Infrastructure Interdependencies - - Coupling and Response Behavior

Coupling refers to how two or more systems are connected or linked together.  Key points relating to coupling are as follows:

• Coupling characteristics include the degree of coupling, the coupling order, and whether the interaction is linear or complex.
• Linkages are either tight or loose depending on the relative degree of coupling.  Tight coupling is characterized by time-dependent relationships - - the natural gas fired power plant and the gas supply pipeline have tight coupling.  Loose coupling implies that the infrastructures or agents are relatively independent of each other and the state of one is weakly correlated to or independent of the state of the other.  A water treatment plant with a three month supply of chemicals has a loose coupling.
• The coupling order indicates whether two infrastructures are directly connected to one another or indirectly through one or more intervening infrastructures.
• Interaction can be either linear or complex.  Linear interactions are those in an expected and familiar production or maintenance sequence and those that are quite visible even if unplanned.  Complex interactions are those of unfamiliar sequences or unplanned and unexpected sequences, and either not visible or not immediately comprehensible.  A gas pipeline can look both linear and complex, depending on the context.  A gas transmission line and facilities transporting gas to your home will look linear.  That same type of gas line going to a power plant, with electricity transmitted to gas conditioning plants and compressor stations, will look very complex.
• A clear understanding of context is vital in analyzing the couplings among infrastructures.
• Coupling is also influenced by adaptive relationships (availability and number of substitutes) and the ability to become flexible when stressed or perturbed.