We first consider debris avalanches on the Soufrière de Guadeloupe volcano (Guadeloupe, Lesser Antilles), and then the combination of rock avalanches and subsequent debris flows in the Prêcheur river (Martinique, Lesser Antilles). It should thus be properly taken into account to calibrate correctly the models and estimate more precisely overflow hazards.Then, we test the feasibility of using SHALTOP for propagation hazard quantification in two case studies. Topography curvature can indeed have a significant influence on the dynamics of rapid gravitational flows. We first highlight the importance of a detailed description of the topography curvature, even with simple rheological laws. This approach is thus easier to use and computationally cheaper than models simulating the dynamics of each solid or fluid particle, and allows to model the dynamics and the geometry of the flows more finely than purely empirical models.Three research issues are considered in this work. We test the SHALTOP numerical model with an empirical rheology described only one or two parameters, in order to facilitate its operational use. Thin-layer equations model the propagation of a flow on a topography and give its thickness, and its depth-averaged velocity. In this work, we assess the possibility to use empirical thin-layer models to enhance propagation hazard estimation for gravitational flows. We estimate that between 3.5 × 10^6 m3 and 8.3 × 10^6 m3 could still be mobilized by future destabilizations in the coming decades.

Our analysis suggests that the destabilizations occurring on the cliff may be associated to the re opening of a paleo-valley filled by pyroclastic materials. We use the Samperre cliff in Martinique (Lesser Antilles, French West Indies) as a study site, where recurrent destabilizations since at least 1988 have produced debris flows that threaten populations and infrastructures.

We use ortho-photographs, aerial views and topographic surveys to (i) describe the different geological units of the cliff, (ii) identify stable and unstable units, (iii) infer the paleo-morphology of the site and (iv) estimate potential unstable volumes. In this work, we show how remote observations can be used to estimate the surface envelope of an unstable mass on a volcanic cliff.

Geometry characterization is all the more complex when unstable masses are located in steep and hardly accessible landscapes, which limits data acquisition. Indeed, it results in complex layering geometries where the interfaces between geological layers may be neither parallel nor planar. This characterization can be particularly difficult in volcanic context due to the succession of deposition and erosion phases. For hazard assessment, it is of prior importance to estimate the geometry and volume of potential unstable masses. Gravitational instabilities can be significant threats to populations and infrastructures. The topics bear on understandin hazardous edifice and dome failures, and the measures to anticipate such failures. Case examples discusse include edifice instability at Mt St Helens, USA, and Soufriere Hills volcano, Montserrat, the stability of lava spines a Mont Pelee, Martinique, and Lamington, Papua New Guinea, and lavadome stability at Soufriere Hills. Some aspect of material property evaluation, analysis procedures, and implications on monitoring are also discussed. The primary focus of the presentatio is on mechanisms and factors associated with collapse, the geometric factors, augmented loading by magma, localized strengt reduction by physical and chemical changes (the latter commonly associated with hydrothermal processes), strain weakening pore-fluid (water or gas) pressure enhancement, retrogressive failure, time-dependent failure, and seismic shaking. This paper examines some aspects of current understanding of edifice and lavadome instability. Collapses of growing lava domes are more frequent, are similar in many ways, to edifice collapse, and can directly generate devastating pyroclastic currents. Hazards derive from the debris avalanches themselves from associated explosive activity that ranges from vertical eruptions (often accompanied by pyroclastic currents) to devastatin directed blasts, from associated lahars, and from tsunamis. More than 20 major slope failures have occurred worldwid over the past 500 years, a rate exceeding that of caldera collapse. Slope failures resulting from structural instability of andesitic volcanic edifices can generate mobile debris avalanche that travel long distances down or beyond the flanks of volcanoes.

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