Environmental Sciences Essay 代写 Extensive Green Roofs For Storm Water Management

Nowadays the global population is concentrating more in urban areas. Urban areas are constantly expanding in terms of space and density. Grassland and forests are displaced by the impervious surfaces of streets, driveways and buildings due to continual demand for urbanization. The level of urbanization is rising and expected to reach 83% by 2030 in developed countries (United Nations, 2002; Antrop, 2004). Urbanization is greatly intensifying storm water runoff, diminishing groundwater recharge and enhancing stream channel and river erosion. This involves an unsustainable use of natural systems and creates numerous problems both within and outside cities. The urban hydrological system faces highly fluctuating amount of surface runoff water all through year; which creates a negative impact for some city infrastructure and surrounding environment. In most cities, conventional stormwater infrastructure failed to convey stormwater quickly from the city to recipients.

Since the 1990s, urban stormwater infrastructure has been exposed to major rethinking when the idea of sustainable development gained more and more ground. A city can be benefited by proper usage of stormwater. Storage reservoirs, ponds as well as green areas are effective tools for reducing high runoff during rainfall. A modification of existing hydrological system is required to cope the situation so that it can play a more active and positive role. Most of the big cities have the high amount of impervious surfaces (Blume, 1998; Ferguson, 1998) and the high land prices make the creation of green areas in urban regions very expensive and sometimes impossible. Solving the high stormwater runoff, a costly and disruptive way is to enlargement or expansion of stormwater infrastructure. Several cities, are commissioning consultants to study environmentally friendly, cost-effective alternatives to solve that problems. Low impact development (LID) and Water sensitive urban design (WSUD) approaches are used in USA and Australia respectively for minimizing impervious cover and maximizing infiltration of rainfall. Although in most locations those approaches are not widely implemented and the vast majority of new stormwater management in US and Australia remains in the form of conventional storm sewers with limited treatment by retention basins (Allison et al., 2007).

In older urban areas, there is a lack of suitable land for creating sustainable stormwater infrastructure, green roofs and vertical green walls can be a variable alternative there. Green roof or vegetated roof can be implemented in the huge amount of unused roof area (about 40-50% of the impermeable surfaces in urban areas (Dunnett and Kingsbury, 2004)). Vegetated roofs can play an important role in modern urban drainage because of their ability to slow down and reduce runoff response. Green roof provides numerous ecological and economic benefits, including stormwater management, energy conservation, mitigation of the urban heat island effect, increased longevity of roofing membranes and mitigation of noise and air pollution as well as improved biodiversity (Getter and Rowe, 2006; Simmons and Gardiner, 2007; Villarreal and Bengtsson, 2005; VanWoert et al., 2005; Liu and Minor, 2005; Meng and hu, 2005; Liesecke, 1998). Out of all proven benefits, the reduction of stormwater runoff is the greatest environmental service that green roofs provide.

In a green roof system, much of the precipitation is captured in the media or vegetation and eventually evaporates from the soil surface or is released back into the atmosphere by transpiration. This is a continuous process. Fig. 1 shows the simple mass balance used to calculate roof runoff.

Fig. 1: Simple mass balance used to calculate roof runoff.

Depending on the type of green roof system (design, substrate depth and plant species), research has shown reductions of 60-100% in runoff (Liesecke, 1998; Moran et al., 2004; VanWoert et al., 2005). Bengtsson (2005) has reported that high evapotranspiration from a vegetated roof can reduce the annual runoff to less than the half precipitation. Local urban flooding and combined sewers overflows (CSOs) can be lessened as water is stored initially in the soil and vegetation which reduces peak flow and extends the time of concentration. Urban runoff pollution can be reduced by green roof as it absorbs the pollutants of wet and dry atmospheric deposition. This pollution runoff is dependent on some factors like the type of surrounding area (industrial, residential or commercial) and local pollution sources (intensity of traffic, type of heating system). When it rains, water comes out of conventional roofs and paved areas mixes with deposited pollutants and carries to rivers and local water sources. Contaminants in stormwater runoff can include fertilizers, herbicides and insecticides; oil and grease from roads and energy production facilities. The resulting contamination not only harms aquatic life but also make water unsafe for human. It also reduces the diversity of insect and fish populations. Sometimes local water treatment systems can be overloaded due to stormwater runoff and in many cases it treats sanitary sewage from showers and toilets and stormwater in the same facilities. During the heavy rainstorm, water may exceed the system's capacity and discharges mixed sewage and stormwater directly into local lakes and rivers. Green roofs not only minimize the stormwater runoff but also act as pollution adsorbents and filters. This study aims to investigate the influence of a green or vegetated roof on Stormwater runoff management. Green roof technology, factors affecting stormwater retention and relationship between green roof and runoff are also discussed.

Green Roof Technology

Green roof is a layered system comprising of a waterproofing membrane, growing medium and the vegetation layer itself. Green roofs typically have four layers of construction. A waterproofing membrane sits immediately on top of the structural roof deck to prevent moisture from entering the building. Typically, above this membrane is a root barrier layer that is designed to prevent roots from penetrating the waterproofing membrane and the structural roof. A drainage layer is next. The drainage layer (realized with either some engineered coarse grained porous media or plastic profiled elements) is typically designed to carry excess runoff to roof drains, and to store water for the plants in dry periods. Next, a filter fabric is installed to prevent soil from washing away and compromising the drainage layer as water drains from the roof. Finally, the growing plants and associated substrate or growing medium (a blend of mineral material enriched with organic material) complete the green roof. The substrate is often a lightweight synthetic soil that is porous and inherently inert, with nutrients added for plant growth. Figure 2 shows a typical green roof system.

Fig. 2: Typical green roof system (Downton, 2010)

A green roof not only acts as an insulation barrier, but the combination of plant processes (photosynthesis and evapotranspiration) and soil processes (evapo-transmission) reduces the amount of solar energy absorbed by the roof membrane, thus leading to cooler temperatures beneath the surface. Based on the depth of the planting medium and maintenance two main types of green roof are usually distinguished; they are intensive and extensive green roofs.

- Intensive green roofs are established with deep soil layers. They can support plants and bushes and typically require maintenance in the form of weeding, fertilizing and watering. Generally intensive green roofs can support complex vegetation like groundcovers, small trees and shrubs which has deeper rooting. Intensive roofs involve a greater load of more than 150 kg/m2 and have more than 200 mm of substrate with higher amount of organic material than extensive systems (GMbh, 2000). Intensive green roofs are typically installed on roofs with a slope of less than 10-.

- Extensive green roofs have a thin substrate layer with low level planting, typically sedum or lawn. They are planted with smaller plants which in the final stage are expected to provide full coverage of the vegetated roof. Extensive roofs are intended to be self sustaining and require minimal maintenance. It can be distinguished by being low cost, lightweight (50-150 kg/m2) and with thin material substrate of up to 150 mm. This type may also be installed on sloped surfaces. The slope angle can be as high as 45-.

3. Factors affecting stormwater retention

Green roof characteristics

Green roof characteristics such as number of layers and type of materials, soil thickness, soil type, vegetation cover, type of vegetation, roof geometry: slope, length of slope, roof position and roof age affects the performance of stormwater retention. Table 1 presented that the annual runoff is mainly determined by the roof type and may be as high as 91% for a traditional non-greened roof and as low as 15% for an intensive green roof; and 19% for an extensive green roof. The different studies on slope influence on green roofs runoff retention capacity bring different results. While some studies find no correlation between roof slope and runoff (Bengtsson, 2005; Liesecke, 1998), the others observe that runoff retention may depend on slopes. Mentens et al. (2006) stated that the annual precipitation, roof type, depth of substrate layers is significantly correlated with the yearly runoff while they did not find any significant relation with the age of the green roof, slope angle and length.

Table 1. Substrate layer depth (mm) and runoff (% of total annual precipitation) characteristics on an annual level (Mentens et al., 2006).

Substrate Layer (mm)

Runoff (%)

Minimum

Maximum

Average

Minimum

Maximum

Average

Intensive Green Roof

150

350

250

15

35

25

Extensive Green Roof

30

140

85

19

73

46

Gravel covered Roof

50

50

50

68

86

77

Non-greened Roof

-

-

-

62

91

77

Getter et al. (2007) has reported that retention values decreased as slope increased and was significant for slopes between 2% and 15% as well as between 2% and 25%. They found that organic matter content and pore space of soil doubled in 5 years time (from 2% to 4% and from 41% to 82%, respectively).

Weather conditions

Weather conditions such as length of proceeding dry period, season/climate (air temperature, wind conditions, humidity), characteristics of rain event (intensity and duration) also affect the stormwater retention. Kaufmann (1999) compared the runoff percentage during winter and summer for both 5 cm gravel roof and 1cm extensive green roof and found the runoff was significantly higher during winter time, 86% for gravel roof and 80% for green roof; where as runoff 70% in the summer for gravel roof and 52% in the summer for extensive green roof. Carter and Rasmussen (2006) found that peak discharge for small storm was much lower from the vegetated roof than a conventional roof but this effect was much reduced for larger storms; 57% of peaks on a vegetated roof were delayed up to 10 min as compared with peaks from a conventional roof. Green roofs generally delayed runoff (peak to peak) by 10 min (VanWoert et al., 2005; Simmons et al., 2008). DeNardo et al. (2005) showed that green roofs reduced the peak intensities from an average rainfall intensity of 4.3 mm/h to an average green roof runoff rate of 2.4 mm/h. Moran et al. (2005) reported that 90% of rain events delays of runoff were observed and minimum 30 min delay for 60% of rain events was observed. Bengtsson et al. (2005) showed that weather conditions (dry or wet) affected the retention capacity of studied green roof; for dry conditions 6-12 mm rain were required to initiate runoff; for wet conditions the response was almost straight.

Green Roof- Stormwater runoff relationship

Green roof reduces stormwater runoff compared with that from a hard roof through lowering and delaying the peak runoff. A certain water volume is detained in a green roof and its substrate layers. A portion of detained water will drain and a portion corresponding to field capacity will be retained. The retained water will evaporate or be used by plants and parts of it will transpire. Through evaporation and transpiration of water runoff volume reduction from green roofs occurred.

Since green roofs retain stormwater, they can mitigate the effects of impervious surface runoff. Peck (2005) estimated that if 6% of all buildings in Toronto had green roofs, it would result in the same stormwater retention impact as building a $60 million (CDN) storage tunnel. Likewise, in Washington, DC, if 20% of all buildings that could support a green roof had one, they would add over 71 million litres (19 million gallons) to the city's stormwater storage capacity and store approximately 958 million litres (253 million gallons) of rainwater in an average year (Deutsch et al., 2005). In the United States, the combined sewer overflows (CSOs) discharge about 850 billion gallons (3.2 trillion Litres) of untreated sewage and stormwater in thirty two states and the District of Columbia every year (EPA 2004a). New York Harbour alone receives more than 27 billion gallons (1 trillion Liters) of sewage and polluted runoff from an average of 460 CSOs every year (Storm water infrastructure matters (SWIM) coalition 2008). In Washington, Seattle city, 815 million gallons (3.1 billion L) from eighty seven CSOs were discharged into local water bodies from June 2007 to May 2008. Sanitary systems can also become overloaded during heavy storms and discharge sewage; the EPA estimates that this happens about 40,000 times every year.

In some highly urbanized societies like Japan, Singapore, Germany and Belgium the advantages of green roofs have already resulted in incentives from the government to encourage or even impose the use of green roofs (Osmundson, 1999; Wong et al., 2003; Dunnett and Kingsbury, 2004). Fig. 1 illustrates the reduction in peak runoff from a green roof, as observed in Belgium during a rainstorm.

Fig. 1. Typical cumulative runoff from a non-greened roof (20 ÌŠslope) and an extensive green roof (20 ÌŠslope) as observed in Leuven (Belgium) during the 24h period of a 14.6 mm rain shower (April 2003, 5 pm-5 pm on the next day) (Mentens et al., 2006).

The vegetated rooftop project at the Fencing Academy of Philadelphia is a 3,000 square-foot vegetated cover installed and monitored by Roofscapes, Inc., on top of an existing structure. The model (Fig. 2) predicted a 54 % reduction in annual runoff volume.

Fig. 2: Runoff attenuation efficiency for a 0.4-inch rainfall event with saturated media.

The model also predicted attenuation of 54 % of the 24 hour, 2 year Type II storm event and 38 % of the 24 hour, 10 year Type II storm event. Additionally, monitoring at a pilot-sized project for real and synthetic storm events was conducted by Roofscapes, Inc. for a period of 9 months at 14 and 28 square-foot trays. The most intense storm monitored was a 0.4 inch, 20 minute thunderstorm. The storm event occurred after an extended period of rainfall had fully saturated the system. Although 44 inches of rainfall was recorded during this period, only 15.5 inches of runoff was generated from the trays. Runoff was negligible for storm events with less than 0.6 inch of rainfall.

Liu (2003) reported that extensive green roof reduced stormwater runoff by 54% (April to September' 2002). Gutteridge (2003) estimated that 3.6 million cubic meters (127 million cubic feet) of stormwater can be retained if 6 percent of the total roof area in Toronto is covered with green roofs (6.5 million square meters or 70 million square feet). Many European cities, as well as several cities in the United States, now charge developers and building owners fees for hook up to the storm water system, based on the amount of discharge produced by the site. In 1996, the State of Illinois passed a law that promotes the planting of buffer zones at grade, to reduce stormwater runoff, in return for a reduction in property taxes. There is also a gradual move in North America toward stormwater user fees, which are based on the degree of impermeable surfaces on a given site.

The city of Cologne, Germany, receives 27% more rainfall than surrounding areas. In cities already plagued by overextended stormwater systems and combined sewage overflows, the problems caused by severe rainfall are likely to worsen with global climate change. If sufficiently implemented in an urban area, green roof systems can help to improve stormwater management. German studies from 1987 to 2003 as summarized by Mentens et al. (2006) report that intensive green roofs showed annual runoff reduction equal to 85%-65% of annual precipitation (100%) and for extensive roofs the corresponding values were 81%-27%.

Some studies find no correlation between roof slope and runoff (Bengtsson, 2005; Liesecke, 1998; Mentens et al., 2006) whereas the others observe that runoff retention may depend on slopes (Getter et al., 2007; VanWoert et al., 2005; Villarreal and Bengtsson, 2005). The effect of the slope on runoff retention combines with the effect of other factors as the physical properties of the roof substrates, length and intensity of precipitation event studied and flow conditions (saturated or unsaturated, overland flow or not), the design of green roof layers and different type drainage materials or systems (Berndtsson, 2010).

Discussion

This review paper addresses the role of green roofs in mitigating urban stormwater runoff problem. In the literature, potential environmental benefits are common but scientific evidence of some benefits is not sufficient. Some contradictory results are presented by different authors in their study. This is due to differences in the study conditions, weather, type of green roofs and plants. On a per roof basis, Fig. 3 shows the storm water mass balance model predicted that an extensive green roof can reduce roof runoff volumes by approximately 65 %, while an intensive roof can reduce runoff by 85 %. Using a combination of 80% extensive and 20% intensive ratio across all green roof-ready buildings in the District, roof runoff volume would decrease by as much as 69% as compared to conventional rooftops (Figure 3).

Fig. 3: Comparison of roof runoff for conventional roofs and green roofs (LIMNO-Tech, 2005, Re-Greening Washington, DC)

From the extensive literature review, it is clear that stormwater retention capability may range from 75% for intensive green roofs to 54% for extensive green roof (Table 1). Although intensive green roofs retain highest water due to its soil thickness and physical properties, it can not be easily retrofitted into the existing roofs. For that reason it is not a suitable option. Intensive green roofs are suitable for new structure which can carry relatively higher load than extensive green roof. For existing building extensive green roof can be an ideal solution for stormwater runoff management.

Concluding Remarks

The review indicates that there is still need of more research on green roof performance in urban environment. It is clear that roof greening alone will never fully solve the urban runoff problem and it needs to be combined with other runoff reduction measures (e.g. storage reservoirs in urban green or under infrastructure, rainwater cisterns, an increase of green areas). Models integrating all these on various time scales are clearly needed if we really want to predict runoff for more efficiently.