Planning considerations - Living Roofs in Integrated Urban Water Systems - page 81

The Rational Formula is primarily intended for use in sizing drainage elements
such as inlets and culverts (McCuen 2004). use of the Rational Formula is limited
to predicting peak low only, and thus may be applied for assessment of runoff
detention. It is appropriate where approximate water balances are required, but
not where the full runoff hydrograph, or total storm volume, is needed for design
(Bedient et al. 2013). Numerical procedures for converting a Rational Formula
peak low to a runoff hydrograph (the Modiied Rational Method) are found in
older hydrology textbooks and design manuals; however, more sophisticated
calculation procedures are now widely accessible. In any case, the total volume of
runoff from a storm event is not predicted by multiplying the Rational Formula
peak low, Q p , by the duration of the storm event.
The FLL (2008) presents a range of C v according to growing media depth and
roof pitch. The FLL values for C v were determined on small plots subjected to
constant rainfall intensity, the latter of which is consistent with the underlying
assumptions of the Rational method. The FLL procedure assumes that the critical
rainfall duration is 15 minutes. To determine C v , the FLL testing procedure
requires media to be saturated and drained (over 24 hours) to establish initial
conditions. The FLL explicitly states that the values for C v are primarily for use in
sizing pipes for vertical drainage in a manner consistent with municipal sewage
system design, which is not necessarily the same as quantifying runoff detention
performance.
Table 3.4 includes results of C p studies performed on living roof plots sub-
jected to simulated constant intensity rainfall events. The choice of simulated
rainfall intensity appears to have a signiicant inluence on results. Alfredo et al.
(2010) demonstrate C p decreases with increasing growing media depth, with
rainfall simulating a 5-year, 6-min storm in New York. From similar size test
plots, DeCuyper et al. (2005) found much higher runoff coeficients from the
250-year, 15-min storm in Belgium. Variation in C p was attributed to variations

Table 3.4 Literature values: peak runoff coeficients for living roofs

Source Rainfall Living roof size and

slope

Growing media
thickness (mm)

C p

DeCuyper et al.
(2005)

Simulated 250-yr, 15-min storm
in Belgium

Multiple Plots: @
7.5 m 2
Slope: 2%

20
40
50
65
80

0.87
0.89 & 0.92
0.53
0.96
0.57 & 0.9

Alfredo et al. (2010) Simulated 5-yr, 6-min storm in

New York

Plot: 0.74 m 2
Slope: 2%

25
63
101

0.53
0.39
0.21

Moran et al. (2005) Field measured for ten storms

with P > 38 mm

70 m 2
Slope: 0%

75 0.50 (average

for 10 events)

Carpenter and
Kaluvakolanu (2011)

Field measured for 21 storms
with 4 mm ≤ P < 75 mm

325.2 m 2
Slope: 4%

102 0.11