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

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

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

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QUANTIFYING RUNOFF IN SEATTLEWhile eco-roof stormwater benefits have been quantified <strong>for</strong> some climatic areas, little in<strong>for</strong>mation isavailable on the per<strong>for</strong>mance of these systems in Seattle. A few cities are collecting monitoring data,although no city has Seattle’s precise climatic conditions. Seattle’s stormwater manual addresses ecoroofs,but the “applicant must model the storage capacity <strong>and</strong> discharge rate of the eco-roof” (3). TheCity’s manual also specifies the hydrologic modeling method, the Santa Barbara Urban Hydrograph(SBUH) method.The SBUH method is an “event-based” hydrologic model used to estimate runoff generated during alimited-duration storm, resulting in a discharge hydrograph <strong>for</strong> a site or basin. The method has beenused in Western Washington since 1990 <strong>for</strong> design analyses (4, 5, 6). More sophisticated continuoushydrologic models, which use long-term precipitation records instead of shorter events, have beenrecently developed <strong>for</strong> designers, but most municipalities <strong>and</strong> county agencies accept SBUH-basedanalyses.Event-based models are most accurate when the initial environmental conditions <strong>for</strong> the storm event areknown. For design storms, these conditions must be estimated by the designer. The st<strong>and</strong>ard approachis to apply assumptions <strong>and</strong> professional judgment to establish initial basin conditions. For back-to-backstorms, assumptions are applied <strong>and</strong> model inputs selected that reflect a relatively high degree of initialsaturation <strong>and</strong> lack of capacity to retain rainwater.To apply SBUH, an engineer must estimate the hydrologic parameters <strong>for</strong> a site, which include extents ofpervious <strong>and</strong> impervious areas, a runoff “curve number,” to represent the moisture-retention capability<strong>and</strong> runoff potential of the soil/l<strong>and</strong>-cover complex, <strong>and</strong> the “time of concentration” that it takes <strong>for</strong>runoff to flow to the basin outlet. Review of the SBUH model basis reveals that the method does notaccount <strong>for</strong> a major process known to occur in eco-roofs: percolation through a freely drained soilmedia. SBUH postulates that once water infiltrates into the soil, it does not re-emerge (6). It is alsobased a relationship between soil moisture storage <strong>and</strong> runoff generation that was not calibrated to ecoroofrunoff. The method does not establish how a designer should estimate these parameters <strong>for</strong> ecoroofs.Since SBUH is not intended <strong>for</strong> eco-roof hydrologic processes, MKA reviewed available literature to findan alternative model suitable <strong>for</strong> eco-roof analysis <strong>and</strong> flow control design in Seattle. Opti-GrunInternational, a German green roof supplier, has a model developed from their experience that isproprietary (7). A regional-scale physical based model was also recently developed <strong>for</strong> Vancouver,Canada, to assist in l<strong>and</strong> use management <strong>and</strong> policy decisions (8). Neither model appears to meet theneed of site designers in Seattle: the ideal model would represent eco-roof processes; be compatiblewith the accepted design method required by code; <strong>and</strong> be within the public domain.Because of the lack of an ideal model MKA developed a site analysis <strong>and</strong> design tool using the methodsdescribed in the following sections.Method <strong>for</strong> Quantifying Runoff Reduction of <strong>Green</strong> <strong>Roofs</strong><strong>Brian</strong> L. <strong>Taylor</strong>, P.E., <strong>and</strong> <strong>Drew</strong> A. <strong>Gangnes</strong>, P.E.2

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