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

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initial infiltration processes (13). For the ERM, rainfall losses due to evapotranspiration, depressionstorage, and canopy interception are estimated by the user using values obtained from other sources andprofessional judgment. Unlike SBUH, the ERM does not compute initial infiltration separately from thesustained infiltration processes discussed below.In SBUH, infiltration and surface runoff are computed from the rainfall and the runoff curve number (CN)selected from standard tables of soil type and cover, initially developed by the Soil Conservation Service(14). Once rainfall exceeds the initial abstraction, the amount of runoff generated is calculated from therainfall and CN, using an empirical-based relationship. In the ERM, the infiltration rate provided by thedesigner is compared to the rainfall intensity to determine if water infiltrates or runs off the surface of theeco-roof. The infiltration rate is established from testing, observed eco-roofs, and/or the soil mediaspecification. The ERM assumes the infiltration rate is constant; the saturated infiltration rate is used forSeattle design storms.Soil moisture is not explicitly modeled in SBUH, but is inherent to the CNs and empirical runoff equation.In the ERM, at each time interval, the ultimate retention capacity of the media is compared to theaccumulated retained water. The first amount of moisture that soaks into the media contributes tosatisfying the ultimate field capacity of the media. Once the field capacity is used, subsequent infiltratedrainwater is held as transient storage in the media voids. Initial saturation and field capacity of the soilmedia determine the ultimate retention capacity for a scenario.Flow attenuation due to water percolating through soil layers is not represented in SBUH because waterthat infiltrates is assumed to go into deep soils and not contribute to runoff (6). In the ERM, an eco-roofsoil media layer is modeled as a reservoir. The storage capacity is determined by the depth and effectiveporosity of the media, where the effective porosity is the portion of the soil volume that provides transientstorage. The attenuation of water flowing through the porous media is computed by the hydrologicrouting procedure described below. Water cascades through the layers, so the discharge from the firstsoil media layer is the inflow to the second layer, and so forth.Surface runoff attenuation is computed in SBUH by the modified Att-Kin hydrologic routing procedure,using a routing coefficient equal to the “time-of-concentration” for the basin (4, 15). Hydrologic routingis a procedure to determine how much inflow peaks are delayed and reduced as water flows throughwater storage reservoirs. The attenuation is based on an open channel with characteristics that matchthe basin. The routed-flow rate is the predicted basin discharge. In the ERM, although surface runoff isnot expected, the same method is applied to surface runoff and separately for seepage flow to accountfor attenuation that occurs as flow is conveyed to the roof drain. The sum of routed surface and seepageflows is the net eco-roof discharge.HYDROLOGIC ROUTING FOR ECO-ROOFSTo account for attenuation as flow percolates through the eco-roof, a modified Att-Kin procedure is usedto model transient storage provided in the eco-roof voids. The procedure uses a storage coefficientbased on the relationship between the volume of water stored and the rate of release (15). For a basinwith open channel conveyances the storage coefficient can be estimated as the time-of-concentration,whereas a different coefficient is needed for freely drained porous media. The theoretical storagecoefficient for an eco-roof soil layer can be computed from the media depth, infiltration rate, andMethod for Quantifying Runoff Reduction of Green RoofsBrian L. Taylor, P.E., and Drew A. Gangnes, P.E.5
effective porosity. Then, the routed discharge rate can be computed using the same procedure as inSBUH. The associated equations and derivation are provided in the Technical Appendix.LEVEL-POOL ROUTING, FLOW SUMMATION, AND NET SITE DISCHARGELevel-pool routing is a standard analysis applicable to detention facilities such as roof-top ponding areasand detention tanks (4, 5, 6). It is integrated as part of this approach to simulate flow attenuation atstormwater detention facilities, allowing designers to account for peak eco-roof discharge rates thatgenerally lag the peak discharge from conventional surfaces – a boon when sizing detention facilities tomeet code requirements. Using level pool analysis allows a detention system to be designed so a sitewill meet the local release standard.When analyzing a project site, the plumbing of the site must be considered. Eco-roof discharges may bedirected to roof ponds or detention tanks if necessary to meet the release standard. An on-site detentiontank can be designed to work in conjunction with eco-roofs or to mitigate a conventional roof basin.Undetained, “bypass,” basins may be included in the site analyses. Developed hydrographs are summedto compute the net release for the developed site. These calculations are facilitated by the spreadsheettool; results can be exported to other modeling software packages for additional analyses, if needed.PERFORMANCE OF THE ECO-ROOF MODELFlow monitoring data obtained from the City of Portland’s Hamilton Apartments eco-roofs were used tovalidate results of the eco-roof model described above (1, 16). Although intended for design, notanalyzing actual storms, Figure 2 demonstrates that the proposed analysis approach and ERM canreasonably match measured roof discharge rates and volumes (13).Flow (cfs)0.00400.00350.00300.00250.00200.00150.00100.00050.0000Total for roof, measuredEco-roof, estimatedTotal for roof, estimated0 360 720Time (min)Figure 2: Calibrated Eco-roof Model Results for Hamilton Apartments, East Roof, May 17, 2003Method for Quantifying Runoff Reduction of Green RoofsBrian L. Taylor, P.E., and Drew A. Gangnes, P.E.6
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 10 and 11: CASE STUDIESCase studies of existin
Page 12 and 13: Coordination with the City of Seatt
Page 14: For a freely drained media the disc

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