The current and pressing issue is the notion that our infrastructure are highly interconnected and mutually dependent in complex ways, both physically and through a host of information and communications technologies. Identifying, understanding, and analyzing such interdependencies are significant challenges. These challenges are greatly magnified by the breadth and complexity of our critical infrastructure.
Infrastructure interdependencies means a bi-directional relationship between multiple different infrastructures in a general system of systems through which the state of each infrastructure influences or is influenced by or correlated to the state of another. A good example is energy and water - - the two systems are interdependent. Energy and power production require water - - thermoelectric cooling, energy minerals extraction/mining, and emissions control. Water production, processing, distribution, and end-use require energy - - pumping, transport, treatment, and use of conditioning. For example, 20% of the total energy use in California is for water transportation, treatment, and use. We can expect growth in energy and water interdependencies - - future energy development will put new demands on water development. Many new technologies will be more water intensive - - the hydrogen economy would require even more water and water constraints will grow for energy development and power plant siting. Future water supplies and treatment will be more energy intensive - - readily accessible fresh water supplies are limited and have been fully allocated in some areas and new technologies to access and/or treat non-traditional water resources will require more energy per gallon of water.
Steven M. Rinaldi probably explains this best with his "Six Dimensions" graphic for describing infrastructure interdependencies. The six dimensions are:
- The Environment - - Comprising concerns that influence normal system operations, emergency operations during disruptions, periods of high stress/repair, and recovery operations.
- Coupling and Response Behavior - - The degree to which the infrastructures are coupled, or linked, strongly influences their operational characteristics. Some linkages are loose and flexible, whereas others are light, leaving little or no flexibility for the system to respond to changing conditions.
- Type of Failure - - Interdependencies increase the risk of failures or disruptions in multiple infrastructures. The subtle feedback loops and complex topologies created by interdependencies can initiate and propagate disturbances in a variety of ways that are unusual and difficult to foresee.
- Infrastructure Characteristics - - Infrastructures have key characteristics that figure in interdependency analyses. These characteristics include spatial, temporal, operational, and organizational dimensions.
- State of Operation - - The state of operation of an infrastructure can be thought of as a continuum that exhibits different behaviors during normal operating conditions, during times of severe stress or disruption, or during times when repair and restoration activities are underway.
- Types of Interdependencies - - Interdependencies vary widely and each has its own characteristics and effects on infrastructure agents. This includes the following four principal classes - - physical, cyber, geographic, and logical.