Modelling runoff reduction through implementation of green and grey roofs in urban catchments using PCSWMM

An increasing interest in retrofitting rooftops as green roofs require improved understanding of the impact on runoff within urban catchments. In this study, a model of Risvollan, an urban catchment in the coastal city of Trondheim, Norway was built using the PCSWMM (Storm Water Management Model) model version 7.2.2780. Rooftops in the catchment were retrofitted as green (vegetated) or grey (non-vegetated - expanded clay, detention-based) roofs. The detention impact was evaluated for nine scenarios using three exceedance thresholds by measuring runoff at the catchment outlet. It was found that implementing green roofs on as little as 11% of the roof area can substantially reduce maximum flows. The implementation of the grey roofs outperformed the green roofs in all the investigated aspects: duration, number, and volume. The study provides insight to urban planners and decision-makers in how to quantify the contribution of rooftop detention to runoff reduction for stormwater management.