Stormwater Treatment and Flow Control
Dan Cloak, P.E.Presentation to the San Diego Region Co-
permitteesHydromodification Workgroup
December 6, 2006
Contra Costa’s Low Impact Development Approach for
Outline
Some key insights into the permit HMP requirements
How Contra Costa co-permittees are implementing the HMP
Low Impact Development (LID)●Philosophy and practice
Possible adaptation to San Diego County
What the permit requires
… post-project runoff discharge rates and durations shall not exceed estimated pre-project discharge rates and durations where the increased discharge rates and durations will result in increased potential for erosion or other significant adverse impacts to beneficial uses...”
Insights from watershed analysis
Most streams are incised and/or are already experiencing accelerated erosion.
Geomorphic assessment is an art as well as a science; methods and conclusions differ.
Local government lacks the resources to conduct a comprehensive analysis of all stream reaches in the County.
Predicting how flows from one development site may affect flows from a whole watershed is complex and uncertain.
Contra Costa HMP Strategy
Accept a presumptive standard that development sites must match pre-project flows
Assist developers with the technical means to comply with that standard
Promote Low Impact Development
Provide developers with options
Options for HMP Compliance
1. Show no increase in directly connected impervious area
2. Use Low Impact Development Integrated Management Practices
3. Use a continuous-simulation model to show runoff does not exceed pre-project flow peaks and durations
4. Show projected increases in runoff peaks and durations will not accelerate erosion of receiving stream
Option 1: No increase in impervious area
Design site to minimize impervious area and maximize time of concentration
Inventory existing vs proposed impervious area
Qualitatively compare pre- to post-project drainage efficiency.
Option 2: Low Impact Development IMPs
Follow the design procedure in the Stormwater C.3 Guidebook
Disperse runoff to impervious areas where possible
Select from a menu of Integrated Management Practices and size according to formulas provided
Option 3: Model Pre- and Post-Project Flows
Continuous simulation using at least 30 years of hourly data
Compare peaks and durations
Instructions for HSPF modeling are in the Stormwater C.3 Guidebook0.00
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Recurrence Interval (years)
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ak
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w (
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IMP OutflowPervious Area Runoff0.5Q2Q10
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% Time Exceeded
Flo
w (
cfs)
IMP OutflowPervious Area Runoff0.5Q2Q10
Option 4: Increased runoff but not erosion
Low Risk: Reaches downstream of project are piped, hardened, tidal, or aggrading; no controls are necessary
Medium Risk: Stream reaches are stable; mitigate additional flows by localized restoration projects
High Risk: Some reaches unstable; plan a comprehensive stream restoration
Option 2: Low Impact Development
Design the site to mimic natural drainage.
Disperse runoff to landscape where possible.
Use Integrated Management Practices distributed around the site.
Example of an engineered LID Integrated Management Practice
“Dry” swale detains and filters runoff
Fitting IMPs into landscaping
Portland, Oregon Albany, California
Fitting IMPs into landscaping
Seattle, Washington
Spokane, Washington
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
Common IMP Locations
Swales in the setback area between parking and lot line
In-ground Planters in parking lot medians and perimeters
Flow-through Planters next to buildings
Bioretention areas receiving piped discharge from upgradient areas
6' to 10' width fits into setback
Underdrain/ overflow to storm drain below
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
Implementing “LID”
IMPs can be effective, attractive, and accepted by developers
Incorporate IMPs in preliminary site, landscaping and drainage design drawings
In-ground planter boxes under construction
Residential subdivision
Clayton, May 2006
How do we know LID works?
Continuous Hydrologic Modeling
Sizing to one ‘design storm’ is not enough
Peak Flow Frequency
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Recurrence Interval (years)
Pea
k F
low
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s)
Impervious
Pre-Project Site
0.5Q2
Identify all HSPF storms in record and rank
Flow Durations
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% Time Exceeded
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w (
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Impervious
Pre-Project SiteQ10
0.1Q2
Rank hourly outputs from HSPF model
Example IMP: In-Ground Planter
18-in sandy loam
Peak Flow Matching Example
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Recurrence Interval (years)
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s)ImperviousMitigated Post-Project SitePre-Project Site0.5Q2
IMP Reduces Impervious Runoff to Less Than Pre-Project Levels
Duration Matching Example
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% Time Exceeded
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w (
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IMP Reduces Impervious Runoff to Less Than Pre-Project Levels
IMP Sizing Factors
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
Infiltration Only:Under-Drain or Infiltration:
Rainfall Variability Adjustment
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
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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
Example Design Using the Sizing Calculator
DMA LS-16,205 SF
DMA ROOF-14,681 SF
DMA PAVE-17,651 SF
DMA PAVE-2
2,737 SFDMA LS-21,112 SF
IMP PL-2530 SF
IMP PL-3515 SF
IMP PL-1825 SF
DMA PAVE-34,826 SF
DMA LS-3
1,207 SF
Adapting to Other Regions
Most aspects are the same:●Regulations are similar●Can use same suite of IMPs●Model stage-storage-discharge
relationships are the same●Stormwater C.3 Guidebook format
and “Stormwater Control Plan” submittal concept has already been reused in Sonoma and Alameda counties
Would need to customize by:●Using local rainfall record to
calculate sizing factors and adjustments