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 Using LID to comply with NPDES Treatment & Hydrograph Modification Management Requirements Design Challenges and Construction Issues Recent LID Projects

A Brief History of LID in CA Contra Costa Approach Hydrograph Modification Mgt. SWRCB Bellflower Decision Portland Stormwater Manual Low Impact Development Manual Imperviousness and flow-control Start at the Source Stormwater NPDES Permits Village Homes, Davis

Village Homes Narrow streets Surface drainage Swales as an amenity

Stormwater NPDES—Early Years Characterization of urban runoff Focused on demonstrating reductions of pollutant loads End-of-pipe treatment vs. BMPs Design criteria for conventional treatment facilities “Do what you can, where you can.”

Start at the Source Preceded by San Francisco Bay RWQCB “Staff Recommendations” (1993) Emphasis on reducing imperviousness to reduce pollutant loading Addressed need to identify site-design alternatives Integrates urban design and site design No regulatory mandate

Imperviousness Importance of Imperviousness (1994) Empirical relationship between watershed imperviousness and stream degradation Awareness of the effects of small storms and increased runoff frequency Peak flow control over a range of storm sizes Continuous simulation Before After

Low Impact Development Developed as an alternative to treatment detention basins Addressed preserving site hydrology and natural functions Site design and bioretention (“rain gardens”) Included hydrologic criteria based on matching curve numbers

Portland Stormwater Manual

Bellflower Decision and HMPs Bellflower made the L.A. RWQCB’s treatment criteria a statewide “maximum extent practicable” standard San Francisco Bay Board added “Hydrograph Modification Management” Before After

Meeting NPDES Requirements LID is a means to achieve compliance with NPDES treatment and flow-control requirements Focus public resources on helping small developments, infill, and redevelopment to comply with NPDES requirements Be pro-active

NPDES requirements in a nutshell Minimize imperviousness Control pollutant sources Treat stormwater prior to discharge from the site Match peaks and durations to pre-project conditions (HMP) Maintain treatment and flow-control facilities in perpetuity

Low Impact Development Stormwater treatment and flow control Minimize imperviousness Disperse runoff Use Integrated Management Practices (IMPs)

Swale

Planter Box

Dry Well

Integrated Management Practices Detain and treat runoff Typically fit into setbacks and landscaped areas Accommodate diverse plant palettes Low-maintenance Don’t breed mosquitoes Can be attractive Soil surface must be 6- 12" lower than surrounding pavement Require 3-4 feet of vertical “head” Can affect decisions about placement of buildings, roadways, and parking Advantages Challenges

Showing Treatment Compliance NPDES Permit sizing criteria for treatment control: “collect and convey” drainage design conventional, “end of pipe” treatment use of “C” factors to determine design inflow or volume

Sizing criterion for treatment Planting medium 0.2 inches/hour i = 5 inches/hour BMP Area/Impervious Area = 0.2/5 = 0.04

Application of sizing factor

LID for flow control Can LID facilities mitigate increased peaks and volumes of flows from impervious areas? How would we demonstrate that? What are the design criteria? Before After

HSPF analysis of unit-acre runoff 33 years hourly rainfall Pre-project condition 100% impervious condition Hydrologic soil groups A, B, C, D Swales, Bioretention Areas, In-ground and Flow-through Planters Underdrain with flow-restrictor in C&D soils Dry wells, infiltration trenches and basins

Results: Control of Peak Flows

Results: Flow Duration Control

Sizing Factors for Flow Control IMPSizing Factors In-Ground Planter Group A: 0.08 Group B: 0.11 Group C: 0.06 Group D: 0.05 Flow- Through Planter Group C: 0.06 Group D: 0.05 Vegetated/ Grassy Swale Group A: 0.10 to 0.14 Group B: 0.14 to 0.21 Group C: 0.10 to 0.15 Group D: 0.07 to 0.12 Bioretention Basin Group A: 0.13 Group B: 0.15 Group C: 0.08 Group D: 0.06 IMPSizing Factors Dry WellGroup A: 0.05 to 0.06 Group B: 0.06 to 0.09 Infiltration Trench Group A: 0.05 to 0.06 Group B: 0.07 to 0.10 Infiltration Basin Group A: 0.05 to 0.10 Group B: 0.06 to 0.16

Adjustment to annual rainfall

Implementing LID — Goals Make it easier for applicants to prepare submittals Make it easier for municipal staff to review submittals for compliance Promote consistent and fair implementation countywide Integrate LID, treatment, and hydrograph modification management requirements

LID Site Design Procedure 1. Divide the site into Drainage Management Areas 2. Use landscape to disperse and retain runoff where possible 3. Route drainage from remaining areas to bioretention facilities 4. Check facility locations for available space and hydraulic head

Four Types of Areas 1. Self-treating areas 2. Self-retaining areas 3. Areas draining to a self-retaining area 4. Areas draining to a treatment facility Only one surface type within each area Many-to-one relationship between drainage areas and facilities Drainage Management Areas

Self-treating areas Must be 100% pervious Must drain offsite Must not drain on to impervious areas Must not receive drainage from impervious areas Must not drain to treatment facilities No treatment or flow control required No further calculations required

Self-retaining areas

Berm or depress grade to retain 1" rain Set area drain inlets above grade Amend soils Terrace mild slopes Have limited applicability in Dense developments Hillsides

Areas draining to self-retaining areas Impervious areas can drain on to self-retaining areas Example: Roof leaders directed to lawn or landscape Maximum ratio is 2:1 for treatment; 1:1 for flow control No maintenance verification required

Areas draining to self-retaining areas

Tabulating Areas Self-Treating Areas DMA NameArea (SF) Self-Retaining Areas DMA NameArea (SF) Areas Draining to Self-Retaining Areas DMA Name Area (SF) Post- project surface type Runoff factor Receiving Self-retaining DMA Receiving DMA Area (SF)

Areas draining to IMPs Areas used to calculate the required size of the facility Where possible, drain only impervious roofs and pavement to facilities Delineate any pervious areas as separate Drainage Management Areas

DMA Name DMA Sq. Ft Surface Type Runoff Factor Area x runoff factor Sizing Factor Min. Size Size Planned Facility A-----  DMAs draining to facilities

Calculating Facility Size A-2: Paving 10,000 SF A-3: Turf 20,000 SF A-1: 5,000 SF Roof Bioretention Facility A

DMA Name DMA Sq. Ft Surface Type Runoff Factor Area x runoff factor A Roof A Paved A Grass Sizing Factor Min. Size Size Planned Facility A-----  DMAs draining to facilities

Design Challenges Residential Subdivisions Aesthetics and landscape design Trip hazards

Residential Subdivisions Where 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 Subdivisions Create a separate parcel for bioretention area Agreement “runs with the land” and is executed prior to subdivision Provisions in CC&Rs describe how homeowners pay for maintenance

Aesthetics More explanations and sketches showing landscape design alternatives Yes, lawns are OK. Yes, trees are OK. Illustrate facility design with structural soil

Trip Hazards

Generic Bioretention Facility 18" specified soil V surface V subsurface h outflow A surface A infiltration Q outflow h overflow

Soil Mix Gravel 18" 10" 2"2" Overflow Under drain “Floodable” Pavement 6"6"

More Storage – Less Aggregate Soil Mix 18" 10" 2"2" Overflow

Cistern

Dry Well with Open Vault Hydrologic Soil Groups “A” and “B” only Treatment of 80% of total runoff Pre-project peak flows and durations not exceeded Drains in 72 hours

Construction Issues Runoff 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 Area Drainage area includes portions of roof and of parking lot

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

Distribute flow evenly

Surface reservoir must fill

Runoff must drain rapidly Typically no native on-site material to be used Imported material to be a mix of sand and organics Minimum infiltration rate 5"/hour Aim for 10"/hour at installation On-site bucket test

No filter fabric

One hot afternoon in Contra Costa