Planning considerations - Living Roofs in Integrated Urban Water Systems - page 82

in growing media thickness, type and/or drainage layer. Test plots under simu-
lated conditions are considered to be of limited use in interpreting ield
conditions.
The volumetric Rational Formula coeficients ( C v ) were investigated for 21 of
the living roof studies described in Section 3.6.2 and Table 3.3 . Empirical data for
15 of the sites suggested that C v increased with increasing rainfall depth, thus
predictive relationships between P and C v were developed using non-linear
regression techniques for those sites (Fassman-Beck et al. in preparation). Results
were organized into curves according to Köppen Geiger climate zones for Figure
3.7 . There was substantial variability in predicted C v , evidenced by the standard
deviations, as there were relatively few sites overall, and a wide range of factors
that can affect day-to-day living roof hydrologic response (antecedent, storm and
other climate conditions, growing media water-holding capacity, moisture
content at the beginning of an event, vegetation vigor and density, etc.). Consid-
ering the regression results together with the standard deviations, the models
appeared to reasonably predict the empirical observations for the purposes of
generating generic runoff coeficients. Regression models follow empirical obser-
vations that runoff is reduced substantially, even in large events in many climate
zones, which would likewise be relected in a reduction in peak low rate when
C v is applied in the Rational Formula.
Despite a reasonable model it amongst averaged site data, the signiicant
site-to-site variation must be acknowledged. An individual regression model that
it the criteria was not identiied for living roofs in Toronto, Shefield, Raleigh,
Goldsboro or Auckland (one site). As would be expected when the averaged

Figure 3.7
Rational Formula
volumetric runoff
coeficients determined
from 15 living roof
studies from the United
States, Canada, Italy, UK
and New Zealand,
organized according to
Köppen Geiger climate
zone

1.0

Csa (n = 1)

0.9

Dfb/Cfa (n = 1)

0.8

Cfa(n = 2)

0.7

Dfa (n = 4)

Cfb (n = 5)

0.6

0.5

Csb (n = 2)

0.4

0.3

0.2

0.1

0.0

0 10 20 30 40 50 60 70 80 90 100

Rainfall depth (mm)

Cfb (n = 5) Dfa (n = 4) Cfa(n = 3) Csb (n = 2) Csa (n = 1) Dfb/Cfa (n = 1)

Vertical bars representing 1 standard deviation are shown only intermittently in the interest of igure clarity.