Ilots de chaleur urbains et Urban Design : recherche d’une urbanité tropicale plus résiliente

Abstract

The fast changing climate has increased the number of weather-related disasters (IPPC, 2014). In Australia, heatwave is a major issue as it is considered the deadliest natural hazard in the country, causing 55% of natural hazard related death (Coates et al. 2014). Often being called the silent killer, heatwave is defined as an unusual long period of excessive heat for the local climate (Perkins & Alexander 2013). Precedent research has demonstrated that urban heat islands (UHIs) exacerbate the extreme heat caused by heatwaves and result in adverse effects to human health and wellbeing. This phenomenon, first revealed in 1818 when an ‘artificial excess of heat’ in the urban area of London was found by Luke Howard, is described by higher air and land surface temperatures in the urban area compared to the rural area (Gartland 2008). Due to urbanization, the rural landscape is altered by more impervious roads, less vegetation, heat retainer materials, high urban building density and excess of anthropologic heat, all of which are producing the UHI effect. In summer nights, as city surfaces releases the excess heat of the day, minimum temperatures are too high for the city to cool down, thus causing heat stress on the urban dwellers. Therefore, mitigation and adaptation strategies have been developed to counteract this UHI effect. Many studies focus on strategies that will help lower the temperatures of UHI by evapotranspiration and shading produced by more green and blue infrastructure, facilitating wind flow with urban morphology and choosing the building materials considering their reaction to heat. Sadly, most of the papers in the literature are often about only one or two of these aspects. But to ensure adaptation and resilience to the UHI effect, all these strategies must be mutualized and combined to a good urban design understanding the residents lifestyle (Greener Places 2017). Indeed, it is needed to improve the life of citizens in the long term by creating a vital and resilient city, thriving even when exposed to an extreme heat event. Yet, little research has been done to analyze the relation the urban dwellers have with public spaces in days of extreme heat. This is surprising considering that urban design strategies could help improve health condition in more way than one. Indeed, many case studies show that urban design can have a huge influence on the physical activity of the people living in the city and thus, on their physical and mental health (Badlang and Schofield 2005). By encouraging city walkability, public transport accessibility, shades and mixed land use, not only can it help decrease the temperatures by reducing car induced excess heat, it can also improve the general air quality of the city and the physical health of the urban dwellers. Consequently, to ensure a strong resilience to heatwaves in Australian cities, UHI mitigation strategies via urban design should be allied to the knowledge and the participation of the people who are going to experience the city (Hatvani-Kovacs and Boland 2015). Furthermore, the UHI effect on people is not only depending on the urban design of the city but also on its geographical location. The strength of the UHI effect on people will likewise be influenced by the weather they are used to. Thus, their involvement is required to understand better their behavior in the urban area in normal settings and in extreme heat situations. In this study, we compare two Queensland tropical towns which were severely impacted by the 2018 heatwaves: Cairns and Townsville. Through the analysis of their urban design features in relation to temperatures, and the assessment of the physical and psychological relationships citizens have with their urban environment, the aim is to identify historical and contemporary spatial dominant discourses, what to improve and how to develop future adaptation strategies. This first phase is part of a larger research that aims at defining a new and more resilient urban tropicality.