Assessing the efficiency and the criticality of the elements belonging to a complex territorial system subject to natural hazards

Abstract. The effects of natural hazards can be mitigated by the use of proper "pre-event" interventions on "key" elements of the territory, that is on elements that are mostly vulnerable to a given catastrophic scenario and whose loss of functionality can cause damages on people, property and environment. In this respect, methodologies and tools should be studied to support decision makers in the analysis of a territory, in order to point out such elements. In this work, vulnerability is taken into account under two aspects: "physical vulnerability", which measures the propensity of a territorial element to suffer damage when subject to an external stress corresponding to the occurrence of a natural phenomenon; "functional vulnerability", which measures the propensity of a territorial element to suffer loss in functionality, even when that is caused by the loss of functionality of other territorial elements. In the proposed modeling approach, vulnerability is represented through the use of a graph-based formalization. A territorial system is represented as a complex set of elements or sub-systems. Such elements have differentiated and dedicated functions, and they may be functionally interconnected among them. In addition, vulnerability is defined through the use of two different variables, namely the criticality and the efficiency. Focusing the attention on the temporal phases corresponding to the occurrence of a calamitous event, the first one measures the service demand of an element, whereas the efficiency is a measure of the service that can be offered by such an element. The approach presented is largely independent from the natural risk considered. Besides, the tools introduced for the vulnerability analysis of the territorial system can also be used to formalize decision problems relevant to the location of the available resources for emergency management. A specific case study pertaining to the hydrological risk in the Val di Vara area (Italy) is presented.