Analytical Equations for Direct Quantification of Green Roofs’ Hydrologic Performance Statistics

Recent studies have provided explicit analytical equations that can be used to quantify directly the hydrologic performance statistics of green roofs, such as runoff-reduction ratios. These equations were obtained based on simplified representations of the hydrologic and hydraulic processes occurring on and inside green roofs, as well as stochastic models describing local rainfall characteristics. To simplify derivations, these studies considered only saturation-excess runoff and neglected infiltration-excess runoff that may be generated from green roofs. We develop a method for considering both saturation- and infiltration-excess runoff; the proposed analytical equations can be used to directly quantify the performance statistics of any type of green roof. Systematic comparisons of analytical and numerical simulation results were also conducted to demonstrate the accuracy of the analytical equations. As an alternative to numerical simulations, the analytical equations can be used by engineers to more conveniently quantify the performances of alternative green roof configurations.