The role of living roofs in holistic stormwater management systems - Living Roofs in Integrated Urban Water Systems - page 31

the inluence of preferential low paths through the growing media, compared to
the built-in-place system. Whether these variables have a meaningful or predicta-
ble impact on assembly stormwater performance has yet to be concluded.
Altogether, retention and detention performance reported as percent differ-
ences from input rainfall or a conventional roof surface monitored concurrently
provide valuable evidence encouraging planning-level decisions in favor of living
roof installations. However, these sorts of metrics are less useful in developing a
rigorous approach for assembly design for stormwater management. Researchers
are now more deeply investigating the mechanics of retention and detention. As
a start, understanding the inluences of speciic growing media composition and
its related pore- and particle-size distributions should prove beneicial for
manipulating retention and detention performance (Berretta et al. 2014; Liu and
Fassman-Beck 2014; Vesuviano et al. 2014; Yio et al. 2013).
2.5.2 Water quality
Basic characterization of living roof discharge (runoff) quality is even less well
understood than its hydrology. Empirical studies of living roof discharge water
quality are dificult to compare and consolidate on a consistent basis. Methods
used to collect data vary, as do data analysis and presentation. The (usually)
exceptional hydrologic performance for the smaller, frequently occurring storms
poses great challenges for water quality researchers who are often externally
constrained by limited durations of funding; few events produce enough dis-
charge to physically collect a sample or enough of a sample to satisfy analytical
requirements for water quality. Indeed, the academic literature to date reports
ield studies where typically 3-10 storm events were sampled, with most unfortu-
nately at the lower end of the range, while the upper end would be considered a
reasonable monitoring program for most SCMs.
Unlike most ground-level SCMs, a living roof usually “treats” only the rain
falling directly over its surface, which begs the question of an appropriate point
of comparison or assessment. Metrics for stormwater quality characterization
may include comparisons to either incoming rainfall, or runoff from a conven-
tional roof surface, or simply the runoff's water quality in isolation. Comparison
to drinking water quality standards is inappropriate, unless evaluating intent as a
potable water source. In general, numerical water quality limits for stormwater
discharges are highly uncommon; but there is growing interest in understanding
SCM discharges in relation to in-stream quality or thresholds for aquatic or
human health impacts. Most often, SCM treatment potential is assessed in com-
parison to incoming untreated runoff and consistency of treated discharge
quality. In the case of living roofs, comparison to a conventional roof surface's
discharge water quality may be the most valuable metric at present, if it is
assumed that the building will be constructed at a given site, regardless of a
potential living roof.
Roof runoff water quality is highly variable, whether from a conventional roof
surface (Clark et al. 2008; Lamprea and Ruban 2011; Wicke et al. 2014), or a