Why Green Roofs? (An Excerpt From Dave Williams’ Thesis Proposal)
Increased urbanization is a seemingly inevitable byproduct of our worldwide population growth. In fact, the United Nations reports that almost all of the projected increase of 12 billion people in the world’s population by 2030 will live in urban areas (UN 2003). 2005 US Census figures predict that by 2030 the US population will top 363 million – an increase of 63 million people over the next 22 years. Ten of those 63 million will end up in Texas’ urban areas (UN 2003; USCD 2005). The logistical and environmental pressures on our urban centers in both Texas and indeed the world will surely grow with these increases in population. Creative problem solving is the key to maximizing urban efficiency, and allowing Texas to become a leading example of growing smart instead of just growing.
In very broad terms, a green roof consists of a growing media and plant material located on top of a building. The idea of using roof space as growing space is by no means an invention of the modern age. The hanging gardens of Babylon date to around 600 BC, Scandinavian cultures have used sod as a rooftop insulator for centuries, and early settlers to the great plains built houses entirely of sod, “soddies,” (Osmundson 1999; Grant, Engleback et al. 2003). These early examples of greening rooftop spaces served multiple functions just as modern green roofs aspire to.
The modern green roof movement has its roots in Germany where studies in the 1950’s and 1960’s demonstrated the viability of Sedum species in gravel covered roofs (Herman 2003). Fundamentally, the modern green roof is comprised of four key items: a water-proof membrane to protect the structure from moisture, a root barrier/drainage layer to facilitate drainage and prevent plant roots from compromising the integrity of the roof, a growing medium for plants, and the plants themselves (Mentens, Raes et al. 2006).
Based on the depth of the growing media, green roofs are divided into two different groups. Extensive green roofs have growing media with depths less than six inches (~150 mm). The thin layers of media restrict the available plant palate to species which can tolerate extremely thin soils, like the species in the genus Sedum. Extensive green roofs have been installed on roofs with pitches ranging from near horizontal to over 40˚. Intensive green roofs have growing media greater than six inches deep, greatly increasing the available plant palate. Some intensive green roofs support shrubs and trees. These systems are generally only practical on roofs with pitches less than 4˚, and require the building to be able to support a substantial amount of weight on the roof (Mentens, Raes et al. 2006)
The Benefits of a Greened Roof
Green roofs can offer numerous benefits to the urban environment ranging from improving air quality to reducing electrical bills (Grant, Engleback et al. 2003). Some of these benefits are described and documented below.
Absorption and mitigation of air pollutants and dust
Plants are able to act as air filters, removing gaseous pollutants and airborne particulates by incorporating them into tissue or passing them through the roots and into the soil (Getter and Rowe 2006). Getter has also translated the results from a German study that showed a significant reduction of diesel engine air pollution by green roof vegetation (Liesecke and Borgwardt 1997). Another study reported a 37% reduction of sulfur dioxide and a 21% reduction in nitrous acid in the air above a green roof when compared to other air samples taken nearby (Yok Tan and Sia 2005). Other studies have estimated that green roofs can remove 0.2 kg of dust particles per year per square meter of vegetated roof (Peck, Kuhn et al. 2003).
Reduction in the urban heat island effect
The urban heat island effect has been well documented by many parties, including NASA, who found via infrared photography that ambient temperatures in downtown Atlanta, Georgia are often upwards of 6˚C (10˚F) warmer than those in surrounding rural areas (Taube 2003). A 2002 study in Phoenix, Arizona found that at Phoenix Sky Harbor Airport the nighttime minimum temperatures have risen by 5˚C, and the average daily temperatures have risen by 3.1˚C (Baker, Brazel et al. 2002). A research team in Ottawa, Canada have shown that while temperatures on a conventional rooftop topped 55˚C (131˚F) during the heat of the day, green-roofed buildings stayed near 21˚C (77˚F) (Liu and Baskaran 2003).
Urban wildlife habitat
A study of 17 green roofs in Basel, Switzerland found 78 spider species and 254 beetle species over a three year period. Of these, 18% of the spiders and 11% of the beetles were considered endangered or rare (Brenneisen 2003). Preliminary results from another Swiss study suggest that ground nesting birds may be able to use extensive green roofs for nesting sites in urban areas (Baumann 2006). Many green roofs have only limited access and as such could function as disturbance free habitats for animals and insects. Some argue that the limited biomass possible on an extensive green roof limits the number of secondary and tertiary consumers that can thrive on a green roof (Baumann 2006). Intensive green roofs may be the ultimate answer if habitat restoration/replication is desired.
At attractive green space in the usually nature-less urban environment
Studies suggest that viewing green space and nature can reduce stress, ease muscle tension, and lower blood pressure (Ulrich and Simons 1986); when viewed at work natural landscapes can increase job satisfaction and reduce reports of headaches and illness (Kaplan, Talbot et al. 1988).
Protecting the building from sunlight and temperature fluctuations
A report from British Columbia showed that green roofs minimize diurnal temperature fluctuations of the underlying roof structure (50˚C change for non-greened vs 3˚C for greened roofs (Connelly and Liu 2005)). They also keep the structure drastically cooler during the heat of summer (Liu and Baskaran 2003). Studies suggest that the thermal protection that the soil and plant material provide can extend the life of the membrane by two to three times (Peck and Callaghan 1999).
Long term cost effectiveness in terms of both longevity and reduction in heating/cooling costs
A study in Singapore directly addressed the issue of green roofs as a cost-effective roofing solution. The study found that one could justify the additional expense of installing a green roof simply considering the longevity of a green roof system. Reductions in heating and cooling costs were an added savings (Wong, Tay et al. 2003).
In Texas, reduction in summer cooling costs would be a huge benefit for many businesses and residences during the hot summer months. Indeed, green roofs can be a way to cut costs of an already existing building. Peck et al (1999) found that indoor temperatures were reduced by around 4˚C when outdoor temperatures were between 25˚C and 30˚C (77˚F and 86˚F). In their book, Dunnett and Kingsbury suggest that every 0.5˚C in internal temperature reduction can result in up to 8% reduction in air conditioning electricity costs – a possible 60% reduction in cooling costs based on the previous results (Dunnett and Kingsbury 2004). Green roof implementation on a significant scale could make a large dent in the 65% of total electricity consumption that buildings are responsible for (Kula 2005). However, it is important to note that these reductions in energy consumption could be more cheaply acquired by simply installing more insulation during construction of the buildings themselves (Getter and Rowe 2006).
Stormwater runoff mitigation
Green roofs provide some solutions for the increasing problem of urban stormwater runoff. As my research will focus on the ability of a variety of green roofs to provide stormwater mitigation, details on these benefits are discussed after the following review of the issues surrounding urban stormwater runoff.
Stormwater Runoff Issues
One of the byproducts of our industrial progress as a society has been the increase in physical, chemical, and biological wastes. These wastes have inevitably, weather accidentally or purposefully, ended up contaminating the ground and surface waters of our nation. The U.S. government has acted to protect its waters, starting with legislation in 1912 which expanded the mission of the United States Public Health Services to include human diseases, sanitation, water supplies, and sewage disposal (Parascandola 1998). However, it took until the 1972 Clean Water Act until point-source polluters were required to have a permit to discharge. In the early 1980’s the then decade-old EPA launched a program called the Nationwide Urban Runoff Program. The results from this study (USEPA 1983) indicated that urban stormwater runoff was indeed a major source of non point-source pollutants in U.S. waters.
Stormwater runoff, for the purposes of this paper, is defined as any precipitation that after contacting the surface of the earth does not infiltrate into groundwater before reaching receiving waters. Urban stormwater runoff is simply stormwater runoff that originated from an urbanized area, and in 2002 the EPA reported that urban stormwater runoff was still one of the major sources of water quality impairment to US surface waters (USEPA 1994; USEPA 2002). The EPA has also found that an average urbanized city block generates approximately five times the volume of stormwater runoff than a wooded lot of the same size (USEPA 2003). This increase in runoff carries with the heavy metals, fertilizer, and other pollutants which accumulate in the urban environment, and make their way into our estuaries and lakes (Mason, Ammann et al. 1999).
Contaminants in urban stormwater runoff are not the only issue; the 500% increase in stormwater runoff volume from urban areas stresses both natural and man-made drainage pathways. Natural channels are always adjusting their shape and form to find equilibrium with their respective flow regimes; a 500% increase in peak flow volumes necessarily creates an imbalance. The result is scouring and erosion of natural channels, at times endangering adjacent property or requiring expensive (and ultimately short-term) stabilization measures. In older cities where storm water and sewage lines converge before entering treatment, these stormwater runoff surges can back up pipes and result in untreated sewage rising into streets and flowing into streams (Osmundson 1999).
The numerous issues with urban stormwater runoff have resulted in the evolution of Best Management Practices (BMP’s) for mitigating its effects. BMP’s are defined as a “device, practice, or method for removing, reducing, retarding, or preventing targeted stormwater runoff constituents, pollutants, and contaminants from reaching receiving waters,” (Strecker, Quigley et al. 2000). Hundreds of examples of BMP’s exist, including, but are not limited to wet ponds, dry ponds, wetlands, sand filters, constructed wetlands, retention basins, silt fences, permeable pavement, retention swales etc… Some of these permanent BMP’s (retention ponds, constructed wetlands) are large scale solutions which can be impractical in an ultra-dense urban setting where real estate is at a premium. Permeable pavements are a BMP which utilize sidewalks, parking lots, and other outdoor hardscapes as runoff reservoirs; effectively creating a surface that supports foot/vehicle traffic but allows water to pass into the soil underneath.
A study in the Pacific Northwest followed four different types of permeable pavements over the course of five years, noting that “virtually all rainwater” infiltrated through all of the systems(Brattebo and Booth 2003). They collected runoff samples from an asphalt parking stall and rainwater that had infiltrated through the permeable pavement and found that copper and zinc were undetectable in the infiltrate, but at near toxic levels in the asphalt runoff. This study also found that the concentration of motor oil was significantly reduced in the infiltrated water (Brattebo and Booth 2003), earlier studies suggest similar results (Pratt, Mantle et al. 1989).
If one imagines oneself as a raindrop falling on an urban area, there are only three main types of hardscapes you might encounter; hardscapes for road traffic, hardscapes for foot traffic, and the tops of buildings. What permeable pavements offer for sidewalks and parking lots, green roofs offer for the 40% to 50% of urban hardscapes that are found on the tops of buildings (Dunnett and Kingsbury 2004).
Mitigation of storm water run-off
Generally, there are two separate benefits green roofs can provide with regard to storm water runoff; reduced volume and delayed peak flows (Getter and Rowe 2006). Precipitation which falls on a green roof can either land on the growing media or land directly on a plant and either fall to the soil or evaporate. The precipitation that enters the soil will eventually leave the medium in one of three ways: it might simply pass through the soil and drainage layers and exit the green roof system as runoff, it might become retained by the soil medium and eventually evaporate back into the atmosphere, or it might be drawn into plant tissue and eventually released into the atmosphere by transpiration.
With regard to runoff quantity, the Department of Land Management in Belgium has recently condensed the German body of data on the subject, and written a summary paper in English; a great help to those interested in Green Roofs in the United States. This summary asserts that an average of 81% of rainwater falling on a standard flat roof leaves as runoff, while76% runs off from gravel covered roofs, 50% for extensive green roofs, and only 25% of rainfall passes through intensive green roofs (Mentens, Raes et al. 2006). Getter and Rowe (2006) provide a concise summary of a number of other studies which give a range from 60% to 100% retention of incident precipitation depending on the type of green roof system and rain regime (Liesecke 1998; Moran 2004; DeNardo, Jarrett et al. 2005; VanWoert, Rowe et al. 2005).
Delayed peak flows are critical to increasing the lifespan of our current stormwater infrastructure; they allow sewer systems to process stormwater surges over a longer period of time (Getter and Rowe 2006). Typical green roof peak flow delays range from 1 ½ hours (Liu 2003) to 4 hours (Moran, Hunt et al. 2005). Moran, Hunt et al (2005) also reported 57% to 87% reduction in runoff rates from green roofs when compared to those of conventional roofs.
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