The role of living roofs in holistic stormwater management systems - Living Roofs in Integrated Urban Water Systems - page 19

Chapter 2: The role of living roofs in
holistic stormwater management systems
Regardless of the growing public awareness of the environmental impacts of
stormwater runoff, government regulation deining requirements for stormwater
management remain the most signiicant driver or potential limitation for design
and installation of all stormwater control measures (SCMs) . 1 At the time of this
writing, many municipalities are found to be in the midst of a paradigm shift for
design of stormwater controls. The shift to green stormwater infrastructure (GSI)
provides an opportunity for living roofs where green infrastructure (GI) is embraced,
and where design manuals offer computational procedures for runoff calculations,
but creates challenges where regulatory agencies are slower adopters. A discussion
of the regulatory context is warranted, as it forms the basis for the technical
objectives adopted by the stormwater designer. It also forms a framework, or set of
rules, against which a building or broader development proposal is evaluated in
terms of creating and mitigating runoff potential.
In GSI, stormwater “management” includes on-site retention to limit runoff
volumes, temporary detention to further reduce peak lows, and source control
to prevent discharge or provide treatment to reduce discharged amounts of a
range of contaminants. These can be further deined as:
• Rainfallorrunoffthatisdesignedtoiniltrateintotheground,evapotranspire
to the atmosphere, or harvest for reuse is considered to be retained . The net
volume of runoff that would otherwise discharge from a site to a sewer or
stream is reduced with stormwater retention. Living roofs, bioretention,
permeable pavements, cisterns with plumbing for reuse, and some swales are
considered retention SCMs. Even where bioretention or permeable pavement
systems have underdrains, signiicant retention can be provided.
• Rainfall or runoff that is captured and slowly allowed to drain into the
receiving environment is considered detained . The low rate of runoff is
mitigated, but the total volume of the discharge is not altered. Ponds, 2 basins
and constructed wetlands are typical detention SCMs. In some cases,
detention alters the time to peak low.
• Self-mitigating surfaces that prevent runoff or contaminants from being
generated at a site are often considered source controls . Living roofs and
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