Multi-risk assessment in a historical city

C Arrighi,M Tanganelli,M T Cristofaro,V Cardinali,A Marra,F Castelli,M De Stefano

doi:10.1007/s11069-021-05125-6

Abstract

Natural hazards pose a significant threat to historical cities which have an authentic and universal value for mankind. This study aims at codifying a multi-risk workflow for seismic and flood hazards, for site-scale applications in historical cities, which provides the Average Annual Loss for buildings within a coherent multi-exposure and multi-vulnerability framework. The proposed methodology includes a multi-risk correlation and joint probability analysis to identify the role of urban development in re-shaping risk components in historical contexts. The workflow is unified by exposure modelling which adopts the same assumptions and parameters. Seismic vulnerability is modelled through an empirical approach by assigning to each building a vulnerability value depending on the European Macroseismic Scale (EMS-98) and modifiers available in literature. Flood vulnerability is modelled by means of stage-damage curves developed for the study area and validated against ex-post damage claims. The method is applied to the city centre of Florence (Italy) listed as UNESCO World Heritage site since 1982. Direct multi-hazard, multi-vulnerability losses are modelled for four probabilistic scenarios. A multi-risk of 3.15 M€/year is estimated for the current situation. In case of adoption of local mitigation measures like floodproofing of basements and installation of steel tie rods, multi-risk reduces to 1.55 M€/yr. The analysis of multi-risk correlation and joint probability distribution shows that the historical evolution of the city centre, from the roman castrum followed by rebuilding in the Middle Ages, the late XIX century and the post WWII, has significantly affected multi-risk in the area. Three identified portions of the study area with a different multi-risk spatial probability distribution highlight that the urban development of the historical city influenced the flood hazard and the seismic vulnerability. The presented multi-risk workflow could be applied to other historical cities and further extended to other natural hazards.

Highlights

Natural hazards pose serious threats worldwide, causing monetary losses, disruption of services and fatalities
The topic covers a gap which is justified by several issues: the complexity of multi-disciplinary subjects, the differences within the metrics associated with the different hazards as the differences involved in the methodologies (Julià and Ferreira 2021; Kappes et al 2012a)
With respect to Grunthal et al (2006), who examined three different risk sources in terms of annual loss (AAL), this method unifies the workflow through exposure modelling instead of using three separated workflows connecting at the end of the risk estimation process

Summary

Introduction

Natural hazards pose serious threats worldwide, causing monetary losses, disruption of services and fatalities. In 2019, 820 events were registered in the NatCatSERVICE database (Munich 2020). 7% were earthquakes, and 45% floods, flash floods and landslides (Munich 2020; Shahri and Moud 2021). Every year disaster cost the global economy US$ 520 billion (UNISDR 2018). Economic losses caused by natural hazards are increasing and climate change and urbanization prospects foresee a worsening of risks in the decades (Dottori et al 2018). The Sendai Framework for disaster risk reduction advocates for the substantial reduction of disaster risk with the priority being the understanding of risk to improve the prevention, mitigation, and preparedness actions (UNDRR 2015; Ferreira et al 2020)

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Discussion

Conclusion