## 2 Critical Infrastructures (CIs)
### Interdependence Types  
Smart ICIs exhibit various types of dependencies and interdependencies. While dependency refers to a unidirectional connection between infrastructures, interdependency implies a bidirectional relationship between them [21]. Numerous scholars have categorized the interdependencies between infrastructures in different ways [22]. In addition to these primary types of interdependencies, it is essential to consider the potential cascading effects that can occur within and between interconnected infrastructures. As the complexity of Smart ICIs increases with the integration of advanced technologies, the possibility of cascading failures across multiple systems becomes a pressing concern. Identifying and understanding the different types of interdependencies can help policymakers, planners, and infrastructure managers develop more effective strategies to mitigate the risks associated with these complex relationships and enhance the resilience of Smart ICIs. Moreover, the increased interconnectivity and digitization of Smart ICIs bring both opportunities and challenges. On the one hand, interdependent infrastructures can improve efficiency, resource optimization, and better decision making. On the other hand, the growing complexity of these systems can make them more vulnerable to disruptions, cyberattacks, and other potential threats. Therefore, it is crucial for stakeholders to strike a balance between leveraging the benefits of interdependent critical infrastructures and addressing the challenges they pose. This can be achieved through continuous research, innovation, and collaboration among various sectors, ultimately contributing to a more resilient and sustainable society; however, in this paper, we classify the interdependencies in critical infrastructures into five primary types:  
* **Physical Interdependence:** Infrastructures exhibit physical interdependence when the functioning of one infrastructure relies on the physical output(s) of another. For example, consider the relationship between a coal-fired power generation plant and its associated railroad system. The power plant generates electricity, which serves as an input for maintaining the railroad control center's operations. Simultaneously, the railroad system is responsible for transporting coal to the power plant, ensuring its fuel supply. In this case, the physical interdependency connection is established through the electricity produced by the power plant and its role in sustaining the railroad system's services. The physical dependence of the renewable energy infrastructure on various factors adds another layer of complexity. For instance, wind turbines are susceptible to changes in wind speed and direction, solar panels depend on sunlight intensity, and hydroelectric systems are reliant on water flow rates. Any disruptions in these physical dependencies can significantly impact the power generation and, in turn, the resilience of the entire system [23]. This underscores the need for an integrative approach to resilience, encompassing not only technical considerations but also the physical dependencies of renewable energy infrastructures. Ensuring resilience in this context requires a thorough understanding of these dependencies and the development of strategies to manage potential disruptions. This might include the diversification of renewable energy sources, strategic placement and design of infrastructure to withstand environmental stresses, and the implementation of systems that can quickly adapt and respond to changes in renewable energy supply. * **Logical Interdependence:** Infrastructures demonstrate logical interdependence when the functioning of one system depends on the functioning of others through a mechanism that is not connected to physical, cyber, or geographic factors. For instance, power system outages can lead to fluctuations in the prices of food and fuel.