.15 5.440k
z=6.5—1.0—1.4=4.1=4.0ft.
HydraulicStructures Page8-52
0.43 6.0
18.B
ValueofZ,ft
Trial2
Trial1
5 6 7 8ValueofD,ftFigure8.28SolutionforVerticalDropwithRiprapBasinDesignExampleDesignExample2Given:8footby6footboxculvert Find:Solutions:Note:
HydraulicStructures
Q=800cfssupercritical?owinculvertnormal?owdepth=brinkdepthy0=4feetTailwaterdepth(TW)=4.2feetDownstreamchannelcantolerate7fpsfordesigndischargeRiprapbasindimensionsfortheseconditions Hightailwaterdepth,TW/yo=1.05>0.75.DesignIiprapbasinusingsteps1through6ofDesignExample1(150=1.8feet;118=6.4feet;L,=64feet;LB=96feetDesignriprapfordownstreamchannelUseFigure8.17toestimatetheaveragevelocityalongthechannel.Computetheequivalentcirculardiameter,De,forthebrinkarea,A,from:
Page8-53
18.B
A= 3.14D62/4=2y02W= (4)(8)=32feet2D= 25fps(DesignExample1) L VL/V oL/De(compute)(Figure6.21)VL10 64 0.59 14.715 96 0.36 9.020 128 0.30 7.521 135 0.28 7.0Thechannelshouldbelinedwiththesamesizerockusedforthebasin.Protectionmustextendatleast135feetdownstreamfromtheculvertbrink.DesignExample3Given:6feetdiametercmpQ =135cfsS =0.004OManningsn=0.024NormaldepthinpipeforQ=135cfsis4.5feetNormalvelocityis5.9fpsFlowissubcriticalTailwaterdepth(TW)is2.0feetFind:Riprapbasindimensionsfortheseconditions.Solution:1 Determiney0andV0:Q/DZ'5=135/(6)2‘5=1.53TW/D=2.0/6=0.33FromFigure6.19,yO/D=0.45yo=(0.45)(6)=2.7feet HydraulicStructures Page8-54
18.B
TW/yo=20/270=0.74,TW/yo<0.7S OKBrinkArea(A)foryo/D=0.45is:A=(O.343)(36)=12.3112[0.343isfromTable8.3]V0=Q/A=135/12.3=11.0fps2 y2=(A/2)°'5=(12.3/2)“5=2.48feet3 F=V ,/[(322)(y,)]O-—5—.11/[322)(2.48)]0—5—1.234 Tryd50/y6=0.25,d50=(0.25)(2.48)=0.623feetFromFigure6.20,hs/ye=0.75,hl9=(0.75)(2.48)=1.86feetcheckzhs/d50=1.86/0.62=3,2<hs/d50<40K5 L,=(10)(h,)=(10)(1.86)=18.6feetorL,=(3)(W0)=(3)(6)=18feet,UseL5=18.6feetLB=(15)(h,)=(15)(1.86)=27.9feetorLB=(4)(Wo)=(4)(6)=24feet,UseLB=27.9feet =0.62feet,Used50=8inchesOtherbasindimensionsdesignedinaccordancewithdetailsareshownonFigure8.14(page8-43).LISTOFV ARIABLESD= Plungeheightds= ScourdepthF = ForcesinpoundspersquareinchFs= Speci?cforceP = Pressureinpoundspersquareinch HydraulicStructures Page8-55
18.B
8.9 g Accelerationofgravity,32.2FT/S2q DischargeperunitwidthincubicfeetpersecondperfootDesigndischargeperunitwidthV Velocityinfeetpersecondy Depthof?owyc CriticaldepthYfVerticalfallatadropinfeetREFERENCESAmericanConcretePipeAssociation,January1988,ConcretePipeHandbook.Chow,VenTe,1959,OpenChannelHydraulics,McGraw-HillPeterk,A.J.,1984HydraulicDesignofStillingBasinsandEnergyDissipators,EngineeringMonographNo.25,UnitedStatesDepartmentoftheInterior,BureauofReclamation.Phone,T.J.,1977,Baf?edApronasSpillWayEntryDissipator,AmericanSocietyofCivilEngineersJournalofHydraulics,No.HY12.US.ArmyCorpsofEngineers,1991,HydraulicDesignofFloodControlChannelsUS.DepartmentoftheInterior,BureauofReclamation,1987,DesignofSmallDams----,1974,DesignofSmallCanalStructuresUS.DepartmentofTransportationFederalHighwayAdministration,September1983,HydraulicDesignofEnergyDissipatorsforCulvertsandChannels,HydraulicEngineeringCircularNo.14.
HydraulicStructures Page8-56
18.B
,mww,
18.B
.
CLOSEDCONDUITS
Stormdrainsandculvertsaregenerallyminorpartsofanoveralldrainagesystem.Stormdrainsarerequiredwhenotherminorpartsoftheoverallsystem,primarilycurb,gutterandroadsideditches,nolongerhavecapacityforadditional?ow.Culverts,"ontheotherhand,aregenerallypartofadrainagesystemthatismorenaturalandexistsbecauseofaninterruptioninthesystemsuchasroadwayorothertypesofmanufacturedembankment.Ineithercase,particularcaremustbetakenduringdesigntoavoidadverseeffectsonthetravelingpublic,receivingwatersorfacilitiesandupstreamanddownstreamproperties.Thefollowingsectionpresentsinformationontheappropriatedesignofstormdrains,culverts,inletsandoutlets,andappurtenantfacilities. .9.1STORMDRAINSAstormdrainsystemreferstoasystemofinlets,conduits,manholesandotherappurtenanceswhicharedesignedtocollectandconveystormrunofffromthedesignstormtoapointofdischarge.Stormdrainsystemdesignisbasedonthedesignstorm.ThedesignstormisthestormassociatedwiththegoverningreturnperiodfromTable6.2(page6.3).Intheupperreachesofasystemthe10-yeardesignstormwillgovern.However,fartherdownstreaminthesystem,thestormdraindesignforthe10-yearstormmaynotmeetthestatedcriteria.Inthiscase,thestormdrainwillneedtobeupsizedtomeetthecriteriaforthe100-yeardesignstorm.However,thisdoesnotnecessarilyimplystormdraindesignedfor100-yeardischarges.Bothreturnperiodsmustbechecked.Thestormconditiongoverningdesignatanypointisthedesignstorm.9.1.1GeneralHydraulicCriteriaClosedconduitsshallbedesignedtoconveythedesignstorm?oodpeakat?111?ow,wheneverpossible,andmaybeallowedto?owunderpressureexceptwhenthefollowingconditionsexist:
Page9
18.B
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9.2 ClosedConduits
O Inareasofhighdebrispotential,thereisapossibilityofstoppageoccurringindrains.Insituationswheredebrismaybeexpected,Citystaffshouldbeconsultedforadeterminationoftheappropriatebulkingfactor. 0 Incertainsituationsopenchannelsectionsupstreamoftheproposedclosedconduitmaybeadverselyaffectedbybackwatereffects.Iftheproposedconduitistobedesignedforpressureconditions,thehydraulicgradelineshallnotbehigherthan1.0footbelowthenormalgutter?owlineatanypointinaspeci?creachofanystormdrain.However,inthosereacheswherenosurface?owwillbeintercepted,ahydraulicgradelinewhichencroacheson,orisslightlyhigherthan,thegroundorstreetsurfacewillbeacceptable.9.1.2HydraulicDesignToensuretheobjectivesofSection91.1areachieved,thehydraulicgradelineshallbecalculatedbyaccountingforpipefrictionlossesandotherminorlossesthatmayoccuralongaparticularreach.Mostproceduresforcalculatinghydraulicgradelinepro?lesarebasedontheBernoulliEquation.Thisequationcanbeexpressedasfollows:V12V22— +D1+SOL=— +D2+S+hmmor(9.1)23 2ginwhichDistheverticaldistancefromtheinverttotheHGL,Soistheinvertslope,Listheprojected,horizontallengthoftheconduit,Sfisthefrictionslope,Vistheaveragevelocityandhminoraretheminorheadlosses.MinorlosseshavebeenincludedintheBernoulliEquationbecauseoftheirimportanceincalculatinghydraulicgradelinepro?lesandareassumedtobeuniformlydistributedalonganypipelength.Thetotalhydrauliclossinasystemisequivalenttothesumofthelossesduetofrictionandtheminorlossesinthatsystem.Minorlossesincludethosecausedbyexpansionorcontractionof?ow,changesinpressureandmomentumatjunctions,pipebendsandmanholes.HEADLOSS1NCLOSEDCONDUITSGenerally,betweentheinletandoutletofastormdrainthe?owencountersavarietyofcon?gurationssuchaschangesinpipesize,laterals,bends,junctions,expansionsandcontractions.Thesevariationsinshapeor?owpatternimposelossesinadditiontothoseresultingfromfriction.Headlossescangenerallybestatedasafunctionofvelocityheadmultipliedbyaparticularconstant.thefollowingdiscussionpresentsthemostcommonheadlossesencounteredinastormdrainsystem.
Page9—
18.B
Qn1.486AR”3
9.2.1FrictionLossFrictionlossforclosedconduitscarryingstormwatercanbecalculatedfromManning'sEquation(seeEquation7.1)oraderivativethereof.Whenrearrangedintoamoreusefulform, sf=( 2=(%)Z(9.2)
inwhich,2/3K=1.486AR (9.3)nThelossduetofrictionthroughoutareachoflengthLis,therefore,calculatedby:
_ _ Q2hf
—
S,L
—
(E)L (9.4)ThevalueofKisdependentupononlytwofactors:thegeometricshapeofthe?owcrosssectionasexpressedbythequantityAR%,andtheroughnesscoe?icient,n.9.2.2TransitionLossTransitionlossshallbecalculatedfromtheEquations9.5and9.6shownbelow.Theseequationsareapplicablewhennochangein?owrateoccursandwherethehorizontalangleofdivergenceorconvergencebetweenthetwosectionsdoesnotexceed5°45'.Forvelocitieswhichincreaseinthedirectionof?ow(V2>V1), 2 2Vht=0.1(2— —1—) (9.5)2g23Forvelocitieswhichdecreaseinthedirectionof?ow(V2<V1), =0.2(—— _ ) (9.6)
Page9-3
18.B
9.2.3JunctionLossIngeneral,junctionlossesshallbecalculatedbyequatingpressureplusmomentumthroughthecon?uencesunderconsideration.ThiscanbedonebyusinganypressureplusmomentummethodortheCityofLosAngeles'Thompsonequation.Bothmethodsareapplicableinallcasesforpressure?owandwillgivethesameresults.Forthespecialcaseofpressure?owwithupstreamanddownstreamareasbeingequivalentand?ictionneglected, 6~D/rcc790/70/f/owV/Figure9.1
V22V52A3V32J x—— ><COSO (9.7)2g2gA22gItisimportanttonotethatthejunctionlosscalculatedusingEquation9.7shouldbeappliedtotheenergygradelineasopposedtothehydraulicgradeline.TheThompsonEquation:TheThompsonEquationforjunctionlossisdescribedbythefollowing:
0/,14,’4/07'1/2v’42
0!!16:43Figure9.2
QV—QV—QVcoseAyxAm.=L_1__1E_i3___(9,8)
Page9-4
18.B
?. "?ush- H1
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9.2.4 9.2.5
9.2.6 wheredeltayisthedifferenceinhydraulicgradientofthetwosectionsinfeetandAW}istheaverageareainsquarefeet.Theaboveequationisapplicableonlytoprismoidalandcircularconduitsorchannels.Thefrictionforcemaybeconsiderednegligibleorcanbecalculatedandtakenintoaccount.TheThompsonEquationshouldnotbeusedforopenchanneljunctionanalysiswhentheopenchannelchangessideslopegoingthroughthejunction.Fordetailsoftheabovemethod,refertoHydraulicAnalysisofJunctions,1968,StormDrainDesignDivision,BureauofEngineering,CityofLosAngeles.ManholeLossManholelossesshallbecalculatedusingEquation9.9andshallbeappliedonlytomanholeswherenolateralconnectionsaremade.hm=0.5(—) (9.9)Whenachangeincrosssectionand/orchange?owrateoccurs,theheadlossacrossthemanholeshallbecalculatedinaccordancewithSections9.2.2and9.2.3.BendLossBendlossesaredeterminedbytheapplicationofthefollowingequation:V2h=K— b b(2g) (9.10)Thebendlosscoef?cient,Kb,isafunctionofthecentralangleofthebend.Foranycentralanglenotexceeding90degrees,Khcanbedeterminedby:V21r<,,=0.2(2—)°-5(9.11)8 Bendlossesshouldbeincludedforallclosedconduits,those?owingpartiallyfullaswellasthose?owingfull.AnglePointLossAnglepointlosseshavebeendeterminedtobea?mctionofthede?ectionangle,(9,andcanbecalculatedusingthefollowingequation:
h=.00336(—) (9.12)
Page9-5
18.B
Inmostcommonapplications,thede?ectionangleshouldnotexceed6degrees.Shouldade?ectionangleofgreatermagnitudebenecessary,approvaloftheCityEngineerisnecessary.9.2.7SpecialCasesBranchingofFlowinPipe-HeadLoss:Thefollowingequationmaybeusedtodeterminethelossofheadincaseswhereitmaybenecessarytosplitorbranchthe?owintoanotherdrain.moLEFTSKEWSKEW SKEW ANGLE
ROADW AY
figure9.3
V?hbr=C— (9.13)2g
‘
Valuesforthecoef?cientCmaybeobtainedfromtheTable9.1belowandapplyonlytostraightreachesofpipeofconstantdiameter.Typicalvaluesforthecoe?icientCarelistedforvariousratiosofQ3/Q1andvariousanglesofdivergence.
Divergence=0,3 9—3=O.7Angle-9 Q1
90° 0.76 0.74 0.80
60° 0.59 0.54 0.52
45° 0.350.320.30Table9.1HeadLossCoef?cient,CForUseWithEquation9.13ForanglesofdivergenceandratiosofQ3/Q1otherthanthoseshown,valuesofCmaybeinterpolated.
Page9-6
18.B
9.3DESIGNREQUIREMENTSFORMAINTENANCEANDACCESS9.3.1ManholesSpacing:
0 Conduit45inchesindiameterofsmaller:Manholesshallbespacedatintervalsofapproximately400feet. 0 ConduitDiameter45inchesorlarger:Manholesshallbespacedatintervalsofapproximately500feet.Thespacingrequirementsshownaboveapplyregardlessofdesignvelocities.DeviationsfromthecriteriashallbesubjecttoapprovalbytheCityEngineer.Location:Manholesshouldnotbelocatedinstreetintersections,especiallywhenoneormorestreetsareheavilytraveled.Insituationswheretheproposedconduitistobealignedwithinaneasementandinstreetright-of—way,manholesshouldbelocatedinthestreetright-of-way,whereverpossible.Manholesshouldbelocatedasclosetochangesingradeas_feasiblewhenthefollowingconditionexists:
0 Whentheupstreamconduitissteeperthanthedownstreamconduitandthechangeingradeisgreaterthan10percent,sedimenttendstodepositatthepointwherethechangeingradeoccurs.ManholeDesign:Whenthedesign?owinapipe?owinglullhasavelocityof20feetpersecondorgreater,orissupercriticalinapartially?illpipe,thetotalhorizontalangleofdivergenceorconvergencebetweenthewallsofthemanholeanditscenterlineshallnotexceed5°45".DeepManholes:Amanholesha?safetyledgeshallbeprovidedinallinstanceswhenthemanholesha?is20feetorgreaterindepth.InstallationshallbeinaccordancewithLosAngelesCountyFloodControlDistrictStandardDrawingNo.2-D430.PressureManholes:ApressuremanholeshaftandapressureframeandcovershallbeinstalledinapipeorboxstormdrainWheneverthedesignwatersurface(HGL)isabovethegroundsurface.
Page9-7
18.B
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9.3.4 9.3.5
MinimumSlopeTheminimumslopeformainlineconduitshallbe0.10percent,unlessotherwiseapprovedbytheCityEngineer.MinimumPipeSizeTheminimumdiameterofmainlineconduitshallbe24inches,unlessotherwiseapprovedbytheCityEngineer.Incaseswheretheconduitmaycarryasigni?cantamountofdebris,theminimumdiameterofthemainlineconduitshallbe30inches.InletStructuresAninletstructureshallbeprovidedforstormdrainsacceptingstorm?owsfromanynaturalconveyance.Thestructureshouldgenerallyconsistofaheadwall,wingwallstoprotectanyadjacentbanksfromerosion,andapavedinletapron.Theapronslopeshouldbelimitedtoamaximumof2:1.Wallheightsshouldconformtotheheightofthewaterupstreamoftheinlet,andbeadequatetoprotectboththe?lloverthedrainandtheembankment.Headwallandwingwallfencingandaprotectionbarriertopreventpublicentryshallbeprovided.Ifdebrisisprevalent,barriersconsistingofvertical3-inchor4-inchdiametersteelpipeat24to36inchesoncentershouldbeembeddedinconcreteimmediatelyupstreamoftheinletapron.OutletStructuresWhenastormdrainoutletsintoanaturalchannelorotherhistoricconveyance,anoutletstructureshallbeprovidedwhichpreventserosionandpropertydamage.Velocityof?owattheoutletshouldagreeascloselyaspossiblewithexistingchannelvelocity.Thefollowingguidelinesshallapply:
0 Whenthedischargevelocityislow,orsubcritical,theoutletstructureshallconsistofaheadwall,wingwallsandanapron.Theapronmayconsistofaconcreteslab,Iipraporgroutedrock. 0 Whenthedischargevelocityishigh,orsupercritical,thedesignershouldconsiderbankprotectioninthevicinityoftheoutletandanenergydissipatorstructure.PleaserefertoSection8fordiscussionofenergydissipators.
Page9-8
18.B
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9.4DESIGNREQUIREMENTSFORJUNCTIONS9.4.1
9.4.2 InletstoMainLineDrainsLateralpipeenteringamainlinestormdrainshallbeconnectedatthespringlineofmainline.Lateralpipeenteringamainlineboxstructureshallconformtothefollowing:
0 Lateralpipe24inchesorlessindiametershallenterthemainlinenomorethan?vefeetabovetheinvert 0 Lateralpipe27inchesindiameterorlargershallenterthemainlinenomorethan18inchesabovetheinvert,withtheexceptionthatcatchbasinconnectorpipeslessthan50feetinlengthshallenternomorethan5feetabovetheinvert.Exceptionstotheaboverequirementsmaybepermittedwhereitcanbeshownthatthecostofbringinglateralsintoamainlineboxconduitinconformancewiththeaboverequirementswouldbeexcessive.AngleofCon?uenceIngeneral,theangleofcon?uencebetweenmainlineandlateralshallnotexceed45degreesand,asanadditionalrequirement,shallnotexceed30degreesunderanyofthefollowingconditions:
0 Wherethe?owintheproposedlateralexceeds10percentofthemainline?ow; 0 Thevelocityof?owintheproposedlateralis20feetpersecondorgreater; 0 Thesizeoftheproposedlateralis60inchesorgreater;and
0 Wherehydrauliccalculationsindicateexcessiveheadlossesmayoccurinthemainlineduetothecon?uence.Connectorpipemaybejoinedtomainlinepipeatanglesgreaterthan45degreesuptoamaximumof90degreesprovidednoneoftheaboveconditionsexist.If,inanyspeci?csituation,oneormoreoftheaboveconditionsdonotapply,theangleofcon?uencefortheconnectorpipesshallnotexceed30degrees.TheaboverequirementsmaybewaivedonlyifcalculationsaresubmittedtotheCityshowingthattheuseofacon?uenceanglelargerthan30degreeswillnotundulyincreaseheadlossesinthemainline.9.5CULVERTSCulvertsareprimarilyusedforconveyingrunoffthrougharoadwayembankment.Theyarenormallyalignedwithanaturalwashordrainagechannel,whichareoftenoutfallsforstorm Page9-9
18.B
drainagesystems.Culvertsaretypicallyassociatedwithdrainsonascalewherebridgesarenotappropriate.Regionaldrainagefacilitiesaregenerallyofamagnitudethatjusti?estheuseofbridges.9.5.1 ClosedConduits
CulvertCriteriaSizing:Crossroadculvertsshouldbesizedaccordingtothefollowingcriteria:OtherdesignfrequenciesshallbeusedonlywiththeapprovaloftheCityEngineer 0 10-yeardischargewithnohead0 lOO-yeardischargeutilizingavailableheadA/IinimumVelocity:Aminimumvelocityof2.5feetpersecondatdesigncapacityisrecommendedtoassureaself-cleaningconditionduringpartialdepth?ow.MaximumVelocity:Asapracticallimit,outletvelocitiesshouldbekeptbelow15feetpersecond.Ingeneral,themaximumvelocityshouldbeconsistentwithchannelstabilityrequirementsattheculvertoutlet.Asoutletvelocitiesincrease,theneedforchannelstabilizationattheculvertoutletincreases.Ifculvertoutletvelocitiesexceedpermissiblevelocitiesfortheoutletchannelliningmaterial,suitableoutletprotectionmustbeprovided.Outletvelocitiesmayexceedpermissibledownstreamchannelvelocitiesbyupto10percentWithoutprovidingoutletprotectioniftheculverttailwaterdepthisgreaterthantheculvertcriticaldepthof?owunderdesign?owconditions.SkewedChannels:Thealignmentofaculvertbarrelwithrespecttoalinenormaltotheroadwaycenterlineisreferredtoasthebarrelskewangle.Aculvertalignednormaltotheroadwaycenterlinehasaskewangleequaltozero.Directionsrightorleftmustaccompanytheskewangle.Theanglefromtheculvertfacetoalinenormaltotheculvertbarrelisreferredtoastheinletskewangle.Thestructuralintegrityofcircularsectionsiscompromisedwhentheinletisskewedduetolossofaportionofthefullcircularsection.Althoughconcreteheadwallshelpstabilizethepipesection,structuralconsiderationsshouldnotbeoverlookedinthedesignofskewedinlets.Incaseswheretheculvertbarrelcannotbealignedwiththechannel?owline,suchaswhenrunoffisdirectedparalleltotheroadwayembankmenttoasuitablecrossinglocation,the?owenterstheculvertbarrelatanangle.Theapproachangleshouldbelimitedtoamaximumof90degrees.Whenhighvelocitiesexist,inletlossesresultingfromturningthe?owintotheculvertshouldbeconsidered.Ifbackwatercomputa-tionsarenotemployedandtheapproachchannelvelocityis6feetpersecondorgreater,thefollowingequationshouldbeusedtoestimatetheloss.Thelossshouldbeaddedtotheotherinletlossesintheculvertdesigncomputation:
Page9-10
18.B
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Iftheseconditionscannotbemet,analysisofbendlossesisrequired.TocalculatebendlossesrefertoEquations9.10and9.11inSection9.2.5TrashRacks:Whenanyofthefollowingconditionsaremet,trashrackswillberequiredontheentrancestoallconduitsinareasadjacenttoschools,recreationalandurbanareas,andhighwayswhichareaccessibletothepublic. 0 Sideslopesinchannelssteeperthan4:1forconcrete,grassandearthliningsand3:1forripraplinings. 0 Conduitssmallerthan7feetindiameter,greaterthan100feetinlengthandwithout12inchesoffreeboardatthedesign?owrate. 0 Conduitswithenergydissipatorsorexitingmulti-usedetentionfacilities. 0 Conduitswithsuf?cientcurvaturethattheoppositeendcannotbeseen.Apluggingfactorof50percentwillbeusedonalltrashracks,andthevelocitythroughtherackshallbelessthanorequalto2.0feetpersecond(afterpluggingfactorsapplied).Fortrashrackswithvelocitieslessthan3.0feetpersecondafterthepluggingfactorhasbeenapplied,thelossescausedbythetrashrackcanbeignoredincomputations.Forgreatervelocities,thelosswillbecomputedandaddedtothecalculatedwatersurface.InletsCulvertinletsareusedtotransitionthe?ow?omapondedconditionupstreamoftheculvertintotheculvertbarrel.Theprovisionforamoregradual?owtransitionwilldecreasetheenergylossandthuscreateamoreef?cientinletcondition.
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Thehydraulicperformanceofculvertsoperatingunderinletcontrolcanbeimprovedbychangingtheinletgeometry.Typicalinletimprovementsincludebevelededges,taperedsidesandslopeddropinlets.Bevelededges:Provideadecreasein?owcontractionlossesattheinlet.Theentrancelosscoe?icientisreducedfrom0.5to0.2,whichcanincreaseculvertcapacityasmuchas20percent.Taperedsides:Provideanenlargedfaceareaaccomplishedbytaperingwingwalls.Thischangeingeometrycanincrease?owcapacityasmuchas25to40percentoversquareedgedinlets.Slopeddrop:Provideadditionalheadatthethroatsection.Thistypeofinletcanaccommodateover100percentgreatercapacitythanaconventionalculvertwithsquareedges.Thedegreeofincreasedcapacitydependsuponthedropbetweenthefaceandthethroatsection.Section9.5.7providesinletcontrolnomographsforvariousculverttypeswhichprovidetheheadwaterdepthrequiredtopassthedesigndischarge.AdditionaldesignchartsforimprovedinletsarecontainedinHydraulicDesignofHighwayCulverts(FHW A,HDSNo.5,September1985).9.5.3OutletsThereceivingchannelatculvertoutletsmustbeprotectedfromthehighculvertoutletvelocitiescausedby?owconstrictionthatisinherentinculvertoperation.Iftheculvertoutletvelocityisgreaterthantheallowablevelocityforthereceivingchannelliningmaterial,protectivemeasuresmustbeprovided.Theminimumrequirementistoprovideariprappad,designedinaccordwiththeprovisionsofthismanual,withacutoffwallprovidedatthedownstreamendofthewall.StandardoutletsforclosedconduitsarepresentedinSection9.3.5.9.5.4DesignProceduresTheCulvertdesignmethodprovidesaconvenientandorganizedprocedurefordesigningculverts,consideringinletandoutletcontrol.The?rststepinthedesignprocessistosummarizeallknowndatafortheculvertatthetopoftheCulvertDesignForm(Figure9.3,page9—19).Thenextstepistoselectapreliminaryculvertmaterial,shape,size,andentrancetype.Theuserthenentersthedesign?owrateandproceedswiththeinletcontrolcalculations.InletControl:Theinletcontrolcalculationsdeterminetheheadwaterelevationrequiredtopassthedesign?owthroughtheselectedculvertcon?gurationiftheculvertopeningisoperatingunderinletcontrol.TheinletcontrolnomographsinSection9.5.7areusedinthedesignprocess.Forthefollowingdiscussion,refertotheschematicinletcontrolnomographshowninFigure9.8,page9-23.
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Conduits
Locatetheselectedculvertsize(point1)and?owrate(point2)ontheappropriatescalesoftheinletcontrolnomograph.(Notethatforboxculverts,the?owrateperfootofbarrelwidthisused.)Extendastraighthorizontallinefromtheculvertsize(point1)throughtheflowrate(point2)andmarkapointonthe?rstheadwater/culvertheight(HW/D)scale(point3).The?rstHW/Dscaleisalsoaturningline.IfanotherHW/Dscaleisrequired,extendahorizontallinefromthe?rstHW/Dscaletothedesiredscale.MultiplyHW/Dbytheculvertheight,D,toobtaintherequired(HW)fromtheinvertofthecontrolsectiontotheenergygradeline.HWequalstherequiredheadwaterdepth.Iftrashracksareincluded,addthelossassociatedwiththetrashracktotheheadwater.OutletControl:Theoutletcontrolcalculationsresultintheheadwaterelevationrequiredtoconveythedesigndischargethroughtheselectedculvertiftheculvertisoperatingunderoutletcontrol.ThecriticaldepthchartsandoutletcontrolnomographsofSection9.5.8areusedtodeterminetherequiredheadwaterforaculvertoperatingundertheseconditions.Forillustration,refertotheschematiccriticaldepthchartandoutletcontrolnomographshowninFigures9.5and9.11,respectively.1. Determinethetailwaterdepthabovetheoutletinvert(TW)atthedesign?owrate.Thisisobtainedfrombackwaterornormaldepthcalculationsofthedownstreamchannel,orfrom?eldobservations.Entertheappropriatecriticaldepthchartwiththe?owrateanddeterminecriticaldepth.IfthecomputedcriticaldepthisgreaterthanD,theheightoftheculvert,useDforcriticaldepth.Note:Thecriticaldepthcurvesaretruncatedforconveniencewhentheyconverge.Ifanaccuratecriticaldepthisrequired,consulttheHandbookofHydraulicsbyKingandBrater,orotherhydraulicreference.Calculate(dc+D)/2.Determinethedepthformtheculvertoutletinverttothehydraulicgradeline(ho).h0=TWor(do+D)/2,whicheverislarger (9,13) Page9-13
18.B
FromTable9.2(page9-39)obtaintheappropriateentrancelosscoef?cient,ke,fortheculvertinletcon?guration.Determinethelossesthroughtheculvertbarrel,H,usingtheoutletcontrolnomograph(Figure9.10)orappropriateequationsifoutsidetherangeofthenomograph.a)IftheManning'snvaluegivenintheoutletcontrolnomographisdifferentthantheManning'snfortheculvert,adjusttheculvertlengthusingtheformula:
1=L[nl/n]2 (9.14)ThenuseL1ratherthantheactualculvertlengthwhenusingtheoutletcontrolnomograph.b)Connecttheculvertsize(point1)withtheculvertlengthontheappropriatekescale(point2).Thisde?nesapointontheturningline(point3).c)Extendalinefromthedischarge(point4)throughthepointoftheturningline(point3)totheHeadLoss(H)scale.ReadH,theenergylossthroughtheculvert,includingentrance,frictionandoutletlosses.Calculatetherequiredoutletcontrolheadwaterelevation. a+H+h0 (9.15)whereEL0istheinvertelevationattheoutletIftheoutletcontrolheadwaterelevationexceedsthedesignheadwaterelevation,anewculvertcon?gurationmustbeselectedandtheprocessrepeated.EvaluationofResults:Comparetheheadwaterelevationscalculatedforinletandoutletcontrol.Thehigherofthetwoisdesignatedthecontrollingheadwaterelevation.Theculvertcanbeexpectedtooperatewiththathigherheadwaterforatleastpartofthetime.Theoutletvelocityiscalculatedasfollows:Ifthecontrollingheadwaterisbasedoninletcontrol,determinethenormaldepthandvelocityintheculvertbarrel.Thevelocityatnormaldepthis Page9-14
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18.B
9.5.5 assumedtobetheoutletvelocity.NormaldepthforcircularandrectangularculvertscanbefoundusingFigure9.8,page9-232 Ifthecontrollingheadwaterisinoutletcontrol,determinetheareaof?owattheoutletbasedontheBarrekgeometryandthefollowing:a)Criticaldepth,ifthetailwaterisbelowcriticaldepth.b)Thetailwaterdepthifthetailwaterisbetweencriticaldepthandthetopofthebarrel.0)Theheightofthebarrelifthetailwaterisabovethetopofthebarrel.Repeatthedesignprocessuntilanacceptableculvertcon?gurationisdetermined.Ifoutletcontrolgovernsandtheheadwaterdepth(referencedtotheinletinvert)islessthan1.2D,itispossiblethatthebarrel?owspartlyfullthroughitsentirelength.Inthiscase,cautionshouldbeusedinapplyingtheapproximatemethodofsettingthedownstreamelevationbasedonthegreateroftailwateror(dc+D)/2.Ifanaccurateheadwaterisnecessary,backwatercalculationsshouldbeusedtochecktheresultfromtheapproximatemethod.Iftheheadwaterdepthfallsbelow0.75D,theapproximatemethodshouldnotbeused.Culvertdesignshouldincludeaculvertratingcurvewhichdisplaysculvertperfor-manceoverarangeofdischarges.DevelopmentofratingcurvesispresentedinthefollowingsectionandanexampleproblemisprovidedinSection9.5.8.CulvertRatingCurvesRatingcurvesarerepresentationsof?owrateversusheadwaterdepthorelevationforaculvert.Becauseaculverthasseveralpossiblecontrolsections(inlet,outlet,throat),agiveninstallationwillhaveaperformancecurveforeachcontrolsectionandoneforroadwayovertopping.Theoverallculvertperformancecurveismadeupofthecontrollingportionsoftheindividualperformancecurvesforeachcontrolsection.InletControl:TheinletcontrolratingcurvesaredevelopedusingtheinletcontrolnomographsinSection9.5.7.Theheadwaterscorrespondingtotheseriesof?owratesaredeterminedandthenplotted.OutletControl:TheoutletcontrolratingcurvesaredevelopedusingtheoutletcontrolnomographsinSection9.5.7.Flowsbracketingthedesign?owareselected.Forthisrangeof?owratesthetotalheadlossthroughthebarreliscalculatedorreadfromtheoutletcontrolnomographs.Thelossesareaddedtotheelevationofthehydraulicgradelineattheculvertoutlettoobtaintheheadwater.
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18.B
9.5.6 Ifbackwatercalculationsarepreformedbeginningatthedownstreamendoftheculvert,frictionlossesareaccountedforinthecalculations.Addingtheinletlosstotheenergygradelineinthebarrelattheinletresultsintheheadwaterelevationforeach?owrate.RoadwayOvertopping:Aratingcurveshowingtheculvert?owaswellasthe?owacrosstheroadwayisause?Jlanalysistool.Theratingcurvedepictsthesumofthe?owthroughtheculvertandthe?owacrosstheroadway.Theoverallratingcurvecanbedeterminedbyperformingthefollowingsteps:1 Selecttherangeof?owratesanddeterminethecorrespondingheadwaterelevationsfortheculvert?owalone.These?owratesshouldfallaboveandbelowthedesigndischargeandcovertheentirerangeof?owsanticipated.Boththeinletandoutletcontrolheadwatersshouldbecalculated.2 Combinetheinletandoutletcontrolratingcurvestode?neasingleratingcurvefortheculvert.3 Whentheculvertheadwaterelevationsexceedtheroadwaycrestelevation,overtoppingwillbegin.Calculatetheequivalentupstreamwatersurfacedepthabovetheroadway(crestofweir)foreachselected?owrate.UsethesewatersurfacedepthsandEquation9.16tocalculate?owratesacrosstheroadway.4.Addtheculvert?owandtheroadwayovertopping?owatthecorrespondingelevationstoobtaintheoverallculvertratingcurve.Usingthecombinedculvertratingcurve,itisaneasymattertodeterminetheheadwaterelevationforany?owrate,ortovisualizetheperformanceoftheculvertinstallationoverarangeof?owrates.RoadwayOvertoppingRoadwayovertoppingwillbeginastheheadwaterrisestotheelevationofthelowestpointalongtheroadway.Thistypeof?owissimilarto?owoverabroadcrestweir.Thelengthoftheweircanbetakenasthehorizontallengthacrosstheroadway.The?owacrosstheroadwayiscalculatedfromthebroadcrestedweirequation:Q=K,C,Il.(1LIW,)1'5(9.16)ThechartsinFigure9.5indicatehowtoevaluatethecorrectionfactorsK1andC,.Iftheelevationoftheroadwaycrestvaries,forinstancewherethecrestisde?nedbyasagverticalcurve,theverticalcurvecanbeapproximatedasaseriesofhorizontalPage9-16
18.B
segments.The?owovereachiscalculatedseparately,andthetotal?owacrosstheroadwayisthesumoftheincremental?owsforeachsegment.Thetotal?owacrosstheroadwaythenequalsthesumoftheroadwayover?owplustheculvert?ow.Aratingcurvemustbeplottedincludingbothculvert?owandroadover?ow.Theheadwaterdepthforaspeci?cdischarge,suchasthe100-yeardischarge,canthenbereadfromthecurve.9.5.7DesignAidsFigures9.3through9.22andTable9-2areprovidedtofacilitatecompletionoftheCulvertDesignFormandthedevelopmentofculvertratingcurves.
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Table9.2EntranceLossCoefficientsOutletControl,FullorPartlyFullEntranceHeadLossofStructureandDesofEntranceCoefficientk.ConcreteProiectingfrom?ll,socketend(groove-end)0.2Proiectingfrom?ll,squarecutend 0.5HeadwallorheadwallandwingwallsSocketendofpipe(groove-end) 0.2Square-edge 0.5Rounded(radius=1/12D) 0.2Miteredtoconformto?llslope 0.7End-Sectionconformingto?llslope 0.5Bevelededges,33.7or45°bevels 0.2Side-orslope-taperedinlet 0.2Pipe,orPipe-Arch,CorrugatedMetalProjectingfrom?ll(noheadwall) 0.9Headwallorheadwallandwingwallssquare-edge0.5Miteredtoconformto?llslope,pavedorunpavedslope 0.7End-Sectionconformingto?llslope 0.5Bevelededges,33.7°0r45°bevels 0.2Side-orslope-taperedinlet 02Box.ReinforcedConcreteHeadwallparalleltoembankment(nowingwalls)Square-edgedon3edges 0.5Roundedon3edgestoradiusof1/12barreldimension,orbeveled0.2edgeson3sidesWingwallsat30°to75°tobarrelSquare-edgedatcrown 0.4Crownedgeroundedtoradiusto1/12barreldimension,or 0.2beveledtopedgeWingwallat10°to25°tobarrelSquare-edgedatcrown 0.5Wingwallsparallel(extensionofsides)Square-edgedatcrown 0.7Side-orslope-taperedinlet 02 ClosedConduits Page9-39
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9.6LISTOFV ARIABLESA = Cross-sectionalareaCT= Lengthoftheroadwaycrestalongtheroadway,feetD = Depthof?owinfeeth = HeadlossHWr= FlowdepthabovetheroadwayinfeetKE= SubmergencefactorL = LengthinfeetLs= Lengthoftheroadwaycrestalongtheroadway,feet 11 = Manning’sRoughnesscoef?cientR = HydraulicradiusSf= FrictionslopeinfeetperfootS0= LongitudinalslopeinfeetperfootTW= TailwaterV = Velocityinfeetpersecond 9 = Inletskewangle9.7REFERENCESAmericanSocietyofCivilEngineersandtheWaterPollutionControlFederation,1969,DesignandConstructionofSanitaryandStormSewersBrater,ErnestF.andHoraceWilliamsKing,1976,HandbookofHydraulicsfortheSolutionofHydraulicEngineeringProblemsSixthEdition.LosAngelesCountyFloodControlDistrict,HydraulicDesignManualUnitedStatesDepartmentoftheInterior,BureauofReclamation,1974,DesignofSmallCanalStructures.
ClosedConduits Page9-40
18.B
UnitedStatesDepartmentofTransportation,FederalHighwayAdministrationSeptember1873,HydraulicDesignofEnergyDissipatorsforCulvertsandChannels,HydraulicEngineeringCircularNo.14.----,September1984,HydraulicDesignofHighwayCulverts,HydraulicsDesignServicesNo5 ClosedConduits Page9-
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18.B
10NTIONDETENTION/RETE
Aspartofatotalsystemofurbanstorrnwatermanagement,detentionandretentionfacilitiesareman-madestoragemeasuresintendedtomitigatethenegativeimpactsofurbanization,whichinclude:Increasedpeak?owrates.Lossofnaturaldepressionstorage.Reductionofin?ltrationcapacityinadrainagebasin.Reductionofnaturalvegetation.Increasedpollutantloadinsurfacerunoff.Thissectionpresentsthemethodsassociatedwiththeplanning,analysisanddesignofdetentionandretentionfacilities.Theguidelineshereinareintendedtoachievethefollowinggoals:12 Designofdetention/retentionfacilitiesthatsatisfytheordinanceprovisionsoftheCity;Designofdetention/retentionfacilitiesthatareamenitiesand,wherepossible,incorporatemultiple-useconcepts;andDesignoffacilitiesthatwillnotjeopardizethequalityofsurfacewaterorgroundwa-terresources.10.1LIMITATIONSONUSEOFDETENTION/RETENTIONFACILITIESTherequirementsforadevelopmenttoprovidestorageofexcessrunoffbydetentionorretentionfacilitiesshallnotbewaivedunlessdeterminedotherwisebytheDirectorofPublicWorksonacase-by-casebasis.Ingeneral,storagefacilitiesaretobelocatedsotheycaninterceptthe?owfromtheentiredevelopmentarea.Theobjectiveistoprovidestorageforexcessrunoffwithaminimumnumberofdetention/retentionfacilitieslocatedatoptimumpointswithinadevelopedarea.Wheneverpossible,thefacilitiesshallbedesignedformultipleuses,suchasparksorotherrecreationalfacilities,tooffsetthecostofopenspaceandtoencourageimprovedmaintenance.10.1.1RegionalDetention/RetentionFacilitiesRegionaldetention/retentionfacilitiesarelargestoragesiteswithinadrainagebasinprovidedtocontrolexcessruno?‘andtoachievethemostcost-effectivedrainagesystem.Advantagesofthistypeoffacilityincludethefollowing:
Detention/Retention Page10-1
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O Thesitinganddesignofregionalstoragefacilitiesarenormallyincorporatedaspartofanoveralldrainagemasterplan. 0 Operationandmaintenancecostsarereduced.
0 Regionalfacilitiesaremoreeffectiveandreliablebecausetheyareplanned,designedandmaintainedaspartofatotaldrainagesystem.10.2DESIGNCRITERIA10.2.1CriteriaforDetention/RetentionFacilitiesDesignFrequency:Alldetention/retentionfacilitiesincorporatedwithinnewdevelopmentsshallbedesignedtoretainthepeak?owandvolumeofrunofffromthelOO-year,3-hourdurationstormevent.Storrnwaterdetentionfacilitiesforwatershedareaslargerthan100acresshallbedesignedtoensurethat,ataminimum,thepost-developmentlOO-year,3-hourdurationpeakdischargedoesnotexceedthepre-developmentcondition.Hydrology:Proceduresfordevelopmentofin?owhydrographsfordeten-tion/retentionfacilitiesareintroducedinSection5ofthisManualandintheRiversideCountyFloodControlandWaterConservationDistrictHydrologyManual.Incaseswheredetention/retentionfacilitiesarebeingevaluatedforextremelysmallwatersheds,areaslessthan100to200acreswithlagtimeslessthan7to8minutes,ashort-cutUnitHydrographMethodmaybeuseful.Themethodisbasedontheassumptionthatinasmallwatershed,whichhasahighpercentageofimperviousarea,responsetimetoeffectiverainfallisveryshort.Therefore,runoffratesforgivenperiodsoftimecanbeassumedtobedirectlyproportionaltoe?‘ectiverain.Itshouldbeemphasizedthatthismethodyieldsconservativeresultsandshouldonlybeusedforwatershedswhichmeetthelimitationsnotedabove.Whendesigningatypicalstormwaterdetentionfacility,therearethreevariablestobeconsidered:1.In?owtothefacility,whichvariesasafunctionoftime;2.Out?owfromthefacility,whichvariesasa?anctionoftime;and,3.Storagewhichistheresultofthedifferencebetweenthein?owandtheout?owforperiodoftimeortimeinterval.AsillustratedinFigure10.1Theout?owhydrographfromaproposedstormwaterdetentionfacilityshallbedeterminedusingthe"StorageIndication"or"Modi?edPuls"methodof?oodrouting.Section10.6offersadetaileddescriptionofthisroutingprocedure.
Detention/Retention Page10-2
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Ifacomputerprogramistobeused,Citysta?'shallbeconsultedontheapplicabilityoftheprogrambeingproposed.
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TIMEFigureI0.1FloodRouting(In?owandOut?owHydrograph)Geometry:Basicrequirementsregardingfacilityshape,sideslopes,depthandbottomcon?gurationareprovidedbelow.0 Shape:Asageneralrule,curvilinear,irregularlyshapedfacilitieswillhavethemostnaturalcharacter.Awiderangeofshapescanbeconsideredandutilizedtointegratethedetentionfacilitywiththesurroundingsitedevelopment. 0 SideSlopes:Wheregrassisintendedtobeestablished,sideslopesshallnotbesteeperthan4horizontalto1vertical.Whereotherprotectionmeasuresareintended,suchasshrubplanting,rockriprapandotherstructuralmeasures,slopesshallnotexceed3horizontalto1verticalunlessapprovedbytheCityEngineer.Whereslopesabutthestreetright-of-way,theminimumslopeshallbe4horizontalto1verticalregardlessofsurfacetreatment.0 DepthandBottomCon?guration:Withrespecttograding,deepfacilitiesshouldbeavoided,ifpossible.Forfacilitiesinexcessofeightfeetdeep,considerationshouldbegiventotheuseof?attersideslopesortheprovisionofintermediatebenchesalongthesideslopes.Thebottomshallbedesignedtodraintoalow?owchannelforadetentionfacility.DrainTime:Thedesignofalldetention/retentionfacilitiesshallbesuchthatthestoredrunoffshallbedischargedcompletelyfromthefacilitywithin72hoursofthestormevent.
Detention/Retention Page10-3
18.B
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Lining/SurfaceTreatment:Asageneralrule,grassandplantspeciesusedforlandscapedevelopmentandrevegetationshouldbenativetoRiversideCounty.Permanentirrigationsystemsarerequiredforgrassareasandmosttypesofbasinrevegetationandlandscaping.However,useofnativeanddroughttolerantspeciesmayonlyrequireatemporarysystemtoobtaine?‘ectivegerminationandestablish-ment.Theuseofinertmaterialsisappropriateforstabilizationanderosioncontrolwheresteepslopesareunavoidable,alongchannels,atpointsofin?ow,attheoutletcontrolstructureandanyotherlocationwhere?owingwatermaythreatenstability.Inertmaterialsforerosioncontrolinclude:
0 Looserockriprapwithaspeci?c,engineeredgradation.0 Looseorgroutedboulders(minimumdimension18inchesorlarger)0 Riverstone.LowFlowChannels:Alow?owchannelisrequiredinthebottomofadetentionfacilitytoprovidepositiveroutingofdrainagetotheprimaryoutletstructure.Anexampleofarectangularconcretelow?owchannelisprovidedinFigure10.2.Thechannelshallhavea0.5percentmaximumlongitudinalslope.InletandOutletStructures:Thedesignofaninletstructureshallbesuchthatin?owisdirectedintothefacilityinanon-erosivemannerandwithoutadverseimpactstotheretentionfacilityortoupstreamareas.Outletstructuresareclassi?edas:1)primaryoutletstructuresthatprovidehydrauliccontrolforthespeci?cdesignevent(s),and2)emergencyspillwaysthatprovidesaferoutes,typicallyviasurfaceover?ow,forstormeventsinexcessofthedesignfrequencyorinthecaseofdebrisblockageormalfunctionoftheprimaryoutletstructure.Theminimumallowablepipesizeforprimaryoutletstructuresshallbe18inchesindiameter.Ifthecapacityofanoutletpipemustbefurtherreduced,anori?ceplatemaybeinstalled,asshownonFigure10.3(a).Theori?ceplatemaybeconstructedofheavy,galvanizedsteelandattachedwithtamper-proofboltsorbyformingasmallerdiameteropeningintheconcreteheadwallattheoutletstructure.
Detention/Retention Page10-4
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EnergyDissipationatOutlet:Adequateenergydissipationmeasuresshallbeprovidedatthedownstreamendofprimaryoutletstructures.Suchmeasuresshallbedesignedtocontrollocalscouratthepipeoutletandtoreducevelocitiestopre-developmentconditionspriortoexitingontodownstreamproperties.EmergencySpillways:Emergencyspillwaysarenormallysurfaceover?owweirs,channels,orcombinationsthereof,providedforthesafeover?owandroutingof?oodwatersunderunusualcircumstances.Considerationmustbegiventothelayoutoftheemergencyspillwaysothatexcess?owisroutedinthesamemanneranddirectionaswouldhaveoccurredunderpre-developmentorhistoricconditions.Emergencyspillwaysmustbedesignedtoconveytheunattenuated100-yearpeakdischargeatnon-erosiveveIOCities.10.2.2CriteriaforSpecialDetention/RetentionMethodsSpecialmethodsforstormwaterdetention/retentioninclude,roadwayembankmentstorageandstorageinparkinglotsandgreenbeltareas.Theuseofundergroundstoragefacilities,fordetention/retentionofexcessrunoffispermitted,subjecttoreviewandapprovalbytheCityEngineer.TheuseofrooftopareasforstorageofrunoffisnotpermittedintheCityofHemet.Detention/Retention Page10-5 Figure102RectangularConcreteChannelSection(Adaptedfrom:WRCEngineering,Inc.1985)
18.B
Sincethefollowingmethodsoftenresultinfacilitiesnearbuildings,itshouldbeemphasizedthatthepadelevationsadjacentthesestructuresshall.beaminimumofonefootabovethe100-yearwatersurfaceofthedetention/retentionfacility.ConveyanceStorage:Duringtheperiodthatchannelsand?oodplainsare?llingwithrunoff,thestormwaterisbeingstoredintransientform.Thistypeofstorageisknownasconveyancestorage.Constructionoflowvelocitychannelswithlargecrosssectionalareasassistsintheaccomplishmentofsuchstorage.Conveyancestoragesystemsareusuallyfeasibleonlyonlargeprojects,andrequiredetaileddynamicmodelingforanalysis.RoadwayEmbankmentStorage:Whenfeasible,useofroadway?llslopesasanembankmentforadetentionbasinprovidesaneconomicalmeansofstormwaterstorage.Specialconsiderationsmustbegivenbothtothestabilityoftheembankmentandtotheprotectionoftheembankmentfromerosion.Additionally,StateofCalifornia,DivisionofDamSafetyrequirementsmayneedtobeaddressediftheembankmentheightand/orthepotentialstoragevolumeexceedcertainlimits.
Detention/Retention Page[0-6
18.B
(a)OrificePlateDetails Tamper-proofbolt(2each)Trashrack Tamper-proof.. O?ce(hinged) boltPrllateA - OutletPi9Floorof OrificeOutletPipe 12'em?i.Outlet Plate ‘ 11'..4 ‘A. 1 .A
A 1 : ‘ AA 4
Recessforplate Floorof AOutletsectionA-A _Ori?ceopening(withtrashracknotshown)NOTE'Trashrackcapacitytobe' 10timesori?cecapacity.
Figure10.3. . DetentionFacilityOutletDetailsDetention/Retention(AdaptedWRCEngineering1987) Page10-7
18.B
. am...pm...“
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InclulluoutnelllnlulIsI noonIn a!lnun-Ilu. pm."gumorlunchuuu 20“notI.nonllro(IIIatnrvuurl.um ' ' ‘ -I ' --- ' ' '~-noyounurlllorll N".no:IruluIMI ISmalI00m!"0‘CONHOIIGd' :MIIweulnrlorcan:uororI(nun-non .uuuuIM menourrnr '3'IDYthe099‘.OfWaterl'lllCMM our“.orIn!munM nclonlpurge-cavurlcullunuu.IIIn:on- 5nouourvunInIn:mml':IIIIItI orIIIInMumrrum-lo nonofIn.onInmunnan.vhlrnru-“5'Il?l an:InnnurIlll’lln[MonorIn."I.“ unllrnurunInstunting-Iforurn new"I“In:onllduuIen. lo I (e)luurvoIn'Iu-rvoIr'nuumyru-rvolrmlencont-InnorIlllcmulnnuIn."InpaumubyI ‘ ___a... ..'ovn-r' orIn. 5centralpapropu-Lecal-Irv“Ian.ornurvnlr:(l)TM5!: IIndun-nun“,lnltlluuo-u.Ann-cl.m4polnlullubul'lllml. 0(2)mm»,nmmpuorqvnll- 0 I0I520 no 7D 30l‘lll?lpllcarver-um(J)BuryuI“:unuu. STORAGECAPACITY'ACI’BFL(I)Innalnrlcl.(5)Iurypcrlu?.TMdul“thorn“unnu.er\I-u-uuornnyofIn.for-um.(7)II-uInnortrul-IppollnolI)laycan".{orM]oftn:roruoln‘.'a—nu'on:not.lr-eluloIn:unIua“nu.(:ueum-600:-6005.Inn-rcan.)302.Pam-cIM[fr-e:ofRaul-Haul.Thu.nxulnlanlIr.adopt-nforlMmaroonofany“;uulIn:wan-IanIrun1nl’Dlvlllon39!"InIurcan._Un?t?n9(Irl‘lll.{ln:'l.MdInnoI)rlleullrGI IMllInu-rural-Hanarnny:.'u" gun-tru-Ill Inorrerlcl'cnupontn runof-y"ouran:lanuul noInIn. ru-nl.notnullKM]Inmyrun::nnurucaloaprIuIn.dun-nun:atmynunorpour:dun"ma_‘Irlldl‘llnncan!"=1 noru:nuornurlntn.u-ount?r(Mr-cur-I‘ erInfannuancinchI Inrnnulnaf"\VIIBrunoa - DAMSaIMPOUNDMENT*VIII!SHOULDASCEITAIIl!STATUTESHA VEIKE!ltVlllB. RESV UNDERRCFCawco ER0””0LI'IYDROLOGYMANUALSTATECONTRFigure10.4Detention/RetentionJurisdictionalDam Page10-8
SPILLW AY
(a)OIMV . yfolio-Ir“who
18.B
ParkingLotStorage:Usingparkinglotsfordetention/retentionisaspecialcaseofsurfacestorage.Itisaneconomicaloptionformeetingdetention/retentionrequirementsinhighdensitycommercialandindustrialdevelopments.Planningofareaswithinaparkinglotwhichwillacceptpondingshouldbesuchthatpedestriansareinconveniencedaslittleaspossible.Thefollowingcriteriashallbeconsideredwhendevelopingparkinglotstorage 0 Themaximumdepthofpondedwaterwithinanyparkinglotlocationshallbeeightinches. 0 TheminimumlongitudinalslopepermittedWithinparkinglotstoragefacilitiesis0.01fr/ft,unlessconcretevalleyguttersareprovided.Withconcretevalleygutters,aminimumlongitudinalslopeof0.005?/?maybepermitted.Drainageofparkinglotscanbeaccomplishedbymeansofcurbopenings,weirs,stormdrainsorori?cesinwalls.10.2.3EmbankmentDesignCriteriaWheneverpossible,detention/retentionfacilitiesshouldbeconstructedwiththestoragevolumelocatedentirelybelowthenaturalgroundsurfaceadjacenttothebasin.However,insomeinstancesthismaynotbepossible,andembankmentsmaybenecessarytoprovidetherequiredstoragevolume.Sincetheuseofembankmentsmaycreateapotentialdownstream?oodhazardduetofailureoftheembankment,thefollowingdesignconsiderationsmustbeaddressedinconjunctionwiththeiruse.Foradditionalinformationandguidelinesforthedesignofdetention/retentionfacilities,refertoDesignofSmallDams(USBR1987).StateDamSafetyRequirements:TheStateofCalifornia,DivisionofDamSafety(DSD)haslegaljurisdictionoveralldams(embankments)whichexceedcertainheightandstoragelimits.TheDSDde?nesajurisdictionaldamaseither25feetormoreinheightorstoresmorethan50acre-feet.Ifitislessthansixfeetinheightregardlessofstoragecapacityordoesnotstoremorethan15acre-feetregardlessofheight,itisnotjurisdictiona."Figure10.4illustratesthedifferencebetweenajurisdictionalandnon-jurisdictionaldam.GeotechnicalEngineeringStudies:Analysesshallbeconductedtoevaluateconditionssuchasembankmentslope,foundationstability,embankmentandfoundationseepage,internalandexternalerosionpotentialandembankmentsettlements.Theresultsoftheseanalysesareusedtodevelopcriteriaforeconomicandsafedesignandconstructionofembankmentdams.Thesecriteriaincludethetypesandzonesofembankment?llmaterialsbasedonusingavailableborrow Detention/Retention Page10
18.B
materials,upstreamanddownstreamembankmentslopes,andrecommendedmeasuresforcontrolofseepage.EmergencySpillway:Allembankmentdamsfordetention/retentionfacilitiesshallincorporateanemergencyspillwayforthesafeover?owandroutingof?oodwatersunderunusualcircumstances.Suchconditionsincludetheblockageormalfunctionoftheprimaryoutletstructureortheoccurrenceofastormeventlargerthanthatforwhichthefacilitywasdesigned.Thedesignofemergencyspillwaysshallincorporateadequateerosioncontrolandenergydissipationmeasurestoinsurethestabilityoftheembankment.TheminimumdesignstandardforemergencyspillwaysshallbeasindicatedinTable10.1.PrimaryOutletStructure:Theprimaryoutletstructureisthemainoutletstructurebywhichstormwaterisdischargedfromthedetention/retentionfacility.Itistypicallyaclosedconduitstructurewithaninletspeci?callydesignedtocontrolasinglefrequencystormormultipleevents.Table10.1EmergencySpillwayDesignCapacityRequirements DamHeight SnillwavDesignCanacitvHs6it Unattenuated100-vearin?ow6ft<H<25ft 1/2ProbableMaximumFlood
10.3DESIGNCONSIDERATIONSThefollowingisanoutlineofdesignconsiderationsandrecommendationswhichfacilitatespeci?cmaintenanceactivities.10.3.1AccessAccessroadsforserviceandmaintenancevehiclesshouldbemaintainedtoallowforequipmentaccesstothefacility,wheneverneeded.Accesscontrolgatesshouldbeprovidedifrestrictedaccessisrequired.DesignRecommendations:
0 Accessrampsintothefacilityshallbegradedat10percentorless.Turningradiishallbe50feetorgreater. 0 Servicedrivesandgatesshallbelocatedinreadilyaccessiblelocations.
Detention/Retention Page10-10
18.B
0 Accesscontrolgatesandadjacentareasshallbeassecureaseconomicallyfeasible.10.3.2SedimentRemovalSedimentwillinevitablybedepositedinthedetention/retentionfacility.Conditionswillbeworstduringyearswhenconstructionactivityinthewatershedisgreatest.DesignRecommendations:
0 Providestillingbasinsorfore-basincollectionpointswheremostsedimentwillbedeposited. 0 Providecontrolledvehicularaccessintothefacilityfortrucksandfront-endloaders.10.3.3MaintenanceofLowFlowChannelsandDrainageStructuresIn-basindrainagestructuresandfacilitiesmustbemaintainedtoinsuretheirproperoperation.Structuredesigncanin?uencemaintenancerequirements.DesignRecommendations 0 Provideenergydissipatorstopreventdamagetothechannelordrainagestructuresduringhighin?owconditions. 0 Designstructuressothattheywillnotcollectdebriswhichcouldimpactproperoperation.10.4MULTIPLE-USECONCEPTSFloodcontrol?rnctionsandotherusesindetention/retentionfacilitiesaregenerallycompatible.Rationaleformultiple-usefacilitiesincludesdecreasedfacilitymaintenancecostsandanincreasedcommunityacceptance.Combining?oodstoragewithrecreationuseorothercommunityfacilitiesonasinglesitedecreasestotalcostsforlandacquisitionandsitedevelopment.Thedevelopmentofdetention/retentionfacilitiesasparksorurbangreenspaceincreasestheacceptancebyarearesidentsandencouragesbetteroverallmaintenance.10.4.1PotentialUsesAppropriateusesfordetention/retentionfacilitiesincludeactiveandpassiverecreationandurbangreenspace.Usesinadditionto?oodcontrolshouldaddressspeci?ccommunityneedsandbeclearlyidenti?edbeforethefacilityisdesigned.ActiveRecreation:Activerecreationincludesawiderangeofactivitiesthatinvolvessometypeofphysicalmovement.Thistypeofrecreationalactivity-bothindividualandgroup-generallyrequireslargerareasthanpassiverecreationuses.Becauseoftheirsize,regionaldetention/retentionfacilitiescanprovidemoreopportunitiesforgroupsportswithlargespacerequirements.
Detention/Retention Page10-11
18.B
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PassiveRecreation:Passiverecreationgenerallyinvolvesindividualsorsmallgroupsandaminimalamountofphysicalactivity.Typically,passiverecreationdoesnotrequirelargeopenspaceandis,therefore,appropriateforbothlargeandsmalldetention/retentionfacilities.UrbanGreenSpace:Urbangreenspaceprovidesavisualresourcewithinthecommunity.Asurbanizationcontinues,thevalueofgreenspacewillincrease.Greenspaceprovidesvisualbreaksfromtheurbanenvironment,actsasa?ltertocleanairandcanreduceerosionfromwindandrain.Landscapematerialsinadeten-tion/retentionfacilityshouldrespondtotherecessednatureofthelandform,thescaleofthefacilityandtheoccurrenceoffrequent?ooding.Theuseofnativeandnon-nativespeciesforlandscapeplantingishighlyrecom-mended.Thefollowingbasiczonesshouldbeconsideredinthelandscapedesignforadetention/retentionfacility. 0 Channels:Plantingintheseareasshouldbelimitedtograsses,groundcoversandlowgrowingshrubs,withpreferencegiventovegetationwith?exiblebranchingandresilientgrowthhabits.0 BasinAreas:Theremaybeinundationandstandingwaterinbasinareasatsometimeduringtheyear.Choiceofplantmaterialsshouldre?ecttheseconditions.Trees,shrubsandgrassescanbeplantedjudiciouslyintheseareas. 0 ElevatedAreas:Theseareasmaybeoccasionallyinundated.Thechoiceofplantmaterialwilldependontheuseassignedtothearea.Trees,shrubsandgrassescanbeplantedandmoreeasilymaintainedinareasofhighergroundelevation.10.5W ATERQUALITYUrbanruno?‘isdistinguishedfromundevelopedarearuno?‘intwoprincipalways:itoccursatgreaterdischargeratesandvolumes,anditcontainsvaryingbutcommonlyhigherconcentrationsoftoxicsubstances,bacteria,anddissolvedorganicmatter.Deten-tion/Retentionfacilitiescanplayasigni?cantroleinmitigatingthepollutionproblemsassociatedwithurbanrunoff.10.5.1MethodforControlofSedimentationSedimentremovalwithinadetention/retentionfacilitymaybefacilitatedbytheuseofa"sedimenttrap"attheinlet,whichwillconcentratethemajorityoftheincomingsedimentbedloadtoasmallportionofthefacility.Sedimenttrapsshouldbeprovidedinconjunctionwithalldetention/retentionfacilitieswhichareintendedasmulti-usefacilities.Thefollowinglistprovidesguidelinesforthedesignofef?cientsedimenttraps.
Detention/Retention Page10-12
18.B
1 Anadditionalsedimentationvolumeshouldbeprovidedwithinthesedimenttrapatanelevationbelowtheinvertofthein?owchannel.2 Thelength/widthratioofthesedimenttrapshouldbeaminimumof2:1,withthelengthmeasuredalongalinebetweentheinletandoutlet.3 Thebasinshouldbewedge-shaped,withthenarrowendlocatedattheinlettothebasin.4 Provisionsfortotaldrainageandaccumulatedsedimentremovalofthesedimenttrapmustbeprovided.Maintenanceaccessshouldalsobeprovidedanddesignedtoaccommodateheavytrucksandotherequipmentnecessaryforremovalofaccumulatedsediment.10.6FLOODROUTINGCharacteristically,thestorageofareservoiriscloselyrelatedtoitsout?owrate.Inreservoirroutingmethods,thestorage-dischargerelationisusedforrepeatedlysolvingthecontinuityequation;eachsolutionisastepdelineatingtheout?owhydrograph.10.6.1FloodRoutingthroughDetentionFacilitiesbytheStorage-IndicationMethod 3=I(t)-0(1) (10.1)
Thecontinuityequationusedinreservoirroutingmethodsisconcernedwithconservationofmass:foragiventimeinterval,thevolumeofin?owminusthevolumeofout?owequalsthechangeinvolumeofstorage.Thecontinuityequationforreservoirroutingis:whereS = Storagevolumeinthesystem,incubicfeet,t = Time,inhours,I(t)= In?owtothesystem,incubicfeetpersecond,and0(t)= Out?owfromthesystem,incubicfeetpersecond.Ifthetimeisbrokenintointervalsofduration"At"andindexedbyj,Equation10.1canberewrittenforthechangeinstorageovertheinterval:
. . O.+O.: J 1+1At_ J 1+1A] (10.2)2 2
Detention/Retention Page10-13
18.B
Detention/Retention
Thevaluesof1jandIj+1areobtainedatthejthtimeinterval?omcal-culationduringtheprevioustimeinterval.Equation10.3resultsfrommultiplyingEquation10.2throughby2/AtandthenisolatingtheunknownsOj+1andSH.25.(11+11“)E0].(10.3) Equation10.3canbeusedtofacilitatethestorage-out?owfunctionsolutionintabularform(seesampleproblem,Section10.6.2).Inordertocalculatetheout?ow(OJ-+1)?'omEquation10.3,astorage-out?owfunctionrelatingZS/At+OandOisneeded.Themethodfordevelopingthisrelationshipusingelevation-storageandelevation-out?owdataisshowninFigure10.5.ThefollowingstepsareusedintheStorage-IndicationMethodof?oodrouting:1.Developthein?owhydrographasdirectedinSection52 Developanelevation-storagerelationship(Figure10.5a)forthestructure.Thestoragewillnormallybedevelopedinacre-feetwhichwillthenbeconvertedtocubicfeetintheworkingtable(seeTable10.3,page10-21).
Page10-14
18.B
Table10.2SpillwayDischargeEquations(fromAppliedHydrology,Chowcta]1988) SplilwayType EquationNotationUncontrolledoverflowogeeO_CLH%O= Discharge,cfs — . .-.(1)crest C=DischargecoerncrentL= EffectivelengthofcrestH=TotalheadonthecrestincludingvelocityofapproachheadBroadcrestedweirO=CLHVZCo=Dischargecoef?cient“)0 L= EffectivelengthofcrestHTotalheadonthecrestincludingvelocityofapproachhead/777775Culvert(submergedinlet0=CdA VZgHA=Cross-sectionalarea3control) Cd= Dischargecoef?cient()Source:HandbookofHydraulics6thEdition,Braterandlong1982.(1)Cvalueforogeecrestvariesfrom3.1to39dependinguponthehead,thedepthofapproachtheslopeofupstreamfaceandthecon?gurationofthedownstreamapron.(2)Covalueforbroadcrestedweirvariesfrom2.3to3.3dependinguponthehead,thebreadthofweircrest,andtheshapeoftheupstreamcomer.(3)Cdvalueforculvertvariesfrom0.2to0.9dependinguponthehead,pipesize,pipelength,materialofthepipe,andtheshapeoftheinletedge.3 Developanelevation-dischargerelationship(Figure10.5b)forthestructurefromhydraulicequationsrelatingheadanddis-chargeforvarioustypesofspillwaysandoutletworks,Table10.2listsvariousequationsthatcanbeused.ForvaluesofC,Co,andCd,seeDesignofSmallDams(USBR1987).4 Selecttheroutinginterval,"At".Theshortertheintervalselected,themoreprecisetheresultswillbe.5 Usingtheresultsofsteps2and3,makeafour-columntablewiththefollowingheadings:1)ElevationH,h;2)Storage81‘?)DischargeQ,cfs;and,4)ZS/At+OForanexample,seeTable10.4,page10-22.6 PlotthevaluesofZS/At+Oonthehorizontalaxisofagraphwiththevalueoftheout?ow,O,ontheverticalaxis(seeFigure10.6).
Detention/Retention Page10-15
18.B
7 ComputethevalueofZSj+1/At+OHusingEquation10.3.AllthetermsontherightsideofEquation10.3areknownforthetimeintervalj.Obtainthevalues0ijandIj+1fromthein?owhydrograph(Step1),asdoneinTable10.5.8 DeterminethecorrespondingvalueofOHtoZSJ-H/At+Oj+lfromthestorage-out?owrelationshipZS/At+0versus0.Thiscanbedonebyeitherusingtheplotofstep6orbylinearinterpolationoftabularvaluesfromstep5.9 CalculatethevalueofZSjH/At-Oj+1tosetthedatarequiredforthenexttimeintervalby:
2SJ+12SJ+1At
—
0h1= At+0h1
—
20h1(10.4)
10Repeatthecomputationforsubsequentroutingperiodsandplotthein?owandout?owhydrographs.
Outflow(e) _|
————————— -o-05WatersurfaceIelevation: Storage-out?ow: ’2'?”functIonI: At'1'OL __lj Watersurfaceelevation
Figure10.5DevelopmentoftheStorage-Out?owFunctionforLevelPoolRoutingontheBasisofStorage-ElevationandElevation-Out?owCurves Outflow(b)
O lIIIH
Storage(0)
S
H
Detention/Retention(from:AppliedHydrology,Chowelal1988) Page10-16
18.B
Time, In?ow, Time, In?ow,
10.6.2FloodRoutingSampleProblem
Steps7,8and9aredemonstratedinTable10.5.WiththeexceptionofStep8,allofthesestepscanbeeasilyperformedusingaspreadsheet.
Adetentionbasinisproposed.Determinethepeakout?owandthepeakwatersurfaceelevationinthebasin.10.6.2.1 Detention/Retain"on
Given:1.hrs0.000.2250.500.751.001.251.501.75
In?owHydrograph
102550100220610450
hrs2.002.252.502.753.003.253.503.75
cfs25016011070402010
Page10-17
18.B
2.Stage-SurfaceAreaRelationshipSurfaceArea,Elevation,ft Acres1100 4.61100.5 4.81101 5.21101.5 5.41102 5.61103 5.81104 6.21105 6.61106 7.01107 7.51108 7.83.Out?owStructuresa PrincipalSpillway,ogeecrestDischargecoef?cient=3.5Widthofweir=20ftElevationofweircrest=1104itb.LowFlowStructure,culvert(corrugatedmetalpipe)Dischargecoef?cient=0.5Diameter=15inches=1.25ftElevationofculvertinlet=11001’:Elevationofculvertcenter=1100.63it(usedtodetermineH2)Inflow \ Culvertoutflow__\_..
Detention/Retention Page10-18
Elev.1104'WeiroutflowDetentionBasin
18.B
10.6.2.2Solution:Anin?owhydrographforthissampleproblemisgiven,sothesolutionprocedurebeginswithStep2.However,forapracticalproblem,thein?owhydrographneedstobedevelopedforStep1.Step2.DevelopaWaterSurfaceElevation-StorageRelationship,asinTable10.3Step3.DevelopaWaterSurfaceElevation-DischargeRelationship,asinTable10.4 WeirFlowOlzClLH3{2=3.5x20xH3{2‘l'l2><1.252CulvertFlow02=C2A‘/2g—H2=0.5x_4__/——_——2x32_2xH2(10.5)TotalOug?ow0=01+O2Step4.At=0.25hrs=900seconds Step5.DevelopaStorage-Out?owRelationshipforadetentionreservoir,asinTable10.5.Step6.PlotthevalueofZS/At+Oonthehorizontalaxisofagraphwiththevalueoftheout?owOontheverticalaxis.SeeFigure10.6.
Detention/Retention Page10-19
18.B
Table10.3WaterSurfaceElevation-StorageRelationship
AverageDl?erenceIntervalSurfaceSurfaceInEevatlon,Storage,Storage,3ElevationArea,AcmArea,Acresft Acre-11Acre-11Storage.ft‘ 1100.04.6 __,_0 o4.70.52.35110054.8 2.35102.3705.00.5251101.05.2 4.85211,2705.305 2.651101.55.4 7.5326,7005.505 2.75110205.6 10.25446,4905.71.05.71103.05.8 15.95694,7806.01.06.01104.062 21.95956,1406.41.06.41105.06.6 28351234,9306.81.06.81106.07.0 35.151,531,1307.251.07251107.075 42.41846,9407.651.07.651108.07.8 50.052,180,180'1Acre-11-43,56003 Detention/Retention Page10—
18.B
Table10.4Storage-Out?owRelationshipforaDetentionReservoir 3 DischargeO,Elevatlon11.11Storage8,11 1:13(ZS/At)+0,C131100.00 0 01100.5102.3700 2271101.0211.2703 4721101.5326,7005 7311102.0446,4906 9981103.0694.7808 1.5521104.0956.1409 2.1341105.01.234.93080 2.8241106.01.531.130209 3.6121107.01846.940376 4,4801108.02.180.180573 5.418 600
3500Uo‘400
(D9‘300O_CU.9200o100
0 100020003000400050006000
Figure10.6Detention/RetentionStorage-OutflowFunctionSampleProblemPage10-21 2S/At+O,cfs
18.B
Steps7through9:
Time,hrs0.000250.500.751.001.251.501.752.002.252.502.753.003.253.503.754.004.254.50 Detem‘ion/Retention
Table10.5RoutingofFlowthroughDetentionBasin
28 26—1-0],Cf8fwmmsIn?ow,1:13I]+11.1,cfsAt Out?ow,c1:1—° \Stgp7g10—-—.10/-\1o25 35 45 453‘599050 75 120 120 0100150 268 270 1220320 580 588 4610830 1,3941,410a4501,0602,3702,45442250700 2,8303,070120160410 2,9423,240149110270 2,928321214270 180 2,8583,10812540 110 2.7622,96810320 60 .2,6622,8228010 30 2,5562,69268o 10 2,4502,566530 2,3682,450412,3042,368322,30426 Page10-22
18.B
Step10.
700600500
:2400E300200100
0 Detention/Retention
Plotthein?owandout?owhydrographs.
In?owOut?ow
1.02.03.0 4.0Tune.hrs.Figure10.7ComparisonofIn?owandOut?owHydrographsPeakOut?owDischarge:Opeak=149cfs(fromTable10.6).PeakWaterSurfaceElevation:Bylinearinterpolationofval-uesinTable10.5andUsingOM.(10.6)1105+(149—80)X(1106— 1105)=1105.53ft(209—80)TheshadedareainFigure10.13istherequiredstoragevolumecapacityforthedetentionbasin. Page10-23 peak
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10.7LISTOFV ARIABLESds/dtChange1nstorageasafunctionoftimeI = In?owincubicfeetpersecond1(1)= In?owattimeintervalt.O= Out?owincubicfeetpersecond0(t)= Out?owattimeintervalt.S = Storagevolumeincubicfeett = Time,inhours10.8REFERENCESChow,V .T,Maidment,DR.andMays,LW.1988AppliedHydrology,McGrawHillUrbanos,RR,1985,StorrnwaterDetentionOutletControlStructures,AmericanSocietyofCivilEngineers.US.ArmyCorpofEngineers,January1990,I-IEC-lFloodHydrographPackageUsersManual.US.DepartmentoftheInterior,BureauofReclamation,1987,DesignofSmallDams,ThirdEdition.US.DepartmentofTransportation,FederalHighwayAdministration,February1980,Underground.
Detention/Retention Page10-24
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18.B
EROSIONANDSEDIMENTCO
Theclearingandstrippingoflandfordevelopmentcancausehighlocalizederosionrateswithsubsequentdepositionanddamagetooff-siteproperties.Whereaserosionandsedimentationisanaturalprocess,theintensityisincreasedbydevelopmentwhichcandestroytheaestheticandpracticalvalueofotherproperties,streamsandlakes.Thepurposeofimplementinganerosionandsedimentationcontrolplanistoreducetheprocesstoanacceptablelevelwithoutplacingundueburdensonthehomeowner,builderorcommunity.A?erOctober1,1992,constructionactivitiesmustbecoveredbyaNationalPollutantDischargeEliminationSystem(NPDES)permit.Failuretocomplywiththeserequirementsmayresultina?neofupto$25,000perdayofviolationandpossibleimprisonment.TheprimaryobjectivesoftheNPDESstormwaterpermitforconstructionactivitiesareto:Reduceexcessiveerosionpotential;Minimizeexcessivesedimentation;Preventothermaterialsusedataconstructionsitefromcausingo?‘sitecontamination;Eliminatenon-stormwaterdischargesfromtheconstructionsite;Installappropriatemeasurestoreduceimpactsonwaterwaysresultingfromthe?nishedproject,andprovideacommitmentthatthesemeasureswillbemaintained;andEstablishmaintenancecommitmentsonpost-constructionsites.11.1ErosionandSedimentationProcessSoilerosionistheprocessbywhichsoilparticlesareremovedfromthelandsurfacebyWind,waterorgravity.Mostnaturalerosionoccursatslowrates;however,therateoferosionincreaseswhenlandisclearedoralteredandle?disturbed.The?vemostcommontypesoferosionincludeerosionfromrainfallimpact,sheeterosion,rillorgullyerosion,streamandchannelerosion,andwinderosion.Sedimentationisde?nedasthesettlingoutofsoilparticlestransportedbywater.Sedimentationoccurswhenthevelocityofwaterinwhichsoilparticlesaresuspendedisslowedsu?icientlytoallowparticlestosettleout.Largerparticles,suchasgravelandsand,settlemorerapidlythan?neparticlessuchassiltandclay.Effectiveconstructionsitemanagement?rstminimizesexcessivesoilerosionbykeepingthesoilstabilizedaslongaspossible,andseconddirectsrunoiffromremainingdisturbedareastolocationswheresedimentsareremovedpriortodischargetowatercourses.
Page11-1
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11.1.1FactorsIn?uencingErosionTherearefourprimaryfactorsthatin?uenceerosion:soilcharacteristics,vegetativecover,topographyandclimate.Soilcharacteristicswhichdeterminetheerodibilityofthesoilareparticlesizeandgradation,organiccontent,soilstructureandsoilpermeability.Soilswithahighproportionofsiltandvery?neclaysaregenerallythemosterodible.Organicmattercreatesfavorablesoilstructure,improvingitsstabilityandpermeability.Thisincreasesin?ltrationcapacity,delaysthestartoferosionandreducestheamountofrunoff.Soilcharacteristicsthata?‘ectsoilstabilityincludepermeabilityandin?ltrationcapacity;thelesspermeablethesoil,thehigherthelikelihoodoferosion.Vegetativecoverplaysanextremelyimportantroleincontrollingerosionbyshieldingthesoilsurface?omtheimpactsoffallingrain,slowingthevelocityofrunoff,maintainingthesoil'scapacitytoabsorbwaterandholdingsoilparticlesinplace.Topography,slopelengthandsteepnessarekeyelementsindeterminingthevolumeandvelocityofruno?‘.Asslopelengthand/orsteepnessincrease,therateofruno?‘increasesandthepotentialforerosionismagni?ed.Rainfallfrequency,intensityanddurationarefundamentalfactorsindeterminingtheamountoferosionTable11.1produced.Whenstormsarefre- Allow,“NonsmrmWateruntintenfnrain Dischargesforqe’ seoro10gduto’ ConstructionSiteserosionrisksarehigh.InCalifornia,theerosionriskperiodistypically' “SmartiesW”?re?ghting
11.2 highinthewinterrainyseason(Oc-toberthroughApril)exceptinandneartheSierraNevadaMountainsandsoutherndeserts,wheresummerthunderstormsmayoccur.StormWaterProgramsTheStateconstructiongeneralpermitprohibitsdischargeswhichdonotresultfromrainfall.However,certainnon-stormwaterdischargesareallowediftheydonotcauseasigni?cantpollutionproblem.Table11.1providesexamplesofnon-stormwaterdischargesallowableundercurrentlaw.Theneedtoprotecttheenvironmenthasresultedinanumberoflawsandsubsequentregulations/programs.Attimesthishas ErosionandSedimentControl
Firehydrant?ushingPotablewatersourcesincludingdechlorinatedwaterline?ushingUncontaminatedgroundwaterresultingfromdewateringactivitiesFoundationorfootingdrainswherethe?owsarenotcontaminatedwithprocessmaterialsNaturaloccurringwatersuchassprings.wetlandsandriparianhabitatIrrigationwaterdischargedduringseeding,plan-ting,andmaintenance,providedfertilizerandpos-ticidesareappliedcorrectlyPavementwashwatersfordustcontrolandgeneralhousekeepingpracticesprovidedspillsorleaksoftoxicorhazardousmaterialshavenotoccurredandwheredetergentsarenotusedIndividualcarwashingAirconditioningcondensation Page11-2
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resultedinoverlapandambiguitybetweenprograms.Thissituationprograms.TheFederalCleanWaterAct(CW A),asamendedin1987,forthecontrolofstormwaterpollutants.Thereare,however,other istrueforstormwateristheprincipalvehicleprogramsthatdirectlyorindirectlydealwiththecontrolofstormwaterpollutants.
I
11.2.1FederalNPDESProgramIn1972,theFederalCW AwasamendedtoprovidethatthedischargeofpollutantstowatersoftheUnitedStatesfromanypointsourceiseffectivelyprohibited,unlessthedischargeisincompliancewiththeNationalPollutantDischargeEliminationSystem(NPDES)permit.OnNovember16,1990,US.EnvironmentalProtectionAgency(EPA)published?nalregulationsthatestablishapplicationrequirementsforstormwaterpermitsforspeci?ccategoriesofindustries.Constructionactivitiesof?ve(5)acresormore(orlessthan5acresifpartofacommonplanofdevelopmentorsale)arede?nedintheregulationsasanindustrialactivity.11.2.2StateNPDESProgramInCalifornia,theNPDESstormwaterpermittingprogramisadministratedbytheStateWaterResourcesControlBoard(SWRCB)throughnineRegionalWaterQualityControlBoards.TheSWRCBhasestablishedaconstructiongeneralpermitthatcanbeappliedtomostconstructionactivitiesintheState.11.2.3MunicipalNPDESProgramMunicipalitieswithapopulationofover100,000,drainagesystemsinterconnectedwiththesemunicipalities'systemsormunicipalitiesdeterminedtobesigni?cantcontributorsofpollutantsarerequiredtoobtainanNPDESstormwaterpermit.InCalifornia,mostofthemajorurbanizedcountieshavealreadyobtainedanNPDESstormwaterpermit.MunicipalitieswithNPDESstormwaterpermitsfortheirownmunicipalseparatestormsewersystemareresponsiblefordevelopingamanagementprogramforconstructionactivitiesintheirjurisdiction.Theprogramaddressesappropriateplanningandconstructionprocedures;ensurestheimplementation,inspectionandmonitoringofconstructionsiteswhichdischargestormwaterintotheirsystems;andprovidesforeducationandtrainingforconstructionsiteoperators. andSedimentControl Page11-3
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mural. proteaim
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QualityMmgemem
REGULATION
GeneralPlanLawCaliforniaEnvironmentalQualityAct(CEQA)SubdivisionMapAct FloodPlainManagement&DrainageOrdinances
CleanWaterAct401and404Permits
ZoningOrdinances
SewerUseOrdinanceUniformBuildingCode
UniformPlumbingCode
FireCodeAir Plans1601and1603StreamBedAlternationAgreementsCoastalZoneAct
ACTIVITY
MunicipaldevelopmentobjectivesAdoptionofordinancesEnvironmentalreviewofprojeds Adoptionofordinances
Controlofvelocity
Detention/retentionBankstabilizationandoutletcontrolsPermitsDredgingandFillingin"WatersoftheUnitedStates" ClusterDevelopment
HillsideDevelopment
Landscape/Openspace
ControlofillicitconnectionsChapter70-Excavating&Grading PreventionofillicitconnedionVariousChaptersonmaterials&anolication/useStorageofMaterialsEmissionAlternationstocreekandstreambeds Regulationofnon-pointpollutionneartheoceanTable11.2
POTENTIAL
Stormwatermanagemmtcontrols,cg,pol-lutantswatershedMitigationmeasuresforredudionofpollutantsStandard/Regulationsforgrading,erosionprotection,detention/retentiondesign,and Controloferosion
Controlofsediment,pollutants,andquantityErosionandSedimentcontrolsErosioncontrol,sedimentcontrol,long-termsedimentbalance&minimizepollutantsVegetativecontrolstopreserveriparianareasMinimizeBasin-wideruno?‘&impervious VegetativeBMP‘sperimetercontrolsreduc-tionofuracil9PollutantcontrolsMinimizeerosion&sedimentationStandardsforstablecut&?llslopesPollutantcontrolsPollutantcontrols PollutantcontrolsSediment&duncontrolsPollutantcontrols
Pollutantcontrols
ExistingRegulationsforStormWaterPollutionControl
andSedimentControl Page11-4
Slope&erosionrash-idiomMayincluderevegetationorstabilizalim.
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11.2.4OtherStateandLocalProgramsInmanycases,stormwaterpollutioncontrolmayalreadybeachievedbyexistingregulationsorprograms.InCalifornia,theGeneralPlanLawandtheCaliforniaEnvironmentalQualityAct(CEQA),provideabasisformunicipalitiestoreviewandcommentonallprojectswithintheirjurisdiction.UndertheGeneralPlanLaw,municipalitiesarerequiredtodeveloppoliciesandregulationswhichguidedevelopmentwithinthemunicipality.Eachdevelopmentprojectisthenreviewedforconformancewiththesepolicies.UnderCEQA,projectsarealsosubjecttoreviewandcommentforanyadverseimpacttheprojectsmayhaveontheenvironment,includingimpactsfromstormwaterdischarges.OtherregulationswhichmaybeapplicablearesummarizedoninTable112.ActionstakenundertheseregulationsshouldbeintegratedwiththeStormWaterPollutionPreventionPlanSWPPPtopromoteconsistencybetweenactivitiesrequiredbylocalandStateauthorities.11.3StormWaterPollutionPreventionPlansThefollowingdiscussionprovidesTabieil.f3guidelinesforthedevelopmentand Baszmag??gfsi?epgg?mpppimplementationofastormwaterpol-lutionpreventionplanforaconstruction0Usescalelargeenoughtodistinguishexistingand. _ _ . proposedfeaturesofthesitepr0ject.PreparingandimplementingLt. txtd“41bedm- .ocalonmamuseenmeoneretheSWPPPneednotbetime-consum-boundaryp y ppenying,andcanbestbeaccomplishedusing-. .. . 0Sitemap(s)shouldshow:and/orslightlyrewsmgcurrentplanmng,_Topographydesignandconstructionactivitiesem—- Limits“Construdim1db . - ConceptualproiectlayoutpOyeymOStpI‘OJCCtS.- Surfacewaterbodies.watercourses,knownwet-lands,springsandwells- Locationswheredrainageleaves/WillleavetheTheSWPPPmustbepreparedbeforepropertyconstructionbegins,ideallyduringthe- Eastinglanduse. . . - Bustingvegetationpr0jectplanmnganddes1gnphases.Or,_Steepo,unstaueslopesitmaybecompletedattheendofthe' Areas“59leStore5°“Sandwast?- AreasofcutandtilldesignphaseorearlyintheconstructionExistingandplannedpavedareasandbuildingsphase.ImplementationoftheSWPPPbeginswhenconstructionbegins,typi-callybeforeinitialclearing,grabbingandgradingoperationssincetheseactivitiesusuallyincreaseerosionpotentialonthesite.DuringconstructiontheSWPPPshouldbereferredtofrequentlyandrefinedbytheownerandcontractorsaschangesoccurinconstructionoperationswhichsigni?cantlyaffectsthepotentialfordischargeofpollutants.Thefollowingguidelinesareorganizedaccordingtothephasesofaconstructionproject:planning,designandconstruction.Foreachphase,guidelinesaregivenonhowto SedimentControl Page11-5
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incorporateSWPPPpreparationintotheworkeffort.AseriesofworksheetsisincludedattheendofthischaptertoaidinpreparationoftheSWPPP.11.3.1PlanningPhaseTheplanningphaseisthesourceofmuchoftheinformationneededfortheSWPPP.Thebasisforstormwaterpollutioncontroldecisionsarealsomadeatthisphaseviathenormalreviewprocess.ThefollowingdiscussionaddressesfouractivitieswhichoccurduringtheplanningphasethatareimportanttopreparationoftheSWPPP:AssessSiteConditionsDevelopBaseMapSelectPost-ConstructionBestManagementPlan(BMP)EstablishLong—TermMaintenanceAgreementsAssessSiteConditions:Theplanningphaseofanyconstructionprojectde?nesthecharacteristicsofthesiteandhowthesecharacteristicswillimpacttheproject.Infor-mationonwhatwillbebuilt,howitwillbeconstructed,drainagepatterns,soils,topography,rainfallandspecialsiteconditionsisusuallyobtainedandusedforinitialplanningofpublicworksordevelopmentprojects.DevelopaBaseM@(S)iThedesignerwillgenerallyprepareasiteanalysisaseitheraformaldocumentorasaninformalplan.Thesiteanalysisreviewsthesite'sphysicalconditions,adjacentareas,constraints,applicablezoninganddevelopmentre-quirements.Toavoidduplicationofeffortandreducecosts,thedesignershouldprepareasitebasemaptobesharedbytheSWPPPandothersiteanalyses(seeTable11.3).Worksheet2attheendofthissectionincludesitemsconsiderednecessaryforpreparingatopographybasemap.SelectPost-ConstructionBMPs:Post-constructionBMPsarethe?nalimprovementstoandcon?gurationoftheprojectwhicharedesignedtocontrollong-termstormwaterpollution.Post-constructionBMPsarenormallyselectedintheplanningphaseinconjunctionwiththeapprovalofthetentativemap,designedduringthedesignphaseofaprojectandcompletedtothesatisfactionofthemunicipalityand/ortheultimateowner.EstablishMaintenanceAgreements:Thegeneralpermitrequiresthattheprojectownerarrangeforthemaintenanceofanydrainage/stormwaterpollutioncontrolmeasuresafterconstructioniscomplete.11.3.2DesignPhaseTherearethreeprincipalactivitieswhicharetypicallyincorporatedintotheSWPPPduringthedesignphase:
0 PrepareProjectSitePlan0 De?neBMPObjectives
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0 DesignPost-ConstructionBMPsPrepareProjectSitePlan:Numerousstudies,reportsanddocumentsaretypicallypreparedduringpreliminaryand?naldesignasthebasisformanydecisionsabouttheproject.AHydrologyReport,DrainageStudyorsimilardocumentistypicallyrequiredaspartofprojectinfrastructure.TheHydrologyReportshouldaddressthedesignstormwhichwillbeusedforerosionandsedimentationcontrol.TheSoilsReportisnormallypreparedbaseduponsitesoilsamplingwhichwillidentifysoilconstraints,designcriteria,slopestability,etc.Bothofthesereportsareusedbytheengineerinpreparationofthepreliminarygradinganddrainageplan.TheyalsoformthetechnicalbasisforselectionoferosionandsedimentationcontrolBMPsandpost-constructionBMPs.ThispreliminarysiteplanincludesseveralitemswhicharerequiredfortheSWPPP:
0 Locationsofbuildingsandpavedareas0 Proposed?owpaths:0 On-site?owpathswhereerosionduringconstructionmayoccuranderosionandsedimentationBMPsshouldbeapplied.Locationswhererunoffwillleavetheproperty.Diversionof,orconveyanceforupstreamrunolfLocationsof?oodcontrolfacilitiesandpost-constructionBMPs.Approximatelocationswherecutand?llwilloccur.Accesspointsforconstructiontra?ic.Areaswhereexistingvegetationmaybepreserved.AreasmostsuitablefortheContractor'syard,materialstorageareaandvehiclemaintenancearea(considerlocationinareastobepaved).DefineBMPObjectives:Duringthe?nalprojectdesignprocesstheengineerwillpreparedetailedgradingplans,pavinganddrainageplansandotherplansnecessaryforthesuccessfulconstructionoftheproject.FortheSWPPPtobecompatiblewiththeotherengineeringplans,themostpracticalprocessmaybefortheengineertodevelopBMPobjectivesfortheconstructionperiodbasedoncontractoractivities,andthegradinganddrainageplanforthesite.TheBMPobjectivesshouldinclude,ataminimum,thefollowingitems:PracticeGoodHousekeepingContainWasteMinimizeDisturbedAreaStabilizeDisturbedareaProtectSlopesandChannelsControlPerimeterofSiteControlInternalErosion Sediment Page11
18.B
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AnarrativediscussionoftheseobjectivesshouldbepreparedsuitableforinclusionintheSWPPP,aswellastoguidetheBMPselectionprocess.Worksheets3,4and5attheendofthissectionmaybeusedinconjunctionwiththesitemaptode?neBMPobjectives.11.3.3ConstructionPhaseFouractivitieswhichmustbeaddressedduringthebiddingandmobilizationperiodoftheconstructionphaseincludeselectionofBMPs,CompletionoftheSWPPP,submittaloftheNoticeofIntent(N01)andtrainingof?eldpersonnel.SelectBWS:Eithertheowner,theowner'sdesignconsultant,orthecontractormayperformtheseactivities,atthediscretionoftheowner.Theconstructiondocumenta-tionshouldspecifywhattheresponsibilitiesoftheownerandthecontractorarewithregardtostormwaterpollutioncontrolbefore,duringandafterconstruction.CompletetheSWPPP:TheSWPPPshouldbedirectedatpersonnelontheconstructionprojectsuchassupervisors,foremenorinspectors.Itshouldprovidespeci?cguidanceonactionstobetakenbythesepersonnelandshouldbepresentedinaformatwhichaccommodatesdailyuse.ContentsoftheSWPPP:TheSWPPPshouldprovideasimplenarrativeanddiagramthatlocatestheconstructionsite,identi?espotentialpollutantsourceson-site,showsthelocationofthemanagementpracticestobeusedtominimizeerosionandsedimentationduringconstruction,describesmeasureswhicheliminatepollutionofstormrunoffbyanychemicalsandmaterialsusedduringtheconstructionprocessandshowsareasforlong-termpost-constructioncontrolmeasures.GuidelinesforSWPPPPreparation:TheSWPPPshouldbeanindependentdocumentwhichconciselyprovidestheerosion,sedimentationandpollutioncontrolmeasurestobeused.Allnecessaryplanningworkhasbeendoneduringthesiteplanninganddesignprocess.The?nalstepconsistsofconsolidatingthepertinentinformationanddevelopingitintoaspeci?cerosionandsedimentcontrolplanfortheproject.Theplanconsistsofthreeparts:asiteplan,narrativeandaninspectionandmaintenanceplan.Thesiteplanisoneofaseriesofmapsordrawingspictoriallyexplaininginformationcontainedinthenarrative.Thenarrativeverballyexplainstheproblemsandtheirsolutionswithallnecessarydocumentation.TheinspectionandmaintenanceplandescribestheprocedureswherebytheBMPsaremaintainedingoodande?‘ectivecondition. 0 SitePlan:TheSWPPPsiteplanshouldbeasimpleillustrationoftheprojectsiteshowingkeyphysicalfeaturesandthelocationoferosionandsedimentcontrolmeasuresandallconstructionoperationmeasuressuchasentrance Control Page11-
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drivesandconstructionyards.TotheextentpossiblelocallyrequiredgradingplansshouldbeusedinpreparingtheSWPPP. 0 ProjectLocationMap:A?gureshowingtheprojectsiteandthesurroundingarea(one—quartermilebeyondtheprojectlimitsandwithadditionalareasifnecessarytoshowlocaldrainagepatterns).AUSGS1:24,000scaletopographicmapisoftenusedasaprojectlocationmap. 0 DetailedSitePlan:Aseriesofoneormoredrawingsoftheconstructionsiteatascale,typically,nosmallerthan1inch=400feet,suf?cienttoclearlyshowon—sitedrainagepatternsandthelocationoferosionsedimentationcontrols.Worksheet2maybeusedasachecklistforpreparingasiteplan.Whenthelayoutofthesitehasbeendecidedupon,aplantocontrolerosion,sedimentationandotherpollutantsfromthedisturbedareasmustbeformulated.
O Narrative:Thenarrativeisawrit-tenstatementwhichexplainsthe Table114erosion,sedimentandotherpollut-Contentsoi'theantcontroldecisionsmadefora N°?°°“Wentparticularprojectandthejusti?ca-oOwnerIDeveloper'sName,Address,tionforthosedecisions.Thenarra-andTe'epmne“WWtiveshouldcontainconcisecondi-0constructionSiteInformationtions,constructionschedulesand'PWtName?“Address.. . . - ConstructionStartandCompletionotherpertinentitemswhicharenot Date'n . . .contaiedonatypicalsueplan.SiteMapWorksheets3,4,5and6arein— - Existingandproposedbuildings ' ' andpavedareastendedforIguidance‘onprepanng_Stormdrainagesystemthenarrativeportionsofthe— AdjacentstreetsSWPPP. oReceivlngWaterInformation- MunicipalitywhereprojectislocatedThenarrativeisimportanttothe- Nearestdownstreamwatercourseconstructionsuperintendentandorwaterbody(fromusesMaporinspectorwhoareresponsiblefor “lumen”implementingtheplan.ItprovidesoTypeofConstructlon- Sizeofsitesthemwithasinglereportthatde-scribeswhereandwhenthevariousBMPsforcontractoractivitiesand - Changeinimpervioussurfacearea
0MaterialHandllnglManagementPracticespost-constructioncontrolsshouldbe - Materialstobeusedduring- constructionmStaued' - Managementpracticestocontrolpossiblepollutants0 Monitoring,InspectionandMaintenancePlan:EveryBMPinstalledonaconstructionsitemustbe 0RegulatoryStatus
andSedimentControl Page11-
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checkedperiodicallyandmaintainedsuf?cientlytoensureproperperfor—mance.TheconstructiongeneralpermitrequiresthatanInspectionandMaintenancePlanbeimplementedwhichassignspersonnelresponsibleforBMPinspectionandmaintenance,requiresmaintenanceofanyBMPwhosee?eaivenessiscompromised,andkeepsinspectionandmaintenancerecordson?leforatleastthreeyears. 0 NoticeofIntentW01):TheconstructiongeneralpermitrequiresthatanNOIbesubmittedtotheStateWaterResourcesControlBoard(SWRCB)priortothestartofconstruction.TheN01isnotanapplicationforapermitandisnotsubjecttoapproval-itsimplynoti?estheSWRCBthataconstructionprojectisabouttobegin,thelocationoftheproject,theresponsiblepartiesandacer-ti?cationthataSWPPPhasbeenpreparedandwillbefollowed.TheowneroftheconstructionprojectisresponsibleforsubmittingtheN01.OwnerswhosubmitNOIsareauthorizedtodischargestormwaterundertheconditionsoftheconstructiongeneralpermit.ThecompletedNOImustbeattachedtotheSWPPP.Priortoissuanceofagradingpermit,anexecutedcopyoftheN01shallbeon?lewiththeCity.IfaconstructionprojectsbeginsbeforetheN01issubmitted,theN01shouldbesubmittedassoonaspossiblethereafter.Allconstructionprojectsaresubjecttothegeneralpermitincludingprojectswhichbeganbeforethegeneralpermitwasissued.OwnersofactiveconstructionprojectswithoutaSWPPPinplacearedischargingstormwaterwithoutanNPDESpermitandareinviolationoftheCleanWaterAct.Violationsmayresultin?nesorimprisonment.FailuretocomplywiththeCleanWaterActmayresultin?nesupto$25,000perdayofviolationoranyotherappropriatesanction. 0 ContentsoftheN01:TheconstructiongeneralpermitandN01areincludedattheendofthissection.MuchoftheinformationprovidedontheN01shouldhavebeendevelopedfortheSWPPP.Table11.4summarizesthisinformation. 0 TrainPersonnel:TrainingisimperativetothesuccessoftheBMPs.AdequatetrainingisrequirediftheseBMPsaretobeinstalledandmaintainedproperly.TheseBMPswillfailifnotproperlyinstalledandmaintained.Aneffectivetrainingprogramisbasedonfourobjectives:
0 Howtoidentifyastormwaterpollutionproblem;0 Howtode?nesolutions; andSedimentControl Page11—
18.B
0 Makingeveryemployeeresponsibleforstormwaterpollutionanditssolution;andO SolicitingfeedbacktoimprovetheBMPs. 0 ConstructionOperations:Theconstructionbidpackagewillusuallycontainspeci?cationsthatthecontractorcomplywiththeconstructiongeneralpermitbyimplementingtheSWPPP.Thismaybecontractually,ortheownermayrequirethecontractortopreparesomeoralloftheSWPPP,andsubmittheN01tocovertheparticularconstructionactivity.Duringconstruction,thecontractorthenimplementstheBMPsaccordingtotheSWPPP.Theowner/contractorisalsoresponsibleformaintenanceandinspectionoftheBMPsforerosionandsedimentcontrolandothercontractoractivities,andtheinstallationofthepost-constructionBMPs.Becausesiteconditionswillvaryduringconstruction,theSWPPPshouldberevisedasnecessary,withanychangeshighlightedonthecopymaintainedattheconstructionsite.Thereisnoformalrevisionprocess;uponinspectiontheSWPPPmustre?ecttheexistingstatusandconditionofthesite. 0 ConstructionCompletion:Duringconstruction,thepost-constructionBMPsshouldhavebeeninstalledaccordingtoconstructionplansandspeci?cations.Thegeneralconstructionpermitremainsinforceuntilconstructioniscomplete.Post-constructionBMPsshouldbeproperlyinstalled,andresponsiblepartiesmustbedesignatedandfundscommittedtothelong-termmaintenanceneedsoftheseBMPs.TheSWRCBmustbenoti?edoftwoconditions:
0 Whenconstructioniscomplete;and0 Whenchangeofownershipoccurs.TheSWRCBrequiresthataNoticeofTerminationbeprovidedforsuchnoti?cation.Ifanewownerisassumingresponsibilityforsomeoralloftheconstructionsite,arevisedN01andarmualfeemustalsobesubmitted,andtheSWPPPmustberevisedbythenewownerforanychangesinconstructionconditions.11.4BestManagementPracticesSelectionBestmanagementpracticesaregenerallyselectedinathreestepprocess:
0 De?neBll/?JObjectives:De?nelocationswhereerosionislikelytooccurandwhereotherconstructionrelatedpollutionmaybegenerated. 0 Identijj/BA/IPCategory:SelecttheappropriatecategoryorcategoriesofBMPswhichaddresseachobjective.
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0 SelectAppropriateBAlPs:BMPsforcontractoractivitiesanderosionandsedimentationcontrolarechosenfromeachcategorybasedon,siteconstraints,constructionrequirementsandcost-effectiveness.11.4.1DefineBlVIPObjectivesEachconstructionprojectisuniqueTherefore,anunderstandingofthepollutionrisksoftheconstructionactivityisessentialforselectingandimplementingBMPs.De?ningtheserisksrequiresreviewofthecharacteristicsofthesiteandthenatureoftheconstruction.Oncethesepollutionrisksarede?ned,BMPobjectivesaredevelopedandBMPsselected.TheBMPobjectivesforconstructionprojectsareasfollows:PracticeGoodHousekeeping:Performactivitiesinamannerwhichkeepspotentialpollutantsfromdrainingorbeingtransportedoff—sitebymanagingpollutantsourcesandmodifyingconstructionactivities.ContainWaste:Disposeofallconstructionwasteindesignatedareas,andkeepstormwaterfrom?owingontoorofftheseareas.MinimizeDisturbedAreas:Onlyclearlandwhichwillbeactivelyunderconstructioninthenearterm,minimizenewlanddisturbanceduringtherainyseasonandavoidclearing/disturbingsensitiveareasandotherareaswheresiteimprovementswillnotbeconstructed.StabilizeDisturbedAreas:Providetemporarystabilizationofdisturbedsoilswheneveractiveconstructionisnotoccurringonaportionofthesite.Providepermanentstabilizationduring?nishgradingandlandscapethesite.ProtectSlopesandChannels:Outsideoftheapprovedgradingplanarea,avoiddisturbingsteeporunstableslopes.Safelyconveyrunolffromthetopoftheslopeandstabilizedisturbedslopesasquicklyaspossible.Avoiddisturbingnaturalchannels.Stabilizetemporaryandpermanentchannelcrossingsasquicklyaspossibleandensurethatincreasesinrunoffvelocitycausedbytheprojectdonoterodethechannel.ControlSitePerimeter:Upstreamruno?‘shouldbedivertedaroundorsafelythroughtheconstructionproject.Localcodesusuallystatethatsuchdiversionsmustnotcausedownstreampropertydamageorbedivertedintoanotherwatershed.Runofffromtheprojectsiteshouldbefreefromexcessivesedimentandothercontaminants.ControlInternalErosion:Detainsediment-ladenwatersfromdisturbed,activeareaswithinthesite. andSedimentControl Page11-
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11.4.2IdentifyBMPCategoriesOncetheBMPobjectivesarede?ned,itisnecessarytoidentifythecategoryofBMPsthatisbestsuitedtomeeteachobjective.AcategoryisagroupingofBMPswhicharerelatedinhowtheycontrolstormwaterpollution,seeTable11.5.TheparticularBMPselectionfromeachcategorydependsonthespeci?csiteconditions,construc-tionactivitiesandcost-effectivenessconsiderations.
ErosionandSedimentControl Page11-
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TodeterminewheretoplacecategoriesofBMPs,amapoftheprojectsiteispreparedwithsuf?cienttopographicdetailtoshowexistingandproposeddrainagepatternsandexistingandproposedpermanentstormwatercontrolstructures.Theprojectsitemapshouldidentifythefollowing:Locationswherestormwaterentersandexitsthesite.Includebothsheet?owandchannel?owfortheexistingand?nalgradedcontours.Locatepermanentstormwatercollectionandcontrolstructures.Identifylocationssubjecttohighratesoferosionsuchassteepslopesandunlinedchannels.Long,steepslopesover100'inlengthareconsideredasareasofmoderatetohigherosionpotential.Categorizeslopesas 0 Lowerosionpotential:0to5percentslope.
0 Moderateerosionpotential:5to10percentslope.
0 Higherosionpotential:slopegreaterthan10percent.Identifysensitiveareaswhichmustnotbedisturbed,aswellasotherareaswheresiteimprovementswillnotbeconstructed.Establishclearinglimitsaroundtheseareastopreventdisturbancebytheconstructionactivity.Identifytheboundariesofdrainageareasifthesitehasmorethanonedrainageoutlet.Thencalculatetheappropriateareaofeachdrainagearea.De?neareaswherevariouscontractoractivitieshavealikelyriskofcausingapollutantdischarge.Withthissitemapinhand,categoriesofBMPscanbeselectedandlocated.Ingeneral,thecategoriesofBMPsshouldbeconsideredintheorderlistedinTable11.5.Thisisbecauseitismoreeffectivetopreventerosion/pollutionthantoremovesediment/pollutants,andbecauseerosionpreventionisachievedmostcost-e?ectivelybyplanningbeforeconstructionbeginsandphasingconstructionactivities.AsshowninTable11.5,mostBMPsachievemorethanoneobjective.ThisshouldbetakenintoaccountwhenselectingBMPstoachievemaximumcost-effectiveness.
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11.4.3SelectingBMPsforContractorActivitiesCertaincontractoractivitiesmaycausepollutionifnotproperlymanaged.Contractoractivitiesthatimpactstormwaterqualityinclude:ConstructionPractices0 DewaterOperations0 PavingOperations0 StructureConstructionMaterialManagement0 MaterialDeliveryandStorage0 MaterialUse0 SpillPreventionandControlWasteManagement0 SolidWaste0 HazardousWaste0 ContaminatedSoils0 Sanitary/SepticWastesVehicleandEquipmentManagement0 Cleaning0 Fueling0 MaintenanceDuringtheselectionoftheproposedBMPs,eachcontractoractivityshouldbegivenconsideration.Theseconsiderationsshouldincludethefollowing:1W?BMPsmaybedifferentonrainydaysvs.drydays,winterdaysvs.summerdays.Forinstance,materialstorageareasmaybecoveredwithtarpduringtherainyseason,butnotinthesummer.Hewmuehmaterialisused?LessstringentBMPsmaybeusedifa"small"amountofmaterialisused.Hey!muehwaterisusgl?Themorewaterusedandwastewatergenerated,themorelikelythatpollutantstransportedbythiswaterwillreachthedrainagesystemorbetransportedolf-site.Washingoutoneconcretetruckona?atareaofthesitemaybesufficient,butacontainmentpitshouldbeconstructedifanumberoftruckswillbewashedoutatthesamesite.Considerationshouldbegiventorecyclingwaterwheneverpossible.Whatarethesiteeenditiens?BMPsselectedwilldifferdependingonwhethertheactivityisconductedonaslopeor?atground,nearadrainagestructureor Page11-16
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watercourse.Anticipatingproblemsandconductingactivitiesawayfromcertainsensitiveareawillreducethecostandinconvenienceofperforming’BMPs.Whataboutaccidents?Pre—establishingaBMPforeveryconceivablepollutantdischargemaybeverycostlyandsigni?cantlydisruptconstruction.Asarule,establishcontrolsforcommonactivitiesandbepreparedtorespondquicklytoaccidents.KeepinmindthattheBMPsaresuggestedpracticeswhichmayormaynotapplyineverycase.11.4.4SelectingBNIPsforErosionandSedimentControlBMPsforerosionandsedimentcontrolareselectedtomeettheBMPobjectivesbasedonspeci?csiteconditions,constructionactivitiesandcost-effectiveness.DifferentBMPsmaybeneededatdifferenttimesduringconstructionconstantlychangingsiteconditions.TheremainderofthissectiondiscusseshoweachBMPobjectiveshouldbemettoaddressthedynamicsofatypicalconstructionproject.MinimizeDisturbedAreasThe?rststepforselectingBMPsistocomparetheprojectlayoutandschedulewithon-sitemanagementmeasuresthatwhereappropriate,canlimittheexposureoftheprojectsitetoerosionandsedimentation.TheseBMPsallhaveasimilargoal,whichistominimizetheamountofthesitesubjecttoerosion.ConsiderthefollowingBMPs:1 Donotdisturbanyportionofthesiteunlessanimprovementistobeconstructedthere.2 Thestagingandtimingofconstructioncanminimizethesizeofexposedareasandthelengthoftimetheareasareexposedandsubjecttoerosion.3 Thestagingofgradingoperationsshouldlimittheamountofareasexposedtoerosionatanyonetime.Onlytheareasthatareactivelyinvolvedincutand?lloperationsorareotherwisebeinggradedshouldbeexposed.Exposedareasshouldbestabilizedassoonasgradingiscompleteinthearea.4 Retainexistingvegetationandgroundcoverwherefeasible,especiallyalongwatercoursesandthedownstreamperimeterofthesite.5.Donotclearanyportionofthesiteuntilactiveconstructionbegins.6.Quicklycompleteconstructiononeachportionofthesite.
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7.Installlandscapingandotherimprovementsthatpermanentlystabilizeeachpartofthesiteimmediatelyafterthelandhasbeengradedtoits?nalcontour.8.MinimizetheamountofdenudedareasandanynewgradingactivitiesduringthewetmonthsofOctoberthroughApril.9.Constructpost-constructionstormwatercontrolfacilitiesearlyintheprojectanduseforsedimenttrapping,slopestabilizationandvelocityreductionduringtheconstructionperiod.StabilizeDisturbedAreasThepurposeofsitestabilizationBMPsistopreventerosionbycoveringdisturbedsoil.Thiscoveringmaybevegetative,chemicalorphysical.Locationsonaconstructionsitewhichareparticularlysubjecttoerosionandshouldbestabilizedassoonaspossibleinclude:SlopesHighlyerosivesoilsConstructionentrancesStreamchannelsSoilstockpilesStabilizedisturbedareaskeepinginmindthefollowing:1.Asarule,nativevegetationinundisturbedareasrepresentsthe"baseline"forerosioncontrol.TheSubdivisionMapActprovideslanguagetolimiterosiontopre—developmentlevels.Thus,retainingnativevegetationinundisturbedareasprovidesthe?rstandbestlineofdefenseagainsterosionandsedimen-tationanddoessoatleastcosttothecontractor,whileminimizingtherequirementtorevegetateorprovidestructuralcontrols.2.Temporarygroundcoversprotectsoilfromerosionuntilpermanentvegetationcanbeestablishedorpermanentconstructionisinstalled.3.Timingofrevegetationeffortsiscriticalinaridregions.Revegetationshouldbeperformedjustbeforeorduringtherainyseasonunlessanirrigationsystemexistson—site.4 Certainrelativenon-toxicchemicals,suchasmagnesiumchlorideorlignumsulfate,maybeusedforsoilstabilizationwhererevegetationisnotpractica—ble.Thesetwochemicalsdonothaveanadverseimpactonplantlifeandarealow-coststabilizationtreatment. andSedimentControl Page11-
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5 Groundcoverofgravel,decomposedgranite,woodchipsormulchmaybeused.ProtectSlopesandChannelsThenextstepistoprovideslopeprotectionindisturbedareas.ConsiderthefollowingBMPs:l. Avoidconstructingsteepslopes.Increasingslopelengthandsteepnessincreasesthevelocityofrunoff,whichgreatlyincreasesitserosiveenergy.Ifslopesteepnessisdoubledwhileotherfactorsareheldconstant,soillosspotentialisincreased2.5times.Ifbothslopesteepnessandlengtharedoubled,soillosspotentialisnearly4timesgreater.Divertorinterceptstormwaterbeforeitreacheslongand/orsteepslopesusingtemporarydikes,swalesorpipeslopedrains.Longsteepslopesshouldbeterracedatregularintervals.Terraceswillslowdowntherunoffandprovideaplaceforsmallamountsofsedimenttosettleout.Slopebenchesmaybeconstructedwitheitherditchesalongthemorbackslopedatagentleangletowardthehill.Thesebenchesandditchesinterceptrunoffbeforeitcanreachanerosivevelocityanddivertittoastableoutlet.Slopestabilityforcutsand?llsshouldconformtogradingordinances,theUniformBuildingCodeand/orsoilsreportrecommendations.Overland?owvelocitiescanbereducedbycreatingaroughsurfaceforrunofftocross.Drivingabulldozerupanddownaslopecreatestreadmarksparalleltoslopecontours.Theseminiatureterracesbothslowrunoffvelocityandprovide?atplacesforvegetationtohold.Rakingordiskingthesoilsurfacebeforeseedingalsokeepsruno?‘velocitiesdownandincreasesplantestablishmentrates.Vegetation,onceestablished,will?irtherreducerunoffrates.Buildcheckdamsorotherenergydissipationstructuresinunlineddrainagechannelstoslowrunoffvelocityandencouragesettlementofsediments.ConstructvelocityreducingstructuresorIiprapliningsassoonasstormwateroutfallsareconstructed.
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ControlSitePerimeterPerimetercontrolestablishesbarriersalongtheperimeteroftheprojectsite.SeveralstrategiesforusingBMPsforperimetercontrolmaybeconsidered.1.Divertupstreamrunoffaroundorthroughtheconstructionsitewithnocontactwithexposedsoilsorconstructionactivities.2 Runofffromareasupslopeofthosethathavebeendenudedshouldnotbeallowedtocrosstheexposedsoils,particularlywhenthedenudedareasareonslopes.3 Leaveavegetativebarrieralongthepropertyboundaryandinteriorwatercourses.4 Trapandremovesedimentinrunofffromtheconstructionsitebeforeitleavestheproperty.Perimetercontrolshouldbeplacedeverywhererunoffentersorleavesthesite.Perimetercontrolsforallbutthesmallestprojectswillbecomeoverloadedbybothrunoffandsediment,andshouldbesupplementedbyadditionalcontrolswithintheinterioroftheconstructionsite.Runoffwhichis?eeofexcessivesedimentshouldbedivertedarounddisturbedareasoftheconstructionsiteandotherlocationshighlysusceptibletoerosion.Ifpossible,theruno?‘shouldbedivertedtostabilizedareaswheretherunoffcanberetainedandallowedtosoakintotheground,orintostabilizeddrainagewayswithsuf?cienthydrauliccapacitytoconveyanyincreasesin?ow.Whenpossible,permanentdrainagestructuresshouldbebuiltearlyintheconstructionprojecttocollectandconvey?owdiversions.Runoffdiversionsshouldbeconsideredinthefollowingsituations:1 Divertupslopewateraroundunvegetatedareasoftheconstructionsitewithdikes,swalesortemporarystormdrains.2.Divertrunofftosoilsabletoabsorbwater3 DikesandditchesarethetwomostcommonBMPstodivertuplandruno?“awayfromadisturbedarea;tointerceptrunofffromadisturbedarea;tointerceptruno?‘oncutorfillslopes,andtopreventrunofffromenteringadisturbedarea,suchasagroupofbuildingpads.4 Upstream,off-site?owcanbetakentothedownstreamareaoftheprojectsiteandreleasedbackintothenaturaldrainagepattern.Aspreadingbasinor ErosionandSedimentControl Page11-20
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othertemporaryformofenergydissipatormaybeneededforlargediverted?owsoratthebaseofsteepslopes.Perimeterchannelorbermtodivert?owshouldnot?ood,causeerosionorotherwiseadverselyimpactsurroundingproperties.Whilestructuresdesignedforsedimentcontrolaretemporaryandaredesignedtoremovesedimentfromfrequentstorms,theymustbedesignedtosafelydetainand/orcarrymajor?oodswithoutincreasedriskofpropertydamage.ControlInternalErosionOnceallothererosionandsedimentcontrolBMPshavebeenexhausted,excessivesedimentshouldberemovedfromthestormwaterbothwithinandalongtheperimeteroftheprojectsite.Theappropriatecontrolsworkonthesameprinciple:thevelocityofsediment—ladenrunoffisslowedbytemporarybarrierbasinwhichpondthestormwatertoallowsedimentstosettleout.SedimenttrappingBMPsincludesiltfence,strawbales,sandbags,brushorrock?lters,stormdraininletprotection,sedimenttrapsandbasins.Appropriatestrategiesforimplementingsedimenttrappingcontrolsinclude:1.2.Directsediment-ladenstormwatertotemporarysedimenttraps.Locatesedimentbasinsandtrapsatlowpointsbelowdisturbedareas.Protectallexistingornewlyinstalledstormdrainagestructuresfromsedimentcloggingbyprovidinginletprotectionforareadrainsandcurbinlets.Directoff—siteand/orsediment-?eestormwateraway?omdenudedareasandawayfromtemporarysedimenttraps.Constructtemporarysedimenttrapsorbasinsatthedrainageoutletforthesite.Thedischargefromonebasinshouldnotentertheinletofanotherbasin.Excavatepost-constructionstormwaterdetentionbasinsearlyintheproject,usethemassedimentationbasinsduringconstruction,removeaccumulatedsediment,andlandscapethebasinwhentheupstreamdrainageareaisstabilized.Temporarysedimentbarrierssuchassiltfences,strawbaleandsandbagbarriersandbrushorrock?ltersshouldonlybeusedinareaswheresheet?owrunoffoccurs.Temporarysedimentbarriersshouldonlybeinstalledalongacontourlineonrelatively?atslopeswheresuf?cientareaforpondingsediment-ladenwaterexists.Thesebarriersarenotappropriateforlargedrainageareasunlessthedrainageareaissplitandadditionalbarriersare ErosionandSedimentControl Page11-
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placedupstreamsothattheincrementaldrainageareatoeachbarrierislessthanoneacre.Protectallexistingornewly-installedstormdrainagestructuresfromsedimentcloggingbyprovidinginletprotectionforareadrainsandcurbinlets.11.4.5SelectionofPost-ConstructionBMPsPost-constructionBMPsaregenerallyorientedtowardmunicipalrequirementsestablishedinamasterplanorordinance.Formostprojects,therewillbenosingleBMPwhichaddressesallthepost—constructionstormwaterqualityproblems.Instead,amulti—levelstrategywillbeworkedoutwhichincorporatessourcecontrols,on—sitetreatmentcontrolsandcommunity-widetreatment.SourceControlBMPsPlanningManagement:Municipalitiesmayregulatethroughzoningordinances,subdivisionordinancesorbuffer/setbackrequirementstheamountofrunofffromasite.Themunicipalitymaydiscouragedevelopmentinenvironmentallysensitiveareas.Foraspeci?cproject,thedevelopermayachievethesegoalsbyminimizingtheimperviousareathatisdirectlyconnectedtothedrainagesystem,andbyincorporatingsetbacksfromsurfacewaters.MaterialsManagement:TheobjectiveoftheseBMPsistominimizetheopportunityforrainfallorrunofftocomeintocontactwithpollutants.Commercialandindustrialprojectdesignsshouldbeadoptedtokeeppollutantstorage/use/disposalareasoutof?oodplains,awayfromdrainagepaths,undercoverand/orincontainmentareas.SpillPrevmtien:SpillpreventionBMPsshouldbeincorporatedintoprojectdesignsbyusingcontainments,leakresistantmaterialsandaccesswaysforpropercleanup.Illegalgimping:Signsmaybeinstalledatinletsoralongdrainageways,andbrochuresmaybedistributedtoresidents/tenantsofnewfacilitiestoincreaseawarenessaboutillegaldumping.Illegalgenneetiens:Mostmunicipalitieswillhaveordinances,andmayrequireinspectionofconstructionsitestopreventconnectionstothestormwatersystemfromnon-stormwatersources.Thisincludesconnectionsof?oordrains,washdownareas,septictankover?owsandthelike.Street/StormDrg'nMaintenance:Infrastructuremaintenanceafterconstruc-tionwillberequiredandmaybeperformedbythemunicipalityorpropertyowner.SuchBMPswouldincludestreetsweeping,catchbasincleaningand SedimentControl Page11-22
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drainagestructurecleanouts.Maintenanceconsiderationsshouldbeintegratedintotheinfrastructurewhendesigned.11.5MeasuringBMPPerformanceOneofthe?nalstepsinthepreparationoftheStormWaterPollutionPreventionPlan(SWPPP)istodevelopaprogramtomonitorhowwelltheBMPswork,andtoevaluateWhetheradditionalBMPsarerequired.Theconstructiongeneralpermitrequiresthatamonitoring,inspectionandmaintenanceplanwiththefollowingobjectivebeacomponentoftheSWPPP:
0 ToinspectBMPsannually,aswellaspriortoandafterastormevent. 0 ToaidinimplementationoftheSWPPP.
0 TomeasuretheeffectivenessoftheBMPs.Tomeettheseobjectivesthemonitoringefforthastheseelements 0 SiteInspection0 Certi?cationofCompliance0 BMPsMonitoring0 RecordKeepingo SWPPPReviewandModi?cations11.5.1SiteInspectionsInspectionsbeforeandafterastormeventarerequiredbytheconstructiongeneralpermit.Attheonsetofaconstructionproject(e.g.,clearing,grubbing,earthmovement)itmaybemoreappropriatetoperforminspectionoftheBMPsonaregularbasisinsteadofjustbeforeandafterastorm.Thiswillallowsuf?cienttimeforanycorrectionsorimprovementstobemadeintimebeforeastorm.Inspectioncanusuallybeperformedaspartofaregularconstructioninspectionprogram.Accordingtothegeneralpermit,atrackingorfollow-upproceduremustfollowaninspectionwhichdiscoversde?cienciesintheBMPs.Theresultsoftheinspectionandassessmentmustbewritten.Includethedateoftheinspection,theperson(s)whoperformedtheinspectionandtheobservations.Inspectionrecordsmustberetainedforthree(3)years.AsampleinspectionformhasbeenprovidedasWorksheetNo.7.11.5.2CertificationofComplianceEachsitemustbecerti?edbasedontheannualinspection,conductedbytheOwner,orhisrepresentative,thatthefacilityisincompliancewiththerequirementsofthegeneralpermitandtheSWPPP.Iftheinspectionindicatesnoncompliance,thegoverningRegionalWaterQualityControlBoardistobenoti?edimmediately.The Sediment Page11-23
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noti?cationshallidentifythetype(s)ofnoncompliance,theactionsrequiredtocomeintocomplianceandatimescheduletoachievecompliance.11.5.3BMPsMonitoringThetypeofBMPmonitoringdependsonwhichBlVIPisimplemented.InthecaseofBMPsinstalledduringconstructionthemonitoringconsistsofvisualinspectiontoensurethattheBMPwasimplementedandmaintainedaccordingtotheSWPPP.Suchinspectionwouldinclude:Lookingforevidenceofspillsandresultingclean-upprocedures;Examiningtheintegrityofcontainmentstructures;Verifyinguseofemployeeseducationprogramsforthevariousactivities;Notingthelocationofactivity;Verifyingadequacyoftrashreceptacle;andVerifyingwastedisposalpractices.InthecaseofsedimentanderosioncontrolBMPs,themonitoringprogramshouldconsistofregularinspectiontodeterminethefollowingAretheinstalledBA/IPse?ective?TheeffectivenessofaBMPwouldbebasedonthepresenceofsiltbehindorwithincontroldevices,thepresenceofsiltdownstreamofthesiteandsignsoferosioninstabilizedareasafterastormevent.Thesystemmaybedeemedineffectiveif:1.2.Siltispresentoutsideofthecontrolarea;Structuralcontrolsarebreachedorfailunderstormeventsofminor,lessthan2-year,24-hour)intensity;Rillsorgulliesarepresentinstabilizedslopes;Evidenceofsiltbuildupindownstreamstormdrainsanddrainagewaysisapparent;andControlsarenotmaintainedinaccordancewithdesignguidelines.Havedrainagepatternschanged?Ifthesitehasundergonesigni?cantgradingoperations,changingthedrainagepatterns,adjustmentstotheBMPcontrolswilllikelyberequiredtoaddressthischange.Theinspectorshalldeterminetheextentofthedrainagepatternchanges,ifthechangesareaddressedintheSWPPPandifmodi?cationstotheerosionandsedimentcontrolsarerequiredtoaddressthischange.
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AresedimentanderosionBMPinstalledproperly?TheSWPPPBMPsshouldincludedetailsorreferencestoallowfortheproperconstructionofstructuralorvegetativeerosioncontroldevices.TheinspectorshouldensurethatthesesystemsareinstalledintheproperlocationsaccordingtotheSWPPP.Areareasstabilizedasquicklyaspossibleinanareaaftercompletionofconstruc-tionactivities?Activeconstructionareas(inactiveconstructionareasmaybede?nedasareasinwhichnoconstructionactivitywilloccurforaperiodof30daysorlonger)whichhavebeendisturbedshouldbestabilizedthroughtheuseofvegetation,mulch,erosioncontrolmattingorstructuralmethodswithinsevencalendardaysfromthelastconstructionactivityinthearea.Ifconstruction,climatological,orothersiteconditionsdonotallowstabilizationwithinthistimeframe,theSWPPPshouldde?nealternativeapproaches(eg.wateringorchemicalsfordustcontrol).AretheBlMPsproperlymaintained?Maintenanceoftheerosionandsedimentdevicesisthemostcriticalaswellasthepotentiallymostexpensivepartoftheerosioncontrolplan.Theinspectorshouldinspectthesiteonaregularbasisandalteranystormof0.5inchesorgreatertodeterminemaintenancerequirementsandgeneralconditionofthesystem.Thefollowingmaintenancetasksshouldbeperformedonaregularbasis:1.Removalofsiltfrombarriersandsedimentationdevices.2.Replacementorrepairofwornordamagedgeotextilefabric.3.Repairorreplacementofdamagedstructuralcontrols.4.Seedingormulchingofdamagedstabilizedareas.5.BMPsforanychemicalsorfuelsnotaddressedintheSWPPPmustbedeveloped.6.Othercontrolmaintenanceasde?nedintheBMPfactsheetoftheapprovedSWPPP.Allmaintenancerelatedtoastormeventshouldbecompletedwithin48hoursofthestormevent.
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11.5.4RecordKeepingRecordsofallinspections,compliancecerti?cationsandnoncompliancereportingaretoberetainedforatleastthree(3)yearsbytheOwner/Developer.Itissuggestedthatrecordsofincidentssuchasspills,orotherepisodicreleases,bekept.AnalyzingahistoryofthisinformationcanprovideinsightintomodifyingBMPs.AlsokeeparecordofmaintenanceactivitiesoranyotherBMPsthatareofan"action"nature.ItiseasytodemonstratethataBMPwhichinvolvesaphysicalchange,suchasbermingorcovering,hasbeenaccomplished.Butactionsthatrelatetogoodhousekeepingcanonlybedemonstratedbyrecordkeeping.11.5.5SWPPPReviewandModi?cationsDuringthecourseofconstructionunanticipatedchangesmayoccurwhichaffecttheSWPPP,suchasschedulechanges,phasingchanges,stagingareamodi?cations,off-sitedrainageimpactsandrepeatedfailuresofdesignedcontrols.ThesechangesmustbemadeknownandtheSWPPPrevisedaccordingly.Duringthepreparationandreviewofthemodi?edSWPPP,constructionmaycontinuewithtemporarymodi?cationstotheerosionandsedimentcontrolBMPs.RevisionstotheSWPPParealsorequiredwhentheproperlyinstalledsystemsareineffectiveinthepreventionofsilttransporto?‘thesite.RevisionstotheSWPPParealsorequiredifthereisanew,deleted,ormovedactivitythatcouldresultinasigni?cantamountofpollutantsdischargedinthestormwater.11.6WorksheetsTheprevioussectionsprovideguidelines,organizedbyphase:planning,designandconstruction,directedatincorporatingpreparationoftheSWPPPintotheoverallworkeffort.ThefollowingworksheetsareincludedtoaidinpreparationoftheSWPPP.
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Owner:
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StormWaterPollutionPlan(SWPPP)WorksheetCaliforniaConstructionGeneralPermitWorksheet1.ProjectInformationProjectName:ProjectLocation:StreetAddress(orEquivalent)City County: ZipCode:ProjectContactPerson: PhoneOwner'sMailingAddress:StreetAddress(orEquivalent):City: County: ZipCode:
[] Identifyresponsiblepersonnel:[]Implementingandrevisingthe[]Inspectingequipment:[]RegularinspectionsofBMPs:[]TrainingemployeesaboutBMPsa?'ectingtheirjob:
ListallContractorsandSubcontractorsresponsibleforimplementingSWPPPfortheproject:
NAMECONTACTPERSONDATEWORKBEGINSDATEWORKENDS
Page11-27
18.B
Worksheet2.ProjectSiteMapRequirementsPleaseChecktheBoxes,andprovidesupportinginformationasrequested:[] TopographicBaseMapAttached?Mapshows:[] Anareaextendingone-quartermilebeyondthepropertyboundariesoftheconstructionsite:[] Theboundaryoftheconstructionsite.ConstructionArea=_Acres.[] Nearbysurfacewaterbodies,includingwatercourses,wetlands,springsandwells.[] Thelocation(s)wherestormwaterdrainsontoor011’oftheproperty.[] Boundaryofo?—siteareasthatdrainintotheconstructionsite.[] Sitemags)attached?Mapsshow:[] Temporarystormwaterstructuresusedduringconstruction.[] Areasusedtostoresoilsandconstructionwaste.[] Areasofcutand?ll.[] Drainagepatternsandslopesanticipatedaftermajorgradingactivities,includingthelocationofstormwaterstructurestobeconstructedontheproperty(e.g.,stormdrains,detentionponds,channels).[] Areasofsoildisturbance.[] LocationsofpotentialsoilerosionrequiringBMPsduringconstruction.[] Existingandproposedpavedareasandbuildings.ExistingArea:_percentofsiteProposedArea:_percentofsite[] Estimatedruno?‘coe?icientbeforeconstruction:afterconstruction(Seethelocalmunicipalityforapprovedruno?'coe?icientsforyourcommunity.)[] Locationswherestormwaterstructuresandcontrolswillbebuilttocontrolstormwaterpollutionafterconstructioniscomplete.[] Theboundaryofthedrainageareaupstreamofeachlocationwherestormwaterleavestheproperty.[] Anyvehiclesstorageandserviceareas.[] Areasofexistingvegetation.
ErosionandSedimentControl Page11-28
18.B
Worksheet3.InventoryofContractor'sActivitiesandSpecialSiteConditionsProvideadescriptiontocontractor'sactivitiesthatcouldresultinthedischargeofpollutantsinthestormwaterrunofffromthesite.Inaddition,provideadescriptionofspecialsiteconditionsthatmayimpactpollutantsinstormwaterdischarges.Contractor'sActivities[] Describetoxicmaterialsthatareknowntohavebeenstored,disposed,spilled,orleakedinsigni?cantquantitiesontotheconstructionsite:
[] Describeconstructionmaterials,equipmentandvehiclesthatcomesincontactwithstormwater:
[] Describeconstructionmaterialloading,unloadingandaccessareas/activities:
[] Describeequipmentstorage,cleaning,andmaintenanceareas/activities:
[] Describestorageanddisposalofconstructionmaterials(on-siteandoff-site) SpecialSiteConditions[] Describestormwaterstructuresandcontrolsonthesitepriortoconstructionandhowthesestructures/controlswillbeintegratedintotheSWPPPtoreducesedimentandotherpollutantsinstormwaterdischarge:
[] Listmaterials/watersotherthanstormwaterwhichwill?owfromthesiteduringdryweather,theapproximateamountof?ow,andmethodsforpreventingotherdryweather?ows:
ErosionandSedimentControl Page11-29
18.B
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18.B
Worksheet5.BMPsforErosionandSedimentationControl[] Describethesourceandcompositionoftheexistingsoiland?llmaterial(SoilReportAttached?Yes_No) [] ProvideasitemapshowinglocationswhereBMP§forerosionEdg?imgltcontrolareplaced.ThismapshouldbeupdatedwhenBMPsarerevisedtomeetevolvingconstructionconditions.ProvideabriefdescriptionofBMPselected,and,ifappropriate,attachmodi?edfactsheetsoradditionalinformationforerosionandsedimentationcontrolBMPs.
DESCRIPTIONOFBMPsFOREROSION&/ BMPsSELECTEDSEDHVIENTATIONCONTROLSITEPLANNINGCONSIDERATIONSScheduling PreservationofExistingVegetation
SOILSTABILIZATIONSeedingandPlanting
Mulching
PHYSICALSTABILIZATIONGeotextilesandMats
DustControl
TemporaryStreamCrossing
ConstructionRoadStabilization
StabilizedConstructionEntrance
ErosionandSedimentControl Page11-31
18.B
Worksheet6.Post-ConstructionBMPSProvideasitemaplocatingtreatmentcontrolBMPswhichwillbeconstructedaspartofthisprojecttoreducestormwaterpollutionafterconstructioniscomplete.Selectionoftheseandotherpost-constructionBMPsmaybeguidedusingtheMunicipalBMPHandbook,andmustconsidersite-speci?candseasonalconditions.ProvideontheworksheetbelowtheBMPselected,theresponsiblepartyformaintenanceandoperations,andsourceforfundingtheoperationandmaintenance.
MAINTENANCEFUNDINGSOURCE/ BMPSSELECTEDRESPONSIBILITYFORO&MTREATMENTCONTROLBMPS(SeeChapter5,MunicipalHandbook)In?ltrationWetPondsConstructedWetlandsVegetatedSwalesandStripsExtendedDetentionBasinsMediaFiltrationOil/WaterSeparatorsandWaterQualityMultipleSystemsSOURCECONTROLBMPS(SeeChapter4,MunicipalHandbook) Describeothermeasureswhichwillbeemployedontheprojectsitetocontrolstormwaterpollutionafterconstructioniscomplete,andstepstobetakenbythecurrentownertoensurethatthesemeasuresareconducted.
ErosionandSedimentControl Page11-32
18.B
Inl—
Worksheet7.Monitoring,InspectionandMaintenancePlan(Tobecompletedbypersonnelresponsibleformonitoring,inspectionandmaintenance) [] Describemaintenance/repaire?ortstoensureBMPsareingoodandeffectivecondition:
[] Describeinspectionproceduresandrecordkeepingefforts:
[]AnnualInspection:
[]Pre-stormInspection:
[]Post-stormInspection:
[] Describetrainingprogram/materialforsitepersonnelresponsibleforinstalling,inspecting,andmaintainingBMPs:
ErosionandSedimentControl Page11-33
18.B
ame Title
Signature Date
Name Title
Signature Date
Certi?cationIcertifyunderpenaltyoflawthatthisdocumentandallattachmentswerepreparedundermydirectionorsupervisioninaccordancewithasystemdesignedtoassurethatquali?edpersonnelproperlygatherandevaluatetheinformationsubmitted.Basedonmyinquiryofthepersonorpersonswhomanagethesystemorthosepersonsdirectlyresponsibleforgatheringtheinformation,theinformationsubmittedistothebestofmyknowledgeandbelief,true,accurate,andcomplete.Iamawarethattherearesigni?cantpenaltiesforsubmittingfalseinformation,includingthepossibilityof?neandimprisonmentforknowingviolations.
ThisSWPPPwaspreparedby:
ErosionandSedimentControl Page11-
18.B
18.B
Appendix1
RiprapQuality:Riprapqualitydeterminationreferstothephysicalcharacteristicsontherockparticlesthatmakeupthebankprotection.Qualitiesdeterminedtobemostimportantincludedensity,durabilityandshape.Requirementsforeachofthesepropertiesareoutlinedinthespeci?cations:Speci?cGravity(Density):Thedesignstonesizeforachanneldependsontheparticleweight,whichisa?inctionofthedensityorspeci?cgravityoftherockmaterial.Theminimumvalueforspeci?cgravityis2.5.Allstonescomposingtheriprapshouldhaveaspeci?cgravityequaltoorexceeding25,asdeterminedbystandardtestproceduresinASTMC127.Durability:Durabilityaddressesthein-placeperformanceoftheindividualrockparticles.Therockparticlesmusthavesu?icientstrengthtowithstandabrasiveactionwithoutreducingthegradationbelowspeci?edlimits.Qualitatively,astonethatishard,dense,andresistanttoweatheringandwateractionshouldbeused.Laboratorytestsshouldbeconductedtodocumentthequalityoftherock.Speci?edteststhatshouldbeusedtodeterminedurabilityinclude:thedurabilityindexandabsorptiontest(ASTMC127).Basedontheseteststhedurabilityabsorptionratio(DAR)iscomputed,usingEquation7.8,asfollows:DurabilityIndex%Absorption+1DA.R.= (A.l) Thefollowingspeci?cationsareusedtoacceptorrejectmaterial:1.DARgreaterthan23,materialisaccepted;2.DARlessthan10,materialisrejected;3.DAR10through23:a.DurabilityIndex52orgreater,materialisaccepted;andb.DurabilityIndex51orless,materialisrejected.Shape:Therearetwobasicshapecriteria.First,thestonesshouldbeangularand,second,nomorethan25percentofthestonesshouldhavealengthmorethan2.5timesthebreadth.Angularityisaqualitativeparameterthatisassessedbyvisualinspection.Therearenostandardtestsusedtoevaluatethiscriteria.
Appendix1,Page]
18.B
RiprapLayerCharacteristics:Themajorcharacteristicsoftheripraplayerinclude:characteristicsize;gradation;thickness;and?lter-blanketrequirements.CharacteristicSize:ThecharacteristicsizeinariprapgradationistheD50.Thissizerepresentstheaveragediameterofarockparticleforwhich50percentofthegradationis?ner,byweight.Gradation:Toformaninterlockingmassofstones,arangeofstonesizesmustbespeci?ed.Theobjectistoobtainadense,uniformmassofdurable,angularstoneswithnoapparentvoids.Therecommendedmaximumstonesizeis2timestheD50,andtherecommendedminimumsizeisone-thirdoftheD50.Agradationcoef?cientof1.5isrequiredandcanbedeterminedbythefollowingequation:G:(D84/D50+D50/D16) (A-Z)Table7.4providesdesigngradationsforspeci?edclassesofriprapThickness:Theriprap-layeristobeatleast1.5timestheD100value,butneednotexceedtwicethesamevalue.Thethicknessismeasuredperpendiculartotheslopeuponwhichtheriprapisplaced.Riprapliningsshallhaveaminimumthicknessof1.5timesD100.FilterBlanketRequirements:Theneedfora?lterblanketisa?inctionofparticlesizeratiosbetweentheriprapandtheunderlyingsoilwhichcomprisethechannelbank.Theinequities,Equations7.10and7.11thatmustbesatis?edareasfollows:(D15)Filter5(D15)Filter(D85)Base(D15)Base (A.3) (D50)Filter(D50)Base (A.4)
Therelationshipmustholdbetweenthe?lterblanketandbasematerialandbetweentheriprapand?lterblanket.Iftheinequitiesaresatis?edbytheriprapitself,thenno?lterblanketisrequired.Ifthedifferencebetweenthebasematerialandtheriprapgradationisverylarge,thenmultiple?lterlayersmaybenecessary.
Appendix1,Page2
18.B
Table7.4DesignGradationforSpeci?edClassesofRip-RapPercent D50ClassIn“)PassmgSize6 8 1218243o36100to902.0Dm1216243648607285to701.5Dm9 12182736455450to301.0D‘n6 8 121824303615to5 0.67D.m4 5 8 121620245to0 0.33Dg2 3 4 6 8 1012Stonesizeininchescanbeconvertedtoanequivalentsphericalstonebyapplyingtheequation:
_ 6W1D- (HY)3 (A5)Where:DsEquivalentvolumesphericalstonediameter,infeet.W= Weight,inlbs.y = Saturatedsurfacedryspeci?cgravityofstoneinlbs/ft2'ycanrangefrom145lbs/ft3to160lbs/R3.Tosimplifytheuseofagravel?lterlayer,Table7.5outlinesrecommendedstandardgradations.TheTypeIbeddinginTable7.5isdesignedtobethelowerlayerinatwo-layer?lterforprotecting?negrainedsoils.Whenthechannelisexcavatedincoarsesandandgravel(i.e.,50percentormore,byweightretainedonaNo.40sieve),onlytheTypeII?lterisrequired.Otherwise,twobeddinglayers,(TypeItoppedbyTypeH)arerequired.Fortherequiredbeddingthickness,seeTable7.6.
Appendix1,Page3
18.B
Table7.5GradationofGravelBeddingStandardTypeI TypeIISieveSize _ (PercentPassingbyWeight)3inches-- 90-1001-1/2inches~- -- 3/4-inch-- 20-90#4 95—1000-20#16 45-80 --
#50 10-30 —-
#100 2—10 --
#200 0-2 0-3
Table7.6ThicknessRequirementsforGravelBeddingMinimumBeddingThickness,InchesCoarseGrainRip-RapFineGrainNativeSoilsNativeSoilClassi?cationTypeI TypeII TypeIII6-8 6 6 912 9 9 1218 9 12 1224 9 12 123O 12 14 1236 12 14 15 FilterFabricRequirements:Thedesigncriteriafor?lterfabricarea?rnctionofthepermeabilityofthefabricandtheeffectiveopeningsize.Thepermeabilityofthefabricmustexceedthepermeabilityoftheunderlyingsoil,andtheapparentopeningsize(AOS)mustbesmallenoughtoretainthesoil.Thecriteriaforapparentopeningsizeareasfollows:1.Forsoillessthan50percentoftheparticles,byweight,passingaNo.200sieve,theADSshouldbelessthan0.6mm(aNo.50sieve).Appendix1,Page4
18.B
2.Forsoilwithmorethan50percentoftheparticles,byweight,passingaNo.200sieve,theAOSshouldbelessthan0.3mm(aNo.50sieve).Thesiteconditionsandspeci?capplicationsandinstallationproceduresmustbecare?lllyconsideredinevaluating?lterfabricasareplacementforgranularbeddingmaterial.HydraulicDesignRequirements:Channelliningsconstructedofplaced,gradedripraptocontrolchannelerosionhavebeenfoundtobecosteffectivewherechannelreachesarerelativelyshort,0.25milesorless.Situationswhereriprapliningsmaybeappropriateare:1 Major?owsarefoundtoproducechannelvelocitiesinexcessofallowablenon-erodingvalues;2 Channelsideslopesof3:1;and3 Rapidchangesinchannelgeometryoccur,suchaschannelbendsandtransitions.RiprapSizing:TheproceduresdescribedinHydraulicDesignofFloodControlChannels(CorpsofEngineers,EM-l110-2-1601,1991)shallbeusedforthedesignofgradedriprapchannelliningsorbankprotection.
Appendix1,Page5
18.B
5.10 6.8
7.8 8.9
References
REFE
ReferencesRiversideCountyFloodControlandWaterConservationDistrict,April1978HydrologyManualReferencesU.S.DepartmentofTransportationFederalHighwayAdministration,March1984HydraulicEngineeringCircularNo.12,DrainageofHighwayPavementsLosAngelesCountyFloodControlDistrict,HydraulicDesignManualReferencesBrater,ErnestF.andHoraceWilliamsKing,1976,HandbookofHydraulicsfortheSolutionofHydraulicEngineeringProblems,SixthEdition.Chow,VenTe,1959OpenChannelHydraulics,McGrawHillDavidian,J.,1984,ComputationofWaterSurfacePro?lesinOpenChannels,U.S.GeologicSurvey,TechniquesofWaterResourcesInvestigations,Book3,Chapter415LosAngelesCountyFloodControlDistrict,HydraulicDesignManualUS.ArmyCorpsofEngineers,July1991,HydraulicDesignofFloodControlChannels,EngineerManualEM1110-2-1601.US.DepartmentofTransportation,FederalHighwayAdministration,1990,HighwaysintheRiverEnvironment,HydraulicandEnvironmentalDesignConsideration.----,April1988,DesignofRoadsideChannelswithFlexibleLinings,HydraulicEngineeringCircularNo.15PublicationNo.FHW A—IP-87-7.ReferencesAmericanConcretePipeAssociation,January1988,ConcretePipeHandbook.
18.B
10.8 Chow,VenTe,1959,OpenChannelHydraulics,McGraw-HillPeterk,A.J.,1984HydraulicDesignofStillingBasinsandEnergyDissipators,EngineeringMonographNo.25,UnitedStatesDepartmentoftheInterior,BureauofReclamation.Phone,T.J.,1977,Baf?edApronasSpillWayEntryDissipator,AmericanSocietyofCivilEngineersJournalofHydraulics,No.HY12.US.ArmyCorpsofEngineers,1991,HydraulicDesignofFloodControlChannelsUS.DepartmentoftheInterior,BureauofReclamation,1987,DesignofSmallDams----,1974,DesignofSmallCanalStructuresUS.DepartmentofTransportationFederalHighwayAdministration,September1983,HydraulicDesignofEnergyDissipatorsforCulvertsandChannels,HydraulicEngineeringCircularNo.14.ReferencesAmericanSocietyofCivilEngineersandtheWaterPollutionControlFederation,1969,DesignandConstructionofSanitaryandStormSewersBoater,ErnestF.andHoraceWilliamsKing,1976,HandbookofHydraulicsfortheSolutionofHydraulicEngineeringProblemsSixthEdition.LosAngelesCountyFloodControlDistrict,HydraulicDesignManualUnitedStatesDepartmentoftheInterior,BureauofReclamation,1974,DesignofSmallCanalStructures.UnitedStatesDepartmentofTransportation,FederalHighwayAdministrationSeptember1873,HydraulicDesignofEnergyDissipatorsforCulvertsandChannels,HydraulicEngineeringCircularNo.14. --—,September1984,HydraulicDesignofHighwayCulverts,HydraulicsDesignServicesNo.5.ReferencesChow,V .T,Maidment,DR.andMays,L.W.,1988AppliedHydrology,McGrawHill
18.B
Urbanos,BR,1985,StormwaterDetentionOutletControlStructures,AmericanSocietyofCivilEngineers.US.ArmyCorpofEngineers,January1990,I-IEC-lFloodHydrographPackageUsersManual.US.DepartmentoftheInterior,BureauofReclamation,1987,DesignofSmallDams,ThirdEdition.U.S.DepartmentofTransportation,FederalHighwayAdministration,February1980,Underground.
18.B
Storm Drain Criteria
and
Drainage Design Manual
PROPOSED ADOPTION JUNE 11, 2024
CITY OF HEMET
18.B
Table of Contents Page 1
TABLE OF CONTENTS
Section No. Description Page No.
SECTION 1 GENERAL PROVISIONS ................................................................................ 1-1
1.1 Short Title ................................................................................................ 1-1
1.2 Jurisdiction ............................................................................................... 1-1
1.3 Purpose ..................................................................................................... 1-1
1.4 Enactment Authority ................................................................................ 1-1
1.5 Amendment and Revisions ...................................................................... 1-1
1.6 Enforcement Responsibility ..................................................................... 1-2
1.7 Review and Approval ............................................................................... 1-2
1.8 Interpretation ............................................................................................ 1-2
1.9 Relationship to Other Standards .............................................................. 1-2
1.10 Waivers from these Criteria ..................................................................... 1-2
1.11 Abbreviations ........................................................................................... 1-3
SECTION 2 DRAINAGE PLANNING SUBM ITTAL REQURIEMENTS .......................... 2-1
2.1 Review Process ........................................................................................ 2-2
2.2 Preliminary Drainage Report ................................................................... 2-2
2.2.1 Preliminary Report Contents ....................................................... 2-3 I. Narrative ............................................................................... 2-3 II. Hydrological Parameters ...................................................... 2-3 III. System Schematic ................................................................. 2-3 IV. References ............................................................................ 2-4
2.2.2 Drawing Contents ....................................................................... 2-4 A. General Location Map .......................................................... 2-4 B. Floodplain Information ......................................................... 2-5 C. Hydrology Map and Drainage Plan ...................................... 2-5
2.3 Final Drainage Report .............................................................................. 2-5
2.3.1 Final Report Contents ................................................................. 2-6 I. Narrative ............................................................................... 2-6 II. Hydrology Calculations ........................................................ 2-7 III. Hydraulic Calculations ......................................................... 2-7 IV. Hydrology Map .................................................................... 2-7 V. Drainage Facilities Map ....................................................... 2-8 VI. Hydrologic Soils Map........................................................... 2-9 VII. Land Use Map ...................................................................... 2-9 VIII.
Rainfall Maps/Tables ............................................................ 2-9 IX. References ............................................................................ 2-9
2.3.2 Drawing Contents ....................................................................... 2-9 A. General Location Map .......................................................... 2-9 B. Floodplain Information ......................................................... 2-9 C. Drainage Facilities Map ....................................................... 2-9
SECTION 3 DRAINAGE POLICY ....................................................................................... 3-1
3.1 Facilities Planning .................................................................................... 3-1
3.1.1 Master Planning .......................................................................... 3-1
18.B
Table of Contents Page 2
3.1.2 Special Planning Areas ............................................................... 3-3
3.1.3 Floodplain Management ............................................................. 3-4
3.2 Storm Runoff Retention ........................................................................... 3-5
3.2.1 Storm Runoff Retention .............................................................. 3-5
3.2.2 Multi-Purpose Concept ............................................................... 3-6
3.2.3 Design Development Criteria ...................................................... 3-6
SECTION 4 FLOODPLAIN REQUIREMENTS................................................................... 4-1
4.1 National Flood Insurance Program (NFIP) .............................................. 4-1
SECTION 5 HYDROLOGY AN D HYDRAULICS .............................................................. 5-1
5.1 Hydrology ................................................................................................ 5-2
5.1.1 Rational Method .......................................................................... 5-2
5.1.2 Synthetic Unit Hydrograph ......................................................... 5-2
5.2 Hydraulics ................................................................................................ 5-3
SECTION 6 STREET DRAINAGE ....................................................................................... 6-1
6.1 Street Classification ................................................................................. 6-2
6.2 Design Criteria for Streets ........................................................................ 6-3
6.2.1 Design Frequency ....................................................................... 6-3
6.2.2 Pavement Encroachment ............................................................. 6-4
6.3 Catch Basins ............................................................................................. 6-4
6.4 Design Criteria ......................................................................................... 6-4
6.4.1 General ........................................................................................ 6-4
6.4.2 Limitations .................................................................................. 6-5
6.4.3 Length Requirements .................................................................. 6-5
6.4.4 Series Catch Basins ..................................................................... 6-5
6.4.5 Local Depressions ....................................................................... 6-5
6.4.6 Connector Pipe ............................................................................ 6-5
SECTION 7 DETENTION/RETENTION ............................................................................. 7-1
7.1 Limitations on Use of Detention/Retention Facilities .............................. 7-2
7.1.1 Regional Detention/Retention Facilities ..................................... 7-2
7.2 Design Criteria ......................................................................................... 7-3
7.2.1 Criteria for Detention/Retention Facilities .................................. 7-3
7.2.2 Criteria for Special Detention/Retention Methods ...................... 7-5
7.3 Multiple-Use Concepts ............................................................................ 7-5
7.3.1 Potential Uses .............................................................................. 7-6
7.4 Water Quality ........................................................................................... 7-7
7.5 Flood Routing .......................................................................................... 7-7
SECTION 8 EROSION & SED IMENT CONTROL ............................................................. 8-1
8.1 Erosion and Sedimentation Process ......................................................... 8-2
8.1.1 Factors Influencing Erosion ........................................................ 8-2
8.2 Storm Water Programs ............................................................................. 8-3
8.2.1 Federal NPDES Program ............................................................ 8-3
8.2.2 State NPDES Program ................................................................ 8-4
8.2.3 Municipal NPDES Program ........................................................ 8-4
8.3 Storm Water Pollution Prevention Plans.................................................. 8-4
8.4 City Erosion Control Plan (ECP) Requirements ...................................... 8-5
18.B
Table of Contents Page 3
TABLES
Table Page No.
6.1 Traffic and Drainage Classification ................................................................... 6-3
6.2 Design Storm Frequencies for Street Drainage ................................................. 6-4
18.B
Section 1
GENERAL PROVISIONS
18.B
General Provisions Page 1-1
1 GENERAL PROVISIONS
1.1 SHORT TITLE
These regulations together with all future amendments shall be known as the "City of Hemet, Storm Drain Criteria and Drainage Design Ma nual" (hereafter called MANUAL) being part of and subject to the City of Hemet Municipal Code (hereafter called CODE).
1.2 JURISDICTION
This MANUAL shall apply to all land within the incorporated areas of the City, including any public lands. This MANUAL shall apply to all facilities constructed on City right-of- way easements dedicated for public use, and to all privately owned and maintained storm water drainage facilities.
1.3 PURPOSE
This MANUAL shall establish the minimum desi gn and technical criteria for the analysis and design of all hydraulic structures and storm water drainage and detention facilities. All subdivisions, planned unit developments, building permit applications or any other proposed construction submitted for appr oval under the provisions of the CODE, shall include an adequate storm drainage system analysis an d appropriate drainage system design. Such analysis and design shall meet or exceed the criteria set forth within this MANUAL. Options to the provisions of this MANUAL may be suggested by the applicant. The burden of proof that the options are equal to or better than the criteria in this MANUAL, is the responsibility of the applicant. Policies and technical criteria not specifically addressed in this document shall follow the provisions specified by the City of Hemet. Drainage facilities in place or under construction shall be accepted without regard to the provisions of this MANUAL.
1.4 ENACTMENT AUTHORITY
The MANUAL has been adopted pursuant to the authority conferred upon the City of Hemet through the California Government Code. As part of the authority provided to the City by the State and specified within the CODE, this MANUAL was adopted by resolution and is considered part of the CODE.
1.5 AMENDMENT AND REVISIONS
The policies and criteria contained in the MANUAL may be amended as new techniques are developed and/or if experience indicates a need for revision. Amendments and revisions to this MANUAL shall be made on a periodic basis, as needed, and shall be enacted by a resolution of the City Council.
18.B
General Provisions Page 1-2
1.6 ENFORCEMENT RESPONSIBILITY
It shall be the responsibility of the City of Hemet Public Works Director/City Engineer or their representative to enforce the provisions of this MANUAL.
1.7 REVIEW AND APPROVAL
The City of Hemet will review all drainage subm ittals for general co mpliance with this MANUAL. Project approval by the City of Hemet does not relieve the owner, engineer, or designer from the responsibility of ensuring that the calcula tions, plans, specifications, construction, and recorded draw ings are in compliance with th e criteria contained in the MANUAL.
The City of Hemet may request other responsible agencies to review any reports or plans to determine their consistency with the requirements of this MANUAL. '
1.8 INTERPRETATION
In the interpretation and appl ication of the provisions of this MANUAL, the following provisions shall apply:
• In its interpretation and application, the provisions shal l be regarded as the minimum requirements for the protectio n of the environment and the public health, safety, and welfare of the residents of the City of Hemet.
• Whenever the provisions of this MANUAL conflict with the CODE or other applicable law, the most restrictive standards or requirements shall apply.
• This MANUAL shall not abrogate or annul any permits or approved drainage reports or construction plans issued before the effective date of this MANUAL.
1.9 RELATIONSHIP TO OTHER STANDARDS
If the Riverside County Flood Control and Water Conservation District, State or Federal Government imposes stricter criteria, standards, or requirements, than those specified in this MANUAL, those criteria shall be incorporated into the MA NUAL after appropriate due process and public hearing(s) are held.
1.10 WAIVERS FROM THESE CRITERIA
Waivers from this MANUAL will be considered on a case-by-case basis by the Public Works Director/City Engineer or their representative. An appeal of a decision by the Public Works Director/City Engineer shall be prep ared and processed consistent with the provisions of the CODE.
18.B
General Provisions Page 1-3
1.11 ABBREVIATIONS
As used in these criteria, the following abbreviations shall apply:
FEMA Federal Emergency Management Agency
FIRM Flood Insurance Rate Map
RCFC&WCD Riverside County Flood Control and Water Conservation District ROW Right-of-Way
USDOT United States Department of Transportation
USGS United States Geological Survey
18.B
Section 2
DRAINAGE PLANNING
SUBMITTAL REQUIREMENTS
18.B
Drainage Planning Submittal Requirements Page 2-2
2 DRAINAGE PLANNING
SUBMITTAL REQUIREMENTS
2.1 REVIEW PROCESS
All subdivisions, planned uni t developments, or other development proposals shall submit drainage reports, construction drawin gs/specifications, storm drain CAD files, and as-built information in accordance with the requirements of this section. Digital copies shall be submitted to the City of Hemet.
Photostatic copies of charts , tables, nomographs, calcula tions, or any other reference material shall be legible. Blurred or unread able portions of the report are unacceptable and could warrant resubmittal of the report. The submittal shall include a declaration of the type of report submitted (i.e., Preliminary or Fina l). Incomplete or absent information may result in the rejection of the report.
A pre-application conference with a represen tative of the Public Works Department is recommended for all applicants prior to submittal of a Pre liminary or Final Drainage Report. At this meeting, general informatio n regarding development regulations, required procedures, possible drainage problems and solutions, and specific submittal requirements for the subject site will be discussed. When the only development approval required is a building permit, this conference will provide information as to whether or not a Preliminary and/or Final Drainage Report will be re quired by the Public Works Department.
2.2 PRELIMINARY DRAINAGE REPORT
A Preliminary Drainage Report, is the first step in the approval process. A preliminary Drainage Report is required to be submitted as part of any development
project (unless specifically waived). This report is intended to determine the feasibility and design characteristics of the proposed development, at a conceptual level. However, those problems that exist on site prior to development must be addressed within the preliminary report.
The report shall include a cover sheet and shall be prepared or supervised by an engineer licensed in California. The report shall contain a certification which shall read as follows:
"I hereby certify that this repo rt (plan) for the Preliminary Drainage design of (Name of Development) was prepared by me (or under my direct supervisi on) in accord ance with the provisions of the City of He met Storm Drain Development Standards, Storm Drain Criteria and Drainage Design Manual for the owners thereof. I understand that the City of Hemet does not
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Drainage Planning Submittal Requirements Page 2-3
and will not assume liability for drainage facilities designed by others."
Registered Professional Engineer
State of California No. -------
(Affix Seal)
2.2.1 Preliminary Report Contents
The Report shall be in accordance with the following outline and contain the applicable information listed:
I. Narrative
A. Introduction
B. Scope of project
C. Site description
1. General information
2. Vegetative cover
3. Prominent geographic features
II. Hydrological Parameters
A. Hydrology Methodologies
B. Rainfall Data and Source
C. Soils Data and Source
D. Runoff Index and Land Cover Assumptions
1. Ground Cover Condition
2. Impervious Area
E. Approved Flow Rates/User Defined Conditions
III. System Schematic
A. Proposed Surface Improvements
1. Streets/roads
a) centerline
b) right-of-way
c) curb and gutter
2. Grading
a) contours/spot elevations
b) slopes
c) retaining walls
B. Proposed Flood Control Facilities
1. Storm Drains
a) location/alignment
b) diameter
c) flow rate
d) right-of-way
e) outlet geometry
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Drainage Planning Submittal Requirements Page 2-4
f) catch basin location
1) type
• curb inlet
• grated inlet
• riser
• size (length/depth)
• intercepted/flow-by
2. Open Channels
a) location/alignment
b) cross section
c) flow rate
d) velocity
e) lining (earthen, rock, grass, concrete, etc)
f) side inlets
1) geometry
2) flow rate
3) right-of-way
3. Detention/Retention Basins
a) Purpose of Basin
b) Basin Design Criteria
c) Mitigation Measure Requirements
C. FEMA Flood Hazards
1. Flood Hazard Zone
2. Impacts to Flood Hazard
3. Mitigation Measure
IV. References
All criteria, master plans, and technical information referenced in support of the drainage concept shall be identified or included as excerpts.
2.2.2 Drawing Contents
A. General Location Map: All drawings shall be 24" x 36" in size. A map shall be provided in sufficient detail to identify drainage flows entering and leaving the development and ge neral drainage patterns. The map should be at a legible scale and show the path of all drainage from the upper end of any off-site basins to the nearest adequate outlet. The map shall identify any majo r facilities from the subject property along the flow path to the nearest adequate outlet, such as existing improved channel, regional detention facilities, culverts, and storm sewers. Basins and divides are to be identified and topographic contours are to be included.
B. Floodplain Information: An exhibit displaying the location of the
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Drainage Planning Submittal Requirements Page 2-5
subject property shall be included with the report as outlined in Section
2.2.1. All major drainageways with a mapped floodplain or floodway shall have the limits of flooding shown and included in report drawings.
C. Hydrology Map and Drainage Plan: Map(s) of the proposed development at a scale of l" = 20' to l" = 200' on a 24" x 36" drawing shall be included. The plan shall show the following:
1. Existing topographic contours at 1 foot minimum intervals. In terrain where the slope exceeds 15%, the maximum interval is 10 feet. The contours shall extend a minimum of 100 feet beyond the property lines.
2. All existing drainage facilities.
3. Approximate flooding limits based on available information.
4. Conceptual drainage facilities including detention basins, storm drain, swales, riprap, and outlet structures in the detail consistent with the proposed development plan.
5. Major drainage boundaries and sub-boundaries.
6. Any off-site feature influencing development.
7. Proposed flow directions and, if available, proposed surface improvements and/or contours.
8. A legend to define map symbols.
9. A title block in lower right comer, north arrow and graphic scale.
10. Seal and signature of a professional engineer.
2.3 FINAL DRAINAGE REPORT
The purpose of the Final Drainage Report is to update the concepts, analysis, and design details discussed in the Preliminary Drainage Report. Also, any significant change to the Preliminary concept must be noted in the Final Report.
The Final Drainage Report sh all be submitted with an application for a grading permit. The Final Drainage Report must be reviewed and accepted by the Public Works Department prior to accepting the final development application for approval.
All reports shall be submitted digitally to the public works department.
The report shall include a cover letter presenting the final design for review and shall be prepared by or under the direction of an engineer licensed in California and certified as shown above in Section 2.2, Preliminary Drainage Report. The report shall also contain a developer certification sheet as follows:
"(Name of Developer) hereby certifies that the drainage facilities for (Name of Devel opment) shall be constructed according to the design presented in this report. I understand
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Drainage Planning Submittal Requirements Page 2-6
that the City of Hemet does not and will not assume liability for the drainage facilities designed and/or certified by my engineer. I understand that the City of Hemet reviews
drainage plans but cannot, on behalf of (Name of
Development), guarantee that final drainage design review will absolve (Name of Developer) and/or their successors and/or assigns of future liability of improper design. I further understand that approval of the final development application and/or final development plan does not imply approval of my engineer's drainage design."
Name of Developer
Authorized Signature
The Final Drainage Report shall be pre pared in accordance with the following outline and include the material completed as part of the Preliminary Drainage Report.
2.3.1 Final Report Contents
I. Narrative
A. Introduction
B. Scope of project
C. Site description
1. General information
2. Vegetative cover
3. Prominent geographic features
D. Hydrology
1. Methodology Used
2. Input parameters
3. Summary of calculations
E. Hydraulics
1. Methodology
2. Input parameters (starting WSE, Manning’s “n” values, etc.)
3. Summary of calculations and facilities
F. Conclusions/Recommendations
II. Hydrology Calculations
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Drainage Planning Submittal Requirements Page 2-7
A. Soils map
B. Rainfall data
1. Rational Method
a) Time of concentration
b) Runoff coefficients
c) Initial subarea
1) Area 10 acres
2) Length 1000 feet
3) Change in elevation
d) Travel Time
1) Street flow calculations
2) Pipe flow calculations
3) Channel flow calculations
2. Synthetic Unit Hydrograph
a) Lag time
1) Length of longest watercourse
2) Length to the centroid of the area
3) Average roughness coefficient
4) Slope along longest watercourse
b) Unit time (25% to 40% of lag)
c) Area adjustment
d) Runoff index
1) Vegetative cover
e) Low loss rate calculation
f) Storm duration
III. Hydraulic Calculations
A. Street Flow
B. Catch Basin Capacity
C. Storm Drains
D. Culverts Capacity
E. Open Channel
F. Inlet Capacity
G. Intercepted and Flow-by Summary Table (if applicable)
IV. Hydrology Map (One for Rational Method and One for Unit Hydrograph Separately)
A. Bar scale/north arrow
B. Legend
C. Title block
D. Watershed identification
1. Sub-area delineation
2. Sub-area in acres
3. Flow path
4. Centroid (hydrograph only)
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Drainage Planning Submittal Requirements Page 2-8
5. Incremental flow
E. Proposed onsite improvements
1. Streets
2. Grading
F. Storm flows
1. Entering project
2. Leaving project
3. Major intersections
V. Drainage Facilities Map
A. Bar Scale/North Arrow
B. Legend
C. Title Block
D. Proposed Surface Improvements
1. Streets/roads
a) centerline
b) right-of-way
c) curb and gutter
2. Grading
a) contours/spot elevations
b) slopes
c) retaining walls
E. Proposed Flood Control Facilities
1. Storm drains
a) location/alignment
b) diameter
c) flow rate
d) right-of-way
e) outlet geometry
f) catch basin location
1) type
• curb inlet
• grated inlet
• riser
• size (length/depth)
• capacity
• intercepted/flow-by
2. Open Channels
a) location/alignment
b) cross-section
c) flow rate
d) velocity
e) lining (earthen, rock, grass, concrete, etc.)
1) side inlets:
• geometry
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Drainage Planning Submittal Requirements Page 2-9
• flow rate
• right-of-way
VI. Hydrologic Soils Map
VII. Land Use Map
A. Bar Scale/North Arrow
B. Legend
C. Title Block
D. Watershed boundaries
E. Proposed surface improvements
VIII. Rainfall Maps/Tables
IX. References
All criteria, master plans, and technical information referenced in support of the drainage plan shall be identified.
2.3.2 Drawing contents
A. General Location Map (See Section 2.2.2):
B. Floodplain Information (See Section 2.2.2):
C. Drainage Facilities Map; Map(s) of the proposed development at a legible scale on a 24" x 36" drawing shall be included. The plan shall show the following:
1. Existing and proposed cont ours at 1-foot maximum
intervals. The contours sha ll extend a minimum of 100
feet beyond the property lines. The contour interval may be increased at the discretion of the City Engineer.
2. Property lines and easements with their intended use shall be noted.
3. Streets, indicating right-of- way width, flowline width, curb type, and approximate slopes.
4. Existing drainage facilities and structures, including irrigation ditches, roadside ditches, crosspans,
drainageways, and culverts. All pertinent information
such as material, size, shape, slope, and location shall also be included.
5. Overall drainage area boundary and drainage sub- area boundaries.
6. Proposed type of street fl ow, such as vertical or
combination curb and gutter, roadside ditch, gutter, slope and flow directions, and crosspans.
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Drainage Planning Submittal Requirements Page 2-10
7. Proposed storm drains and open drainageways, including outlets, manholes, culverts, and other appurtenances,
including riprap protection.
8. Proposed outfall point for runoff from the developed area and facilities to convey flows to the final outfall point without damage to downstream properties.
9. Routing and accumulation of flows at various critical points for the initial storm runoff listed on the drawing.
10. Volumes and release rates for retention or detention storage facilities and information on outlet works.
11. The location and elevations, if available, of all existing floodplains affecting the property.
12. Routing of offsite drainage flow through the development.
13. The definition of flow path leaving the development through the downstream properties ending at a major
drainageway or facility.
14. A legend to define map symbols.
15. A title block in lower right hand comer.
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Section 3
DRAINAGE POLICY
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Drainage Policy Page 3-1
3 DRAINAGE POLICY
Flooding is a significant concern which affects the use of property, the provision of public services and the health of City residents. Provision for adequate drainage in urban areas is necessary to preserve and promote the general health, welfare and economic well-being of the region. Drainage is a regional feature that affects
all governmental agencies and all parcels of land, making it necessary to formulate a program that balances both public and private interests.
The City of Hemet has expended a significant amount of resources to plan and develop a drainage system that provides considerable benefit to the public. The long-term goal of the City is to complete the improvements specified in the City's Master Flood Control and Draina ge Plan or any update thereto.
To achieve this goal, the following objectives have been established:
• Reduce the hazards to individuals and property caused by undertaking both short- and long- term improvements to protect the health, safety and welfare of area residents.
• Improve vehicular and pedestrian circulation.
• Improve the ability of emergency services to provide timely and necessary assistance.
• Control the quality of water that is either released onto downstream properties or allowed to percolate into groundwater basins.
• Ensure downstream properties are not adversely affected by upstream improvements.
These objectives can be made operational through a set of policy statements. The application of the policy is in turn facilitated by technical criteria and data. The following sections present policy statements to be enacted as part of the manual. E ach statement is preceded by a brief discussion of the requirements or basis for the policy, followed by the actual policy statement.
3.1 FACILITIES PLANNING
3.1.1 Master Planning
As the City of Hemet continues to grow, drainage and flooding will remain significant concerns. The planning of drainage facilities must be included in the urbanization process. The first step is to include drainage planning with all regional and local development master plans.
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Drainage Policy Page 3-2
Storm water management facilities, such as storm drains and channels provide both a conveyance and storage function. When a channel is planned as a conveyance feature, it requires an outlet as well as downstream storage space. When the space requirements are considered, the provision for adequate drainage becomes a competing use for space along with other land uses. If adequate provision is not made in a land use plan for drainage requirements, storm water runoff will conflict with either land uses and will result in storm water damage, and will impair or even disrupt the ability of other infrastructure systems to function.
The policy of the City shall be to re quire storm drainage planning, including allocation of space for drainage facilities, for all development and redevelopment.
In recognition that drainage boundaries are non-jurisdictional, Master Drainage Plans are recognized for regional boundaries and facilities. In order to implement an effective regional flood control system, development must consider the Riverside County Flood Control and Water Conservation District (RCFC&WCD) Master Drainage Plans.
The policy of the City shall be to en courage the development, and periodic update, of detailed regional drainage master plans which set forth requirements for new development and identify the required public improvements.
As a result of the planning process a fee structure shall be developed to provide funding for the public improvements. These Development Impact Fees (DIF) fees will be based upon the current approved Development Impact Fee Nexus Study. As fees are collected, they will be applied to the construction of master planned facilities, or the acquisition of facilities, on a priority basis.
The policy of the City shall be to apply DIF fees collected for the installation of master planned drainage facilities toward those locations or projects which exhibit the following characteristics:
A. The construction or purchase of downstream facilities/properties deemed to be of the highest importance.
B. Projects which are needed to alleviate a safety hazard.
C. Projects with the maximum benefit for the cost of improvements.
D. The purchase of vacant downstream property which does not have a strong likelihood for development and has been identified as important to the completion the master planned system.
E. The construction of storm water/drain age facilities within existing street or road rights-of-way.
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Drainage Policy Page 3-3
F. The construction of storm water/drainage facilities on land which has been dedicated to the City for use as part of the master planned system and which is adjacent to or a logica l extension of an existing master planned facility.
G. The construction of storm water/drainage facilities on land which has been dedicated to the City, but is no t a logical extension of an existing master planned facility.
Recognizing that the cost to construct drainage facilities is generally high and that drainage DIF fees are slow to accrue, it will be necessary for the City to rely on development to carry its fair share of the cost burden of the construction of master planned facilities.
The policy of the City shall be to require all development projects adjacent to master planned facilities to construct said facilities. Development projects shall bear the total cost of constructing a ma ster planned facility excluding land costs.
This cost shall be credited against the tota l amount of drainage impact fees due on the project. Dependent upon the area and the benefit provided, the City may use a recapture or reimbursement agreement to appropriately allocate construction costs based on benefit received.
Master planned facilities constructed for development projects shall also:
A. Provide for the retention of the increase in storm water runoff to the extent regional retention basins are added to the master plan.
B. Provide an inlet for tributary upstream runoff and a method of metering the downstream discharge.
Realizing that a master plan is a document used to guide the development of an adequate drainage system, provisions must be made for the introduction of alternate methods of controlling storm water. There must be enough flexibility in the plan that the creativity of the engineers and planners preparing development master plans is not stifled.
It shall be the policy of the City to allow for the installation of drainage facilities which differ from the design characteristics containe d in the City's Master Flood Control and Drainage Plan, provided such facilities have the same design capacity, function and purpose. In addition, the type of facility proposed shall be consistent with the types of facilities or improvements which exist in the area.
3.1.2 Special Planning Areas
Currently, there are areas in the City where significant drainage problems exist. Any new development or redevelopment in these areas may compound the existing
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Drainage Policy Page 3-4
drainage problems.
The policy of the City will be to condition development projects to construct the drainage facilities necessary to meet the needs and mitigate the impacts created by each development proposal. A project which is phased or constructed in an incremental manner shall either be designed to accommodate, or have in place, a mechanism to ensure that each portion of the project bears its proportionate cost of the project improvements.
In addition to these areas, there are areas in the C ity where presen tly no adequate outlet exists for drainage by gravity. In general, there are no major drainage facilities or natural water courses in these areas.
The policy of the City shall be to require development projects to construct additional drainage facilities to meet one or more of the following needs:
A. A development project exacerbates existing drainage problems.
B. A development project creates drainage problems downstream.
C. Facilities are necessary to protec t downstream properties from storm water runoff caused by development.
D. Facilities are necessary to mitigate identified environmental impacts caused by development.
There also exist development projects which do not produce a significant impact on the environment. In these cases, it may be not necessary to provide a high level of analysis to arrive at measures which will adequately remove storm water from the project site.
It shall be the policy of the City to require a hydrology study for all development projects, except the following:
A. Single family home construction on existing lots.
B. Subdivisions that create more than tw o (2) separate parcels without any proposed improvements.
3.1.3 Floodplain Management
The City has adopted a floodplain ordinance which sets forth regulations for the development of mapped floodplains. The ordinance has been written in a manner which enforces federal regulations related to the development of floodplains. The designer is referred to the City's ordinance for guidance during the planning phase of development projects in flood-prone areas.
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Drainage Policy Page 3-5
It shall be the policy of the City to have a development review process consistent with FEMA guidelines that includes the filing of the following documentation by the applicant:
A. A development permit consistent with FEMA criteria.
B. Certification of the lowest floor elevation by an engineer and/or architect depending upon the location of the property and its relationship to the floodplain.
3.2 STORM RUNOFF RETENTION
3.2.1 Storm Runoff Retention
The value of storm runoff retention as part of a master plan has been explored by many individuals, agencies and professional societies. Retention is considered a viable method to reduce the costs associated with the development of a drainage system.
Temporarily retaining a few acre-feet of runoff can significantly reduce downstream flood hazards as well as downstream facility requirements. Storage also provides for sediment and debris collection which helps maintain the quality of water in streams and rivers. Thus, public health benefits may accrue from storage of storm runoff.
However, all benefits can only be obtained through consistent administration of the retention policy.
The goal of the City is to require the installation of on-site retention facilities. The realization of this goal will allow for the development of land while producing no net increase in storm runoff. However, the implementation of retention basins must ensure adverse impacts affect ing the public are not create d, such as v ector related issues.
The policy of the City requires all development projects to provide for the on- site retention of storm water runoff. In those cases where projects are not located adjacent to a master planned drainage facility, the City Public Works Director/City Engineer, or their represen tative, shall have the option of either:
A. Requiring the development to provide onsite retention, or;
B. Requiring the installation of drainage facilities which connect to Master Plan or other facilities.
As retention facilities increase in size, it may become reasonable to incorporate them into the existing master drainage plan.
3.2.2 Multi-Purpose Concept
Design development of retention facilitie s should not on ly consider their primary
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Drainage Policy Page 3-6
function. During the early stages of desi gn development consideration should be given to additional uses for these facilities. Considerable effort should be expended to develop a facility that can be used as a park, or integrated into the development such that additional us es can be identified and planned. This is not to say that the primary purpose of the facility should be overlooked; however, whenever possible these facilities should serve multiple purposes to the community.
The policy of the City shall encourage ret ention facilities to be designed as usable open space or formal park site s for complementary land uses, such as single family residential tracts. To implement this policy the following shall apply:
A. All facilities shall have appropriate pedestrian access, landscaping and irrigation systems.
B. Park development impact fees may be applied to those facilities which are specifically designed as park sites consistent with the City's General Plan.
In addition to thos e considerations note d above, thought shou ld be given to the development patterns in the area. If deve lopment patterns and timing are such that a group of developments can utilize an appropriately located retention facility, every effort should be to develop a site agreeable to all concerns. These efforts will help reduce the number of minimally sized retention basins while respecting the intent of the City's policy decision.
It shall be the policy of the City to encourage property owners who are seeking to develop adjacent parcels to join togeth er to construct a single retention facility that will be of mutual benefit. The City will assist in providing a mechanism to equitably distribute the facility's development costs to all property owners benefiting from such an agreement. Such a mechanism could include:
A. The reduction or elimination of development impact fees.
B. A recapture or reimbursement agreement.
C. A memorandum of understanding.
D. A combination of all the above.
3.2.3 Design Development Criteria
Previous discussion has centered on the usefulness of retention facilities as part of an overall drainage system, the goals of City, and general guidelines to be considered during design development of retention facilities. To further clarify the design development process, certain general criteria must be defined.
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Drainage Policy Page 3-7
The policy of the City requires on-site retention facilities to meet the following criteria:
A. The facility shall be integrated into the project's design using one of the methods listed below, which may render certain portions of the property unusable for short periods after each storm. If the type of facility used is intended primarily to retain st orm water runoff, th en a method shall be employed to meter the discharge and ensure all water is removed within 72 hours after the storm.
1. Facilities for single family subdivisions shall be constructed and deeded to the City on one or more parcels. The size of the property to be deeded shall be in conformance with the City development standards and subdivided consistent with the existing subdivision pattern. Up on completion of the necessary downstream drainage system, the property can be sold by the City for conversion to residential uses.
2. Facilities for multiple family projects shall be integrated into on- site landscaping, parking or re creation areas, where feasible.
3. Facilities for commercial or industrial projects shall be integrated into parking and/or landscape areas, where feasible.
B. All facilities shall be constructed in accordance with the requirements of this manual.
C. All retention facilities sh all have an inlet for tributary runoff and a method of ensuring the basin discha rge is metered to drain the basin within 72 hours. The characteristic s of such facilities shall be determined based upon the followin g general criteria, in order of priority:
1. The facility shall be consistent with the needs of the RCFC&WCD Master Drainage Plans.
2. The facility will not have adverse affects upon the health, safety or welfare of area residents.
3. The facility shall provide for dual use, such as open space or park facilities.
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Section 4
FLOODPLAIN REQUIREMENTS
18.B
4 FLOODPLAIN REQUIREMENTS
The regulation of flood plains is necessary to pr eserve public safety and general health and welfare and to promote the economic well being of the region. The genera l intent of current floodplain regulations is summarized below:
• To reduce the hazard of floods to life and property;
• To preserve the hydraulic characteristics of water courses used for conveyance of flood waters; and
• To protect the public from excessive fi nancial expenditures for the development of flood control facilities.
The City of Hemet’s Floodplain Ordinance is Chapter 14, Article V. It is the Engineer's responsibility to utilize the most current adopted floodplain ordinances and regulations.
4.1 NATIONAL FLOOD INSURANCE PROGRAM (NFIP)
The City is a participant in the National Fl ood Insurance Program which is administered by the Federal Emergency Management Agency (FEMA). This program's basic function is to designate flood-prone areas throughout the City of Hemet, and the County of Riverside, and subsequently make available to the general public varying degrees of flood insurance. FEMA develops certain floodpl ain designations by first completing a Flood Insurance Study and later identif ies these designations on re lated Flood Insurance Rate Maps.
A Flood Insurance Study for the City of Heme t was originally completed in September, 1977 with several revisions completed up to the current approved study from March 22, 2022. In addition, the associated Flood Insurance Rate Maps were also revised in 2008..
Issues involving FEMA should be identifi ed and addressed during the conceptual development and in the Preliminary Drainage Re port. It is considered the responsibility of the developer, or the deve loper's representative, to fulfill all of FEMA's rules and regulations and to prepare any appeals or revisions that may be required as a result of a proposed development. If a Letter of Ma p Revision (LOMR) or a Letter of Map Amendment (LOMA) is required by FEMA prior to the removal of a special flood hazard
18.B
zone (100-year floodplain), it will be the responsibility of the developer to obtain any and all such documents prior to issuance of any permits.
18.B
5 HYDROLOGY AND
HYDRAULICS
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Rainfall and Runoff Page 5-2
5 HYDROLOGY AND
HYDRAULICS
The purpose of this section of this manual is to establish the design hydrology methods and procedures currently in use in the City of Hemet and Riverside County.
The materials contained in this section are intend ed for the use of both City staff and engineers submitting hydrologic an d hydraulic computatio ns to the City. The methods presented are considered to produce hydrologic design parameters acceptable for use during the planning and design of underground storm drains, open channels, retention and detention facilities and debris basins, as well as subdivision revision and floodplain mapping.
Prior to continuing with this section, the engineer should become thorou ghly familiar with the Riverside County Flood Control and Water Conser vation District Hydrology Manual and the Riverside County Flood Control and Water Conservation District.
5.1 HYDROLOGY
The City of Hemet utilizes the Riverside County Flood Control and Water Conservation District’s Hydrology Manual, which is located here: https://rcflood.org/engineering-tools. The manual documents the design methods and criteria currently used by RCFC&WCD.
RCFC&WCD utilizes two primary methods to determine design discharges: the Rational Method and the Synthetic Unit Hydrograph Method. Both of these methods are described in the RCFC&WCD Hydrology Manual, and the approved software for modeling is also provided at the link above
.
5.1.1 RATIONAL METHOD
Rational Method calculations are utilized for th e sizing of stor m drain facilities, including catch basins, inlets, and storm drains, as well as determining storm drain inlet locations based upon street design criteria. The calculations must be performed utilizing the Ri verside County Flood Control and Water Conservation District Hydrology Manual methodology and input parameters. Software utilized for performing rational method calculations must be one of the District Approved Softwares listed on the rcflood.org website.
5.1.2 SYNTHETIC UNIT HYDROGRAPH
The Synthetic Unit Hydrograph is utilized for flood cont rol facilities in excess of 300 to 500 acres, sizing re tention basins and other fa cilities used to address increased runoff mitigation (see Sectio n 7). Additionally , unit hydrograph calculations are utili zed for addressing the Hydr ologic Condition s of Concern
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Rainfall and Runoff Page 5-3
(HCOCs) for the current Water Quality Management Plan requirements (this may be subject to change pend ing updates to the MS4 Pe rmit). The uni t hydrograph calculations must be prepar ed per the Riverside County Flood Control and Water Conservation District Hydrology Manual methodology and in put parameters.
Software utilized for performing unit hydrograph calculations must be one of the District Approved Softwares listed on the rcflood.org website.
5.2 HYDRAULICS
Hydraulic calculations are performed to demonstrate that proposed facilities have been designated to adequately convey, retain, or intercept the design flow rate tributary to the system. District Approved Software on the rcflood.org website include programs currently accepted for hydraulic modeling. The following are typical calculations required:
1. Inlet calculations need to be provided that include interception, flow-by and upstream tributary flows.
2. Storm drain calculations need to be performed, typically with the WSPG (Water Surface Profile Gradient) program.
3. Any other hydraulic calculations necessary to demonstrate that facilities have been designed appropriately.
Hydraulic calculations must be performed per the Riverside County Flood Control and Water Conservation District Hydraulic Design Manual.
18.B
6 STREET DRAINAGE
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Street Drainage Page 6-2
6 STREET DRAINAGE
Urban streets with curb and gutter serve an important and necessary drainage service, even though their primary function is for the movement of traffic . Traffic and drainage uses are compatible up to a point, beyond which drainage is, and must be, subservient to traffic needs .
Gutter flow in streets is necessary to transport runoff water to storm drain catch basins and to major drainage channels. Good street planning can substantially help in reducing the size of, and sometimes eliminate the need for, a storm drain system in newly urbanized areas.
Drainage design for the collection and conveyance of storm water on public streets is based on a reasonable frequency and magnitude of traffic interference. That is, depending on the character of the street, certain traffic lanes can be fully inundated once during the design storm return period. However, during less intense storms, runoff will inundate traffic lanes but to a lesser degree. Therefore, one of the primary functions of streets is to convey nuisance flows quickly and efficiently to a storm drain or other drainage facility without obstructing traffic movement. During a major storm event, the function of streets is to provide an emergency escape for flood flows with minimal damage to urban environment.
6.1 STREET CLASSIFICATION
The streets in the City are clas sified for drainage purposes as Local, Collector or Arterial according to the average daily traffic (ADT) for which the street is designed. The larger the ADT, the more restrictive the allowable drainage encroachment into driving lanes. Traffic
classifications and typical cross sections of
the three drainage classifications are presented in Table 6.1 and Figure 6.1, respectively.