Cities are hotter than their surrounding countryside, maybe as much as 10F hotter. Impervious surfaces like buildings, sidewalks, roads and parking lots are primarily to blame. Louisville can be as much as 20F hotter than the surrounding countryside.
High city temperatures are bad for people: One study in California showed a 2.3% bump in mortality for every 10F increase in temperature throughout the day. The 1995 Chicago heat wave is blamed for 700 deaths. Not only is city heat bad for people, but it’s bad for transportation infrastructure, too. Prolonged high temperatures causes asphalt to soften, expand and form potholes and ruts. Railroad tracks warp. Hot air impacts airplane liftoff performance. Electrical transmission lines and transducers are less efficient. Louisville Urban Heat Management Study is currently out for public comment. Developed by Dr. Brian Stone, regional planning professor at Georgia Tech University, the study is the first heat management assessment ever done by a major U.S. city.
Dr. Stone was quoted in the Louisville Courier-Journal, “We find that overall, through actions that are achievable through public policy, more than 20% of the annual heat-related deaths in Louisville could be avoided,” he said in an interview. “We see reductions in summer averages of 1-degree Fahrenheit or more, which is significant when averaged over several months.”
Five primary actions are presented as ways the city can mitigate heat.
1. Use cool materials in areas with extensive impervious surfaces where vegetation isn’t cost effective. This can include cool, white or light-colored roofs, permeable pavement, etc.
2. Use vegetation where possible, especially where people live. Residential areas offer low-cost planting options. The report recommends planting as many as 450,000 trees.
3. Pursue energy efficiency.
4. Manage heat in every zone of the city with a plan for heat adaptation.
5. Develop economic and regulatory programs and incentives to encourage desired land cover changes and energy efficiency.
Plants are a big piece of the solution. Evapotranspiration, when plants take water from the ground and transpire it into the atmosphere through their leaves, has a cooling effect. The report states that one oak tree transpires 40,000 gal. of water a year – that’s a lot of cooling! Trees and plants also cool by shading the ground. Green roofs can also play an important role in cooling cities: A green roof can be as much as 90F cooler than a conventional roof in the summer.
The draft report offers that increasing urban tree canopy and grass cover can be accomplished through a new zoning tool, “green area ratio.” Such a policy would set minimum green cover targets for all parcels. Targets could be met through any number of ways: Lawn, planting trees, gardens, green roofs, green walls, planters, permeable pavement, rainwater capture, etc. Such a policy would be applied to new development. Two cities that have adopted similar tools are Seattle and Washington, D.C.
A second new policy would also help to cool the city using plants. In this case, the city would limit the amount of barren land allowed on a parcel. These non-vegetated portions of a site add to heating much like roadways. These areas can be converted to grass.
Other policies that will help areas unable to meet green area requirements include converting parking lots to tree canopy and non-residential buildings to green roofs.
The assessment estimates that 23.5 square miles can be converted to new green cover.
Director of Sustainability Maria Koetter said, “We are reviewing the data and evaluating which strategies can be implemented near-term and long-term. Combatting our heat island issues will be a multi-year process and will require communitywide action.“
Importantly, the report asks the question: Are cool materials better in cooling Louisville than vegetated strategies? On the surface, it would look to be so since there’s so much potential: almost 70 square miles of impervious surfaces that could be converted to cool surfaces. However, it’s impractical and prohibitively expensive to convert all roofing and pavement to green and cool materials.
Besides that, vegetated solutions are better on a per area basis: Each new square mile of tree or grass cover is 1.2 times more effective in lowering temperatures than each new square mile of cool materials. The challenge, especially with green roofs, is lowering costs to be comparable to cool materials.
NewTerrain June 15, 2016.