The role of living roofs in holistic stormwater management systems - Living Roofs in Integrated Urban Water Systems - page 29

day of the storm event, and living roof design between studies. Peak low reduc-
tion reported in literature from an individual living roof ranges from 31 to 100
per cent (Berghage et al. 2007; DeNardo et al. 2005; Fassman-Beck et al. 2013;
Hathaway et al. 2008; Uhl and Schiedt 2008). Peak low should always be well
mitigated (even during large storms) as high retention capability means there is
simply less water to discharge, and adequately designed permeability ensures
rainfall percolates through the growing medium, slowing it down, rather than
lows across the vegetated surface (Fassman and Simcock 2012).
Fassman-Beck et al. (2013) found that the frequency spectrum of peak lows
and runoff volumes discharged from four living roofs in Auckland were less than
the frequency spectrum of rainfall ( Figure 2.3 ). In other words, runoff from the
living roofs occurred much less frequently than rainfall, thus effectively achieving
the objective of preventing rooftop runoff generation. Long-term data sets incor-
porating multiple ield sites in New York City and Auckland (Carson et al. 2013;
Fassman-Beck et al. 2013a), as well as a short-term study of test plots in College
Station, Texas (Volder and Dvorak 2013) showed empirically that precipitation
was a strong predictor of runoff retention. As-yet mostly unpublished data com-
piled from Chicago, multiple sites in North Carolina, multiple sites in Michigan
and Portland (OR) show similar trends (personal communication Carpenter 2013;
Hunt 2013; Kurtz 2013).
The increased depth of an intensive living roof does not strictly correspond to
increased stormwater control. Retention performance depends more speciically
on the growing media's available water-holding capacity, which results from a
combination of its composition and depth, followed by ET and the frequency
spectrum of rainfall events. Figure 2.1 demonstrated that the majority of individual

Figure 2.3
Runoff volume and peak
low frequency spectrum
for a living roof in
Auckland with average
growing media depth of
60 mm (adapted from
Fassman et al . 2013)

5

Rainfall peak low
Rainfall depth

Living roof peak low
Living roof runoff depth

60

50

4

40

3

30

2

20

1

10

0 0

1 5 10 50 100

Percent of rainfall or runoff events equalled or exceeded (%)