An investigation into the sustainable design of green roofs in relation to their hydrological performance

It became paramount for resilient cities to mitigate negative effects of climate change such as extreme weather, heat waves or flooding. Implementation of green roofs in urban regions could help to improve local microclimate through evapotranspirationfrom green roof surfaces and vegetation, and mitigate flood risk by providing additional storage for stormwater surface runoff. This research investigates the sustainable design of green roofs using conventional and alternative materials, in relation to their hydrological performance under UK climatic conditions. The assessment of the hydrological performance of green roofs was performed by means of laboratory-based and in-situ experiments.

This research has identified and selected the alternative materials, suitable for the use in
extensive green roof systems. Subsequently, the properties of these materials were assessed using appropriate British Standards, showing that properties-based, as opposed to type-based, selection of the materials is of high importance to the sustainable green roof design. The in-situ experiment demonstrated high retention performance across eight green roof designs with median retention above 99% and cumulative retention for the entire monitoring period of 4 years ranging from 61.5% to 77.9%. The highest retention was recorded for the green roof design of the deepest substrate (100mm) and drainage layer (40mm). Green roofs investigated in the laboratory under extreme rainfall events demonstrated much lower hydrological performance (6% - 11.5% of median retention) than these assessed insitu. However, their maximum retention capacity ranged from 61% to 78%, given specific conditions such as long inter-event dry period prior to the extreme rainfalls. The green roofs made of alternative materials performed as well as or better than the conventional green roofs in regards to retention. The preliminary multiple linear regression models confirmed the significance of the rainfall depth and temperature in predicting runoff depth and retention as well as porosity of the substrate material and water absorption of drainage layer material.

These models could be the basis for further development of tools for accurate prediction of green roof responses to rainfall events in order to assist green roof designers, standardisation bodies, specifiers, manufacturers, and contractors.