A Framework for Probabilistic Multi-Hazard Assessment of Rain-Triggered Lahars Using Bayesian Belief Networks

Tierz, P. , Woodhouse, M., Phillips, J., Sandri, L., Selva, J., Marzocchi, W., & Odbert, H. (2017). A Framework for Probabilistic Multi-Hazard Assessment of Rain-Triggered Lahars Using Bayesian Belief Networks. Frontiers in Earth Science, 5, [73]. https://doi.org/10.3389/feart.2017.00073 Publisher's PDF, also known as Version of record License (if available): CC BY Link to published version (if available): 10.3389/feart.2017.00073 Link to publication record in Explore Bristol Research PDF-document This is the final published version of the article (version of record). It first appeared online via Frontiers at http://journal.frontiersin.org/article/10.3389/feart.2017.00073/full. Please refer to any applicable terms of use of the publisher. University of Bristol - Explore Bristol Research General rights This document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Full terms of use are available: http://www.bristol.ac.uk/red/research-policy/pure/user-guides/ebr-terms/ ORIGINAL RESEARCH published: 11 September 2017 doi: 10.3389/feart.2017.00073 A Framework for Probabilistic Multi-Hazard Assessment of Rain-Triggered Lahars Using Bayesian Belief Networks Pablo Tierz 1*†, Mark J. Woodhouse 2, Jeremy C. Phillips 2, Laura Sandri 1, Jacopo Selva 1, Warner Marzocchi 3 and Henry M. Odbert 2, 4 1 Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Bologna, Bologna, Italy, 2 School of Earth Sciences, University of Bristol, Bristol, United Kingdom, 3 Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Roma, Rome, Italy, 4 Met Office, Exeter, United Kingdom Volcanic water-sediment flows, commonly known as lahars, can often pose a higher threat to population and infrastructure than primary volcanic hazardous processes such as tephra fallout and Pyroclastic Density Currents (PDCs). Lahars are volcaniclastic flows Edited by: Roberto Sulpizio, of water, volcanic debris and entrained sediments that can travel long distances from their Università degli studi di Bari Aldo source, causing severe damage by impact and burial. Lahars are frequently triggered by Moro, Italy intense or prolonged rainfall occurring after explosive eruptions, and their occurrence Reviewed by: Eisuke Fujita, depends on numerous factors including the spatio-temporal rainfall characteristics, the National Research Institute for Earth spatial distribution and hydraulic properties of the tephra deposit, and the pre- and Science and Disaster Prevention, post-eruption topography. Modeling (and forecasting) such a complex system requires Japan Raffaello Cioni, the quantification of aleatory variability in the lahar triggering and propagation. To fulfill University of Florence, Italy this goal, we develop a novel framework for probabilistic hazard assessment of lahars *Correspondence: within a multi-hazard environment, based on coupling a versatile probabilistic model for Pablo Tierz [email protected] lahar triggering (a Bayesian Belief Network: Multihaz) with a dynamic physical model for †Present Address: lahar propagation (LaharFlow). Multihaz allows us to estimate the probability of lahars of Pablo Tierz, different volumes occurring by merging varied information about regional rainfall, scientific British Geological Survey, The Lyell knowledge on lahar triggering mechanisms and, crucially, probabilistic assessment of Centre, Edinburgh, United Kingdom available pyroclastic material from tephra fallout and PDCs. LaharFlow propagates the Specialty section: aleatory variability modeled by Multihaz into hazard footprints of lahars. We apply our This article was submitted to framework to Somma-Vesuvius (Italy) because: (1) the volcano is strongly lahar-prone Volcanology, a section of the journal based on its previous activity, (2) there are many possible source areas for lahars, and (3) Frontiers in Earth Science there is high density of population nearby. Our results indicate that the size of the eruption Received: 21 April 2017 preceding the lahar occurrence and the spatial distribution of tephra accumulation have Accepted: 23 August 2017 Published: 11 September 2017 a paramount role in the lahar initiation and potential impact. For instance, lahars with ≥ 5 3 Citation: initiation volume 10 m along the volcano flanks are almost 60% probable to occur Tierz P, Woodhouse MJ, Phillips JC, after large-sized eruptions (∼VEI ≥ 5) but 40% after medium-sized eruptions (∼VEI4). Sandri L, Selva J, Marzocchi W and Some simulated lahars can propagate for 15 km or reach combined flow depths of 2 m Odbert HM (2017) A Framework for Probabilistic Multi-Hazard Assessment and speeds of 5–10 m/s, even over flat terrain. Probabilistic multi-hazard frameworks like of Rain-Triggered Lahars Using the one presented here can be invaluable for volcanic hazard assessment worldwide. Bayesian Belief Networks. Front. Earth Sci. 5:73. Keywords: probabilistic hazard assessment, volcanic multi-hazard, lahar triggering, Bayesian belief network, doi: 10.3389/feart.2017.00073 Somma-Vesuvius Frontiers in Earth Science | www.frontiersin.org 1 September 2017 | Volume 5 | Article 73 Tierz et al. Probabilistic Multihazard Framework for Lahars INTRODUCTION landsliding on the loose tephra deposits and (2) this generates an initial volume of water-sediment mixture that flows downslope Explosive eruptions can produce volumes of pyroclastic material and transforms into a lahar. Hazard analyses of lahars have of up to thousands of cubic kilometers (Newhall and Self, often focused on one of these two components, either the 1982). A substantial proportion of the material erupted during temporal and/or spatial features of the lahar triggering, or the these eruptions is deposited over the areas surrounding the calculation of the hazard footprints (i.e., inundated areas) of volcano as tephra fallout (e.g., Bursik, 1998) or transported in lahars of specified total volumes. For temporal triggering, rainfall Pyroclastic Density Currents (PDCs; e.g., Sulpizio et al., 2014). Intensity-Duration (I-D) thresholds have been one of the most The deposits from tephra fallout typically affect areas of hundreds utilized methods (Tuñgol and Regalado, 1996; Lavigne et al., to thousands of square kilometers, while PDC deposits usually 2000a; Van Westen and Daag, 2005; Capra et al., 2010). Some have areal extents of tens to hundreds of square kilometers studies have improved the classical I-D approach (in which just (e.g., Orsi et al., 2004). This input of fresh loose pyroclastic one line marks the occurrence or not of lahars) to produce material into the drainage basins around an erupting volcano probabilistic thresholds by performing logistic regressions on the alters the hydrogeological equilibrium of the basins, leading binary dataset, i.e., event/no event, given an I-D combination commonly to the formation of volcanic water-sediment flows (e.g., Frattini et al., 2009), or through Bayesian data analysis or lahars (Pierson and Major, 2014), which can be considered (e.g., Berti et al., 2012). Other studies have used several a cascade effect in response to intense or sustained events of diagnostic triggering variables (e.g., antecedent and total rainfall), rainfall (Van Westen and Daag, 2005; Pierson et al., 2013). in addition to I-D, and have explored their potential use in The degree to which the drainage basins are disturbed, and probabilistic forescasts of lahar occurrence in nearly real time their subsequent hydrogeomorphic response, depend on many (Jones et al., 2015). For spatial triggering, deterministic physical different factors (Pierson and Major, 2014) including total tephra models of slope stability and/or overland erosion have been volume deposited, grain size distribution of the deposits, changes employed to delimit the areas that can act as sources of water- in the morphology of the catchments, and the type and extent sediment flows (e.g., Iverson, 2000; Frattini et al., 2004). Very few of vegetation damage/loss. Moreover, the rainfall characteristics of these studies have considered the spatial availability of tephra associated with the climate of the volcano’s region have a before the lahar triggering (e.g., Bisson et al., 2007; Volentik et al., profound impact on the frequency and volumes of the generated 2009). Some probabilistic assessments of the areal susceptibility lahars (e.g., Pierson et al., 1992). of such flows have been implemented by exploring the epistemic The risk associated with the occurrence of lahars can be very uncertainty on soil properties (e.g., Frattini et al., 2009) or by high. In the last four centuries, the number of fatalities caused by coupling the results from deterministic susceptibility models with lahars is second only to PDCs when the largest volcanic disasters a probabilistic measure based on the recurrence interval for are removed from the dataset (Auker et al., 2013). Therefore, a specific event of triggering rainfall (e.g., Mead et al., 2016). quantifying the hazard due to cascading processes, for instance However, such studies did not merge the lahar triggering with lahars, is crucial to produce complete and robust multi-hazard the lahar propagation to compute hazard footprints. and multi-risk assessments (e.g., Selva, 2013; Mignan et al., Diverse approaches have been developed to model lahar 2014; Liu et al., 2015). Similarly to other volcanic hazards, lahar hazard footprints, including statistical/empirical models (e.g., hazard assessment is affected by deep uncertainties, from the LAHARZ: Iverson et al., 1998) and two-dimensional

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