Modeling the Stormwater Benefits of Green Roofs in the City of Toronto
Stormwater best management practices (BMPs) provide a number of tools to reduce the quantity and improve the quality of stormwater runoff at the source, along the drainage system and at the drainage outlet. These include devices such as downspout disconnection, stormwater gardens, green roofs, rain barrels, infiltration trenches, stormwater exfiltration/filtration systems, sand filters, bio-retention areas, wet and dry detention ponds, and constructed wetlands (Banting et al. 2005). Most end-of-pipe BMPs require a significant amount of land to host them, which is not generally available in downtown urban environments. The opportunity for green roofs to act as source level viable BMPs is logical in cities such as Toronto, since flat rooftops recreate the open space, previously at ground level, that has otherwise been eliminated for vegetation (Jennings et al. 2003). This chapter focuses on green roofs and presents a case study of the stormwater and combined sewer overflow control benefits of implementing GRT over the City of Toronto (Banting et al 2005).
Green roof technologies (herein after GRT) are not totally new and can be defined as a green space created by adding layers of growing medium and plants on top of a traditional roofing system. This should not be confused with the traditional roof garden, where planting is done in freestanding containers and planters, located on an accessible roof terrace or deck. GRT have been around in different forms all over the world for years and their insulative qualities have been used to keep dwellings cool in Tanzania, and warm in Scandinavia. The stormwater performance of GRT is dependent upon the soil substrate and the vegetation which in combination provide interception, storage, evapo-transpiration, and attenuation of stormwater.
There are two categories of GRT. Intensive GRT are characterized by deep substrate (greater than 300 mm) and large vegetations (e.g. shrubs and trees) while extensive GRT are characterized by shallow substrate (less than 300 mm) and ground covering vegetation (e.g. sedum and turf). Extensive GRT are primarily used in existing buildings with structural limitations. Based on the Ontario Building Codes’s loading requirements, Au (2007) estimated the maximum substrate depth of extensive GRT for residential, commercial, and industrial buildings in Ontario are 150 mm, 200 mm, and 200 mm respectively. Intensive GRT are usually designed for new buildings as the loading support can be incorporated in the structural design of the new buildings. For instance, the Isuzu dealership building in Singapore has a SUV driving test range on the top of the six storey building with 1 m depth substrate and safari vegetations. Extra structural support was incorporated into the structural design of the building.
Studies have shown that GRT could provide both quantity and quality control of storm runoff in urban areas (Dramstad et al. 1996; Graham and Kim 2003; TRCA 2006). While greenroofs may not provide adequate control over large storms, they have the ability to intercept, store, and attenuate most of the moderate and small events (Li 2006). Thompson (1998) found that a typical extensive GRT can retain 60 to 100% stormwater. According to the Zinco planning guide (1998), GRT are expected to retain 70-90% and 40-50% stormwater during the summer and winter months respectively. Liesecke (1993; 1998) estimated that a GRT of 2 to 4 cm and 60% of substrate can retain 40-50% and 60% of the annual rainfall respectively. Based on a 15-month field monitoring results, Liptan et al. (2003) found that 69% of the rainfall was retained by a GRT. Jennings et al. (2003) and Rowe et al. (2003) found that a GRT can retain up to 100% of rainfall volume dependent upon the volume, intensity, and interevent time of rainfall events. Additionally, Jennings et al. (2003) and Liu (2003) found that a GRT can also attenuate the peak flow of a light rain (19 mm in 6.5 h) to up to almost 95 minutes.
While the above research studies have indicated the stormwater control performance of a GRT, there are not many studies that focus on their performance on a watershed basis. Marshall Macklin Monaghan Ltd. (2004) developed HSPF’s Unit Response Functions (URF) of GRT using the 2003 field monitoring data of the York University GRT (TRCA 2006). Using the HSPF’s URF of GRT, Aqua Beech Inc. (2004) simulated the effectiveness of implementing GRT within the Markham Branch of Highland Creek and found annual runoff volume could be reduced by 5%. Casey Trees and LimnoTech (2007) developed a green build-out model to quantify the stormwater management benefits of trees and green roofs in Washington, DC. A methodology to estimate the stormwater benefits of greenroofs in the City of Toronto, Ontario, Canada is described below.
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