ASCE LID Conference LID Analysis Considerations in Western Washington November 17, 2008

1. ASCE LID ConferenceLID Analysis Considerations in Western WashingtonNovember 17, 2008 Doug Beyerlein, P.E. Clear Creek Solutions, Inc.

2. This presentation was originally given at the ASCE LID Conference in Seattle, WA, in November 2008by Doug Beyerlein, P.E. Clear Creek Solutions, Inc.

3. Presentation Introduction Permeable Pavement Modeling Rain Garden/Bioretention Modeling Impervious Runoff Dispersion Modeling Green Roof Modeling Summary

4. Introduction There is nothing magical about LIDs. Water must go somewhere. Water must either: Infiltrate into the soil. Evaporate/transpire into the atmosphere. Runoff.

5. Introduction Key analysis considerations: Type of soil. Meteorological conditions precipitation evapotranspiration (ET)

6. Introduction Single-event design storm methodology doesn’t work for LID modeling because: • Single-event flow frequency standards are based on inappropriate assumptions. • Single-event modeling does not include the long-term effects of evapotranspiration and infiltration.

7. Introduction LID hydrologic modeling requires continuous simulation: WWHM (HSPF) Continuous simulation hydrology models the entire hydrologic cycle for multiple years.

8. Introduction WWHM: Western Washington Hydrology Model • Developed for the State of Washington Department of Ecology. • Has the ability to model a full range of LID facilities and practices. • Other versions for California.

9. Permeable Pavement Modeling

10. Permeable Pavement Reduces Runoff Volume: • Infiltration to native soil. • Evaporation.

11. Permeable Pavement Flow Paths Evaporation from pavement Rain on pavement Surface Runoff Infiltration through pavement Infiltration to gravel subgrade Underdrain Flow Infiltration to native soil

12. Permeable Pavement Flow Paths Infiltration to native soil is dependent on native soil characteristics. Infiltration to native soil (range of rates: zero to 0.01 in/hr)

13. WWHM Permeable Pavement Modeling

14. WWHM Permeable Pavement Modeling Table 1. Permeable Pavement Reduction of Total Runoff Volume

15. WWHM Permeable Pavement Modeling Figure 1. Permeable Pavement Reduction of Total Runoff Volume

16. WWHM Permeable Pavement Modeling • Reasons why permeable pavement is a good LID option in Western Washington: • Ratio of drainage area to infiltration facility area is 1 to 1. • Western Washington rainfall volumes and intensities are relatively low. • Subsurface storage provides water for long-term slow infiltration and evaporation.

17. Rain Garden/Bioretention Modeling

18. Bioretention includes planter boxes

19. Bioretention and Rain Gardens Reduce Runoff Volume: • Evaporation. • Transpiration. • Infiltration to native soil.

20. WWHM Rain Garden/Bioretention Modeling Downstream control structure: Water infiltrates into the soil before runoff.

21. Bioretention Flow Paths Inflow to Bioretention Facility Weir Flow Infiltration to Amended Soil Vertical Orifice Flow Underdrain Flow Infiltration to Native Soil

22. Bioretention Flow Paths Infiltration to native soil is dependent on native soil characteristics. Infiltration to Native Soil

23. WWHM Bioretention Modeling

24. WWHM Rain Garden/Bioretention Modeling Table 2. Rain Garden Reduction of Total Runoff Volume

25. WWHM Rain Garden/Bioretention Modeling Figure 2. Rain Garden Reduction of Total Runoff Volume

26. WWHM Rain Garden/Bioretention Modeling • Reasons why bioretention is a good LID option in Western Washington: • Regardless of volume reduction there are good water quality benefits. • Works best with outwash soils; less benefit with till/poor draining soils • Soil moisture storage provides water for long-term slow infiltration and evapotranspiration.

27. Impervious Runoff Dispersion Modeling Dispersion of impervious roof runoff on adjacent pervious lawn allowing some water to infiltrate before becoming stormwater runoff.

28. Impervious runoff dispersion reduces runoff volume by slowing the runoff velocity and allowing: • Evaporation from adjacent pervious land. • Transpiration from adjacent pervious land. • Infiltration on adjacent pervious land.

29. Impervious Runoff Dispersion Flow Paths Impervious Surface Surface Runoff Pervious Surface Interflow Infiltration to Native Soil Infiltration to native soil is dependent on native soil characteristics.

30. WWHM Impervious Runoff Dispersion Modeling

31. WWHM Impervious Runoff Dispersion Modeling Table 3. Impervious Area Reduction of Total Runoff Volume

32. WWHM Impervious Runoff Dispersion Modeling Figure 3. Impervious Area Reduction of Total Runoff Volume

33. WWHM Impervious Runoff Dispersion Modeling • Reasons why impervious runoff dispersion is a good LID option in Western Washington: • Turns impervious runoff into pervious runoff. • Works best with outwash soils; less benefit with till/poor draining soils • Soil moisture storage provides water for infiltration and evapotranspiration.

34. Green Roof Modeling

35. Green Roofs Reduce Runoff Volume: • Evaporation. • Transpiration.

36. Green Roof Flow Paths

37. Green Roof PET Limitations Potential Evapotranspiration (PET) Seattle November-March rainfall = 27 inches Seattle November-March PET = 3 inches (0.02 in/day) Excess runoff = 24 inches

38. WWHM Green Roof Element

39. WWHM Green Roof Modeling Table 4. Green Roof Reduction of Total Runoff Volume

40. WWHM Green Roof Modeling Figure 4. Green Roof Reduction of Total Runoff Volume

41. WWHM Green Roof Modeling • Reasons why green roofs are a good LID option in Western Washington: • Can be used in highly developed urban areas. • Soil moisture storage provides water for evapotranspiration.

42. Summary • In Western Washington: • Permeable pavement can provide 100% infiltration at a low infiltration rate of 0.01 inches per hour. • Rain gardens work best in outwash soils, but have some benefit in till soils. • Impervious dispersion can reduce total runoff volume by up to 25%. • Green roofs can reduce total runoff volume by 20% to 30%.

43. Summary • In Western Washington: • Soil infiltration rates play a major role in determining LID effectiveness. • Low winter evapotranspiration rates combined with non-stop, continuous, never-ending, will-we-ever-see-the-sun-again?, winter rainfall limit the effectiveness of green roofs in reducing total runoff volume.

44. AcknowledgementsSeattle Public Utilities provided much of the funding for the modeling of green roofs and bioretention.Taylor Associates of Seattle provided info and photos of Seattle green roofs.The City of Portland, Oregon, Bureau of Environmental Services (BES) provided the Hamilton green roof monitoring data.

45. Questions?Contact: Doug Beyerlein 425.892.6454beyerlein@clearcreeksolutions.com