Integrating stormwater performance and architectural design - Living Roofs in Integrated Urban Water Systems - page 102

Table 4.2 Water holding capacities of growing media reported in the literature.

Source Growing Media Composition

(% by volume)

Plant Available Water * (%)

Fassman and Simcock
(2012); Fassman et al .
(2013)

80% pumice, 20% composted pine bark ines 24.2 (FLL = 49.6)

50% pumice, 30% zeolite, 20% composted pine bark
ines

23.1 (FLL = 46.6)

70% pumice, 10% zeolite, 20% blend of composted pine
bark, mushroom compost, spaghnum peak

28.9 (FLL = 63.0)

60% pumice, 20% expanded clay, 20% compost-based
garden mix

20.2

Farrell et al . (2013) 80% scoria, 20% coir 40.1

80% crushed terracotta roof tiles, 20% coir 43.5

Graceson et al . (2013) 70-90% coarse or ine crushed brick, 10-30% composted

green waste

23.1-24.7 (coarse)
24.1-28.2 (ine)

70-90% coarse or ine crushed tile, 10-30% composted
green waste

9.9-12.3 (coarse)
29.8-32.5 (ine)

70-90% ly ash, 10-30% composted green waste 23.4-30.0

Stovin et al . (2012) ** Commercial (UK) assembly based on crushed brick and

ines

~20-25%

Notes
* Various speciic methods for measurement, but each represents a laboratory measure between ield capacity and the permanent
wilting point.
** Estimated from empirical rainfall-runoff observations, but acknowledging ield capacity and permanent wilting point.

laboratory-measured PAW (Fassman and Simcock 2012). Graceson et al. (2013)
and Bengtsson (2005) also used metrics analogous to PAW to assess media
water-holding capacity against storm event retention performance. Substantial
variability in actual storm-to-storm retention has been measured for all living
roofs documented in the literature, in part because the growing media is not
always as dry as the permanent wilting point at the onset of rain.
Until further research emerges to reliably quantify stormwater retention and
detention dynamics in living roof systems, regulatory approaches and designers are
largely reliant on estimators of the “ill and drain” scenario of rainfall capture and
storage in the growing media, as described in Section 2.7 . This approach is predi-
cated simply on the difference between a media's ield capacity and permanent
wilting point - i.e., PAW . Within the living roof industry, FLL tests are used fairly
widely, particularly as a marketing tool outside of Germany, and are suitable for
determining the ield capacity. However, a measure of wilting point must also be
provided. PAW provides a better estimator for stormwater retention than just ield
capacity, a more conservative design approach, and an important metric for horti-
cultural consultants to help identify a range of plants suitable for individual living
roof applications. A speciic design procedure based on PAW is detailed in
Section 4.1.3 .
Additional research is needed to provide conidence in a more reined design
approach. Testing of model formulation (e.g., those presented in Section 2.7 )