Water attenuation

...and stormwater management implications
   of Green roofs.

Rainfall in Urban areas is typically more problematic than in rural environments. Under natural conditions, precipitation is impeded from running off by vegetation, ground surface retention and subsurface storage. The retained rainwater will contribute to the soil moisture and ground water replenishment.

Urban landscapes are dominated by impervious surfaces, such as pavements, building walls and roofs and paved car parks and roads. These collect the flow and direct it into storm drains, sewers and engineered channels (the urban drainage system) urban runoff eventually reaches receiving waters as sudden uncontrolled surges. Many surface contaminants are carried with this torrent of stormwater. Common contaminants include suspended solids, heavy metals, chlorides, oils and grease and other pollutants that arise from the use of roads and other surfaces that water has passed over. Increased land development results in increased runoff volumes, runoff velocities and peak discharge rates.

Green roofs act as a stormwater management device, quite simply because they replicate the open space previously at ground level. Vegetated roofs play an important role in modern urban drainage because of their ability to slow down and reduce runoff response. Unlike some other best management practices, green roofs may be able to offer controls and improvements in both the quantity and quality of stormwater runoff. The temporal storage of water in the soil and vegetation reduces peak flow, which prolongs the time-of-concentration, which means that local urban flooding and combined sewers overflows can be lessened. (CSO)

Green roofs have great potential benefit in terms of protecting water health and reducing flood risk to urban areas. It has also been shown that green roof redevelopment on existing buildings could help to restore watershed health over time. Graham and Kim (2003) Not only are green roofs able to filter contaminants out of rainwater that has flowed across the roof surface (Dramstad et al., 1996), but they can also degrade contaminants, either by direct plant uptake, or by binding them within the growing medium itself (Johnston and Newton, 1996).

Numerous studies have demonstrated quantitatively that a properly installed and maintained green roof will absorb water and release it slowly over a period of time, as opposed to a conventional roof where stormwater is immediately discharged. Typical extensive green roofs, depending on the substrate depth, can retain 60 to 100% of the stormwater they receive (Thompson, 1998). In addition, living roofs are normally able to retain 70 to 80% of the stormwater that falls on them during the summer months, depending on the frequency of rain and drying rates. In winter months, green roofs are predicted to retain 25 - 40% of the stormwater. These data are subject to variation based on variations in climatic conditions. The amount retained also depends on numerous factors such as the volume and intensity of rainfall, the amount of time since the previous rainfall event, and the depth and saturation level of the existing substrate (Monterusso, 2003).

Green roofs can retain up to 60% of stormwater on an annual basis (Liesecke, 1993). Liesecke also indicated that there were noticeable differences between retention in warm weather and in cool weather. In warm weather, shallow substrate depth can retain 11% more stormwater than it can during cold weather (Liesecke, 1993). Liptan et al, (2003) demonstrated similar findings. Within a 15-month monitoring period, they found that precipitation retention was approximately 69% of the total. However, between December and March the rainfall retention was 59%, while from April to November, rainfall retention was 92%. Research conducted by Jennings et al. (2003) in North Carolina showed that a green roof can retain up to 100% of the precipitation that falls on it in warm weather. However, the percentage retained for each storm decreased when there had not been an adequate amount of time between each storm event.

During a heavy downpour, pollution is mobilized from impermeable surfaces and transported with the water. Green roofs not only reduce the quantity of runoff from roofs but can also filter contaminants from rainwater. Runoff from urbanized areas is the leading source of water quality impairment. Most of the stormwater runoff enters water bodies directly without any treatment.

The substrate on the Green roof has the ability to retain particulate matter in the stormwater and to reduce the quantity of the runoff and, as a result the total mass of pollutants that flow off the roof. Thus, the stormwater runoff quality as well as the receiving surface water quality can be improved.

A number of studies have looked at green roof runoff quality. Dramstad et al. 1996 showed that the physical and chemical properties of the growing substrate, and the vegetation itself, help to control the nitrogen phosphorous and contaminants. These are either broken down by the plants themselves, or more usually bound in the substrate instead of being discharged in the runoff.

Of the many benefits of green roofs the benefits from stormwater flow reduction including impact on the combined sewer overflow (CSO) are included in those with the most quantifiable monetary value. Capital expenditures and operating costs for wastewater treatment in combined sewer areas and stormwater treatment in separated sewer areas are typically assumed to be lessened by the rainfall captured by green roofs. Acks (2003)