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Home My WebLink About2013-01-15 Planning Board Supplemental Materials (11) Eggleston Environmental January7,2013 NorthAndoverPlanningBoard 1600OsgoodStreet NorthAndover, MA01845 Attn:JudyTymon,TownPlanner RE:1018OsgoodStreet StormwaterManagementReview DearMs. TymonandBoardMembers: Iamwritingthisletterinfollow-uptomyNovember14,2012reviewletterontheabove- referencedproject.SincethattimeIhavereceivedandreviewedtheDecember7,2012 revisedplansandStormwaterManagementReportsubmittedbyMHFDesign Consultants.Ialsoparticipatedinameetingwithtownstaffandtheapplicantson November19,2012todiscussmycommentsandtheissuesonthe site. Thedesignrevisionsreflectadecreaseinoverallimperviousareaonthesiteandprovide stormwaterrechargethroughtwoproposedinfiltrationsystems,oneintheareadraining totheonsitewetlandandoneinthefrontportionofthesite,intheWatershedProtection District.TheproposedStormceptorunithasbeeneliminated,aswaterqualitytreatment ofrunoffflowsistobeprovidedthroughtheoil/gritchambersandinfiltrationsystems. Thereviseddrainageanalysisreflectsthesedesignrevisionsandusesthemore conservativeRawlsrateformodelingofexfiltration.InotethattherevisedStormwater Reportalsoincludesdatafromfouradditionaltestpitsconductedonthe siteinNovember 2012. Mycommentsontherevisedsubmittalareoutlinedbelow: 1.Atourmeetingon11/19,wediscussedtheneedfordistributingrechargeacross theprojectsite,andparticularlywithintheWatershedProtectionDistrict.I indicatedtoMHFthateveniftheycouldnotprovideatwo-footseparationtothe highgroundwaterelevation,aninfiltrationsystemwithintheWPDshouldstillbe abletoprovideeffectiverechargeandfilteringofflowundermostgroundwater conditions.Withtheadditionaltestpitdata,however,aclearerpictureofthe groundwaterconditionsonthesiteemerges,andIquestionwhetherany subsurfaceinfiltrationsystemcanbeusedtoeffectivelyrechargeortreatflow withintheWPD.Sevenoftheeighttestpitsconductedonthe site(fourinAugust 2012andanotherfourinNovember2012)indicatethatthehighgroundwater elevationisapproximatelythreetofourfeetbelowexistinggradeacrossthesite. Evidenceofgroundwaterwasnotseenintheeighthtestpit,howeverbasedonthe othertestpitstheESHGWwasestimatedtoalsobefourfeetbelowgrade.Thus, the ESHGWelevationacrossmostoftheproject siterangesfromelevation144to 32OldFramingham RdUnit29SudburyMA01776tel508.259.1137fax866.820.7840 1018OsgoodStreet,StormwaterReview2 January7,2013 140.InfiltrationSystem#1(nearthewetland)islocatedinaportionofthesite wheretheproposedgroundelevationwillberaised.Thebottomofthestonein thesystemwouldonlybe1.2feetabovetheESHGW,butthesystemshouldbe abletofunctionadequatelymostofthetime.ProposedInfiltrationSystem#2, however,islocatedinacutportionofthesite,wheretheproposedsurfacegrades willbeloweredbyapproximatelythreefeet.Theentiresubsurfaceinfiltration system,withabottominvertof136.67,wouldthereforebeseveralfeetbelowthe ESHGWelevationinthatarea,andlikelytobeingroundwatermostoftheyear. 2.Evenwithoutthegroundwaterissue,theproposedplanonlyattenuatestherateof runoffflowleavingthesite,notthevolume.Thevolumeofflowdischargedto OsgoodStreetunderpost-developmentconditionswouldmorethantwicethe existingrunoffvolumeundereverydesignstormcondition,andcouldexacerbate anydownstreamflooding. 3.Theproposeddesignalsodoesnotprovidesufficientdeadstoragevolumeinthe infiltrationsystemstotreatevena½-inchwaterqualityvolume.TheTSS calculationsusethestoragevolumeintheOil/GritChamberstoreducethewater qualityvolumeintheinfiltrationsystems.Thisisaninvalidassumptionfortwo reasons.First,theoil/gritchambersaresolidstructuresandaredesignedto operatefullandtoremainfullbetweenstorms.Second,inordertoprovidethe treatmentnecessarytogetthe80%TSSremovalcreditintheinfiltrationsystems theentirewaterqualityvolumemustbeinfiltrated. 4.Itisnotclearwhatthebasisisforthesaturatedthicknessesof129and133ftused inthemoundingcalculations.Asindicatedinmypreviouscomments,saturated thicknessshouldbethedifferencebetweenESHGWandthebottomofthe aquifer,probablysomethingintherangeof10to15feet.Themodelmaycallfor anelevation,butitalsoneedsthesaturatedthicknessoftheaquiferasthisispart ofwhatdetermineshowquicklytheadditionalgroundwaterflowisdispersed. Theseelevationsarenotevenconsistentwiththe ESHGWonthe site. 5.OnSheet8,thedetailfortheSnoutOil/Waterseparatorshouldbe clarified/relabeledtoindicatethatitistheoutlethoodforthecatchbasins,soas nottoconfuseitwiththedetailforthelargeroil/waterseparatorchambers.I wouldactuallysuggestthatthelargerchambersbecalledoutasoil-gritchambers. ThedetailfortheStormceptorunitsshouldalsoberemovedfromtheplan. 6.AnapplicationforaWatershedSpecialPermit,includingawrittencertification byaprofessionalengineerstatingthattheprojectwillnotcauseanysignificant degradationinthequalityorquantityofwaterinorenteringLakeCochichewick isstillneeded. Onceagain,IappreciatetheopportunitytoassisttheNorthAndoverPlanningBoardwith thereviewofthisproject,andhopethatthisinformationissuitableforyourneeds.Please 1018OsgoodStreet,StormwaterReview3 January7,2013 feelfreetocontactmeifyouortheapplicantshaveanyquestionsregardingtheissues addressedherein. Sincerely, EE GGLESTONNVIRONMENTAL LisaD. Eggleston,P.E. C:JenniferHughes,ConservationCoordinator