Green Roofs and Stormwater Management
Using Living Roofs to Reduce Flood Risk and Damage to Ecosystems
As urban areas continue to expand, stormwater runoff has become an increasing concern. Through the construction of city infrastructures, naturally porous landscapes that allowed water to be absorbed into the ground have been replaced with impervious structures such as buildings, roads, and parking lots. Since water from rainfall and snowmelt cannot be absorbed, it collects on the surface and flows into waterways at increasing volumes. The traditional methods for handling runoff by diverting it through underground pipes and storage tanks has caused additional environmental damage by eroding riverbanks, transferring pollution into waterways, and damaging wildlife habitats.
Integrated Stormwater Management
Today many city planners and stormwater managers are looking for ways to control wet weather flow (rainfall and snowmelt) at its source. These integrated stormwater management plans attempt to emulate the natural flow of water before the area was developed. Green roofs are an important element in these plans.
How Green Roofs Control Runoff
A green roof is a building covering that incorporates vegetation in the engineered design. The plants included in the green roof essentially replace the natural vegetation that was in the area before the building was constructed. The materials used to construct the green roof work as a water retention system that reduces the amount of runoff from the roof. In their report for the Environmental Protection Agency, Robert Berghage et al compared a green roof with an asphalt roof and determined that the green roof reduced the water runoff by 50%. ("Green Roofs for Stormwater Control", Feb 2009) Similar studies with other traditional roofing materials have demonstrated runoff reduction rates between 50 and 70%.
Other Effects of Green Roofs
In addition to reducing the volume of runoff, green roofs also affect runoff peak flow and water quality.
Peak Flow
At the beginning of a rainfall, the green roof retains all the precipitation until it exceeds the storage capacity. This process delays the start of the runoff and releases runoff more gradually than traditional roofing systems. Berghage et al noted that when compared to asphalt roofs, the runoff from green roofs at the peak flow period was reduced by half.
Water Quality
Water runoff from traditional roofs often picks up contaminants from roofing materials. In his examination of research results on various traditional roofing systems including galvanized metal, tar paper, and shingles, Thomas Schueler found reports of increased levels of zinc, copper, and lead (“Is Rooftop Runoff Really Clean”, The Practice of Watershed Protection, 2000). These findings have been replicated in many other studies of rooftop runoff. In some cases, the runoff contained these substances at concentrations known to be toxic to fish in neighboring habitats.
Green roofs are not immune to leaching substances into the runoff. Berghage et al reported increased levels of several nutrients (phosphorous, potassium, calcium, iron, magnesium, manganese, sodium, and zinc) in runoff from green roofs as compared to asphalt roofs. They indicate that these values are similar to the levels expected in leaching from any planted area and may not have a negative impact on receiving waters. However, they do recommend that runoff from green roofs receive additional treatment, such as routing it through a swale, bioretention system, or rain garden as part of an overall stormwater system.
Green Roofs as Part of an Overall Stormwater Management Plan
Green roofs offer many benefits to effective stormwater management systems; however, they do not preclude the use of other best management practices. The green roof implementation must consider the usual precipitation in the area, the type of growing medium and plants, the effect of fertilizers, the sensitivity of surrounding habitats, and the need for additional water treatment for the runoff. With proper implementation and maintenance, green roofs can help to manage wet weather flow from urban areas that often leads to flooding and damage to the ecosystem.
River
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Runoff Draining into River
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Green Roof Drainage Layer
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