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Home My WebLink About2018-2 Cedar Ridge Short Plat - Basin Diversion.pdfDenisLawMayorCommunity&EconomicDevelopmentC.E.“Chip”Vincent,AdministratorApril6,2018JingsongFeng,P.E.PacificEngineeringDesign1544553rdAvenueS.Suite100Seattle,WA98188RE:CedarRidgeShortPlat(C17004067)—BasinDiversion-Adjustment2018-02DearMr.Feng:TheCityofRentonhascompletedreviewoftheadjustmentrequestforCedarRidgeShortPlat(C17004067)inaccordancewiththeCityadopted2017RentonSurfaceWaterDesignManual(RSWDM).Astheapplicant’sengineer,youarerequestinganadjustmentfromthe2017RSWDMtoSection1.2.1,CoreRequirement#1,DischargeattheNaturalLocation.Morespecifically,youarerequestinganadjustmenttodivertstormdrainagefromapproximately1.71acresoflandareafromdrainingatitsnaturaldischargelocationtoanotherlocation.Findings:1.CoreRequirement#1,DischargeattheNaturalLocation,requiresthatallstormwaterrunoffandsurfacewaterfromaprojectbedischargedatthenaturallocationsoasnottobedivertedontoorawayfromdownstreampropertiesperSection1.2.1oftheCityadopted2017RSWDM.2.Theprojectsiteislocatedat13609l56”AvenueSEand6315SE2’Place.TheprojectisasinglefamilyresidentiallanduseandislocatedintheR-4residentialzone.Theprojectsiteisapproximately1.69acresinsizebeforerightofwaydedicationandisdevelopedwithtwoexistingsinglefamilyhomes.Theprojectproposestodemolishtheexistingsinglefamilyhomesandsubdividethesiteintosixlotsforfuturesinglefamilyhomesandonetractforastormwaterdetentionvault.Theentiresiteoutsideoftreeretentionareaswillbedisturbedaspartofthisproject.1055SouthGradyWay,Renton,WA98057.rentonwa.gov Mr.JingsongFengPage2of4April6,20183.ThesiteislocatedwithintheCity’sFlowControlDurationStandardMatchingForestedConditionsArea.Thedevelopmentmustmatchtheflowdurationofpre-developedratesforforested(historical)siteconditionsovertherangeofflowsextendingfrom50%ofthe2-yearuptothefull50-yearflowandmatchpeaksforthe2-and10-yearreturnperiods.4.Thenaturalstormwaterdischargepointfromthesiteisatthesite’ssouthwestcorner.Thenaturaldischargepointistoasinglefamilyresidentialpropertywiththeaddressof27$ToledoAvenueSE.Thereisnoexistingpublicorprivatestormwaterconveyancesystematorinthevicinityofthedischargepointtothisproperty.5.Thereisnotadequatespaceontheprojectsitetodispersestormwaterflowsfromtheproposeddetentionvault.6.ACityofRentonstormwatereasementwithapipedstormwaterconveyancesystemwouldberequiredfromthenaturaldischargepointatthesite’ssouthwestcornertotheexistingCitystormwaterconveyancesystemin154thAvenueSE/ToledoAvenueSEtoallowfordischargetofollowthenaturaldownstreamdrainagepath.7.Theexistingdownstreamstormwaterconveyancesystemin154thAvenueSEIToledoAvenueSEexperiencessignificantcapacityissuesapproximately1,000feetsouthof278ToledoAvenueSE.TheconveyancesysteminthisareaislocatedinunincorporatedKingCounty.Theconveyancesystemisdocumentedbyhomeownersintheareaasoverwhelmedinheavyandprolongedraineventsandduringsnowmelt.Theconveyancesysteminthisareaconsistsmostlyofshallowswales.ExistingissuesweredocumentedinthedownstreamanalysisprovidedintheTechnicalInformationReportandAdjustmentRequestpreparedbyMr.FengatPacificEngineeringDesign.ThenaturaldownstreamdrainagepatheventuallydrainsintoanunnamedstreamsouthofSE142rStreetinunincorporatedKingCounty.Thepointofdischargeintothestreamislocatedapproximately2,600feetsouthIsouthwestofthesite.8.TheAdjustmentRequestproposestodivertallstormwaterfromtheprojectsitetoanexistingCity12”pipedconveyancesystemnorthofthesiteinSE2idPlace.ThisconveyancesystemflowswesttoasectionofunimprovedCityofRentonrightofwaywestofHighlandEstatesandsouthoftheintersectionofRosarioAvenueSEandSE2ndPlace.Fromthispoint,drainageflowssouthinanopenconveyancechanneltotheHighlandEstatesdetentionpond.TheHighlandEstatesdetentionpondhastwopointsofdischarge.OnepointofdischargeiswestintoanunnamedstreaminKingCountyParksproperty.TheH:\FileSys\SWP-SurfaceWaterProjects\SWP-27-SurfaceWaterProjects(CIP)\27-3129RentonStormwaterManual\ADJUSTMENTS\2018\2018-2CedarRidgeShortPlat-BasinDiversion.doc1055SouthGradyWay,Renton,WA98057rentonwa.gov Mr.JingsongFengPage3of4April6,2018otherpointofdischargeissouthtoapipedKingCountystormwaterconveyancesystemin152AvenueSE.TheunnamedstreamontheKingCountyParkspropertyeventuallydischargestoapipedKingCountystormwaterconveyancesystemin143thPlaceSE.Thetwoconveyancepathsconvergeatapipedconveyancejunctionneartheintersectionof149thPlaceSEandSE139thPlaceinunincorporatedKingCounty.Fromthepointofconvergence,drainagecontinuesthroughastormwaterdetentionpondandthensouthinapipedKingCountystormwaterconveyancesystemin149thPlaceSE.ItthencontinueseastinSE142Streettoanotherstormwaterdetentionpond.Fromthispond,drainagedischargestothesameunnamedstreamdescribedinthenaturaldownstreamdrainagepath.Theproposedpointofdischargetothestreamisapproximately800’upstreamfromthepointofdischargefromthenaturaldownstreamdrainagepath.9.CityMaintenancestaffhavedocumentedproblemswiththeexistingswaleintheunimprovedCityofRentonrightofwaywestofHighlandEstatesthatispartoftheproposeddownstreamdrainagepath.Problemsintheswalearerelatedtodebrisaccumulatingandrestrictingthecapacity.10.AdetaileddescriptionoftheproposeddownstreamdrainagepathfromthesitetothestreamthatservesastheultimatenaturaldischargepointisprovidedintheTechnicalInformationReportandAdjustmentRequest.ThedownstreamanalysisnotesthatthedrainagesystemdownstreamoftheHighlandEstatespondisinadequateconditionwithadequatecapacity.NoexistingdrainageproblemswereidentifieddownstreamoftheHighlandEstatespond.ThesizeofthepipesdownstreamoftheHighlandEstatespondare12”—30”indiameter.As-builtdrawingsforthesepipeswereprovidedintheTechnicalInformationReportandAdjustmentRequest.11.Theproposeddownstreamdrainagepathwoulddivertstormwaterawayfromtheproblematicsystemin154thAvenueSEdownstreamofthesiteinunincorporatedKingCounty.Theproposeddownstreamdrainagepathwouldintersectwiththenaturaldrainagepathapproximately5,000feetdownstreamofthesiteasmeasuredalongtheproposeddownstreamdrainagepathand2,600feetdownstreamofthesiteasmeasuredalongthenaturaldownstreamdrainagepath.Decision:Basedontheinformationprovidedintheadjustmentrequest,theadjustmenttodischargeallstormwaterfromtheCedarRidgeShortPlatprojecttoanalternateH:\FileSys\SWP-SurfaceWaterProjects\SWP-27-SurfaceWaterProjects(CIP)\27-3129RentonStormwaterManual\ADJUSTMENTS\2018\2018-2CedarRidgeShortPlat-BasinDiversion.doc1055SouthGradyWay,Renton,WA98057rentonwa.gov Mr.iingsongFengPage4of4April6,2018downstreamdrainagepathasdescribedpreviouslyinthisdocumentisapprovedwithconditions.Conditions:1.TheproposeddetentionvaultshallbedesignedtodischargeinaccordancewiththeFloodProblemFlowControlStandard(Level3FlowControl)foundinSection1.2.3.1ofthe2017RSWDM.ThesitemustmeettheflowcontrolrequirementoftheFlowControlDurationStandardMatchingForestedConditionsAreawhilematchingthe100-yearpeakforexistingsiteconditions.2.TheexistingopenstormwaterconveyancechannelintheunimprovedCityofRentonrightofwaywestofHighlandEstatesshallbepipedfromtheexistingpipedCitystormwaterconveyancesysteminSE2ndPlacetotheHighlandEstatesdetentionpond.Ifyouhaveanyquestionsaboutthisadjustment,pleasecontactBrianneBannwarth,DevelopmentEngineeringManager,at(425)430-7299orRonStraka,SurfaceWaterUtilityEngineeringManager,at(425)430-7248.Sincerely,,,4ABrianneBannwah,P.E.DevelopmentEngineeringManagercc:IanFitz-James,P.E.,CivilEngineerIllMattHerrera,SeniorPlannerLysHotnsby,P.E.,UtilitySystemsDirectorRonStraka,P.SurfaceWaterUtilityEngineeringManagerH:\FileSys\SWP-SurfaceWaterProjects\SWP-27-SurfaceWaterProjects(CIP)\27-3129RentonStormwaterManual\ADJUSTMENTS\2018\2018-2CedarRidgeShortPlat-BasinDiversion.doc1055SouthGradyWay,Renton,WA98057rentonwa.gov