Green roofs as stormwater measures

Urban growth, densification and climate change put increased pressure on existing drainage systems. The natural drainage patterns change with consequences for, among other things, groundwater level and water flow in urban waterways. One way to reduce these unfortunate consequences is to base stormwater management on nature's water management principles. Green roofs are an example of such measures where water is temporarily stored in the roof structure and later removed by evapotranspiration (evaporation and plant water consumption). In addition, the water flowing from the roof will be delayed in time compared to runoff from a regular roof. A new PhD Thesis by Birgitte Gisvold Johannessen shows that extensive green roofs can remove a significant proportion of the rainwater from a roof. This will result in reduced volumes of water to be transported and purified, which is particularly useful in areas with a common drainage system (where stormwater and sewage are fed into the same pipe).