The effect of minimum inter-event dry period definition for storm event identification and SuDS hydrological performance evaluation

Abstract

Urban drainage practitioners and academics increasingly recognise the value of using continuous rainfall/outflow data to derive a comprehensive understanding of how vegetated Sustainable Drainage Systems (SuDS) respond to long time-series rainfall inputs. For event-based analysis, an agreed definition of ‘an event’ is crucial. Typically events are defined based on a Minimum Inter-Event Time (MIT). The objective of this research was to understand how the specific MIT impacts upon event metrics and on the selection of suitable initial conditions for design storm simulations. This was achieved using modelled outflow from a representative green roof (GR) and bioretention cell (BIO) combined with MITs of 6, 12, 24 and 48 hours. The choice of MIT used to separate events in rainfall/outflow analysis has an impact on the derived performance metrics, particularly for devices offering significant detention. The analysis has also highlighted that – for SuDS that do not include infiltration – the most probable retention storage is small, around 5 mm for typical green roofs and 0 mm for bioretention cells in a temperate climate.