Abstract
Abstract. The characterization of triggering dynamics and remobilized volumes is crucial to the assessment of associated lahar hazards. We propose an innovative treatment of the cascading effect between tephra fallout and lahar hazards based on probabilistic modelling that also accounts for a detailed description of source sediments. As an example, 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 the La Fossa cone (Vulcano, Italy) in the last 1000 years: a long-lasting Vulcanian cycle and a subplinian eruption. The spatial distribution and volume of 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. Model input data were obtained from both geotechnical tests and field measurements (e.g. hydraulic conductivity, friction angle, cohesion, total unit weight of the soil, and saturated and residual water content). TRIGRS simulations show how shallow landsliding is an effective process for eroding pyroclastic deposits on Vulcano. Nonetheless, the remobilized volumes and the deposit thickness threshold for lahar initiation strongly depend on slope angle, rainfall intensity, grain size, friction angle, hydraulic conductivity, and the cohesion of the source deposit.
Highlights
Lahars, an Indonesian term to indicate volcanic debris flows and hyper-concentrated flows with various amounts of volcanic solid content, can cause loss of life and damage to infrastructure and cultivated lands; they represent one of the most devastating hazards for people living in volcanic areas (Pierson et al, 1990, 1992; Janda et al, 1996; Scott et al, 1996, 2005; Lavigne et al, 2000; Witham, 2005; De Bélizal et al, 2013)
We presented a detailed analysis of the volume of tephra fallout deposit that could be potentially remobilized by rainfall as a result of two likely eruptive scenarios of the La Fossa volcano, the main volcanic system on Vulcano island: a longlasting Vulcanian eruption and a short-lived eruption (VEI 2 and VEI 3; using the PAL-D eruption as the reference event) (Fig. 1 and Table 1)
The great novelty of this work is the assessment of compounding hazards based on both numerical modelling and field and geotechnical characterization of the source deposit
Summary
An Indonesian term to indicate volcanic debris flows and hyper-concentrated flows with various amounts of volcanic solid content, can cause loss of life and damage to infrastructure and cultivated lands; they represent one of the most devastating hazards for people living in volcanic areas (Pierson et al, 1990, 1992; Janda et al, 1996; Scott et al, 1996, 2005; Lavigne et al, 2000; Witham, 2005; De Bélizal et al, 2013). The most destructive lahars are caused by the breakout of crater lakes or volcano dammed lakes (e.g. Mt. Kelud in Indonesia; Thouret et al, 1998) and by the interaction of hot pyroclastic density currents (PDCs) with glacial ice and snow at ice-capped volcanoes (e.g. Nevado del Ruiz in Colombia; Pierson et al, 1990). The most common lahars are those generated by heavy rainfall on tephra fallout and PDC deposits emplaced on volcano slopes (e.g. Casita Volcano, Nicaragua; Scott et al, 2005; Panabaj, Guatemala; Charbonnier et al, 2018). Janda et al, 1996; Newhall and Punongbayan, 1996) Despite their relatively small volumes, over 6 years, these lahars have remobilized 2.5 km of the 5.5 km of primary pyroclastic material, inundated 400 km of villages and Published by Copernicus Publications on behalf of the European Geosciences Union
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