Slope stability models for rainfall-induced lahars during long-lasting eruptions

Abstract

In this study we analyse the spatial distribution of potential lahar sources during long-lasting eruptions using two slope stability models (SHALSTAB and TRIGRS). The analysis is based on observed deposit grain-size and thickness, rainfall data and slope angle of the lahar events, in the area surrounding the Cordón Caulle volcano, Chile, that occurred during the 2011 eruption. The main phase of the eruption (4-7 June) was characterized by eruptive plumes from 7 to 11 km high which dispersed most of the tephra eastward toward Argentina, with a total estimated volume of about 1 km 3. Tephra fall blanketed the region from ESE to ENE of the volcano with the thickness of the tephra layers between approximately 1 m (15 km from vent) and 0.06 m (240 km from the vent). On 10 June 2011, a major lahar occurred close to the Argentina-Chile border that reached the National road 231 (28 km from the vent). Three field campaigns were undertaken to collect samples and data from tephra deposits and triggering mechanisms in the lahar source area. Model input parameters were obtained from geotechnical test and field measurements. Several metrics are used for model evaluation and best fit to the data are obtained for simulations considering non-cohesive ash layers for SHALSTAB and a cohesion of 0.5 kPa for TRIGRS. Both models are sensitive to the physical properties of the tephra deposit and the hydraulic and materiel strength properties of the study area. They both also show good agreement with field data but provide different information: TRIGRS provides an estimate of the timing (based on a storm event) and location of a potentially unstable area, while SHALSTAB simulations result in landslide susceptibility classes based on critical rainfall value. These outcomes provide fundamental insights into lahar triggering during long-lasting volcanic eruptions and are crucial to the compilation of lahar hazard maps and emergency management plans in the South Andes volcanic region.

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

“…The mean hydraulic saturated conductivity of the Pal D primary deposit could not be measured in the laboratory because the deposit grainsize (Mdφ = -3.4φ) is too coarse for the apparatus measuring hydraulic conductivity; instead we use here both values one from the literature for other coarsegrained volcanic soils and the second the value obtained through field measurements. A value of 1 × 10 -2 m s -1 (Table 4) inferred from the hydraulic conductivity measured in the field on 2011 Cordon Caulle (Chile) eruption lapilli deposits (Baumann et al, 2018) and from the lower pumice deposit from Vesuvius (Crosta and Dal Negro, 2003). These values show a large discrepancy with our field measurements of 6.8 × 10 -4 m s -1 .…”

“…Several slope stability models have been used to predict lahar initiation processes as shallow landslides in volcanic areas (e.g. Cascini et al, 2010;Frattini et al, 2004;Crosta and Dal Negro, 2003;Cuomo and Iervolino, 2016;Cuomo and Della Sala, 2016;Cascini et al, 2011;Sorbino et al 2007Sorbino et al , 2010Mead et al, 2016, Baumann et al, 2018. Among those, the model TRIGRS (Baum et al, 2002) can be used for computing transient pore pressure and the related changes in the factor of safety due to rainfall infiltration.…”

“…Here, TRIGRS is used to investigate the timing and location of shallow landslides in response to rainfall in large areas (e.g. Baumann et al, 2018).…”

Determination of Primary and Secondary Lahar Flow Paths of the Fuego Volcano (Guatemala) Using Morphometric Parameters

“…These areas of blue color were observed to have been directed by the lahars that destroyed the populated areas. These three morphometric indices [23][24][25][26] are valid parameters to determine the flow paths of primary and secondary lahars. The areas with high values of these indices are areas of moisture concentration in the slopes, flow paths and concentration of sediments that can move from the high parts by intense rains and gravity generating secondary lahars, generally on the slopes.…”

“…These areas of blue color were observed to have been directed by the lahars that destroyed the populated areas. These three morphometric indices [23][24][25][26] The field control was carried out by technicians of CONRED.…”