Dan Cloak, Principal Dan Cloak Environmental Consulting

1. Low Impact Development for Stormwater Treatment and Hydrograph Modification Management in California Dan Cloak, PrincipalDan Cloak Environmental Consulting

2. 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

3. 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 2003 2000 1999 1994 1978

4. Village Homes • Narrow streets • Surface drainage • Swales as an amenity

5. 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.”

6. 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

7. 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 After Before

8. 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

9. Portland Stormwater Manual

10. 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

11. 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

12. 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

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

14. Swale

15. Planter Box

16. Dry Well

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. 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

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

20. Application of sizing factor

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

22. 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

23. Results: Control of Peak Flows

24. Results: Flow Duration Control

25. Sizing Factors for Flow Control

26. Adjustment to annual rainfall

27. 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

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

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

30. 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

31. Self-retaining areas

32. 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

33. 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

34. Areas draining to self-retaining areas

35. Tabulating Areas

36. 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

37. DMAs draining to facilities

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

39. DMAs draining to facilities

40. Design Challenges • Residential Subdivisions • Aesthetics and landscape design • Trip hazards

41. 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?

42. 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

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

44. Trip Hazards

45. Generic Bioretention Facility hoverflow Vsurface Asurface 18"specified soil houtflow Vsubsurface Qoutflow Ainfiltration

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

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

48. Cistern

49. 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

50. 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.