Green Roof Hydrology by Doug Beyerlein, P. E. Joe Brascher Clear Creek Solutions, Inc. Mill Creek, wa green Roof Hydrology Presented at the 2005 awra annual Conference, Seattle, wa 2006 Clear Creek Solutions, Inc. All Rights Reserved


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


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



Green/Eco-Roof Cover



Green/Eco-Roof Design Principal Green Roof Technology Components



Green Roofs

      • Many benefits:
        • Increased roof life
        • Lower energy costs
        • Improved roof runoff water quality
        • Reduced stormwater detention (?)


Green Roof Stormwater Benefits



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


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)


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


Green Roof Stormwater Modeling

      • Need to model the entire hydrologic cycle:
        • Runoff = Rainfall - Evapotranspiration - Storage
      • Need continuous simulation hydrology


Hydrologic Cycle:

  • Rainfall

  • Interception

  • Soil moisture

  • Evapotranspiration

  • Runoff



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


WWHM Green Roof Stormwater Modeling Results: Storage Reduction

      • Seattle 21%
      • Olympia 23%
      • Port Angeles 31%
      • Bellingham 17%
      • Vancouver, WA 20%


WWHM Green Roof Stormwater Modeling Results: The Numbers



WWHM Green Roof Stormwater Modeling Results: The Chart



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


Green Roof Reality

      • No groundwater storage
      • All runoff must be stored in the soil column or in a stormwater detention facility (for example, vault)


Green Roof Reality

      • 50% reduction is impossible in Western Washington.
      • 20% reduction is realistic.


Blue Roofs?



Blue Roof Stormwater Modeling Results: Rooftop Storage

      • Seattle 0.40 feet depth
      • Olympia 0.41 feet depth
      • Port Angeles 0.56 feet depth
      • Bellingham 0.35 feet depth
      • Vancouver, WA 0.48 feet depth
    • No additional stormwater storage is required.


Conclusions:

      • Green is good.
      • Blue is better.


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





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