With the end-of-winter thaw just around the corner, melting snow and ice will have nowhere to go but the sewer system. This can create problems in urban environments with large amounts of impenetrable surfaces like roads and roofs, as precipitation does not naturally soak into the ground. Stormwater management is therefore a priority in order to prevent flooding and damage.
In natural conditions, stormwater — either rain or melted snow — is absorbed into the ground, where it is filtered before either replenishing aquifers or flowing into streams and rivers.
According to the Toronto and Region Conservation Authority, the runoff from precipitation in urban areas runs through storm drains, municipal sewers and drainage ditches into natural bodies of water. In doing so, it picks up pesticides, road salts, heavy metals, oils and bacteria. The polluted runoff can cause downstream flooding, aquatic habitat destruction, sewage overflow and infrastructure damage.
There are systems that can improve stormwater management, which also have added benefits to the buildings around them. Two projects in Southern Ontario — Environment 3 (EV3) at the University of Waterloo and the North Toronto Collegiate Institute — have utilized two different types of systems that do just this.
At EV3, a constructed wetland mimics a natural wetland by filtering water. Constructed wetlands are beneficial, as they increase water efficiency and decrease the load on municipal stormwater systems.
Located at grade level, the constructed wetland filters both rainwater funneled off the roof, and greywater from kitchen and bathroom sinks through a vertical flow system. The vertical flow system has water applied to the wetland surface, which flows downward. The plants filter out contaminants from the two water sources before pumping it back into EV3 to flush toilets and water a living wall.
There are two types of constructed wetlands: those that are located at ground level, and those that are located at roof level (and are often integrated into green roofs). There are advantages to both systems, but there is more maintenance associated with roof systems.
“The advantage of the ground level system is twofold,” says Paul Parker, professor and director of the economic development program at the University of Waterloo. “Capacity can be larger on the ground without concerns about structural strength on the roof; maintenance is easier and the potential damage is much less if a leak occurs on the ground versus the roof.”
Wetland roofs are a special type of extensive green roof system that includes wetland or marsh plants. Because of the types of plants, the systems have to be irrigated on a daily basis. This differs from the standard extensive green roof, which can be self-sustaining and not require irrigation.
At the North Toronto Collegiate Institute (NTCI), a redevelopment included the construction of a new four-storey school, which utilized original historical elements, the development of two residential condo towers, as well as an artificial turf field and a tree-lined public walkway. Covering 80 per cent of the school’s roof is an extensive green roof system, which is connected to a stormwater management system.
“The green roof layers are as follows: drainage layers, filter fabric, water retention fleece, pre-cultivated vegetation mat, growing medium, and finally ballast. The plant material is contained on pre-cultivated vegetation mats,” says Susan Spencer Lewin, principal and director of sustainable strategies at CS&P Architects.
In this system, rainwater is stored under the growing medium in the retention fleece. Water that gets through to the drainage layer moves to roof drains, which slowly discharge the water using control flow features. The water then travels through internal rainwater leaders within the building to the storm system.
Lewin says that the artificial turf, associated open space and walkways at the NTCI generate considerable stormwater. Under the south end of the field is a system of catch basins that lead to a three-chamber stormtech system, which manages the water and provides retention and detention systems.
“The open bottom chamber is used for retention, where storm water infiltrates into the underlying soils, and for detention where peak flows are attenuated using an outlet control structure,” Lewin says.
In the event of a major storm that exceeds the storage capacity of the roof, there are overflow roof scuppers in place, which creates an overland flow route to a flow outlet.
Both the extensive green roof system at NTCI and the constructed wetland at EV3 ultimately use different methods to achieve the same results: to control the amount of stormwater entering into the system, and reduce the burden on the municipal stormwater management system.
Leah Wong is the online editor of Building Strategies & Sustainability magazine.