Vulnerability and risk
Vulnerability and risk are closely related to the concepts of resilience and resistance. There is an abundance of peer-reviewed and gray literature on vulnerability and risk. The definitions of these concepts given here correspond closely to those used by the Intergovernmental Panel on Climate Change. The concepts of vulnerability and risk, and their relationship with the concepts of exposure, resilience and resistance, are further clarified in this article. The relationships are illustrated with examples related to the coastal zone.
Vulnerability results from the combination of exposure and lack of resilience to hazards (potentially disruptive events). Vulnerability can be expressed as the estimated amount of damage that the coastal community will suffer as a consequence of certain hazards.
The amount of damage that will be inflicted on the coastal community as a result of its vulnerability to (a certain type of) disruptive events. Risk = likelihood x consequence. It can be expressed as the probability that a certain level of damage is exceeded.
A coastal region situated far below sea level protected by high dikes ('bath tube') has a low level of resilience because dike failure inevitably leads to drowning. If this region hosts a large population and many economic activities, it is highly exposed. High exposure and low resilience imply high vulnerability; flood events will produce great damage. However, if the dikes are very strong and well maintained, their resistance against extreme storms is high. The probability of a flood event is small and therefore also the flood risk.
Floods are generally the most threatening hazard in coastal areas. Flood vulnerability can be expressed as the estimated amount of damage caused to the coastal community by a flood, depending on the flood extent. Flood risk is the estimated amount of damage occurring as a result of vulnerability to flooding. Flood risk is evaluated by adding up the probabilities of all flood events that result in damage exceeding a certain level. The evaluation must take into account evolving probabilities and damages over time due to climate change and socio-economic developments.
Another example refers to ecosystems. High biodiversity generally implies strong resilience and low vulnerability. Such an ecosystem has the potential to recover from occasional temporary disturbances. However, if this ecosystem is located near to an industrial port, it is threatened by chemical nondegradable pollution, by invasive species from ballast water or by reclamation for port extension. In this case, the risk - the probability of permanent major loss of biodiversity - can be high.
Estimating vulnerability and risk
A first step in assessing vulnerability is exposure analysis: an inventory of people and tangible and intangible values at risk. A second step is the analysis of resilience: the capacity of institutions, communities, individuals, infrastructures and ecosystems to deal with the impact of hazards, together with the various factors (e.g. social, economic, geographical and environmental conditions) that may hamper adaptation and mitigation strategies. Geographical Information System (GIS) are an important tool to analyze and assess exposure and resilience by mapping information on various exposure and resilience factors at all relevant scales. This information provides an indispensable basis for estimating the potential damage caused by various hazards.
Damage can encompass loss of life, loss of means of subsistence and well-being, loss of property, damage to infrastructure, loss of cultural values, loss of ecosystems and ecosystem services. Estimating damage is far from obvious. Some types of damage can be quantified, for example in money or in number of casualties. Relative figures are often more relevant than absolute figures, for example, the percentage of destroyed habitats or houses. For some damages that cannot be quantified in this way, indirect valuation methods have been developed, see for example the articles Contingent Valuation Method, Travel cost method and Hedonic Evaluation Approach. However, there are also important non-quantifiable values, also called bequest and existence values that have to be considered.
The probability of occurrence of disruptive events must generally be inferred from model simulations, unless long time series are available from which the mean recurrence intervals can be determined. This is, however, seldom the case as conditions evolve in the course of time. For example, the likelihood of extreme high waters susceptible to cause flooding increases with sea level rise. The likelihood of pollution accidents susceptible to ruin marine ecosystems increases due to the production of new chemicals or decreases because of safety standards and enforcement. The likelihood of extreme waves susceptible to destroy coastal infrastructures evolves with climate change. Studies for estimating the probability of occurrence of disruptive events usually require process-based simulation models and statistical analyses. Many decision support tools have been developed to facilitate these studies, see for example Decision Support Systems for coastal risk assessment and management and Decision support tools. Most of these tools are complex and require the involvement of experts.
Because risks relate to events that may occur in the future with some probability, assumptions must be made about the evolution of risk factors. Such assumptions are called scenarios. Since we do not know how the future will unfold, different scenarios are usually considered. These scenarios include assumptions on the evolution of exposure, on the evolution of factors related to the resilience of the coastal region under consideration and on the evolution of factors related to the occurrence of potentially disruptive events.
The article Flood risk analysis study at the German Bight Coast provides a good example of estimating vulnerability and risks in practice.
Risk reduction policy
Risk awareness is essential for the development and implementation of risk reduction policies. Risk reduction policies must compete for priority and funding with policies in other areas. The occurrence of a devastating hazard is usually sufficient to put risk reduction high on the political agenda, but anticipation is obviously preferable. A coastal vulnerability and risk analysis is therefore a better means to stimulate the development and implementation of a risk reduction strategy. Such an analysis identifies vulnerabilities and risks and describes possible ways to address identified vulnerabilities. Indicators (Coastal Vulnerability Indices) have also been developed for coastal vulnerability assessment.
For risk reduction being effective, a complete inventory of all risk factors must first be made. Identifying all risk factors can be a difficult task. Risk studies tend to underestimate the actual risks. Well-identified risk factors can be addressed in a risk reduction strategy. However, the greatest risks often come from overlooked risk factors.
Low-damage high-probability risks are more easy to identify than high-damage low-probability risks. Strategies to reduce the former type of risks are also more easily supported politically and financially. Incentives to tackling high-damage low-probability risks are generally not that strong, as they cost money with no short-term reward. Measures to tackling these risks often have to wait until a disaster has occurred.
In cases where vulnerability cannot easily be reconciled with economic development, a choice has to be made. A thorough quantitative risk analysis is necessary to correctly estimate the trade-offs between vulnerability and development. In some cases, opting for economy may the best choice.
There are two approaches to reduce risk. One approach consists in reducing exposure and vulnerability and another approach in reducing the probability of occurrence of devastating hazards. Policies that follow both approaches in mutual coherence can be most advantageous. This is illustrated below for some examples.
The vulnerability of ecosystems can be reduced by reducing their exposure and increasing their resilience. The exposure to hazards can be reduced by regulating land-use in the neighborhood of vulnerable ecosystems. Possible measures to increase resilience involve strengthening the biodiversity, for example by restoring habitats, by creating corridors between habitats or by avoiding disturbance. The probability of hazards can be reduced by measures such as establishing and enforcing regulations that minimize industrial pollution accidents.
The strategy for reducing flood risks in the low-lying parts of the Netherlands below sea level is based on three pillars. The first two pillars are aimed at reducing vulnerability. One pillar addresses the built infrastructure and comprises measures to reduce exposure, for example by raising the ground level with sand before new urban extensions are built. Resilience is improved by several measures such as creating space for water, constructing canals that allow better control of the water level and the construction of floating houses in new residential areas. A second pillar concerns the harnessing of early warning systems, the creation of safe refuge areas and the establishment of a governance structure for emergency interventions. The third pillar is aimed at minimizing the flood risk through the construction and regular upgrade of flood defenses. Soft flood defenses (beach-dune system maintained with sand nourishments) are privileged along the open sea coast in the Netherlands, while along the estuaries and tidal rivers, dikes are constructed that leave sufficient space for flood expansion within the streambed. The third pillar is by far the most crucial for the land below sea level. All sea defenses are designed for a very low failure probability (see Risk and coastal zone policy: example from the Netherlands).
Many studies related to flood risk can be found on the Floodsite webpage.
- Resilience and resistance
- Flood risk analysis study at the German Bight Coast
- Decision Support Systems for coastal risk assessment and management
- Environmental risk assessment of marine activities
- Risk and coastal zone policy: example from the Netherlands
- Climate adaptation measures for the coastal zone
- Climate adaptation policies for the coastal zone
- Integrated Coastal Zone Management (ICZM)
- IPCC, 2022: Climate Change 2022: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [H.-O. Pörtner, D.C. Roberts, M. Tignor, E.S. Poloczanska, K. Mintenbeck, A. Alegría, M. Craig, S. Langsdorf, S. Löschke, V. Möller, A. Okem, B. Rama (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA, 3056 pp
- Rocha, C., Antunes, C. and Catita, C. 2023. Coastal indices to assess sea-level rise impacts - A brief review of the last decade. Ocean and Coastal Management 237, 106536
- Merz, B., Elmer, F. and Thieken, A. H. 2009. Significance of 'high probability/low damage' versus 'low probability/high damage' flood events. Natural Hazards and Earth System Sciences (NHESS) 9 :1033-1046
- Bosoni, M., Tempels, B. and Hartmann, Y. 2023. Understanding integration within the Dutch multi-layer safety approach to flood risk management. International Journal of River Basin Management 21: 81-87
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