Agriculture Reference
In-Depth Information
(up to approximately 67 percent annual retention over multiple years) because PAW
ranged from about 11 mm to 36 mm, depending on the speciic growing media's
composition and depth. The living roofs were so effective because 94 percent of
individual events delivered less than 25 mm of rainfall (accounting for 58 percent of
the total depth of rainfall received), thus even when the media was not fully dry,
adequate storage capacity was available to capture the next small storm (Fassman-
Beck et al. 2010).
For a given depth of growing medium, a high water-holding capacity is usually
associated with low permeability, low aeration, high wet weight, and can be det-
rimental to plant survival. Low water-holding capacity limits stormwater retention
and plant viability in the absence of regular irrigation. The FLL (2008) recom-
mends that suitable growing media for extensive living roofs should store water
at a minimum of 20 percent v/v and a maximum of 65 percent v/v, but it is noted
that the test method used in the FLL includes water that is held below the
nominal permanent wilting point, and is not the same as the agronomists' PAW
method previously mentioned.
As a minimum criteria for living roofs to provide stormwater control, the
growing media should be able to store rainfall from at least the smallest design
storm depth (DSD) advocated by the local regulatory authority for GI (e.g.,
25 mm). The minimum growing media depth for stormwater retention is strongly
encouraged to be given by the deeper of:
D LR = DSD
_____
PAW
(4.1)
D LR 100 mm for new construction, or 50 mm for retroits.
Where:
D LR = inished (naturally settled) living roof growing media depth (mm).
DSD = design storm depth, limited to P 25 mm.
PAW = plant available water (fraction or decimal) as determined by agro-
nomic methods (e.g., tension test over range 10-1,500 kPa [0.1-15 bar]
[Gradwell and Birrell 1979], or equivalent).
Measuring PAW for the speciic growing media to be used on a project is impor-
tant. If this data is not readily available from the growing media supplier, it should
be obtained from a reliable or certiied laboratory. In the United States, most
land-grant universities (there is one in every state) provide soil testing services.
The relationship between PAW , DSD and D LR is illustrated in Figure 4.1 . PAW typi-
cally increases with smaller particle (and pore) size distribution and greater
organic content (Graceson et al. 2013; Liu and Fassman-Beck 2014). PAW may
be manipulated by design, but decreasing particle size distribution and increasing
organic content has negative consequences for growing media weight and per-
meability (Fassman and Simcock 2012), as discussed in Section 4.1.2 .
The growing media depth is determined by the combination of design storm
depth and PAW , according to Equation 4.1. Regardless of the calculation's results,
some rule-of-thumb limitations should be considered. For example, in Denver,
 
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