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Landslides and flash floods are geomorphic hazards (GHs) that often co-occur and interact. They generally occur very quickly, leading to catastrophic socioeconomic impacts. Understanding the temporal patterns of occurrence of GH events is essential for hazard assessment, early warning, and disaster risk reduction strategies. However, temporal information is often poorly constrained, especially in frequently cloud-covered tropical regions, where optical-based satellite data are insufficient. Here we present a regionally applicable methodology to accurately estimate GH event timing that requires no prior knowledge of the GH event timing, using synthetic aperture radar (SAR) remote sensing. SAR can penetrate through clouds and therefore provides an ideal tool for constraining GH event timing. We use the open-access Copernicus Sentinel-1 (S1) SAR satellite that provides global coverage, high spatial resolution (∼10–15 m), and a high repeat time (6–12 d) from 2016 to 2020. We investigate the amplitude, detrended amplitude, spatial amplitude correlation, coherence, and detrended coherence time series in their suitability to constrain GH event timing. We apply the methodology on four recent large GH events located in Uganda, Rwanda, Burundi, and the Democratic Republic of the Congo (DRC) containing a total of about 2500 manually mapped landslides and flash flood features located in several contrasting landscape types. The amplitude and detrended amplitude time series in our methodology do not prove to be effective in accurate GH event timing estimation, with estimated timing accuracies ranging from a 13 to 1000 d difference. A clear increase in accuracy is obtained from spatial amplitude correlation (SAC) with estimated timing accuracies ranging from a 1 to 85 d difference. However, the most accurate results are achieved with coherence and detrended coherence with estimated timing accuracies ranging from a 1 to 47 d difference. The amplitude time series reflect the influence of seasonal dynamics, which cause the timing estimations to be further away from the actual GH event occurrence compared to the other data products. Timing estimations are generally closer to the actual GH event occurrence for GH events within homogenous densely vegetated landscape and further for GH events within complex cultivated heterogenous landscapes. We believe that the complexity of the different contrasting landscapes we study is an added value for the transferability of the methodology, and together with the open-access and global coverage of S1 data it has the potential to be widely applicable.

The 2017 Mw 7.1 Puebla–Morelos intraslab earthquake (depth: 57 km) severely hit Popocatépetl Volcano, located ∼ 70 km north of the epicenter. The seismic shaking triggered shallow landslides on the volcanic edifice, mobilizing slope material saturated by the 3 d antecedent rainfall. We produced a landslide map based on a semi-automatic classification of a 50 cm resolution optical image acquired 2 months after the earthquake. We identified hundreds of soil slips and three large debris flows for a total affected area of 3.8 km2. Landslide distribution appears controlled by the joint effect of slope material properties and topographic amplification. In most cases, the sliding surfaces correspond with discontinuities between pumice-fall and massive ash-fall deposits from late Holocene eruptions. The largest landslides occurred on the slopes of aligned ENE–WSW-trending ravines, on opposite sides of the volcano, roughly parallel to the regional maximum horizontal stress and to volcano-tectonic structural features. This suggests transient reactivation of local faults and extensional fractures as one of the mechanisms that weakened the volcanic edifice and promoted the largest slope failures. The material involved in the larger landslides transformed into three large debris flows due to liquefaction. These debris flows mobilized a total volume of about 106 m3 of material also including large wood, were highly viscous, and propagated up to 7.7 km from the initiation areas. We reconstructed this mass wasting cascade by means of field evidence, samples from both landslide scarps and deposits, and analysis of remotely sensed and rainfall data. Although subduction-related earthquakes are known to produce a smaller number of landslides than shallow crustal earthquakes, the processes described here show how an unusual intraslab earthquake can produce an exceptional impact on an active volcano. This scenario, not related to the magmatic activity of the volcano, should be considered in multi-hazard risk assessment at Popocatépetl and other active volcanoes located along volcanic arcs.

Debris flows are one of the most complex and devastating natural phenomena, and they affect mountainous areas throughout the world. Structural measures are currently adopted to mitigate the related hazard in urbanized areas. However, their design requires an estimate of the impact force, which is an open issue. The numerous formulae proposed in the literature require the assignment of empirical coefficients and an evaluation of the kinematic characteristics of the incoming flow. Both are generally not known a priori. In this article, we present the Grand Valey torrent site (Italian Alps). A monitoring system made up of strain gauges was installed on a filter barrier at the site, allowing the evaluation of impact forces. The system provides pivotal information for calibrating impact formulae. Two debris flows occurred during the monitoring period. We present the interpretation of videos, impact measurements, and the results of numerical analyses. The combined analysis allows a back calculation of the events in terms of forces, flow depth, and velocity. Thus, we investigate the applicability of the impact formulae suggested in the literature and of the recommended empirical coefficients. The results highlight that hydrostatic effects dominated the impact during the first event, while hydrodynamic effects prevailed in the second one.

In recent decades, slope instability in high‐mountain regions has often been linked to increase in temperature and the associated permafrost degradation and/or the increase in frequency/intensity of rainstorm events. In this context we analyzed the spatiotemporal evolution and potential controlling mechanisms of small‐ to medium‐sized mass movements in a high‐elevation catchment of the Italian Alps (Sulden/Solda basin). We found that slope‐failure events (mostly in the form of rockfalls) have increased since the 2000s, whereas the occurrence of debris flows has increased only since 2010. The current climate‐warming trend registered in the study area apparently increases the elevation of rockfall‐detachment areas by approximately 300 m, mostly controlled by the combined effects of frost‐cracking and permafrost thawing. In contrast, the occurrence of debris flows does not exhibit such an altitudinal shift, as it is primarily driven by extreme precipitation events exceeding the 75th percentile of the intensity‐duration rainfall distribution. Potential debris‐flow events in this environment may additionally be influenced by the accumulation of unconsolidated debris over time, which is then released during extreme rainfall events. Overall, there is evidence that the upper Sulden/Solda basin (above ca. 2500 m above sea level [a.s.l.]), and especially the areas in the proximity of glaciers, have experienced a significant decrease in slope stability since the 2000s, and that an increase in rockfalls and debris flows during spring and summer can be inferred. Our study thus confirms that “forward‐looking” hazard mapping should be undertaken in these increasingly frequented, high‐elevation areas of the Alps, as environmental change has elevated the overall hazard level in these regions., Global warming has a far‐reaching impact on high‐mountain regions, where rising temperatures affect permafrost thaw, frost‐cracking mechanisms, and the frequency of convective storms. These changes impact the frequency of rockfalls and debris flows and thus lead to a dynamization of geological hazards over a wide range of elevations. Future management plans for Alpine regions should consider these changes to reduce the associated risks and ensure adequate safety measures. image, Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659

In this study, an inventory of storm-triggered debris flows performed in the area of the San Vicente volcano (El Salvador, CA) was used to calibrate predictive models and prepare a landslide susceptibility map. The storm event struck the area in November 2009 as the result of the simultaneous action of low-pressure system 96E and Hurricane Ida. Multivariate Adaptive Regression Splines (MARS) was employed to model the relationships between a set of environmental variables and the locations of the debris flows. Validation of the models was performed by splitting 100 random samples of event and non-event 10 m pixels into training and test subsets. The validation results revealed an excellent (area under the receiver operating characteristic (ROC) curve (AUC) = 0.80) and stable (AUC std. dev. = 0.01) ability of MARS to predict the locations of the debris flows which occurred in the study area. However, when using the Youden&rsquo, s index as probability threshold to discriminate between pixels predicted as positives and negatives, MARS exhibits a moderate ability to identify stable cells (specificity = 0.66). The final debris flow susceptibility map, which was prepared by averaging for each pixel the score of the 100 MARS repetitions, shows where future debris flows are more likely to occur, and thus may help in mitigating the risk associated with these landslides.

During an extensive grid-mapping campaign on Mars' southern hemisphere, we have detected so-called sheet flow deposits, which are defined by distinctive lobate fronts, thin and mostly planar layers, and faint sub-parallel surface lineations. They originate from the high plains of Dryas Mons, a large massif of impact and tectonic origin, and follow the topographic gradient down into the adjacent basins. Their sources often coincide with steep and multi-layered outcrops. This work addresses the formation of these deposits, and if they are related to the unique tectonic and endogenic environment of Dryas Mons. We applied photogeological mapping, age determinations by crater counts, as well as topographic and heat flow measurements in order to reconstruct the evolution of these landforms. The calculated ages (mid to late Amazonian), their location in the mid-latitudes, as well as some specific morphologies like lobate flow fronts are typical for standard viscous-flow features on Mars. In contrast to such viscous-flow features, the sheet flow deposits occur isolated in the Dryas Mons region. Aside from that, major other landforms, typical for viscous-flow features are lacking, like sublimation pits, brain terrain or arcuate deformations. Considering these similarities and differences, we suggest that both landforms viscous-flow features and the sheet flow deposits of Dryas Mons were formed by the involvement of volatiles; however, at varying amounts and by different emplacement processes. As the formation of the sheet flow deposits is located in an area with one of the highest heat flow values planet-wide, we calculated if the heat flow could be a potential trigger for the release of outcropping volatile-rich layers. However, our calculations have shown that the heat flow is still insufficient to enable near-surface melting. Instead, we suggest that the volatiles originate from deep layers affected by magmatic and/or intrusive activities in a tectonically active environment. These conditions led to melting and mobilisation of the volatiles by advective hydrothermal processes. Thus, the volatiles migrated upward along hypothesized deep-seated fault systems, formed by Noachian/Hesperian impacts and tectonic activities, until they reached conductive layers outcropping at Dryas Mons, enabling them to drain. After their release to the cold Amazonian atmosphere, this mixture of volatiles and solids partially froze, resulting in a comparatively high viscosity, and hence, a slow and laminar (non-turbulent) movement. This flow behaviour might have led to the formation of the lineations at their surface along shear zones. Hence, we suggest that the sheet flow deposits may have originally formed as slowly moving icy debris flows during the mid- to late Amazonian. This work has been funded by the Research Fellowship Program through the European Space Agency. J. Ruiz contribution was supported by the project PGC2018-095340-B-I00 from the Spanish Ministry of Science, Innovation and Universities.

We analysed spatial development of traditional and modern settlements on active alluvial fans in the Upper Sava Valley (NW Slovenia), by using old cadastral data from the beginning of the 19th century, time series of aerial photographs from the middle of the 20th century and recent building cadastre. The valley is surrounded by the mountainous Julian Alps in the south and the Karavanke Mountains in the north where there is a lack of space for settlements due to steep slopes that are increasing the danger of slope processes, torrential processes and floods. By using a very high-resolution 1m LiDAR digital elevation model, we defined the morphometry of alluvial fans and the characteristics of the drainage system of contributing tributaries. We classified the areas according to the threat posed by the modelled torrents and debris flows. We analysed the resilience of settlement in different periods from this perspective and evaluated the integration of natural processes effects in modern spatial planning projects. We found that geomorphic processes threaten a relatively large proportion of some new and old settlements on alluvial fans and that safe planning of areas of settlement has been successful neither in the past nor in recent decades.

When dealing with natural geo–hazards, it is important to understand the influence of sediment sorting on debris flows. The presence of coarse fraction is one of the aspects which affects the rheological behaviour of natural viscous granular fluid mixtures. In this paper, experiments on reconstituted debris flow mixtures with different coarse–to–fine sediment ratios are considered. Such mixtures behave just as non–Newtonian yield stress fluids and their rheological behaviour is largely affected by the presence of coarse fraction. Experimental results demonstrate that yield stress is very sensitive not only to bulk sediment concentration but also to coarse sediment fraction. A novel yield stress model is presented. It accounts for an empirical grading function depending on the coarse–to–fine grain content. The yield stress model performed satisfactorily in comparison with the experiments, showing that it is almost independent of the coarse–to–fine grain fraction in case of dominant coarse sediment content.

The China–Pakistan Economic Corridor (CPEC) project passes through the Karakoram Highway in northern Pakistan, which is one of the most hazardous regions of the world. The most common hazards in this region are landslides and debris flows, which result in loss of life and severe infrastructure damage every year. This study assessed geohazards (landslides and debris flows) and developed susceptibility maps by considering four standalone machine-learning and statistical approaches, namely, Logistic Regression (LR), Shannon Entropy (SE), Weights-of-Evidence (WoE), and Frequency Ratio (FR) models. To this end, geohazard inventories were prepared using remote sensing techniques with field observations and historical hazard datasets. The spatial relationship of thirteen conditioning factors, namely, slope (degree), distance to faults, geology, elevation, distance to rivers, slope aspect, distance to road, annual mean rainfall, normalized difference vegetation index, profile curvature, stream power index, topographic wetness index, and land cover, with hazard distribution was analyzed. The results showed that faults, slope angles, elevation, lithology, land cover, and mean annual rainfall play a key role in controlling the spatial distribution of geohazards in the study area. The final susceptibility maps were validated against ground truth points and by plotting Area Under the Receiver Operating Characteristic (AUROC) curves. According to the AUROC curves, the success rates of the LR, WoE, FR, and SE models were 85.30%, 76.00, 74.60%, and 71.40%, and their prediction rates were 83.10%, 75.00%, 73.50%, and 70.10%, respectively, these values show higher performance of LR over the other three models. Furthermore, 11.19%, 9.24%, 10.18%, 39.14%, and 30.25% of the areas corresponded to classes of very-high, high, moderate, low, and very-low susceptibility, respectively. The developed geohazard susceptibility map can be used by relevant government officials for the smooth implementation of the CPEC project at the regional scale.

Over the last decades, numerous extreme climate events such as extreme temperatures, droughts, heavy precipitation, and storms associated with climate change have been recorded in many countries, including Bulgaria. As a result, geomorphological hazards such as landslides, debris flows, mudflows, high-speed soil erosion, etc. often occur on the territory of the country. The debris flow is one of the most common hazardous processes in small catchments of the main river basins in Bulgaria. The Kresna Gorge located in the middle part of Struma River valley is a typical area with such processes which often cause the damages to the E79 international highway. The purpose of the present study is to characterize debris flows in Kresna Gorge (southwestern part of Bulgaria) by comparative analysis between the two events (occurred on May 24, 2009 and July 28, 2019). In order to achieve the aim of the study the geomorphological features and flow type of 2019 event were identified and the results were compared with the previous publications which investigated the event on May 24, 2009. In the present paper, the sediments and the type of transportation of the investigated event (July 2019) were determined by grain-size and clast-shape analysis. The impact of weather conditions on debris flow occurrence was shown by the analysis of the synoptic conditions on the day before the event. The results of the study bring to clarifying the geological-geomorphological and meteorological factors for the occurrence of debris flow and are important for geomorphological hazard management.

Alluvial (torrential) fans, especially those created from debris-flow activity, often endanger built environments and human life. It is well known that these kinds of territories where human activities are favored are characterized by increasing instability and related hydrological risk; therefore, treating the problem of its assessment and management is becoming strongly relevant. The aim of this study was to analyze and model the geomorphological aspects and the physical processes of alluvial fans in relation to the environmental characteristics of the territory for classification and prediction purposes. The main geomorphometric parameters capable of describing complex properties, such as relative fan position depending on the neighborhood, which can affect their formation or shape, or properties delineating specific parts of fans, were identified and evaluated through digital elevation model (DEM) data. Five machine learning (ML) methods, including a hybrid Euler graph ML method, were compared to analyze the geomorphometric parameters and physical characteristics of alluvial fans. The results obtained in 14 case studies of Slovenian torrential fans, validated with data of the empirical model proposed by Bertrand et al. (2013), confirm the validity of the developed method and the possibility to identify alluvial fans that can be considered as debris-flow prone.

In mountain basins, long-term instrumental monitoring coupled with high-resolution topographic surveys can provide important information on sediment yield. The Gadria catchment, located in the eastern Italian Alps, typically features several low-magnitude flood episodes and a few debris-flow events per year, from late spring to late summer. Beginning in 2011, sensors devoted to debris-flow detection (geophones, video cameras, flow stage sensors) were installed along the main channel, upstream of a retention basin. In case of debris flows, high-resolution topographical surveys of the retention basin are carried out multiple times per year. Rainfall is measured in the lower part of the catchment and at the headwaters, while passive integrated transponder tracing of bedload was performed in the main channel during spring and summer 2014. In this work, we present the reconstruction of the sediment dynamics at the catchment scale from 2011 to 2017. Results show that (i) coarse sediment yield is dominated by the few debris flows occurring per year; (ii) debris-flow volume estimations may be significantly different—up to 30 percent lower—when performed through a digital elevation model of difference analysis, compared to the time-integration of the debris-flow discharge estimates; (iii) using this latter method, the volumes are affected by significant uncertainties, particularly for small values of flow depth; and (iv) rainfall analysis permits us to characterize debris-flow initiation but also highlights difficulties in discriminating triggering from non-triggering rainstorms if based on rainfall duration and intensity only.

The amount of water runoff need to evolve landscapes is rarely assessed. Empirical studies correlate erosion rate to runoff or mean annual precipitation, but rarely is the full history of a landscape known such that it is possible to assess how much water was required to produce it. While this may not seem to be of primary importance on Earth where water is commonly plentiful, on Mars the amount of water to drive landscape evolution is a key question. Here we tackle this question through a series of five chapters, one devoted to field work at Meteor Crater, another to laboratory experiments about controlling processes, and then two chapters on analysis of landforms and implications of water runoff on Mars (associated with the Mars Science Laboratory mission to Gale Crater), and then we complete this effort with a consideration of how we can reliably assign relative timing between events resulting in small depositional features. What follows below is a summary of what is found in each chapter.Meteor Crater, a 4.5 km2 impact crater that formed ~50,000 years ago in northern Arizona, has prominent gully features on its steep walls that appear similar to some gullies found on Mars. At the crater bottom, there are over 30 meters of lake sediments from a lake that disappeared ~10,000 to 11,000 years ago, indicating the transition from the Pleistocene to the current, drier climate. A combination of fieldwork, cosmogenic dating, and topographic analysis of LiDAR data show that debris flows, not seepage erosion and fluvial processes as previously suggested in the literature, drove gully incision during their formation period of ~40,000 years before the onset of the Holocene. Runoff from bare bedrock source areas high on the crater wall cut into lower debris mantled slopes, where the runoff bulked up and transformed into debris flows that carried boulders down to ~5 to 8 degree slopes, leaving distinct boulder lined levees and lobate tongues of terminal debris deposits that crisscrossed on the lower slopes. We hypothesize that the fine material, likely generated in the impact, and deposited with the coarse debris on the lower portion of the crater wall, is key to this bulking up process as flows cut across the deposits. Fluvial processes following the debris flow gullies extended alluvial deposits to the crater floor and contributed to lake infilling. Cosmogenic dating confirms that most of the modification of the crater walls occurred before the early Holocene. To account for the 75 distinct deposits currently lying on the crater floor, debris flow frequency would be about 1 event every 17 years, assuming debris flow activity terminated ~10,000 years ago. Assuming a water-to-rock ratio of 0.2 at the time of transport, it would have taken ~100,000 m3 of water to transport the ~500,000 m3 of debris flow deposits on the crater floor. Given the 4.5 km2 size of the crater, this extensive erosion would require less than 0.02 m of total runoff, or the equivalent of just 0.001 mm/year over a 40,000 year period. This insignificant amount of water was likely packaged into short-lived storm or snow-melt events when debris flows were generated. Much more runoff did occur, as evidenced by the lake and fluvial deposits, as well as the likely cool, wet conditions of the late Pleistocene. This suggests only a small fraction of the total runoff is needed to do considerable geomorphic evolution, producing strongly gully-scared crater walls. Currently, only minor fluvial modification of the gully networks occurs.To test the hypothesis that fine-grained material is a necessary component behind sustained mobilization in granular debris flows, we performed a series of experiments in a 4 m diameter vertically rotating drum. Specifically, we assessed the role of fines and coarse grain size distribution on the rate of pore fluid pressure generation and its dissipation upon cessation of movement of the granular mass. We varied the amount of fines, from no fines to amounts found in debris flow deposits, and varied the coarse grain size distribution, from a single grain size (10 mm) to a range found in a natural flow deposit. The mixtures with fines contents close to those found in actual debris flows had elevated pore pressures, indicating they were almost fully liquefied, though the highest excess pore pressures were generated with the combination of fine material and a wide coarse grain size distribution. A boulder was also placed upon the fines-rich mixtures just after cessation of motion and it was observed that the pore fluid pressure rose instantly, bearing most of the grain's weight, and then the pressure slowly declined. When the same boulder was placed on the water-gravel flows, there was no change in fluid pressure, as the mass was supported entirely by grain-grain contacts. Our observations, combined with observations of others in the literature, inform the conceptual model presented herein where we hypothesize that the dilational separation of particles during debris flow shear leads to coarse particles (in this case particles >12 mm) slowly settling through the highly viscous non-Newtonian fluid. As the fines-rich mixture has a yield strength of ~5 Pa, we calculate that particles ~12 mm and smaller remain suspended. This combined effect requires the fluid to sustain the weight of the particles, leading to pore pressures equal to the entire weight of the solid and fluid mass.As part of the MSL science team, we focused on understanding the geomorphology and its implications for the hydrologic history of the landing site for the Curiosity rover, which was located at the distal end of the Peace Vallis fan in Gale Crater. The Peace Vallis fan covers 80 km2 and is fed by a 730 km2 catchment. Valley incision into accumulated debris delivered sediment through a relatively low-density valley network to a mainstem channel to the fan. An estimated total fan volume of 0.9 km3 matches the calculated volume of removal due to valley incision (0.8 km3). The fan profile is weakly concave up with a mean slope of 1.5% for the lower portion. Runoff (discharge/watershed area) to produce the fan is estimated to be more than 600 m, perhaps as much as 6000 m, indicating a hydrologic cycle that likely lasted at least 1000's of years. Atmospheric precipitation, possibly snow, not groundwater seepage produced the runoff. Based on topographic data, Peace Vallis fan likely onlapped the rise onto which Curiosity landed, Bradbury Rise, and spilled into a topographic low to the east of the rise. This argues that the light-toned fractured terrain within this topographic low corresponds to the distal deposits of Peace Vallis fan and in such a setting lacustrine deposits were predicted (and later confirmed by observations from the rover).Previous studies had suggested that there might have been large lakes in Gale. This history is important to understanding Mars' climate history and the volumes of water that may have passed through or covered the sediments encountered by Curiosity. Here we use improved imagery, topography, and ground-based observations from the Curiosity rover to map deltas, fans and gullies, which leads to a proposed history of lakes in Gale. We report evidence for at least three distinct large lakes within Gale, all which occurred after the crater's central mound, Aeolis Mons, reached close to its current topographic form. From corresponding deltaic deposits off the southern rim of Gale crater and the southern flank of Aeolis Mons, we identified the highest lake level at -3280 meters, which would have had a mean depth of 700 meters. The larger context provided by craters near Gale suggests that this lake was derived from regionally sourced water from the south. The next lake level, which was established after a period of drying with subsequent lake level decline before rewetting and lake level rise, is defined by four deltaic features as well as the termination of gullies around the northern rim of Gale. This second lake corresponds to an elevation of -3980 m, has a mean depth of 300 meters, and was likely sourced from water more local to Gale. Lake levels then rose to the third lake level, at -3780 m, with a mean depth of 400 meters. Two deltaic deposits derived from sediment from the rim of Gale define this lake. This last lake declined sufficiently quickly to prevent incision into any of the deltaic forms around the crater, though hydrologic activity likely continued as evidenced by a time of fan building, including the Peace Vallis fan, around the crater. Quantification of the fan and delta volumes and their gully sources on the crater wall and rim, and on Aeolis Mons, suggest that these discrete deposits contain essentially all of the sediment mobilized from the upslope source areas. Importantly for the MSL mission, this suggests that most of the sediments the rover has and will encounter experienced at least a few cycles of drying and wetting.The depositional features (i.e. alluvial fans and deltas) within Gale Crater are small but they likely record a time history of lakes and other depositional events within the crater. An obvious question to ask is: can crater counting, which has been used to date much of Mars' surface, be used on such small surfaces to provide accurate age dating? Here we introduce a simple simulation model to quantify the effects of sample area size and crater obliteration effects on age estimates derived from crater size-frequency distributions. Our results show, quantitatively, that crater counting to estimate age of small surfaces (

Today, Bagnold&rsquo, s theory is still applied to gravity-driven flows under the assumption of uniform sediment concentration. This study presents findings of flume experiments conducted to investigate the velocity and concentration distributions within the debris body by using high-resolution images. The analysis has shown that the concentration and mobility of grains vary along the depth. A linear law to interpret the grains concentration distribution, starting from the knowledge of the packing concentration and of the surface concentration, Cs, has been identified. By considering such a law, modified expressions of the Bagnold&rsquo, s number and the velocity in stony-type debris flows are also presented. By using these expressions, three regimes of motion have been identified along the depth, and the velocity profile within the debris body is determined as a function of the parameter Cs. It has been verified that the velocity profiles estimated by using the modified equation compare well (mean square error less than 0.1) with the literature&rsquo, s measured profiles when Cs is correctly measured or estimated. Results of cutting tests, conducted for a sample of the used material, have also allowed us to verify that Cs could be determined as a function of the static friction angle of the material.

In the studies of landslide susceptibility assessment, which have been developed in recent years, statistical methods have increasingly been applied. Among all, the BLR (Binary Logistic Regression) certainly finds a more extensive application while MARS (Multivariate Adaptive Regression Splines), despite the good performance and the innovation of the strategies of analysis, only recently began to be employed as a statistical tool for predicting landslide occurrence. The purpose of this research was to evaluate the predictive performance and identify possible drawbacks of the two statistical techniques mentioned above, focusing in particular on the prediction of debris flows. To this aim, an inventory of debris flows triggered by the passage of the hurricane IDA and the low-pressure system associated with it 96E, on 7 th and 8 th November 2009, in an area of about 26 km 2 close to the Caldera Ilopango, El Salvador (CA), was employed. Two validation strategies have been applied to both statistical techniques, thus obtaining four models – BLR (I), MARS (I), BLR (II) and MARS (II) – to be compared in pairs. Model performance was assessed in terms of AUC (area under the receiver operating characteristic (ROC) curve), Sensitivity, Specificity, Positive Prediction Value and Negative Prediction Value. Moreover, to evaluate the robustness of the modelling procedure, 50 replicates were created for each model and standard deviation was calculated for each of them. The results show that both techniques allow for obtaining good or excellent performances so that it is not possible to define one of the two techniques as absolutely better. However, the validation procedure reveals slightly better performance of the MARS models, with greater sensitivity and greater discrimination among True Negatives (TNs).

Although the present environmental conditions on Mars prohibit the generation of significant volumes of liquid water, observations of several very young landforms, such as gullies and recurrent slope lineae, have been interpreted as signals for aqueous processes. To explore the range of conditions under which such features can be formed on Earth, a field site in northern Victoria Land, East Antarctica, was geomorphologically investigated. Despite the small size of the icefree area, the site displays gullies, water tracks and other traces of liquid water. The gullies show clear evidence of sediment transport by debris flows, and are typical of paraglacial processes on steep slopes in a recently deglaciated area. Water tracks appear in different forms, and seem to recur seasonally in the austral summer. Melting of snow and surface glacier ice is the major water source for both debris flows and water tracks. The observations presented here highlight the potential for hyperarid polar deserts to generate morphogenetically significant amounts of meltwater. The gullies are morphologically analogous to Martian gullies, and water tracks on steep slopes appear very similar to recurrent slope lineae. The observations suggest that even small icefree sites in continental Antarctica may enable observations which can serve as a basis for working hypotheses in Mars analogue studies, and future field work should consider more areas in Antarctica in addition to the McMurdo Dry Valleys to search for Mars analogue landforms.

The sediment deposited by debris flows in rivers is a common cause of mountain disasters. The deposition characteristics of debris flows are strongly affected by the entrainment during the transportation process and the water depth in the river channel, but how is poorly understood. Here, the effects of the released flow volume, flow density, bed sediment and water depth on the deposition process of debris flows are investigated for a total of 22 different tests. Detailed topographic surveys of deposit lobes are conducted using a 3D laser scanner with high precision. The slopes of lobes are largely determined by the flow density and bed-sediment composition. However, the effect of released flow volume on the lobe slope is relatively limited. The lobe areas projected onto the horizontal plane display dumbbell or trapezoidal shapes depending on deposit volume, and the cross sections along the lateral flume present a Gaussian distribution for flows after bed-sediment entrainment. The lobe area is reduced and lobe height is enhanced with an increase of water depth, contributing to a steeper deposit. Partial, submersed or complete blockages occur, depending on the debris-flow volume and water depth. This study improves understanding of the deposition features and may aid future hazard assessments of debris flows.

Debris flows can quickly destroy basic facilities in mountainous areas where suitable land resources are limited and towns are located on gentle slopes. To alleviate the threat of debris flows two main approaches based on debris storage or debris-flow excretion have been proposed in recent decades in China. In August 2015 a debris flow occurred in the Zongzhai Valley for the first time in the last 100 years. Field surveys section measurements and discharge calculations indicated that the bulking of the debris flow and the sediment entrainment were serious resulting in the widening and deepening of the channel along the moraine terrace. Therefore the two traditional mitigation methods appear to be inappropriate for the debris flows in Zongzhai Valley because this region lacks sufficient space for debris storage and economic support for a long drainage canal. This study analyzed the supply mechanism of the bed material and bank sediments along the moraine terrace and found that the bank collapse was mainly controlled by bed scour. Based on a literature review we determined that the channel bed strength in the terrace should be increased and proposed that shallow dams be constructed to maintain the stability of the channel bed. The parameters of these shallow dams are provided in this article. © 2018 The Author(s).

In watershed mountain basins, affected in the last decades by strong rainfall events, the role of debris-flow and debris flood processes was investigated. Morphometric parameters have proven to be useful first-approximation indicators in discriminating those processes, especially in large areas of investigation. Computation of morphometric parameters in 19 watershed mountain basins of the western side valley of the Vallo di Diano intermontane basin (southern Italy) was carried out. This procedure was integrated by a semi-automatic elaboration of the potential susceptibility to debris flows, using Flow-R modelling. This software, providing an empirical model of the preliminary susceptibility assessment at a regional scale, was applied in many countries of the world. The implementation of Flow-R modelling requires a GIS application and some thematic base maps extracted using DEMs analysis. A 5-meter-resolution DEM has been used in order to produce the susceptibility maps of the whole study area, and the results are compared and discussed with the real debris flow/flood events that occurred in 1993, 2005, 2010, and 2017 in the studied area. The results have provided a good reliability of Flow-R modelling within small catchment mountain basins.

The different types of calcareous exotic clasts (fragments of pre-existing rocks), embedded in the Paleocene siliciclastic deposits of the Istebna Formation from the Beskid Mały Mountains (Silesian Unit, Western Outer Carpathians), were studied and differentiated through microfacies-biostratigraphical analysis. Calcareous exotics of the Oxfordian- Kimmeridgian age prevail, representing a type of sedimentation comparable to that one documented for the northern Tethyan margin. The Tithonian exotic clasts (Štramberk-type limestones), which are much less common, were formed on a carbonate platform and related slope. The sedimentary paleotransport directions indicate the Silesian Ridge as a main source area for all exotics, which were emplaced in the depositional setting of the flysch deposits. The exotics constitute a relatively rare local component of some debrites. Proceedings of the sedimentological facies analysis indicate that these mass transport deposits were accumulated en-masse by debris flows in a deep-water depositional system in the form of a slope apron. Exotics prove that clasts of the crystalline basement and, less common, fragments of the sedimentary cover, originated from long-lasting tectonic activity and intense uplift of the source area. Mass transport processes and mass accumulation of significant amounts of the coarse-grained detrital material in the south facial zone of the Silesian Basin during the Early Paleogene was due to reactivation of the Silesian Ridge and its increased denudation. Relative regression and erosion of the emerged older flysch deposits were also forced by this uplift. These processes were connected with the renewed diastrophic activity in the Alpine Tethys.

Catastrophic collapse of large rock slopes ranks as one of the most hazardous natural phenomena in mountain landscapes. The cascade of events, from rock-slope failure, to rock avalanche and the near-immediate release of debris flows has not previously been described from direct observations. We report on the 2017, 3.0 × 10^6 m3 failure on Pizzo Cengalo in Switzerland, which led to human casualties and significant damage to infrastructure. Based on remote sensing and field investigations, we find a change in critical slope stability prior to failure for which permafrost may have played a destabilizing role. The resulting rock avalanche traveled for 3.2 km and removed over one million m3 of glacier ice and debris deposits from a previous rock avalanche in 2011. Whereas this entrainment did not lead to an unusually large runout distance, it favored debris flow activity from the 2017 rock avalanche deposits: the first debris flow occurred with a delay of 30 s followed by ten debris flows within 9.5 h and two additional events two days later, notably in the absence of rainfall. We hypothesize that entrainment and impact loading of saturated sediments explain the initial mobility of the 2017 rock avalanche deposits leading to a near-immediate initiation of debris flows. This explains why an earlier rock avalanche at the same site in 2011 was not directly followed by debris flows and underlines the importance of considering sediment saturation in a rock avalanche’s runout path for Alpine hazard assessments., Geomorphology, 351, ISSN:0169-555x, ISSN:1872-695X

On the night of March 31, 2017, a series of mass movements took place in the Mocoa Basin and devastated the northwestern part of its urban area. The mass movements were triggered by a 4-day rainfall accumulation and high rainfall intensities on the night of the event. After the trigger, about 420 mass movements transported material as landslides, debris flows, and hyperconcentrated flows along the Taruquita and Taruca creeks and Sangoyaco River on the northern side, and as landslides and mudflows along the Mulato River on its southern side, resulting in major damages to both its population and infrastructure. Previous reports by Cheng et al. (2018) and García-Delgado et al. (2019) described the event from a hydrological and geological perspective, connecting them with the damage caused by the event, but leaving aside an analysis of the series of mass movements and its characteristics. This work presents an extended review of these mass movements and studies them via a statistical analysis, providing a detailed review of the event’s characteristics and their relationship with the resultant damage. These characteristics are compared with the existing hazard maps and they provide valuable information and awareness on hazard management for communities settled in the northern Andes.

This work focused on a post-wildfire landslide hazard assessment, applied to the 2017 Montagna del Morrone fire. This wildfire increased the possibility of landslides triggering, as confirmed by the occurrence of a debris flow, triggered by an intense, short duration rainfall event in August 2018. The study area was investigated through a detailed analysis incorporating morphometric analysis of the topography and hydrography and geomorphological field mapping, followed by the landslide hazard assessment. In detail, the analysis was performed following a heuristic or expert-based approach, integrated using GIS technology. This approach led to the identification of five instability factors. These factors were analyzed for the construction of thematic maps. Hence, each factor was evaluated by assigning appropriate expert-based ranks and weights and combined in a geomorphology-based matrix, that defines four landslide hazard classes (low, moderate, high, and very high). Moreover, the morphometric analysis allowed us to recognize basins prone to debris flows, which, in relevant literature, are those that show a Melton ratio of &gt, 0.6 and a watershed length of &lt, 2.7 km. Finally, all the collected data were mapped through a cartographic and weighted overlay process in order to realize a new zonation of landslide hazard for the study area, which can be used in civil protection warning systems for the occurrence of landslides in mountainous forested environments.

ISSN:0148-0227 ISSN:2169-9003 ISSN:2169-9011

Debris flows are natural hazards causing fatalities and damages to infrastructures every year. One of the current challenges is to improve the predictability of such events using simulation tools. In this direction, the paper aims to model debris-flow generation starting from the water component and then simulating the motion of the bulked solid-fluid mixture mass flow. The debris component is progressively increased through entrainment of the channel bed material. The simulation has been performed exploiting the tool r.avaflow, which implements a physically-based model (Pudasaini and Mergili, 2019) for the flow propagation and an empirical multi-phase model for the entrainment processes. The investigated study case occurred in the Northeastern Alps of Italy, near the town of Cortina d'Ampezzo (Veneto Region), during the summer of 2017. The debris flow was triggered by a heavy rainstorm that caused extreme surface runoff, leading to entrainment of sediment from the channel bed. The debris flow obstructed the bridge of a regional road and consequently flooded the adjacent areas. Different types of debris flow simulations are performed, testing four specific functions to compute the entrainment rate. The simulated results are then compared against field observations. The analysis considers the differences in volume and depth of entrainment and in the output hydrograph. We conclude that entrainment is correlated with the terrain slope, particularly if it is calculated on a smoothed digital elevation model, which dilutes a less significant local steepness. We calibrate a spatially distributed slope-dependent erosion coefficient that successfully reproduced the observed entrainment volumes. The outcomes highlight the great importance of simulating debris flow entrainment processes adopting a multiphase model, which resulted particularly suitable for an accurate reproduction of the investigated event. The results, corroborated by further verifications, can improve the reliability of challenging predictive simulations on debris flow erosion.

Debris-flow frequency, discharge, and travel distance are highly catchment dependent and typically controlled by topography, hydrological conditions, and sediment supply. As a consequence, detailed and case-specific investigations are needed to decipher debris-flow histories in order to improve hazard mitigation. This study documents past (ca. 10 years) debris-flow occurrences originating from the Ohya landslide, central Japan, by using a large set of methods including field monitoring, repeat airborne LiDAR, orthophoto interpretation, and tree-ring reconstructions. We demonstrate that the different approaches generally agree on the occurrence of debris flows but that mismatches may exist when it comes to the assessment of areas affected by individual events. These differences may even exceed the usual errors in precision inherent to each of the methods used. In the present case, high-resolution orthophoto interpretation tends to underestimate areas affected by debris flows, especially in the vertical direction, in the absence of lateral movement of the channel bed and as a result of shade and areas under trees. On the other hand, we realize that LiDAR data cannot necessarily be used to distinguish local changes in topography from noise. Tree-ring analyses can help to improve the temporal resolution of the analysis, but may have limitations when it comes to the definition of areas affected by an event because of the point-type nature of data. We conclude that the best and most complete results are obtained by combining multiple methods to improve the spatial and temporal resolution of past debris flows and to delimit areas affected by individual events.

The principal objective of this study was to investigate the types of large-scale meteorological situations that are conducive to the precipitation and temperature conditions most likely to trigger debris flows in the Zermatt valley, Switzerland, under current and future climates. A two-dimensional Bayesian probability calculation was applied to take account of uncertainties in debris-flow triggering. Precipitation quantities exceeding the 95th percentile of daily precipitation amounts were found to have a significantly higher probability to coincide with observed debris flows. A different relationship exists for extreme temperatures, however. Southerly air flows, weak horizontal pressure gradients over Europe, and westerly flows are mostly associated with observed debris flows and 95th precipitation percentile exceedances. These principal flow directions are well represented in the regional climate model (RCM) HIRHAM control simulations for events exceeding the 95th precipitation percentile and the 30th temperature percentile. Under the IPCC A2 emission scenario, westerly and southerly flows are mostly responsible for these precipitation and temperature conditions under the hypothesis of slow adaptation to climate change (HS1/HC1). Under the hypothesis of rapid adaptation to climate change (HS1/HS1), southerly flows and weak horizontal pressure gradients are likely to gain in importance. In both scenarios for the future, southeasterly flows are among the principal flow directions responsible for the joint exceedance of the 95th precipitation percentile and the 30th temperature percentile, while these were absent in observations and the control simulation.

Debris flow is a very common and destructive natural hazard in mountainous regions. Pore water pressure is the major triggering factor in the initiation of debris flow. Excessive pore water pressure is also observed during the runout and deposition of debris flow. Debris materials are normally treated as solid particle–viscous fluid mixture in the constitutive modeling. A suitable constitutive model which can capture the solid-like and fluid-like behavior of solid–fluid mixture should have the capability to describe the developing of pore water pressure (or effective stresses) in the initiation stage and determine the residual effective stresses exactly. In this paper, a constitutive model of debris materials is developed based on a framework where a static portion for the frictional behavior and a dynamic portion for the viscous behavior are combined. The frictional behavior is described by a hypoplastic model with critical state for granular materials. The model performance is demonstrated by simulating undrained simple shear tests of saturated sand, which are particularly relevant for the initiation of debris flows. The partial and full liquefaction of saturated granular material under undrained condition is reproduced by the hypoplastic model. The viscous behavior is described by the tensor form of a modified Bagnold’s theory for solid–fluid suspension, in which the drag force of the interstitial fluid and the particle collisions are considered. The complete model by combining the static and dynamic parts is used to simulate two annular shear tests. The predicted residual strength in the quasi-static stage combined with the stresses in the flowing stage agrees well with the experimental data. The non-quadratic dependence between the stresses and the shear rate in the slow shear stage for the relatively dense specimens is captured.

Recent severe rainstorms events in October 2014 and September 2015 triggered more than a hundred debris-flows in the western part of the Apennines of Emilia-Romagna (Italy). In this work, we tested a novel method to define debris flows rainfall thresholds for the 2014 and 2015 rainstorms (which have been used as reference events) and to extend these results across the Apennines of Emilia-Romagna, making use of long-term rainfall data of the regional raingauges network. Results are compared, for validation, to rainfall rates recoded during other past rainstorm-debris flows events (which have been used as validation events). At first, the method involves a spatial discriminant analysis between the spatial distribution of debris flows and the high frequency weather-radar rainfall data for the 2014 and 2015 reference events. The analysis defines rainfall cutoff values over rainfall durations from 30′ to 6 h, related to verification indices in the ROC curves, which are used as debris flows rainfall thresholds. Exceedance ratios are calculated between the computed rainfall thresholds and the rainfall rates at 10 years return periods at corresponding rainfall durations computed for raingauges located in the areas affected by the 2014 and 2015 events. The ratios are then used as multipliers of the rainfall rates at 10 years return periods over rainfall durations from 30′ to 6 h calculated for all other raingauges in the regional study area. To spatialize thresholds calculation to the regional scale, the computed thresholds are interpolated across the Apennines of Emilia-Romagna. The research resulted in the assessment of two-levels debris flows rainfall thresholds curves which seem to be adequate to discriminate rainfall rates recorded during past debris flows events used for validation. Discussion evidences advantages and limits of our approach, compares results to existing debris flows thresholds and highlights their possible use in a multi-stage warning procedure at regional scale.

Debris flows represent a major hazard in mountainous areas, due to their rapid motion along steep channels and to the transport of large sediment volumes, including large boulders. In this paper, we present data of channelized debris flows characterized by different velocities and sediment concentrations recorded in an instrumented channel reach of the Gadria basin (eastern Italian Alps). From the analysis of the seismic energy produced by the interaction of solid particles with channel boundaries, we show that (i) the peak amplitudes are representative of the kinetic energy of each surge and (ii) most energy transfer occurs during the passage of the surge fronts. Then, we propose a debris flow detection algorithm based on the amplitude information gathered from a linear array of geophones installed along the channel. The short time average over long time average ratio of the seismic signal is used to early detect the debris flow occurrence in a continuous stream of seismic data. The algorithm recognizes moving, long-lasting sources of ground vibration (i.e., debris flows) and filters out different seismic sources (i.e., anthropic noise, earthquakes, and rockfalls). The alarm is triggered when the short time average/long time average threshold is exceeded on two geophones, progressively with time from upstream to downstream. The algorithm is employed in the early warning system installed for research purposes at Gadria. Complementary data (rainfalls, flow stage measurements, and video recordings) permitted a detailed event characterization and alarm validation. During three monitoring seasons, all debris flows were successfully detected, with the alarm lasting for their entire duration, and no false positives were produced.

A storm, composed of two delayed cells, hit the Monte Antelao slopes (Dolomites, North Eastern Italy) in the early morning of July 18th, 2009. The resulting runoff triggered two consecutive debris flows along the Rovina di Cancia channel. The detailed topographic data collected before and just after this event allowed an accurate reconstruction of the morphological changes experienced by the channel bed. These data are here used as benchmark to test the ability of numerical models to reproduce the dynamics of a real event, taking into account the morphology changes of the channel bed. The aim is to provide an efficient model for engineering applications on large scales, such as those required by debris flow hazard assessment. A rainfall-runoff transformation is applied to reconstruct the solid-liquid hydrograph needed for computing the debris flow propagation. Two routing models are used: a GIS-based movable bed model, and a widely used fixed bed model (FLO-2D). Although similar results are obtained in terms of areas subjected to deposition, significant differences emerge in terms of mobilized volumes. Only the simulation of both the deposition and entrainment processes allows to reliably reproduce the sediment volumes estimated from the pre- and post-event topographic data. This information is fundamental in any hazard assessment because the volume of sediment mobilized by debris flow events exerts a fundamental control on the extension of areas subjected to inundation and on the thickness of sediment deposits. The capability to reproduce correctly the mobilized volumes also entails a more reliable simulation of the evolution of the peak and volume of the solid-liquid hydrograph as the debris flow propagates downstream, allowing the identification of the channel reach where banks could be overflowed. Conversely, adopting a fixed bed model leads mainly to an underestimation of the both the transported sediments volumes and the area subjected to deposition. As a consequence, the maximum debris flow depth in the portion of the channel subjected to erosion is underestimated and that in the portion of the channel subjected to deposition is overestimated. All these types of information are of great importance for an effective hazard assessment.

Landslides are a common natural hazard in Costa Rica, recurrently triggered by seismicity and extraordinary rainfall. Here, we investigate the coalescence of both processes and their ability to trigger massive landslides and debris flows in Costa Rica. The study focuses on Miravalles Volcano, affected by an earthquake of 5.4 Mw on July 2, 2016, and by intense rainfalls related to Hurricane Otto only 4 months later, on November 24, 2016. During the passage of Hurricane Otto, ~300 mm of rain were recorded in the study region. We use logistic general linear regression models (GLM) to represent the statistical relationships between the factors controlling landslides (such as epicenter distance, rainfall during Hurricane Otto, altitude, and slope). The compound 2016 event triggered 942 landslides, of which 62% were located within 3–6 km from the Bijagua earthquake epicenter, and on the eastern, southeastern, and southern slopes of Miravalles Volcano, i.e., in the zone where the density of local faults is highest and rainfall reached maximal values during the hurricane. The statistical analysis supports the existence of coupled earthquake-hurricane dynamics with higher landslide densities close to the epicenter and at sites receiving larger rainfall totals, but also showing higher slopes and altitudes. Debris flows affected an area of ~27 km2 and moved down the river systems, leaving eight casualties around the volcano and ca. 103 million US$ of losses in Upala and Bagaces. Results of this study can be useful for the assessment and understanding of geological and hydrometeorological hazards in Costa Rica and other tropical countries.

Ash-fall pyroclastic deposits covering Campanian mountain slopes (southern Italy) are very prone to instability under heavy and prolonged rainfall. In such a geo-hazard framework, to understand hydrological dynamics of pyroclastic mantle is a further step toward the assessment of rainfall thresholds and landslide hazard. In this research, the hydrological and stability modelling of representative ash-fall pyroclastic soil mantled slopes of the Sarno Mountains is proposed to assess the role of seasonal hydrological variability of the pyroclastic cover on rainfall triggering debris flows. The approach is based on the numerical modelling, from seasonal to inter-annual timescales, of unsaturated/saturated flows occurring into the pyroclastic mantle upslope of a source area of a debris flow. Modelling results were calibrated by means of field measurements achieved by a monitoring station equipped with tensiometers and Watermark sensors. Among the main results, the recorded pressure head time series showed a dominant unsaturated condition and a very relevant decrease in the summer season, whose effects are not limited to the root zone but extended down to the bedrock interface, about 4 meter deep. This seasonal hydrological regime of the ash-fall pyroclastic mantle was correlated both to the distinctive water retention properties of pyroclastic soils and to the existence of a deciduous forest, which concentrates the evapotranspiration demand during the dry season. Hydrological and stability modeling of the representative slopes allowed the reconstruction of deterministic rainfall thresholds for both dry and wet seasons and the assessment of the significant predisposing role of antecedent hydrological conditions to slope instability during short and heavy rainstorms.

Many debris flows have occurred in the areas surrounding the epicenter of the Wenchuan earthquake. Susceptibility assessment of debris flows in this area is especially important for disaster prevention and mitigation. This paper studies one of the worst hit areas, the Subao river valley, and the susceptibility assessment of debris flows is performed based on field surveys and remote sensing interpretation. By investigating the formation conditions of debris flows in the valley, the following assessment factors are selected: mixture density of landslides and rock avalanches, distance to the seismogenic fault, stratum lithology, ground roughness, and hillside angle. The weights of the assessment factors are determined by the analytic hierarchy process (AHP) method. Each of the assessment factors is further divided into five grades. Then, the assessment model is built using the multifactor superposition method to assess the debris flow susceptibility. Based on the assessment results, the Subao river valley is divided into three areas: high susceptibility areas, medium susceptibility areas, and low susceptibility areas. The high susceptibility areas are concentrated in the middle of the valley, accounting for 17.6% of the valley area. The medium susceptibility areas are in the middle and lower reaches, most of which are located on both sides of the high susceptibility areas and account for 45.3% of the valley area. The remainders are classified as low susceptibility areas. The results of the model are in accordance with the actual debris flow events that occurred after the earthquake in the valley, confirming that the proposed model is capable of assessing the debris flow susceptibility. The results can also provide guidance for reconstruction planning and debris flow prevention in the Subao river valley.

This study describes an example of analysis of morphological changes by using high resolution DEMs derived from LiDAR data. The evaluation of eroded or deposition volumes is based on the comparison between topographical profiles derived from two time-series of high resolution DEMs.The proposed method has been applied to selected areas of the Vernazza basin (Cinque Terre, eastern Liguria) that suffered strong morphological changes due to the 25 October 2011 very intense rainfall event. Three human embankments areas located along channels, completely or partially destroyed during the event, were selected for calculating eroded volumes whereas five areas in the valley floor were investigated for estimating deposition volumes. The obtained results have shown the impact of human embankments in increasing the effects of very hazardous phenomena typical of hilly or mountain areas, such as debris flows and debris floods. The proposed method, although approximate, can be very useful when a rapid estimation of volume mobilized in specific areas is required.

The synonymous use of the general term “landslide”, with a built-in reference to a sliding motion, for all varieties of mass-transport deposits (MTD), which include slides, slumps, debrites, topples, creeps, debris avalanches etc. in subaerial, sublacustrine, submarine, and extraterrestrial environments has created a multitude of conceptual and nomenclatural problems. In addition, concepts of triggers and long-runout mechanisms of mass movements are loosely applied without rigor. These problems have enormous implications for studies in process sedimentology, sequence stratigraphy, palaeogeography, petroleum geology, and engineering geology. Therefore, the objective of this critical review is to identify key problems and to provide conceptual clarity and possible solutions. Specific issues are the following: (1) According to “limit equilibrium analyses” in soil mechanics, sediment failure with a sliding motion is initiated over a shear surface when the factor of safety for slope stability (F) is less than 1. However, the term landslide is not meaningful for debris flows with a flowing motion. (2) Sliding motion can be measured in oriented core and outcrop, but such measurement is not practical on seismic profiles or radar images. (3) Although 79 MTD types exist in the geological and engineering literature, only slides, slumps, and debrites are viable depositional facies for interpreting ancient stratigraphic records. (4) The use of the term landslide for highvelocity debris avalanches is inappropriate because velocities of mass-transport processes cannot be determined in the rock record. (5) Of the 21 potential triggering mechanisms of sediment failures, frequent short-term events that last for only a few minutes to several hours or days (e.g., earthquakes, meteorite impacts, tsunamis, tropical cyclones, etc.) are more relevant in controlling deposition of deep-water sands than sporadic long-term events that last for thousands to millions of years (e.g., sea-level lowstands). (6) The comparison of H/L (fall height/runout distance) ratios of MTD in subaerial environments with H/L ratios of MTD in submarine and extraterrestrial environments is incongruous because of differences in data sources (e.g., outcrop vs. seismic or radar images). (7) Slides represent the pre-transport disposition of strata and their reservoir quality (i.e., porosity and permeability) of the provenance region, whereas debrites reflect post-transport depositional texture and reservoir quality. However, both sandy slides and sandy debrites could generate blocky wireline (gamma-ray) log motifs. Therefore, reservoir characterization of deep-water strata must be based on direct examination of the rocks and related process-specific facies interpretations, not on wireline logs or on seismic profiles and related process-vague facies interpretations. A solution to these problems is to apply the term “landslide” solely to cases in which a sliding motion can be empirically determined. Otherwise, a general term MTD is appropriate. This decree is not just a quibble over semantics; it is a matter of portraying the physics of mass movements accurately. A precise interpretation of a depositional facies (e.g., sandy slide vs. sandy debrite) is vital not only for maintaining conceptual clarity but also for characterizing petroleum reservoirs.

The Apuan Alps are characterized by frequent heavy rainfall. In several cases this triggered many shallow landslides (soil slips). With the aim of contributing to the landslide hazard evaluation of the southern Apuan Alps (upper Versilian area), a detailed analysis of the main pluviometric events was carried out. Data recorded at the main raingauge of the area from 1975 to 2002 were analysed and compared with the occurrence of soil slips, in order to examine the relationship between soil slip initiation and rainfall. Some thresholds for soil slip-debris flow activity in terms of mean intensity, duration and mean annual precipitation (MAP) were defined for the study area.

Palynological interpretations of both spores and acritarchs from the Devonian Tiffrit Formation located in Saida (North-Western Algeria) support identical conclusions. Species of spores and acritarches that appear in the upper layers of the younger reference unit, named Boukourdene (BK1), are compatible with a Pragian-Emsian age, based especially on the presence of the miospores Dibolisporites wetteldorfensis, Dictyotriletes emsiensis, D. subgranifer, Verrucosisporites polygonalis, and the acritarchs Evittia crucistellata, E. spicifera, Fimbriaglomerella aulerca, Florisphaeridium toyetae, and Veryhachium aff. V. vandenbergheni. On the basis of occurrences of microconglomeratic levels with elements of Silurian phtanites found along the sedimentary units of Tiffrit, we consider that Silurian sediments as well as Lower Devonian sediments have been reworked. We thus propose a Praguian-Emsian age for the Boukourdene units. From a sedimentological point of view, the Devonian sedimentary units of the Tiffrit Massif are linked to a gravitational deposition controlled by sea level rise and significant subsidence. Facies seems to change along an East-West axis. The particle size, the nature of the facies, and their extreme variability over short distances indicate a slope apron environment of deposition. The lithological, sedimentological and environmental characteristics of the studied system are consistent with deposition in an active tectonic margin setting., D'un point de vue paléontologique, les interprétations palynologiques basées sur les miospores et les acritarches du Dévonien de Tiffrit (Saïda, Algérie nord occidentale) sont similaires. Aucune des espèces qui apparaît dans les couches supérieures de l'unité de référence dite unité de Boukourdène ou (BK1), d'âge plus récent, n'est incompatible avec un âge Praguien-Emsien, comme le dénote la présence de miospores, surtout de Dibolisporites wetteldorfensis, Dictyotriletes emsiensis, D. subgranifer et Verrucosisporites polygonalis, ainsi que des acritarches tels que Evittia crucistellata, E. spicifera, Fimbriaglomerella aulerca, Florisphaeridium toyetae, Veryhachium aff. V. vandenbergheni. Au vu des niveaux microconglomératiques à éléments de phtanites siluriens, retrouvés le long des unités sédimentaires de Tiffrit, nous considérons qu'un remaniement a affecté aussi bien le Silurien que le Dévonien inférieur. Nous plaçons les unités sédimentaires de Boukourdène à l'étage Praguien-Emsien. D'un point de vue sédimentologique, les unités sédimentaires du Dévonien du môle de Tiffrit sont liées à une dynamique gravitaire, dominée par un accroissement du niveau marin et rythmée par une subsidence accrue. Les faciès semblent évoluer selon une polarité Est-Ouest. La taille des particules ainsi que la nature des faciès et leur grande et rapide variabilité permettent d'évoquer un corps sédimentaire de type "slope apron". Les caractères lithologiques, sédimentologiques et environnementaux du dispositif étudié plaident pour une marge tectonique active.