Lava flow hazard map of Piton de la Fournaise volcano

Abstract

Abstract. Piton de la Fournaise, situated on La Réunion island (France), is one of the most active hot spot basaltic shield volcanoes worldwide, experiencing at least two eruptions per year since the establishment of the volcanological observatory in 1979. Eruptions are typically fissure-fed and form extensive lava flow fields. About 95 % of some ∼ 250 historical events (since the first confidently dated eruption in 1708) have occurred inside an uninhabited horseshoe-shaped caldera (hereafter referred to as the Enclos), which is open to the ocean on its eastern side. Rarely (12 times since the 18th century), fissures have opened outside of the Enclos, where housing units, population centers, and infrastructure are at risk. In such a situation, lava flow hazard maps are a useful way of visualizing lava flow inundation probabilities over large areas. Here, we present the up-to-date lava flow hazard map for Piton de la Fournaise based on (i) vent distribution, (ii) lava flow recurrence times, (iii) statistics of lava flow lengths, and (iv) simulations of lava flow paths using the DOWNFLOW stochastic numerical model. The map of the entire volcano highlights the spatial distribution probability of future lava flow invasion for the medium to long term (years to decades). It shows that the most probable location for future lava flow is within the Enclos (where there are areas with up to 12 % probability), a location visited by more than 100 000 visitors every year. Outside of the Enclos, probabilities reach 0.5 % along the active rift zones. Although lava flow hazard occurrence in inhabited areas is deemed to be very low (< 0.1 %), it may be underestimated as our study is only based on post-18th century records and neglects older events. We also provide a series of lava flow hazard maps inside the Enclos, computed on a multi-temporal (i.e., regularly updated) topography. Although hazard distribution remains broadly the same over time, some changes are noticed throughout the analyzed periods due to improved digital elevation model (DEM) resolution, the high frequency of eruptions that constantly modifies the topography, and the lava flow dimensional characteristics and paths. The lava flow hazard map for Piton de la Fournaise presented here is reliable and trustworthy for long-term hazard assessment and land use planning and management. Specific hazard maps for short-term hazard assessment (e.g., for responding to volcanic crises) or considering the cycles of activity at the volcano and different event scenarios (i.e., events fed by different combinations of temporally evolving superficial and deep sources) are required for further assessment of affected areas in the future – especially by atypical but potentially extremely hazardous large-volume eruptions. At such an active site, our method supports the need for regular updates of DEMs and associated lava flow hazard maps if we are to be effective in keeping up to date with mitigation of the associated risks.

Reappraisal of gap analysis for effusive crises at Piton de la Fournaise

“…As a result it is now used operationally to make model-based hazard assessments for any new effusive event (Peltier et al 2020 ). Since 2014, this has been embraced as part of an effusive crisis response protocol (Harris et al 2017a , 2019 ), and contributed to the update of the long term lava flow hazard map (Chevrel et al 2021 ). This has resulted in generation of a GIS (Geographic Information System) based on lava flow modelling below which there are layers including vulnerable structures and highways to allow risk assessment (cf.…”

“…Piton de la Fournaise (La Réunion island, Indian Ocean, Fig. 1 a) is a highly active dominantly effusive volcano, which has experienced two eruptions per year on average since the creation of Observatoire Volcanologique du Piton de la Fournaise – Institut de Physique du Globe de Paris (OVPF-IPGP) in 1979 (Peltier et al 2009 ; Roult et al 2012 ; Chevrel et al 2021 ). Many actions are implemented by OVPF-IPGP to mitigate hazard prior to and during any eruption at Piton de la Fournaise, where operations carried out by the OVPF-IPGP are mandated by the French government via a national emergency plan.…”

“…A public report (in French), published in 2012, evaluated volcanic hazards at Piton de la Fournaise, and presented a comprehensive lava flow hazard map (Di Muro et al 2012 ; Davoine and Saint-Marc 2016 ). This hazard map has recently been updated by Chevrel et al ( 2021 ). Indeed, for all eruptions since 1931, lava flow inundation areas during all eruptions have been mapped to allow generation of an exhaustive database for lava flow coverage (Derrien 2019 ; Chevrel et al 2021 ).…”

“…This hazard map has recently been updated by Chevrel et al ( 2021 ). Indeed, for all eruptions since 1931, lava flow inundation areas during all eruptions have been mapped to allow generation of an exhaustive database for lava flow coverage (Derrien 2019 ; Chevrel et al 2021 ). We see that, since 1950 (i.e., the starting date of the gap analysis as for Tsang and Lindsay 2020 ), only three eruptions have occurred in inhabited areas outside of the Enclos (Fig.…”

“…In addition to the lava flow hazard maps published by Di Muro et al ( 2012 ) and Chevrel et al ( 2021 ), a response protocol is in place that involves real-time model-based assessment of likely lava flow inundation areas. This has been in place since 2014 (Harris et al 2017a , 2019 ), and has been implemented for all eruptions since that of June 2014.…”