- Open Access
Improving volcanic ash fragility functions through laboratory studies: example of surface transportation networks
Journal of Applied Volcanology volume 6, Article number: 16 (2017)
Surface transportation networks are critical infrastructure that are frequently affected by volcanic ash fall. Disruption to surface transportation from volcanic ash is often complex with the severity of impacts influenced by a vast array of parameters including, among others, ash properties such as particle size and deposit thickness, meteorological conditions, pavement characteristics, and mitigation actions. Fragility functions are used in volcanic risk assessments to express the conditional probability that an impact or loss state will be reached or exceeded for a given hazard intensity. Most existing fragility functions for volcanic ash adopt ash thickness as the sole hazard intensity metric that determines thresholds for functional loss. However, the selection of appropriate hazard intensity metrics has been highlighted as a crucial factor for fragility function development and recent empirical evidence suggests that ash thickness is not always the most appropriate metric. We review thresholds of functional loss for existing published surface transportation (i.e. road rail, maritime and airport) fragility functions that use ash thickness. We then refine these existing functions through the application of results from a series of recent laboratory experiments, which investigate the impacts of volcanic ash on surface transportation. We also establish new fragility thresholds and functions, which applies ash-settling rate as a hazard intensity metric. The relative importance of alternative hazard intensity metrics to surface transportation disruption is assessed with a suggested approach to account for these in existing fragility functions. Our work demonstrates the importance of considering ash-settling rate, in addition to ash thickness, as critical hazard intensity metrics for surface transportation, but highlights that other metrics, especially particle size, are also important for transportation. Empirical datasets, obtained from both post-eruption field studies and additional laboratory experimentation, will provide future opportunities to refine fragility functions. Our findings also justify the need for rapid and active monitoring and modelling of various ash characteristics (i.e. not ash thickness alone) during volcanic eruptions, particularly as potential disruption to surface transportation can occur with only ~ 0.1 mm of ash accumulation.
Surface transportation including road, rail and maritime networks (see Table 1 for terminology) are critical for many social and economic functions. Disruption to surface transportation can affect commuter travel, access for emergency services, distribution and provision of goods and services, other infrastructure (e.g. electricity systems, water and fuel) and the economy. Damage and loss of function to surface transportation networks from historical volcanic eruptions worldwide has been qualitatively and semi-quantitatively recorded for roads and airports (e.g. Blong 1984, Nairn 2002, Barnard 2009, Guffanti et al. 2009, Wilson et al. 2014). Quantitative data sourced from historical eruptions or controlled laboratory experimentation has been lacking, meaning that there have been limited empirical or hybrid datasets to develop robust relationships between hazard intensity and network impact (damage and disruption) (Wilson et al. 2017).
Figure 1 summarises recorded impacts for road, rail and airports, caused by various volcanic hazards with a focus on tephra, following historical eruptions since 1980. Impacts to transportation networks can be complex, particularly when exposed to multiple volcanic hazards during eruptions causing a range of impact states. Volcanic hazards such as pyroclastic density currents (PDCs) and lava flows (shown in the top sections of the charts in Fig. 1) are geographically well constrained. Volcanic ash (i.e. the component of tephra with particle size < 2 mm) however, is often widespread (Blong 1984) and generally has far-reaching and complex interactions with surface transportation. Studies since the 1980 Mount St Helens eruption (e.g. Blong 1984, Johnston 1997, Guffanti et al. 2009, Horwell et al. 2010, Wilson et al. 2011, Dunn 2012, Wardman et al. 2012, Wilson et al. 2012, Stewart et al. 2013, Wilson et al. 2014, Blake et al. 2016, 2017a) demonstrate that volcanic ash frequently reduces skid resistance and covers markings on paved surfaces. Reduced visibility caused by airborne ash and the abrasion or cracking of vehicle windscreens are also common, and engine failure may result if vehicle air intake filters are not adequately maintained. All of these impacts can affect transportation functionality, whether it is by reduced vehicle volumes and speed, an increase in accident rates and congestion, or network closures. As such, we focus on the impacts associated with volcanic ash in this paper.
Globally there is limited quantitative data for the impact of ash on surface transportation networks. Similarly, experimental data is sparse due to the complexities of replicating infrastructural components and volcanic ash properties in laboratories (Jenkins et al. 2014a, Wilson et al. 2014). Where quantitative data exist, impacts on transportation, as well as other critical infrastructure, have generally been related to the thicknesses of ash on the ground. For example, Wilson et al. 2017 use the ash thickness variable (defined as a Hazard Intensity Metric (HIM); Table 1) to produce a series of volcanic ash fragility functions for different infrastructure types. Although adopting ash thickness as a HIM has distinct advantages, particularly in that it is a frequently modelled and often relatively readily measured variable following eruptions, it is not always appropriate to consider this metric alone. Characteristics such as ash particle size, ash type, the quantity of soluble components, wetness and airborne concentration or ash-settling rate may have large effects on overall impact intensity and subsequent loss of functionality in some cases. Recent work by Blake (2016), Blake et al. (2016 and 2017a) has focused on targeted experiments investigating common surface transportation impact types for volcanic ash under controlled laboratory conditions through a series of targeted experiments. New quantitative data available from these studies enables analysis of HIM importance and appropriateness, the refinement of thresholds for functional loss (termed Impact State (IS)), and opportunities to improve fragility and vulnerability functions. Such approaches, whereby the vulnerability of infrastructure is assessed using laboratory studies to supplement field-based empirical observations, has been proven in other disciplines including earthquake engineering and for structural loading in tsunami (e.g. Rossetto et al. 2013, Nanayakkara and Dias 2016).
In this paper, we summarise existing IS thresholds for surface transportation from previous, largely qualitative, post-eruption literature, and using ash thickness on the ground as the HIM. Next we adopt the empirical results from Blake (2016), and Blake et al.’s (2016 and 2017a) (see Additional file 1 for key findings summary) suite of targeted laboratory experiments to refine these established thresholds for ash thickness, and to develop new IS options for visibility impairment based on ash-settling rate as the HIM. The importance of additional HIMs (such as ash particle size and colour) as measures of functional loss for specific impact types is investigated through relative comparisons to one another. This allows us to propose a credible strategy to enhance fragility functions for surface transportation networks, by means of incorporating related uncertainty. We only consider discrete and direct ash fall events and not effects that may occur from remobilised ash. Our focus is on road disruption as most garnered data is directly related to road infrastructure. However, disruption to airports and rail and maritime transportation are also discussed, particularly as recent empirical studies of visibility reduction apply to all surface transportation modes.
Background: Quantitative volcanic impact assessments
Risk assessments may incorporate vulnerability functions to describe the likelihood that an asset will sustain varying degrees of loss over a range of hazard intensities (Rossetto et al. 2013). The ‘loss’ may be expressed as economic cost, damage (e.g. physical damage of a sealed road surface from ballistics) and/or functionality (e.g. reduced speeds on roads from volcanic ash). However, vulnerability functions are less common in volcanic risk assessments than they are in risk assessments for many other disciplines such as seismic engineering due to the variety of volcanic hazards and associated complexities (Jenkins et al. 2014a).
Qualitative data obtained following eruptions is often considered sufficient to establish and communicate information relating to expected impacts during future events. For example, exclusion zones may be implemented in the immediate vicinity of the vent due to qualitative knowledge gained from past eruptions about the high likelihood of severe damage from proximal hazards such as pyroclastic density currents (PDCs) and lahars, and advice can be issued to avoid travel in relatively short-lived and localised ashfall events until ash has been cleared. Residents often heed such advice due to health concerns (Stewart et al. 2013) and to avoid potential damage to their vehicles (e.g. Blake et al. 2015). However, the volcanic ash hazard can have complex impacts on infrastructure networks causing widespread disruption (Johnston and Daly 1997), potentially affecting thousands of kilometres of surface transportation routes. Loss of functionality can also be prolonged due to on-going volcanic activity and the remobilisation, re-suspension and secondary deposition of ash (sometimes for months to years after an eruption has ceased) by wind, fluvial processes, and/or anthropogenic disturbance. As such, it is beneficial to establish thresholds from semi-quantitative and/or quantitative data (e.g. ash thickness measurements) to indicate when specific impact types (e.g. road marking coverage or visibility impairment), and of what severity, occur. These impact thresholds can in turn inform damage ratios, which express the economic cost required to restore infrastructure (i.e. absolute damage) by indicating the damaged proportion of the infrastructure (i.e. relative loss) (Reese and Ramsay 2010, Tarbotton et al. 2015). Impact thresholds and damage ratios can be adopted by emergency management officials and in transportation maintenance guidelines such as for informing when to commence road sweeping or implement road closures following volcanic ashfall (Hayes et al. 2015). Sometimes however, a more gradational approach to assess the vulnerability of infrastructure to volcanic ash is required and fragility functions can be used in such situations.
Fragility functions are probabilistic vulnerability models that describe the probability that a damage or functional state will be reached or exceeded for a given hazard intensity (Singhal and Kiremidjian 1996, Choi et al. 2004, Rossetto et al. 2013, Tarbotton et al. 2015). They allow the quantification of risk and provide a basis for cost-benefit analysis of mitigation strategies (Jenkins et al. 2014a, Wilson et al. 2014). Data used to create fragility functions can be derived from a variety of sources and is generally classified into four types: 1) empirical data from field and/or laboratory observations, 2) analytical data from numerical modelling, 3) data from expert opinions / judgement, 4) hybrid data from a combination of these approaches (Porter et al. 2012, Wilson et al. 2017). Variables can take on either discrete values or a continuous range of values, and as such fragility functions can comprise a number of forms (Porter et al. 2007, Wilson et al. 2017). The reader is referred to Wilson et al. (2017) for more detailed discussion on impact data types and fitting of volcanic fragility functions. To date, most volcanic fragility functions have focused on damage, particularly the physical damage to buildings and roofs. However, loss of infrastructure functionality may be as, if not more, important than damage in some cases. The loss of infrastructure functionality can have potentially large implications for governments and local authorities (e.g. deciding whether to shut down parts of a network) and cause substantial, sometimes unexpected, effects on end-users of critical infrastructure such as drivers and residents through a reduced ‘level of service’. It is important to note that there are often many impact types, along with factors such as infrastructure characteristics and decision-making by authorities, which influence whether networks remain open. For example, in New Zealand a main state highway was closed following < 3 mm of ash accumulation from the 2012 Tongariro eruption (Jolly et al. 2014, Leonard et al. 2014), but in Argentina after the 2011 Cordón Caulle eruption, many key roads remained open despite receiving up to 50 mm of ash (Craig et al. 2016a). Such differences are likely due to duration of disruption, threat of future ashfall, criticality of the road, previous experiences with volcanic ash and different tolerance levels in different regions (Craig et al. 2016a).
It is difficult to incorporate all factors which contribute to surface transportation closure (Table 2) into volcanic fragility functions. However, these variations in damage and disruption can be accounted for by introducing estimates of uncertainty within fragility functions. Uncertainties include aleatory uncertainties such as natural variations between volcanic eruption hazard severity and resulting infrastructure response, and epistemic uncertainties such as those associated with limited data or choosing appropriate HIMs and ISs (Rossetto et al. 2014, Wilson et al. 2017). These uncertainties are outlined more fully by Wilson et al. (2017). Sometimes, HIMs cannot be measured in the field in real time (Jenkins et al. 2013, Wilson et al. 2017); for example, it may be dangerous to measure ash characteristics due to the ash or other volcanic hazards potentially impacting health. Laboratory experimentation can be used to reduce epistemic uncertainty through the provision of larger impact data sets. Additionally, the controlled nature of laboratory experimentation means that particular conditions can be assessed, and uncertainty can often be reduced in this respect as well. However, the introduction of new data that differs from previous data may reflect either aleatory or epistemic uncertainty. As was conducted by Wilson et al. (2017), where possible, we account for uncertainties by calculating the probability that the surface transportation mode could be in each IS at each HIM value. Binning the HIM values and adopting the median HIMs on each chart accounts for the variation in values (Wilson et al. 2017). It is important that uncertainties are subsequently transferred across to plans and strategies that utilise fragility functions, ideally using probabilistic techniques to ensure that different outcomes are considered (Jenkins et al. 2014a). However, as new qualitative field data and quantitative data from further laboratory experiments becomes available, existing datasets can be reviewed and fragility functions adjusted accordingly, thus reducing overall uncertainty.
Selection of appropriate HIMs and establishment of representative IS thresholds are crucial to produce robust fragility functions (Rossetto et al. 2013). Wilson et al. (2014) and Wilson et al. (2017) highlight that fragility functions in volcanology are poorly developed compared to those from other natural hazard disciplines. They also outline that the range of intrinsic volcanic hazard properties, such as the particle size of ash, can cause different impacts, leading to difficulties in deriving functions. Additionally, much of the data that has informed volcanic fragility functions is qualitative or semi-quantitative with limited quantitative empirical, analytical or theoretical data from field studies or laboratory experiments.
To date, the most common HIM for volcanic ash fragility functions to assess surface transportation disruption is the thickness of ash on the ground (Wilson et al. 2014). This is largely due to its extensive use in existing impact datasets and applicability to hazard model outputs at the time. Previous IS thresholds that have been defined using thickness as the HIM (Wilson et al. 2017) are shown in our results (section 4.1) for comparative purposes. Of particular note is that IS1 (reduced visibility, loss of traction, covering of road markings and/or road closures) was previously identified as occurring with thicknesses of ~1 mm or more, due to reduced traction (technically known as skid resistance) and impaired visibility disrupting most transportation types. Impacts to maritime transportation have not been considered in detail in relation to thickness, as most ash types (with the exception on pumiceous material, which can form pumice rafts) do not accumulate, or are readily dispersed, on water and are thus difficult to monitor. The majority of data used to inform previous ISs was from qualitative post-eruption impact assessments and media reports. Observations from Barnard (2009), who conducted a number of semi-quantitative field experiments on Mt. Etna, Italy, also informed ISs for road transportation where thicknesses exceed 50 mm.
Blake (2016), and Blake et al. (2016 and 2017a) conducted targeted experiments under controlled laboratory conditions to investigate the most frequent surface transportation impact types identified from post-eruption assessments: skid resistance reduction, visibility impairment and road marking coverage (see Additional file 1 for key findings summary). The studies assessed the effect of key HIMs (Table 3) on functionality and we refer the reader to each of the corresponding papers for detailed information on the methodologies adopted for the experiments including different approaches used to measure HIMs.
The studies by Blake (2016), Blake et al. (2016) and 2017a) suggest that ash thickness and ash-settling rate are the most critical HIMs for the assessment of surface transportation functionality during initial volcanic ashfall events, particularly as they are two of the most readily measured variables in the field. There would rarely be impacts to transportation when there is no ground accumulation of ash and no suspended ash in the atmosphere, which further emphasises the importance of these two HIMs. However, recent laboratory work has also revealed that alternative HIMs to ash thickness and settling rate (Table 3) should not be disregarded.
Figure 2 summarises previous and current developments to volcanic ash fragility functions for surface transportation. Most steps in the diagram indicate anticipated improvements to data accuracy. However, as fragility functions are developed, requirements for more impact data are often introduced to test and improve new findings and reduce uncertainty.
Impact state thresholds
Using the key findings of the skid resistance and road marking coverage laboratory studies, we refine the IS thresholds for surface transportation established by Wilson et al. (2017) which adopt ash thickness as the HIM (Fig. 2). New thresholds are applied directly from laboratory study analysis results but some require rounding to the nearest order of magnitude to account for uncertainties such as those associated with the variation in results between individual tests and lack of extensive datasets in some cases. This is achieved using the authors’ expert judgement, particularly through application of existing knowledge of surface transportation impacts by volcanic ash gained from field and laboratory observations. We remove previously suggested correlations between visibility and ash thickness because, as stated by Blake (2016), it is “illogical to associate an atmospheric-related impact to a ground-based measurement”, especially as we do not consider effects from remobilised ash in this paper. Laboratory work using ash thickness as the core HIM considered paved surfaces on roads and at airports. Railway tracks were not considered in detail, partly because there has been only one recorded instance of track-wheel adhesion loss following ashfall (Fig. 1b) and the effects were complicated by snowfall at the time. As such, we do not provide any refinements for rail transportation ISs in relation to ash thickness and the previously established thresholds for rail transportation are therefore unchanged by our study. For maritime transportation, a challenge for fragility function development is that due to ash dispersing in water, impact mechanisms from tephra cannot easily be linked to deposition thickness as they can for road, rail and airports. However, as with other forms of transportation, and as occurs in dense fog, it is likely that navigation by sea can be disrupted or even temporarily halted by visibility impairment during ashfall. Therefore, maritime transportation impacts are segregated from the ash thickness HIM and assessed solely in relation to visibility impairment.
As ash deposit thickness has a debatable impact on visibility impairment, and due to recent developments in both field monitoring equipment, and ash dispersion and fallout models which provide settling-rate outputs (Blake 2016), we consider ash-settling rate as an alternative HIM (Fig. 2) and propose new IS thresholds. Our settling rate IS thresholds are developed using a hybrid approach. They are informed by (a) direct empirical laboratory results, adjusted using expert judgement and rounding, (b) literature for shipping in Auckland’s Waitemata Harbour (e.g. Harbourmaster and Maritime New Zealand information for maritime impacts (Auckland Council 2014, MNZ 2015)) and Auckland Airport and CAA guidelines for airport impacts (Auckland Airport 2008, CAA 2008), and (c) expert consultation with critical infrastructure managers (Deligne et al. 2015, Blake et al. 2017b, Deligne et al. 2017). We establish IS thresholds for all modes of surface transportation related to visibility impairment. This is achieved by means of comparison with operational guideline information and impact states expected for corresponding visual ranges in foggy conditions, the data sources of which are discussed in Blake (2016):
IS thresholds for roads are largely based on comparisons with empirical studies involving driver simulations in fog.
Comparisons with operational procedures for fog in Auckland are used to establish thresholds for airports and maritime transportation, and thus these thresholds should be treated as more area-dependent than for road.
IS thresholds for rail are the most subjective of the four transportation modes: we implement higher threshold values than for road due to the often automated controls for the spacing of locomotives along the network and additional technological safety systems which visibility impairment does not affect.
Hazard intensity metric analysis
We conduct a comparative analysis of HIMs other than ash thickness by assessing their relative importance to surface transportation disruption. Without extensive datasets for all HIMs, this is achieved by applying simple rank values to each HIM for the core HIMs of ash settling rate and at different ash thicknesses. HIMs are ordered by relative importance to one another and given a rank value of between 1 and 6. Although somewhat subjective, the lower the rank value applied, the greater the influence of that HIM on surface transportation disruption. HIMs of similar importance are given the same rank value.
Fragility function development
We use procedures described by Wilson et al. (2017) for volcanic fragility function development, the basic methodological principles of which are summarised as follows:
Assign each data point a HIM value and IS value;
Order data set by increasing HIM value;
Group into HIM bins, such that each bin has approximately the same number of data points;
Calculate probability of being greater than, or equal to, each IS of interest;
Obtain discrete HIM values by taking the median of each HIM bin.
Apply functions as linear segments defined by the available data points after the HIM binning process (as described by Wilson et al. 2017).
New road and airport fragility functions for ash thickness are established through modification of those proposed by Wilson et al. (2017). All points representing median thickness within the HIM bins obtained from post-eruption data are retained as the number of post-eruption records remains unchanged. New points are added to the chart to appropriately display the new findings from IS threshold adjustment following laboratory work, with a focus on improving functions for relatively thin deposits (the focus of laboratory work). More substantial updates are made to airport fragility functions as we also incorporated the duration of airport closure. However, we stress that some points have been corrected using best judgement in order to fit with guidelines outlined by Wilson et al. (2017).
The IS thresholds for ash-settling rate are used to establish separate fragility functions for road, rail and maritime transportation, and at airports. Without reliable field data it is difficult to follow Wilson et al.’s (2017) methodology for fragility function production, especially to accurately calculate probabilities of ash-settling rate values equalling or exceeding each IS. However, we produce functions using empirical laboratory studies and comparisons to research for fog, to indicate expected impact on visibility and vehicles at near-ground level. This is achieved through adopting the basic principles and rules outlined by Wilson et al. (2017). As we cannot group data into HIM bins and obtain discrete HIM values, specific ash-settling rates are chosen based on key changes in impact states instead. We use best judgement to assign probabilities and these are open to revision in future.
Limitations of methodology
Besides the general limitations outlined by Wilson et al. (2017) for fragility function production, our methodology for fragility function improvement through empirically informed data contains several additional limitations which may also introduce uncertainty:
The laboratory experiments used to inform fragility functions were based on the assessment of key impact types previously identified from post-eruption observations. However, observations of volcanic ash impacts to transportation are relatively limited (at least compared to impacts from other hazards such as earthquake damage to buildings) with an apparent increase in frequency of events after 1980. We suggest that this increase is due to heightened awareness and land-monitoring of volcanic hazards following the 1980 Mount St Helens eruption, and recent increases in the number of motor vehicles and general population growth and infrastructure development in volcanically active areas worldwide (TRB 1996). Additionally, there is a higher frequency of impacts recorded for roads than for other modes of surface transportation, likely due to more road networks in the areas affected by volcanic activity. Therefore, the relevance of further impact types may be underestimated by our study, and future observations and additional laboratory testing will verify the extent of this.
The empirical datasets we use are constrained by the equipment and set-ups that were adopted in the laboratory studies. For example, the skid resistance testing used a Pendulum Skid Resistance Tester, which was restricted to investigating small (< 10 mm) ash thicknesses (Blake et al. 2017a). Furthermore, it was unfeasible to investigate all possible ash characteristics (e.g. every soluble component option, all moisture regimes) during laboratory testing, so our results are limited to those characteristics that we did investigate.
Laboratory experiments are generally time and resource intensive. As the experiments by Blake (2016), and Blake et al. (2016 and 2017a) were the first to be developed and conducted to specifically assess ash impacts on individual transportation components, the datasets are currently relatively small. The repetition of laboratory experiments will help to reduce uncertainty in the future but our results are limited to those characteristics investigated to date.
Results and discussion
Ash thickness fragility function improvements
Figure 3 shows IS thresholds for surface transportation, which were defined using ash deposit thickness as the HIM. It includes thresholds for rail that were unmodified from Wilson et al. 2017, and original (grey) and newly revised (red) thresholds for roads and airports; the revised thresholds were informed by key findings from recent laboratory experiments that can be directly related to ash accumulation (i.e. skid resistance reduction and road marking coverage (Blake et al. 2016, 2017a), in addition to new post-eruption data where available. Figure 3 illustrates that some disruption to roads and airports can occur with an ash thickness of ~ 0.1 mm, an order of magnitude less than previously suggested by most anecdotal data. Figure 3 also suggests that larger thicknesses of ash may not always result in greater disruption. For example, an ash thickness of ~ 10 mm on roads could potentially lead to less disruption than a thickness of ~ 5 mm as skid resistance reduction is more likely at 5 mm. Although the impacts of reduced visibility (accounted for separately) may mask such effects overall, we suggest particularly elevated disruption to road transportation from ash thicknesses between ~ 2.5 and 5.0 mm. At this range, all road markings are covered and especially reduced skid resistance occurs. Although limited, the post-eruption data available (Blake 2016, Blake et al. 2016 and 2017a) indicate that fewer impacts are identified when ash is ~ 5–20 mm thick, supporting the hypothesis of elevated disruption regions on the thickness scale. Fluctuating intensities of road transportation disruption with thickness have not been identified in the past, highlighting the importance of laboratory testing and the complexities that can be involved in determining accurate IS thresholds.
Figure 4 shows corresponding fragility functions for roads, updated from Wilson et al. 2017. Two new points (at 0.1 and 5.0 mm ash thickness) have been added to appropriately account for new findings from laboratory work for IS1 (i.e. disruption in the form of skid resistance reduction (Blake et al. 2017a) and road marking coverage (Blake et al. 2016)). The decrease in function observed for IS1 when ash thickness exceeds 5.0 mm is due to the potential increase in skid resistance; it is largely informed by recent laboratory findings (which do have limitations – see section 3.4) but is somewhat supported by semi-quantitative field observations. A decreasing fragility function breaks one of the core guidelines established by Wilson et al. 2017, which states that “functions should not decrease as the HIM value increases”. However, this guideline was established for damage rather than functional loss, and for when there is limited data to base vulnerability estimates on (i.e. not accounting for detailed empirical studies).
Airports can be closed due to ash in nearby airspace, without any ground accumulation of ash (Guffanti et al. 2009). Indeed, the International Civil Aviation Organisation (ICAO) advise that “aircraft should avoid volcanic ash encounters” (p.1–1), although “the operator is responsible for the safety of its operations” (p.2–1) and is required to complete a risk assessment as part of its safety management system, and have satisfied the relevant national (or supra-national) CAA before initiating operations into airspace forecast to be, or at airports known to be, contaminated with volcanic ash (ICAO 2012). Aircraft will likely become grounded due to a reduced runway friction coefficient when ash deposits exceed 1 mm (ICAO 2001, Wilson et al. 2017). Furthermore, severe deterioration in local visibility can result when engine exhausts from aircraft taxiing, landing and taking off disturb ash on the runway (ICAO 2001).
We assess functionality loss of airfields by applying the key findings from skid resistance and road marking coverage experiments for airfield concrete surfaces covered by ash (Fig. 3). It is important to consider such impact types, as aircraft operation may be possible when airborne ash concentrations are below aviation authority, and airline and airport guideline values. Although vehicle operation on airfields by ground staff could occur, even when aircraft are grounded, we focus on aircraft operations for the fragility function chart (Fig. 5). These fragility functions estimate the temporal duration of airport function assuming that the surrounding airspace is open and prior to any clean-up. Some functionality loss of the airport surface is possible between 0.1 and 1.0 mm due to markings becoming covered and reduced skid resistance (Blake et al. 2016, 2017a) before the airport is likely closed if ash accumulates to > 1 mm thickness. We display the temporal component for airport closure graphically as separate ISs (Fig. 5) as such information may be beneficial for end-users of fragility functions.
New ash-settling rate fragility functions
Figure 6 shows IS thresholds for visibility with ash-settling rate adopted as the HIM. Forecasts for visibility disruption are particularly useful for areas where there is minimal ash accumulation on the ground (i.e. during initial ashfall events or subsequent events following thorough clean-up), as well as for maritime transportation and where surfaces are wet due to any re-suspension of ash into the atmosphere being minimised by water. Many of the thresholds established in Fig. 6, and depicted in new fragility function charts in Fig. 7, are particularly influenced by decisions made by local transportation authorities and we stress that our established thresholds are preliminary and open for improvement. Best judgement by the authors is used to determine some thresholds. For example, IS1 starts at 10 g m−2 h−1 for road, but at 20 g m−2 h−1 for rail, due to the relative resilience of rail to airborne ash, which results from more automated controls and fixed paths of travel (Blake et al. 2017b). IS thresholds may require adaptation to be compatible in other areas, especially where infrastructure types and associated technology differ. Thresholds are established for visibility only and do not consider other potential disruption caused by airborne volcanic ash such as ingestion into engines or the abrasion of windscreens.
A literature search revealed no quantitative or semi-quantitative data for visual ranges at specified ash-settling rates following previous eruptions worldwide. Figure 7 shows fragility functions for the ash-settling rate HIM, based entirely on empirical laboratory studies and comparisons to research for fog, to indicate expected impact on visibility and vehicles at near-ground level. Further extensive laboratory testing, in addition to syn- and post-eruption field surveys, will help to refine probabilities. Studies of ash remobilisation and re-suspension will likely improve our understanding of potential links between ash thickness and settling rate.
We emphasise that our studies were carried out in the context of transportation infrastructure found in New Zealand (e.g. ash characteristics and pavement properties found in the country) and that fragility functions may vary in different parts of the world. However, we suspect the trends will remain similar.
Multiple hazard intensity metrics
Figure 8 presents the results of comparative analysis of six additional HIMs identified during laboratory experimentation as having effects on surface skid resistance and road marking coverage.
This was achieved by using best judgement considering recent laboratory experiments to apply simple rank values to each HIM. The core HIM of ash thickness was used with the values of alternative HIMs dependent on relative importance to one another.
It is clear from Fig. 8 that as ash increases in thickness on the ground, the effect of different HIMs on surface transportation functionality changes. For example, particle size and colour play an important role below ~1.0 mm thickness due to the effect of fine-grained and light-coloured ash on road marking coverage, but less of a role when ash thickness exceeds ~1.0 mm. However, the wetness of ash is important compared to other HIMs when ash thicknesses are >10 mm because it influences how readily ash binds together, in turn affecting how easily vehicles can drive through thicker deposits.
Arguably even more relevant for fragility functions is the relative importance of additional HIMs for visibility impairment (Fig. 9). Unlike ash thickness, there is no evidence to suggest that the importance of different HIMs relative to one another changes as settling rate changes. However, results from Blake (2016) indicate that, as for thickness, the effect of additional HIMs has a lesser effect on functionality loss for greater ash-settling rates. This is likely due to the more dominant effect of there simply being more ash particles in the atmosphere. The HIM characteristics responsible for greater disruption are largely the same as for ash thickness (Fig. 8), with the exception of ash particle shape; irregular-shaped ash particles may lead to greater disruption when airborne due to more light reflectance and subsequently lower visual range, whereas spherical-shaped ash particles can lead to greater disruption when on paved surfaces as a result of lower skid resistance. Particle size is clearly a crucial ash characteristic to consider when assessing surface transportation disruption, especially for < 10 mm thicknesses.
Further repeated laboratory experiments to investigate the effect of each ash characteristic on every surface transportation impact type, along with detailed post-eruption field sampling and analysis, and subsequent computational probabilistic modelling will assist to fill this gap in knowledge. In the meantime, and in the absence of extensive datasets, it is difficult to evaluate the precise quantitative effect of alternative HIMs (i.e. those other than ash thickness and settling rate) on surface transportation disruption from volcanic ash and perform meaningful statistical analysis. However, we suggest that the importance of multiple HIMs can be accounted for by considering ‘error boundaries’ that illustrate uncertainty around existing functions for ash thickness and settling rate (Fig. 10) (although other uncertainties also exist). The conceptual diagram (Fig. 10) shows that with thicker ash or greater ash-settling rates, alternative HIMs (e.g. particle size, colour, shape) become less important with it being more beneficial to solely consider the core HIMs of ash thickness or ash-settling rate when forecasting impact levels.
We conclude that ash thickness and settling rate are the critical HIMs for the assessment of surface transportation functionality during volcanic ashfall events. However, due to current difficulties in quantifying the impact that ash thickness has on visibility impairment (the key impact type relatable to ash-settling rate and relevant for all modes of surface transportation), the two HIMs are not directly comparable and should be considered separately. For the ash thickness HIM, we identify the potential for fluctuating intensities of road transportation disruption as thickness increases, a feature that has not been identified in the past from empirical studies and is a product of experimental data obtained from targeted laboratory testing for specific impact types. We highlight that disruption can occur at an order of magnitude less than previously indicated (i.e. for thicknesses of ~0.1 mm rather than 1.0 mm) due to the potential for surface marking coverage; fragility functions for road and airports have been updated accordingly. Although highly subjective, preliminary fragility functions for visibility with ash-settling rate adopted as the HIM have been established using empirical data alone and by making comparisons to impacts previously identified in fog.
Our analysis of alternative HIMs (i.e. other than the critical HIMs of ash thickness and settling rate) and their effect on volcanic ash fragility function development for surface transportation leads to several key findings:
Although ash thickness and settling rate should be treated as core HIMs for the assessment of surface transportation disruption, alternative HIMs should not be overlooked.
Ash particle size is identified as the next most important HIM for functionality loss, especially when airborne concentrations and accumulations of ash on the ground are relatively small.
For different ash thicknesses, the relative importance of alternative HIMs may be different. However, for different ash-settling rates there is no evidence to suggest that alternative HIMs change in their relative importance to one another.
As ash thickness and ash-settling rates increase, alternative HIMs have less of an influence on surface transportation functionality loss. This confirms that it is indeed appropriate to consider ash thickness and settling rate as core HIMs for surface transportation impact assessments.
Without extensive datasets, it is difficult to accurately model the effect of alternative HIMs on disruption. However, we suggest that they could be incorporated into fragility functions by implementing ‘error boundaries’, alongside descriptors for the specific ash characteristic features responsible for increased probabilities of impact states being reached or exceeded.
Our findings support the need to provide forecasts and actively monitor a range of ash characteristics in areas that may be affected by volcanic ashfall, especially the thickness of deposits on the ground and ash-settling rate, but also other ash properties including particle size distributions, colour, and shape. This should be prioritised where there are abundant exposed surface transportation networks and populations: potential disruption can occur with ~0.1 mm ash thickness on the ground, depending on the ash characteristics present. Additional (particularly quantitative) datasets derived from new eruptions and laboratory tests will assist with the advancement of volcanic ash fragility functions for surface transportation, thus allowing further improvements in risk assessments and contingency planning in volcanically active regions.
Civil aviation authority
Hazard intensity metric
International civil aviation organisation
Maritime New Zealand
Pyroclastic density current
Transport research board
United nations international strategy for disaster reduction
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We acknowledge all the technical and financial support received to conduct the extensive laboratory experimentation on surface transportation impacts from volcanic ash. Although the results were not directly covered in this paper (they are covered in separate publications), many of the key findings would not have been possible without these initial studies. Particular thanks are expressed to the New Zealand Earthquake Commission (EQC), Determining Volcanic Risk in Auckland (DEVORA) project, Natural Hazards Research Platform (NHRP), University of Canterbury Mason Trust, and GNS Science Core Programme funding scheme for their recent support to the authors. Also, the financial support from the University of Canterbury Department of Geological Sciences is greatly appreciated, which allowed the purchase of key laboratory equipment. Daniel Blake would like to thank his additional PhD co-supervisors, Jan Lindsay (The University of Auckland) and Jim Cole (University of Canterbury), for their edits, guidance and advice throughout this study. This paper was improved by the comments of two anonymous reviewers.
The authors declare that they have no competing interests.
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Blake, D.M., Deligne, N.I., Wilson, T.M. et al. Improving volcanic ash fragility functions through laboratory studies: example of surface transportation networks. J Appl. Volcanol. 6, 16 (2017). https://doi.org/10.1186/s13617-017-0066-5
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