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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0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60

Recurrence Interval (years)

Pe

ak

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w (

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)

IMP OutflowPervious Area Runoff0.5Q2Q10

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0.00% 0.02% 0.04% 0.06% 0.08% 0.10% 0.12% 0.14% 0.16% 0.18% 0.20%

% 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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0 1 2 3 4 5 6 7 8 9 10

Recurrence Interval (years)

Pea

k F

low

(cf

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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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

Flo

w (

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)ImperviousMitigated Post-Project SitePre-Project SiteQ100.1Q2

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