1. Green Roof Hydrology by Doug Beyerlein, P.E. Joe Brascher Clear Creek Solutions, Inc. Mill Creek, WA

2. Green Roof Hydrology Presented at the 2005 AWRA Annual Conference, Seattle, WA © 2006 Clear Creek Solutions, Inc. All Rights Reserved.

3. Green/Eco-Roof Cover

4. Green/Eco-Roof Design Principal Green Roof Technology Components

5. Green Roofs  Many benefits:  Increased roof life  Lower energy costs  Improved roof runoff water quality  Reduced stormwater detention (?)

6. Green Roof Stormwater Benefits  Claim: 50% reduction in runoff and storage  Actual: 20% reduction

7. Green Roof 50% Stormwater Claims  Based on inappropriate model assumptions:  Models use single event SCS hydrology  Models do not look at the entire hydrologic cycle including PET limitations

8. Green Roof PET Limitations  Potential Evapotranspiration (PET)  The maximum rainfall that can be evaporated and transpired.  Western Washington November- March PET = 3.3 inches (0.02 inches/day)

9. Green Roof PET Limitations  Potential Evapotranspiration (PET)  Seattle November-March rainfall = 27.1 inches  Western Washington November- March PET = 3.3 inches (0.02 inches/day)  Excess runoff = 24 inches

10. Green Roof Stormwater Modeling  Need to model the entire hydrologic cycle:  Runoff = Rainfall - Evapotranspiration - Storage  Need continuous simulation hydrology

11. Hydrologic Cycle:  Rainfall  Interception  Soil moisture  Evapotranspiration  Runoff

12. Green Roof Stormwater Modeling  We used the Western Washington Hydrology Model (WWHM) at five locations to model green roofs:  Seattle  Olympia  Port Angeles  Bellingham  Vancouver, WA

13. WWHM Green Roof Stormwater Modeling Results: Storage Reduction  Seattle21%  Olympia23%  Port Angeles31%  Bellingham17%  Vancouver, WA20%

14. WWHM Green Roof Stormwater Modeling Results: The Numbers

15. WWHM Green Roof Stormwater Modeling Results: The Chart

16. WWHM Green Roof Modeling  Based on DOE flow duration standard  Local long-term rainfall data (35-50 yrs)  Puyallup pan evap data  10 acre rooftop surface  8 inches of rooftop soil  4 inches of water storage in the soil column

17. Green Roof Reality  No groundwater storage  All runoff must be stored in the soil column or in a stormwater detention facility (for example, vault)

18. Green Roof Reality  50% reduction is impossible in Western Washington.  20% reduction is realistic.

19. Blue Roofs?  Remove the dirt  Store water on the rooftop (no additional stormwater detention is required)

20. Blue Roof Stormwater Modeling Results: Rooftop Storage  Seattle0.40 feet depth  Olympia0.41 feet depth  Port Angeles0.56 feet depth  Bellingham0.35 feet depth  Vancouver, WA0.48 feet depth No additional stormwater storage is required.

21. Conclusions:  Green is good.  Blue is better.

22. Questions? Call Doug Beyerlein 425.892.6454 beyerlein@clearcreeksolutions.com Joe Brascher 360.943.0304 brascher@clearcreeksolutions.com