Hydrological Performance Evolution of Extensive Green Roof Systems

Global urbanisation has resulted in a reduction of urban green spaces, replacing moisture permeable landscapes with impermeable surfaces that quickly convey rainfall to receiving drainage systems. Sustainable drainage systems (SuDS) are a selection of devices that can be used to complement and/or replace existing urban drainage structures to meet future demands. SuDS can control rainfall at source, infiltrating it into the ground, thereby replicating the natural hydrological processes of a pre-developed urban site. Green roofs are one example of a source control SuDS device, capable of restoring green spaces to urban environments without requiring land space. As green roof systems age, there are several temporal processes than can lead to changes in their hydrological performance. Current knowledge of hydrological performance evolution is conflicting; this study presents a coupled non-invasive imaging and long-term monitoring study to provide the missing knowledge required to better inform the long-term maintenance, future development, and modelling of green roof systems. A 6-year long-term record of rainfall, runoff, climate and substrate moisture data for a field research site in Sheffield has been analysed to identify temporal trends in green roof hydrological performance. This monitoring study is coupled to two green roof microcosm studies which non-invasively characterise differently aged substrate properties and identify the impacts on long-term hydrological performance. Both methods of investigation identify that for conventional green roof system configurations, a crushed brick substrate with Sedum vegetation, there are small improvements to potential hydrological performance year-on-year. These performance improvements arise from a rearrangement of the substrates pore spaces, with smaller pore sizes in aged substrates. However, seasonal variations in substrate properties were identified to be more significant than year-on-year increases. Stormwater practitioners may be encouraged by no evidence of any decline in performance with time for conventional green roof configurations.