Two-phase SPH modelling of a real debris avalanche and analysis of its impact on bottom drainage screens

Rapid flow-like landslides, particularly debris flows and debris avalanches, cause significant economic damage and many victims worldwide every year. They are usually extremely fast with the capability of travelling long distances in short times, sweeping away everything in their path. The principal objective of this paper is to test the ability of the ‘GeoFlow-SPH’ two-phase model developed by the authors, to reproduce the complex behaviour of natural debris avalanches where pore-water pressure evolution plays a key role. To reach this goal, the model is applied to reproduce the complex dynamic behaviour observed in Johnsons Landing debris avalanche including the observed bifurcation caused by the flowing out of part of the moving mass from the mid-channel. Initial thickness deposit trim-line, distribution of deposit volume, and the average velocities were provided for this real case, making it an appropriate case to validate the developed model. The paper also contributes to evaluate the SPH-FD model’s potentialities to simulate the structural countermeasure, like bottom drainage screens, used to reduce the impact of debris flows. The analysis of the results shows the adequacy of the proposed model to solve this complicated geophysical problem.

DebrisFlow Predictor: an agent-based runout program for shallow landslides

Credible models of landslide runout are a critical component of hazard and risk analysis in the mountainous regions worldwide. Hazard analysis benefits enormously from the number of available landslide runout models that can recreate events and provide key insights into the nature of landsliding phenomena. Regional models that are easily employed, however, remain a rarity. For debris flows and debris avalanches, where the impacts may occur some distance from the source, there remains a need for a practical, predictive model that can be applied at the regional scale. We present, herein, an agent-based simulation for debris flows and debris avalanches called DebrisFlow Predictor. A fully predictive model, DebrisFlow Predictor employs autonomous subroutines, or agents, that act on an underlying digital elevation model (DEM) using a set of probabilistic rules for scour, deposition, path selection, and spreading behavior. Relying on observations of aggregate debris flow behavior, DebrisFlow Predictor predicts landslide runout, area, volume, and depth along the landslide path. The results can be analyzed within the program or exported in a variety of useful formats for further analysis. A key feature of DebrisFlow Predictor is that it requires minimal input data, relying primarily on a 5 m DEM and user-defined initiation zones, and yet appears to produce realistic results. We demonstrate the applicability of DebrisFlow Predictor using two very different case studies from distinct geologic, geomorphic, and climatic settings. The first case study considers sediment production from the steep slopes of Papua, the island province of Indonesia; the second considers landslide runout as it affects a community on Vancouver Island off the west coast of Canada. We show how DebrisFlow Predictor works, how it performs compared to real world examples, what kinds of problems it can solve, and how the outputs compare to historical studies. Finally, we discuss its limitations and its intended use as a predictive regional landslide runout tool. DebrisFlow Predictor is freely available for non-commercial use.

The role of sedimentology in the mobility of debris avalanches: Evidence from their deposits and granular flow experiments

<p>Debris avalanches and lahars are among the most destructive and hazardous mass flows in volcanic environments making them important to understand from a hazard assessment perspective. Sedimentological characteristics of their deposits are important for assessing their propagation and emplacement mechanisms. Here, we compare the sedimentology of nine volcanic debris avalanches and eight lahars, by the descriptive statistics: median grain size, sand, gravel and finer particle proportion, skewness, and sorting.</p><p>Results suggest that lahars and debris avalanches diverge in their grain size distribution evolution during propagation, even when sourced from the same material. Increasing bimodality, evolution to negative skewness, with decreasing sediment size, accompanied by very poor sorting suggest comminution of particles due to particle-particle interactions in debris avalanches. Instead, preferential deposition of the coarsest particles and improved sorting suggest that the decrease in grain size of lahars is the result of debulking. The divergence is mainly caused by the high water content in lahars, which introduce different processes during propagation. This suggests, in agreement with previous studies, that debris avalanches can be considered as dense granular flows where the effect of inertial collisions of solid fragments are more important than fluid effects.</p><p>Present findings and previous sedimentological studies suggest that both volcanic and non-volcanic debris avalanches exhibit bimodal grain-size distributions, at least locally, in areas of high shear accommodation. Following these results, an experimental campaign has been carried out to test the effect of bimodality on the propagation of granular flows. These experiments are flows of bidisperse granular material on an initial inclined plane, with a horizontal accumulation surface at the bottom.  Findings confirm that the bimodality of the grain size distribution generates a more efficient shearing arrangement, which can increase the mobility of granular flows in the same way recorded in debris avalanche deposits.</p>

The Bontău Volcano, Apuseni Mts. (Romania), source for numerous debris avalanche deposits

<p>The Neogene volcanism in the western part of Romania is confined to the Apuseni Mountains and surrounding areas. The largest volcanic area is mostly developed in the WNW-ESE oriented, ca. 120 km in length Zărand-Brad-Zlatna Basin.</p><p>The Bontău Volcano (Seghedi et al., 2010) is located inside the western part of the Zărand-Brad-Zlatna Basin and it is strongly affected by erosional processes, being crossed in its northern part, from east to west, by the Crișul Alb River.</p><p>The Bontău Volcano is known to be active roughly between 14-10 Ma (according to the available K/Ar data) and it has been characterized as a composite or stratovolcano volcano associated with dome complexes, built by calc-alkaline andesitic lavas and pyroclastic deposits (andesite to basaltic andesite). The long-lasting volcanism developed in the Bontău area has a complex build up stages that we recently have found were interrupted by a series of destructive failure events. Several important volcanic collapses of the volcanic edifice took place producing large volcanic debris avalanches followed by numerous debris flows which produced various secondary volcaniclastic deposits that can be observed in different places all around the Bontău volcano. The debris avalanches deposits have not yet been known up to this study. The distribution of the debris avalanche deposits and associated volcaniclastic deposits is the main target of this study. In order to reconstruct Bontău Volcano activity and reconstruct its original morphology we done field observations and sampled the main lithologies to perform petrographic observations and geochemical and isotopic analyses (for the main lithologies).</p><p>During our field observations we tried to identify the relationships between debris avalanche deposits and older volcanic bodies (lavas, domes, volcaniclastic). One main important remark is related with the presence of several small basins at the margin of the volcano consisting of a succession of thin planar and cross-bedded sandstone in an alternation of coarse and fine layers associated with discontinuous lapilli trains (including pumices); The deposits are poorly to moderately sorted; with low angle cross lamination in lenses or pockets. Such deposits, as closely associate with debris avalanche deposits have been interpreted as small intra-hummocky basins formed after debris avalanche generation; they are mostly situated at the margins of the volcano.</p><p>The presence of multiple debris avalanche deposits can be connected with volcano growing in an extensional environment. We may assume that the long-lived Miocene rift graben system of the Zărand-Brad-Zlatna Basin experienced numerous changes in the fracture propagation and vertical movements that promoted repeated dyke intrusion and facilitated generation of numerous debris avalanches.</p><p>Acknowledgements: This work was supported by a grant of the of Ministry of Research and Innovation, CNCS – UEFISCDI, project number PN-III-P4-ID-PCCF-2016-4-0014, within PNCDI III.</p>

A probabilistic model for assessing debris flow propagation at regional scale: a case study in Campania region, Italy

<p>Debris flows and debris avalanches are rapid to extremely rapid landslides that tend to travel considerable distances from their source areas. Interaction between debris flows and elements at risk along their travel path may result in potentially significant destructive consequences. One of the critical challenges to overcome with respect to debris flow risk is, therefore, the credible prediction of their size, travel path, runout distance, and depths of erosion and deposition. To these purposes, at slope or catchment scale, sophisticated physically-based models, appropriately considering several factors and phenomena controlling the slope failure mechanisms, may be used. These models, however, are computationally costly and time consuming, and that significantly hinders their applicability at regional scale. Indeed, at regional scale, debris flows hazard assessment is usually carried out by means of qualitative approaches relying on field surveys, geomorphological knowledge, geometric features, and expert judgement.</p><p>In this study, a quantitative modelling approach based on cellular automata methods, wherein individual cells move across a digital elevation model (DEM) landscape following behavioral rules defined probabilistically, is proposed and tested. The adopted model, called LABS, is able to estimate erosion and deposition soil volumes along a debris flow path by deploying at the source areas autonomous subroutines, called agents, over a 5 m spatial resolution DEM, which provides the basic information to each agent in each time-step. Rules for scour and deposition are based on mass balance considerations and independent probability distributions defined as a function of slope DEM-derived values and a series of model input parameters. The probabilistic rules defined in the model are based on data gathered for debris flows and debris avalanches that mainly occurred in western Canada. This study mainly addresses the applicability and the reliability of this modelling approach to areas in southern Italy, in Campania region, historically affected by debris flows in pyroclastic soils. To this aim, information on inventoried debris flows is used in different study areas to evaluate the effect on the predictions of the model input parameter values, as well as of different native DEM resolutions.</p>

Geomorphometric descriptions of archipelagic aprons off the southern flanks of French Frigate Shoals and Necker Island edifices, Northwest Hawaiian Ridge

This study describes the geomorphometries of archipelagic aprons on the southern flanks of the French Frigate Shoals and Necker Island edifices on the central Northwest Hawaiian Ridge that are hotspot volcanoes that have been dormant for 10−11 m.y. The archipelagic aprons are related to erosional headwall scarps and gullies on landslide surfaces but also include downslope gravitational features that include slides, debris avalanches, bedform fields, and outrunners. Some outrunners are located 85 km out onto the deep seafloor in water depths of 4900 m. The bedforms are interpreted to be the result of slow downslope sediment creep rather than products of turbidity currents. The archipelagic aprons appear to differ in origin from those off the Hawaiian Islands. The landslides off the Hawaiian Islands occurred because of oversteepening and loading during the constructive phase of the islands whereas the landslides off the French Frigate Shoals and Necker Island edifices may have resulted from vertical tectonics due to the uplift and relaxation of a peripheral bulge or isolated earthquakes long after the edifices passed beyond the hotspot. The lack of pelagic drape in water depths above the 4600 m depth of the local carbonate compensation depth suggests that the archipelagic apron off the French Frigate Shoals edifice is much younger, perhaps Quaternary in age, than that off the Necker Island edifice, which has a 50 m pelagic drape. The pelagic drape off the Necker Island edifice suggests that the landslides may be as old as 9 Ma. The lack of pelagic drape off the French Frigate Shoals edifice suggests that the most recent landslides are more recent, perhaps even Quaternary in age. The presence of a chute-like feature on the mid-flank of the French Frigate Shoals edifice appears to be the result of rejuvenated volcanism that occurred long after the initial volcanism ceased to build the edifice.

Disaster Storytelling and Volcanic Eruptions Caused by Debris Avalanches on Mt. Bandai in Aizu and Mt. Unzendake and Mt. Mayuyama in Shimabara

People tend to forget the past. For example, nine years have passed since the Great East Japan Earthquake and Tsunami of 2011, and memories of the disaster have begun to fade even in the disaster-stricken areas. People who have experienced disasters directly have tried to spread the memories of those events in various ways because they do not want their children and grandchildren to endure what they did. One of the most impressive ways of sharing these memories is for witnesses of disasters to communicate how they directly experienced them. There is a challenge in handing down these stories because people directly affected by the disasters will die within the next ten years. This paper takes up two examples of volcanoes in Japan, and examines how stories of these disasters were passed on to people who have not experienced them directly. We proceed by investigating common points in these stories and comparing them, and also by exploring the activity of passing how these disaster stories have been passed down after more than 100 years since its occurrence when there are no more survivors who have any direct memory of it.