Integrating stormwater performance and architectural design - Living Roofs in Integrated Urban Water Systems - page 101

The main difference between the sources is that the FLL quantiies numerical
objectives regarding suitability (it for purpose) in relation to the German climate .
For example, minimum allowable saturated hydraulic conductivities according to
living roof assembly (intensive versus extensive, with or without a separate drain-
age layer) are deined to prevent ponding subject to typical German rainfall inten-
sity. Compliance with several FLL numerical objectives is required in Germany.
The FLL speciies an acceptable range for the growing medium's particle size
distribution (PSD); ASTM does not. The FLL PSD guidance is mostly in relation to
plant health, but PSD also has important implications in terms of permeability
and weight (Fassman and Simcock 2012). Smaller particles create denser growing
media, with reduced pore space and size, leading to heavier media with reduced
permeability. In New Zealand, the FLL PSD guidelines were slightly relaxed to
create acceptable growing media using existing, readily available aggregates
(Fassman et al. 2010), whose success in stormwater management and plant
establishment were proven in multiple ield applications (Fassman et al. 2013;
Fassman-Beck et al. 2013).
Neither the FLL nor ASTM living roof-related guidance provide methods for
measuring the permanent wilting point. However, standard methods are found
in common soil science, horticultural and agricultural reference texts, and are dis-
cussed further in Section 4.1.3 .
4.1.2.1 Discussion: measurements of moisture storage potential in the design
process
To predict water storage capacity and hence rainfall retention, both the FLL and
ASTM use methodologies typical of geotechnical engineering moisture content
assessments. In this approach, a sample of the growing medium after two
hours' drainage from saturation is oven-dried. The weight of the sample before
and after oven-drying yields the moisture content. In planted environments,
agronomists and horticulturalists are aware that not all of the moisture stored in
a media is accessible to plant roots. Some moisture is held tightly to the soil
matrix (to charged surfaces and in very small pores). This moisture is unable to
be extracted by plant roots and thus not available for transpiration. An agro-
nomic laboratory test of “plant available water” ( PAW ) would therefore likely
predict less potential for water storage than would a test where the media is
subjected to oven-drying. The PAW for a variety of living roof media reported in
the literature is found in Table 4.2 . The various tests are useful to compare dif-
ferent media, although little side-by-side testing of living roof growing media
moisture storage characteristics according to different methodologies has been
published to date. However, in a comparison in Auckland of three growing
mediums' characteristics, in the laboratory the FLL and ASTM methodology esti-
mated approximately twice as much moisture storage capacity compared to
measures of PAW for each medium ( Table 4.2 ). At the ield scale, each of four
living roofs' maximum storm event retention (rainfall capture) was not
statistically different on average from the storage capacity predicted by the