A key point of landslide hazard assessment is the estimation of their runout. Empirical relations linking angle of reach to volume can be used relatively easily, but they are generally associated ...with large uncertainties as they do not consider the topographic specificity of a given study site. On the contrary, numerical simulations provide more detailed results on the deposits morphology, but their rheological parameters can be difficult to constrain. Simulating all possible values can be time consuming and incompatible with operational requirements of rapid estimations. We propose and compare three operational methods to derive scaling power laws relating the landslide travel distance to the destabilized volume. The first one relies only on empirical relations, the second one on numerical simulations with back-analysis, and the third one combines both approaches. Their efficiency is tested on three case studies: the Samperre cliff collapses in Martinique, Lesser Antilles (0.5 to 4×106 m3), the Frank Slide rock avalanche (36×106 m3) and the Samperre cliff collapses in Martinique, Lesser Antilles (0.5 to 4×106 m3) the Fei Tsui debris slide in Hong Kong (0.014×106 m3). Purely numerical estimations yield the smallest uncertainty, but the uncertainty on rheological parameters is difficult to quantify. Combining numerical and empirical approaches allows to reduce the uncertainty of estimation by up to 50%, in comparison to purely empirical estimations. However, it may also induces a bias in the estimation, though observations always lie in the 95% prediction intervals. We also show that empirical estimations fail to model properly the dependence between volume and travel distance, particularly for small landslides (<20,000 <0.02×106 m3).
Validation and benchmarking of pyroclastic current (PC) models is required to evaluate their performance and their reliability for hazard assessment. Here, we present results of a benchmarking ...initiative built to evaluate four models commonly used to assess concentrated PC hazard: SHALTOP, TITAN2D, VolcFlow, and IMEX_SfloW2D. The benchmark focuses on the simulation of channelized flows with similar source conditions over five different synthetic channel geometries: (1) a flat incline plane, (2) a channel with a sharp 45° bend, (3) a straight channel with a break-in-slope, (4) a straight channel with an obstacle, and (5) a straight channel with a constriction. Several outputs from 60 simulations using three different initial volume fluxes were investigated to evaluate the performance of the four models when simulating valley-confined PC kinematics, including overflows induced by topographic changes. Quantification of the differences obtained between model outputs at
t
= 100 s allowed us to identify (1) issues with the Voellmy-Salm implementation of TITAN2D and (2) small discrepancies between the three other codes that are either due to various curvature and velocity formulations and/or numerical frameworks. Benchmark results were also in agreement with field observations of natural PCs: a sudden change in channel geometries combined with a high-volume flux is key to generate overflows. The synthetic benchmarks proved to be useful for evaluating model performance, needed for PC hazard assessment. The overarching goal is to provide an interpretation framework for volcanic mass flow hazard assessment studies to the geoscience community.
Over the past 9,150 years, at least 9 flank collapses have been identified in the history of La Soufrière of Guadeloupe volcano. On account of the volcano's current unrest, the possibility of such a ...flank collapse should not be dismissed in assessing hazards for future eruptive magmatic as well as non-magmatic scenarios. We combine morphological and geophysical data to identify seven unstable structures (volumes ranging from 1 × 10
m
to 100 × 10
m
), including one that has a volume compatible with the last recorded flank collapse in 1530 CE. We model their dynamics and emplacement with the SHALTOP numerical model and a simple Coulomb friction law. The best-fit friction coefficient to reproduce the 1530 CE event is tan(7°) = 0.13, suggesting the transformation of the debris avalanche into a debris flow, which is confirmed by the texture of mapped deposits. Various friction angles are tested to investigate less water-rich and less mobile avalanches. The most densely populated areas of Saint-Claude and Basse-Terre, and an area of Gourbeyre south of the Palmiste ridge, are primarily exposed in the case of the more voluminous and mobile flank collapse scenarios considered. However, topography has a prominent role in controlling flow dynamics, with barrier effects and multiple channels. Classical mobility indicators, such as the Heim's ratio, are thus not adequate for a comprehensive hazard analysis.
Gravitational instabilities can be significant threats to populations and infrastructure. For hazard assessment, it is important to estimate the geometry and volume of potential unstable masses. This ...characterization can be particularly difficult in volcanic contexts due to the succession of deposition and erosion phases. Indeed, it results in complex layering geometries in which the interfaces between geological layers may be neither parallel nor planar. Geometry characterization is all the more complex when unstable masses are located in steep and hard to access landscapes, which limits data acquisition. In this work, we show how remote observations can be used to estimate the surface envelope of an unstable mass on a volcanic cliff. We use ortho-photographs, aerial views and topographic surveys to (i) describe the different geological units of the cliff, (ii) characterize the stability of geological units, (iii) infer the paleo-morphology of the site and (iv) estimate potential unstable volumes. We investigate 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 infrastructure. Our analysis suggests that the destabilizations occurring on the cliff may be associated with the re-opening of a paleo-valley filled by pyroclastic materials. We estimate that between 3.5×106 and 8.3×106 m3 could still be mobilized by future destabilizations in the coming decades.
•Successful mass flow simulations with up to two rheological parameters.•Extensive use of field data for model calibration and scenario definition.•Mapping of areas exposed to high discharge debris ...flow, for hazard assessment.
High discharge debris flows in mountainous and volcanic areas are major threats to populations and infrastructures. Modeling such events is challenging because the associated processes are complex, and because we often lack data to constrain rheological parameters. In this work, we show how extensive field data can help model a rock avalanche, and the subsequent remobilization of the deposits as a high discharge debris flow, with a single one-phase thin-layer numerical code, SHALTOP, and up to two rheological parameters. With the Prêcheur river catchment (Martinique, Lesser Antilles) as a case study, we use geological and geomorphological data, topographic surveys, seismic recordings and granulometric analyses to define realistic simulation scenarios and determine the main characteristics of documented events for model calibration. Then, we model a possible 1.9×106 m3 rock avalanche. The resulting deposits are remobilized instantaneously as a high discharge debris flow. We show that, for a given unstable volume, successive collapses allow to better reproduce the dynamics of the rock avalanche, but do not change the geometry of the final deposits, and thus the initial conditions of the subsequent debris flow simulation. The location of the debris flow initiation has also little influence on simulation results. However, progressive remobilization of materials slows down the debris flow and limits overflows, in comparison to an instantaneous release. Nevertheless, high discharge debris flows are well reproduced with an instantaneous initiation. Besides, the range of travel times measured for other significant debris flows in the Prêcheur river is consistent with our simulation results.
Depth‐averaged thin‐layer models are commonly used to model rapid gravity‐driven flows such as debris flows or debris avalanches. However, the formal derivation of thin‐layer equations for general ...topographies is not straightforward. The curvature of the topography results in a force that maintains the velocity tangent to the topography. Another curvature term appears in the bottom friction force with frictional rheologies. In this work, we present the main lines of the mathematical derivation for these curvature terms that are proportional to the square velocity. With the SHALTOP numerical model, we quantify their influence on flow dynamics and deposits over synthetic and real topographies. This is done by comparing simulations in which these terms are exact, disregarded or approximated. With the Coulomb rheology, for slopes θ = 10 and for friction coefficients below μ = tan (5°), neglecting the curvature force increases the simulated travel times by up to 10% and 30%, for synthetic and real topographies respectively. When the curvature in the friction force is neglected, the travel distance may be increased by several hundred meters on real topographies, whatever the topography slopes and friction coefficients. We observe similar effects on a synthetic channel with slope θ = 25° and μ = 15°, with a 50% increase of the kinetic energy. Finally, approximations of curvature in the friction force can break the noninvariance of the equations and decelerate the flow. With the Voellmy rheology, these discrepancies are less significant. Curvature effects can thus have significant impact for model calibration and for overflows prediction, both being critical for hazard assessment.
Key Points
The rigorous derivation of thin‐layer equations yields two curvature terms: one in the bottom friction and one independent of the rheology
Neglecting (respectively approximating) the curvature term in friction generally accelerates (respectively decelerates) the flow
Neglecting the curvature term that is, independent from the chosen rheology generally slows down channelized flows
Quantifying the propagation of landslides is a key step for analyzing gravitational risks. In this context, thin-layer models have met a growing success over the past years to simulate the dynamics ...of gravitational flows, such as debris flows. They are easier to use and require less computing ressources than 3D models, and provide more detailed estimations of flow thicknesses and velocities than purely empirical methods. In this literature review, we present the main rheologies used to model homogeneous gravitational flows, and describe a practical case study with debris flow modeling in the Prêcheur River (Martinique, Lesser Antilles). Then, we discuss possible developments for operational hazard and risk assessment with thin-layer models.
La quantification de la propagation des glissements de terrain est une étape clé de l’analyse des risques gravitaires. Dans ce contexte, les modèles d’écoulement en couche mince sont de plus en plus utilisés pour simuler la dynamique d’écoulements gravitaires comme les coulées de débris. Ils sont plus souples d’utilisation et moins coûteux en temps de calcul que des modèles 3D plus complexes, et fournissent des informations plus précises sur les vitesses et les épaisseurs des écoulements que des méthodes purement empiriques. Dans cette revue de la littérature, nous présentons les principales rhéologies utilisées pour modéliser des écoulements gravitaires homogènes, et donnons un exemple d’application pratique avec la Rivière du Prêcheur (Martinique, Petites Antilles). Nous discutons ensuite les principales pistes de développements permettant d’utiliser ces modèles dans le cadre d’études opérationnelles d’analyse d’aléas et de risques.
La quantification de la propagation des glissements de terrain est une étape clé de l’analyse des risques gravitaires. Dans ce contexte, les modèles d’écoulement en couche mince sont de plus en plus ...utilisés pour simuler la dynamique d’écoulements gravitaires comme les coulées de débris. Ils sont plus souples d’utilisation et moins coûteux en temps de calcul que des modèles 3D plus complexes, et fournissent des informations plus précises sur les vitesses et les épaisseurs des écoulements que des méthodes purement empiriques. Dans cette revue de la littérature, nous présentons les principales rhéologies utilisées pour modéliser des écoulements gravitaires homogènes, et donnons un exemple d’application pratique avec la Rivière du Prêcheur (Martinique, Petites Antilles). Nous discutons ensuite les principales pistes de développements permettant d’utiliser ces modèles dans le cadre d’études opérationnelles d’analyse d’aléas et de risques.
Quantifying the propagation of landslides is a key step for analyzing gravitational risks. In this context, thin-layer models have met a growing success over the past years to simulate the dynamics of gravitational flows, such as debris flows. They are easier to use and require less computing ressources than 3D models, and provide more detailed estimations of flow thicknesses and velocities than purely empirical methods. In this literature review, we present the main rheologies used to model homogeneous gravitational flows, and describe a practical case study with debris flow modeling in the Prêcheur River (Martinique, Lesser Antilles). Then, we discuss possible developments for operational hazard and risk assessment with thin-layer models.
Since May 2018, Mayotte Island has been experiencing seismo-volcanic activities that could trigger submarine landslides and, in turn, tsunamis. To address these hazards, we use the HySEA numerical ...model to simulate granular flow dynamics and the Boussinesq FUNWAVE-TVD numerical model to simulate wave propagation and subsequent inundations. We investigate 8 landslide scenarios (volumes from \(11.25 \times 10^6~\text{m}^3\) to \(800 \times 10^6~\text{m}^3\)). The scenario posing the greatest threat involves destabilization on the eastern side of Mayotte’s lagoon at a shallow depth and can generate sea-surface deformations of up to 2 m. We show that the barrier reef surrounding Mayotte plays a prominent role in controlling water-wave propagation and in protecting the island. The tsunami travel time to the coast is very short (a few minutes) and the tsunami is not necessarily preceded by a sea withdrawal. Our simulation results provide a key to establishing hazard maps and evacuation plans and improving early-warning systems.
Granular flows occur in various contexts, including laboratory experiments, industrial processes, and natural geophysical flows. To investigate their dynamics, different kinds of physically based ...models have been developed. These models can be characterized by the length scale at which dynamic processes are described. Discrete models use a microscopic scale to individually model each grain, Navier-Stokes models use a mesoscopic scale to consider elementary volumes of grains, and thin-layer models use a macroscopic scale to model the dynamics of elementary columns of fluids. In each case, the derivation of the associated equations is well-known. However, few studies focus on the extent to which these modeling solutions yield mutually coherent results. In this article, we compare the simulations of a granular dam break on a horizontal or inclined planes for the discrete model convex optimization contact dynamics (COCD), the Navier-Stokes model Basilisk, and the thin-layer depth-averaged model SHALTOP. We show that, although all three models allow reproducing the temporal evolution of the free surface in the horizontal case (except for SHALTOP at the initiation), the modeled flow dynamics are significantly different, and, in particular, during the stopping phase. The stresses measured at the flow's bottom, reflecting the flow dynamics, are in relatively good agreement, but significant variations are obtained with the COCD model due to complex and fast-varying granular lattices. Similar conclusions are drawn using the same rheological parameters to model a granular dam break on an inclined plane. This comparison exercise is essential for assessing the limits and uncertainties of granular flow modeling.
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