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Discussion papers
https://doi.org/10.5194/nhess-2019-77
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/nhess-2019-77
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 10 Apr 2019

Research article | 10 Apr 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Natural Hazards and Earth System Sciences (NHESS).

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

Valérie Baumann1, Costanza Bonadonna1, Sabatino Cuomo2, Mariagiovanna Moscariello2, Sebastien Biasse3, Marco Pistolesi4, and Alessandro Gattuso5 Valérie Baumann et al.
  • 1Department of Earth Sciences, University of Geneva, Rue des Maraîchers 13, 1205 Geneva, Switzerland
  • 2Laboratory of Geotechnics, University of Salerno, Via Giovanni Paolo II 132, 84081 Fisciano Salerno, Italy
  • 3Earth Observatory of Singapore, Nanyang Technological University, Singapore, Singapore
  • 4Dipartimento di Scienze della Terra, Università di Pisa, Pisa, Italy
  • 5Istituto Nazionale di Geofisica e Vulcanologia, Sezione Palermo, Italy

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 < 10 cm for a subplinian event depending on the hydraulic conductivity. The scenario remobilizing the largest unstable volumes by rain-triggered lahars is, therefore, that associated with a Vulcanian cycle with duration of 18 months and a subplinian eruption of VEI 3 (for low hydraulic conductivity). TRIGRS simulations show that shallow landsliding is an effective process for eroding the primary tephra-fallout deposits in combination with high-intensity rainfall events with short duration, such as those occurring on Vulcano every year. Our results provide a new innovative treatment of the cascading effect between tephra fallout and lahar susceptibility that also accounts for detailed characterization of source sediments based on field observations and geotechnical tests.

Valérie Baumann et al.
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Valérie Baumann et al.
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Short summary
Lahars are fast moving mixture of volcanic debris and water propagating downslope volcanoes that can be very dangerous for people and properties. Identification of lahar source areas and initiation mechanisms is crucial to comprehensive lahar hazard assessments. We present the first rain-triggered lahar susceptibility maps for La Fossa volcano (Vulcano, Italy) combining probabilistic tephra modelling, slope-stability modelling, precipitation data, field characterizations and geotechnical tests.
Lahars are fast moving mixture of volcanic debris and water propagating downslope volcanoes that...
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