EXH0460022925

Work is underway by the Port of Seattle to revisethe LowStreamflowAnalysisand Summer Low FlowImpactOffsetFacilityProposalper 401 Permitconditions.In theprocessofpreparing the reviseddocument, Port of Seattle consultantsidentifiederrors in the low streamflow modelingthatrequirecorrectionin thereviseddocument,andthatwillaffect the conclusionsof the lowstreamflowanalysis. This meetingwas called by the Portto allow_hePort consultingteam to explainthe modeling errorsandrevisionsthatwillbemadeto correcttheerrors. DESCRIPTION OF ERRORS AND ASSOCIATED REVISIONS 1. Conversionfactorerrorin embankmentfill inputto HSPF: When outputfrom the embankmentmodelingwas input to HSPF, an error of 1/24= magnitudewasmade. Conversionof dailyoutputto hourlyoutputwas occurringtwice- oncebythemodeler(Joe Brascher,Aquaten',a)andonceautomaticallywithintheHSPF ;',_ogram.This error affects all areas where embankmentdischarge is inputto low streamflowanalysis. The errorhasbeen correctedinrevisedmodelingwhichshowsthe contdbutionof flow from the embankmentflu to lowstream flow is now 24 timesthe previousvalue. C,'[;o¢_ and ._gmr_'ak4m 48l'd,.o_M .._ngs',Temlxxary ,,emelRe 'IOLKlILLowFlow F!NAL e.==_ to_ _¢ PageI of3 n 11/26/2001 •" 401 Permit.IssuanceClarification, Sea-TacAirport Third Runway FloydSnidcrMcCarthy,Inc. October 30, 2001 2. Incorrectinput files for embankmentmodeling: • : Inthepreviousmodeling,AquaterTagavePacificGroundwaterGroupdaily"AGWO"files as inputto the embankmentmodeling, Instead,hourly"AGWI" filesshouldhave been provided. t 3. Revisedapproachto modelingof imperviousarea at embankmentfilterstrips: The err_ listed above in #2 has a Jevelof significancethat has led the modelers to proposemore directmodelingof the imperviousarea that runsoff tofilter stdpsat the top of the embankment. In 401 permitdecision-makingdiscussionsbetweenthe Port, KingCountyand Ecology,several alternativeswere discussedfor how to model the imperviousarea tributaryto the filterstrips. It was decided then that rainfallon the perviousarea of the embankmentwouldbe "scaledup" to addressthe impe_ous area. With the revisionin embankmentmodelinginputfiles to hourly"AGWI"files,more direct modeling of the imperviousarea and filter skips will be performed by the Ports consultingteam, Inthismoredirectmodeling,AquaterrawillgivePacificGroundwaterGroupthe "AGWI" time sedes data for the pervious embankment,and "SURO" time series data for imperviousareas on the embankment,bothon a…

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PSCAA Advisory Council Meeting Agenda
Add to CalendarMWednesday Mar 11, 9:00am - 11:35am
1904 3rd Avenue, Suite 105, Seattle, WA 98101
Call to order – Roll Call – Quorum Established – Land Acknowledgement We would like to acknowledge that our Agency operates in four present-day counties, which are the traditional lands of the Coast Salish people, past and present. This acknowledgement does not take the place of authentic relationships with indigenous communities in our jurisdiction and [...]