DESIGN OF LID SYSTEMS - Low Impact Development and Sustainable Stormwater Management - page 155

Dual Media with a Synthetic Retention Layer
Dual media assemblies with a synthetic retention layer introduce plastic panels
with cuplike receptacles on their upper surface (i.e., a modified geocomposite
drain sheet). The panels are in-filled with coarse lightweight mineral aggregate.
The cups trap and retain water. They also introduce an air layer at the bottom of
the assembly. A typical profile would include:
• A waterproofing membrane
• A felt fabric
• A retention/detention panel
• A coarse-grained drainage medium
• A separation geotextile
• A fine-grained growth medium layer
• A vegetation layer
These assemblies are suitable on roof with pitches less than or equal to 1 : 12
( 4 . 8 ◦ ) . Due to their complexity, these systems are usually 5 in. or deeper. If
required, irrigation can be provided via surface spray or midlevel drip.
Potential Applications
Assuming that a roof is structurally adequate to support a vegetated roof load
(Figure 7-27), and that the roof slope is less than 20% (although not abso-
lutely flat, as this may hinder proper drainage), vegetated roofs can potentially
be used for most buildings. Vegetated roofs can be employed for all land uses;
the greatest stormwater benefits will be realized in highly developed sites where
the building footprint comprises a significant portion of the site, and for sites
with poorly drained soils. Buildings on sites with greater perviousness may
also use vegetated roofs, although it may be less expensive to use at-grade LID
measures for stormwater management purposes; vegetated roofs can be used in
conjunction with at-grade measures. Vegetated roofs may be used to help achieve
benefits other than mitigating stormwater runoff. These include evaporative cool-
ing (which lowers building energy use for HVAC systems and helps minimize
the urban heat island effect), noise reduction inside the building, increased roof
longevity, habitat provision, and improved aesthetic value.
7.6 CAPTURE-REUSE
There are many designs and manufactured systems whose function is to inter-
cept and store runoff, primarily from rooftops [21-24]. The stormwater that is
contained can be used for irrigation or other nonpotable water needs without
disinfection. For potable systems, the stored rainfall is further filtered and disin-
fected (by an ultraviolet tube) with in-line units prior to consumption. This design