It has previously been discussed that the severity and lasting impact that the effects of disasters have on our communities are often linked to the resilience of the underpinning infrastructure systems. Therefore, to ensure that these systems are resilient, and can protect our communities, the term ‘resilience’ and what makes a system resilient must first be identified. In a similar manner to the term ‘infrastructure’ the term ‘resilience’ is also seemingly difficult to define and the definition often depends upon the context in which it is being used. The Oxford English Dictionary defines ‘resilience’ as:
‘The ability of a substance or object to spring back into shape’ or ‘the capacity to recover quickly from difficulties.’ (Oxford Dictionaries 2012)
Whilst, previous studies in ecology, systems and information engineering and risk management have defined ‘resilience’ as:
Ecology – ‘Measures of the persistence of systems and of their abilities to absorb change and disturbance and still maintain the same relationships between populations or state variables.’ (Holling 1973)
Systems and information engineering – ‘The ability of the system to withstand a major disruption within acceptable degradation parameters and to recover within an acceptable time and composite costs and risks.’ (Haimes 2009)
Risk management – ‘The uncertainty about and severity of consequences of the activity given the occurrence of any types of events.’ (Aven 2011)
These definitions all differ, however, in a similar manner to the term ‘infrastructure’ there is a common theme connecting all definitions. With regards to infrastructure systems, the Australian Government does not have a universally used definition of ‘resilience’ (Rogers 2011) and the US Government has only short definition of the term: ‘The ability to resist, absorb, recover from or successfully adapt to adversity or a change in conditions.’ (Homeland Security Advisory Council 2011)
The UK Government has the most detailed definition of the term which was formed as part of its plans to increase the resilience of national infrastructure systems (shown in
Figure 2.1). These plans are a direct result of the disruption caused by the summer 2007 floods, which caused 13 deaths, flooded 44,660 homes and cost the UK economy over £4 billion (BBC 2008; Environment Agency 2010a). After this flood event a detailed report was commissioned, the Pitt Review (2008), which called for ‘a more systematic approach to building resilience in critical infrastructure’ (Cabinet Office 2011a) and highlighted the need for:
• Improved understanding of the level of vulnerability to risk to which
infrastructure and hence wider society is exposed; • More consistent emergency planning for failures;
• Improved sharing of information at a local level for emergency response
planning.
In the Pitt Review the term ‘resilience’ was defined as:
‘The ability of a system or organisation to withstand and recover from adversity.’ (Cabinet Office 2008c)
In response to the recommendations in the Pitt Review, the UK Government published the consultation document ‘Strategic Framework and Policy Statement’ in 2010 (Cabinet Office 2010a) which set out the process and timescale for a Critical Infrastructure Resilience Programme. This report adopted the definition of the term ‘resilience’ from the Pitt Review and emphasised that in the Government’s view resilience encompasses activity to prevent, protect and prepare for natural hazards. Responses to this consultation document were obtained from all nine areas of the national infrastructure (Figure 2.1) and all areas highlighted the need for a clearer definition of the term ‘resilience’ (Cabinet Office 2010b). From these responses the Government produced a further document for consultation defining ‘resilience’ in greater detail (Cabinet Office 2011a), to which responses were again gathered and a final report produced: ‘Keeping the Country Running’ (Cabinet Office 2011b). In this report the UK Government identifies four components needed to build resilience into infrastructure (shown in Figure 2.2 and defined in Table 2.2).
Figure 2.2: The four components of infrastructure resilience, according to Cabinet Office (2011b).
Table 2.2: A summary of the definitions of the four components of resilience, as given by Cabinet Office
(2011b).
Resistance Reliability
The Resistance element of resilience is focused on providing protection. The objective is to prevent damage or disruption by providing the strength or protection to resist the hazard or its primary impact.
The Reliability component is concerned with ensuring that the infrastructure components are inherently designed to operate under a range of conditions and hence mitigate damage or loss from an event.
Redundancy Response & Recovery
The Redundancy element is concerned with the design and capacity of the network or system. The availability of backup installations or spare capacity will enable operations to be switched or diverted to alternative parts of the network in the event of disruptions to ensure continuity of services.
The Response and Recovery element aims to enable a fast and effective response to and recovery from disruptive events. The effectiveness of this element is determined by the thoroughness of efforts to plan, prepare and exercise in advance of events.
From these definitions it can be deduced, that in the view of the UK Government, for an infrastructure system to be resilient it must have increased strength to resist the primary impact of the hazard (resistance), to have some ability to maintain function in a reduced capacity (reliability), have increased or backup capacity (redundancy) and be quickly repaired back to normal operation (response and recovery). It can also be stated that the lack of one of these four elements could result in the decrease of the resilience of the system as a whole. For example, if the components of a system lacked the resistance (strength) to resist the primary impact of the hazard and even though there was an effective management plan in place (response and recovery) there would be an increased recovery time, due to the increased initial damage to system components. Likewise, if a system lacked redundancy (capacity) the flow of service to communities could be restricted, or even interrupted, if system components were slightly damaged and flow along them could not be redistributed.
The definition of ‘resilience’ may not have received much attention from individual Governments, but it has been debated by many academic researchers. The most notable are the studies by Bruneau et al. (2003) and O'Rourke (2007) who base their definition upon that of Comfort (1999):
‘The capacity to adapt existing resources and skills to new situations and operating conditions.’ (Comfort 1999)
Bruneau et al. (2003) state that resilience can be understood:
‘As the ability of the system to reduce the chance of shock, to absorb a shock if it occurs (abrupt reduction of performance) and to recover quickly after a shock (re-establish normal performance).’ (Bruneau et al. 2003)
Unlike the UK Government, these studies also express their idea of resilience graphically (Figure 2.3), capturing the initial damage to the system (the loss of quality of the infrastructure of a community from 100% to 50% at t0) and the time taken to restore the infrastructure (from t0 to t1). It can also be seen from this graphical view that the resilience of a system is directly affected by the initial damage to the system and also the time taken to restore functionality to the system. However, less apparent, in the figure, is the impact of redundancy to the resilience of the system; although, it
could be deduced that this is implied and accounted for in the measure of the quality of infrastructure (y-axis).
Figure 2.3: A conceptual definition of the resilience of an infrastructure system (Bruneau et al. 2003).
These studies also quantify the idea of resilience by using formulae to measure the size of the expected degradation in quality over time (from initial impact, at t0, to full recovery, at t1) as shown by Equation 2.1. Using this equation the resilience of different infrastructure systems can be quantified and compared.
𝑅𝑅 = � [100 − 𝑄𝑄(𝑡𝑡)]𝑡𝑡1
𝑡𝑡0
𝑑𝑑𝑡𝑡 2.1
In a similar manner to the UK Government, Bruneau et al. (2003) also defined four elements of resilience, which were later refined by O'Rourke (2007) and are shown in Table 2.3. Although these definitions seem similar to those of the UK Government there are subtle differences. Both parties agree that the infrastructure system must include redundancy and that the system must also have sufficient strength to withstand the impacts of hazard, but each give this component a different name (resistance / robustness). The UK Government’s element of response and recovery splits the idea behind O’Rourke’s rapidity into two distinct areas, where recovery is concerned with pre-event planning and response the time taken to restore service after the event. The idea of pre-planning is not explicitly stated in the elements used by O’Rourke, although it is implied in the idea of rapidity (as the speed with which disruption can be overcome can only be improved through better planning). Also, the idea of early response by the emergency services is not explicitly stated by the UK Government and forms part of the idea of resourcefulness by O’Rourke. The two parties also differ in the concept of reliability, which encompasses the idea that infrastructure components should be designed to operate under a range of conditions
and is specifically stated by the UK Government and is again only implied by O’Rourke in the resourcefulness element.
Table 2.3: The four dimensions of resilience, according to O'Rourke (2007).
Robustness Redundancy
The inherent strength or resistance in a system to withstand external demands without degradation or loss of functionality.
System properties that allow for alternate options, choice and substitutions under stress.
Resourcefulness Rapidity
The capacity to mobilise needed resources and services in emergencies.
The speed with which disruption can be overcome and safety, series and financial stability restored.
Many studies have used these four elements of resilience (Table 2.3) and the quantifying equation (Equation 2.1) of Bruneau et al. (2003) and O'Rourke (2007) to assess and quantify the resilience of systems. These studies include: comparing seismic retrofit strategies in water distribution systems (Chang and Shinozuka 2004), seismic resilience assessment for acute care facilities (Bruneau and Reinhorn 2007) and the assessment of the resilience of networked infrastructure (Reed et al. 2009). One study, by Ouyang et al. (2012), expanded upon the graphical representation of resilience (Figure 2.3) to visually show three distinct stages of resilience (Figure 2.4).
Figure 2.4: The typical performance response curve of an infrastructure system following the
occurrence of a hazard, according to Ouyang et al. (2012).
Unlike the figure developed by Bruneau et al. (2003) and O'Rourke (2007), this figure incorporates an assessment of the damage to infrastructure before recovery can take place. This inclusion is logical as it will take time to assess the damage to the system before a plan for recovery can implemented. However, it is unclear whether the figure used by Bruneau et al. (2003) and O'Rourke (2007) accounts for temporary infrastructure measures, as their figure described the quality of infrastructure. For example, if the power supply to a community is disrupted the emergency plans could include the provision of generators to provide temporary power, subsequently causing the quality of the infrastructure to increase (as there is now provision of some power, although it may be restricted). In their study Ouyang et al. (2012) also state that many infrastructure systems are constantly evolving and that the resilience of the system will change depending on the time interval between 0 and t0 (in Figure 2.4). Due to the recent publication of this study, there have currently been no other published studies which have adopted the model of Ouyang et al. (2012) and used it to assess the resilience of infrastructure systems, other than the authors themselves (Ouyang and Duenas-Osorio 2012). Therefore, it is unclear whether this method is more accurate, or has been adopted by other researchers, in place of the model of Bruneau et al. (2003).
Whilst many studies have used the models of Bruneau et al. (2003) and O'Rourke (2007) to quantify the resilience of infrastructure systems, to the authors’ knowledge there does not currently exist a study which has used this method to inform decisions
on how to increase the resilience of an infrastructure system. These methods are useful at giving an indication of where the vulnerabilities may lie within the management of an infrastructure system (e.g. is the speed of recovery hampering the resilience of the system?) and are also useful to provide a quantification of resilience that can be used to compare different infrastructure systems. However, it is unclear whether this information could be used to provide information to minimise the impact to the quality of infrastructure (Figure 2.3) or the drop in performance level (Figure 2.4).
To conclude, many definitions of the term ‘resilience’ have been presented and debated; in a similar manner to the term ‘infrastructure’, the definition of the term ‘resilience’ used in this thesis will be the same as that stated by the UK Government. It has also been established that to be ‘resilient’ an infrastructure system must have sufficient strength to resist the initial impact of the hazard (resistance) and have additional capacity to reroute flow if necessary (redundancy). There is some debate regarding the other elements of resilience; however, it can be concluded that an effective management plan is needed to ensure a speedy recovery (rapidity / response) and that this plan must include details regarding the mobilisation of resources (resourcefulness). Additionally, it can be argued that to be resilient an infrastructure system must be capable of operating under a range of conditions (adaptable / reliable). It has also been established that the failure of one of these elements could prove to be detrimental to the long term functioning of the system (making for a longer recovery time).