A seismic network reliability evaluation on a GIS environment – a case study on Catania province S. Cafiso, A. Condorelli, G. Cutrona & G. Mussumeci Department of Civil and Environmental Engineering, University of Catania, Viale Andrea Doria, 6, 95125 Catania, Italy Abstract Human society is nowadays strongly dependent on an articulated and complex network of road infrastructures. Essential services for current users as well as for every kind of human activity have been entrusted to this network that takes the name of “lifeline”. Network Reliability Analysis mainly measures network performance related to its capability to oppose or react against the failure of single elements. When a catastrophic event strikes a wide area, it is necessary that the infrastructure system is designed with a high redundancy, to have effective alternatives in choosing a route to maintain network function. However, if in mathematical analysis of a simple structure, a redundancy rate could clearly be defined, it would not be sufficient to quantify the redundancy effect in a more complex and real-life structure such as a road network. In this paper, we propose a GIS (Geographic Information System) based methodology to study the road Network effectiveness after a seismic event. The method is based on the concepts of Encountered Reliability and Terminal Reliability and it has been applied to the Catania Province area. The results show which towns and links are in the most critical condition and must be considered for road planning and management prioritization. Keywords: road network, lifelines reliability, seismic risk, GIS, bridge vulnerability. Risk Analysis IV, C. A. Brebbia (Editor) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-736-1 132 Risk Analysis IV 1 Reliability and risk of lifelines network The analysis of the effects induced by calamities on the transport system clearly shows how the destruction or severe damage of even one element has serious consequences for all the territory, such as the impossibility to access the urban network, interruptions or overloading of part of the transport system or on other means of transport, delay in emergency services arriving at the scene of the calamity, etc. The examination of the transport system and its functionality is thus very important for the wider evaluation of territorial risk. The transport network is affected by two different phenomena that can modify its reliability: 1) Variation in what is offered for transportation; 2) Variation in the demand on the transport services. In the case of damage produced by seismic events it is obvious that the effects of the interruption of the local network and the consequence reduction in what remains available profoundly affect the overall performance of the system (increase in travel time, distance and costs). Furthermore, the effects of this damage induce the users to change their behavior which then changes the demand on the system. The effects of the damage to the system can be reduced by the capacity of the users to adapt to the new circumstances. This means that the user should have a high degree of information so as to be able to awarely decide beforehand from among the available options. On the other hand, if there is a low degree of available information the only option is that of changing route when an interruption is encountered, when the user is already involved in the congestion of the system [1]. The reduction of the risk on the transport network has generally been made by the structural improvement of some of its components, but other aspects should also be taken into consideration such as: - The improvement of the global configuration of the network; - The construction of alternative infrastructures that can guarantee the redundancy of the system; - The monitoring of critical components; - The carrying out of regular preventative maintenance; - The identification of the priority for repair of the damaged components. Operations of this type can be carried out using the methodologies of analysis for reliability engineering, applied to the road network and to the relative traffic flow both in normal conditions and in the case of an emergency [2]. Reliability can be defined as the “the probability of a device performing its purpose adequately for the period of time intended under the operating conditions encountered” (Wakabayashi and Idia, 1992 [3]), accepting that this is with the various meanings of the term “correctly” assumed by the different users. A road network, in particular, will be reliable if “…. provides a safe and not fluctuating service for the traffic and offers the users alternative routes, even when some parts of the system are not available due to road accidents, Risk Analysis IV, C. A. Brebbia (Editor) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-736-1 Risk Analysis IV 133 maintenance or natural disasters”. In some cases, however, it could be of primary importance that the journey finishes in a determined period of time, while in other cases it is more important to evaluate if there are interruptions along the route. With this aim in mind there are two concepts of reliability. Terminal reliability is “the probability that nodes are connected, i.e. it is possible to reach the destination” and this is surely the parameter that is easier to evaluate. With this approach reliability assumes the value of one or zero, in relationship to the probability that it is possible or not to reach the destination by any route, but it is not possible to describe the situations in which the system works in intermediate conditions within the range 0-1. The encountered reliability can be defined as “the probability of not encountering a link degradation on the path with least (expected) cost”. Another concept that is complimentary that will be used in the proposed methodological approach is the reliability of the time and cost of the journey, commonly defined as “the probability that a trip can be successfully finished within a specified time interval”. In parallel to the concepts of reliability it is indispensable to address the problems linked to the network risk. It should be remembered that the risk, following an approach that has been well consolidated in literature, can be seen as the product of three independent factors: - hazard, linked to the probability that in a certain place there will be an event of a certain intensity with a given return time; - exposure, given by the number of people (and goods) that can be damaged by the event; - vulnerability, that defines the propensity of the infrastructural element that can undergo damage during the event [4,5]. 2 Study area and the road lifeline network The province of Catania has an area of about 3,552 Km2 with a population of 1,054,778 inhabitants, in which there are 58 towns. Among these the most important is the province capital (Catania), with 313,110 inhabitants. There are also five urban centres with a population of more than 30,000 inhabitants (Acireale, Paternò, Misterbianco, Caltagirone, and Adrano). A value was assigned to each city for its “direct exposure ” equal to the number of inhabitants in the towns multiplied by the “index for seismic risk” defined in Italy proportionally to the values of the expected losses and with a value varying between about 0 and 0.8 [6]. The GIS, developed using Arcview®, contains all the data necessary for the analysis of risk and emergency management, organized in shape-files and Relational data bases [7]. In particular, the shape-file relative to the roads present in the province of Catania (classified as urban, communal, provincial, state and motorways) and the segmentation of the network and stretches allow the application of the functionality of the Network Analyst®. This was carried out integrating the road network Tele Atlas® with the information obtained from maps and recent orthophotographs, providing all the Risk Analysis IV, C. A. Brebbia (Editor) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-736-1 134 Risk Analysis IV information requested by the detailed analysis: toponymy, length of the branch, width of the carriage ways, number of carriage ways, direction, speed limits, functional classification of the road stretch, eventual presence of restrictions for some types of traffic, eventual need to pay a transit toll, belonging to the lifeline network or not and the time necessary to complete the route at the design speed. All the bridges and viaducts present on the network were positioned within the GIS with the help of provincial maps with a scale of 1:10,000. Based on this information all the routes that could be used by emergency services were identified. For the definition of these routes, which provide communication for all the 58 towns of the Province, three hypotheses of traffic accessibility were considered, having identified in the areas of Messina, Palermo and Ragusa/Gela the possible origins (O) for emergency services [8]. The scheme produced allowed the determination of the network of lifelines of the province of Catania, on which to analyze risk and reliability (Figure 1). Messina m Palermo 247ֹ252 m 214ֹ291˙1 m 664ֹ883˙2 m 239ֹ663˙1 m 334ֹ544 m 64ֹ339 m 71ֹ967 Ragusa - Gela Figure 1: The road network diagram created by Arcview®. 3 Indirect exposure on the stretches of the road network The indirect exposure on the single stretches of the road network can be defined in relationship to the number of people who would experience a delay in the arrival of emergency services due to an interruption of that given stretch of the road network. The shortest route between each origin and each of the 58 towns of the province was calculated by ArcView®, both in terms of distance and time needed to cover the route.