Stormwater Treatment and Flow Control

1. Contra Costa’s Low Impact Development Approach for Stormwater Treatment and Flow Control Dan Cloak, P.E.Presentation to the San Diego Region Co-permitteesHydromodification Workgroup December 6, 2006

2. 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

3. 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...”

4. 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.

5. 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

6. Options for HMP Compliance • Show no increase in directly connected impervious area • Use Low Impact Development Integrated Management Practices • Use a continuous-simulation model to show runoff does not exceed pre-project flow peaks and durations • Show projected increases in runoff peaks and durations will not accelerate erosion of receiving stream

7. 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.

8. 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

9. 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 Guidebook

10. 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

11. 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.

12. Example of an engineered LID Integrated Management Practice • “Dry” swale detains and filters runoff

13. Fitting IMPs into landscaping Portland, Oregon Albany, California

14. Fitting IMPs into landscaping Seattle, Washington Spokane, Washington

15. Planter Box

16. 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

17. 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 Integrated Management Practices Advantages Challenges

18. 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

19. How do we know LID works?

20. Continuous Hydrologic Modeling • Sizing to one ‘design storm’ is not enough

21. Peak Flow Frequency • Identify all HSPF storms in record and rank

22. Flow Durations • Rank hourly outputs from HSPF model

23. Example IMP: In-Ground Planter 18-in sandy loam

24. Peak Flow Matching Example

25. Duration Matching Example

26. IMP Sizing Factors Infiltration Only: Under-Drain or Infiltration:

27. Rainfall Variability Adjustment

28. Example Design Using the Sizing Calculator

30. DMA PAVE-3 4,826 SF DMA PAVE-2 2,737 SF DMA LS-3 1,207 SF IMP PL-2530 SF DMA LS-2 1,112 SF IMP PL-3515 SF DMA ROOF-1 4,681 SF DMA PAVE-1 7,651 SF DMA LS-1 6,205 SF IMP PL-1 825 SF

44. 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