Low Impact Development for Stormwater Treatment and Hydrograph Modification Management in California
Dan Cloak, PrincipalDan 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 1978
1994
1999
2003
2000
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 requirementsin 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
Reservoir, 12" min. depth
Reverse bend trap or hooded overflow
18" sandy loam, minimum infiltration rate 5" per hour
12" open-graded gravel, approx. ½" dia.
Perforated pipe
Downspout
Building exterior wall
Cobbles or splash block
Filter fabric
Concrete or other structural planter wall with waterproof membrane
Additional waterproofing on building as needed
Drain to storm drain or discharge; bottom-out or side-out options
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
0.00
0.20
0.40
0.60
0.80
1.00
1.20
0 1 2 3 4 5 6 7 8 9 10
Recurrence Interval (years)
Pea
k F
low
(cf
s)
ImperviousMitigated Post-Project SitePre-Project Site0.5Q2
IMP Reduces Impervious Runoff to Less Than Pre-Project Levels
Results: Flow Duration Control
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40
% Time Exceeded
Flo
w (
cfs
)
ImperviousMitigated Post-Project SitePre-Project SiteQ100.1Q2
IMP Reduces Impervious Runoff to Less Than Pre-Project Levels
Sizing Factors for Flow Control
IMP Sizing Factors
In-Ground Planter
Group A: 0.08Group B: 0.11Group C: 0.06Group D: 0.05
Flow-Through Planter
Group C: 0.06Group 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: 0.13Group B: 0.15Group C: 0.08Group D: 0.06
IMP Sizing Factors
Dry Well Group A: 0.05 to 0.06Group B: 0.06 to 0.09
Infiltration Trench
Group A: 0.05 to 0.06Group B: 0.07 to 0.10
Infiltration Basin
Group A: 0.05 to 0.10Group B: 0.06 to 0.16
Adjustment to annual rainfall
Group A, y = 0.0020x + 0.08Group B, y = -0.0005x + 0.11Group C, y = -0.0022x + 0.06Group D, y = -0.0022x + 0.05
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
-14 -12 -10 -8 -6 -4 -2 0 2 4 6
Mean Annual Rainfall (MAP) Relative to Martinez Gauge (in)
Siz
ing
Fac
tor
Group A soils
Group B soils
Group C soils
Group D soils
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 Areas1. Self-treating areas2. Self-retaining areas3. Areas draining to a self-retaining area4. 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
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
1Pervious
Impervious
Areas draining to self-retaining areas
Tabulating AreasSelf-Treating Areas
DMA Name Area (SF)
Self-Retaining Areas
DMA Name Area (SF)
Areas Draining to Self-Retaining AreasDMA 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
DMANam
e
DMASq. Ft
Surface
Type
RunoffFactor
Area x runoff factor
Sizing Factor
Min. Size
SizePlanne
d
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
DMANam
e
DMASq. Ft
Surface
Type
RunoffFactor
Area x runoff factor
A-1 5000 Roof 1.0 5000
A-2 10000 Paved 1.0 10000
A-3 20000 Grass 0.1 2000 Sizing Factor
Min. Size
SizePlanne
d
Facility A----- 17000 0.04 680 800
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 HazardsReservoir, 12" min. depth
Reverse bend trap or hooded overflow
18" sandy loam, minimum infiltration rate 5" per hour
12" open-graded gravel, approx. ½" dia.
Perforated pipe
Downspout
Building exterior wall
Cobbles or splash block
Filter fabric
Concrete or other structural planter wall with waterproof membrane
Additional waterproofing on building as needed
Drain to storm drain or discharge; bottom-out or side-out options
Generic Bioretention Facility
18"specified soil
Vsurface
Vsubsurfacehoutflow
Asurface
Ainfiltration
Qoutflow
hoverflow
Soil Mix
Gravel 18"
18"
10"
2"
Overflow
Under drain
“Floodable” Pavement
6"
More Storage – Less Aggregate
Soil Mix 18"
10"
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
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 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
One hot afternoon in Contra Costa
One hot afternoon in Contra Costa
One hot afternoon in Contra Costa
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