As we head into summer, Europe is sweating under another severe, record-breaking heatwave, with France, Germany, Austria, and the Czech Republic all reporting new national temperature records. Temperatures on the West and East Coasts of the US are also expected to be warmer than average this year.
Urban centers can get particularly hot, thanks to urban heat islands. Humans, cars, buses, and the like all give off heat, and there are a lot of them in cities. Buildings, which are often made of glass and metal, and pavement parking lots and sidewalks can hold on to and reflect heat. Since higher temperatures are likely the new norm, cities are starting to think about how to manage and prevent extreme heat on the local level.
The most popular city-level solution to the urban heat island effect, increasing green infrastructure, has the added benefit of improving stormwater runoff quality as well. Planting urban trees is the most obvious green infrastructure plan. In addition to improving air quality, trees and grasses don’t reflect heat the way man-made materials do. And, of course, they retain stormwater and reduce peak flows.
In Paris, there are plans to plant trees and grasses around four of the city's landmark sites and 28 children’s playgrounds, reports CityLab. The plan is part of the city’s goal to replace 50% of the city’s surfaces with permeable pavement or vegetation by 2050. Part of this plan also includes reducing parking in the city—the mayor of Paris notes that during her term, traffic has dropped 5% per year. Replacing parking lots with green spaces would serve a dual purpose of improving runoff quality and reducing heat in the city.
However, surfaces may only be replaced with trees and grasses to a certain point. Dense shady forest canopies are simply not achievable throughout a city. But some research suggests housing density can also affect heat island effects. Data collected in Washington DC, and reported by Greater Greater Washington, suggest temperatures were highest in “low-slung, high-density areas with very little tree cover and wide, multi-lane streets,” but varied more in areas with narrower streets and taller, more varied building heights. Taller buildings over narrower streets offer more shade, which may be mitigating some of the man-made effects of urban heat islands. Adding green roofs or rain gardens to the tops of high-density buildings would certainly help as well.
What insights do you think stormwater professionals have to offer in the efforts to combat urban heat islands? As the new editor of Stormwater magazine, I want to hear your thoughts. Send your comments to email@example.com.