Through a
variety of physical, biological and chemical treatment processes that
filter pollutants and reduce the volume of precipitation runoff, green
roofs reduce the amount of pollution delivered to the local drainage
system and, ultimately, to receiving waters. One pollutant that green
roofs help control, for example, is nitrogen.
While
nitrogen gas occurs naturally as a major component of the atmosphere,
nitrogen compounds from automobile exhaust, agricultural fertilizers and
industrial activities can create a significant pollution problem.
Airborne nitrogen compounds can fall to the ground in dust, raindrops,
or simply due to gravity. When these compounds are carried away with
stormwater runoff, they contribute to eutrophication problems in surface
water. Green roofs, however, can help make nitrogen pollution less of
a water quality problem.
The following figure shows how the breakdown of nitrogen occurs in the soil. The end
result of this process is the NO3 (nitrate) form of N that
plants can take up as they grow.
Green
roofs contribute to improved water quality not only by
retaining and filtering the rainwater through the soil and
root uptake zone, but also through:
·The
vegetation, which slows down the water through friction and
root absorption.
·The
foliage in particular, which collects dust, transpires
moisture and provides shade.
·The
binding of potential pollutants to clay and organic matter in
the roof top soil matrix (Dramstad, et al, 1996).
In addition, the
temperature of the water not retained on the rooftop is moderated
before draining downstream.
1
Results of the
limited number of studies pertaining to the water quality benefits
of green roofs are promising. Typical study results indicate that
most of the heavy metals and other nutrients present in stormwater
are bound in the soil substrate instead of discharged into streams
or rivers. It was estimated that over 95% of cadmium, copper and
lead and 16% of zinc can be taken out of rainwater by green roofs
and that nitrogen levels can also be diminished (The London Ecology
Unit, 1993).
A major
objective of LID site design is to minimize, detain and retain the
post-development runoff uniformly throughout a site so as to mimic
the site’s predevelopment hydrologic functions.2
This is achieved by infiltrating and temporarily storing runoff
water using one or more Integrated Management Practices (IMPs) that help maintain the
predevelopment peak discharge rate and timing.
The storage
provided by green roofs helps to reduce the volume of runoff that
would otherwise need to be controlled elsewhere in order to
replicate natural watershed conditions and attenuate peak flows.
Water quantity control by green roofs is well documented. In recent years its use has increased due to its
potential for addressing urban flooding and in reducing hydraulic
loads on combined storm sewer systems. The quantity of rainfall
retained or detained by a green roof can vary, but for small
rainfall events little or no runoff will occur and the majority of
the precipitation will return to the atmosphere through evapotranspiration. For storms of greater intensity and duration a
vegetated roof can significantly delay and reduce the runoff peak
flow that would otherwise occur using conventional roof design. This
helps to reduce the risk of flash flooding and the frequency of
combined sewage overflow events. 3
Water Quantity
Benefits
Green Roofs
As with
natural soil/plant systems, green rooftops reduce runoff
problems by a variety of means, including:
The
storage of water in the substrate
Absorbing water in the root zone
Capturing and holding precipitation in the plant foliage
where it is returned to the atmosphere through transpiration
and evaporation
Slowing
the velocity of direct runoff as it infiltrates through
layers of vegetated cover
4
Green Roof Top
Processes For Stormwater Management (Source:
Roofscapes, Inc.)
Greater grass &
plant diversity provides better plant uptake and increased friction,
creating less erosion and more water retained on the green roof
surface.
1
The
characteristics of the soil substrate have a major influence on the
effectiveness of a green roof as a whole. The soil layer traps
sediments, leaves and other particles, thereby treating the runoff
before reaching an outlet. The water retention capacity of the soil
is dependent upon both the properties of the soil substrate and the
vegetative cover. For example:
·A
1-inch deep moss and sedum layer over a 2-inch gravel bed retains
about 58% of the water.
·A
2.5-inch deep sedum and grass layer retains about 67% of the water.
·A
4-inch layer of grass and herbaceous vegetation retains about 71% of
the water.
·A
major 2-inch rainstorm, generating about 1.25 gallons of water per
square foot, on a 2.5-inch thick extensive green roof would retain
approximately 0.50 gallons of water per square foot, or 40% of it.
Besides the
depth and composition of both the soil substrate and vegetation,
environmental factors that affect stormwater management efficiency
of green roofs include air and roof temperature, sun and wind.
Rooftop
runoff interception rates may vary between 15 and 90 %. Average
runoff absorption rates may vary between 50 and 60 % due to
differences in the soil matrix and roof top cover conditions.
Absorption rates may also vary considerably, by as much as 50 %,
between summer and winter cover conditions due to temperature, wind,
evapotranspiration rates and plant uptake through both the root zone
and plant foliage.
1
Stormwater
runoff for a 3.35-inch, 24-hour rainfall event.
(Source:
Roofscapes, Inc.)
Studies in
both North America and Europe have provided ample evidence as to the
effectiveness of green roof tops in controlling stormwater. These
studies have determined that the greatest cost benefit for the
control of stormwater is provided by the first inch of soil and
vegetation cover.
In
Toronto, Canada, where the average rainfall event is 1.6 inches, a
three-month long summer study showed that a green roof with a 2.8
inch deep vegetation layer produced no runoff, while the soil
surface at grade, without planting, produced 42% runoff and a gravel
surface produced 68% runoff.
4
A
study in Berlin, Germany showed that green roofs can absorb 75% of
the precipitation that falls on them, which translates into an
immediate discharge reduction to 25% of normal levels.
In
Portland, Oregon a garage roof top planted with a mixed layer of
sedum and grass retained up to 90% of all the rain that fell on it,
becoming less effective only during continuous and heavy rainfall.5 The effectiveness of the roof top could have been
improved upon if additional storage had been provided through the
use of additional detention devices such as modified roof drains and
water tanks.
Additional Ecosystem
and Environmental Benefits
Green
roofs, beyond their use for stormwater management, provide a number
of ecological amenities. They can help to preserve habitat and
biodiversity and provide an oasis of life in an otherwise sterile
urban environment. Even in densely populated areas, birds, bees,
butterflies and other insects can be attracted to green roofs and
gardens up to 20 stories high (The London Ecology Unit, 1993). Some
of the ecological and environmental amenities of green roofs are
summarized as follows:
Green rooftops
can provide a micro 'stepping stone' habitat for birds and
insects, connecting natural isolated habitat pockets with each
other, or provide an 'island' habitat above those at ground level.
6
Green roofs
can be specifically designed to resemble endangered ecosystems or
habitats, i.e., prairie grasslands or desert xeriscapes.
A green roof
designed for minimal maintenance is very protected and can provide
habitat to both plants easily damaged by walking and/or to ground
nesting birds.
Green roofs
can improve air quality through increased evapotranspiration and
the filtering and shading effect of their foliage, thus helping to
ameliorate the urban heat island effect, especially in sparsely
vegetated areas.
1 Roofscapes,
Inc., Green Technology For the Urban Environment. www.roofmeadow.com
C. miller.
2 Coffman, L.S., R. Goo and R.
Frederick, 1999: Low impact development an innovative
alternative approach to stormwater management. Proceedings of
the 26th Annual Water Resources Planning and Management
Conference ASCE, June 6-9, Tempe, Arizona.
3
Exploring the Ecology of Organic Green Roof
Architecture,Green Roofs Web
Site www.greenroofs.com
L.
Velazquez.
4
Peck, S.W. and
C. Callaghan, 1999: Greenbacks from Green Roofs: Forging a New
Industry in Canada. Prepared for: Canada Mortgage and Housing
Corporation. Environmental Adaptation Research Group,
Environment Canada.
5 Scholz-Barth,
K., 2001: Green Roofs, Stormwater Management From the Top Down.
Environmental Design and Construction. Accessible at
www.edcmag.com/.
6
Bass, B., 1999: Modeling the Impact of Green Roofs on Toronto’s
Urban Heat Island. Environment Canada, Green Roofs for Healthy
Cities.
