Detention Performance of Green Roof Systems: Experimental Characterisation and Numerical Modelling

Green roofs, as an example of Sustainable Drainage Systems (SuDS), can benefit stormwater management through retention and detention processes. Retention in a green roof refers to the rainwater that is retained in the system, and detention is the process that leads to lag and attenuation effects in the system runoff hydrograph. The understanding of retention in green roof systems has been well-established. However, the understanding and modelling of green roof detention processes are less developed. The physical properties of green roof substrates that contribute to detention performance have not been fully characterised to date. Current research in detention modelling lacks a generic physically-based model capable of modelling the detention processes in a complete green roof system. In this study, the physical properties of green roof substrates were characterised, and a physically-based detention model capable of representing the detention processes in a complete green roof system was developed.
The Soil Water Release Curves (SWRC) for four representative green roof substrates were determined using the hanging column method, and the Hydraulic Conductivity Function (HCF) for the substrates was characterised in an infiltration column using steady-state and transient techniques. The conventional natural soil-derived HCF model — Durner-Mualem model, for which the model parameters were from the measured SWRC data, was shown to provide a poor fit to the measured HCF data. A new three-segment HCF curve was, therefore, proposed to fit measured HCF data for the green roof substrates. Detention tests were carried out on 100 mm and 200 mm deep substrates using four simulated design storms. The runoff and moisture content data collected during the detention tests were used to validate the HCFs using the Richards Equation. The results showed that the new HCF provides a better estimation of the runoff and moisture content profiles than the Durner-Mualem model.
A two-stage physically-based detention model was developed for complete green roof systems in this study. In the model, the vertical flow in the substrate is represented by the Richards Equation, and the horizontal flow in the underlying drainage layer is modelled by the Saint Venant equation. This two-stage physically-based model, together with the green roof model in SWMM (Storm Water Management Model) were validated using measured runoff profiles from two contrasting green roof systems: a conventional green roof system; and an innovative system. Both models showed a reasonable estimation of the runoff profiles from the green roof systems. However, due to the limitation that the models are not capable of representing the flow conditions in the innovative green roof system’s detention layer, the model results were less accurate than for the conventional green roof system.