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Low Impact Development for Stormwater Treatment and Hydrograph Modification Management in California. Dan Cloak, Principal Dan Cloak Environmental Consulting. Topics. Brief History of LID in California - PowerPoint PPT Presentation

Text of Dan Cloak, Principal Dan Cloak Environmental Consulting

  • Low Impact Development for Stormwater Treatment and Hydrograph Modification Management in CaliforniaDan Cloak, Principal Dan Cloak Environmental Consulting

  • TopicsBrief History of LID in California Using LID to comply with NPDES Treatment & Hydrograph Modification Management Requirements Design Challenges and Construction IssuesRecent LID Projects

  • A Brief History of LID in CAContra Costa Approach Hydrograph Modification Mgt. SWRCB Bellflower DecisionPortland Stormwater Manual Low Impact Development Manual Imperviousness and flow-controlStart at the Source Stormwater NPDES Permits Village Homes, Davis 19781994199920032000

  • Village HomesNarrow streetsSurface drainageSwales as an amenity

  • Stormwater NPDESEarly YearsCharacterization of urban runoffFocused on demonstrating reductions of pollutant loadsEnd-of-pipe treatment vs. BMPsDesign criteria for conventional treatment facilitiesDo what you can, where you can.

  • Start at the SourcePreceded by San Francisco Bay RWQCB Staff Recommendations (1993)Emphasis on reducing imperviousness to reduce pollutant loadingAddressed need to identify site-design alternativesIntegrates urban design and site designNo regulatory mandate

  • ImperviousnessImportance of Imperviousness (1994)Empirical relationship between watershed imperviousness and stream degradationAwareness of the effects of small storms and increased runoff frequencyPeak flow control over a range of storm sizesContinuous simulation


  • Low Impact DevelopmentDeveloped as an alternative to treatment detention basinsAddressed preserving site hydrology and natural functionsSite design and bioretention (rain gardens)Included hydrologic criteria based on matching curve numbers

  • Portland Stormwater Manual

  • Bellflower Decision and HMPsBellflower made the L.A. RWQCBs treatment criteria a statewide maximum extent practicable standardSan Francisco Bay Board added Hydrograph Modification ManagementBeforeAfter

  • Meeting NPDES RequirementsLID is a means to achieve compliance with NPDES treatment and flow-control requirementsFocus public resources on helping small developments, infill, and redevelopment to comply with NPDES requirementsBe pro-active

  • NPDES requirementsin a nutshellMinimize imperviousnessControl pollutant sourcesTreat stormwater prior to discharge from the siteMatch peaks and durations to pre-project conditions (HMP)Maintain treatment and flow-control facilities in perpetuity

  • Low Impact DevelopmentStormwater treatment and flow control Minimize imperviousnessDisperse runoffUse Integrated Management Practices (IMPs)

  • Swale

  • Planter Box

  • Dry Well

  • Integrated Management PracticesDetain and treat runoffTypically fit into setbacks and landscaped areasAccommodate diverse plant palettesLow-maintenanceDont breed mosquitoesCan be attractive

    Soil surface must be 6-12" lower than surrounding pavementRequire 3-4 feet of vertical headCan affect decisions about placement of buildings, roadways, and parking


  • Showing Treatment ComplianceNPDES Permit sizing criteria for treatment control:collect and convey drainage designconventional, end of pipe treatmentuse of C factors to determine design inflow or volume

  • Sizing criterion for treatmentPlanting medium0.2 inches/houri = 5 inches/hourBMP Area/Impervious Area =0.2/5 = 0.04

  • Application of sizing factor

  • LID for flow controlCan LID facilities mitigate increased peaks and volumes of flows from impervious areas?How would we demonstrate that?What are the design criteria?BeforeAfter

  • HSPF analysis of unit-acre runoff33 years hourly rainfallPre-project condition100% impervious conditionHydrologic soil groups A, B, C, DSwales, Bioretention Areas, In-ground and Flow-through PlantersUnderdrain with flow-restrictor in C&D soilsDry wells, infiltration trenches and basins

  • Results: Control of Peak Flows

  • Results: Flow Duration Control

  • Sizing Factors for Flow Control

    IMPSizing FactorsIn-Ground PlanterGroup A: 0.08Group B: 0.11Group C: 0.06Group D: 0.05Flow-Through PlanterGroup C: 0.06Group D: 0.05Vegetated/ Grassy SwaleGroup A: 0.10 to 0.14Group B: 0.14 to 0.21Group C: 0.10 to 0.15Group D: 0.07 to 0.12Bioretention BasinGroup A: 0.13Group B: 0.15Group C: 0.08Group D: 0.06

    IMPSizing FactorsDry WellGroup A: 0.05 to 0.06Group B: 0.06 to 0.09Infiltration Trench Group A: 0.05 to 0.06Group B: 0.07 to 0.10Infiltration Basin Group A: 0.05 to 0.10Group B: 0.06 to 0.16

  • Adjustment to annual rainfall

  • Implementing LID GoalsMake it easier for applicants to prepare submittalsMake it easier for municipal staff to review submittals for compliancePromote consistent and fair implementation countywideIntegrate LID, treatment, and hydrograph modification management requirements

  • LID Site Design ProcedureDivide the site into Drainage Management AreasUse landscape to disperse and retain runoff where possibleRoute drainage from remaining areas to bioretention facilitiesCheck facility locations for available space and hydraulic head

  • Drainage Management AreasFour Types of AreasSelf-treating areasSelf-retaining areasAreas draining to a self-retaining areaAreas draining to a treatment facilityOnly one surface type within each areaMany-to-one relationship between drainage areas and facilities

  • Self-treating areasMust be 100% perviousMust drain offsiteMust not drain on to impervious areasMust not receive drainage from impervious areasMust not drain to treatment facilitiesNo treatment or flow control requiredNo further calculations required

  • Self-retaining areas

  • Self-retaining areasBerm or depress grade to retain 1" rainSet area drain inlets above gradeAmend soils Terrace mild slopes Have limited applicability in Dense developmentsHillsides

  • Areas draining to self-retaining areasImpervious areas can drain on to self-retaining areasExample: Roof leaders directed to lawn or landscapeMaximum ratio is 2:1 for treatment; 1:1 for flow controlNo maintenance verification required

  • Areas draining to self-retaining areas

  • Tabulating Areas

    Self-Retaining AreasDMA NameArea (SF)

    Areas Draining to Self-Retaining AreasDMA NameArea (SF)Post-project surface typeRunoff factorReceiving Self-retaining DMAReceiving DMA Area (SF)

  • Areas draining to IMPsAreas used to calculate the required size of the facilityWhere possible, drain only impervious roofs and pavement to facilitiesDelineate any pervious areas as separate Drainage Management Areas

  • DMAs draining to facilities

  • Calculating Facility Size

    A-2: Paving 10,000 SFA-3: Turf 20,000 SFA-1: 5,000 SFRoofBioretention Facility A

  • DMAs draining to facilities

  • Design ChallengesResidential SubdivisionsAesthetics and landscape designTrip hazards

  • Residential SubdivisionsWhere to drain the street?Who owns the facilities?How does the municipality verify maintenance and compel owners to fix problems?What if there is no HOA?

  • Residential SubdivisionsCreate a separate parcel for bioretention areaAgreement runs with the land and is executed prior to subdivisionProvisions in CC&Rs describe how homeowners pay for maintenance

  • AestheticsMore explanations and sketches showing landscape design alternativesYes, lawns are OK.Yes, trees are OK.Illustrate facility design with structural soil

  • Trip Hazards

  • Generic Bioretention Facility18" specified soilVsurfaceVsubsurfacehoutflowAsurfaceAinfiltrationQoutflowhoverflow

  • Soil MixGravel18"18"10"2"OverflowUnder drainFloodable Pavement6"

  • More Storage Less AggregateSoil Mix18"10"2"Overflow

  • Cistern

  • Dry Well with Open VaultHydrologic Soil Groups A and B onlyTreatment of 80% of total runoff Pre-project peak flows and durations not exceededDrains in 72 hours

  • Construction IssuesRunoff from the intended tributary area must flow to the facility.The surface reservoir must fill to its intended volume during high inflows.Runoff must filter rapidly through the soil layer.Filtered runoff must infiltrate into the native soil to the extent possible.Remaining runoff must be captured and drained to daylight or a storm drain.

  • Tributary AreaDrainage area includes portions of roof and of parking lot

  • Ensuring flow to the facilityRunoff may enter by sheet flow or be piped.Roof leaders can be piped directly or spill across pavement

  • Distribute flow evenly

  • Distribute flow evenly

  • Surface reservoir must fill

  • Surface reservoir must fill

  • Surface reservoir must fill

  • Surface reservoir must fill

  • Surface reservoir must fill

  • Runoff must drain rapidlyTypically no native on-site material to be usedImported material to be a mix of sand and organicsMinimum infiltration rate 5"/hourAim for 10"/hour at installationOn-site bucket test

  • No filter fabric

  • One hot afternoon in Contra Costa

  • One hot afternoon in Contra Costa

  • One hot afternoon in Contra Costa

  • One hot afternoon in Contra Costa


    [email protected]

    Runoff Factor

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