Identifying and modelling interdependencies and cascading failures between critical urban services: positioning of the RESCCUE approach

Urban technical networks, which are essential for the functioning of cities, are still vulnerable to various climatic hazards. These vulnerabilities are increased by the links of dependencies and interdependencies between urban services and their infrastructures. For example, a hydraulic pump is dependent on the electricity supply, signal light in the underground transit system is dependent on its centralized commands, an underground electric cable is dependent on the state of the street beneath which it lies, and the communication system is dependent on a potential panic reaction triggering network overload. Cascading failures may then happen, corresponding to the phenomenon of propagation of the failures within one system, generated by an initial event, to other systems through dependencies. 

These complex phenomena and the subject of numerous researches to identify, model and simulate them. Because this topic is at the juncture of numerous disciplines, issues and sectors, we observe a wide variety of definitions, related frameworks, tools and methodologies. These different researches can be highly varied in terms of the disciplines involved (geography, industrial safety, IT, civil engineering, etc.), target findings, objects studied, data required, scales studied, methods deployed, implementation processes and degree of stakeholder involvement.

These initiatives are often complex, combining several tools and modelling techniques, and are highly diverse. They make it possible (partially or completely) to identify interdependencies, to formalize them, to model relations between components and related cascading failures, to simulate failure scenarios, to analyze and prioritize interdependency scenarios or measures. 

It seems that the development of a single and universal solution for the analysis and the modelling of interdependencies and cascading failures is hardly achievable today. Numerous scientific, technical and practical difficulties make the construction and implementation of a single model, faithful to the complexity and dynamics of the reality of interdependent urban technical networks, laborious.

It is therefore advisable to build dedicated approaches adapted to a particular context, following local practices, supported by actors and decision-makers, but also composite and flexible. 

Within these initiatives, the RESCCUE project aims to provide various innovative models and tools to improve the ability of cities to withstand and recover quickly from multiple shocks and stresses and maintain continuity of services, with a multi-scale, multi-sectorial, multi-granularity and multi-hazard approach. The project seeks to make these tools and methods work together, around a central methodology called Hazur. This methodology brings together various stakeholders, networks experts, critical infrastructures, scales and objects to help identify, formalize, and model interdependencies and cascading failures within urban networks considering several triggering hazards. It offers a simple functional formalism, making it possible to make intelligible and compatible a large number of information necessary for the study of cascading effects, coming from various sources and formats.

Finally, the RESCCUE approach is interdisciplinary, global and flexible as it allows working on several territories, scales, granularities, networks, hazards and within a different group of stakeholders. The collaborative approach also allows simulating a real collective dynamic, to obtain concrete results that can be used operationally, and to create and engage a community of local stakeholders to tackle urban resilience.