Ilots de chaleur

Heat islands: when water refreshes the city

Due to their high mineralization*, city centers experience greater temperature rises in summer, creating urban heat islands (UHI). This phenomenon can have heavy consequences, especially in terms of health, ranging up to an increased mortality rate for inhabitants.


Veolia pilot projects underway or completed

2011: microclimate simulation for the Saint- Augustin PEM in Nice (France)
2012: road humidification pilot scheme in Lyon (France)
2013: start-up of the EVA project with IRSTV in Lyon (La Part-Dieu district) (cf. article opposite)
2014: development of a health vulnerability diagnosis tool
2015: field tests of the vulnerability diagnosis tool in Lyon; EVA project results
2016: first commercial deployment of the diagnosis and urban cooling solution design offerings
2018: start-up of the first urban cooling demonstrator based on evaporative pavements in Toulouse Montaudran Aerospace mixed development zone (unique in Europe)
2018: series of measures in the PEM in Nice over the summer
2019: pilot scheme underway in Milan (Italy)

In response, for the past several years Veolia has been developing diagnostic tools and innovative solutions to mitigate UHIs, based in particular on the use of non-potable water. Developing these solutions, which can be equally applied to a neighborhood or an agglomeration, requires experiments to be carried out on the ground.

In Lyon, France, Veolia devised a UHI characterization method in the Part-Dieu district, before testing a road humidification solution there. This method overlays two types of mapping: firstly, the city’s exposure to UHIs, weighting several indicators (surface temperatures, presence of vegetation and water, etc.) correlated with in situ measures; and secondly, the sensitivity of populations in terms of health impact, using indicators such as population type (especially the elderly and children) and type of housing (precarious housing amplifies the health impact of UHIs). The aim of this mapping is to identify vulnerable areas to be treated as a priority.

In the wake of this mapping, Veolia joined forces with the Institute for Research on Urban Sciences and Techniques (IRSTV) to develop EVA (water, vegetation, albedo), a multi-criteria decisionmaking tool for cooling solutions aimed at developers. EVA models cooling solutions and compares their impact on reducing UHIs and the water consumption that they involve. Two complementary approaches are prioritized when it comes to water solutions: humidification for roads, and evaporative pavements – which are more fragile due to their porosity – solely for pedestrian areas.

In Nice in the south of France, Veolia uses a combination of cooling solutions that were studied as part of the creation of public spaces in the Var plain’s multimodal transit hub (PEM). The end result has been the implementation of two cooling solutions: 250 m2 of evaporative pavements at tramway stops and 300 m2 of road humidification, all connected to the city’s raw water distribution network. Ahead of the next series of measures scheduled for summer 2019, September 2018’s campaign has already led to a perceived temperature drop of 5 to 6°C in the test area.

* Mineralized surfaces absorb heat during the day and release it back into the atmosphere at night.

Chiffres clés

Almost 30% of the global population is exposed to climate conditions exceeding a potentially deadly threshold for at least 20 days per year. By 2100, this figure is set to reach 48% – in a scenario of drastically reduced greenhouse gas emissions – and 74% in a scenario of increased emissions.
Source: Nature Climate Change, “Global risk of deadly heat,” June 2017

By 2100, two in three Europeans will be affected by climate disasters. The number of annual deaths due to the latter will rise from 3,000 – observed from 1981 to 2010 – to 152,000 at the end of the century. While floods and storms represent very serious threats for the European population, heat waves will be the deadliest climate events. They will cause no fewer than 99% of the total deaths expected. In other words, 151,500 deaths (uncertainty bounds: between 80,000 and 239,000) by 2071.
Source: The Lancet Planetary Health, “Increasing risk over time of weather-related hazards to the European population: a data-driven prognostic study,” August 2017

The urban heat island phenomenon in the United States concerns over 80% of the population living in urban areas.
Source: Physical Review Letters, study March 2018