Stormwater Management Manual - CivicPlus

Public Works DepartmentCity of Billings, Montana

Stormwater Management Manual

February 2018

Stormwater Management Manual

Public Works DepartmentCity of Billings, Montana

Developed by

With assistance from

Debi Meling, PECity Engineer

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TableofContentsChapter1‐Introduction........................................................................................1‐1

1.1 OVERVIEW.................................................................................................................................1‐1 1.2 AUTHORITY...............................................................................................................................1‐2 1.3 ADMINISTRATIVEVARIANCE...........................................................................................1‐2 1.4 APPLICABILITY........................................................................................................................1‐2 1.5 SEVERABILITY.........................................................................................................................1‐2

Chapter2‐PolicyRequirements.........................................................................2‐1

2.1 PROJECTCLASSIFICATIONS...............................................................................................2‐1 2.1.1 GeneralRequirementsforAllDevelopments...........................................................2‐1

2.1.2 CityContractedProjects....................................................................................................2‐2

2.1.3 ResidentialLotDevelopments........................................................................................2‐3

2.1.4 CommercialPropertyDevelopment.............................................................................2‐3

2.1.5 SubdivisionDevelopment.................................................................................................2‐4

2.1.6 BusinessDistrictDevelopment/Redevelopment....................................................2‐4

2.2 MAINTENANCEACTIVITIES..............................................................................................2‐7

Chapter3‐Plan/PermitSubmittalRequirements........................................3‐1

3.1 PLANSUBMITTALREQUIREMENTS..............................................................................3‐1 3.1.1 SiteStormwaterPlan(SSP)..............................................................................................3‐1

3.1.2 ComprehensiveDrainagePlan(CDP)..........................................................................3‐2

Chapter4‐Rainfall..................................................................................................4‐1

4.1 APPLICATION...........................................................................................................................4‐1 4.2 MINORANDMAJORDRAINAGESYSTEMS..................................................................4‐1 4.3 DESIGNSTORMFREQUENCY............................................................................................4‐1 4.4 DESIGNSTORMDEPTHANDINTENSITY.....................................................................4‐2 4.5 WATERQUALITYSTORM....................................................................................................4‐3

Chapter5‐Runoff.....................................................................................................5‐1

5.1 APPLICATION...........................................................................................................................5‐1 5.2 DRAINAGEBASINAREA......................................................................................................5‐1 5.3 SELECTIONOFRUNOFFCALCULATIONMETHODS................................................5‐2 5.4 NRCS(SCS)HYDROGRAPHMETHOD.............................................................................5‐2

5.4.1 SoilsTypes...............................................................................................................................5‐2

5.4.2 TimeofConcentration........................................................................................................5‐2

5.4.3 CurveNumbers.....................................................................................................................5‐3

5.5 RATIONALMETHOD..............................................................................................................5‐4

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5.5.1 FrequencyCorrectionFactors........................................................................................5‐4

5.5.2 TimeofConcentration........................................................................................................5‐4

5.5.3 RunoffCoefficients...............................................................................................................5‐5

5.6 EPASWMMRUNOFFMETHOD.........................................................................................5‐5 5.7 MODIFIEDRATIONALMETHOD......................................................................................5‐5 5.8 COMPUTERAIDEDDESIGNSOFTWARE......................................................................5‐6

Chapter6‐HydraulicAnalysisandDesign......................................................6‐1

6.1 APPLICATION...........................................................................................................................6‐1 6.2 GUTTERFLOW.........................................................................................................................6‐1

6.2.1 Hydraulics...............................................................................................................................6‐1

6.2.2 MinimumGutterSlope.......................................................................................................6‐2

6.2.3 InletSpacingandLocation...............................................................................................6‐2

6.3 STORMDRAIN..........................................................................................................................6‐3 6.3.1 Hydraulics...............................................................................................................................6‐3

6.3.2 Materials..................................................................................................................................6‐4

6.3.3 AccessManholes...................................................................................................................6‐4

6.3.4 ClearancefromOtherUtilities........................................................................................6‐5

6.3.5 PrivateDrainageSystemConnections.........................................................................6‐6

6.3.6 Outfalls......................................................................................................................................6‐6

6.4 OPENCHANNELCONVEYANCES.....................................................................................6‐7 6.4.1 General......................................................................................................................................6‐7

6.4.2 Clearance.................................................................................................................................6‐7

6.4.3 ErosionControl.....................................................................................................................6‐7

6.4.4 FreeboardRequirements..................................................................................................6‐8

6.4.5 Low‐FlowChannels.............................................................................................................6‐8

6.4.6 FrictionFactors(n)..............................................................................................................6‐8

6.4.7 SideSlopes...............................................................................................................................6‐8

6.4.8 MaintenanceAccess............................................................................................................6‐8

6.4.9 Operation&MaintenanceofPrivateOpenChannels............................................6‐8

6.5 CULVERTS..................................................................................................................................6‐9 6.5.1 StreetOvertopping..............................................................................................................6‐9

6.5.2 HeadwaterDepth.................................................................................................................6‐9

6.5.3 AllowableVelocities............................................................................................................6‐9

6.5.4 Materials................................................................................................................................6‐10

6.5.5 EndTreatments...................................................................................................................6‐10

6.5.6 MaintenanceAccess..........................................................................................................6‐10

6.6 BRIDGES...................................................................................................................................6‐10

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Chapter7‐RunoffControlFacilities..................................................................7‐1

7.1 APPLICATION...........................................................................................................................7‐1 7.2 DETENTIONBASINS..............................................................................................................7‐1

7.2.1 Groundwater..........................................................................................................................7‐1

7.2.2 BasinGeometry.....................................................................................................................7‐1

7.2.3 MaximumDrain‐DownTime...........................................................................................7‐2

7.2.4 Low‐FlowChannels.............................................................................................................7‐2

7.2.5 Multi‐PurposeUse................................................................................................................7‐2

7.2.6 SetBacks..................................................................................................................................7‐2

7.2.7 WaterQualityTreatment..................................................................................................7‐2

7.2.8 OutletControlStructures..................................................................................................7‐3

7.2.9 EmergencyOverflow&Spillways.................................................................................7‐3

7.2.10 Vegetation&Landscaping................................................................................................7‐3

7.2.11 MaintenanceandMaintenanceAccess........................................................................7‐4

7.3 RETENTION/INFILTRATIONBASINS............................................................................7‐4 7.3.1 Geotechnical/HydrogeologicalEvaluation................................................................7‐4

7.3.2 Limitations..............................................................................................................................7‐5

7.3.3 Groundwater..........................................................................................................................7‐5

7.3.4 BasinGeometry.....................................................................................................................7‐5


7.3.6 Multi‐PurposeUse................................................................................................................7‐6

7.3.7 SetBacks..................................................................................................................................7‐6

7.3.8 InjectionWells.......................................................................................................................7‐7

7.3.9 WaterQualityTreatment..................................................................................................7‐7

7.3.10 Maintenance&MaintenanceAccess............................................................................7‐7

7.4 BOULEVARDSWALES...........................................................................................................7‐8 7.4.1 Locations..................................................................................................................................7‐8

7.4.2 Geotechnical/HydrogeologicalEvaluation................................................................7‐8

7.4.3 Groundwater..........................................................................................................................7‐8

7.4.4 BasinGeometry.....................................................................................................................7‐9


7.4.6 Culverts.....................................................................................................................................7‐9

7.4.7 LandscapingandVegetation............................................................................................7‐9

7.4.8 Maintenance&MaintenanceAccess............................................................................7‐9

Chapter8‐PermanentWaterQualityTreatment.........................................8‐1

8.1 APPLICATION...........................................................................................................................8‐1

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8.2 WATERQUALITYVOLUME.................................................................................................8‐1 8.3 WATERQUALITYFLOW......................................................................................................8‐1 8.4 SELECTIONOFPERMANENTWATERQUALITYBMPS..........................................8‐2

Chapter9‐ErosionandSedimentControl......................................................9‐1

9.1 APPLICATION...........................................................................................................................9‐1 9.2 SWPPPANDNOIREQUIREDSUBMITTALS.................................................................9‐1 9.3 CONSTRUCTIONLESSTHANONE‐ACRE......................................................................9‐2 9.4 BESTMANGAGEMENTPRACTICES(BMP)..................................................................9‐2 9.5 CONSTRUCTIONADJACENTTOWATERWAYS..........................................................9‐2

Chapter10‐Bibliography...................................................................................10‐1

ListofTablesTable2.1–RequirementsforResidentialLotDevelopments.................................................................................2‐3 Table2.2–RequirementsforCommercialPropertyDevelopments...................................................................2‐5 Table2.3–RequirementsforSubdivisionDevelopment..........................................................................................2‐6 Table3.2–ComprehensiveDrainagePlan(CDP)Submittals.................................................................................3‐2 Table4.1–DesignStormFrequencybyDevelopmentType...................................................................................4‐1 Table4.2–DesignStormFrequencybyStreetClassification.................................................................................4‐2 Table4.3–PrecipitationDepth–Duration....................................................................................................................4‐2 Table4.4–PrecipitationIntensity‐Duration...............................................................................................................4‐3 Table5.1–AcceptableRunoffCalculationMethods...................................................................................................5‐2 Table5.2–RunoffCurveNumbersforUrbanAreas...................................................................................................5‐3 Table5.3–FrequencyCorrectionFactorsforRationalMethod............................................................................5‐4 Table5.4–RunoffCoefficients("C")fortheRationalMethod...............................................................................5‐5 Table6.1–AllowableUseOfStreetsForMinorStormRunoff...............................................................................6‐1 Table6.2–AllowableUseOfStreetsForMajorStormRunoff...............................................................................6‐1 Table6.3–Manning’sCoefficients(n)forStormDrainConduits.........................................................................6‐4 Table6.4–MinimumAllowableManholeSize..............................................................................................................6‐4 Table6.5–MaximumAllowableManholeSpacing.....................................................................................................6‐5 Table6.6–AllowableStreetOvertoppingDepthsatCulvertCrossings.............................................................6‐9 Table8.3–NRCSUnitPeakDischarge..............................................................................................................................8‐2

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Appendices

AppendixAAdditionalRequirementsforCommercialSiteDevelopments

AppendixBPreliminaryDrainageReport

AppendixCFinalDrainageReport

AppendixDGeotechnical/HydrogeologicalReport

AppendixEOperationsandMaintenanceRequirements

AppendixFHOAAgreementRequirements

AppendixGTemplateForms StormwaterFacilityMaintenanceAgreement

BMPInspectionForm

ReportCertificationForm

Injectionwellexample

Boulderpitexamples

AppendixHSoilInfiltrationTestingRequirement

AppendixISpecialDischargeArea

AppendixJProjectSubmittalChecklist

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Chapter1‐Introduction

1.1 OVERVIEWThepoliciesandrequirementsofthismanualshallapplytodevelopmentsandconstructionactivitiessubmittedafterthedate listedonthefrontcover. Developmentsandconstructionactivitiescreatedpriortothisdateshallbegovernedunderpreviousappropriatemanualsandassociatedaddendums.ThismanualshallbereadinrelationtoCityCode,Chapter28,toensureproperstormwatermanagementpracticesaremet.

ThepurposeofthisManualistoprovidetheminimumstandardstobeusedfortheanalysisanddesignofstormdrainagesystemsforprivatedevelopmentprojectsandCitycontractedprojectswithintheCityof Billings. Thismanual provides direction and guidance to allow responsible development in andaroundtheCityofBillingswhileimprovingwaterqualitywithintheYellowstoneRiverdrainage.Thismanualprovidesguidanceforthepolicy,designandpermittingprocesstoaddressstormwaterrunoffandtreatmentfromproposeddevelopmentandlanddisturbance.

Thedesigncriteriapresentedinthismanualarebasedonindustrystandardengineeringpracticeforstormwatermanagement,modifiedtosuittheneedsoftheCityofBillings.Dependingonspecificsiteconditions,thedesignofstormdrainagesystemsmayneedtoexceedtheminimumstandardspresentedhereinordertoprovideadequateprotectionfromflooding.CriterianotspecificallydetailedhereinshallbedeterminedinaccordancewithsoundengineeringpracticeswiththeCity’sapproval.

TheCitywillconductalimitedreviewofDrainageplansforcompliancewithrequirementssetforthinthisManual.Contentandscopeofthedrainageplanwillvarybasedupondevelopmenttype,locationand site characteristics. The Applicant is exclusively responsible for ensuring that the design,construction drawings, completed construction, and record drawings comply with acceptableengineering practices and this Manual. The City’s limited plan review of Drainage plans is not asubstantivereviewoftheplansandengineering.TheCity’sultimateapprovalofaDrainageplanisnotanendorsementoftheplanorapprovalorverificationoftheengineeringdataandplans.NeithertheApplicantnoranythirdpartymayrelyupontheCity’slimitedrevieworapproval.

This manual is written for use by engineers who are familiar with generally accepted hydrologic,hydraulic,waterqualityandhydrogeologicdesignpractices.Adetailedpresentationofdesignmethodsand procedures is not included, as this information is readily available through industry‐acceptedpublications.Thismanualrelies,inpart,onmethodsandprocedurespublishedintheFederalHighwayAdministration (FHWA) Hydraulic Engineering Circular No. 22 (HEC‐22), “Urban Drainage DesignManual”,PublicationNo.FHWA‐NHI‐10‐009,datedAugust2013.ReferencestospecificsectionsofHEC‐22tobeusedforthedesignofstormdrainagesystemsintheCityofBillingsareincludedthroughoutthis document. HEC‐22 is available as a PDF document from the FHWA Website(http://www.fhwa.dot.gov/engineering/hydraulics/pubs/10009/10009.pdf). This manual alsoreferences theMontana Post‐Construction StormWater BMPDesign GuidanceManual for design ofstormwater quality improvements. This document is available from the City of Billings website

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(http://ci.billings.mt.us/567/Stormwater‐Management). In addition, this manual relies on generalapplication,design,andinstallationproceduresfortemporaryBMPsfromthemostcurrenteditionofthe Montana Department of Transportation “Erosion and Sediment Control Management PracticeManual”(http://www.mdt.mt.gov/publications/docs/manuals/env/bmp‐manual‐dec16.PDF).

TheCityreservestherighttomakeperiodicchangesandmodificationstotheguidelinesandtemplateformstoreflectupdatesofcitypractices.Itistheresponsibilityoftheusertodeterminethattheyareutilizingthemostcurrentversionofthesestandardsandforms.

1.2 AUTHORITYThisManualhasbeenadoptedbytheCityEngineer’soffice,andshallbereadwiththeBillingsMunicipalCityCodeandallotherlistedappurtenantreferenceddocumentsandmanuals.FailuretocomplywithanyportionofthismanualmayresultinrejectionofallpermitsandactionsasdescribedwithintheCityCode.Inaddition,Developermayberequired,attheirowncosts,tomitigatedesigndeficienciesfoundwithinthesystemoutsidetherequiredwarrantyperiod,ifthedeficiencieswerefoundtobetheresultofimproperconstructionand/ordesignassumptions.

1.3 ADMINISTRATIVEVARIANCETheCityEngineershallhavetheauthoritytoapproveavariancefromanyorallpartsofthismanualifitisdeemedavariancewillbeinkeepingwiththeintentofthemanualandwillbeinthebestinterestoftheCityofBillingsandtheexistingsystemadequacy.Suchvariancemaybegrantedonacase‐by‐casebasisandshallnotapplytopreviousorfutureconstructionordevelopmentactivities.TheCityEngineerdoesnothavetheauthoritytograntavariancefromtherequirementsofStateandFederalcodesandconditionssuchas,butnotlimitedto,theCity’sGeneralPermitorconditionsofDEQCircular8.

1.4 APPLICABILITYThismanualshallapplytoalldevelopment,redevelopmentandconstructionactivitiesonpublicandprivatepropertywithintheCityofBillings.

1.5 SEVERABILITYIfanysection,clause,sentence,orphraseoftheseregulationsisheldtobeinvalidorunconstitutionalby any court of competent jurisdiction, then said holding will in no way affect the validity of theremainingportionsoftheseregulations.

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Chapter2‐PolicyRequirementsThefollowingsectionsprovidetherequirementsforstormwatermanagementrelatedtoconstructionanddevelopmentactivitiesasdefinedbytheCityEngineer’sOffice. Detailsofthedesigncriteriaforstormwatermanagementfacilitiesareprovidedinsubsequentchapters.

The rate and volume of stormwater runoff fromproposed landdevelopments shall be estimated inaccordancewiththisManualandshallbethefoundationofthedrainageplan.DrainageplansshallbepreparedbyaProfessionalEngineerlicensedintheStateofMontanaandshallbesubmittedtotheCityEngineer’sOfficeforreviewandapproval.

Throughout thismanual, references aremade to theCity’s stormdrain systemornetwork. For thepurposesofthismanual,theCity’sstormdrainsystemornetworkisdefinedasanypipe,drain,pond,lakeorstreamownedandoperatedbytheCityofBillings.Roadsideborrowditches,irrigationditchesandirrigationwastedrainsshallnotbeconsideredapartoftheCity’sdedicatedstormdrainsystemornetworkunlesssuchfacilityhasbeenpreviouslyidentifiedandformallyincorporatedassuch.PleasecontacttheCityEngineer’sOfficeforthecurrentmapoftheCity’sstormdrainsystem.

2.1 PROJECTCLASSIFICATIONSStormwater management requirements vary based on the classification of the project. The CityEngineer’s Office has classified construction and development projects into five categories: 1) CityContracted Projects; 2) Residential Lot Developments; 3) Commercial Property Developments; 4)Subdivisions;and5)BusinessDistrictDevelopment/Redevelopment.Thefollowingsectionsdescribeandsummarizeeachclassificationandtheassociatedstormwaterrequirements.

2.1.1 GeneralRequirementsforAllDevelopments Natural drainages, such as depressions, swales, ditches, drains, channels, etc. shall be

preserved to the maximum extent possible. If preservation of existing drainages is notpossible,Developershallprovideadequateaccommodationsensuringflowsthroughnaturaldrainagesareproperlymitigated.Applicableregulationsfromotheragencieswillneedtobemetintheeventexistingdrainagesarenotpreserved.

Non‐stormwaterdischarge (illicitdischarge) isprohibited fromentering the stormdrainsystemasdefinedanddetailedbyChapter28oftheBillingsMunicipalCityCode.

BillingsMunicipalCityCode,Chapter6,Section1209 identifies specific subdivisions thatshallhavenooff‐siterunoffasaresultofdevelopment.Runoffwithintheseareasshallbeentirelymitigatedonsite. InadditiontothespecificareaslistedbyCityCode,developmentofallareasbetween

theRimsandHighway3,notcurrentlyidentifiedbyCityCode,shallresultinnooff‐siterunofffromtheMajorstormasdefinedinsubsequentsectionsofthismanual.

Stormwater discharge to private irrigation ditches, drains and laterals is acceptableprovidedapprovalhasbeengrantedbysuchfacilityowner/operator,thedischargerateis

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metered per the requirement of Sections 2.1.4 and 2.1.5 and discharge is delayed(controlled)toensuretheditch,drainorlateralfacilityisnotadverselyimpactedbeyondexistingconditions.Anagreementshallbesignedbetweenthefacilityowner/operatoranddeveloperwiththeCitynamedasathirdparty. Agreementmuststateataminimumthefollowing: Development’sdischarge requirements, conditionsofuse, termof agreement,maintenance responsibilities and a note stating that theCity shall have the first right toaccepttheautomatictransferofallinterestsandeasementsoftheditch/drainfacilityshouldtheditch/drainfacilityoperatorabandontheirfacilityadjacenttothedevelopment.

Subdivisions or commercial property developing adjacent to ditches and drains shallprovideright‐of‐wayoraneasementfromthetopofbankalongthepropertyfrontageformaintenanceaccess.Thewidthofright‐of‐wayoreasementshallbespecifiedbytheditchordrainfacilityownertoaccommodatebothappropriateheavyequipmentaccess,cleaningandmaintenanceactivities.However,innocaseshallthewidthbelessthan20‐feetoneithersideof theditch/drainfacility. Easementdocumentand/orplatSIAshallspecifythatnofences,trees,structuresorotherpermanentimprovementscanbeconstructedwithintheeasementarea.

Developersshallacknowledgethatthereexiststhepotentialforditches/drainstoovertop

duringrainevents,resultinginfloodingtoadjacentproperties. Thelocationsandrateofovertopping is unknown; however, developers shall account for this potential whensubdividing, constructing homes or businesses and shall use flood proofing or establishelevationsandgradingplansaccordingly.Thedegreeofimprovementsrequiredwilldependon the character of the adjacent ditch/drain and the location of development along theditch/drain.

Inaddition,developersshallacknowledgethatthereexiststhepotentialforadjacent

ditches/drains to leak contributing to seasonally high groundwater conditionswithin the development; which will need to be accommodated in the design ofstormwatermanagementfacilitiesandwithintheSIA.

2.1.2 CityContractedProjectsThe following requirements apply to projects contracted by the City of Billings, including roadreconstruction projects, new road construction, and other projects that impact existing drainagepatternswithintheCityofBillingsrightofway.

CityContractedprojectsshallfollowdesigncriteriaincludedinthismanual.RunoffcontrolandwaterqualitytreatmentrequirementswillbeestablishedbytheCityEngineermanagingtheproject.

SubmitaStormDrainageMemoforprojectswhichresultinonlyminorimpactstoexisting

drainagepatterns. TheCityEngineer’sOfficewill determinewhether the impactwill beconsidered“minor”or“major”onacase‐by‐casebasis.

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SubmitaComprehensiveDrainagePlan(CDP),inaccordancewithChapter3ofthismanual,for projects which result in major impacts to existing drainage patterns. The CDP shallidentifyanychangesinfloodhazardsduringtheMajorStormforprojectswhichresultinmajorimpactstoexistingdrainagepatterns.

2.1.3 ResidentialLotDevelopmentsTherequirementspresentedinTable2.1applytoresidentiallotdevelopment,includingsinglefamilyhomes,duplexes,townhomes,andcondodevelopmentsconstructingtwoorlesslivingunitslocatedona single lot and for “minor” construction and building projects in which either the improvementsnecessitateabuildingpermitand/orimprovementscreatemorethan2,500SFofimpervioussurface.

Table2.1–RequirementsforResidentialLotDevelopmentsDrainageDesignCriteria Reference

Site grading requirements shall follow specific requirements established in/on thesubdivisionplat,SIAoranycovenantswithinthesubdivision.

N/A

Unmitigatedrunoffgeneratedfromaresidentialsite(ornewimprovements)shallnotdrainfromthatsitetoaneighboringproperty.

N/A

If a commondrainage approach is usedwhich develops a solution for side and backyardrunoff, developer shall specify in the subdivision plat/SIA the details and practices to befollowed by subsequent property owners. For existing developments, a Declaration ofCommonDrainageshallbecompletedidentifyingdrainagesolutions.

N/A

Hard surfaces, including gutter downspouts shall drain onto lawns or pervious areasprovidingaminimumlengthof15‐feetforrunofftodispersepriortoreachingthepropertylinewithslopesnogreater than5%for lawnsandnogreater than2%forotherperviousareas.

N/A

PropertyownersmaynotalterexistingdrainagepatternsoftheirlotwithoutpriorapprovalfromtheCityEngineer.

N/A

ResidentiallotownersareencouragedtouseLIDmethodsontheirlot. Section 8.16

The elevation of residential dwellings and other lot features must be established inrecognitionoftheCity’spolicythatstormrunoffflowsareallowedtoadepthof18‐inchesinthegutterflowlineofadjacentstreetsduringtheMajorstorm.

Section 6.2

Verify that your project requires a Stormwater Pollution Prevention Plan (SWPPP); if sosubmitSWPPPandNOIForms.

Chapter 9

Downspouts with unfinished landscaping shall be equipped with sediment bags and/orenergydissipatersuntillandscapingestablishes.

Stormwaterfeaturesshallbepreservedpertheinitialdesignandmaintainedbythepropertyowner.

SIA & Section 7.4

Submittals Reference

SubmitaSiteStormwaterPlan(SSP)detailinglotgradinganddrainageplan. Section 3.1.1

2.1.4 CommercialPropertyDevelopmentTherequirementspresentedinTable2.2applytocommercialandindustrialdevelopmentprojectsandresidential home construction projects including single family homes, duplexes, townhomes, condodevelopmentsandresidentialmanufacturedhomesconstructingthreeormorelivingunitslocatedonasinglelotorthatarepartofaMasterSitePlanDevelopment. Fordevelopmentsconstructingprivatestreets,theprivatestreetcorridorsshallbeconstructedinaccordancewithSection4.3ofthismanualanddesignedtothe50‐yearstorm.

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Commercial expansion creating more than 2,500 square feet of new impervious surface shall begovernedbythismanual.Runofffromexistinghardsurfaceswillnotbegovernedbythismanualifthenewhardsurfaceisadequatelyhydraulicallydisconnectedfromtheexistinghardsurface.However,forcommercialsiteswheretheexistingcommercialfacilityandrelatedimpervioussurfacesareremoved,or substantially removed (asdefinedby theCity), andnewor redevelopment is occurring, the newcommercial development, or reconstructed portion thereof, shallmeet the full requirements of thissection.

IfanewconnectiontotheCity’sstormdrainsystemisinstalled,regardlessoftheactivitiescompletedunder Section2.2 of thismanual, the development’s stormwater systemwhich connects to theCitystormdrainsystemshallbebroughtintocompliancewiththerequirementsofthissection.

Some commercial project developments may be classified as “Business District Development/Re‐Development.”ForadescriptionofBusinessDistrictDevelopment/Redevelopmentandtheassociatedrequirements,seeSection2.1.6.

Duetothepotentialforincreasedpollutantrunoff,somesitesmayrequireadditionalregulatoryanddesignrequirements.RefertoAppendixAforthefollowingfacilities:

Fuelingstation,facilitystoring/transportingmorethan1,500gallonsofpetroleumproducts,hydraulic equipment storage, property zoned controlled or heavy industrial, vehiclemaintenance/repair, nurseries, lawn care/fertilizer facility, agricultural or animal carefacility,orothersimilarfacilities.

ThetableidentifyingRequirementsforCommercialPropertyDevelopmentscanbefoundwithinTable2.2onPage2‐5.

2.1.5 SubdivisionDevelopmentThetableidentifyingrequirementsforSubdivisionDevelopmentcanbefoundwithinTable2.3onPage2‐6.TherequirementsofthissectionandTable2.3applytosubdivisionswithinCityLimitsaswellastractsoflandunderreviewforannexation.

2.1.6 BusinessDistrictDevelopment/RedevelopmentWhere existingbuildingswithin the generaldowntown limits are currently constructed topropertylinesanddevelopmentisbeingreviewedbytheCityEngineer’sOfficebecauseofachangeinuseorotherqualifyingalteration,thefollowingcriteriashallapply:

IfidentifiedbytheCity,Developershallinstallthenecessaryimprovementstoensurerunofffromthesiteismanagedinamannerthatimprovesoffsitedrainagefromthatofexistingconditions.Anyknow,existingstormdrainproblemswillbeaddressedatthistime.

Reconstructionofthesesites(asdefinedbytheCity)shallbegovernedbySection2.1.4.Priortodesign,Developer shall meet with City Engineer’s Office to discuss site‐specific stormwater improvementrequirements.

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Table2.2–RequirementsforCommercialPropertyDevelopmentsGeneralCriteria Reference

DesignStormFrequencies. Section 4.2 – 4.5

AcceptableRunoffCalculationMethods. Section 5.3

ContributiontoregionalfacilityifidentifiedinSIAorotherprojectagreements N/A

WaterQualityRequirements Reference

WaterQualityVolumeandFlow(WQVandWQF). Section 8.2 & 8.3

WaterQualityTreatmentMethods. Section 8.4

RunoffControlRequirements Reference Runoffgeneratedfromthe50‐year,24‐hourstormshallbemitigatedonsite,withnodischargeontoneighboringpropertyorthepublicright‐of‐way.ThevolumeofrequiredstorageshallbebasedupontheassumptionofnodischargetotheCity’sstormdrainsystem.Infiltrationtonativesoilsisallowedinthecalculations.ThisrequirementappliestobothareasinsideandoutsideoftheSpecialDischargeArea,whichisfoundwithinAppendixI.

N/A

ForpropertiesnotidentifiedinAppendixIasbeingwithintheSpecialDischargeArea:WithCityapproval,waterqualitymitigated,andon‐sitestorageprovidedfor50‐year,24‐hrstorm,aconnectiontothestormdrainsystemmaybeallowed,withdischargemeteredtonotexceedpre‐developedrunoffratesforthe2,10and50‐yearstorms.

Section 6.3.5 Appendix I

ForpropertiesidentifiedinAppendixIasbeingwithintheSpecialDischargeArea:WithCityapproval,waterqualitymitigated,andon‐sitestorageprovidedfor50‐year,24‐hrstorm,aconnectiontothestormdrainsystemmaybeallowed,withdischargemeteredtoaproratedvalueof1cfsper10acresofdevelopment.i.e.a0.5acredevelopmentwouldbealloweda0.05cfs(appx.22gpm)discharge.

Appendix I

CommercialdevelopmentconstructedwithinanysubdivisionshallbegovernedbytherequirementsofthissectionindependentoftherequirementsofSection2.1.5.However,fornewdevelopments,acomprehensivesolutionthatcombinesthesubdivisionstormwatermanagementfacilitieswiththecommercialstormwatermanagementfacilitiesmaybeconsideredifanupfrontplanisdeveloped.

N/A

Overland/sheetflowtotheCity’sstormdrainsystem,includingstreetsandalleys,isnotallowed,unlessapprovedbytheCityEngineer.

N/A

Designstormwaterrunoffcontrolfacilitytomeetabovecriteria. Chapter 7

StormwaterFacilityDesignRequirements Reference Abovegroundstorageinparkinglotsmaynotpondtoadepthgreaterthan12inchesduringtheMajorStorm.

N/A

TheelevationofcommercialpropertiesmustbeestablishedinrecognitionoftheCity’spolicyallowingforan18‐inchdepthofflowinthegutterflowlineofadjacentstreetsduringtheMajorStorm.

Section 6.2

GeotechnicalandHydrogeologicEvaluationandRecommendations. Appendix D, H

Submittals,Permits,andEasements Reference

SubmitaComprehensiveDrainagePlan(CDP). Section 3.1.2

ASWPPPandNOImayberequiredtoaddressstormwaterrunoffduringconstructionanduntillandscapinghasestablished.Determinewhetheryourprojectwillrequirethesepermitsandsubmitasnecessary.

Chapter 9

Commercialpropertydevelopingadjacenttoditches/drains. Section 2.1.1

OwnershallcallforCityinspectionofdrainagefeaturespriortobackfilling. N/A

TVreportshallbesubmittedforconnectionswithinthepublicright‐of‐way. COB Standard Mods

OperationandMaintenance Reference

Allfacilitiesshallbeowned,operatedandmaintainedbythedevelopment.Appendix E, F Section 6.3.5

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Table2.3–RequirementsforSubdivisionDevelopmentGeneralCriteria Reference

DesignStormFrequencies. Section 4.2 – 4.5

AcceptableRunoffCalculationMethods. Section 5.3

HydraulicAnalysisandDesign. Chapter 6

ContributiontoregionalfacilityifidentifiedinSIAorotherprojectagreements. N/A

WaterQualityRequirements Reference

Adjacent subdivisions may work in conjunction to develop a common solution to achieve WQV and WQFdrainagerequirements,withapprovalfromtheCityEngineer’sOffice.

Chapter 8

WaterQualityVolumeandFlow(WQVandWQF). Section 8.2 & 8.3

WaterQualityTreatmentMethods. Section 8.4

RunoffControlRequirements Reference

ForpropertiesnotidentifiedinAppendixIasbeingwithintheSpecialDischargeArea;runoffduringthe2,10and 100‐year storm events shall not exceed the runoff rate of natural (pre‐developed) conditions, prior tosubdividing and developing the land. Off‐site runoff from the subdivision must maintain existing drainagecharacteristicsofpre‐developedconditions.

N/A

ForpropertiesidentifiedinAppendixIasbeingwithintheSpecialDischargeArea;runoffgeneratedbythe100‐yeareventshallbemitigatedandstoredonsite.Themaximumallowabledischargeshallbelimitedtoameteredrateof1cfs/10acresofsubdivisiondevelopment.

Appendix I

Runoffenteringthesubdivisionfromupstreampropertiesshallalsobeevaluatedandincludedinthecompositerateandvolumeofrunofffromthesubdivision.

N/A

Designstormwaterrunoffcontrolfacilitytomeetabovecriteria. Chapter 7.0

StormwaterFacilityDesignRequirements Reference

Theelevationofresidentialdwellings,buildings,orotherpermanentfacilitiesmustbeestablishedinrecognitionoftheCity’spolicyallowingforan18‐inchdepthofflowinthegutterflowlineofadjacentstreetsduringtheMajorStorm.EstablishedminimumbuildingelevationsshallbedocumentedintheSIA,platorotherapplicablerecordeddocument.

Section 6.2

Consider individual lot grading and drainage issues and provide necessary drainage easements to provideeffectivedrainagetopublicright‐of‐wayorapproveddownstreamareas.

N/A

GeotechnicalandHydrogeologicEvaluationandRecommendations. Appendix D

Submittals,Permits,andEasements Reference

SubmitaComprehensiveDrainagePlan(CDP)andGeotechnical/HydrogeologicalEvaluation. Section 3.1.2

SubmitHOAAgreement,O&MManualandBMPInspectionChecklist. Appendix F, G

A Stormwater Pollution Prevention Plan (SWPPP) and Notice of Intent (NOI) may be required to addressstormwaterrunoffduringconstructionanduntillandscapinghasestablished.Determinewhetheryourprojectwillrequirethesepermitsandsubmitasnecessary.

Chapter 9

Ifoff‐sitedischargeontoneighboringpropertiesisrequired,adrainageeasementmustbeobtainedthroughthedownstreamneighboringpropertiestothepointatwhichtherunoffiscollectedinapublicdrainagefacility.

Appendix G

Dischargeleavingsitemustleaveinsamemanor/characteristicsaspre‐developedconditions. N/A

Subdivisionsdevelopingadjacenttoditches/drains. Section 2.1.1

Ifcurbcutsfordriveapproachesareincludedduringinitialsubdivisionconstruction,theassociateddrivewayaprons(orotherapprovedfeature)shallalsobeconstructedtoaccommodategutterflowtothefullgutterdepththroughoutthesubdivision.

N/A

As‐Builts(PDF)shallbestampedandsubmittedbytheProfessionalEngineerofrecord. N/A

TVreportshallbesubmittedforallpipewithinthepublicright‐of‐way. COB Standard Mods

OperationandMaintenance Reference

Stormwater facilities within a subdivision (excluding mainline and inlets) shall be owned, operated andmaintainedbythesubdivisionHOA. Stormwater facilitiesoutsideof theR.O.W.shallbe locatedwithina lotownedbytheHOAandshallincludeaplattedeasementandassociatedaccesstotheR.O.W.Accesseasementshalldetailpropertyowner/HOAuseandmaintenanceofeasementarea.Landscapeplansshallbesubmittedwiththedevelopmentplans.

Appendix E, F, G

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2.2 MaintenanceActivitiesTheactivitieslistedbelowareconsideredtobe“maintenance”andarethereforenotgovernedbytherequirements of thismanual. Exclusion from these stormwatermanagement requirementsdoes notrelievethedevelopmentofotherrequiredpermitsandsubmittals.ContacttheCityEngineer’sOfficetodeterminewhat(ifany)permitsorsubmittalswillberequired.

Replacementofexistinginfiltrationfacilities;i.e.,boulderpitsorFrenchdrains. Resurfacingofanexistingparkinglot,includingreconstructionofbasegravelifgradesofthe

parkinglothavenotaltereddrainagepatterns. CityofBillingsmaintenanceprojects. Privateutilityimprovementprojectsdisturbinglessthanone‐acreoflandsurface.

Ifanewconnection ismadetotheCity’sstormdrainsystem,thedevelopmentshallmeettherequirementofSection2.1.4.FailuretoproperlymaintainastormwaterfacilityisgovernedbytherequirementsofChapter28oftheBillingsMunicipalCityCode.

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Chapter3‐Plan/PermitSubmittalRequirementsTheintentofthischapteristoprovideaframeworkforuniformityinplanpreparation,submittal,andreviewtopromoteefficiencyinthereviewprocess.Inaddition,properlydevelopeddrainageplanswillfacilitateproperoperationandmaintenanceofdrainagefacilitiesfollowingconstruction.AcceptancebytheCitydoesnotrelieveapplicantsfromresponsibilityforensuringsystemperformance,safety,andcompliancewithotherlocal,state,andfederalregulations.

TheCitywillconductalimitedreviewofDrainageplansforcompliancewithrequirementssetforthinthisManual.Contentandscopeofthedrainageplanwillvarybasedupondevelopmenttype,locationand site characteristics. The Applicant is exclusively responsible for ensuring that the design,construction drawings, completed construction, and record drawings comply with acceptableengineering practices and this Manual. The City’s limited plan review of Drainage plans is not asubstantivereviewoftheplansandengineering.TheCity’sultimateacceptanceofaDrainageplanisnotanendorsementoftheplanorapprovalorverificationoftheengineeringdataandplans.NeithertheApplicantnoranythirdpartymayrelyupontheCity’slimitedrevieworacceptance.

3.1 PLANSUBMITTALREQUIREMENTSStormwater Drainage Plans are divided into two categories based upon the development type; SiteStormwaterPlan(SSP)andComprehensiveDrainagePlan(CDP).Theapplicabilityandrequirementsforeacharedescribedasfollows:

3.1.1 SiteStormwaterPlan(SSP)TheSSPappliestoalldevelopmentslistedinSection2.1.3andshallbereviewedandacceptedpriortoissuanceofabuildingpermit.ThefollowingshallbeaddressedinanSSPsubmittal:

Inform theCity as to thedrainageplan, thenatureof the construction,project schedule,downstreamconveyances,andprojectcontact information.Planshall includeall finishedfloorelevations,drainageflowpaths,topbackofcurbelevations,downspout,windowwelllocationsandsimilarcriticalelevations.

Identifythedrainagepatternofadjacentlotstoensureacommondrainageapproachwithinthesubdivisionisbeingmet.

Showalleasementswithinlotandshow/identifyallsitespecificcriteriaandrequirementslistedwithinthesubdivisionSIA.

IfafterreviewoftheSSP,theCitydeterminesthatmoredetailorinformationisrequired,theCitymayrequireaComprehensiveDrainagePlan(CDP).

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3.1.2 ComprehensiveDrainagePlan(CDP)TheCDPapplies toalldevelopments listed inSection2.1.4 through2.1.6andshallbereviewedandacceptedpriortoissuanceofabuildingand/orright‐of‐waypermit.Table3.2shallbeusedtoidentifyrequired information to be submitted for various development activities. Additional information toguidethesesubmittalsisprovidedinthereferencedappendices.

Table3.2–ComprehensiveDrainagePlan(CDP)Submittals RequiredSubmittal

DevelopmentActivity

PreliminaryDrainageReport

FinalDrainageReport

Geotechnical/Hydrogeological

ReportO&M HOA SIA Reference

Commercial X X X

AppendixA,C,D,E,G,H

PreliminaryPlatX X X Appendix

B,D,G,H

PrivateContract X X X X Appendix

C‐H

3.1.2.1 PreliminaryDrainageReportThepurposeofthePreliminaryDrainageReportistoidentifyanddescribesitedrainageimpactsandillustratepreliminarysolutionstothedrainagesystemandanyproblemswhichmayoccuron‐siteandoff‐siteasaresultofthedevelopment,oranyphaseofthedevelopment.

Thereportshallprovideanappropriatelevelofdetailtoaddressdrainageissuesandpresenttheoverallplanfortheproperty.ThereportshallbebasedontheoutlineinAppendixBandincludeappropriate background information, supporting data, preliminary calculations and preliminaryplandrawings.

3.1.2.2 FinalDrainageReportThepurposeoftheFinalDrainageReportistoprovideindepthdetailsandcalculationstoaddressthedrainageissuesandpresentsizingandlocationsforallproposedimprovements.Thereportshallbe based on the outline provided in Appendix C and shall include appropriate backgroundinformationandsupportingdata,calculationsandfinalplandrawings.

Inadditiontodetailsandcalculations,theFinalDrainageReportshallincludeanarrativedescribingindetailhowthesiteandsitefeatureswillfunctionforthewaterqualitystormandtheMinorandMajorstormevents.Itisimperativethatthewrittennarrativeprovidesufficientinformationanddetailssuchthatinreadingthemainbodyofthereport,thereaderhasaclearunderstandingofhowthe site’s stormwater system functions foreach stormevent. Insufficient report contentwillbegroundsforrejectionofthesubmittal.

The Final Drainage Report shall contain the signed project certification cover sheet found inAppendixG.

If infiltration to underlying soilswill be used tomanage any portion of the site runoff, refer toproceduresoutlinedinAppendixDandGandthegeotechnical/hydrogeologicalrequirementsofthismanual.

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3.1.2.3 Geotechnical/HydrogeologicalReportThepurposeoftheGeotechnical/HydrogeologicalReportistoprovidesufficientinformationsuchthatreviewerhasaclearunderstandingofunderlyingsoilsandgroundwatercharacteristicsandhowthosewillinteractwithandbeimpactedbytheproposeddevelopment. Thereportshallbebased upon the outline provided in Appendix D and shall include appropriate backgroundinformationandsupportingdata,calculationsandplandrawings.

Inaddition to the report, a letter from thegeotechnical orhydrogeologicalprofessional shall besubmittedstatingtheimpactsthatthestormwaterrunoffwillhavetogroundwaterlevels,structuresandfacilitiesbothwithinandoutsidethelimitsofdevelopments.Ifimpactsareidentified,thereportshallprovidemitigationsolutionsforthedevelopment.

A groundwater mounding calculation shall be provided to identify the impacts of infiltratedstormwater runoff. Anexamplecalculationmethodandspreadsheet ismadeavailable fromtheUnitedStatesGeologicalSurvey(USGS);however,otherapproved,similarcalculationmethodsmaybeaccepted.

TheGeotechnical/HydrogeologicalReportshallcontainthesignedprojectcertificationcoversheetfoundinAppendixG.

3.1.2.4 OperationandMaintenance(O&M)ManualThe general purposeof theO&Mmanual is to identify theparty responsible for operations andmaintenance of the stormwater facility, detail maintenance schedules/activities and to ensureadherencewithapproveddesignoperatingconditions.

Appendix E outlines further requirements and information that shall be included in the O&Mmanual.

3.1.2.5 Homeowners’Association(HOA)AgreementFor subdivision development, an HOA agreement shall be submitted and approved to ensureperpetual legal validity and financial stability of the party responsible for ownership andmaintenanceofthestormwaterfacilityandthetemplateformfoundinAppendixG.

Appendix F outlines further requirements and information that shall be included in the HOAagreement.

3.1.2.6 SubdivisionsImprovementsAgreement(SIA)RequirementsThe SIA shall include language describing HOA agreements, O&M requirements, easements,property owner responsibilities and any other subdivision or building‐specific stormwatermitigationrequirements.

3.1.2.7 TemplateFormsDependingonthespecificrequirementsofthedevelopment,additionalagreementsorformsmayberequiredforsubmittal.TemplateformsareprovidedinAppendixG.

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Chapter4‐Rainfall

4.1 APPLICATIONThis chapter provides design storm frequency and precipitation data to be used in the design ofstormwatermanagementfacilitiesforCityofBillings’contractedprojects,residentiallotdevelopments,commercial property developments, subdivision developments, and Business DistrictDevelopment/Redevelopment and miscellaneous developments. The information provided for theWaterQualityStormisintendedforuseinthedesignofpermanentwaterqualitytreatmentfacilitiesforcommercial property, subdivisions, and Business District Development/Redevelopment andmiscellaneousdevelopments.

4.2 MINORANDMAJORDRAINAGESYSTEMSEveryurban areahas two separate anddistinct drainage systems,whetherornot they are actuallyplannedordesigned.OneistheMinorDrainageSystemandtheotheristheMajorDrainageSystem,whicharecombinedtoformtheTotalDrainageSystem.

TheMinorDrainage System is designed to transport the runoff from storm eventswith recurrenceintervalsfrom2‐yearto10‐yearwithaminimumofdisruptiontotheurbanenvironment.Minorstormdrainagecanbeconveyedinthecurbandgutterareaofthestreet(subjecttostreetclassificationandcapacityasdefinedherein),aroadsideditch,intheundergroundstormdrain,openchannels,orotherconveyancefacilities.

TheMajorDrainageSystemisdesignedtoconveyrunofffromthe100‐yearrecurrenceintervalstormtominimizehealthandlifehazards,damagetostructures,andinterruptiontotrafficandservices.Majorstormflowscanbecarriedintheurbanstreetsystem(withinacceptabledepthcriteria),openchannels,stormsewers,andotherfacilities.

Drainage planning and design shall include consideration for both the Minor and Major DrainageSystems.

4.3 DESIGNSTORMFREQUENCYThe design storm frequency varies depending on the development type as well as the streetclassificationasshowninTables4.1and4.2.

Table4.1–DesignStormFrequencybyDevelopmentType

DevelopmentTypeDesignStormFrequency(RecurrenceInterval,Year)Minor Major

ResidentialSubdivision 2 100CommercialLotDevelopment 50 N/A

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Table4.2–DesignStormFrequencybyStreetClassification

PublicStreetClassification2DesignStormFrequency(RecurrenceInterval,Year)

Minor1 MajorLocalStreets 2 100Collector/CommercialSubdivisionStreet 5 100Industrial/CentralBusinessStreets 10 100ArterialStreets 10 100150‐yeardesignstormshallbeusedfordepressedroadcrossings.2PrivatestreetsshallbegovernedasCommercialLotDevelopmentperTable4.1.

DrainagesystemsshallbeevaluatedfortheMajorstormtoidentifypotentialfloodhazards.Drainagesystemsmayneedtobedesignedtoconveyaportion,orall,oftheMajorstormflowsifMajorstormflowscannotbesafelyconveyedtoasuitablereceivingsystem,orifallowableflowdepthsinstreetscannotbemaintained(seeSection6.2).

4.4 DESIGNSTORMDEPTHANDINTENSITYRainfall depths and intensities are provided in Tabled 4.3 and 4.4 for the City of Billings, includingdurationsfrom5minutesupto24hoursandrecurrenceintervalsfrom2yearsupto100years.Thisinformation was derived using precipitation data available from the National Climatic Data Center(NCDC) forBillingsLogan InternationalAirport (NCDCCooperative StationNumber240807 (NCDC,2014)fortheperiodofrecordfromJuly1948throughSeptember2013.

Table4.3–PrecipitationDepth–Duration(DepthInInches)

Duration 2‐year 5‐year 10‐year 25‐year 50‐year 100‐year

5‐min 0.27 0.42 0.51 0.65 0.75 0.8510‐min 0.39 0.58 0.70 0.87 1.00 1.1315‐min 0.47 0.68 0.83 1.03 1.18 1.3320‐min 0.50 0.75 0.91 1.13 1.30 1.4625‐min 0.54 0.80 0.98 1.21 1.39 1.5630‐min 0.56 0.84 1.02 1.28 1.47 1.6635‐min 0.59 0.89 1.08 1.34 1.53 1.7240‐min 0.61 0.92 1.12 1.39 1.59 1.7845‐min 0.63 0.95 1.16 1.43 1.64 1.8450‐min 0.65 0.97 1.19 1.47 1.68 1.8955‐min 0.67 1.00 1.22 1.50 1.72 1.931‐hr 0.68 1.03 1.26 1.55 1.76 1.972‐hr 0.76 1.11 1.34 1.63 1.85 2.073‐hr 0.85 1.18 1.40 1.68 1.88 2.096‐hr 1.05 1.38 1.60 1.88 2.08 2.2812‐hr 1.29 1.67 1.92 2.23 2.46 2.7024‐hr 1.57 2.05 2.37 2.78 3.08 3.38

BasedonDOWLPrecipitationAnalysis(2015)

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Table4.4–PrecipitationIntensity‐Duration(IntensityInInchesperHour)

Duration 2‐year 5‐year 10‐year 25‐year 50‐year 100‐year

5‐min 3.26 5.02 6.18 7.75 8.96 10.16

10‐min 2.33 3.45 4.19 5.20 5.98 6.75

15‐min 1.87 2.74 3.31 4.11 4.72 5.32

20‐min 1.50 2.24 2.73 3.39 3.89 4.38

25‐min 1.29 1.93 2.35 2.91 3.33 3.76

30‐min 1.12 1.68 2.05 2.55 2.94 3.33

35‐min 1.01 1.52 1.85 2.29 2.62 2.95

40‐min 0.92 1.38 1.68 2.08 2.38 2.68

45‐min 0.84 1.26 1.54 1.91 2.18 2.45

50‐min 0.78 1.17 1.43 1.76 2.02 2.27

55‐min 0.73 1.09 1.33 1.64 1.88 2.11

1‐hr 0.68 1.03 1.26 1.55 1.76 1.97

2‐hr 0.38 0.55 0.67 0.82 0.93 1.03

3‐hr 0.28 0.39 0.47 0.56 0.63 0.70

6‐hr 0.18 0.23 0.27 0.31 0.35 0.38

12‐hr 0.11 0.14 0.16 0.19 0.21 0.22

24‐hr 0.07 0.09 0.10 0.12 0.13 0.14 BasedonDOWLPrecipitationAnalysis(2015)

ThedepthversusdurationinformationinTable4.3shallbeusedtogetherwiththeSCS(NRCS)TypeIIrainfall distribution to develop the 24‐hour stormhyetograph for runoff hydrograph analyses. TherainfallintensitieslistedinTable4.4forthecorrespondingdurations(timesofconcentration)shallbeusedintheRationalMethodtodeterminepeakrunoffrates.FortimesofconcentrationotherthanthoselistedinTable4.4,donotinterpolatewithinTable4.4directly;rather,thecorrespondingintensityshallbeestimatedusinga linear interpolationofprecipitationdepths. (i.e., the5‐year,22‐minuterainfalldepthinterpolatedfromthe5‐year,20‐minuteand25‐minutedepthsis0.77inches,whichtranslatestoanintensityof2.10inchesperhour).

4.5 WATERQUALITYSTORMThewaterqualitydesignstormshallbeusedtosizerunofftreatmentandwaterqualityBMPs.RunofftreatmentBMPs should be sized based on either thewater quality volume or flow rate in order toachievetherequiredtreatmentefficiencies.

Thewaterqualityrunoffvolumeand/orflowrateforpost‐developmentconditionsshallbebasedonthe0.5‐inchrainfallevent. ThisstormwasselectedbytheMontanaDEQand issued in theGeneralPermitandhasbeenadoptedbytheCityofBillingsasthewaterqualitydesignstorm.Industrystandardforwaterqualitytreatmentistocaptureandtreatrunofffromatleast90percentofthestorms.BasedonanalysisofprecipitationdatafortheCityofBillings,a0.5‐inchdailyprecipitationrepresentsthe94percentnon‐exceedancevalue(only6%ofdailystormtotalsexceed0.5inches).FurtherdetailsonhowtheWaterQualityDesignStormistobeappliedisdiscussedinChapter8–PermanentWaterQualityTreatment.

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Chapter5‐Runoff

5.1 APPLICATIONThischapterdiscussescriteriafordrainagebasindelineationandforselectionofacceptablestormwaterrunoffcalculationmethodstobeusedfordrainagedesignwithintheCityofBillings.

5.2 DRAINAGEBASINAREAThetotalarea,includingupstreamoffsiteareas,contributingtothepointofinterestshallbeincludedinthedelineationofdrainagebasins.Runofffromupstreamundevelopedland,notpartoftheproposedproject,shallbeincludedinthedesigncalculations.Runofffromupstreamdevelopedpropertymustbedeterminedbasedonexistingconditionsorapproveddrainageplans.Adetailedcontouredmapwiththebestinformationavailableshallbeusedtoidentifyoff‐siteareas.

There are several irrigationditcheswithin theCity ofBillings. The impact of these ditchesmust beconsideredinevaluatingdrainagebasinboundariesandinanalyzingoff‐sitecontributingrunofftothesiteandoff‐sitedischargefromthesite.Thedrainageanalysisshallincludeanevaluationofthecapacityoftheirrigationditchforcarryingstormwaterrunoffinadditiontoirrigationflowsandshallalsoincludean evaluation of potential flow contributions from irrigation deliveries or waste‐way structures.Irrigationditchesshouldgenerally,notbeconsideredtobeabasinboundarysincestormrunofffromup‐basinmayovertoptheditchandflowintotheareaofdevelopment.Irrigationditchesshouldonlybeconsidereddrainagedividesifitcanbedemonstratedthatadequatesurchargecapacityisavailableinexcessofthatrequiredtocarryirrigationflows.

Large drainage basins will typically need to be divided into sub‐basins to effectively evaluate thestormwaterrunoffconditions.Sub‐basinboundariesshallbeconstructedtorepresentthecontributingareatoindividualstormdraininletsandshallconsidertheroadwaycrown,roadsideditches,orothertopographicordrainagefeatureswhichdictaterunoffpatterns.Largesubdivisionsshouldbedividedintosub‐basinsreflectingpostdevelopmentbuild‐outconditionswhichwilldictatebasinsboundaries.Drainage sub‐basin boundaries for subdivisions shall reflect assumed post‐developed topographicdrainageboundaries,whichtypicallydonotfollowlegallotlines.Sub‐basinsforusewiththeRationalMethodshallbedelineatedtorepresenthomogenouslanduses.Wherebasinsarenothomogenous,theRationalMethodmayresultinunrealisticallylowpeakflowratesduetouseoflowweighted‐averagerunoffcoefficients.

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5.3 SELECTIONOFRUNOFFCALCULATIONMETHODSTable5.1–AcceptableRunoffCalculationMethodsRunoffCalculationMethod

Applications Limitations/Notes

RationalMethod

‐ Used for determining peak runoff rates for sizing conveyance systems ‐ Should not be used when routing of runoff hydrographs is required

‐ Should only be used for developments and basins of 5 acres or less ‐ Should only be used for basins with homogeneous land uses

ModifiedRationalMethod

‐ A simplified method used to approximate storage requirements for small drainages

‐ Should only be used for developments and basins of 5 acres or less ‐ The Modified Rational Method spreadsheet developed by the City of Billings shall be used (https://ci.billings.mt.us/567/Stormwater‐Management)

NRCS(SCS)Method

‐ Used for determining peak runoff rates and runoff hydrographs for large drainage basins ‐ Used for determining storage requirements for detention or retention facilities

‐ Should be used for developments and basins larger than 5 acres

EPASWMM‐ Used for complex drainage systems requiring modeling of a network of system components

‐ Consult with City Engineer for Approval to use this method

5.4 NRCS(SCS)HYDROGRAPHMETHODTheSCSHydrographMethodshallbeemployedusingtheproceduresdetailedinSection3.2.4oftheHEC‐22Manual(http://www.fhwa.dot.gov/engineering/hydraulics/pubs/10009/10009.pdf).

5.4.1 SoilsTypesUse site‐specific soils information for the project site when available, or the Natural ResourcesConservationService(NRCS)SoilSurveyofYellowstoneCountytoidentifythesoilsandcorrespondinghydrologicsoilgroupsforeachdrainagebasin

5.4.2 TimeofConcentrationThe time of concentration shall be calculated using the procedures detailed in TR‐55 Method(https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1044171.pdf). Time of concentration(Tc)shouldrangefromfiveminuteto20minutesformostbasins.Totheextentpossible,theresultingpeakflowsshallbeverifiedagainstobservedrunoffduringhistoricrainfalleventsinBillingstoensurereasonableness.Thefinalsubdivisionbuild‐outshallbeconsideredindeterminingappropriateTcflowpath. Sheet flow lengths shall be limited to no more than 150’ and, unless approved by the CityEngineer’sOffice,flowpathsfrombackyardsshallnotbeconsideredinthedeterminationofTc.Multipleflowpathscenariosshallbeevaluatedinselectingtheflowpathwhichresultsintheshortesttimeofconcentration,whichwillgoverndeterminationofthepeakflow.

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5.4.3 CurveNumbersCurvenumbers(CNs)tobeusedshallbeassetforthinTable5.2.Dependingonbasinarea,multipleCNswith associated flow properties may be required. A conservative approach shall be used whendeterminingCNvalues.

Table5.2–RunoffCurveNumbersforUrbanAreas(AverageWatershedCondition,Ia=0.2SR)

LandUseDescriptionCurveNumbersforHydrologicSoilGroup

A B C DFullyDevelopedUrbanAreas(VegetationEstablished)Lawns,openspaces,parks,golfcourses,cemeteries,etc. Goodcondition;grasscoveron75%ormoreofthearea 39 61 74 80 Faircondition;grasscoveron50to75%ofthearea 49 69 79 84Pavedparkinglots,roofs,driveways,etc. 98 98 98 98 Streetsandroads Pavedwithcurbsandstormsewers 98 98 98 98 Gravel 76 85 89 91 Dirt 72 82 87 89 Pavedwithopenditches 83 89 92 93

Average%impervious* Commercialandbusinessareas 85 89 92 94 95Industrialdistricts 72 81 88 91 93ResidentialLots:Average%impervious*

65 77 85 90 9238 61 75 83 8730 57 72 81 8625 54 70 80 85

*InterpolateasnecessaryDevelopingUrbanAreas(NoVegetationEstablished) Newlygradedarea 77 86 91 94NativeGroundCover Herbaceous–mixtureofgrass,weedsandlowgrowingbrush,withbrush

theminorelement(30%‐70%groundcover) 71 81 89

Herbaceous–mixtureofgrass,weedsandlowgrowingbrush,withbrushtheminorelement(>70%groundcover)

62 74 85

Sagebrushwithgrassunderstory 35 47 55Pasture,grasslandorrange–continuousforageforgrazing 39 61 74 80Meadow–continuousgrass,protectedfromgrazing,mowedforhay 30 58 71 78CultivatedAgriculturalLands Rowcrops 67 78 85 89Close‐seededorbroadcastlegumesorrotationmeadow 58 72 81 85 Smallgraincrops 63 75 83 87

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5.5 RATIONALMETHODOneofthemostwidelyusedequationsforthecalculationofpeakrunofffromsmallbasinsistheRationalformula,givenasfollows:

Q=CfCIA

Where:Q=Flowincfs, Cf=correctionfactorforinfrequentstorms,

C=adimensionlessrunoffcoefficient, I=rainfallintensityininchesperhour,and A=drainageareainacres

The RationalMethod shall be applied using the procedures detailed in Section 3.2.2 of theHEC‐22Manual (http://www.fhwa.dot.gov/engineering/hydraulics/pubs/10009/10009.pdf). To the extentpossible,theresultsshallbeverifiedagainstobservedrunoffduringhistoricrainfalleventsinBillingstoensurereasonableness.

5.5.1 FrequencyCorrectionFactorsTable5.3liststhecorrectionfactortobeusedforinfrequentstormevents.

Table5.3–FrequencyCorrectionFactorsforRationalMethodRecurrenceInterval(years) AdjustmentFactorCf

2and10‐year 1.0025‐Year 1.1050‐Year 1.20100‐Year 1.25

NOTE:C*Cfshouldnotexceed1.00

5.5.2 TimeofConcentrationThetimeofconcentrationshallbecalculatedusingtheproceduresdetailedinSection3.2.2.3oftheHEC‐22 Manual (http://www.fhwa.dot.gov/engineering/hydraulics/pubs/10009/10009.pdf). Time ofconcentration(Tc)shouldrangefromfiveminutesto20minutesformostbasins.Totheextentpossible,the resulting peak flows shall be verified against observed runoff during historic rainfall events inBillings to ensure reasonableness. The final subdivision or site build‐out shall be considered indeterminingappropriateTcflowpath. Sheetflowlengthsshallbelimitedtonomorethan150’and,unlessapprovedbytheCityEngineer’sOffice,flowpathsfrombackyardsshallnotbeconsideredinthedeterminationofTc.Multipleflowpathscenariosshallbeevaluatedinselectingtheflowpathwhichresultsintheshortesttimeofconcentration,whichwillgoverndeterminationofthepeakflow.

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5.5.3 RunoffCoefficientsRunoff Coefficients to be used shall be as set forth in Table 5.4. Depending on basin area,multiplecoefficientswithassociatedflowpropertiesmayberequired. Aconservativeapproachshallbeusedwhendeterminingcoefficientvalues.

Table5.4–RunoffCoefficients("C")fortheRationalMethodTypeofDrainageArea RunoffCoefficient,C*

Residential: Neighborhoodareas 0.70Single‐familyareas 0.50Multi‐units,detached 0.60Multi‐units,attached 0.75Apartmentdwellingareas 0.70Industrial: Lightareas 0.80Heavyareas 0.90Parks,cemeteries 0.10–0.25Playgrounds 0.20–0.40Railroadyardareas 0.20–0.40Unimprovedareas(forest) 0.10–0.30Lawns: Sandysoil,flat,2% 0.10Sandysoil,average,2‐7% 0.15Sandysoil,steep,7% 0.20Heavysoil,flat,2% 0.17Heavysoil,average2‐7% 0.22Heavysoil,steep,7% 0.35Streets,parkinglotsandotherpavedareas: Asphalticandconcrete 0.95Brick 0.85Drives,walksandroofs 0.95GravelAreas 0.70‐.85*Highervaluesareusuallyappropriateforsteeplyslopedareasandlongerreturnperiodsasinfiltrationandotherlosseshaveaproportionallysmallereffectonrunoffinthesecases.

5.6 EPASWMMRUNOFFMETHODTheEPASWMMrunoffmethodiswidelyacceptedandistypicallyusedforlargecomplexprojectsorsubdivisions.Theresultsfromthismethodshallbeverifiedforreasonablenessthroughcomparisontothe results of other accepted methods including the SCS Hydrograph Method and/or the RationalMethod.Totheextentpossible,theresultsshallalsobeverifiedagainstobservedrunoffduringhistoricrainfalleventsinBillingstoensurereasonableness.CheckwiththeCityEngineerforapprovaltousethismethod.

5.7 MODIFIEDRATIONALMETHODTheModifiedRationalMethodwasdeveloped(Poertner,1974)with the intentofusing therationalmethodforsizingofstoragefacilitiesonsmallwatersheds.TheModifiedRationalMethodapproximatesthevolumeofrunoffforvariousstormdurations,withpeakflowsbasedonthetimeofconcentrationforeachrespectivedurationandatriangularapproximationof therunoffhydrograph foreach. The

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differencebetweenthevolumeofrunoffintothefacilityandtheoutflowfromthefacility,computedforthevariousstormdurationsisusedtoestablishthemaximumrequireddetentionstorage.

Thissimplifiedapproachismorevalidforsmallbasinssuchasrooftopdrainage,parkinglots,orothersmallcomponentsofdevelopment,withcontributingareaslessthan5acres.

TheCitydevelopedandpreferstheuseoftheirModifiedRationalMethodspreadsheet.ThisspreadsheetisavailableontheCity’swebsite(https://ci.billings.mt.us/567/Stormwater‐Management).

5.8 COMPUTERAIDEDDESIGNSOFTWARETheCity of Billingswill allow the use of other computer aideddesign softwareprograms for large,complexdevelopments. However,designer shall provideadetailed,writtenexplanation,within thebodyoftheCDP,detailingselectionofinputparameters,descriptionforhowprogramcalculatesresults,andadetailedexplanationofthoseresults.AppurtenantresultspagesshallbeprovidedinanappendixoftheCDP.

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Chapter6‐HydraulicAnalysisandDesign

6.1 APPLICATIONThischapterprovidescriteriatobeusedinthedesignofpublicstormdrainageinfrastructureincludinginlets,manholes,stormdrain,openchannels,culverts,andbridgestosafelyconveystormrunoff forprojectswithintheCityofBillings.

6.2 GUTTERFLOWTheprimarypurposeofstreetsisfortrafficandtheuseofstreetsforstormrunoffmustthereforeberestricted.However, streets are an integral part of the urbandrainage systemandmaybeused fortransportingalimitedamountofstormrunoff.TheCityallowstheuseofstreetsfordrainagewithinthelimitationsspecifiedinTables6.1and6.2.StreetclassificationsarespecifiedonthecurrentFunctionalClassification Map developed by the City of Billings. This map can be found on the City’s website(http://ci.billings.mt.us/DocumentCenter/View/1998).

Table6.1–AllowableUseOfStreetsForMinorStormRunoffStreet

ClassificationMaximumStreetEncroachment

Local Nocurbovertopping.Flowmayspreadtocrownofstreet.1

CollectorsNocurbovertopping.Flowspreadmustleaveatleastone,11’lanefreeofwater,fivefeeteithersideofthestreetcrown.1

ArterialsNocurbovertopping.Flowspreadmustleaveatleasttwo,11’lanesfreeofwater,tenfeeteachsideofthestreetcrownormedian.1

Arterials(morethan6lanes)

Nocurbovertopping.Flowspreadmustleaveatleastfour,11’lanesfreeofwater,twentyfeeteachsideofthestreetcrownormedian.1

1Wherenocurbingexists,encroachmentshallnotextendbeyondpropertylines,exceptatdrainageeasements.

Table6.2–AllowableUseOfStreetsForMajorStormRunoffStreet

ClassificationMaximumDepth

LocalandCollectorsThe depth of water at the gutter flowline shall not exceed 18 inches.Residential dwellings and public, commercial, and industrial buildingsshallnotbeinundatedatthegroundlineunlessflood‐proofed.

Arterials

Toallowforemergencyvehicles,thedepthofflowatthestreetcrownshallbenomorethansixinches.Residentialdwellingsandpublic,commercial,andindustrialbuildingsshallnotbeinundatedatthegroundlineunlessflood‐proofed.

6.2.1 HydraulicsGutterflowencroachmentandhydraulicsshallbeevaluatedusingthemethodspresentedinSection4.3oftheHEC‐22Manual(http://www.fhwa.dot.gov/engineering/hydraulics/pubs/10009/10009.pdf).

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6.2.2 MinimumGutterSlopeGuttersshallbeconstructedatslopesnoflatterthan0.4percentforretrofitconditionsand0.6percentfornewconstruction.Fornewguttersectionsinverticalcurves,thek‐valueshallbelessthanorequalto167toprovideforadequatedrainage.Ifthek‐valueexceeds167,specialconsiderationshallbegiventopromotedrainage.

6.2.3 InletSpacingandLocation

6.2.3.1 GeneralTheinterceptioncapacityofinletsandrequiredspacingshallbedeterminedinaccordancewiththeproceduresdescribedinSections4.3and4.4oftheHEC‐22Manual.

RecommendedLocationsforinlets Priortopedestriancrossings Atlowpointsintheguttergrade Wheresignificantflowsfromofftheright‐of‐wayareexpected On horizontal curveswhere a change from normal crown to super‐elevationmay cause

watertosheet‐flowacrosstheroad Wherelay‐downcurb(e.g.,atapproaches)mayallowtheflowtoescapeandcauseflooding Wherenecessarytomaintaingutterflowwidthsanddepthswithintheallowablelimitsset

forthinTables6.1and6.2 Mid‐block inlets within subdivisions, shall be located along property lines to minimize

impactstofuturedrivewaysandotherdevelopmentfeatures Whereacurbedroadwaycrossesabridge, thegutter flowshouldbe interceptedandnot

permittedtoflowontothebridge.

6.2.3.2 InletTypesAllowedstorminlettypesincludegratedandcombination(gratedwithcurbopeningorgratedwithcurbopeningplusslotteddrain)inlets.TheCityofBillingsstandardinletsincludeTypeIIinletsinsag locations and Type III inlets for on‐grade installations (See City of Billings StandardModifications;https://ci.billings.mt.us/569/Standard‐Mods). Forstreetswithoutcurbandgutter,theseinletsmaynotbeappropriateandanotherinletmaybeselectedwithCityapproval.

Inlets in sag locations– inlet capacity in sump locations shall reflect50percentpluggingbydebris(designcapacityequals50percentofthetheoreticalcapacity).

Inletsaton‐gradelocations–inletcapacityon‐gradeshallreflect25percentpluggingbydebris,(designcapacityequals75percentofthetheoreticalcapacity).

Inletsinstalledwithintheright‐of‐way,orareadjacenttotrails,sidewalks,andbikelanesmusthave grates that aredesignated forpedestrianandbicycle traffic. Approval from theCity isrequiredforinletswithintheright‐of‐waythatarenotdesignatedforpedestrianandbicycle.

Thecapacityofaninletshallbecheckedagainstthecapacityoftheinletlateralpipetoensurethatthelateralpipehasadequatecapacitytoconveytheinlet(s)capacity.

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6.2.3.3 SedimentFiltersSedimentfiltersmayberequiredbytheCityinlocationswithhighsedimentloads.Sedimentfiltersmustbeproperlymaintained,includingsemiannualreplacementschedules.ImpropermaintenanceofsedimentfilterswillbesubjecttoCityrecourseasoutlinedinCityCode,Chapter28.Sedimentfiltersmaydecreaseflowcapacityoftheinlet.Theinletcapacitycalculationsshouldconsiderthefiltermanufacturer’scapacityrestrictionsoftheinlet,iffiltersarerequired.

6.3 STORMDRAIN

6.3.1 HydraulicsUsethemethodssetforthinChapter7oftheHEC‐22Manualforthehydraulicdesignofstormdrains,exceptasmodifiedherein(http://www.fhwa.dot.gov/engineering/hydraulics/pubs/10009/10009.pdf).

6.3.1.1 FreeboardRequirementsStormdrainsshallbedesigned tooperate inanon‐pressurized(non‐surcharged) flowconditionduringtheMinorstorm.StormdrainsmaybedesignedtosurchargeduringMajorstormevents;however, surcharging shall not result in a hydraulic grade line (HGL) elevation higher than thecriterialistedinTable6.2.

6.3.1.2 Diameter,Slope,andVelocityLimitsMinimumslopesforstormdrainpipesshallbeasrequiredtomaintainafull‐flowvelocityofatleast2.5feetpersecondduringtheMinorStormEvent.Maximumfull‐flowvelocityshallbelimitedto12feetpersecondinstormdrainmainsduringtheMinorStormEvent.

Minimumdiameterforstormdrainmainlinesandlaterals,whichwillbepartofthepublicstormdrainage system, shall be12 inches. Minimumdiameter forprivate connections into thepublicstormdrainagesystemshallbe6 inchesasdiscussed inSection6.3.5. Pipesizesshallnormallyincreaseinthedownstreamdirectionandtransitionsfromsmallerpipestolargerpipesshalloccurbymatchingtheinsidetop(crown)ofthepipeswherepracticable.Whereitisnotpossibletomatchcrowns,the80‐percentdiameterpointsofthepipesshallbematched,wherepracticable;or,uponapproval fromtheCityEngineer’sOffice, invertsmaybematchedif theHGLdoesnotexceedthestreetelevation.

6.3.1.3 MaintenanceAccessAllstormwaterpipeandfacilitiesshallbeaccessibleforoperationandmaintenancebytheCityofBillings,Homeowners’Associationorprivatebusiness.

Whenvehicleaccessisnecessary,forfacilitiesconstructedoutsideofthestreetsection,accessroadsshallbeprovidedindedicatedaccesseasements.Theminimumcleardrivinglanewidthofaccessroadsis12feet.Accessroadsshallhaveamaximumgradeofninepercentandshallbeconstructedwithgravel,pavement,concreteoranappropriateall‐seasonsurface.

Gatesand/orbollardsarerequiredwhennecessarytorestrictaccesstostormwaterfacilities.Cablesand/orchainsstretchedacrossaccessroadsarenotacceptable.

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6.3.1.4 Manning“n”ValuesTheManning’s“n”valueusedforthedesignofstormdrainsshallbeasshowninTable6.3.

Table6.3–Manning’sCoefficients(n)forStormDrainConduitsPipeMaterial Roughnessor

CorrugationManning’sn

ConcretePipe Smooth 0.013ConcreteBoxes Smooth 0.015

SpiralRibMetalPipe Smooth 0.013

CorrugatedMetalPipe,Pipe‐ArchandBox

2‐2/3by1/2inAnnular2‐2/3by1/2inHelical

6by1inHelical5by1in3by1in

6by2inStructuralPlate9by2‐1/2inStructuralPlate

0.0270.0230.0250.0260.0280.0350.037

PolyBasedThermoplasticSmooth 0.015

Corrugated 0.025PVCBasedThermoplastic Smooth 0.011

*Publishedvaluesmaydiffer;however,valuespresentedinthistableassumelongtermuseofpipewhichleadstoincreasedroughness.Manufacturerrecommendationsshallbeusedifvaluesarehigherthanpresentedabove.

6.3.2 MaterialsStorm drains shall be constructed and installed as represented by the City of Billings StandardModifications to the Montana Public Work Standard Specifications (MPWSS)(https://ci.billings.mt.us/569/Standard‐Mods).

6.3.3 AccessManholes

6.3.3.1 GeneralAccessmanholesarerequiredwhenjoiningpipesofdifferenttypes,sizes,athorizontalorverticalbendsinthealignment,atlateralconnections,andattheupstreamterminusofstormdrainmains.

6.3.3.2 RequiredSizeTherequiredminimummanholesizeshallbeasshowninTable6.4.

Table6.4–MinimumAllowableManholeSize

StormDrainDiameterManholeDiameter

12"to24" 4’27"to36" 5’42" 6’48"andlarger JunctionboxorTeeManhole

Largermanholediametersorajunctionboxmayberequiredwhenstormdrainalignmentsarenotstraightthroughorwherelateralpipesenterthemanhole.Thenumberandsizeofpipesthatmaybeconnectedtoanysinglemanholeislimitedinordertomaintaintheintegrityofthestructure.Forangledconnectionsorthosewithseveralpipesonthesameplane,alargermanholethansetforthintheTable6.4mayberequired.Forstructural integrity,minimumundisturbedwall(edgeofpipe

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openingtoedgeofpipeopening)shallbe8inchesandfor72‐inchandlargerdiameterstructures,theminimumundisturbedwallbetweenopeningsshallbe12inches;unlessotherwiseallowedbythemanufacturer’sengineer.

6.3.3.3 RequiredSpacingThemaximummanholespacingalongstormdrainsisassetforthinTable6.5.

Table6.5–MaximumAllowableManholeSpacingStormDrainDiameter MaximumSpacing

12”to36” 400’42”to60” 500’66”andLarger 600’

6.3.3.4 MaximumManholeDepthManhole depths shall not exceed 20 feetwithout special safety provisions such as intermediateplatformsandminimumdiameterrisersof48inches.

6.3.3.5 DropManholesThedifferencebetweenthehighesttrunklinepipeinvertenteringamanholeandtheinvertleavingshallnotexceed24inches.Manholesexceeding24inchesoffallshallbedesignedasdropmanholes.Dropmanholeswithdropheightsexceedingsixfeetshallbedesignedwithhighstrength(6,000psi)concrete.

6.3.4 ClearancefromOtherUtilitiesThe followingutilityclearancesshallbemaintainedwherepossible. Allclearancesarebasedontheoutsideedgeofthestormdraintotheoutsideedgeoftheotherutilities.Foradditionalinformation,refertotheInfrastructureLayoutdiagramlistedintheCity’sStandardModificationsDrawings.

Horizontalclearancesfromstormmain:CableTV,Gas,Power 5feetSewer 6feetTelephone,FiberOptics 5feetWater 12feet

Verticalclearancesfromstormmain:CableTV,Gas,Power 1feetSewer 1feetTelephone,FiberOptics 1feetWater 1.5feetMisc.PrivateUtilities 1feet

Watermaincrossingsshallbedesignedtopreventfreezingduetominimalclearancefromstormdrains.

Avoidcrossingotherutilitiesathighlyacuteangles.Theanglemeasurebetweenutilitiesshallbebetween45and90degrees.

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6.3.5 PrivateDrainageSystemConnectionsPrivatedrainagesystemconnectionstothepublicstormdrainsystemshallbeapprovedbytheCityandshallcomplywiththefollowingcriteria.Suchconnectionsshallbeentirelyownedandmaintainedtothemainbydevelopmentinwhichtheconnectionwasinstalledand/orservices.

All private stormwater connections shall include backflow prevention to preventstormwater from the City’s stormdrain system from surcharging onto private property.Backflowpreventermustbeinstalledon‐siteandnotwithinthepublicright‐of‐way. Minimum pipe diameter discharging to the City’s storm drain system shall be 6

incheswithaminimumslopeof1percenttoprovideadequatescourvelocity.ThemaximumpipediameterallowedwilldependonanevaluationofthecapacityoftheCity’sstormdrainsystemandapprovalfromtheCityEngineer’sOffice.

Directly connectedpumped connections to the City’s stormdrain system are notallowed.Developmentsmayinstallapumptomitigatestormwaterrunoffpertherequirements of this manual; however, stormwater runoff shall be pumped to amanholeorotherfeaturepriortomakingagravityconnectiontotheCity’ssystem.

Lateralconnectionswithinthepublicright‐of‐wayshallbemadeatrightangles. Private connections to the City storm drain shall bemade by the following (in order of

preference): Core‐drillorappropriatefittingdirectlyonthemainline;or Connectingtoanadjacentcatchbasin/manhole.Connectionshallonlybemadewith

approvalfromtheCityEngineer’sOfficeifitisdeemedsuchconnectionisinthebestinterestoftheCity.

6.3.6 Outfalls

6.3.6.1 General UsethemethodssetforthinChapter7.1.5oftheHEC‐22Manual,asmodifiedherein. Invertelevationsofoutfallsshallbenolowerthanthebank‐fullwatersurfaceelevation(2‐

yrflood)inopenchannelsorstreams,wherepracticable. Outfallsdownstreamofdetentionfacilitiesshallbedesignedtopreventbackwaterintothose

facilities. Outfallswithinditches/drainsshallbeconstructedwithfire‐proofmaterial.

6.3.6.2 ErosionProtection Erosionprotectionisrequiredattheoutlettopreventerosionoftheoutfallchannelbedand

bank.

6.3.6.3 EnergyDissipation/Rip‐Rap Where flow velocities exceed 10 feet‐per‐second at the outfall, during theMinor Storm

(whentheoutfallconduitisrunningatdesigncapacity),energydissipation,inadditiontoerosion protection may be required to minimize erosion. Design energy dissipationmeasures inaccordancewithFHWAHEC‐14, “HydraulicDesignofEnergyDissipaters forCulverts and Channels”(https://www.fhwa.dot.gov/engineering/hydraulics/pubs/06086/hec14.pdf).

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Rip‐Rapsizeandclassificationshallbebaseduponflowratestobemitigated.Rip‐RapsizingshallfollowMDT’sguidelineswithintheirStandardSpecifications,Division700:

http://www.mdt.mt.gov/other/webdata/external/const/specifications/2014/division_700.pdf

6.3.6.4 MaintenanceAccess Allstormwaterpipeandfacilitiesshallbeaccessibleforoperationandmaintenancebythe

CityofBillings,Homeowners’Associationorprivatebusiness.When vehicle access is necessary, for facilities constructed outside of the street section,accessroadsshallbeprovidedindedicatedaccesseasements.Theminimumcleardrivinglanewidthof access roads is12 feet. Access roads shallhaveamaximumgradeofninepercent and shall be constructedwith gravel, pavement, concrete or an appropriate all‐seasonsurface.

Gates and/or bollards are required when necessary to restrict access to stormwaterfacilities.Cablesand/orchainsstretchedacrossaccessroadsarenotacceptable.

6.4 OPENCHANNELCONVEYANCES

6.4.1 GeneralNeworalteredchannelsshallbelinedwithgrass,rocksorothererosionresistantmaterials.ConcreteorasphaltshallnotbeusedunlessapprovedbytheCityEngineer.

Design open channels in accordancewith themethods provided in Chapter 5 of HEC‐22 except asmodifiedherein.BoulevardswalesareconsideredacategoryofOpenChannelConveyancesandarediscussedinmoredetailinChapter7.

6.4.2 ClearanceChannelsshallbelocatednocloserthantenfeetfromanystructurefoundationasmeasuredhorizontallyfromtheedgeoftheswaleatthetopoffreeboardelevation.

6.4.3 ErosionControlChannel segments shall be designed according to the permissible tractive force (shear stress)methodology set forth in Section 5.3 of HEC‐22 and Hydraulic Engineering Circular 15(http://www.fhwa.dot.gov/engineering/hydraulics/pubs/10009/10009.pdf,http://www.fhwa.dot.gov/engineering/hydraulics/pubs/05114/05114.pdf,respectively)

Both the bare soil condition immediately following construction and the anticipated vegetatedconditionsof thechannelshallbeevaluated. If thechannel isdeterminedtobeunstableduring theMinor Storm for either of these conditions, the appropriate long‐term, temporary, and transitionalliningsshallbeinstalled.

Erosion control structures, such as check drops or check dams, may be required to control flowvelocities.

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ThegrassspeciesselectedforseedingshallconformtorequirementssetforthintheMontanaPublicWorksStandardSpecificationsandtheCityofBillingsStandardModifications.

6.4.4 FreeboardRequirementsAminimumfreeboardofone‐footfromthewatersurfaceduringMajorStormeventtothetopofbankshallbeprovidedforopenchannelconveyances.

6.4.5 Low‐FlowChannelsLow flow channels shall be included in the channel cross section to carry sustained low flows andfrequent storm events in a confined sub‐section of the larger channel. A pipemay also be used tomitigatesustainedlowflows.

6.4.6 FrictionFactors(n)UseManning’sroughness factors (n)set forth inTable5‐1ofHEC‐22.Thedesignshall consider thechannel roughness both immediately after construction and when vegetation is fully established.Roughnessfactors,whicharerepresentativeofunmaintainedchannelconditions,shallbeusedfortheanalysis of water surface profiles. Roughness factors, which are representative of well‐maintainedchannelconditions,shallbeusedtodeterminemaximumvelocity.

6.4.7 SideSlopesSideslopesshallbenosteeperthan4H:1Vformaintainedgrass‐linedchannels,3H:1Vforunmaintainednativegrass‐linedchannelsand2H:1Vforriprap‐linedchannels.

6.4.8 MaintenanceAccessProvidemaintenanceaccess for inspection,mowingoperations,anddebris removalbyconventionalequipment along the length of the conveyance channel. The type of equipment needing access isdependentonthesizeofthechannel.Largechannelswillneedaccessfordumptrucksandloaders.Forsmall ditches, foot or pick‐up truck accessmay suffice. Channels may need to be offset within theeasementtofacilitymaintenance.

6.4.9 Operation&MaintenanceofPrivateOpenChannelsOpenchannelsrequireperiodicmaintenance.Thedegreeofmaintenanceisdependentonthelocation,thespecifictypeoffacility,andthelinermaterial(grass,rock,etc.).

Maintenanceofopenchannelsisrequiredtoinsuretheconveyancecapacityofthefacilityismaintainedandthatchannelerosiondoesnotoccur.Theconditionofopenchannelsshouldbecheckedonaperiodicbasis,especiallyafterlargestormsorextendedperiodsofhighfloworimmediatelyfollowingperiodsofhigh intensity winds (erosion may occur during high flows, from scour caused by localized debrisblockageorfromdebrisblownintothechannel).Debrisshouldberemovedtopreventchannelplugging,channelscourandlossofchannelconveyance.Erosionshallberepairedorstabilized.

Vegetatedchannelsshallbemaintainedtoensurethatvegetationdoesnotlimittheconveyancecapacityof the facility. If conveyance restrictions are apparent, the vegetation should be trimmed to restorecapacity.

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Emergentvegetation(spirogyra,elodea,watercress,etc.) inconveyancechannelsmayalsobecomeaproblemifitisallowedtoconstricttheconveyancecapacityofthefacility.Vegetationabovetheordinaryhighwatermarkshallalsobemonitoredprimarilyforitsabilitytoretainbankstabilitywithoutreducingchannelcapacityatmaximumdesignflows.

6.5 CULVERTSCulvertsareusedtoconveywaterinirrigationditchesandnaturaldrainage‐waysunderCityStreetsandtrails.CulvertsshallbedesignedusingthemethodssetforthintheFederalHighwayAdministration(FHWA)HydraulicDesignSeriesNo.5(HDS‐5),“HydraulicDesignofHighwayCulverts”,PublicationNo.FHWA‐NHI‐01‐020 except as modified herein(http://www.fhwa.dot.gov/engineering/hydraulics/pubs/12026/hif12026.pdf).

6.5.1 StreetOvertoppingCulvertsshallbesizedsuchthatthedepthofstreetovertoppingislimitedassetforthinTable6.6.

Table6.6–AllowableStreetOvertoppingDepthsatCulvertCrossings

StreetClassification MinorStorm MajorStorm

LocalandCollector NoneSix inches at the street crown. Residential dwellings and public,commercial, and industrial buildings shall not be inundated at thegroundlineunlessflood‐proofed.

Arterial NoneNo overtopping allowed. Provide 1‐ft of clearance between thecrown of the culvert and the water surface elevation wherepracticable,fordrainagebasinsgreaterthanonesquaremiles.

6.5.2 HeadwaterDepthTheheadwater(HW)depthshallbelimitedaccordingtothefollowingratiostodiameter(D):

Forculvertswithacrosssectionalarealessthanorequalto30squarefeet:HW/D≤1.5 Forculvertswithacrosssectionalareagreaterthan30squarefeet:HW/D≤1.2

Culvertsmustalsobesizedwithoutcreatingsignificantflowconstriction,suchthatexistingchannelsupstreamarenotovertoppedduringthedesignflowevent.

6.5.3 AllowableVelocitiesCulverts shall be designed tomaintain aminimumvelocity of 2.5 feet‐per‐secondduring theMinorStormtopreventsedimentaccumulationandshallbedesignedwithaminimumslopeof0.5percent,wherepracticable.

CulvertsshallbesizedtolimitvelocitiesinordertominimizeerosionpotentialduringtheMajorStormEvents.Forexitvelocitiesinexcessof10feet‐per‐secondduringtheMajorStorm,energydissipation,in addition to erosion protection may be required to minimize erosion. Design energy dissipationmeasuresinaccordancewithFHWAHEC‐14,“HydraulicDesignofEnergyDissipatersforCulvertsandChannels”(https://www.fhwa.dot.gov/engineering/hydraulics/pubs/06086/hec14.pdf).

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6.5.4 MaterialsCulvertsshallbeconstructedofmaterialsapprovedbytheCityasrepresentedbytheCityofBillingsStandard Modifications to the Montana Public Work Standard Specifications (MPWSS)(https://ci.billings.mt.us/569/Standard‐Mods).

Culvertwallstrengthsandcoatingsshallbesuitableforthesoilconditions,designdepths,andtrenchdetails.CulvertstrengthshallbedesignedassumingHS‐20liveloadcapacityunlessuniqueconditionsofthecrossingwarrantahigherloadcapacity(i.e.,HS‐25orE‐80).

When an abrasive bed load is anticipated orwhen velocities exceed 10 feet per second, protectivemeasures shall be implemented to limit pipe damage. Corrosion, abrasion and other appropriateobservationsoffieldconditionsshallalsobeconsideredindeterminingappropriateculvertmaterialsand joint types. Corrosion resistance shall be evaluated based onminimum resistivity, pH, sulfatecontentandchlorinecontentofthesoilandgroundwater.

6.5.5 EndTreatmentsCulvertsshallbedesignedwithappropriateendtreatmentsattheirinletsandoutletssuchasflaredendsections,headwalls,orwingwallstoprovidesmoothtransitionsto/fromthedrainagechannelorditchand to conform to embankment slopes. In addition to the pipes, end treatments installed withinditches/drains shall be fire‐proof. Erosion protection or energy dissipaters shall be provided asnecessarytolimiterosionduetoturbulentflowandhighvelocities.Dependingontheculvertlocation,asafetygrateortrashrackmayneedtobeinstalled.

6.5.6 MaintenanceAccessProvidemaintenanceaccesstotheupstreamanddownstreamendsofculvertsforinspectionanddebrisremoval.

6.6 BRIDGESHydraulicsizingforbridgesacrossmajordrainagesshallconformtotherequirementsoftheMontanaDepartmentofTransportation(MDT).

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Chapter7‐RunoffControlFacilities

7.1 APPLICATIONThis chapter provides criteria for design of runoff control facilities including detention basins,retention/infiltrationbasins,andboulevardswales.Thesefacilitiesareusedtocontrolthequantityofrunoffdischargedfromtheprojectsitebystoringrunoffandslowlyreleasingwaterthroughanoutletstructurebyevaporationandinfiltrationorconveyancemeasures.Theserunoffcontrolfacilitiesmayalso be used in conjunctionwithwater quality treatment facilities discussed in Chapter 8. StoragerequirementsrelatedtorunoffcontrolvarywiththeprojectclassificationasdiscussedinChapter2.

7.2 DETENTIONBASINSDetentionbasinscanbedesignedasastandalonefacility,alsoknownasadrybasin,whererunoffisroutedovertimeuntilthebasincompletelydrainsout.Theycanalsobedesignedto“stack”ontopofwaterquality facilities such as retention/infiltrationbasins orwetbasinswhere temporarily storedrunoff will drain down to the original wet basin water surface elevation or to the top of theretention/infiltrationbasindesignedtostorethewaterqualityvolume(WQV).FurtherdetailsofthesewaterqualityfacilitiesareprovidedinChapter8.

DesignofdetentionbasinsshallfollowtheprocedurespresentedinChapter8oftheHEC‐22Manual,asmodifiedherein(http://www.fhwa.dot.gov/engineering/hydraulics/pubs/10009/10009.pdf).

7.2.1 GroundwaterAnticipatedgroundwaterlevelsmustbeaddressedinthedesigntoensurethatsufficientcapacitywillbe available in the basin, above the historic, seasonally‐highwater table, for storage of stormwaterrunoff. For standalone detention basins, the historic, seasonally‐high water table level shall be aminimumofthreefeetbelowthebottomofbasintoavoidsaturatedconditionswhichinterferewithpropermaintenanceofthefacility.

FurtherdetailsonrequiredgroundwaterevaluationsandsuggestedsourcesofinformationareprovidedinAppendixD.

7.2.2 BasinGeometry Maximumwaterdepthatanytimeshallnotexceed6feetfordetentionbasins.Themaximum

waterdepthcanexceed6feetonlyinthosecaseswhereadetentionbasinis“stacked”ontopofawaterqualitywetbasinsorwetland.Freeboardabovethedesignstormintheamountof1footisrequiredwhendesigningdetentionponds.

Side slopes shall be no steeper than 4H:1V for maintained grass‐lined sections, 3H:1V for

unmaintainednativegrass‐linedsectionsand2H:1Vforriprap‐linedsections.

Theslopeof thepondbottomshallbeno less than2%topromotedrainageacrossgrassedsurfaces.

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ThemaximumwatersurfaceelevationduringtheMajorStormshallbenolessthanonefootbelowtheadjacentground,windowwell,finishedfloor,topoffoundationoranyotherentrypointvulnerable to flooding forresidentialdwellingsandpublic,commercial,and industrialbuildings.

Safetybenchesshouldbeconsideredwithlargerdetentionbasinstoprovideashallowareaforpeopleandanimalsthatinadvertentlyentertheopenwater,toexitthebasin.

7.2.3 MaximumDrain‐DownTimeDetentionbasinsareusedtolimitdischargefromthesitetoanallowablerunoffrateasdiscussedinChapter2. Thewatersurfaceinthefacilityshallreturntothepre‐stormlevelwithin72‐hoursaftercessationofthemajorstorm.

7.2.4 Low‐FlowChannelsDrydetentionbasinsshallbeconstructedwithlow‐flowchannelsthathaveacapacityof1to3percentoftheMinorStorminflowrateandwithaminimumlongitudinalslopeof0.5percent.Thebottomofthebasinsshallbeslopedata2percentminimumgradetowardsthelow‐flowchanneltofacilitatedrainage.Apipedconfigurationmaybeusedinlieuofadedicatedabove‐groundlow‐flowchannel.

7.2.5 Multi‐PurposeUseDetentionfacilitiesdesignedformulti‐purposeuse(sportcourts,neighborhoodparks,playareas,picnicareas,etc.)areallowed.Multi‐useamenitiesshallbeanchoredtopreventfloatation.

Runoff from more frequent storms shall be stored separately from the multiple use areas. Theseseparatestorageareasshould,ataminimum,besizedtostorethewaterqualitystormvolume.

ThedevelopershallmakearrangementformaintenanceofsuchamenitiesunlesssuchresponsibilityisacceptedbytheCityofBillings.

7.2.6 SetBacks Detentionbasinsshallbelocated:

Suchthatthefacilitydoesnotinterferewithundergroundutilities,utilityeasements,floodplains,etc.;and,

Such that adequate access, maintenance and operations needs (includingconstructionconsiderationswith full replacement)aremetandadjacent facilitiesareprotected.Thegeotechnicalandhydrogeologicalreport,detailedinAppendixD,shall identifysubsurface impacts tosurroundingsoils,groundwater,andadjacentfacilitiesorstructures.GuidelinesinAppendixDwillaffectthesetbackfordetentionbasins.

Designer shall check ARM 17.36.323 and DEQ Circular 8 for additional setbackstandards.

7.2.7 WaterQualityTreatmentStormwater runoff from thewater quality event shall be routed through a sediment trap, sedimentforebay, or other appropriatewater quality BMP, detailed in Chapter 8, prior to discharging to the

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detentionbasin inorderto facilitateremovalof transportedsedimentsanddebris. Ifotherpotentialpollutantssuchasoils,grease,orfuel(gasolineanddiesel)couldbepresentinthesiterunoff,itmayalsobenecessarytoprovideaddedmeasurestoremovethesecontaminants.

Thefacilityshallbeprotectedfromhighsedimentloadsduringconstructionanduntilsitevegetationhasestablished.

7.2.8 OutletControlStructuresOutletcontrolstructuresshallbedesignedusingthecriteriaandmethodssetforthinChapter8oftheHEC‐22Manual,asmodifiedherein.

Minimumorificediameterwithoutscreeningissixinches. Screeningshallbeprovidedtopreventblockagefororificesofsmallerdiameter.

Wheresafetyordebrisisanissue,installremovabletrashandsafetyracksatoutletorifices,pipes,andweirs.

Designtheoutletstructuretominimizethepotentialforclogging.Considerusingperforatedpipeswithgravelsasafiltertopreventclogging,whereappropriate.

Theminimumdiameter foroutlet conduits shallbe12 inches. Anti‐seepcollars shallbeplacedonoutletconduitsthroughembankments.

7.2.9 EmergencyOverflow&SpillwaysUse the criteria set forth inChapter8.4.4.4 of theHEC‐22Manual asmodifiedherein.All detentionstorage facilities shall include a provision for non‐erosive control of overflows. Overflows from theMajor Storm event shall be directed to a safe discharge path to protect adjacent and downstreampropertiesfromdamage.

Discharge leaving the site shall be at the same location and have the same characteristics as pre‐developedflows.OffsiteeasementsmayberequiredperTable2.3.

7.2.10 Vegetation&LandscapingDetention basins shall be landscaped to provide for slope stability, erosion control, and lowmaintenance.Landscapematerialsshallbecompatiblewithuseinastormwaterdetentionfacilityandassociated water quality treatment facility. Utilize plant species native to the Billings area to themaximumextentpracticable.Ingeneral,basinsshallbeirrigatedandeitherseededorinstalledwithsodtoprovideanamenitytothecommunity.

Floatableorerodiblematerial(i.e.,woodchips,strawmulch,etc.)shallnotbeallowedwithinthebasin.Theinteriorsofthebasinshallbestabilizedwithgravel,rock,andvegetation.

Vegetationonbasinembankmentsshallbelimitedtoshallowrootedvarieties.Pointsofinflowtothebasinshallbearmoredtopreventerosion.

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7.2.11 MaintenanceandMaintenanceAccess MaintenanceshallbeperformedbytheHOAorcommercialsiteowner,unlessthisresponsibility

isacceptedbytheCity.FurtherdetailsareprovidedinAppendicesEandF.

Maintenancewillberequired toremove invasiveplantsanddebrisaccumulatedat inletandoutletstructures.

Stormwaterdetentionbasinoutletcontrolstructuresshallbeaccessibleformaintenanceand

operation.Whenvehicleaccessisnecessary,accessroadsshallbeprovidedindedicatedaccesseasementsorright‐of‐wayofatleast20feetwide.Theminimumcleardrivingwidthofaccessroadsshallbe12feetandtheminimumturn‐aroundradiusshallbe25feetorhammerhead.Accessroadsshallhaveamaximumgradeofninepercentandshallbeconstructedwithgravel,pavement,concreteoranappropriateall‐seasonsurface. Gates and/or bollards are requiredwhen necessary to restrict access to stormwater

facilities.Cablesand/orchainsstretchedacrossaccessroadsarenotacceptable.

7.3 RETENTION/INFILTRATIONBASINSRetentionbasinsandinfiltrationbasinsareverysimilarinfunctionandconsistofabasinwiththeonlymeansofemptyingbeingthroughevapotranspirationandinfiltration.RetentionandInfiltrationbasinsareusedtocontrolrunoff,butarealsousedtoprovidewaterqualitytreatmentbyfiltrationintothesoil.Thissectiondiscussesthedesigncriteriaforretention/infiltrationbasinsforbothrunoffcontrolandwaterqualitytreatmentofthewaterqualityvolume(WQV).Furtherdetailsofthewaterqualitydesignare discussed in Chapter 8. A subsurface infiltration facility (boulder pit) example is foundwithinAppendixG.

Retention/infiltrationbasinscaneitherbeconstructedasopensystemsorsubsurfacesystems.Opensystemstypicallyincludesexcavationofabasindesignedtoslowlyinfiltratethecollectedrunoffintotheunderlyingsoil.Subsurfaceretention/infiltrationbasinstaketheformofboulderpits,vaultsystems,andinjectionwellswhicharetypicallyusedonsiteswithlimitedavailablespace.

7.3.1 Geotechnical/HydrogeologicalEvaluationAgeotechnical/hydrogeological evaluation conducted in accordancewithAppendixD is required tosupportthedesignofretention/infiltrationbasinsforbothrunoffcontrolandwaterqualitytreatmentpurposes. Theevaluationshallbeconductedatandbelowthebottomelevationtominimumdepthsspecifiedatthelocationoftheproposedbasintocharacterizenativesoils,groundwaterconditions,andadjacentfacilitiesandstructures.Theassessmentshalldemonstratethefeasibilityofinfiltrationandtoassess potential risk of stormwater infiltration. Infiltrated stormwater must not adversely affectgroundwaterlevelsorflowatpropertyboundaries.Thelevelofdatarequiredwillbedependentontheamountofstormwater tobemanaged, the typeof infiltrationsystemproposed,and thesurfaceandsubsurface soil conditions at the site. The assessment will be conducted by a professional withexperiencecollectingandanalyzinghydrogeologicaldata.

TheDevelopershalldemonstratethrough:1)infiltrationtesting;2)soillogs;and3)awrittenopinionofalicensedcivil/geotechnicalengineerorqualifiedhydrogeologicprofessionalthat

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sufficientlypermeablesoilsexistonthesiteforaninfiltrationsystemmeetingtherequirementsherein to function properly with the site‐specific conditions. The preferred methods fordetermininginfiltrationratesarepresentedinAppendixH.

Theinfiltrationrateshallbemeasuredatadepthequaltotheproposedbottomgradeofthefacility.Afactorofsafetyof2.0shallbeappliedtothemeasuredinfiltrationrateinarrivingatthedesigninfiltrationrate.FollowproceduresinAppendixHfordetermininginfiltrationrates.

Geotechnical/Hydrogeological report shall identify the appropriate proximity of infiltrationsysteminrelationtoadjacentfacilitiesandstructures.Developershallcollectsufficientdatatoidentifyhydrogeologicalconditionsthathavethepotentialtoallowshallowlateralmovementof infiltratedwater; (i.e., high permeability soil layer, underlain by a low permeability soillayer). The report will contain sufficient data and detail to demonstrate that proposedinfiltrationsystemswillnot impactany facilitiesandthatany impactswillbe limited to thesubject property.Where hydrogeological conditions exist that indicate potential impacts tofacilitiesorstructuresbyinfiltratedstormwater,infiltrationsystemsmaynotbeconsideredasappropriateforstormwatermanagement.

RefertoAppendixDandHforadditionalrequirements.

7.3.2 Limitations Retention/Infiltrationbasinsmaynotbeapprovedforusewherehydrogeologicalconditions

existthatindicatethepotentialforinfiltratedstormwatertoimpacton‐oroff‐sitefacilitiesorstructuresandwherepotentialimpactswillnotbeconfinedtotheproperty.

Notappropriateforusewithtightclaysorothersoilswithlowinfiltrationratesorinareaswithashallowwatertable.

7.3.3 GroundwaterThedepthtothehistoric,seasonally‐highwatertable,bedrock,hardpanorotherimpermeable layershallbenolessthanthreefeetbelowthebottomofbasintoallowforinfiltrationoftherunoff.Whereimpermeablesoilsarenearthebottomoftheretention/infiltrationbasin,itmaybepossibletouseofapermittedClassVinjectionwelltoaugmentinfiltration(SeeSection7.3.8).

Insomeinstancesitmaybeappropriateto“key”thebottomofthesub‐surfaceinfiltrationsystemintoasandorgravellens,whichmaycoincidewithgroundwater,toachievemanageableinfiltrationrates.Insituationssuchasthis,therunoffgeneratedbythewaterqualitystormshallbemitigatedinaseparatesystemwheredirectinteractionwithgroundwaterisavoided.SeeAppendixGforreference.

FurtherdetailsofrequiredgroundwaterevaluationsareprovidedinAppendixD.

7.3.4 BasinGeometry Maximumwaterdepthatanytimeshallnotexceed6 feet foranopenretention/infiltration

basin.Safetybenchesshouldbeconsideredwithlargerretention/infiltrationbasinstoprovideashallowareaforpeopleandanimalsthatinadvertentlyentertheopenwater,toexitthebasin.

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Openretention/infiltrationbasin side slopes shallbe4H:1Vor flatter formaintainedgrass‐

linedsections,3H:1Vforunmaintainednativegrass‐linedsectionsand2H:1Vforriprap‐linedsections.

Embankmentfillslopesshallbenosteeperthan3H:1Vandpreferably4H:1Vorflatter.

Subsurface retention/infiltration basins shall be sized based upon the porosity of the fillmaterialwithinthefacilityandnotonthevoidratioofthefillmaterial.

ThemaximumwatersurfaceelevationduringtheMajorStormshallbenolessthanonefootbelowtheadjacentground,windowwell,finishedfloor,topoffoundationoranyotherentrypointvulnerable to flooding forresidentialdwellingsandpublic,commercial,and industrialbuildings.

7.3.5 MaximumDrain‐DownTime Retention/Infiltrationbasinsareusedtotreatandcontrolrunofffromthesitetomeetcriteria

discussedinChapter2.ThebasinshallcompletelydrainthecapturedrunofffromtheMajorstormwithin72‐hoursaftercessationofthestorm.

Iftheretention/infiltrationbasinisusedincombinationwithadetentionbasintocontrolthequantityofrunoff,thetotaldraw‐downtimeforthefacilityshallnotexceed72‐hours.

7.3.6 Multi‐PurposeUseRetention/infiltrationfacilitiesdesignedformulti‐purposeuse(sportcourts,neighborhoodparks,playareas,picnicareas,etc.)maybeallowedwherethehydrogeologicalconditionsareappropriate.Multi‐useamenitiesshallbeanchoredtopreventfloatation.

Runofffrommorefrequentstormsshallbestoredseparatelyfromthemultipleuseareas.Ataminimum,usethewaterqualitystormtosizetheseseparatestorageareas.

ThedevelopershallmakearrangementformaintenanceofsuchamenitiesunlesssuchresponsibilityisacceptedbytheCityofBillings.

7.3.7 SetBacks Retention/Infiltrationbasinsshallbelocated:

Suchthatthefacilitydoesnotinterferewithundergroundutilities,utilityeasements,floodplains,etc.;and,

Such that adequate access, maintenance and operations needs (includingconstructionconsiderationswith full replacement)aremetandadjacent facilitiesareprotected.Thegeotechnicalandhydrogeologicalreport,detailedinAppendixD,shall identifysubsurface impacts tosurroundingsoils,groundwater,andadjacentfacilitiesorstructures.GuidelinesinAppendixDwillaffectthesetbackfordetentionbasins.

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Designer shall check ARM 17.36.323 and DEQ Circular 8 for additional setbackstandards.

7.3.8 InjectionWells Stormwaterinjectionwells(asclassifiedbyEPA)maybeconsideredforuseinconjunction

withretention/infiltrationsystemstoimprovetheoverallinfiltrationrate.Thesewellscanbe usedwhere shallow low permeability soils overlymore permeable soils and preventdirectinfiltrationofstormwaterintotheunderlyingsoilswhicharecapableofacceptingthevolume and rate of stormwater infiltration necessary to meet the requirements of thismanual.AnexampleofastormwaterinjectionwellisprovidedinAppendixG.

Dependingondesignof thesubsurfaceretention/ infiltrationsystem, thesystemmaybeclassifiedasaClassVEPAinjectionwell.AClassVwellbydefinitionisanybored,drilled,ordrivenshaft,ordugholethatisdeeperthanitswidestsurfacedimension,oranimprovedsinkhole, or a subsurface fluid distribution system (an infiltration systemwith piping toenhanceinfiltrationcapabilities). Developerisresponsibletodetermineifproposedsub‐surface retention/infiltration systems are coveredunder the EPAUnderground InjectionControlProgramasaClassVwell.DevelopershallobtainandsubmitappropriatepermitasnecessarypriortoapprovalfromtheCity.

7.3.9 WaterQualityTreatmentStormwater runoff from thewater quality event shall be routed through a sediment trap, sedimentforebay, or other appropriate water quality BMP, listed in Chapter 8, prior to discharging to theretention/infiltrationbasininordertofacilitateremovaloftransportedsedimentsanddebris.Ifotherpotentialpollutantssuchasoils,grease,orfuel(gasolineanddiesel)couldbepresentinthesiterunoff,itmayalsobenecessarytoprovideaddedmeasurestoremovethesecontaminants.MoreinformationontheseBMPsisdiscussedinChapter8and/orAppendixAofthismanual.

Retention/InfiltrationbasinsshallhavesufficientcapacitytostoreatleasttheentireWQV(seeChapter8).

Retention/Infiltrationbasinshallbedesignedtofullydrainwithin72‐hours.

Iftheretention/infiltrationbasinisusedincombinationwithadetentionbasintocontrolthequantityofrunoff,thetotaldraw‐downtimeforthefacilityshallnotexceed72‐hours.

The Retention/Infiltration basin shall be protected from high sediment loads during

constructionanduntilsitevegetationisestablished.

7.3.10 Maintenance&MaintenanceAccess Maintenance shall be performed by the HOA or commercial site owner, unless this

responsibilityisacceptedbytheCity.FurtherdetailsareprovidedinAppendicesEandF.

Infiltrationfacilitiesshallbeaccessibleforoperationandmaintenance.Whenvehicleaccessis necessary, for facilities constructedoutsideof the street section, access roads shall be

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provided in dedicated access easements of 20‐footminimumwidth. Theminimum cleardrivingwidthofaccessroadsshallbe12feetandtheminimumturn‐aroundradiusshallbe25feetorhammerhead.Accessroadsshallhaveamaximumgradeofninepercentandshallbeconstructedwithgravel,pavement,concreteoranappropriateall‐seasonsurface.

Approval of retention/infiltration systems shall obligate the owner to repair, replace, orreconstruct the system if it fails to operate asdesigned.Themaintenance andoperationscheduleforretention/infiltrationsystemsshallincludesuchaprovision.AsdescribedinSection1.2,Developermayberequiredtomitigatesystemdeficienciesbeyondtherequiredwarranty period, if the deficiencies were found to be the result of improper design,construction,ormaintenance.Additionally,ifthesystemfailstofunction,duringMinorandMajorstormevents,asdescribedintheanalysisrequiredinSection3.1.2.3,Developermayberequiredtomitigatedeficienciesbeyondthewarrantyperiod.

Failuretomaintainretention/infiltrationsystemswillbesubjecttothetermssetforthintheagreementforthesubdivisionorcommercialsite.

7.4 BoulevardswalesBoulevardswalesareprimarilyusedinsubdivisionswithoutconventionalcurb/gutterstreetsections.Boulevardswalesaredesignedtocontrolrunoffbyeithercollectingrunofffromthestreetandadjacentareas, storing the runoff, and then infiltrating into the native soil or conveying runoff to adetention/retentionorotherapprovedfacility.Boulevardswalestypicallyhavecontributingdrainageareastypicallylessthan1acre. Boulevardswalesshallbedesignedtoaccommodatetheworst‐caserunoffscenariowhichistypicallyimmediatelyfollowinginitialconstructionofthedevelopment,priortobuild‐out.BoulevardswalesshouldnotbeconfusedwithwaterqualityswalesdiscussedinChapter8,whichareusedforwaterqualitytreatmentdesignedaroundawaterqualityflow(WQF).

7.4.1 Locations Boulevardswalesarecommonlyplacedalongroadwaysorpropertylinesinamannerthat

integratestheswalewithadjacentinfrastructure,landscapingandwaterqualityfeatures.

Boulevard swales shall be located within the right‐of‐way or dedicated storm drainageeasement.

7.4.2 Geotechnical/HydrogeologicalEvaluationGeotechnical/Hydrogeological investigations for boulevard swales shall follow the same criteria asretention/infiltrationbasinsdescribedinsection7.3.1.Thegeotechnical/hydrogeologicalinvestigationshallbeconductedatandbelowthebottomelevationandlocationoftheproposedswale.SeeAppendixDandHforadditionalrequirements.

7.4.3 GroundwaterThehistoric,seasonally‐highwatertable levelshallbeaminimumofthreefeetbelowthebottomofboulevardswaletopreventsaturationwhichhamperspropermaintenanceofthefacility.

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FurtherdetailsrequiredforgroundwaterevaluationscanbefoundinAppendixD.

7.4.4 BasinGeometry Theboulevardswaleshalluseatrapezoidalcrosssectionwithsideslopesnosteeperthan

4H:1V for maintained grass‐lined sections, 3H:1V for unmaintained native grass‐linedsectionsand2H:1Vforriprap‐linedsections.

Theminimumbottomwidthshallbe1‐ft.

The swale must be designed to control runoff in accordance with the requirements

presentedinChapter2.

ThemaximumwatersurfaceelevationduringtheMajorStormshallbenolessthanonefootbelowtheadjacentground,windowwell,finishedfloor,topoffoundationoranyotherentrypointvulnerabletofloodingforresidentialdwellingsandpublic,commercial,andindustrialbuildings.

7.4.5 MaximumDrain‐DownTime Boulevardswalesusedforretentionshallbedesignedtofullydrainwithin72‐hoursafter

cessationoftheMajorstorm.

7.4.6 Culverts Culverts installed in boulevard swales shall be designed in accordance with the

requirementsofSection6.5of thismanualand theCityofBillings’ standarddrawing fordriveapproaches.

7.4.7 LandscapingandVegetation The boulevard swales shall be either planted with vegetation sufficient to provide full

groundcoverageorshallbelandscapedwithrock.

Vegetationshallbecomprisedofdrought‐tolerantgrassesorshallbesupplementedwithasprinklersystem.

7.4.8 Maintenance&MaintenanceAccess Boulevardswalesshallbeprotectedfromhighsedimentloadsduringconstructionanduntil

vegetationisestablishedintheswaleandonadjacentareascontributingrunoff.Thismayrequireperiodiccleaningoftheboulevardswaleuntilvegetationisfullyestablished.

Maintenance for swales andassociated culverts shall be addressed in theHOAorby theadjacentpropertyownerandshallfollowtherequirementssetforthintheSIAandAppendixE and F.Maintenance requirements shall address vegetation heights,mowing,watering,fertilizingandfrequencyforsedimentremovalanderosionrepairs.

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Ifaccessfromtheadjacentpublicright‐of‐wayisnotavailable,anaccesseasementtotheswaleshallbeprovidedtofacilitateinspection,monitoring,andmaintenance.Appropriateaccessshallbeconsideredwhenaccountingformaintenanceofculvertswithintheswale.

Failuretomaintainboulevardswaleswillbesubjecttothetermssetforthintheagreementforthesubdivision.

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Chapter8‐PermanentWaterQualityTreatment

8.1 APPLICATIONTheCityofBillingshasanactiveGeneralPermitforStormWaterDischargeswithSmallMS4’sonrecordwiththeMontanaDepartmentofEnvironmentalQuality.Thischapterpresentstherequirementsfortheimplementationanduseofpost‐constructionBestManagementPracticestoensurecompliancewiththeCity’s General Permit requirements. Compliance with this section does not require water qualitymonitoring,orquantitativeestimatesofpollutantloadremoval.However,theuseofaperformance‐basedapproachwherebytheprinciplesandobjectivesofstormwaterpollutantcontrolareaddressedandappliedisrequired.

8.2 WATERQUALITYVOLUMETheWaterQualityVolume(WQV)istheamountofstormwaterrunofffromarainfalleventthatshouldbecapturedandtreatedtoremovethemajorityofstormwaterpollutantsonanaverageannualbasis. TheWQVisbasedonthefirsthalfinchofrainfallpertheCity’scurrentGeneralPermitwithDEQ.Pollutantstypicallycomefromtheimperviousareaandthereforetheunifiedstormwaterequationwillbeusedtocalculatethewaterqualityvolume(WQV):

Where:

WQV=WaterQualityVolume,inacre‐feetP=WaterQualityRainfallDepth,inches(0.5‐inches)Rv=theunitlessrunoffcoefficient,Rv=0.05+0.9(I)I=thepercentimperviouscoverdrainingtothefacility,indecimalA=totalsiteareadrainingtothestructure,inacres

8.3 WATERQUALITYFLOWTheWaterQualityFlow(WQF)isusedtodetermineaflowrateassociatedwiththeWQV,forsizingflow‐based treatment systems (e.g.TreatmentSwales,Flow‐ThroughDevices, etc…). TheWQF is calculatedusing theWQV and the Natural Resource Conservation Service (NRCS) runoffmethod as discussed inChapter5.TheWQFiscalculatedusingthefollowingequation:

Where:

WQF=WaterQualityFlowRate,incfsqu=unitpeakdischarge,incfs/mi2/inch(seeTable8.3)WQV=waterqualityvolume,inacre‐feet

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Table8.3–NRCSUnitPeakDischargeTimeofConcentration

(minutes)*qu

(cfs/mi2/inch)6 101012 80018 67624 59230 52945 42460 357

*Eitherrounddowntothenearesttimeofconcentrationorinterpolateforintermediatetimesofconcentration.

8.4 SELECTIONOFPERMANENTWATERQUALITYBMPSIn coordination with the Montana Department of Environmental Quality and Montana’s other MS4Municipalities,theMontanaPost‐ConstructionStormWaterBMPDesignGuidanceManual(September,2017)wascreatedasameansforthedevelopmentcommunitytodesignandconstructstormwaterfeaturesthatensuresdevelopmentmeetsthewaterqualityrequirementsoftheGeneralPermit.

TheBMP’slistedinChapter5oftheGuidanceManualweredevelopedtospecificallymeettherequirementsof the General Permit. It is the City’s understanding that if a developer constructs post‐constructionstormwaterfeaturesperthemethodologyintheGuidanceManual,thedevelopment’ssitewillbecompliantwith theGeneralPermit. If a stormwater feature isnot constructedper theGuidanceManual, it is thedeveloper’ssoleresponsibilitytoensuretheirsitespecificpost‐constructionBMPmeetstherequirementsoftheGeneralPermit.

TheMontanaPost‐ConstructionStormWaterBMPDesignGuidanceManual(September,2017)canbeviewedontheCity’swebsite(https://ci.billings.mt.us/567/Stormwater‐Management).

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Chapter9‐ErosionandSedimentControl

9.1 APPLICATIONThissectionprovidescriteriaforselection,design,installation,andmaintenanceoftemporarysedimentanderosioncontrolBestManagementPractices(BMPs)toreduceimpactsfromconstructionactivitiestosurfacewaters,publicrights‐ofwayandadjacentproperties.

9.2 SWPPPANDNOIREQUIREDSUBMITTALSInaccordancewithCityOrdinance28‐100, if aprojectmeets thecriteriabelow,aSWPPP(StormwaterPollutionPreventionPlan)andNOI(NoticeofIntent)shallbesubmittedtoboththeCityofBillingsandMontanaDepartmentofEnvironmentalQuality (MTDEQ) foracceptance. TheSWPPPandNOIshallbesubmittedtotheCitypriortoanygradingonthesiteandpriortosubmittaltoMTDEQ.TheSWPPPandNOIsubmittaltoMTDEQshallbethesameaspreparedfortheCityofBillings.

TheMTDEQ,inaccordancewithfederalregulations,requiressubmittalofaSWPPPandNOIincompliancewiththeGeneralPermitforpublicandprivateprojectsthat:

Disturbone(1)acreormoreofland;or Arepartofalargermasterplandevelopmentorsubdivisionthatwillultimatelydisturbone(1)

acre or more of land (including Subdivisions and Master Plan reviews, as required by theBillingsMunicipalCityCode(BMCC))

Section28‐401(a),BMCCrequiresaSWPPPandNOIbesubmittedforanyconstructionactivityonpropertieswithin50feetofanywatercourse.Thisincludesirrigationanddrainageditchesidentifiedasstatewaters.

Forms for the SWPPP and NOI can be found on MTDEQ’s website(http://deq.mt.gov/Water/WPB/mpdes/stormwaterconstruction).TheGeneralPermitrequirestheSWPPPandNOIpermitholderto:

Be identified within the NOI and match the Owner of the SWPPP. A Certified SWPPPAdministratorisrequiredforallconstructionprojectsrequiringanNOIandSWPPPandshallbeidentifiedwithintheNOIandSWPPP.

MaintainandupdatetheSWPPPtoreflectchangesontheconstructionsite. Specify the inspection frequency. Inspections of stormwater and erosion controls shall be

every7daysorevery14daysandfollowingeachstormeventof0.25"ormore. MaintaininspectionrecordsandprovidetherecordsuponrequestbytheCityand/orMTDEQ. MaintainandmodifytemporaryBMPstoreflectcurrentconditionsofthejobsiteandupdate

siteplansandSWPPPlanguageduringinspections. Achieve stabilization and remove all temporary BMPs once 70% of the disturbed area is

revegetated. FileaNoticeofTermination(NOT)uponstabilizationofthesite.

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9.3 CONSTRUCTIONLESSTHANONE‐ACREConstructionactivitydisturbingareaslessthanoneacreshallberequiredtoimplementBMPsandgoodhousekeepingpracticestominimizeimpactsincluding,butnotlimitedto,erosionandsedimenttransportinto public right‐of‐way or onto adjoining property. Homebuilders/contractors are responsible formanagingBMPsonindividuallotswithinasubdivisionand/ormasterplanareaandarerequiredtofollowtherequirementsintheNOIandSWPPPfortheproperty.

9.4 BESTMANGAGEMENTPRACTICES(BMP)Temporaryerosionandsedimentcontrolbestmanagementpractices(BMPs)forconstructionsitesintheCityofBillingsmaybeselected,designed,andinstalledusingthemethodologydiscussedinthemostcurrenteditionoftheMontanaDepartmentofTransportationErosionandSedimentControlBestManagementPracticesManual.Thismanualprovidesguidelinesforapplications,limitations,effectiveness,materials,design&installation,inspection&maintenance,andremovaloftemporaryBMPs.ThesemanualsareavailableonMDT’swebsite(https://www.mdt.mt.gov/publications/docs/manuals/env/bmp‐manual‐jan15.PDF).

Inaddition,theMontanaDepartmentofEnvironmentalQualitydevelopedtheStormWaterManagementDuringConstructionFieldGuideforBestManagementPracticesreferencedocumentwhichmayhelpindevelopingaBMPplan.Pleasecontacttheirofficeforacopyofthisdocument.

9.5 CONSTRUCTIONADJACENTTOWATERWAYSInadditiontoaSWPPPandNOIapplication,ifconstructionactivitiesareplannedonornearawaterwayinMontana,oneorseveralpermitsmayberequired.Forthepurposeofthismanual,awaterwayisdefinedasaditch,drain,creek,stream,riverorwetland.Conservationdistricts,alongwithparticipatingresourceagencies,developedajointapplicationformthatincludestherequirementsofthefollowingagenciesandtheirrequiredpermits:

1. ConservationDistricts(localgovernment)‐310permits

2. MTFish,WildlifeandParks(stategovernment)‐SP124permits

3. FloodplainAdministrators(BuildingDepartment)‐floodplainpermits

4. USArmyCorpsofEngineers(federalgovernment)‐Section404/Section10permits

5. MTDepartmentofEnvironmentalQuality(stategovernment)‐318(turbidity)Authorizations

6. MTDepartmentofNaturalResourceandConservation(stategovernment)‐Navigableriverlanduselicensesandeasements.

The jointapplicationcanbefoundonDNRC’swebsite(http://dnrc.mt.gov/licenses‐and‐permits/stream‐permitting).

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Chapter10‐BibliographyCityofBillings,PublicWorksDepartment(2011/2015)StormwaterManagementManual),Billings,Montana.

CityofGillette,DepartmentofEngineering(2011)StormDrainageDesignManual,Gillette,Wyoming.

CityofKalispell,PublicWorksDepartment(2009)StandardsforDesignandConstruction,Kalispell,Montana.

CityofSheridan,PublicWorksDepartment(2006)StormDrainageDesignCriteria,Sheridan,Wyoming.

CityofPoulsbo,PublicWorksDepartment(2008)ConstructionStandardsandSpecifications,Poulsbo,WA

MontanaDepartmentofTransportation(January,2015),ErosionandSedimentControlBestManagementPracticesManual.Helena,Montana

Montana’sMS4Municipalities (September, 2017)MontanaPost‐Construction StormWaterBMPDesignGuidanceManual,Missoula,Montana.

EnvironmentalQualityApril,2014StormWaterManagementDuringConstructionFieldGuideForBestManagementPractices.Helena,Montana.

PrinceGeorge’sCounty,Maryland,DepartmentofEnvironmentalResourceProgramsandPlanningDivisions,(June1999),Low‐ImpactDevelopment–AnIntegratedDesignApproach

UrbanWaterResourcesResearchCouncil of theAmericanSocietyofCivil Engineers and theWaterEnvironmentFederation(1992)ASCEManualsandReportsofEngineeringPracticeNo.77,WEFManualofPracticeFD‐20,DesignandConstructionofUrbanStormwaterManagementSystems,LibraryofCongressCatalogNo.:92‐36519.

U.S.DepartmentofTransportation,FederalHighwayAdministration(2006)HydraulicEngineeringCircularNo.14,HydraulicDesignofEnergyDissipatersforCulvertsandChannelsPublicationNo.FHWA‐NHI‐06‐086.

U.S.DepartmentofTransportation,FederalHighwayAdministration(2009)HydraulicEngineeringCircularNo.22,Second Edition, Urban Drainage Design Manual Third Edition, Publication No. FHWA‐NHI‐10‐009, NationalHighwayInstitute.

U.S. Department of Transportation, Federal Highway Administration (2012) Hydraulic Design Series Number 5,HydraulicDesignofHighwayCulvertsThirdEdition,PublicationNo.FHWA‐HIF‐12‐026

WaterEnvironmentFederations&AmericanSocietyofCivilEngineers/Environmental&WaterResourceInstitute(2012)WEFManualofPracticeNo.23&ASCEManualsandReportsofEngineeringPracticeNo.87,DesignofUrbanStormwaterControls

Stormwaterassociation.com,March,2015.

TheOhioStateUniversity. StormwaterManagementProgramPostConstructionBMPOperationandMaintenanceGuidanceManual.December1,2009.

GeorgiaStormwaterManagementManual–Volume1StormwaterPolicyGuidebook.FirstEdition.August,2001

AppendixAAdditionalRequirementsforCommercialSiteDevelopments

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PerSection2.1.4,somecommercialsitesrequireadditionalstormwaterrunofftreatment.Thetableandtextbelowidentifiessuchfacilitiesandthepollutantremovalcriteria.

Oil Treatment Nutrient Treatment Metals Treatment

Required if: Required if: Required if:

1. Fueling Stations and similar type facilities

1. Nurseries 1. Fueling Stations and similar type facilities

2. Commercial or industrial properties storing or transferring 1,500 gallons or more of petroleum chemicals

2. Lawn care/ fertilizer facilities

2. Properties zoned control heavy industrial

3. Properties zoned control heavy industrial

3. Agricultural facilities 3. Vehicle maintenance/ equipment repair facilities

4. Vehicle maintenance/ equipment repair facilities

4. Animal Care facilities 4. Hydraulic equipment storage areas

5. Hydraulic equipment storage areas

5. City determined necessary



OilTreatment*

Oil treatment is required for all highuse andhigh traffic areas regardless of the impervious area. Oiltreatment facilities need only be located to treat stormwater which may contain oil and grease. Oiltreatmentfacilitiesarenotrequiredtotreatrunofffromcleanroofs,landscapedareas,orotherareaswhichareseparatedfromornotsubjecttosurfaceswhichmaycontainoilorgrease.Somehightrafficareasmaygeneratesufficientquantitiesofoil to threatenwaterquality,but thequantitiesmaybe insufficient formanyoil controlBMPs tobeeffective; therefore thesepropertiesmayemploydifferentBMPs thanarerecommendedforhigh‐useareas.

Theoilcontrolfacilitiesareintendedtoachievethegoalsofnoongoingorrecurringvisiblesheen,andtohaveaTotalPetroleumHydrocarbon(TPH)concentrationnogreaterthan10mg/l.

NutrientTreatment*

Nutrient, typicallynitrogenandphosphorus, treatment isrequiredatallnurseries, lawncare/fertilizer,agriculture,andanimalcarefacilitiesover1acreinsize.NutrienttreatmentBMPfacilitiesshallbesizedtotreatallstormwaterdischargingtotheBMP.Nutrienttreatmentisnotrequiredforrunoffwhichdoesnotcomeincontactwithchemicalstorageandapplicationareasorareasinaccessibletoanimals.Treatmentshould achieve50%reduction of total phosphorus and35%reductionof total nitrogen, dependingoninfluentconcentrationsanddesign.

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MetalsTreatment*

Metalsaretypicallyintheformofparticles,andaregenerallyremovedwithbasictreatmentrequirementswhichremovetotalsuspendedsolids.Therefore,propertiesrequiringmetalstreatmentshouldalreadybetreated throughTSS treatment. In the event that dissolvedmetals are reachingwater bodies throughstormwaterrunoff,additionalcontrolmeasures,suchasfiltersmayberequiredatsuchfacilities.Metalsremovalshouldbefrom20to85%dependingondesign,metalstate(i.e.,dissolved),influentconcentration,etc.

*ItistheresponsibilityofthedevelopertoensuresitecomplieswithDEQ,EPAorotheragencieswhichmayhaveregulationsnotspecifiedinthismanual.

AppendixBPreliminaryDrainageReport

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Thepurposeofthepreliminarydrainagereportistodescribeandillustratethepreliminarysolutionstothedrainageproblemswhichmayoccuron‐siteandoff‐siteasaresultofthedevelopmentoranyphaseofthedevelopment.ThedrainagereportshallbesubmittedduringthesubdivisionprocesswiththeapplicationforPreliminaryPlat.

Preliminarydrainagereportsshallprovideanappropriatelevelofdetailtoaddressdrainageissuesandpresenttheoverallplanfortheproperty.Thereportshallbebasedonthefollowingoutlineandincludeappropriatebackgroundinformation,supportingdata,calculationsandplandrawing(s).

TITLEPAGE

1. TypeofReport(Concept)2. ProjectName3. Preparedfor/by4. Date5. P.E.SealandSignature

INTRODUCTION

1. Locationa. City,County,StateHighwayandlocalstreetswithinandadjacenttothesite,orthearea

tobeservedbythedrainageimprovements.b. Namesofsurroundingdevelopments,propertiesorlandmarks.

2. DescriptionofPropertya. Areainacresb. Groundcover(typeofgroundcoverandvegetation)c. Existinglandusesandknownandforeseeablefuturelandusesd. Topographicfeatures,steepnessofslopese. Majordrainagewaysandreceivingchannelsf. Existingdrainagefacilitiesg. FloodHazardZonesh. GeologicFeatures(ifapplicable)i. Previousdrainagestudiesfortheproperty(ifany)

3. ProposedProjectDescriptiona. Landusesb. Changestoexistingfacilitiesc. Changestofloodplainsd. Proposedsystemimprovements

4. DrainageCriteriaa. MinorandMajorStormAnalysisb. Geotechnical/HydrogeologicalAnalysisc. HydrologicMethods

i. Rainfallii. DesignStormsiii. Runoffmethodsandcomputermodels

d. HydraulicMethodsi. Designstandardsii. Hydraulicmodels

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iii. DetentionPondsizinge. StateorFederalRegulations(ifapplicable).

HISTORICDRAINAGESYSTEM

1. MajorBasinDescriptiona. Reference tomajor drainageway planning studies such as flood hazard delineation

report,majordrainagewayplanningreports,andfloodinsuranceratemaps.b. Major basin drainage characteristics and structures, existing and planned land uses

withinthebasin.c. Summaryofoff‐siteandon‐sitebasincharacteristicsandrunoffrates.

PROPOSEDDRAINAGESYSTEM

1. DesignConceptsa. Discussionofconceptandtypicaldrainagepatterns.b. Discussionofcompliancewithoff‐siterunoffconsiderations.c. Discussionofproposeddrainagepatternsandimprovementsincludingstreets,storm

sewer,culverts,openchannelsanddetentionstorage.d. Discussionofthecontentoftables,charts,figures,plates,ordrawingspresentedinthe

report.e. Discussionofgeotechnicalandhydrogeologicalimpactsofdevelopment.

SUMMARY

1. Relationtooff‐sitedrainagefeatures.2. Summaryofproposedimprovements.

a. Stormsewerb. Culvertsc. Openchannelsd. DetentionStoragee. On‐siteandoff‐siteimpactandmitigationmeasures

3. Floodplainimpacts.4. StateorFederalregulations.5. Compliancewithapplicableregulationsandstandards.

REFERENCES

Reference all criteria, master plans, and technical information used in support of concepts andcalculations.

APPENDICES BackgroundData

1. Floodplainmaps2. Applicablereportsorreportexcerpts.3. Keycorrespondencewithadjacentpropertyownersorutilities.

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PRELIMINARYREPORTDRAWINGCONTENTS

Alldrawingsshallbesubmittedasback‐upmaterialswiththePreliminaryPlat.Amapshallbeprovidedin sufficient detail to identify drainage flows entering and leaving the development and generaldrainagepatterns. Themap shall identify anymajor facilities from theproperty (i.e., development,existingdetentionfacilities,culverts,stormsewers)alongtheflowpathtothenearestmajordrainageway.

FloodplainInformation:Thelocationofthesubjectpropertyshallbeincludedwiththereport.Allmajordrainagewaysshallhavethefloodplaindefinedandshownonthereportdrawings.

DrainagePlanshallshowthefollowing:

1. Existingtopographiccontoursattwo(2)feetmaximumintervals.Thecontoursshallextendaminimumofone‐hundred(100)feetbeyondthepropertylines.

2. Allexistingdrainagefacilities.

3. Approximatefloodinglimitsbasedonavailableinformation.

4. Conceptualmajordrainagefacilitiesincludingdetentionbasins,stormsewers,swales,riprap,andoutletstructuresinthedetailconsistentwiththeproposeddevelopmentplan.

5. Majordrainageboundariesandsub‐basinboundaries.

6. Anyoff‐sitefeaturesinfluencingdevelopment.

7. Proposedflowdirectionsand,ifavailable,proposedcontours.

AppendixCFinalDrainageReport

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ThepurposeoftheFinalDrainageReportistopresentthefinaldesigndetailsforthedrainagefacilitiesdiscussedinthePreliminaryDrainagePlan.Anychangestothepreliminaryconceptmustbepresentedandfullyexplained.

Drainageplanshallprovideanappropriatelevelofdetailtoaddressthedrainageissuesandpresentsizing and locations for all proposed improvements. The report shall be based on the followingoutlineandincludeappropriatebackgroundinformationandsupportingdataandcalculationsandplandrawing(s).

TITLEPAGE

1. TypeofReport(Final)2. ProjectName3. Preparedfor/by4. Date

CERTIFICATIONPAGE(foundinAppendixG)

1. ProjectName2. P.E.,P.G.,P.H.SealandSignature

INTRODUCTION

1. Locationa. City,County,StateHighwayandlocalstreetswithinandadjacenttothesite,or

theareatobeservedbythedrainageimprovements.b. Namesofsurroundingdevelopments,propertiesorlandmarks.

2. DescriptionofPropertya. Areainacresb. Groundcover(typeofgroundcoverandvegetation)c. Existinglandusesandknownandforeseeablefuturelandusesd. Topographicfeatures,steepnessofslopese. Majordrainagewaysandreceivingchannelsf. Majordrainagewaysandreceivingchannelsg. Existingdrainagefacilitiesh. FloodHazardZonesi. GeologicFeatures(ifapplicable)

3. Previousdrainagestudiesfortheproperty(ifany)a. ProposedProjectDescriptionb. Landusesc. Changestoexistingfacilitiesd. Changestofloodplainse. Proposedsystemimprovementsf. Right‐of‐wayconveyanceoracquisitionrequired

4. DrainageCriteriaa. ApplicationStandardsorexceptionsb. MinorandMajorStormFrequenciesc. HydrologicMethods

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i. Rainfallii. DesignStormsiii. Runoffmethodsandcomputermodelsiv. Geotechnical/HydrogeologicalAnalysis(AttachReports)

d. HydraulicMethodsi. Designstandardsii. Hydraulicmodelsiii. DetentionPondsizing

e. StateorFederalRegulations(ifapplicable)

HISTORICDRAINAGESYSTEM

1. MajorBasinDescriptiona. Reference to major drainage way planning studies such as flood hazard

delineationreport,majordrainagewayplanningreports,andfloodinsuranceratemaps.

b. Majorbasindrainagecharacteristicsandstructures,existingandplanned landuseswithinthebasin.

c. Summaryofoff‐siteandon‐sitebasincharacteristicsandrunoffrates. 2. Sub‐BasinDescription

a. Discussionsofhistoricdrainagepatternsoftheproperty.b. Discussion of off‐site drainage flows and flow patterns and impact on

developmentunderexistingandfullydevelopedbasinconditions.c. Summaryofoff‐siteandon‐sitebasincharacteristicsandrunoffrates.

PROPOSEDDRAINAGESYSTEM

1. DesignConceptsa. Discussionofminorandmajordrainagepatterns,impacts,flowsandvolumes.b. Discussionofcompliancewithoff‐siterunoffconsiderations.c. Discussionofproposeddrainagepatternsandimprovements includingstreets,

stormsewer,culverts,openchannelsanddetentionstorage.d. Discussionofthetables,charts,figures,drawings,etc.presentedinthereport.

2. DesignDetailsa. Discussionofproblemsencounteredandsolutionsatspecificdesignpoints.b. Discussionofdetentionstorageandoutletdesign.c. Discussionofmaintenanceandaccessaspectsofthedesign.d. Discussionofimpactsofconcentratingtheflowonthedownstreamproperties.e. Summaryofbasincharacteristicsandrunoffrates.f. Discussionofgeotechnicalandhydrogeologicalimpactsofdevelopment.g. Discussfloodinghazardsanddescribeminimumbuildingelevations.

SUMMARY

1. Relationtooff‐sitedrainagefeatures.2. Summaryofproposedimprovements.

a. Stormsewer

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b. Culvertsc. Openchannelsd. DetentionStoragee. Geotechnical/Hydrologicimpactsf. On‐siteandoff‐siteimpactsandmitigationmeasures

3. Floodplainimpacts.4. StateorFederalregulations.5. Compliancewithapplicableregulationsandstandards.

REFERENCES

Referenceallcriteria,masterplans,andtechnical informationusedinsupportofconceptsandcalculations.

APPENDICES

1. BackgroundDataa. Floodplainmaps.b. Applicablereportsorreportexcerpts.c. Keycorrespondencewithadjacentpropertyownersorutilities.

2. HydrologicComputationsa. Landusesregardingadjacentproperties.b. Soiltypes,coverageandlosscoefficientsc. Proposedlandusesforprojectbybasin.d. Timeofconcentrationandrunoffcoefficientsforeachbasin.e. Basinparametersusedformodelingincludingbasinarea,length,slope,distance

tocentroidandroutingelements.f. Initial andmajor stormrunoff at specificdesignpoints for off‐site andon‐site

flows.g. Off‐site, historic and fully developed runoff computations at specific design

points.h. Hydrographsatcriticaldesignpoints.i. Schematicdiagramofhydrologymodelshowingbasinsandroutingelementsand

combinationelements.3. HydraulicComputations

a. CulvertCapacitiesandinletandoutletprotection.b. Stormsewercapacity,includingenergygradeline(EGL)andhydraulicgradeline

(HGL)elevations.c. Guttercapacityascomparedtoallowable.d. Storminletcapacityincludingroughnesscoefficients,tricklechannels,freeboard,

hydraulicgradeline,andslopeprotection.e. Checkand/orchanneldropplacement.f. Detention area volume capacity and outlet capacity calculations; depths of

detentionbasins,outletconfiguration.g. Downstream/outfallcapacitytotheMajorDrainagewaysystem.

4. MiscellaneousInformationa. Otherdocumentsrelatingtodrainageconditionsontheproperty.b. Agreementswithpropertyownersorotheragencies.C. Permits,etc.

AppendixDGeotechnical/HydrogeologicalReport

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1. Theevaluationshallincludeataminimum:a. A review of available geologic, hydrogeological, and topographic conditions to

identifyanysiteconditionsthatcouldimpacttheuseofthestormdrainagesystemsortheconstructionofsub‐levelstructures.Thisreviewshallincludeallavailablepreviousgeotechnical engineering reports for the development. Citations to possibly usefulreferencesareprovidedattheendofthisappendix.

b. Whereaccesstoadjacentpropertiesisunavailable,theprojectownershallrelyuponthebestknowninformationforthearea,supplementedwithavailableinformation,includinganyexistingengineeringreportsorstudiesforsitesinthevicinity.

c. A surface and subsurface reconnaissance of the site and an inspection of adjacentpropertiestoassesspotentialimpacts fromtheproposedstormwatersystemandtoverifythattheconditionsareconsistentwiththemappedinformation.

d. Thelevelofdataforthehydrogeologicalassessmentrequiredwillbedependentontheamountofstormwatertobemanaged,thetypeofinfiltrationsystemproposed,andthesurfaceandsubsurfacesoilconditionsatthesite.Theassessmentwillbeconductedbyaprofessionalwithexperiencecollectingandanalyzinghydrogeologicaldata.

e. Anassessmentofhydrogeologicalconditionsthatindicatethepotentialforinfiltratedstormwatertoimpacton‐oroff‐site,facilitiesorstructures.Theassessmentwillalsodemonstrate that impacts to groundwater elevation or flow, resulting from theproposed infiltration system will be confined to the property. A groundwatermounding calculation shall be provided to identify the impacts of infiltratedstormwaterrunoff.Anexamplecalculationmethodandspreadsheetismadeavailablefrom the United States Geological Survey (USGS); however, other approved, similarcalculationmethodsmaybeaccepted.Thisinformationcanbefoundatthefollowinglink:https://pubs.usgs.gov/sir/2010/5102/

f. The Geotechnical/Hydrogeological report will contain the signed projectcertificationcoversheetfoundinAppendixG.

2. TheReportNarrativeshallinclude:a. A brief project description including size, number of lots proposed, project location

(section, township and range), and background information relevant for drainagedesign;

b. A discussion of the study investigations including methods and results of fieldassessments,testingandanalysesperformed;

c. Adescriptionofthesoilunitsandsubsurfacegeologicconditionsonthesiteandinthevicinityofthesite;

d. Adescriptionofthesite includingsurface,soil,andgroundwaterconditions,etc.3. TestMethodDocumentationshallinclude:

a. Amapwiththelocationofallsubsurfacefieldexplorations,samplinglocationsandanyin‐placefieldtests;

b. Adescription of the field testandanydifficulties encountered during excavationandtesting;

c. Adescriptionoftheequipmentusedtoperformthefieldexplorationsortests.Whenapplicable,describethetypeoffabricliningandgravelbackfillused;

d. Logsofsubsurfaceboringsshallidentifythedepthtogroundwater,thepresenceofanylimitinglayersandthetargetsoillayer;include testpitorexcavationdimensions.Borings intended to characterize hydrogeologic conditions for infiltrationssystems should extend a minimum of 10‐feet below the base of the proposed

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infiltrationsystem,oraminimumof25‐feetbelowthegroundsurface,whicheverisdeeper;

e. Report test data documenting any infiltration testing, calculations, results problemsencountered;and,

f. Adescriptionoftheconditionofanyexistingfacilitiesbeingtested,notinganysiltbuild‐up,water level, connections to other structures (including distance to inverts of anyinterconnectingpipes),measureddepthsanddimensions,etc.

4. Resultsoffieldandlaboratorytestingconducted,includingthegrainsizeanalysis representedbothgraphicallyandintabularformat;

5. Asummaryoffieldtestingconductedandthemeasuredandproposeddesigninfiltrationratesforinfiltrationsystems.ApprovedtestmethodsforinfiltrationtestingarefoundinAppendixH;

6. Resultsofthesub‐levelstructurefeasibilitystudyandasummaryofthepropertyboundaryanddown‐gradientanalysisasapplicable;and,

7. Ageologiccross‐sectionofthestormwaterdisposalareadrawntoscale,withtheproposedstormwaterdisposalfacilitiessuperimposedonthecross‐section.Allrelevantgeologicunitsshallbeclearlyidentifiedincludingthetargetdisposallayerandlimitinglayers.

8. Conclusionsandrecommendations.a. TheSitePlanshallinclude:b. Projectboundaries(includingallexistingandproposedpropertylines);c. Labeledtopographiccontours,extendingbeyondtheprojectanddrainagebasin.Projects

inanurbanareashalluseamaximumcontourspacingof1foot;d. Locationofthesoilandgeologicunitsidentified;e. Locationofsignificantstructures,propertiesorgeologicfeaturesonsiteandintheproject

vicinity;f. Locationofexistingnaturalorconstructeddrainagefeaturesonsiteandintheproject

vicinity;and,g. Locationofproposedsiteinfrastructureincludingroadwaysanddrainagefeaturessuchas

ponds,drywells,etc.

SUGGESTEDSOURCES:

MontanaGroundWaterInformationCenterDatabase:http://mbmggwic.mtech.edu/ Lopez, D.A., and Sims,M., 2003, Areas of potential swelling‐clay hazard in the Billings area, Yellowstone

County,Montana:MontanaBureauofMinesandGeologyGeologicMap61D,1sheet,scale1:48,000. Lopez,D.A.,2002,GeologicmapoftheBillingsarea,YellowstoneCounty,Montana:MontanaBureauofMines

andGeologyGeologicMap61A,1sheet,scale1:48,000. Lopez,D.A.,2000,GeologicmapoftheBillings30'x60'quadrangle,Montana:MontanaBureauofMinesand

GeologyGeologicMap59,1sheet,scale1:100,000. Olson,J.L.,andReiten,J.C.,2002,HydrogeologyofthewestBillingsarea:Impactsofland‐usechangesonwater

resources:MontanaBureauofMinesandGeologyReportofInvestigation10,32p.,2sheets. Olson, J.L., and Reiten, J.C., 2001, Basic hydrogeologic data for the West‐Billings area (1999‐2000),

YellowstoneCounty,Montana:MontanaBureauofMinesandGeologyOpen‐FileReport436,110p.UnitedStates Geological Survey (USGS). Scientific Investigations Report 2010‐5102. Simulation of GroundwaterMounding Beneath Hypothetical Stormwater Infiltration Basins. Glen B. Carleton.http://pubs.usgs.gov/sir/2010/5102.

AppendixEOperationsandMaintenance

Requirements

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OPERATION ANDMAINTENANCE

AnOperationsandMaintenanceManualisrequiredforSubdivisionandCommercialPropertydevelopment.TheO&MManualsummarizesthetasksrequiredforperpetualmaintenancetoensuretheproperoperationofstormwaterfacilities.TheO&Mmanualshallincludeataminimum:

ContactinformationforthepartyresponsibleforO&M.

Descriptionofthemaintenancetaskstobeperformedandtheirfrequency.

Aninspectionchecklisttobeusedforannualmaintenance.TemplateformsfoundinAppendixG.

Listoftheexpecteddesignlifeandreplacementscheduleofeachcomponent.

Siteplanshowingtheoveralllayoutofthedevelopment.

CopyofrecordedHOAAgreementandSIA,ifapplicable.

Otherinformationasnecessary.

TheO&MManualshallfirstbesubmittedtotheCity’sEnvironmentalAffairsDivisionforreviewandcomment.AfteracceptancebytheEnvironmentalAffairsDivision,theO&MManualshallberecordedattheYellowstoneClerkandRecordersOfficeinaformatacceptabletothem.

AppendixFHOAAgreementRequirements

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HOMEOWNERS’ASSOCIATIONSREQUIREMENTS

For stormwater systems within subdivisions, a homeowner’s association (HOA) shall be formed tomaintainandoperatethefacilities.

A draft copyof the SIA and/or CC&Rs for theHOA in charge ofoperating andmaintaining the facilitiesassociatedwiththestormwatersystemshallbesubmittedatthetimeofPreliminaryPlatsubmittal.FinalcopiesarerequiredatthetimeofinitialPrivateContractSubmittal.TheSIA/CC&RsshallsummarizethemaintenanceandfiscalresponsibilitiesoftheHOA.Inaddition,theSIA/CC&R’sshallstatethatanyproposedchangestothestormwatersystem/facilitiesshallfirstbeapprovedbytheCityEngineer’sOffice.TheO&Mmanual shall also be submitted at this time. A financialplan is required inorder toprovide theentityresponsible for maintenance with guidance in regard to financial planning for maintenance andreplacementcosts.Thefinancialplanshallincludethefollowingitems:

Alistofallstormwater‐relatedfacilitiesandtheirexpecteddateofreplacementandassociated

replacementcosts.

Sinkingfundcalculationsthattakeintoconsiderationprobableinflationoverthelifeoftheinfrastructureandestimatesthefundsthatneedtobesetasideannually.

Amechanismforinitiatingandsustainingthesinkingfundaccountdemonstratingthatperpetual

maintenanceofallfacilitiesassociatedwiththestormwatersystemwillbesustained.Homeowners’associationsaretobenon‐profitorganizations.Astandardbusinesslicenseisnotacceptableforthispurpose.TheHOAshallremainingoodstandingwiththerequirementsoftheStateofMontana.Developer shall sign HOA Agreement stating ownership and responsibilities prior to approval ofdevelopment.AtemplateagreementisfoundinAppendixG.

AppendixGTemplateForms

STORMWATERFACILITYMAINTENANCEAGREEMENT BMPINSPECTIONFORM REPORTCERTIFICATIONFORM INJECTIONWELLEXAMPLE BOULDERPITEXAMPLES

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STORMWATERFACILITYMAINTENACEAGREEMENT

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The following agreement shall be used for sample purposes only. The document shall not be used

directly for recording. A template document can be downloaded off the City’s website (http://ci.billings.mt.us/567/Stormwater‐Management).

Stormwater Facility Maintenance Agreement

THIS Agreement is made by and between the City of Billings, a municipal corporation of the

State of Montana, hereinafter referred to as the “City” and ____________________________________, a Developer, as owner (including successors and assignee’s of the City as may become applicable including the heirs, executors, administrators, successors and assigns of above owner(s) as may be or may become applicable), hereinafter called “Grantor,” (if more than one grantor is listed above, said language herein referring thereto shall be interpreted in the plural and refer jointly and severally to such grantors).

WHEREAS, the undersigned is proceeding to build on and develop the property; and has submitted the Site Plan/Subdivision Plan know as _______________________________________ (Name of Plan/Development), located on _______________________ (Lot/Block/Subdivision) hereinafter called the "Plan", which is expressly made a part hereof, as approved or to be approved by the City, provides for detention of stormwater within the confines of the property; and

WHEREAS, the City and the undersigned, its successors and assigns, including any homeowners association, (hereinafter the “Landowner”) agree that the health, safety, and welfare of the residents of the City, requires that on‐site stormwater management facilities be constructed and maintained on the Property; and

WHEREAS, the City requires that on‐site stormwater management facilities as shown on the Plan (the “Facilities”) be constructed and adequately maintained by the Landowner.

NOW, THEREFORE, in consideration of the foregoing premises, the mutual covenants contained herein, and the following terms and conditions, the parties hereto agree as follows: 1. The Facilities shall be constructed by the Landowner, in accordance with the plans and specifications identified in the Plan. 2. The Landowner shall at all time, adequately maintain the Facilities. Such maintenance obligation shall include the obligation to properly maintain all surface and subsurface inlets, pipes, channels, structures, rock pits, vegetation, and all other improvements provided to control the quantity and quality of the stormwater within the facility. Adequate maintenance is herein defined as keeping the Facilities and all components thereof in good working condition so that these Facilities continue to perform their design functions. 3. In the event the Landowner fails to maintain the Facilities in good working condition acceptable to the City, the City may enter upon the Property and take such steps as are necessary to correct deficiencies identified in the inspection report and to charge the costs of such repairs to the Landowner. This provision shall not be construed to allow the City to erect any structure of permanent nature on the land of the Landowner outside of the easement for the stormwater management facilities. It is expressly understood and agreed that the City is under no obligation to routinely maintain or repair said facilities, and in no event shall this Agreement be construed to impose any such obligation on the

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City. The Landowner grants to the City, its authorized agents and employees, a non‐exclusive, perpetual easement over, across, under and through the Property for such purposes. 4. The Landowner shall perform all work necessary to keep the Facilities in good working order. In the event a maintenance schedule for the stormwater management facilities (including sediment removal) is outlined on the approved plans, the Landowner shall comply with such schedule. 5. In the event the City performs work of any nature on the Facilities in accordance with this Agreement, or expends any funds in performance of said work for labor, use of equipment, supplies, materials, and the like, the Landowner shall reimburse the City upon demand, within thirty (30) days of receipt thereof for all actual costs incurred by the City hereunder. 6. This Agreement imposes no liability of any kind whatsoever on the City and the Landowner agrees to hold the City harmless from any liability in the event the stormwater management facilities fail to operate properly. 7. This Agreement shall be recorded among the deed records of Yellowstone County, Montana, and shall constitute a covenant running with the land, and shall be binding on the Landowner, its administrators, executors, assigns, heirs and any other successors in interests, including any homeowners association. ** Note to Reviewer: Attached to this agreement shall be required information listed in Appendix F of the City’s Stormwater Management Manual. Remove this text for final document**

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IN WITNESS THEREOF, the parties hereto acting through their duly authorized agents have caused this Agreement to be signed, sealed and delivered: (Insert Company/Corporation/Partnership Name) [SEAL] Date

______________________________________________ _____________

By: (Name and Title)

State of Montana County of _____________________________ This instrument was signed and sworn to before me on _____________ by ____________________ (Name of signer)

______________________________ (Notary Signature) [Affix seal/stamp to the left or below]

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ACKNOWLEDGMENT AND ACCEPTANCE OF CONVEYANCE

The City of Billings acknowledges receipt of this HOA Stormwater Facility Maintenance Agreement and hereby accepts the property interest conveyed through this instrument.

______________________________________

________________________, City Administrator, City of Billings

ATTEST: (name), City Clerk

State of Montana County of _____________________________ This instrument was signed and sworn to before me on _____________ by ____________________

(Name of signer)

______________________________ (Notary Signature) [Affix seal/stamp to the left or below]

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POSTCONSTRUCTIONBMPINSPECTIONCHECKLIST

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Example Inspection Forms for Post‐Construction BMP’s can be found in Appendix F of the Montana Post‐Construction StormWater BMP Design Guidance Manual; September 2017. The table below suppliments theInspectionFormsbyindicatinginspectionactivitiesandfrequencies.

POSTCONSTRUCTIONBMPINSPECTIONCHECKLIST

Monthly

Schedule/Frequency

Annually

Annually

Inspectpondarea,sidewalls,andshorelineforerosion,settlement,androdentdamage

Inspectexteriorofcatchbasins

Annually

Annually

Annually

Quarterly

Monthlyandafterstormevents

Quarterlyandafterstormevents

Quarterly,andafterstormevents

Quarterlyandafterstormevents

Quarterly

Inspectditches,checkdams,andallvisiblepipesandculvertsfortrash,obstructionsand

otherproblems

Inspectbioswalesforvegetationcoverandbareareas

Inspectfences,gatesandlocks

Quarterly

PreparedbyRESourcesforSustainableCommunitiesfortheBirchBayWatershed&AquaticResourcesManagement(BBWARM)District.ThisprojectwasbeenfundedwhollyorinpartbytheU.S.EnvironmentalProtectionAgencyunderassistanceagreementWS‐96073401toWhatcomCounty.ThecontentofthisdocumentdonotnecessarilyreflecttheviewsandpoliciesoftheEnvironmentalProtectionAgency,nordoesmentionoftradenamesorcommercialproductsconstituteendorsementorrecommendationsforuse.

Inspectpondareaforoilsheensortrash

Inspectaccessrampsforeaseofheavyequipmentaccess

Inspectinsidecatchbasins,includingflowrestrictor/orificeplate

Inspectspillwayforvegetationovergrowthandeaseofheavyequipmentaccess

Inspectinteriorofcatchbasinsfordebrisandsediment

Pondareasedimentaccumulation(pondbottom)

Inspectpondareaforundesirableorpoisonousvegetationandnoxiousweeds

Inspectwaterlevelsinthepond

Inspecttrashracks,debrisbarriers,andenergydissipaters

Inspectinletsandoutletsfortrash,obstructions,andvegetation

Activity

Semi‐annually,duringgrowingseason

Afterstormevents

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REPORTCERTIFICATIONFORM

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(Project Title) FINAL DRAINAGE, GEOTECHNICAL OR HYDROGEOLOGICAL REPORT (List one)

(LOCATION)

CERTIFICATION

IherebystatethatthisFinalDrainage,GeotechnicalorHydrogeologicalReport(pickone)hasbeenpreparedbymeorundermysupervisionandmeetsthestandardofcareandexpertisewhichisusualandcustomaryinthiscommunityofprofessionalengineers,geologistsorhydrogeologists(pickone).TheanalysishasbeenpreparedutilizingproceduresandpracticesspecifiedbytheCityofBillingsandwithinthestandardacceptedpractices.

PE, PG or PH STAMP OR SEAL

____________________________________ __________________________

John Doe, P.E.; P.G.; P.H. Date

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INJECTIONWELLEXAMPLE

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BOULDERPITEXAMPLES

AppendixHSoilInfiltration

TestingRequirement

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This Section adds to the infiltration testing section of theMontanaPost‐Construction StormWaterBMPDesignGuidanceManual;however,thissectionshalltakeprecedenceoveranydiscrepancieswiththepracticesorresultsofthewaterqualityGuidanceManual.

Oneofthefollowingmethodsshouldbeusedtodeterminethedesigninfiltrationrate.

DesignInfiltrationRateUsingtheUSCSClassification(non‐fieldmeasured–Thismethodisonlyapplicabletoproposedinfiltrationsystemswithlessthan5,000squarefeetofinfiltrationarea)

EncasedFallingHeadTest(fieldmeasured) PilotInfiltrationTest(fieldmeasured) BoreholeInfiltrationTest(fieldmeasured)

DesignInfiltrationRateUsingtheUSCSClassification

For infiltrationsystemswith less than5,000square feet,adesign infiltrationratecanbeselected fromTable1:InfiltrationRateRangesforUSCSSoilsbasedontheleast‐permeablesoillayerencounteredwithin10feetofthebaseoftheproposedinfiltrationsystem.ThedesigninfiltrationratespresentedinTable1:InfiltrationRateRangesforUSCSSoilsrepresenttherangesofinfiltrationratesforeachsoilclassification.Theminimuminfiltrationrateshallbeselectedasthedesigninfiltrationrate.

Table1:InfiltrationRateRangesforUSCSSoils

SoilDescription USCS

RangeofTypicalInfiltrationRates(inches/hour)

min max*

Wellgradedgravel,sandygravel GW 1.30 137.00

Poorlygradedgravel,sandygravel GP 6.80 137.00

Wellgradedsand,gravellysand SW 0.80 68.00

Poorlygradedsand,gravellysand SP 0.50 68.00

Siltygravel,siltysandygravel GM 1.63 13.50

Clayeysands SC 0.05 0.78

SiltySand SM 0.24 0.70

Clayeygravel,clayeysandygravel GC 0.04 0.50

InorganicSiltsoflowplasticity ML 0.04 0.14

Clay CL 0.00 0.01

InorganicSiltsofhighplasticity MH 0.00 0.01

Inorganicclaysofhighplasticity CH 0.00 0.01 *Ifproposingtousemaximumrates,adetailedexplanationshallbeprovidedwhythemaximumratesapplytodevelopment.

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EncasedFallingHeadTest

Theencasedfallingheadtestisperformedwitha6‐inchcasingthatisembeddedapproximately24inchesintothenativesoil.Thegoalofthisfieldtestistoevaluatetheverticalinfiltrationratethrougha24‐inchplugofsoil,withoutallowinganylateralinfiltration.Thetestisnotappropriateingravellysoilsorinothersoilswhereagoodsealwiththecasingcannotbeestablished.

Aminimumofthreeencasedfallingheadtestsmustbeconductedwithinthefootprintofeachinfiltrationsystem.Forproposed infiltrationsystemswithmore than10,000square feetof infiltrationarea,oneadditionalencasedfallingheadtestisrequiredforeachadditional10,000squarefeet.Differentsoiltypesmaybeencounteredduringthesoilinfiltrationtesting;aminimumoftwoencasedfallingheadtestspersoiltypearerequired. Theencasedfallingheadtestlocationsshouldbeevenlyspacedthroughouttheproposedinfiltrationsystem.Theresultsoftheinfiltration tests will be averaged to determine the measured infiltration rate for the infiltration system. Themeasuredinfiltrationwillbedividedbyasafetyfactorof2.0toarriveatthedesigninfiltrationrate.

EncasedFallingHeadTestProcedure:

Embedasolid6‐inchdiametercasingintothenativesoilattheelevationoftheproposedfacilitybottom.Ensurethattheembedmentprovidesagoodsealaroundthepipecasingsothatpercolationwillbelimitedtothe24‐inchplugofthematerialwithinthecasing.Theminimumcasinglengthmustbe48inches;longercasingscanbeused.

Fill the6‐inchdiametercasingwithcleanwateraminimumof24inchesabovethesoil tobetested,andmaintainthisdepthforatleast4hours(orovernightifclaysoilsarepresent)topresoakthenativematerial.Insandysoilswithlittleornoclayorsilt,soakingisnotnecessary. Ifafterfillingtheholetwicewith24inchesofwater,ifthewaterinfiltratescompletelyinlessthan10minutes,thetestcanproceedimmediately.

Toconductthefirsttrialofthetest,fillthe6‐inchdiametercasingtoapproximately24inchesabovethesoilandmeasurethewaterleveltothenearest0.01foot(⅛inch).Theheadusedinthetestcanbegreaterthan24inches,providedtheheadisnotgreaterthan50percentofthemaximumheadintheproposedinfiltrationsystem.Thepre‐saturationheadmustbethesameastheinfiltrationtesting.Thelevelshouldbemeasuredwithatapeorotherdevicewithreferencetoafixedpoint.Thetopofthepipeisoftenaconvenientreferencepoint.Recordtheexacttime.

Measurethewaterleveltothenearest0.01foot(⅛inch)at10‐minuteintervalsforatotalperiodof1hour(or20‐minuteintervalsfor2hoursinslowersoils)oruntilallofthewaterhasinfiltrated.Infasterdrainingsoils(sandsandgravels),itmaybenecessarytoshortenthemeasurementintervalinordertoobtainawell‐definedinfiltrationratecurve.Constantheadtestsmaybesubstitutedforfallingheadtestsatthediscretionoftheprofessionaloverseeingtheinfiltrationtesting.Successivetrialsshouldberununtilthepercentchangeinmeasuredinfiltrationratebetweentwosuccessivetrialsisminimal.Thetrialshouldbediscountediftheinfiltrationratebetweensuccessivetrialsincreases. Atleastthreetrialsmustbeconducted. Aftereachtrial,thewaterlevelisreadjustedtothe24inchlevel.Enterresultsintothedatatable.

Measurethedepthandapproximatevolumeofanywaterthataccumulatesintheboreholeortrencharoundthetestcasing,indicatingabadsealaroundthepipeorshortcircuitingthroughthesoilbeingtested.

Theaverageinfiltrationrateoverthelasttrialshouldbeusedtocalculatethemeasuredinfiltrationrate ThemeasuredinfiltrationratemustbewithintheratesgiveninTable1:InfiltrationRateRangesforUSCS

Soils,providedinthisappendix,forthenativesoiltested.Ifthemeasuredinfiltrationrateisgreaterthanthe

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maximumlistedrate,additionalinformationmayberequiredbytheCity,uptoandincludingrepeatingthetestprocedureunderobservationbypersonneldesignatedbytheCity

Thelocationofthetestshallcorrespondtotheinfiltrationsystemlocation.

Figure1:EncasedFallingHeadTestDiagram:

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PilotInfiltrationTest

Large‐scale infiltration testing using the Pilot Infiltration Test described below is the preferred method forestimating the measured infiltration rate of the soil profile beneath proposed infiltration systems. The PilotInfiltrationTestisnotastandardtestbutratherapracticalfieldprocedurerecommendedbytheCityofBillings.

A minimum number of two pilot infiltration tests must be conducted within the footprint of each proposedinfiltrationsystem.Forproposedinfiltrationsystemswithmorethan10,000squarefeetofinfiltrationarea,oneadditionalpilotinfiltrationtestisrequiredforeachadditional10,000SF.Thepilotinfiltrationtestlocationsshouldbeevenlyspacedthroughouttheproposedinfiltrationsystem.Theresultsoftheinfiltrationtestswillbeaveragedto determine themeasured infiltration rate for the infiltration system. The pilot infiltration testwill produce ameasuredinfiltrationrateininchesperhour.Themeasuredinfiltrationwillbedividedbyafactorofsafetyof2.0toarriveatthedesigninfiltrationrate.

PilotInfiltrationTestProcedure:

Excavate the testpit to theestimatedsurfaceelevationof theproposed infiltrationsystem.Layback theslopessufficientlytoavoidcavinganderosionduringthetest.Alternatively,considershoringthesidesofthetestpit.

Thehorizontalsurfaceareaofthebottomofthetestpitshouldbeapproximately100squarefeet.Accuratelydocumentthesizeandgeometryofthetestpit.

Installaverticalmeasuringrod(minimum5‐ft. long)marked in1/8” increments in thecenterof thepitbottom

Usearigidpipewithasplashplateonthebottomtoconveywatertothepitandreduceside‐wallerosionorexcessivedisturbanceofthepondbottom.Excessiveerosionandbottomdisturbancewillresultincloggingoftheinfiltrationreceptorandyieldlowerthanactualinfiltrationrates.

Addwatertothepitataratethatwillmaintainawaterlevelbetween6and12inchesabovethebottomofthepit.Arotametercanbeusedtomeasuretheflowrateintothepit.Every15‐30min,recordthecumulativevolumeandinstantaneousflowrateingallonsperminutenecessarytomaintainthewaterlevelatthesamepointonthemeasuringrod.Keepaddingwatertothepituntilonehouraftertheflowrateintothepithasstabilized(constantflowrate;agoalof5%variationorlessvariationinthetotalflow)whilemaintainingthesamepondwaterlevel.Thetotalofthepre‐soaktimeplusonehouraftertheflowratehasstabilizedshouldbenolessthan6hours.

Aftertheflowratehasstabilizedforatleastonehour,turnoffthewaterandrecordtherateofinfiltration(thedroprateofthestandingwater)ininchesperhourfromthemeasuringroddata,untilthewaterhasinfiltratedcompletely.Calculateandrecordthemeasuredinfiltrationrateininchesperhourin30minutesorone‐hourincrements.

ThemeasuredinfiltrationratemustbewithintherangesgiveninTable1:InfiltrationRateRangesforUSCSSoils,providedinthisappendix,forthenativesoiltested.Ifthemeasuredinfiltrationrateisgreaterthanthemaximumlistedrate,additionalinformationmayberequiredbytheCity,uptoandincludingrepeatingthetestprocedureunderobservationbypersonneldesignatedbytheCity.


BoreholeInjectionTest

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Theboreholeinjectiontestwillproduceameasuredinjectionrateingallonsperhourforastormwaterinjectionwell.Themeasuredinjectionratewillbedividedbyafactorofsafetyof2.0toarriveatthedesigninjectionrate.In‐situ injection measurements, using the borehole injection test described below is the preferred method forestimatingthemeasuredinjectionratefordisposalofstormwatertohigherpermeabilitysoilslocatedataminimumdepth of 20 feet bgs. The borehole injection test is not a standard test but rather a practical field procedurerecommendedbytheCityofBillings.

BoreholeInjectionTestProcedure:

Thelocationofthetestmustbewithin20feetoftheproposedlocationofthestormwaterinjectionwell. Drillboreholetotargetinjectiondepth,aminimumof20feet. Installsteelcasing,aminimumof4inchesdiameter,bypushingordrivingitto24inchesbelowthedrillbit.

Casingmustextendtothegroundsurface. Addwatertothecasingataratedeterminedbythetester,basedontheaveragevolumetobedischargedto

thewellover72hours.Arotametercanbeusedtomeasuretheflowrateintothecasing.Every15‐30min,recordthecumulativevolumeandinstantaneousflowrateingallonsperminute.

Thecasingmustremainunderatmosphericpressurethroughoutthedurationofthetest.Apumpmaybeusedtoconveywatertothecasing,butthepumpdischargecannotpressurizethecasing.

Watermustbesuppliedtothecasingforaminimumofonehour.Testsmaybeterminatedafterwaterhasbeensuppliedtothecasingforatotaloffourhours.Documentthetimewaterissuppliedtothecasingandthedurationofanyinterruptions(e.g.changingthewatersupplysource).

Thetestmaybeterminatedanytimeafteronehourifthevolumedischargedduringthetestexceeds5%oftheestimated72‐hourdischargevolumeoftheproposedstormwaterinjectionwell.

Calculateandrecordtheaverageinfiltrationrateingallonsperminuteoverthefinalhourofthetest.Theaverage injection rate over the final hour of the test will be the measured injection rate used for thestormwaterinjectionwell.Themeasuredinjectionratewillbedividedbyafactorofsafetyof2.0toarriveatthedesigninjectionrate.


AppendixISpecialDischargeArea

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The map below identifies the City’s West Billings Special Discharge Area, which identifies that the allowablestormwaterdischargeratewithinthisareafromalltypesofdevelopmentandforalldurationsofstorms,uptoandincludingtheMajorStorm,shallbelimitedtoaproratedvalueof1cfsper10acresofdevelopment.Forexample,a100‐acresubdivisionwouldbeallowedadischargerateof10cfs,whilea0.5‐acrecommercialsitewouldbeallowedadischargerateof0.05cfs(approximately22.4gpm).ThemapbelowindicatesthepropertieslocatedwithintheSpecialDischargeArea.

AppendixJProjectSubmittalChecklist

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Thefollowinginformationisintendedtoprovideaframeworkforrequireddocumentstobesubmittedbasedupondevelopmentactivity.Thislistmaynotbeallinclusiveandadditionalinformationorformsshallbesubmitteduponrequest.

ResidentialLotDevelopment

SSP LotGradingPlan

CommercialPropertyDevelopment

CDP Geotechnical/HydrogeologicReport ProjectPlans OperationsandMaintenanceManual–RecordedCopy PlatandSIA

SubdivisionDevelopment

CDP Geotechnical/HydrogeologicReport ProjectPlans OperationsandMaintenanceManual–RecordedCopy PlatandSIA HOAMaintenanceAgreement–RecordedCopy MaterialsSubmittals(ShopDrawings) FinalAcceptanceSubmittalafterconstruction

Documents

Stormwater Management Manual - CivicPlus