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Home My WebLink About2012-02-07 Planning Board Supplemental Materials (2) Eggleston Environmental January23,2012 NorthAndoverPlanningBoard 1600OsgoodStreet NorthAndover, MA01845 Attn:JudyTymon,TownPlanner NorthAndoverConservationCommission 1600OsgoodStreet NorthAndover, MA01845 Attn:JenniferHughes,ConservationCoordinator RE:StormwaterManagementReview 102PetersStreet DearMs. TymonandMs.Hughes: Peryourrequest,IhaveconductedaninitialtechnicalreviewoftheSitePlanSpecial Permitapplicationfortheproposedcommercialdevelopmentat102PetersStreet. IncludedinthematerialsIreceivedandreviewedwerethefollowing: StormwaterReport,102PetersStreet,preparedforDundeeProperties,LLCby AndoverConsultants,Inc.anddatedDecember14,2011. ProposedSitePlan,102PetersStreet,3sheets,preparedforDundeeProperties. LLCbyAndoverConsultantsInc.anddatedDecember13,2011. ItismyunderstandingthataNoticeofIntent(NOI)applicationfortheprojectisbeingor hasbeenfiledconcurrentlywiththeConservationCommission,andthatmyreviewis intendedtoassistboththePlanningBoardandtheConservationCommissionintheir respectivereviewsoftheproject.Myprimaryfocusinthisreviewisthereforeonthe overallstormwatermanagementapproachanddesignconceptsusedintheproject,as wellasitscompliancewiththeTownofNorthAndover’szoningrequirementsforSite PlanReview,theMassachusettsWetlandsProtectionAct(WPA),theNorthAndover WetlandsBylaw,andtheMassachusettsDepartmentofEnvironmentalProtection(DEP) StormwaterManagementStandardsandRegulations. Theprojectsiteisanapproximately0.3-acreparcelonthesouthsideofPetersStreet, adjacenttotheAndovertownline.Thepropertyiscurrentlyoccupiedbyasinglefamily house.Underpresentconditionsapproximatelyone-thirdoftheexistingsitedrains overlandtowardPetersStreet,andtheremainderdrainstothesouthandwest,towardan existingwetlandtothesouthofthe site. Therearenodrainagestructuresontheproperty. Theproposedplancallsfordemolitionoftheexistinghouseandconstructionofasingle- story2,745sfcommercialbuilding,withanewentrancedrivewayandassociated parking.Asproposed,roofrunofffromthebuildingwouldbeinfiltrateddirectlythrough asubsurfaceinfiltrationsystemcomprisedofeightprecastdrywellstotherear(south)of 32OldFramingham RdUnit29SudburyMA01776tel508.259.1137fax866.820.7840 107PetersStreet,TechnicalReview2 January23,2012 thebuilding.Runofffromthepavedparkinglotinfrontofthebuildingwouldbe collectedinaslottedvanedrainacrosstheentrancedrivewayandasingledeepsump catchbasinanddischargedthroughasubsurfacedetention/infiltrationsystembeneaththe parkinglot. Excessflowfromthedetention/infiltration systemwoulddrainatacontrolled ratetoasingle500-gallondrywell/infiltrationpitatthenorthwestcorneroftheproperty, adjacenttoPetersSt.Whenthecapacityofthedrywell/infiltrationpitisexceeded, overflowwouldbedischargedthroughthesurfacegrateanddrainontoPetersSt. Mycommentsareoutlinedbelow: 1.Theproposedprojectwouldincreasetheimperviouscoveronthesitefrom approximately11percenttoabout43percent;howeveritdoesnotappearthatany seriousconsiderationwasgiventotheuseofLowImpactDevelopment(LID) techniquesasisrequiredbytheDEPStormwaterStandards.Inparticular,theuse ofbioretentionand/orperviouspavementwouldbeappropriateandpotentially moreeffectivealternativestothebestmanagementpractices(BMPs)proposed. 2.ThesubmittalpacketincludesHydroCAD8.0analysesandatabularsummaryof thepeakflowratesunderpre-andpost-developmentconditionsforthe1,2,10and 100-yrdesignstorms,indicatingthattherateofrunofffromthesitewouldnotbe increasedasaresultoftheproject.Asoutlinedbelow,Idohaveanumberof concernsaboutthehydrologicmodelonwhichthisisbased.Inadditionthough, theapplicanthasnotaddressedchangesinthevolumeofrunoffthatwouldresult fromtheproject,asisrequiredbyboththeDEPStormwaterStandardsandthe WetlandsBylaw.Basedonthecalculationssubmitted,theproposed drywell/infiltrationpitlocatedintheexistingdrivewaywouldoverflowinallof thedesignstormconditions,withtheexcessflowdrainingdirectlyontoPeters Street.ThetotalvolumeofflowdischargedtoPetersStreetineachstormwouldbe approximatelydoublewhatitisunderexistingconditions,whilethevolumeof runofftowardthewetlandareawouldbeabouthalfofthepresentvolume. 3.Nocreditshouldbetakenforinfiltrationorstoragevolumewithingroundwater. WhiletheHSGBsoilsatthesitearesuitableforinfiltration,itappearsfromthe limitedsoilsdataprovidedthatallthreeproposedsubsurfaceinfiltrationstructures wouldbeinornearthegroundwatertableduringhighgroundwaterconditions. TheDEPHandbookcallsforaminimumoftwofeetofseparationfrominfiltration structurestotheestimatedseasonalhighgroundwaterelevation(ESHGW).Thisis particularlyimportantwheretheinfiltrationsystemisalsobeingusedtomeetthe 80percentTSSremovalrequirement,asisthecaseforthedetention/infiltration systembeneaththeproposedparkingarea.BasedonthedatafromTestPit#2,the ESHGWwouldbeapproximatelycommensuratewiththeinvertofthe detention/infiltrationsystem,providingnoseparationordeadstorageduringhigh groundwaterconditions. 4.Neitherofthetwotestpitsconductedonthesitewerewithinthelocations proposedforinfiltrationand,inthecaseoftheoverflowdrywell/infiltrationpit,the 107PetersStreet,TechnicalReview3 January23,2012 closesttestpitis75ftawayandfourfeethigherinelevation.Sitespecifictesting withineachproposedinfiltrationlocationwillbeneededpriortosystem installationtoverifysoilsuitabilityandensureadequatedepthtogroundwater and/orbedrock. 5.TheproposedprojectcallsfortheinfiltrationBMPstoattenuatelargestormflows (10-yrandlarger),thereforetheDEPstandardsrequirethatamoundinganalysisbe performedsincetheseparationtoseasonalhighgroundwaterislessthan4feet.In theeventthatacalculatedmoundreachestheinfiltrationstructure,additional storagevolumewouldbelost. 6.Itisnotclearwhytheoverflowdrywell/infiltrationpitismodeledwithan exfiltrationrateof0.17inches/hr,ratherthanthe0.52inches/hrusedelsewhere. Arethesoilsdifferentinthatlocation,oristhereducedinfiltrationratebasedon thefactthatthedrywellislocatedbeneaththeexistingdriveway,thereforethe soilsmaybecompacted? 7.Therateofflowfromtheproposeddetention/infiltrationsystemintheHydroCAD analysisappearstobedrivenmorebythemodeledtailwaterconditionsinthe downstreamdrywell(withitslimitedexfiltrationcapacity)thanbytheoutlet orifice.Irecommendrunningtheanalysiswithoutthetailwaterconditionstobetter assesstheeffectivenessofthedetention system. 8.InaccordancewiththeDEPhandbook,exfiltrationfromthethreeinfiltration structuresshouldbemodeledoverbottomareaonly,notthewettedarea. 9.Itappearsthatpost-developmentsubcatchment“Developed2”intheHydroCAD modelshouldberoutedtoPondP1(thecatchbasin),ratherthandirectlytothe subsurfacedetention system. 10.Thepavementareainpost-developmentsubcatchment“Developed2”shouldhave aCNof98,not95. 11.Thestonedepthintheproposeddetention/infiltrationsystemismodeledasbeing 192.25fthigh,itshouldbe4.25ft. 12.Considerationshouldbegiventoraisingtheoutletinvertfromthesubsurface detentionsystemtoincreasethedeadstoragevolumeinthesystemandenhance infiltration. 13.Theoverflowdrywell/infiltrationpitclosesttoPetersStismodeledashavingarim elevationof188.8,buttheplanshowsarimelevationof189.9andthedetailgives arimelevationof188.9. 14.Asindicatedabove,theproposedtreatmenttrain(deepsumpcatchbasinfollowed byinfiltration)doesnotmeetthe80percentTSSremovalrequirementbecause 107PetersStreet,TechnicalReview4 January23,2012 thereisnotadequateseparationtogroundwater.Inaddition,however,thereisa highrateoffailureofsubsurfaceinfiltrationstructureswherethereisonlyminimal pretreatmentsuchasisproposedforthisproject.Considerationshouldbegivento eitheraugmentingthesolidsremovalprovidedbeforeinfiltration,orimplementing amoreaggressivesweepingprogramtoreducethesolidsloadingtothedrainage system. 15.ThedrywelldetailsshownonSheets2and3shouldbelabeledastowhichsystem theycorrespondto. 16.The ESHGWelevation shouldbe shownoneachinfiltration systemdetail. 17.Thedepthofstoneunderthedetention/infiltrationsystemshouldbeshownonthe designdetails. 18.Theoutletpipefromtheproposeddrainmanholeis shownasbeing8inchesonthe DMHdetail,and10inchesonthedrywelldetail. 19.Inspectionportsareneededonthedetention/infiltrationsystem,andinspectionof that systemincludedintheO&MPlan. 20.TheO&MPlanshouldincludeanO&Mscheduleandlogform(forlong-term maintenancetasks)asrequiredpertheDEPregulations.Igenerallyfinditusefulto includea simplesketchplan showingthelocationsoftheBMPstobemaintained. 21.TheLongTermPollutionPreventionPlancallsforsnowtobestoredjustoffthe edgeofpavement;hencemostofthesnowmeltwillbypassthestormwater treatmenttrain. 22.Theproposedprojectwillnotentaildisturbanceofmorethanoneacre,thusitis notsubjecttocoverageundertheEPAConstructionGeneralPermit.The ConstructionPeriodPollutionPreventionandErosionandSedimentControlPlan shouldidentifyproposedstockpilelocationsandidentifymeasurestoprotectthe proposeddetention/infiltrationareaduringconstruction. IappreciatetheopportunitytoassisttheNorthAndoverPlanningBoardand ConservationCommissionwiththereviewofthisproject,andhopethatthisinformation issuitableforyourneeds.Pleasefeelfreetocontactmeifyouortheapplicantshaveany questionsregardingtheissuesaddressedherein. Sincerely, EE GGLESTONNVIRONMENTAL LisaD. Eggleston,P.E.