Post-event impact assessment

Post-event impact assessment (post-event IA) is designed to assess the impacts that occurred as a consequence of a hazardous event affecting a system characterized by

certain vulnerability conditions (Fig. 1.1). Fed by empirical impact data and experts judgment, post-event IA will provide the information to elicit what types of impact occurred associated to a specific event, and also to prioritise areas of intervention in the case of subsequent cascading consequences. The outcomes of a post-event IA are indispensable to improve IA in the case of future events [Menoni et al., 2017;

Bonadonna et al., in press] (Fig. 1.1).

In volcanology, existing post-event IA, mainly qualitative, describe impacts in relation to a single hazard parameter (e.g. tephra thickness). Quantitatively, the most complete studies have been conducted for the evaluation of physical damage of buildings due to tephra fallout, pyroclastic density currents and lahars (e.g. Blong [1984, 2003]; Spence et al. [1996, 2005]; Jenkins et al. [2015a]; Hayes et al. [2019]);

and these studies have led to the development and improvement of fragility curves as previously discussed. One of the earliest post-event IA dedicated to CI was conducted by Johnston et al. [2000]. Based on a comparison of 2 similar volcanic eruptions but occurring 50 years apart, Johnston et al. [2000] demonstrated the importance of societal vulnerability in indirect impacts involving social and economic losses. Stewart et al. [2006], for the first time, proposed a simple impact model for the water supply system, based on measurements of the tephra composition of several eruptions, and its relation with the acidification and soluble contaminants of potable water. Some other studies compiling impacts on CI, generally related to tephra thickness, are the extensive post-event IA of Wilson et al. [2011, 2012];

Magill et al. [2013]; Wilson et al. [2014]; Craig et al. [2016a]; Elissondo et al. [2016a].

Interestingly, all these studies highlighted the relevance of impacts associated with secondary hazards, in particular, wind-remobilisation of volcanic ash and lahars, as they are a source of important disruptions. Finally, significant advances on the understanding of impacts related to public health (e.g. Horwell and Baxter [2006];

Carlsen et al. [2012]; Baxter and Horwell [2015]), and agriculture (e.g. Thorarinsson [1979]; Wilson et al. [2011]; Craig et al. [2016b]; Forte et al. [2018]), which are particularly important for both society and environment, have been done.

From these post-event IA, we can conclude that most of disaster impacts are related to complex systemic interconnections which cannot be related to a single

hazard parameter or vulnerability aspect, and, therefore, they are very difficult to quantify. A more recent approach to investigate past events, in a more holistic and structured manner, have been developed within the FORensic INvestigation of Disasters program (FORIN) of the Integrated Research on Disaster Risk (IRDR)⁶, launched by the International Council for Science (ICSU), the International Social Science Council (ISSC) and the UNISDR [Burton, 2010]. This approach aims to fully integrate the different aspects of risk, combining natural, socio-economic, health and engineering sciences as well as policy-making. The core of this approach is to an-swer the what, who, when, how and why the different impacts occur, through a systematic and integrative analysis of the main drivers, or the so-called,root causes of impacts [Blaikie et al., 1994; Oliver-Smith et al., 2013]. FORIN framework as a whole provided the first conceptual cornerstone towards more deeper and inte-grative impact studies [Burton, 2010; Fraser et al., 2014]. However, current FORIN application studies are more oriented on the social and political context, rather than developing clear and technical methodologies that can be applied, as also stated by Mendoza [2019].

In the field of volcanic risk, forensic investigation is in its infancy stages. First studies looking from a retrospective and a multi-disciplinary approach have been carried on in the STREVA project (Strengthening resilience in volcanic areas)⁷. Based on dedicated workshops, involving researchers and in-country stakeholders, STREVA methodology combined the physical processes of various past eruptions and its associated impacts into a holistic analysis of the social and physical vulner-abilities of the communities (e.g. [Wilkinson, 2013; Armijos and Few, 2017; Armijos et al., 2017; Few et al., 2017; Hicks and Few, 2015; Sword-Daniels et al., 2015; Pyle et al., 2018; Barclay et al., 2019]). Another study achieved by Wantim et al. [2018]

proposed a first classification of impacts based on a detailed post-event IA.

Despite of the last decades efforts, volcanic post-event IAs have not yet produced the amount of data required to better constrain pre-event impact assessments that allow to forecast impacts. The following limitations are identified:

,→ Due to the lack of quantifiable data collection guidelines, most of analysis

6http://www.irdrinternational.org/projects/forin/about-forin/

7https://streva.ac.uk

are limited to a qualitative (narrative) description. As a result, reliable impact data that can support the development of fragility curves and account for the complex systemic relations that lead to cascading effects is very scarce. Innovative strategies are then required to better understand the causes of impacts, and therefore identify areas where interventions and mitigation measures should be prioritized.

,→ Individual post-event IAs are based on a specific realised event. A complete catalogue based on both systematic data collection and structured analysis is there-fore required to construct robust conclusions in the future (and, eventually, develop accurate fragility curves and impact scales).

Since these needs are essential to DRR, other perspectives linked to the foren-sic investigation of disasters have been implemented by the European Commission.

Notably, the efforts made by the Joint Research Centre of the Institute for the Protection and Security of the Citizen [De Groeve et al., 2013, 2014, 2015] con-cluded in a guidance to record and share impact data. In addition, the EU-funded projects IDEA (Improving Damage Assessments to Enhance cost-benefit Analysis, 2014) focused on floods, MATRIX (New Multi-Hazard and Multi-Risk Assessments Methods for Europe, 2010-2013), and SYNER-G (Systemic Seismic Vulnerability and Risk Analysis for Buildings, Lifeline Networks and Infrastructures Safety Gain, 2009-2012), both focused on seismic risk, provided important insights on forensic methodologies. Finally, from the Reinsurance perspective (Zurich Re)⁸, the need to build more resilient societies against floods drove to the PERC methodology (Post-event Review Capability, 2013) [Venkateswaran et al., 2015].

An integrative forensic analysis framework could help assessing past events, in order to better understand what could be done to reduce potential damages and impacts. This thesis aims to contribute with a novel approach for impact assessment accounting for the entangled relation of physical and systemic vulnerabilities of critical infrastructures, and the long-lasting consequences associated with tephra fallout. The case study, focused on the Cordón Caulle (CC) eruption occurred from June 2011 to December 2012 [OVDAS, 2012], demonstrated that society faces high risk from even small to moderate size eruptions. Indeed, this eruption is a

8https://floodresilience.net/perc

clear example where various sectors, strongly impacted by tephra fallout, triggered a large chain of cascading effects where systemic vulnerability played a major role.

Additionally, the CC eruption demonstrated that secondary hazards, in particular wind-remobilisation of ash, lead to long-lasting consequences that are still generating disruptions and health issues in the communities exposed.