A probabilistic approach in estimating optimal evacuation scenarios for seismic emergency management

In recent earthquake experiences proper evacuation planning and potential emergency actions to be conducted to help the citizens of the quake municipalities were significant issues. In this work the effects of earthquake scenarios were shown in terms of seismic damages to residential and industrial buildings and also to infrastructural networks through the use of probabilistic approaches. The analyses were conducted on the building and infrastructural assets of the municipality of Conegliano, a town of 40000 inhabitants located in the northern part of the province of Treviso, NorthEastern Italy. A preliminary study on historical seismicity and geological substructures of the surrounding areas was performed, followed by the evaluation of the potential seismic damages to the built heritage. On the basis of these results, the issues of the post-quake accessibility and the management of the inhabitants optimal evacuation using the procedure described in Hadas et al. 2013 were performed. Information about building and infrastructural damages, spatial distribution and capacity of harvesting areas, boundary conditions in terms of predetermined maximum timing for carrying out the evacuation and needed costs for the seismic retrofit of bridges belonging to network links involved in the evacuation were the main input data for the simulations. Road network infrastructure is vulnerable for extreme events, and as a result its ability to supply the required capacity, when needed most, can be seriously hampered. Hence, it is crucial to identify those critical segments which prohibit safe evacuation, and find an optimal retrofit scheme at the network level in order to minimize evacuation time. In this work an emergency evacuation model able to consider infrastructures vulnerability caused by bridges and building damages, event location and magnitude, road network, transportation demand and evacuation areas is developed in order to identify the critical infrastructures and recommend budget allocation for increasing network capacity and minimizing evacuation time, given budget alternatives. 12 International Conference on Applications of Statistics and Probability in Civil Engineering, ICASP12 Vancouver, Canada, July 12-15, 2015 2 Evacuation analysis requires a multidisciplinary approach integrating transportation, structural engineering, operations research, and social sciences. With reference to the structural aspects, in emergency situations the first requirement is to investigate the effects induced on infrastructures by hazardous events, and to identify possible relations between these physical and mechanical impacts and the functional characteristics both of single components and of the road network as a whole. It is important that the transportation system remain operative or that its function be repaired or restored as soon as possible (Nicholson and Du 1997). In particular, past experience has shown too often that earthquake damage to road network components can severely interrupt traffic flow, thus negatively impacting on the economic activity of a region as well as on post-earthquake emergency response, evacuation and recovery activities (Franchin et al. 2006). Past works have focused on seismic performance assessment of individual components of road network, whereas few pay attention to system performance assessment and therefore to the optimal economic allocation in the network before the earthquake in order to improve/retrofit the components (Gastaldi et al. 2013; Modena et al. 2014), which is crucial for fast evacuation of the population, if needed. Bridges’ seismic vulnerability assessment is necessary for a proper planning of the emergency response and to define priority on retrofit interventions. Fragility curves allow assessing bridge seismic vulnerabilities (Lupoi and Franchin 2006; Carturan et al. 2014; Zanini et al. 2013; Padgett and DesRoches 2008; Shinozuka et al. 2003; Borzi et al. 2014), taking into account uncertainties of the variables and using probabilistic distributions to describe the properties of the materials composing the structure. These curves can be developed empirically as well as analytically. Regarding the issues concerning network design, evacuation planning can be related to the facility location problem, and network design and flow models. In particular: facility location problems (Nagy and Salhi 2007) aim at locating a set of facilities, both serving and being served, in a network, in order to achieve an objective function with a set of constraints (Avella and Boccia 2009); the network design problem (Magnanti and Wong 1984) is a set of issues designed to construct networks with different objective functions in mind, given the flow which can be served by a network constrained by capacity; network flow models (Ahuja et al. 1993) determine the flow in various network structures, objective functions, and constraints. Network flow models, such as the maximum-flow and minimum-cost problems (Hillier and Lieberman 2005) are well known problems that find the total flow from origin to destination (the former), or the minimal cost for flow from origin to destination, given costs associated with arcs and nodes (the later). These models assume costs per unit, rather than construction costs associated with network design problems and facility location problems. In this context, Hadas and Laor 2013 were the first to present a model for the design of an optimal network in terms of minimizing both evacuation time and network constructions costs. In this paper a revised optimization model that consider retrofit alternatives, in order to minimize evacuation time and budget allocation is presented. The revised procedure takes also into account the capacity reductions induced to each network link in relation to the potential interaction between damaged jutting buildings and the roadways as well as considering potential damages to bridges. 1. INTEGRATED PROCEDURE Figure 1 shows the framework of the integrated procedure; it is composed by four basic components: Bridges Information System (BrIS), 12 International Conference on Applications of Statistics and Probability in Civil Engineering, ICASP12 Vancouver, Canada, July 12-15, 2015 3 Seismic Information System (SIS), Buildings Information System (BuIS) and Transportation and Land-use Information System (TLIS). A description of the procedure components is addressed in the following. Figure 1: General evacuation analysis framework. 1.1. Bridges Information System System components potentially directly subjected to risk in road network risk assessment are usually bridges, tunnels, slopes, retaining walls and roads (Morbin et al. 2014). Each exposed bridge structure is surveyed, its fragility parameters are evaluated, and stored into a specific Bridges Information System (BrIS). For a proper analysis of the potential bridges’ criticalities due to an earthquake occurrence it is necessary to know bridge physical and geometrical characteristics (Pellegrino et al. 2014), which are essential input data for the fragility characterization in probabilistic terms. Another significant element in the BrIS is the collection of the possible retrofit intervention costs (Padgett et al. 2010). They are suitable indexes for assigning the most effective retrofit intervention among possible alternatives. 1.2. Seismic Information System The Seismic Information System (SIS) contains data regarding seismogenetic sources and their parameters to build seismic hazard maps; examples of this information are geo-localized seismogenetic source area, focal mechanism, seismic source depth, annual occurrence ratio. 1.3. Buildings Information System Previous literature studies have underlined the need of deal with the evaluation of the short and long term interaction between road network and damaged buildings (Goretti and Sarli 2006) without deepen this issue at structural level. The Building Information System (BuIS) is organized similarly to the bridges’ one, requiring the knowledge of the physical and geometrical description of the main buildings’ features subsequently functional to their seismic fragility characterization through the use of specific fragility curves sets (Rota et al. 2008). 1.4. Stochastic damage state assessment The level of vulnerability of an infrastructure reflects its attitude in the face of physical damage (physical vulnerability) and/or loss of functionality (functional vulnerability). Structural damage states are defined according to bridge fragility curves for each specific structure with a Montecarlo random number generation. 1.5. Transportation and land-use system analysis With regard to post-earthquake the evaluation of the variation in production and attraction indexes of Origin and Destination zones is of primary interest. The functionality of an element is likely to change as the consequence of a certain event and this represents the functional vulnerability of that element. In this situation, therefore, the physical response of the infrastructure assumes the role of input, as the functional conditions of the single element are evaluated according to a suitably defined capacity function. 1.6. Retrofit strategy A retrofit strategy is a set of possible retrofits coupled with estimated cost and estimated capacity. For each bridge belonging to the analysed transportation network, an evaluation of retrofit intervention types has to be identified. Each retrofit intervention is characterized by a set of corrective coefficients to be applied for deriving retrofitted bridge fragility curves and again to evaluate the new Bridge Damage Indexes (BrDIs) in the case of a quake 12 International Conference on Applications of Statistics and Probability in Civil Engineering, ICASP12 Vancouver, Canada, July 12-15, 2015 4 occurrence on the same retrofitted bridge. BrDIs are then related to Link Damage Indexes (LDIs) by joining a set of possible reduced link capacity values