Volcanic ash fall hazard and risk

All explosive volcanic eruptions generate volcanic ash, fragments of rock that are produced when magma or vent material is explosively disintegrated. Volcanic ash is then convected upwards within the eruption column and carried downwind, falling out of suspension and potentially affecting communities across hundreds, or even thousands, of square kilometres. Ash is the most frequent, and often widespread, volcanic hazard and is produced by all explosive
volcanic eruptions. Although ash falls rarely endanger human life directly, threats to public health and disruption to critical infrastructure services, aviation and primary production can lead to potentially substantial societal impacts
and costs, even at thicknesses of only a few millimetres. Communities exposed to any magnitude of ash fall commonly report anxiety about the health impacts of inhaling or ingesting ash (as well as impacts to animals and property damage), which may lead to temporary socio-economic disruption (e.g. evacuation, school and business closures, cancellations). The impacts of any ash fall can therefore be experienced across large areas and can also be long-lived, both because eruptions can last weeks, months or even years and because ash may be remobilised and re-deposited by wind, traffic or human activities.

Given the potentially large geographic dispersal of volcanic ash, and the substantial impacts that even thin (a few mm in thickness) deposits can have for society, this technical background paper elaborates upon the ash component
of the volcanic contribution to the UNISDR 2015 Global Assessment Report on Disaster Risk Reduction. We focus on the hazard and associated impacts of ash falls; however, the areas affected by volcanic ash are potentially much larger than those affected by ash falling to the ground, as fine particles can remain aloft for extended periods of time. For example, large portions of European airspace were closed for up to five weeks during the eruption of Eyjafjallajökull, Iceland, in 2010 because of airborne ash (with negligible associated ash falls outside of Iceland). The distance and area over which volcanic ash is dispersed is strongly controlled by wind conditions with distance and altitude from the vent, but also by the size, shape and density of the ash particles, and the style and magnitude of the eruption. These factors mean that ash falls are typically deposited in the direction of prevailing winds during the eruption and thin with distance. Forecasting ash dispersion and the deposition ‘footprint’ is typically achieved through numerical simulation.

In this technical background paper, we discuss volcanic ash fall hazard modelling that has been implemented at the global and local (Neapolitan area, Italy) scales (Section 3). These models are probabilistic, i.e. they account for
uncertainty in the input parameters to produce a large number of possible outcomes. Outputs are in the form of hazard maps and curves that show the probabilities associated with exceeding key hazard thresholds at given
locations. As with any natural hazard, these results are subject to uncertainty and the local case study describes how ongoing research is working to better quantify this uncertainty through Bayesian methods and models. Further to
the ash fall hazard assessments, we discuss the key components required to carry these hazard estimates forward to risk: namely identification of likely impacts and the response (vulnerability) of key sectors of society to ash fall
impact. The varied characteristics of volcanic ash, e.g. deposit thickness and density, particle size and surface composition, the context, e.g. timing and duration of ash fall, and resilience of exposed people and assets can all influence the type and magnitude of impacts that may occur. We draw from data collected during and following past eruptions and experimental and theoretical studies to highlight likely impacts for key sectors of society, such as health, infrastructure and the economy (Section 4). In many parts of the world, the failure, disruption or reduced functionality of infrastructure or societal activities, e.g. ability to work or go to school, is likely to have a larger impact on livelihoods and the local economy than direct damage to buildings. Broad relationships between ash thickness (assuming a fixed deposit density) and key levels of damage is also outlined (Section 5); however, vulnerability estimates are typically the weakest part of a risk model and detailed local studies of exposed assets and their vulnerability should ideally be carried out before a detailed risk assessment is undertaken.

Greater knowledge of ash fall hazard and associated impacts supports mitigation actions, crisis planning and emergency management activities, and is an essential step towards building resilience for individuals and communities. This technical background paper concludes with a discussion on where some of the important advances in ash fall hazard and risk assessment may be achieved, providing a roadmap for future research objectives.