Lava flow hazard map of Piton de la Fournaise volcano
Authors: Chevrel Magdalena Oryaëlle , Favalli Massimiliano , Villeneuve Nicolas , Harris Andrew J. L. , Fornaciai Alessandro , Richter Nicole , Derrien Allan , Boissier Patrice , Di Muro Andrea , Peltier Aline 
Published: August 2021

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.