Mapping the susceptibility of syn-eruptive rain-triggered lahars at Vulcano island (Italy) combining field characterization and numerical modelling

Abstract

Abstract. Lahars are a widespread phenomenon on Vulcano island (Italy), where many loose pyroclastic deposits provide a significant source of sediments. In this study we have estimated the volumes of tephra-fallout deposit that could be remobilized by rainfall-triggered lahars in association with two eruptive scenarios that have characterized the activity of La Fossa cone: a long-lasting Vulcanian cycle and a subplinian eruption. The spatial distribution and volume of tephra-fallout deposits that could potentially trigger lahars were analysed based on a combination of tephra-fallout probabilistic modelling (with TEPHRA2), slope stability modelling (with TRIGRS), field observations and geotechnical tests. Field characterization includes tephra-fallout primary deposits in the lahar initiation zones and lahar deposits both on the volcanic cone and in the ring plain. Model input data (hydraulic conductivity, friction angle, cohesion, total unit weight of the soil, saturated and residual water content) were obtained from both geotechnical tests and field measurements. In particular, hydraulic conductivity plays an important role on the stability of tephra-fallout deposits. Our parametric analysis has shown that the tephra-fallout critical thickness required to trigger a lahar for the considered rainfall event is between 20–25 cm for the Vulcanian scenario, and between 10–65 cm or

Lahar Risk Assessment: the Case Study of Vulcano Island, Italy

“…Lahars bed thickness vary between 0.1 and 0.6 m in the middle and lower catchments of the cone (Baumann et al 2019). These loose and incoherent deposits cover the Tufi Varicolori layer, an impermeable substratum that favours the remobilisation of the overlying deposits through lahars generation (Ferrucci et al 2005).…”

“…Vulcanian cycle and subplinian eruptions of VEI (Volcanic Explosivity Index) 2 and 3. Baumann et al (2019) have used the associated probabilistic isomass maps to assess the scenario that would produce the most unstable deposit combining field characterization, geotechnical analysis, and numerical modelling with TRIGRS (Baum et al 2002). For the Vulcanian scenario, the largest unstable volume was reached for an eruption duration of 18 months, while the subplinian deposits, which are characterised by a smaller cohesion, become more stable with thickness increase depending on rainfall intensity and slope angle (Baumann et al 2019).…”

“…Baumann et al (2019) have used the associated probabilistic isomass maps to assess the scenario that would produce the most unstable deposit combining field characterization, geotechnical analysis, and numerical modelling with TRIGRS (Baum et al 2002). For the Vulcanian scenario, the largest unstable volume was reached for an eruption duration of 18 months, while the subplinian deposits, which are characterised by a smaller cohesion, become more stable with thickness increase depending on rainfall intensity and slope angle (Baumann et al 2019). Given that a Vulcanian cycle has the highest probability of occurrence at this volcanic system (Selva et al 2020), in this study we consider the worst-case scenario identified by Baumann et al (2019) for the Vulcanian cycle (i.e.…”

“…18 months of eruption duration with the characteristics of the last cycle occurred on Vulcano in 1888-90). Using a probabilistic isomass map compiled for a 25% probability of occurrence, Baumann et al (2019) used the model TRIGRS to estimate the potential triggering source at 22 upper catchments located in the northern sector of La Fossa cone (Fig. 3).…”

“…Because the island is periodically affected by intense rainy events, lahar remobilization of tephra deposits is still nowadays an active process (e.g. Di Traglia et al 2013, Baumann et al 2019. Galderisi et al (2013) had already shown how the north of Vulcano island is the most exposed to lahars generation and, based on the characteristics of buildings and urban fabrics, is characterised by various degrees of vulnerability.…”