The role of living roofs in holistic stormwater management systems - Living Roofs in Integrated Urban Water Systems - page 27

approximately 1 m (Fassman-Beck et al. 2013). On the other hand, the UK labo-
ratory study of synthetic drainage layers found that the delay in runoff timing
was independent of the distance to the gutter (Vesuviano and Stovin 2013) and
a ield study in Sweden concluded length did not inluence the runoff temporal
distribution (Bengtsson 2005).
Since a living roof typically captures only the precipitation falling directly on its
surface, the extent to which the system can reduce a site's total runoff depends
on the scale of the living roof in relation to the total site area. With lot-line to lot-
line development, as in a central business district or downtown, a living roof
might eliminate site runoff altogether for the small storm events, and alleviate
pressure on combined sewers for all events. As the extent of living roof coverage
throughout the city's core increases, there is an increasingly pronounced positive
effect on the sewershed. Where the building is only one element of a site's devel-
opment, the presence of a living roof should reduce the footprint of ground-level
SCMs needed to satisfy overall site stormwater requirements.
2.5 sTormwaTer performance expecTaTions
Research documenting the stormwater control performance of living roofs is
rapidly growing, with a substantial increase in publication rate since 2010. The
most popular topics in living roof research appear to be (in order) thermal
effects, runoff quality and hydrology (Li and Babcock 2014). Useful reviews and
consolidation of research publications with an emphasis on stormwater
performance began with Getter and Rowe (2006); Berndtsson (2010) proposed
likely inluences on discharge water quality, while Li and Babcock (2014)
updated developments in understanding living roof hydrology. Living roof
research in the stormwater context gained greater attention and momentum in
the United States from frequently cited experimental studies at Penn State Uni-
versity (Berghage et al. 2007) and the University of Georgia (Carter and Rasmus-
sen 2006), among others. Since these studies quantifying retention and
detention performance, more attention has been focused on understanding the
science driving hydrologic processes and generally characterizing water quality
in living roof systems.
Empirical performance measurements are obtained through ield studies (full-
or pilot-scale) and scaled laboratory experiments. Field studies quantify the
“true” response of a living roof to rainfall, yet the variability of roof design and
rainfall patterns complicates the ability to translate empirical results to another
location or living roof. Field studies are typically resource-intensive, thus individual
studies are often limited in the number of storms or systems for which
performance is assessed. In the laboratory, the experimentalist can more easily
isolate speciic factors to quantify their inluence on performance, but is rarely
able to extrapolate to the inherent variability of the natural condition. In the
following section, emphasis has been placed on reporting ield measurements
found in peer-reviewed journal articles and reports, as much as possible.