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Source: ReNatur© 98 Brochure |
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"By working with the patterns and processes favored by the
living world, we can dramatically reduce the ecological impacts of our
designs," (Van der Ryn and Cowan, 1996).
To a certain
measure, we have the opportunity to mitigate the developmental impacts of
construction practices by replicating the environment that we have destroyed
with the building footprint through the design of our rooftops. Greenroofs
embody many environmental benefits, especially when applied to urban settings,
where nature is at a premium. They can help restore the ecological value of open
space to densely developed city centers. Obviously, the larger the greenroof
surface and the plant mass, the greater the ecological benefits.
According to The London Ecology’s book
entitled Building Green: A Guide to Using Plants on Roofs, Walls and
Pavements (1993), “Cities can be viewed from an entirely new, ecological
perspective. Buildings offer
surfaces akin to natural landforms and these can be planted following clues from
nature. The skin of the city can be
transformed into a living landscape.”
Perhaps
the greatest ecological function a greenroof can provide is its stormwater
management capacity.
Impervious cover has become
a function of contemporary land uses. As a result of new land use practices, cities across the nation have developed
over-stressed sewer systems with urgent stormwater management problems.
According to analysis of Lansat Satellite data by NASA climate scientists,
University of Georgia researchers and others, metro Atlanta is losing 50 acres
of tree cover per day. From 1988 to
1998 the 13-county metro area lost approximately 190,000 acres of tree cover to
development (Charles Seabrook, 1999). Lost
green space is then a by-product of the proverbial asphalt jungle, and the
inherent natural processes associated with natural areas are also lost.
The chart below from Bruce Ferguson's Introduction to Stormwater: Concept,
Purpose, Design (1998), shows the amount of impervious cover
that development and the new impervious pavements produce.
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Redrawn from Bruce Ferguson's "Introduction to
Stormwater: Concept, Purpose, Design," 1998. |
“We are obligated to restore the mechanisms of the earth’s
self-maintaining balance. Runoff
must be moderated, treated, and returned to its restorative path in the soil,”
(Ferguson, 1998).
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Source: ZinCo International
3/98 Brochure |
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On-site stormwater retention and
runoff control from expansive roof surface areas of buildings can be
accomplished through greenroofs. According
to civil engineer Charlie Miller, Principal, of Roofscapes, Inc., “Vegetated roof covers may offer the
only practical ‘at-source’ technique for controlling runoff in areas that
already are highly urbanized.” The reversal of damage caused by uncontrolled
storm water runoff and non-point source pollution is possible within our
urbanized watersheds. He believes that the intelligent use of best management
practices (BMPs) can result in significant improvements, as well as long-term
savings to individuals and municipalities (www.roofmeadow.com).
Depending on rain intensity and greenroof
soil depths, runoff can be absorbed between 15 to 90 %, thereby
considerably reducing runoff and potential pollutants from traditional
impervious roofing surfaces. Plants
intercept and delay rainfall runoff and the peak flow rate, alleviating combined
sewer overflows, and eventually return water to the surrounding atmosphere by
evaporation and transpiration. Average
runoff absorption rates are between 50 to 60% (www.roofmeadow.com).
The control of stormwater runoff is
achieved by mimicking natural processes by intercepting and delaying rainfall
runoff.
Greater grass & plant
diversity provides better plant uptake and simple friction, which creates less
erosion, and more water is retained on the greenroof surface.
Stormwater Natural Processes Detail from
www.roofmeadow.com.
According to Charlie Miller, the installation of
greenroofs is “a potential technique for relieving nuisance flooding and
reducing hydraulic loads on combined storm sewer systems.”
He contends that, “In addition to providing immediate relief for
overburdened stormwater management facilities, the deployment of vegetated roof
covers can help reduce the overall costs of infrastructure rehabilitation in our
older cities.”
Greenroofs reduce the volume of stormwater flowing into streams and drainage
channels, resulting in the control of sediment transport and overall soil
erosion. According
to an article in the November/December 1998 issue of Erosion Control Magazine, the
natural carpets provided by greenroofs protect both roofs and the soil below.
Nitrogen, phosphorus and toxins can enter a vegetated stream as dissolved
substances. Greenroofs' vegetated cover properties of friction, root
absorption, clay, and soil organic matter can control these substances from
entering a stream corridor (Dramstad, et al, 1996). In February of 1999, the International Erosion Control
Association’s Conference & Trade Exposition was held in Nashville, TN, and
featured a training workshop and special section regarding the benefits and
applications of roof greening systems. Thomas
Roess of Strodthoff and Behrens GMBH of Germany presented on this subject in
1999, and is a frequent lecturer worldwide on greenroof technology.
Publisher's Note: Thomas Roess is now Managing Director of IGG
Internationale Geotextil GmbH and they are supplying roofgreening systems under
IGG Roofgreening Systems (October, 2006).
Vegetation
absorbs pollutants from rainwater, and greenroofs provide this same amenity.
Heavy metals and nutrients found in stormwater are bound in the soil
instead of being discharged into the groundwater or streams or rivers.
Over 95% of cadmium, copper and lead and 16% of zinc can be taken out of
rainwater. Nitrogen levels can also
substantially fall (The London Ecology Unit, 1993).
An Atlanta area problem common to other
growing cities is the water quality and supply issue.
Extremely important to environmentalists and developers alike, this
situation has potentially critical consequences for all of Atlanta (Bookman,
1999). Metro Atlanta is now the fastest growing metro region in the country, and
the Chattahoochee is the smallest river in the country serving as the primary
water source for a major metropolitan area.
Atlanta’s ever-burgeoning residential and commercial developments’
demand for water also raises the question of water quality. Invariably,
expansion places chemicals, bacteria, sediment and other pollutants into local
waterways.
Tightly
sealed impervious surfaces such as concrete and asphalt, commonly found in urban
areas, greatly contribute to the ever-growing problem of the urban heat island
effect.
Barren
walls, roofs and streets act as reflectors, absorbing some energy and
redirecting a portion to other hard surfaces (The London Ecology Unit, 1993).
“Asphalt in parking lots and on rooftops, in particular, soaks up everything
and reradiates it as thermal infrared radiation.
The heat is released after sunset and forms a dome of higher temperatures
over the cities,” (science.msfc.nasa.gov).
This growth of our cities has resulted in hot spots within otherwise cool areas
of the countryside.
NASA has been conducting a study of
several sprawling U.S. cities contributing to this phenomenon of higher urban
temperatures. Its Project ATLANTA (Atlanta Land-use Analysis: Temperature and
Air-quality) was funded in 1996 as a NASA EOS Interdisciplinary Science (IDS)
investigation. This urban heat island experiment in Atlanta “seeks to observe,
measure, model, and analyze how the rapid growth of the Atlanta, Georgia
metropolitan areas since the early 1970’s has impacted the region’s climate
and air quality. Our key goal is to
derive a better scientific understanding of how land cover changes associated
with the urbanization in the Atlanta area, principally in transforming forest
lands to urban land covers through time, has, and will, effect local and
regional climate, surface energy flux, and air quality characteristics,” (www.ghcc.msfc.nasa.gov/atlanta).
In natural landscapes, vegetative canopy
biomass greatly lowers air temperatures, whereas the artificial, altered
surfaces common in urban landscapes greatly raises them.
“Urban forests are important to keeping cities cool,” says
co-investigator Dr. Jeffrey Luvall of the NASA Global Hydrology and Climate
Center in Huntsville, Alabama. “What’s
important are both the extent and arrangement of these forests.”
Satellite images of Atlanta readily show
how urban sprawl has extended into previous areas of farms and wooded areas,
largely along interstate and other major highways. Aerial photography below illustrates the wide range of thermal energy responses between the May
1998 daytime (Figure 1) and nighttime (Figure 2) Atlanta landscapes.
(See "Why Have A Test Greenroof?" for color thermal photos.) Figure 1 shows intense thermal energy responses from buildings, rooftops,
pavements and other typical urban surfaces.
Maximum daytime air temperature was approximately 25 degrees C (77
degrees F). According to the NASA
study, sample surface temperatures for tree-shaded grass, tree canopy, and
asphalt in full sunlight during the afternoon were 28 degrees C (82.4 degrees
F), 21 degrees C (69.8 degrees F), and 50 degrees C (122 degrees F),
respectively.
Figure
1
Figure 2
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Courtesy NASA, Dr. Jeffrey Luvall |
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Courtesy NASA, Dr. Jeffrey Luvall |
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In contrast, Figure 2 shows the relative
flatness of the Atlanta thermal landscape at night, with an average temperature
of 10 degrees C (50 degrees F). Daytime
temperatures for a commercial building roof comprised of rock/membrane coating
ranged from 49 degrees C (120.2 degrees
F) to 52 degrees C (125.6 degrees F). It
appears that thermal energy responses for vegetation across the image are
relatively uniform at night, regardless of vegetative type, e.g., grass, or
trees (www.ghcc.msfc.nasa.gov/atlanta).
The temperature
in downtown Atlanta is often 10 degrees F warmer than the surrounding outlying
areas. The bottom line is that “asphalt parking lots and roofs soak up
virtually all of the radiation that falls on them and reradiate it as heat,” (www.science.msfc.nasa.gov).
The debate is not over whether the greenhouse effect is being increased,
but to what extent.
The heated air from large expanses of
dark asphalt paving is suspected by meteorologists of causing thunderheads to
develop on summer afternoons. This
may cause moisture-laden air to dump its water on the cities where it loads the
storm sewers, rather than carrying it over to the open lands beyond.
A light-colored pavement should not have this effect (Edmund Thelen,
et. al., 1972). The urban heat island in Atlanta creates thunderstorms in Fayette
and Clayton counties, south of the city (www.cnn.com/NATURE/9903/25/heat.island.enn).
Researchers found an unusual pattern of thunderstorms after dark, from 4 a.m. to
dawn, that are tied to the heat released at night from buildings and roads.
According to Dr. Jeffrey Luvall, the
added heat also contributes to Atlanta’s air quality problem.
“The city already has a serious ozone problem, and the 10-degree rise
in temperature doubles the amount of ozone that is produced,” (CNN
interactive)
The Atlanta Regional Commission (ARC) is
in the process of developing a 20-year growth plan for a 10 county area around
Atlanta and NASA hopes to work with the ARC using data collected from Project
ATLANTA. NASA hopes this information will then be applied by urban planners,
environmental managers and decision-makers to improve our future by modifying
growth plans to design sustainable urban environments.
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Courtesy NASA, Dr. Jeffrey Luvall |
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Salt Lake City also showed its hot and
cold spots, seen at right, from overflight data recorded with infrared photography
in July 1998. Its rooftops and
other structures reached a blistering 71 degrees C (160 degrees F) (www.science.msfc.nasa.gov).
For more information regarding urban heat
island studies of Atlanta, Salt Lake City, and New York City, click
here
for an October 2000 CNN.com In-Depth Special article.
Greenroofs
can reduce ambient air temperatures and increase humidity levels in the
surrounding areas. Differences in cooling and heating
between the natural and manmade surfaces can affect city temperatures.
Even when the air is clean, dry air can put a strain on our breathing during
periods of higher temperatures. Due to the capturing and holding of precipitation in the plant foliage,
humidity levels increase, and the release of moisture results in a cooling
effect. It has been proven that local microclimates are positively affected by
the presence of green space. In urban settings where a greenroof has been
installed, warmer air above hard surfaces rises, lowering temperatures above the
vegetated roof cover (The London Ecology Unit, 1993).
Greenroofs contribute to the vertical mixing of ambient air, producing
lower air temperatures, and thus, in quantity, can reduce the urban heat island
effect.
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Greenroofs can
filter and bind dust particles, and naturally filter airborne toxins.
Ventilation
is sometimes inhibited due to the vertical design of downtown areas, which
reduces wind speed and traps heat in air pockets. Pollutants can remain suspended for days without relief. Atmospheric dust is held until rain washes it
off into the greenroof soil substrate. Carbon dioxide and other airborne toxins from
the city air are absorbed through the foliage, naturally cleansing the air.
A NASA study for space laboratories found that even a single fig tree can purify
10 m3 of air per day (McMarlin, Western Roofing, May/June, 1996). And Green Roofs
For Healthy Cities states that, "One square meter of grass roof can remove
approximately 0.2 kg of airborne particles from the air every year."
Some
relief of natural areas for wildlife can be sustained through the use of
greenroofs, although they are not intended to be replacements for true natural
areas.
Rather,
they should be part of a system to compliment green corridors and wildlife
habitats within an urban setting. "In the face of continued habitat loss
and isolation, many landscape ecologists stress the need for providing landscape
connectivity," (Dramstad, et al, 1996). In highly populated areas, greenroofs could
represent island habitats, or better yet, stepping stones using a series of them for
wildlife movement. In this way a natural wildlife corridor can be somewhat
replicated.
Greenroofs provide a habitat for a
diversity of wildlife species. In a landscape ecological context,
greenroofs create an artificial or man-made edge, and yet also serve as a
vegetative habitat patch. These greenroof patches are set within the
matrix of a city or urban landscape, and can accomplish ecological functions or
objectives. Even in densely populated areas,
beneficial insects, birds, bees and
butterflies can be attracted to greenroofs. Studies in the U.S. indicate that
butterflies will visit gardens up to 20 stories high, and birds up to 19 (The
London Ecology Unit, 1993).
Native plant selections usually fare better in these instances since they
have evolved together with the animals that depend on them for food, shelter and cover.
Roof vegetation normally encounters less
interference than an equivalent area at ground level, which could be an
important factor to creatures wary of human disturbance in urban settings.
Subsequently, micro habitats can be created for insects and birds.
Greenroof architecture embodies both
physical and culturally sustainable design concepts. The simple definition
of sustainable design or sustainable landscapes is those that "meet the
needs of the present generation without compromising the ability of future
generations to meet their needs," (Morrison UGA lecture, September 1999).
One of greenroofs' greatest
sustainability factors is the reduction or conservation of a structure's heating
and cooling resources. Also, as stated previously, once extensive
greenroofs are established, little maintenance is required. With the
correct plant palette, regeneration will occur naturally by means of seeds or
offshoots, as will growth media enrichment through decomposition.
Certain drainage
products and sustainable water collection techniques can supply additional water automatically
during periods of drought.
The numerous aforementioned environmental
attributes offered by the natural processes present within the design of
greenroofs certainly meet the definition of sustainability. One could
definitely argue the sustainability of some of the very elaborate intensive
greenroofs due to their higher human maintenance requirements. And yet,
the natural
processes of evapotranspiration, stormwater retention, etc. inherent in the
design of every greenroof, cannot be denied.
To ensure culturally sustainable landscapes, it is also our
responsibility to expose healthy areas and then to inspire the public to protect
them. By attracting the attention of humans, an aesthetic of care will
ensure cultural survival (Alfie Vick UGA lecture, September, 1999).
The same thought can be applied to creating ecologically healthy
greenroofs. By using the natural landscape characteristics
present on a particular site as a model, sensitive greenroof design can reflect
its surroundings, establishing a sense of place, and become an integral part of the landscape master plan. In the case of urban settings, we could historically
recreate original landscapes lost through development on the rooftops.
We have the ability (and responsibility) to ask pertinent ecological questions
and therefore raise the overall level of design for the future to reflect
sustainable design practices.
"We do not inherit the earth from
our ancestors; we borrow it from our children." - Andre
Gide
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