The role of living roofs in holistic stormwater management systems - Living Roofs in Integrated Urban Water Systems - page 26

design to some extent; this is addressed in Chapter 4 . For most living roofs, the
growing medium's water storage capacity is less than saturation , the latter condi-
tion is a technical deinition whereby all pore space is occupied by water ( Figure
2.2 ), and thus can be measured as the porosity of the medium. Due to implica-
tions for structural loading, surface erosion and drowning of plant roots, living
roof growing media should be designed so that it cannot physically achieve
saturation.
The materials comprising the drainage layer may or may not inluence runoff
detention. Laboratory testing in the UK showed that runoff patterns through
several types of synthetic drainage layers were similar to the runoff pattern across
a simulated bare roof deck (Vesuviano and Stovin 2013). Vertical low through
the growing media has been found to dominate the living roof's inluence on
runoff detention (Bengtsson 2005; Vesuviano et al. 2014). On the other hand, a
granular (aggregate) drainage layer may enhance peak low control, as runoff
has to low through a porous media to reach the gutter. Studies have not yet
documented whether this leads to a meaningful difference in overall system
detention, and thus advice for design choice.
A common hydrologic premise holds that a longer distance for water to travel
results in a longer time to reach the peak low rate, and possibly a lower magni-
tude of peak low. Whether the orientation of a living roof's drainage path with
respect to the location of gutters and downspouts inluences peak low attenua-
tion or runoff timing has only been investigated to a limited degree. For living
roofs subject to the same storm events, a ield study measured lower peak lows
from living roofs where runoff traveled several meters through the drainage layer
than from living roofs where the horizontal distance to the gutter was only

Saturation
All pore spaces are lled with water. Engineered
media designed for extensive living roof applications
should drain freely so that it never reaches saturation.

Field capacity
Pore space is air- and water- lled. Moisture is held
against gravity by the media matrix in small and
medium-sized pores. Large pores are air- lled.

Permanent wilting point
Pore space is mostly air- lled, but some water is held
tightly to the media particles. Moisture content below
this point (“hygroscopic water”) cannot be readily
accessed by the plants for transpiration, and is only
“lost” to the atmosphere very slowly.

Figure 2.2
States of soil moisture