Practical modelling approach for assessing cascading effects of critical infrastructures

A practical modelling approach for assessing cascading effects of critical infrastructures (CIs) is presented. The overall objective in the modelling is to specify threats and dependencies leading to the cascading effects of identified CIs. The threats are presented as threat functions describing the expected level of intensity of threats at a certain time and location. The dependencies are presented in interdependency tables stating the rules of CI dependencies. The predicted cascading effects are set on a timeline in order to create a common picture of the situation. The timeline can be used to understand the incident evolution and to mitigate the consequences. For the modelling approach two initial requirements were set. The modelling approach should be versatile to be used in different accident scenarios implementing information from different kinds of threat functions. It should be scalable in the level of detail so that the spatial modelling of the accident scenario concurs with the overall assessment objectives. In an accident scenario, the overall assessment objective is often two-fold. It can be the crisis management during the accident, or it can be the accident recovery to normal life with functioning CIs. In the crisis management during the accident, the main focus is in the short-term operations trying to minimize the injury of people and the damage to the environment and property. In the accident recovery, the focus is more in the long-term operations trying to repair the subsequent damages. The two overall objectives are of course overlapping and their actual difference is in the time span of the operations. The modelling approach was developed using a flooding scenario in a densely populated area as an example. The crisis management during the accident was chosen as the overall assessment objective. An accident scenario map, locating the initiating events and CIs, was defined and a hexagonal grid was laid on the map. The relevant CIs for each hex were identified, and their interdependencies and vulnerabilities were defined in the model. A reference point was chosen for each hex. The threat function results on the reference point were then applied to all CIs in the hex. The threat function describing the expected level of water in the different locations of the polder area at a certain time was provided by a separate modelling tool. The initial failure times (i.e. not considering the interdependencies) of the CIs were determined. The final failure times were determined taking into account the interdependencies between the CIs, and the cascading effects were assessed. The modelling approach results are best utilized in the preparedness and training phase of crisis management. It gives guidance for the planning of emergency response by revealing the CIs that are important to protect in order to prevent or mitigate the escalation of the accident. The failure times can be compared with the estimated response times to evaluate the feasibility of different accident responses and what kind of impact the responses can have to the overall accident sequence.