Stormwater Capture Efficiency of Bioretention Systems

Bioretention systems are increasingly being used to control the adverse effects of urbanization on stormwater quantity and quality. The stormwater capture efficiency of a bioretention system, defined as the fraction of stormwater volume captured by the system, can be used as an important index of its stormwater management performance. In this paper, an analytical probabilistic expression (APE) is derived for estimating the long-term average stormwater capture efficiency of bioretention systems. The derivation is based on the probability distribution functions of the input rainfall event characteristics and the rainfall-runoff-overflow transformations occurring on a bioretention system and its contributing catchment. In the derivation, instead of simply adopting the Howard's conservative assumption as used in many previous studies, an approximate expected value of the surface depression water contents of a bioretention system at the end of a random rainfall event [denoted as E(S-dw)] is derived and used. The accuracy of the resulting APE is verified by comparing its results with those determined from continuous simulations. The use of E(S-dw) is proven to be advantageous than the use of the Howard's conservative assumption, it demonstrates that similar methods may be developed to analytically evaluate the stormwater management performance of other types of storage facilities for which the Howard's conservative assumption was employed previously.