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

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