Urban Fire (During/Following Volcanic Eruption)

Primary reference(s)

Baxter, P.J., R. Boyle, P. Cole, A. Neri, R. Spence and G. Zuccaro, 2005. The impacts of pyroclastic surges on buildings at the eruption of the Soufrière Hills volcano, Montserrat. Bulletin of Volcanology, 67:292-313.

ISO, 2020. ISO/TS 17755-2:2020. Fire Safety – Statistical data collection – Part 2: Vocabulary. Accessed 18 October 2020.

Additional scientific description

All fires, regardless of trigger, need three elements to sustain themselves: fuel, oxygen, and heat. The heat thermally decomposes (pyrolysis) the fuel into a hot gas (volatiles) which mixes with the oxygen which then creates a combustible gas namely the flame, the edge of which is where the combustion reaction happens. The flame can then transfer the heat through: radiation to other objects that it has a line of sight to; convection of hot gases; and conduction through the fuel that is pyrolysing (Drysdale, 2011).

Most cellulosic materials pyrolyse between 150 and 500°C to producing volatiles (Zhou et al., 2013). These volatiles will spontaneously ignite if the surface of the pyrolysing object reaches between 450 and 600°C, or between 300 and 450°C if there is a flame already present (Drysdale, 2011).

As the fire grows within a room, the rate at which fuel is consumed increases if there is sufficient oxygen within the room. If oxygen levels are low, then the fire will ‘move’ towards more oxygen-rich environments, this causes a phenomenon called flashover, where flames (typically about 1 m long under laboratory conditions for a standard door and a 9 m2 room) are ejected from compartment openings (Drysdale, 2011). In windier conditions, these can increase up to 3 m for a standard door-sized opening (de Koker et al., 2020), however this is not a linear relationship, and above a certain wind speed (dependent on the size of flame and other spatial and material properties) the length of the flame will not increase any further as convective cooling due to the wind reduces the amount of heat energy within the flame. If the spatial distribution of homes is close, then fires can spread from one building to another. The separation distance will be determined by the specific typology of the compartment/ room, its openings, and the fuel therein.

Four areas should be considered in relation to fire triggered specifically by volcanic eruptions: lava flows, pyroclastic density currents (PDCs), hot tephra, and ground shaking.

• Lava flows: high temperature lava (1000–1200°C), moving usually in the order of 4–5 km/hr but exceptionally up to tens of kilometres per hour (e.g., the lava lake at Nyiragongo; Tedesco et al., 2007; Balagizi et al., 2018) may interact with combustible materials in its path and cause ignitions.
• Pyroclastic Density Currents: PDCs are hot, unstoppable, gas-particle mixtures that race across the ground surface at velocities of tens to hundreds of kilometres per hour and have temperatures typically between 200 and 600°C (Dufek et al., 2015). At these temperatures pyrolysis will occur for most cellulosic materials and fire damage will be observed (Baxter et al., 2005). The noxious gases often replace oxygen within the PDCs and this, combined with the flow speed which takes energy away from the pyrolysing surface (Babrauskus, 2003), limits the probability of ignition during the immediacy of the current. However, the energy stored in the materials and residual temperatures when oxygen is present can cause ignitions. PDCs can cause large amounts of structural damage creating openings and breaking of windows for hot ash to ingress and they can also redistribute fuel load (trees etc.) facilitating urban fire spread. PDCs can also cause ignitions in a similar manner to ground shaking (see below).
• Hot tephra: large lapilli, rocks and bombs, have the ability to cause ignition of dry combustible materials in and around urban structures, while the accumulation of hot volcanic ash (>300–400°C) could accumulate on surfaces to cause fires and as for fire brands created by wildfires, could ingress into structures through openings and cause ignitions (Baxter et al., 2005).
• Ground shaking: caused by the eruption and potential to be felt over large distances, ground shaking can unsettle open flames or disrupt energy supplies within buildings and could thus be a triggering event for a fire.

If fires are triggered in one or more rooms in a home, then homes can be severely affected by fire damage during/following a volcanic eruption.

Urban fires during/following a volcanic eruption have not been systematically recorded in detail to date. Baxter et al. (2005) have created a six-point damage scale which incorporates fire as an observed effect for PDCs.

Metrics and numeric limits

Not available.

Examples of drivers, outcomes and risk management

Urban fires can cause large losses of life and livelihood: 95% of global deaths (180,000 to 300,000 people per year) and injury (10 million Disability-Adjusted Life Years lost each year) from fire occur in low and middle income countries (Mock et al., 2008; WHO, 2018); fire costs 1% of global GDP (The Geneva Association, 2014); and those who are at greatest risk (the urban poor) generally have little in means of protection against losses. In addition, those at greatest risk of death and injury are the old and the young due to lack of knowledge in how to respond and lack of mobility when trying to respond (Rush et al., 2020).

Urban fires are linked to density of structures and type of construction. Highly dense settlements (i.e., informal settlements or slums) are likely to have large areas of structures that are in close proximity to one another which will facilitate fire spread. This, when combined with combustible construction can lead to large-scale fire events. Combustible construction here refers not only to the material used in construction but also how the structure is sealed against the weather. For instance, a steel walled structure that has any gaps at joints sealed with paper or plastic materials would be susceptible to fire attack from another structure (Walls et al., 2017, 2018; Kahanji et al., 2019).

The density of settlements and the construction of the buildings is also inextricably linked to the wealth of the inhabitant with the urban poor being less able to live in space and less able to live in non-combustible buildings or to maintain buildings in such a way that fire events are more readily controlled. There are also areas, historic in nature, that have high structure density and combustible construction such as the fire in Shangri-La that occurred on 11 January 2014 (Associated Press, 2014).

Baxter et al., (2005) gave a good summary of urban fires known to have followed volcanic eruptions. Volcanic eruptions which create PDCs can break windows in homes. This will allow hot ash and other tephra to ingress the homes and, if combustible materials are present, can cause large fires to occur. The high temperatures of the PDCs can also pyrolyse and char roof structures. This was seen at the Montagne Pelée and St Vincent eruptions in 1902, Vesuvius in AD79, Montserrat in 1997; while extensive scorch zones were observed 1–3 km from the periphery of the PDC margin following the eruptions at both Mount St Helens in 1980 and Mt Lamington in 1951.

Jenkins et al. (2017) noted a few instances of fires caused by the lava flow following the Fogo eruption in 2014–2015. They highlighted that although minimal damage due to fire occurred in this eruption due to lava flows, this could have been greater if the urban area were made of more flammable construction (such as seen in Hawai’i) and fuel (such as gas canisters) had not been removed in a timely manner.

During the 2002 eruption of Nyiragongo volcano, between 60 and 100 people were killed owing to the explosion of a gas station surrounded by lava, and about 470 were injured with burns, fractures and/or gas intoxication (Tedesco et al., 2007; Balagizi et al., 2018).

Use of non-combustible construction materials and ensuring that buildings remain well sealed during volcanic eruptions are key control measures. This combined with preparedness in dealing with lava flows and securing energy supplies can reduce the impact of urban fires during/following volcanic eruptions.