A risk index for assessing heat stress mitigation strategies. An application in the Mediterranean context

In many Mediterranean cities, there are densely built up settlements characterized by the lack of green spaces and the abundant use of impervious construction materials, so the phenomenon of heat stress is exacerbated and mitigation strategies are key to improve thermal conditions of pedestrians. The present study aims to evaluate the effectiveness of heat stress mitigation strategies by proposing a new heat stress risk index that comprehensively accounts for hazard, vulnerability and exposure of the urban context. The index is tested with the assessment of two different greenery scenarios based on the introduction of vegetation in the existing street spaces rearranged according to the principles of traffic calming and vertical vegetated surfaces. These scenarios have been simulated in a district of the metropolitan city of Catania (Italy) through the combined use of GIS software and Computational Fluid Dynamics (CFD) simulations in ENVI-met. Results showed that mitigation scenarios improve outdoor thermal conditions for pedestrians because about 66% of the domain area showed Tmrt reductions in the range of 2.5°C to 7.5°C with respect to the existing scenario. Furthermore, the no hazard area for 80+ residents is increased from 63% of current scenario to 83% of the advanced scenario, thus demonstrating the usefulness of the new index in giving a synthetic measure of effectiveness to decision makers. Further studies are planned to stress the capabilities of the index in contrasting heat stress risk and mitigation solutions for different urban tissues and climates.