The domino effect of natural hazards: Why cascading impacts demand alignment and consistency in resilience efforts

Cascading failures present a major risk to our increasingly interdependent and interconnected infrastructure. Such failures could increase the impact of extreme weather or hazards by more than 200%. For instance, impact to an electrical asset may result in cascading power loss that could result in a water pump losing electricity, resulting in a hospital or school losing power. Another example is the interdependence between our electricity, telecommunications, and transport systems. 

Cyclone Gabrielle made these interdependencies critically apparent and further highlighted the need for resilience planning to account for the interconnections within and between infrastructure systems. When preparing for extreme events, businesses must not only consider their own asset risk, but the risk to the infrastructure that their assets and interests depend on. This information needs to be relayed to these asset owners so that external criticality can be considered. Understanding the dependencies between these systems is crucial for building wider community and economic resilience, forecasting impacts, responding to outages and losses, and recovering from the acute events. However, although this is critically important, it is not commonplace in risk assessment guidance or practice and therefore is absent from subsequent planning.

This is the focus of ongoing research funded by MBIE at the University of Canterbury’s engineering school and the outputs of this research are being translated into practice in the Resilience Explorer. The Resilience Explorer, provided by Urban Intelligence Ltd, is a tool designed to assess direct and indirect risk, and enable agencies to operate on consistent information and share their information between relevant groups. Users can choose what information to share with others to support collaborative resilience planning and aligned decision-making based on consistent data. Such integration is crucial to enable infrastructure providers to make aligned decisions, efficiently build resilience, and create effective adaptation plans.

The research backing this tool is the first available in the world to apply a cascading failures model to real-world infrastructure (previous studies have been limited to synthetic networks due to data sensitivity). To assess these cascading impacts, we created a network model or digital twin of the interdependent infrastructure and then stress tested it with different hazard scenarios. Any impact cascades through the connected infrastructure and onto properties/end-users. 

The study shows that when considering infrastructure interdependence, the impacts are significantly higher and their spatial distribution is strikingly heterogeneous in utility outages. For example, considering direct risks alone, a 1-in-10-year coastal flood in Christchurch threatens 3,600 homes. However,  an additional 7,800 homes are at risk from losing one or more utilities (Figure 1). The multiple colours shown in Figure 1 indicate that there are spatially specific vulnerabilities throughout the city; some residents lose just one utility (such as water, shown in light blue) and might be able to remain in their house following an event, while other residents lose all three utilities (shown in black) and might need to be housed elsewhere. These are in addition to the flooded households, which are shown in yellow. While most risk assessments focus on impacts to infrastructure (and typically to a single infrastructure), our recently published study shows the broader, often underestimated indirect impacts to both property and interdependent infrastructure.

Fig 1. Exposure and indirect outages across Ōtautahi Christchurch due to a coastal flooding event with an ARI of 10 years and no sea-level rise. Indirect outages mean that the impact is 216% higher. (Brunner et al., 2024)

Considering infrastructure dependencies is critical for our infrastructure and economic resilience planning. These indirect impacts can easily outweigh the direct impacts in terms of the number of residents impacted and help to understand the heterogeneity of utilities available following a natural hazard. Understanding them is essential to ensure decisions around emergency preparedness, adaptation planning, and building organisational and community resilience are effective and efficient.

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Dr Logan is the Technical Director of Urban Intelligence, Senior Lecturer of Civil Systems Engineering at the University of Canterbury, a Rutherford Discovery Fellow, and the President of the Australia NZ Society of Risk Analysis.