The role of living roofs in holistic stormwater management systems - Living Roofs in Integrated Urban Water Systems - page 34

Quite substantial additional research is needed regarding living roof water
quality. Excellent advice for stormwater quality monitoring campaigns is found
from the freely available International Best Management Practices website ( www.
bmpdatabase.org ). The website also provides statistical summaries of SCM water
quality performance, and the (USA) National Stormwater Quality Database which
is likely the largest and most up-to-date database of untreated runoff water
quality, consolidated according to land use.
Nonetheless, some general trends of living roof water quality may be suggested
as a starting point, but certainly signiicant additional research is necessary. Nitro-
gen does not seem to be a prevalent issue, although “improvement” compared to
conventional roof runoff is marginal. Phosphorus is potentially problematic in
phosphorus-sensitive receiving environments, as even signiicant retention is
unlikely to offset the total mass of phosphorus discharged because the concentra-
tions are quite substantially elevated. In this case, living roof runoff should be
routed through an at-grade SCM capable of reducing phosphorus, such as biore-
tention, or harvested for reuse. Bioretention with an internal water storage zone
would also supplement nitrogen improvement (see for example, Brown et al. 2009
for design guidance). In the case of heavy metals, the signiicant long-term hydro-
logic control means that covering, replacing or substituting a metal roof with a
living roof would likely reduce the long-term mass loading from the site.
Uncertainty around living roof water quality is high, and assembly inluences
are largely not understood. If it is assumed that a building will be constructed
regardless of the presence of a living roof, the pertinent question is whether a
living roof's discharge will be of comparable or better quality to a conventional
roof surface, noting that conventional roof surfaces generally do not provide any
water quality improvement, and generate substantial volumes of runoff. In CSO
areas, the potential receiving water quality could improve with living roof installa-
tions, as they will reduce the frequency and volume of CSO discharges simply by
keeping runoff from the rooftops out of the combined sewer. Ultimately, in terms
of stormwater impacts, living roofs should primarily be considered as a tool for
runoff volume and peak low reduction, with some associated beneit in contam-
inant mass reduction, rather than as a speciic water quality control tool.
2.6 evapoTranspiraTion
ET plays a signiicant role in a living roof's day-to-day functionality. ET encom-
passes evaporation directly from the growing media and transpiration by plants.
It is the process by which a living roof “empties” stored water (retained rainfall),
thereby enabling the system to capture the next rainfall event. Over the long
term, relative variations in day-to-day ET may not have a signiicant inluence on
the metric of annual performance (DiGiovanni et al. 2013; Stovin et al. 2013;
Voyde 2011). On the other hand, ET will inluence the extent to which back-to-
back storm events may be retained, or partially retained by a living roof system,
as well as the overall appearance and health of the plant community.