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.