The influence of substrate and vegetation on extensive green roof hydrological performance

The objective of this research was to investigate the hydrological processes occurring in extensive green roof systems through data collected during a continuous monitoring programme of different green roof configurations. Nine green roof test beds (TB) which vary systematically in their substrate composition and vegetation options have been monitored since April 2010 at the University of Sheffield, UK. Three green roof substrates were tested: two commercial substrates manufactured by Alumasc – Heather with Lavender (HLS) and Sedum Carpet (SCS) Substrate were considered alongside a Lightweight Expanded Clay Aggregate (LECA)-based substrate. Three vegetation treatments have been tested: a drought tolerant specie (sedum), a meadow flower mixture and a no vegetation option. Per event retention performance varied depending on the initial water content within the substrate and the characteristics of the rainfall event. Consistent behaviour was observed among the tested green roof configurations with respect to per event retention. Greater retention was associated with HLS and SCS substrates when compared with LECA. Vegetated configurations showed consistently higher retention performance. Sedum vegetation resulted in higher retention performance than Meadow Flower. This was particularly evident on the LECA substrate.