Low Impact Development

1. Low Impact Development Construction of Facilities Dan Cloak Environmental ConsultingConstruction Workshop, May 2, 2012

2. Topics • Low Impact Development: What it is, what it does • Runoff dispersal (“self-retaining areas”) • Pervious pavements • Bioretention • How facilities work and how they are sized • What to watch for during construction

3. Conventional Urban Drainage • Features • Impervious surfaces: roofs and pavement • Catch basins and piped drainage • “Collect and convey” design objective

4. Drainage Impacts

5. LID Design Objectives

6. LID Drainage Principles • Instead of “collect and convey,” “slow it, spread it, sink it.” • Avoid concentrating flows • Keep drainage areas small • Promote infiltration • Detain • Treat

7. LID Design Steps • Optimize the site layout

8. LID Design Steps • Optimize the site layout • Use pervious surfaces and green roofs where possible

9. LID Design Steps • Optimize the site layout • Use pervious surfaces and green roofs where possible • Disperse runoff to landscaping

10. LID Design Steps • Optimize the site layout • Use pervious surfaces and green roofs where possible • Disperse runoff to landscaping • Direct drainage from impervious surfaces to bioretention facilities

11. Self-retaining Areas • Rationale • Produce no runoff from “small” storms • Also called “zero discharge” areas • Criteria • Bermed or graded concave • Slopes do not exceed 4% • Amended soils and vegetation to maintain soil stability and permeability • Inlets of area drains, if any, 3" minimum above surrounding grade

12. Draining to Self-Retaining • Rationale • Self-retaining areas are capable of ponding and infiltrating some additional runoff • Criteria • Ratio is not greater than 2:1 if treatment-only requirements apply • Ratio is not greater than 1:1 if flow-control requirements apply • Provide sufficient ponding/storage depth • Facilitate infiltration

13. Pervious Pavements • Variations • Pervious concrete • Porous asphalt • Porous pavers • Gravel • Open pavers • Solid unit pavers set on sand/gravel • Artificial turf • Common Characteristics • Require a base course • Base course depth varies with soil type • More costly than conventional concrete or asphalt • Installation on slopes requires careful design

14. Pervious Pavements • Minimal compaction of underlying soil • Check base course depth • 4 to 12 inches or more, depending on competence of underlying soils • Base course underdrains • Consider omitting the underdrainand relying on infiltration and lateral flow • Perforated SDR 35 or equivalent • Outlet elevation minimum 3" above bottom of base course • Underdrain may be routed to storm drain

15. Pervious Pavements • Pavers • Rigid edge required • Solid pavers require 3/8" minimum gap • Joints filled with open-graded aggregate free of fines • Permeable concrete and pervious asphalt • Installed by industry-certified professionals according to vendor’s recommendations.

16. Installation on slopes

17. Bioretention

18. Bioretention Overflow Underdrain

19. Treatment-Only Criterion i = 0.2 inches/hour BMP Area/Impervious Area = 0.2/5 = 0.04 Surface Loading Rate i = 5 inches/hour

20. Flow Duration Criteria

21. Standard Section A 0.04 2" Overflow V1 4" Soil Mix 18" Gravel V2 12" Φ = 0.4

22. Sizing Factors

23. Tributary Area • Drainage area includes portions of roof and of parking lot

24. Drainage Management Areas

25. Drainage Management Areas

26. Grading and Paving

27. Grading

28. Roof leaders • Runoff may enter by sheet flow or be piped. • Roof leaders can be piped directly or spill across pavement • Ensure is consistent with DMA delineation

29. Protect during grading

30. Observe elevations • Outlet structure • Top of overflow grate • Underdrain connection • Inlet • Flow line at inlet • Top of curb • Top of adjacent paving • Soil layers • Top of soil layer • Bottom of gravel layer • Bottom of soil layer

31. Structural 15"

32. Structural

33. Inlets

34. Outlets Overflow elevation

35. Fixing a low outlet grate

36. Gravel and Underdrain • Class 2 permeable • Caltrans spec 68-1.025 • Typical to be slightly off gradation spec on delivery • No filter fabric • Underdrain • Near top of gravel layer • PVC SDR 35 or equivalent; holes facing down • Solid pipe for 2' closest to outlet structure • Cleanout

37. Soil Specification • Now included in NPDES Permit • 60-70% Sand • ASTM C33 for fine aggregate • 30-40% Compost • Certified through US Composting Council Seal of Testing Assurance Program • Submittal per Guidebook • Option to accept test results for a “brand-name” mix if volume is less than 100 cubic yards • Install in 8"-12" lifts • Do not compact • Do not overfill • Leave room for mulch

38. Plantings • Maintain design top of soil elevation • Trees • Incorporate into bioretention facility • Account for surface roots

39. Maintenance Inspections • Inspect each new facility within 45 days of installation • Inspect 20% of all facilities each year • Inspect every facility at least once every five years

40. Resources • Stormwater C.3 Guidebook, 6th Edition • www.cccleanwater.org/c3.html • Or search “Contra Costa Stormwater” • See in particular: • Chapter 5, Construction Checklist • Design sheets at end of Chapter 4 • Appendix B, Soils/Plantings/Irrigation for Bioretention Facilities