Green Roofs - Living Architecture Monitor - Spring 2018 - 7

calculation into the context of the whole site (i.e. not just the building footprint), or possibly a whole catchment or sewershed.
Flow processes through an engineered green roof (and
many other green infrastructure systems) are very complicated.
Unfortunately, current design guidance offered by many regulatory agencies either neglects calculation instruction altogether,
or offers very simplified approaches which are akin to a bucket
filling up and emptying or overflowing. This approach neglects
the dynamic operation of a green roof, and tends to underestimate
performance.
The US Environmental Protection Agency's Stormwater Management Model (SWMM), is the only model of which I am aware
that specifically incorporates calculation routines that might accurately represent green infrastructure technologies. I say "might"
because its accuracy hasn't been widely tested (yet) against
observed data. SWMM can be applied at any scale - from a single
building to an entire watershed. SWMM also offers the benefit
of being an open-source, publicly available model. We're working
on generating field performance data that can evaluate SWMM's
accuracy for a variety of green infrastructure technologies.
MH: Green roofs in Florida typically require irrigation during
the dry season. Additionally, to provide a water quality benefit,
it may be necessary to capture and reuse green roof runoff.
For this reason, when designing green roofs in Florida, it is
recommended to provide some water storage to capture this

runoff. To effectively size the cistern a long term, 10 years or
greater of precipitation data, continuous simulation model is
recommended. There are many software programs available to
perform this modeling such as EPA SWMM, ICPR, or even
using Microsoft Excel.
8. Are you aware of any developments coming down the pipe that will
either strengthen or weaken the use of green infrastructure, and if so what
are they?
EFB: At the moment, major investments in green infrastructure
are occurring across the USA in order to comply with regulations,
and avoid potential massive environmental fines, by reducing the
frequency and volume of combined sewer overflows (CSOs). In
cities like New York and Philadelphia, investments are anticipated
at over US$1B. This isn't a problem unique to large cities though.
The US EPA indicates that most communities with CSO problems
have fewer than 10,000 people. Widespread stormwater GI implementation has been shown to be a critical contributor to technically
achieving the required level of runoff mitigation.
MH: I would say resiliency and sea-level rise might be an ally to
the green roof industry. This is because green roofs can reduce
runoff at a given site without additional footprint. In a future where
stormwater attenuation and storage will be harder to find, green
roofs might offer a solution in space limited urban areas.

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