From single to multi-risk perspective: How heatwaves risk mitigation solutions can reduce terrorist risk in historic outdoor open areas

• A workflow for multi-mitigation strategies for historic squares is proposed. • Heatwave and terrorism risks methods are combined to test multi-risk scenarios. • Behavioural simulation-based approach is used for assessment goals. • Sustainable, compatible and redundant heatwaves-oriented solutions are defined. • Solutions impact is verified for terrorist acts, and then for multi-risk mitigation. Historical outdoor Open Areas (hOA) are relevant “hot-spots” in urban built environments, attracting many users due to morphological and use-related features. Besides significant heritage vulnerability, hOAs can also be affected by critical levels of users’ exposure and vulnerability, exacerbating the effects of natural and anthropogenic risks. Mitigation solutions for one risk may impact others due to mutual interactions among different phenomena, necessitating multi-risk evaluations. This work focuses on how heatwaves-mitigation solutions (“slowly” impacting how users behave and gather in hOAs) can support risk-reduction for terrorist acts (“suddenly" striking users and implying evacuation) emerging in heatwaves-affected scenarios and, thus, multi-risk mitigation. To this end, an innovative approach for hOAs multi-risk analysis methodology is applied to a relevant case study (Piazza dell’Odegitria, Bari, Italy), using previously validated behavioural-based simulators. Original and post-retrofit (involving sustainable/highly reversible/compatible strategies with historical and cultural relevance of the place) scenarios are compared through multi-risk metrics, by analysing effects on hosted users. Results suggest remarkable and effective multi-risk reduction (>15 %) by combining, at least, cool pavement and barriers implemented with green elements. Moreover, the findings highlight the approach's capability to support policymakers in sustainably evaluating and comparing different scenarios for preliminary analyses of mitigation strategies.