Probabilistic analysis of the retention time in stormwater detention facilities

Stormwater detention facilities are often used in modern drainage systems to reduce the hydraulic load on existing sewers, due to the increase of impermeable surfaces and to the more frequent extreme rainfalls, consequence of climate changes. Although their design is mainly aimed to limit uncontrolled spills into receiving water bodies, storage capacity for water quality enhancement is often considered, mainly with the purpose of increasing the retention time. Standard analysis is usually based on empirical methods or on continuous simulations. This paper focuses on the probabilistic analysis of retention times aimed to provide guidance to engineers for the design of stormwater detention facilities. In particular, the influence on retention time of the possibility of water mixing from consecutive rainfall events, due to the pre-filling of the storage capacity from previous runoffs has been investigated. Derived expression has been tested by their application to a case study. (C) 2015 The Authors. Published by Elsevier Ltd.