Brian L. Taylor, P.E., and Drew A. Gangnes, P.E. - Green Roofs for ...

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CASE STUDIESCase studies of existing and proposed projects in Seattle were analyzed using the approach and eco-roofmodel presented previously to determine how an eco-roof would have affected site runoff characteristicsand the detention volume required to comply with City release standards. The case studies selectedrepresent projects ranging from one third to a full city block in size, and include office, lifescience/biotechnology, and residential building types (see Table 1 on the following page).The City of Seattle and Washington Department of Ecology provide runoff CNs to use with the SBUHmethod that take into account antecedent moisture conditions expected in the region. To match localantecedent conditions, the initial saturation was estimated to be 81 percent of the eco-roof field capacity(17). Ultimate field capacity and transient storage were estimated to be 20 percent of the media volume.The saturated infiltration rate was specified at 3 inches per hour for a single layer of media above a freeflowingdrainage layer. The eco-roof thickness and coverage varied in the analyses – the building typedetermined the amount of coverage possible. Projects are “zero lot line” developments, so project androof areas are equivalent.Each project drains to Seattle’s combined sewer system, for which the release standard is 0.20 cfsmaximum per acre of the site for the 25-year, 24-hour design storm (3.125 inches of rainfall). Fordetention design, the City uses a unique rainfall distribution similar to the SCS Type 1A storm, thestandard for the Pacific Northwest, except the City increases the peak rainfall to reduce the potential ofover-topping detention systems during “back to back” storms.For a one-half block site, a typical project size, detention tanks were sized to estimate effects of varyingeco-roof thickness and area coverage. These results, shown in Figure 3, show that for Seattle’s flowcontrol design criteria, the greatest stormwater benefits are obtained when the media thickness is 6inches or more. Eco-roofs less than 6 inches thick are saturated before the peak of the storm, and donot offer as much detention reduction benefit as thicker sections.Tank Length (versusconventional roof design)100%90%80%70%60%2-inch4-inch6-inch8-inch50%0% 20% 40% 60% 80% 100%Eco-roof Coverage for SiteFigure 3: Detention Required for Eco-roof Alternatives vs. Conventional RoofingMethod for Quantifying Runoff Reduction of Green RoofsBrian L. Taylor, P.E., and Drew A. Gangnes, P.E.7
Case study projects with 8-inch eco-roofs were modeled using the above assumptions for significantSeattle design storms. The 6-month, 24-hour storm, 1.075 inches, is typically used to size water qualitytreatment facilities, because 90 percent of total annual rainfall occurs in storms of lesser or equalmagnitude (5, 6); the runoff volumes were reduced by 10 to 20 percent as shown in Table 1. The 2-year, 24-hour storm, 1.680 inches, is linked to streambank erosion processes (3, 5); eco-roofs reducedrunoff rates by 34 to 65 percent.Table 1 shows potential detention reductions for each project. The base for comparison was computedusing the City’s SBUH-based detention design worksheet. Initial construction costs were estimated foreach tank and eco-roof installation using contractor-provided cost data. Based on these data, eco-roofswere considered to cost $5 per square foot more than a conventional roof. Initial costs savings fromreducing the detention tank size could be applied to purchasing the eco-roof. The portion that could befunded by this detention cost offset is shown in Table 1. This shows that when eco-roofs are credited fortheir stormwater benefits, construction cost offsetting could enhance the appeal of eco-roofs fordevelopers and owners. Just to note, the sites identified are from different neighborhood districts ofSeattle, and the overall size of the city blocks on which the projects are located varied.Building Type Residential Life Science Typical Office Large OfficeSize of site28,800 ft²(1/3 block)26,600 ft²(1/2 block)28,800 ft²(1/2 block)57,600 ft²(full block)Eco-roof coverage, % of roof 40% 44% 80% 80%6-mo. volume reduction for roof 10% 11% 20% 20%2-yr. peak rate reduction for roof 34% 38% 65% 65%25-yr. detention volume reduction 35% 36% 50% 42%Portion of eco-roof costs potentiallyoffset by detention tank reduction16% 15% 12% 12%Table 1: Case Study Comparison of Four Sites with Potential Eco-roof ProjectsCONCLUSIONSA site analysis approach and eco-roof hydrology model has been presented to estimate runoff rates andvolumes for building projects that include eco-roofs. The use of a design tool to account for thehydrologic processes occurring on the project site is a way to quantify the benefits of rooftop stormwatermanagement practices such as eco-roofs. Application of the design approach and model to actualSeattle projects demonstrates the potential of eco-roofs to reduce the amount of underground detentionneeded to meet the City’s requirements. Quantifying how eco-roofs contribute to meeting stormwatercode requirements is a way that engineers help promote eco-roof implementation; it is up tomunicipalities and agencies to provide installation incentives, and up to contractors and vendors to lowereco-roof costs, so developers choose to build eco-roofs.Method for Quantifying Runoff Reduction of Green RoofsBrian L. Taylor, P.E., and Drew A. Gangnes, P.E.8
Page 1: METHOD FOR QUANTIFYING RUNOFFREDUCT
Page 4 and 5: INTRODUCTIONGreen roofs, also known
Page 6 and 7: NEW APPROACH TO MODELING MODEL SITE
Page 8 and 9: initial infiltration processes (13)
Page 12 and 13: Coordination with the City of Seatt
Page 14: For a freely drained media the disc

Brian L. Taylor, P.E., and Drew A. Gangnes, P.E. - Green Roofs for ...

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