Construction of On-Site Stormwater Treatment and Flow Control Facilities

1. Construction of On-SiteStormwater Treatment andFlow Control Facilities Dan Cloak, P.E.PrincipalDan Cloak Environmental Consulting

2. Topics • Why do we have permanent on-site facilities to treat stormwater? • Facility designs and how they work • Operating characteristics and errors in design and construction • Draft checklist for facility inspection

3. 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

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

5. Residential Development

6. Large hillside project

7. Swaleor In-groundPlanter

8. Planter Box

9. Dry Well

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

11. Application of sizing factor

12. 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?

13. 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

14. Results: Control of Peak Flows

15. Results: Flow Duration Control

16. Sizing Factors for Flow Control

17. Adjustment to annual rainfall

18. Key Operating Characteristics • 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.

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

21. Ensuring flow to the facility • Runoff may enter by sheet flow or be piped. • Roof leaders can be piped directly or spill across pavement

22. Distribute flow evenly

23. Distribute flow evenly

24. Surface reservoir must fill

25. Surface reservoir must fill

26. Surface reservoir must fill

27. Surface reservoir must fill

28. Surface reservoir must fill

29. Surface reservoir must fill

30. Runoff must drain rapidly • Typically nonative on-site material to be used • Imported material to be a mix of sand and organics • Minimum infiltration rate 5"/hour • Aim for 10"/hour at installation • On-site bucket test

31. No filter fabric

32. Promote Infiltration • Protect excavation from fine sediment and compaction • Re-excavate if used for sediment control during construction • Rip the bottom soils to promote infiltration

33. Underdrain • Perforated pipe • Solid pipe nearest 2' to connections • Bedded in gravel layer • Connected to storm drain or daylight • Cleanouts • Omit in sandy soils

34. Recap • 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.

35. Additional Features • Check dams • Moisture barriers/cutoff walls • Outflow-limiting orifices on underdrains • Specified depths or volumes for surface reservoirs or gravel layers • Check plans for facility cross-sections and details

36. Checklist • Pre-construction • Review facility details • Review required inspections • Site Preparation • Erosion and sediment controls • Runoff diverted from facilities • Facility areas marked • Pipes and appurtenances delivered and inspected

37. Checklist • Excavation and Grading • Grade breaks and elevations correct • Bottom of excavation elevation and dimensions correct • Native soil at excavation bottom ripped • Embankment/wall elevations and widths correct and level. • Drainage material is Class 2 perm • Bucket test on imported soil mix achieves 8-12 inches per hour

38. Checklist • Facility Installation • Location size and depths of appurtenances • Roof leaders and impervious areas drain to facility as intended • Underdrain, inlet, outlet, orifice, cleanouts, and overflow are located and installed correctly • Overflow at correct elevation • Moisture barrier or cut-off walls installed correctly

39. Checklist • Facility Installation • Underdrain base elevation correct and drain bedded properly • Irrigation system on separate zone • Gravel (Class 2 perm) depth correct • Soil media installed and passes bucket test • Soil top elevation and surface reservoir depth correct • Inlets and overflows correctly located

40. Checklist • Landscaping and planting • Vegetation complies with approved planting plan • Woody vegetation won’t block inlets and overflows

41. Checklist • Final check • Areas draining to facility are cleaned (pavement) or stabilized (landscape) • Inlets installed and operational • Configuration, size and depth of facility correct • Vegetation established and irrigation operating • Overflow installed and free of debris • Runoff reaches the facility