Hillslope Debris Research Articles

Conventional data-driven landslide susceptibility models may only tell us half of the story: Potential underestimation of landslide impact areas depending on the modeling design

Data-driven landslide susceptibility models (LSM) have been widely used to spatially predict areas likely affected by landslides. Even though such approaches are used to estimate the spatial likelihood of landslide occurrence, they are frequently trained with observations that rather represent landslide release zone conditions without explicitly considering the potential landslide downslope propagation into flat terrain. Landslide runout models, instead, are designed to route the downslope propagation of masses from given release zones but are seldom coupled with landslide release susceptibility models. This research compares the prediction pattern of three conceptually different models: (i) a model trained on landslide release observations only (Model R), (ii) a model trained on landslide body observations only (Model B), and, (iii) a model that explicitly couples landslide-release susceptibility and the subsequent runout zones (Model R + R). The aim is to explore and interpret the implications of differences in the susceptibility classes and associated model performance metrics. In this context, the interpretation focuses on the results observed for gentle terrain that is located downslope of the prevalent steeper hillsides. These partly built-up areas (i.e., buildings, linear infrastructures) are frequently within the reach of open-hillslope debris flows and, therefore of particular interest from a hazard and risk viewpoint.The study area for this research is the 54 km2 catchment Córrego Dantas, situated in the mountainous region of Rio de Janeiro, SE Brazil. After a heavy rainfall event in 2011, an inventory composed of 313 landslides was mapped and used as input for this work. First, a landslide release susceptibility model (Model R) based on an established data-driven method (Generalized Additive model, GAM) was created to explore the spatial likelihood of landslide release. Model R was then combined with r.randomwalk to compute the propensity of downslope regions to be affected by the released landslides, resulting in an integrated landslide impact susceptibility score (Model R + R). In order to achieve this, a constrained top-down random walk approach was used. Furthermore, a GAM model built upon observations from the entire landslide body (Model B) was used for comparison. Results reveal a potential underestimation of the landslide threat in flatter areas. This tendency is most pronounced for the release-trained model (Model R). The runout impact susceptibility scores produced by the random walk (Model R + R) were observed to better represent the actual impact threat of the whole geomorphological phenomena. From a statistical viewpoint, also the simpler model (Model B) exhibited a satisfactory performance to predict the full landslide area that includes release and deposit zones. The outcomes of this research could be of particular interest to modelers aiming to model landslide susceptibility of regions characterized by long-runout hillslope debris flows and for studies focusing on landslide impact and risk.

Practical guide for debris flow and hillslope debris flow protection nets and its application in case studies

Debris flows and hillslope debris flows endanger people and infrastructures. Technical protection measures are important elements in addition to spatial planning (adapted use of space like hazard maps) and organizational measures (warning systems, emergency plan, evacuation). Additional to other rigid protection measures flexible debris flow nets and hillslope debris flow nets were developed in 2008, and 2010 respectively. Such flexible barriers consist of an interception surface of netting spanned between horizontal support ropes including energy devices and posts for large span width. Since then, they are available on the market worldwide. Many projects were successfully installed and already filled by debris flow events. After more than ten years of experience and demonstrating that these measures work against debris flow and hillslope debris flow, they are fully accepted as possible measures for integral protection against debris flow. To provide planners and engineers the experience and know-how of these flexible protection nets, a practical guide was initiated by the Swiss Federal Roads Office (FEDRO) and Swiss Federal Office for the Environment (FOEN). Two case studies with planned and installed flexible nets, in one case for a debris flow and in the other case for a hillslope debris flow, are presented.

Multi-Methodological Investigation of the Biersdorf Hillslope Debris Flow (Rheinland-Pfalz, Germany) Associated to the Torrential Rainfall Event of 14 July 2021

The investigation of mass movements is of major interest in mountain regions as these events represent a significant hazard for people and cause severe damage to crucial infrastructure. The torrential rainfall event that mainly occurred on the 14 July 2021 in western Central Europe not only led to severe flooding catastrophes (e.g., Meuse, Ahr and Erft rivers) but also triggered hundreds of mass movements in the low mountain range. Here, we investigate a hillslope debris flow that occurred in Biersdorf in the Eifel area (Rhenish Massif, Rheinland-Pfalz) using a comprehensive geomorphological–geophysical approach in order to better understand the triggering mechanisms and process dynamics. We combined field studies by means of Electrical Resistivity Tomography (ERT), Direct Push Hydraulic Profiling (HPT) and sediment coring with UAV-generated photogrammetry, as well as debris flow runout modelling. Our results show that for the Biersdorf hillslope debris flow, the geomorphological and geotectonic position played a crucial role. The hillslope debris flow was triggered at a normal fault separating well-draining limestones of the Lower Muschelkalk, from dense weathered clay and sandstones of the Upper Buntsandstein. The combination of a large surface runoff and strong interflow at the sliding surface caused a transformation from an initial translational slide into the high-energy and widespread hillslope debris flow. We further created and validated a stand-alone model of the debris flow on a local scale achieving promising results. The model yields a 97% match to the observed runout area as well as to deposition spreads and heights. Thus, our study provides a pathway for analyzing hillslope debris flows triggered by torrential rainfall events in low mountain ranges. General knowledge on hillslope debris flows, risk assessment and hazard prevention were improved, and results can be transferred to other regions to improve risk assessment and hazard prevention.

2D Runout Modelling of Hillslope Debris Flows, Based on Well-Documented Events in Switzerland

In mountain areas, mass movements, such as hillslope debris flows, pose a serious threat to people and infrastructure, although size and runout distances are often smaller than those of debris avalanches or in-channel-based processes like debris floods or debris flows. Hillslope debris-flow events can be regarded as a unique process that generally can be observed at steep slopes. The delimitation of endangered areas and the implementation of protective measures are therefore an important instrument within the framework of a risk analysis, especially in the densely populated area of the alpine region. Here, two-dimensional runout prediction methods are helpful tools in estimating possible travel lengths and affected areas. However, not many studies focus on 2D runout estimations specifically for hillslope debris-flow processes. Based on data from 19 well-documented hillslope debris-flow events in Switzerland, we performed a systematic evaluation of runout simulations conducted with the software Rapid Mass Movement Simulation: Debris Flow (RAMMS DF)—a program originally developed for runout estimation of debris flows and snow avalanches. RAMMS offers the possibility to use a conventional Voellmy-type shear stress approach to describe the flow resistance as well as to consider cohesive interaction as it occurs in the core of dense flows with low shear rates, like we also expect for hillslope debris-flow processes. The results of our study show a correlation between the back-calculated dry Coulomb friction parameters and the percentage of clay content of the mobilised soils. Considering cohesive interaction, the performance of all simulations was improved in terms of reducing the overestimation of the observed deposition areas. However, the results also indicate that the parameter which accounts for cohesive interaction can neither be related to soil physical properties nor to different saturation conditions.

Schützt der Wald vor Rutschungen? Hinweise aus der WSL-Rutschungsdatenbank

Does the forest provide protection from landslides? Evidence from the WSL Shallow Landslide Database During strong rainfall events, shallow landslides and debris avalanches (hillslope debris flows, or open-slope debris flows) are triggered and sometimes lead to considerable damage. Analysis of damage-causing events show that there are fewer landslides in forested areas compared to non-forested areas, which indicates the generally positive influence of forest vegetation on slope stability. However, these effects depend on the condition of the forest stand and quantification of the effects is difficult. Event documentation contributes to a better understanding of the relevant processes. The information obtained is not only important for the preparation of hazard maps, but also provides valuable insight for assessing the hazard protection provided by the forest. Data from the landslide database of the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) were used to evaluate the influence of the forest on slope stability. Currently, the database contains information on 734 landslides. Of these, 661 were included in the evaluation – 356 landslides in non-forested areas and 305 in forested areas. In areas with slope angles up to 38°, more landslides per unit area are observed in non-forested areas than in forested areas. In areas with steeper slope angles a stabilizing effect of the forest is no longer recognizable. Statistical analyses show that landslides in forested areas are smaller than in non-forested areas and are more frequent on steeper slopes. In general, the landslides become smaller with increasing slope. Multivariate analyses indicate a positive influence of the forest and also somewhat smaller landslides in well-developed forests. Negative effects are evident in non-forested areas and in areas with overly dense forests. In addition to illustrating the importance of the forest condition for slope stability, the paper also discusses how the forest condition can be described.

Numerical modeling using an elastoplastic-adhesive discrete element code for simulating hillslope debris flows and calibration against field experiments

Abstract. This paper presents a discrete-element-based elastoplastic-adhesive model which is adapted and tested for producing hillslope debris flows. The numerical model produces three phases of particle contacts: elastic, plastic and adhesive. A parametric study was conducted investigating the effect of model parameters and inclination angle on flow height, velocity and pressure, in order to define the most sensitive parameters to calibrate. The model capabilities of simulating different types of cohesive granular flows were tested with different ranges of flow velocities and heights. The basic model parameters, the microscopic basal friction (ϕb) and ratio between stiffness parameters k1/k2, were calibrated using field experiments of hillslope debris flows impacting a pressure-measuring sensor. Simulations of 50 m3 of material were carried out on a channelized surface that is 41 m long and 8 m wide. The calibration process was based on measurements of flow height, flow velocity and the pressure applied to a sensor. Results of the numerical model matched those of the field data in terms of pressure and flow velocity well while less agreement was observed for flow height. Those discrepancies in results were due in part to the deposition of material in the field test, which is not reproducible in the model. Results of best-fit model parameters against selected experimental tests suggested that a link might exist between the model parameters ϕb and k1/k2 and the initial conditions of the tested granular material (bulk density and water and fine contents). The good performance of the model against the full-scale field experiments encourages further investigation by conducting lab-scale experiments with detailed variation in water and fine content to better understand their link to the model's parameters.

Wildfires in Grasslands and Shrublands: A Review of Impacts on Vegetation, Soil, Hydrology, and Geomorphology

Wildfires are prevalent in grasslands and shrublands. The objective of this study is to provide land managers with a general overview, by assessing the main impacts of wildfire, including those on plant communities (e.g., secondary succession and species invasion), soil characteristics (e.g., water repellency (hydrophobicity), aggregation and structure stability, and contents of organic carbon and nutrients), and surface processes (e.g., ash deposition, ground surface clogging, water runoff, soil erosion, hillslope debris flow, and dry ravel). Additionally, the study discusses the effects of livestock grazing on the functioning of post-fire grasslands and shrublands. Although mesic regions are mentioned, this review focuses on drylands. The comparatively low-to-moderate fuel loads that characterize grasslands and shrublands generate wildfires of relatively moderate intensity, resulting in moderate burn severity. Yet, it seems that because of decreased soil aggregate stability following burning, the hoof action of livestock that access burnt lands shortly after the fire increases the shearing and detachment of mineral material from the ground surface; this increases soil erodibility, with the possible risk of accelerated land degradation. The review ends with an assessment of general implications for environmental sustainability and health, and provides recommendations on wildfire control in rangelands, and on restoration of burnt lands.

Debris thickness patterns on debris-covered glaciers

Many debris-covered glaciers have broadly similar debris thickness patterns: surface debris thickens and tends to transition from convex- to concave-up-down glacier. We explain this pattern using theory (analytical and numerical models) paired with empirical observations. Down glacier debris thickening results from the conveyor-belt-like nature of the glacier surface in the ablation zone (debris can typically only be added but not removed) and from the inevitable decline in ice surface velocity toward the terminus. Down-glacier thickening of debris leads to the reduction of sub-debris melt and debris emergence toward the terminus. Convex-up debris thickness patterns occur near the up-glacier end of debris covers where debris emergence dominates (ablation controlled). Concave-up debris thickness patterns occur toward glacier termini where declining surface velocities dominate (velocity controlled). A convex-concave debris thickness profile inevitably results from the transition between ablation-control and velocity-control down-glacier. Debris thickness patterns deviating from this longitudinal shape are most likely caused by changes in hillslope debris supply through time. By establishing this expected debris thickness pattern, the effects of climate change on debris cover can be better identified.

Hillslope coupled stream morphology, flow conditions, and their effects on detrital sedimentology in Garnet Canyon, Teton Range, Wyoming

Characterizing stream erosion in any steep mountain landscape is arduous, but the challenge level increases when the stream flows through a glaciated catchment frequently modified by hillslope debris. Glacial landforms and stochastic mass wasting in alpine systems may interfere with sediment delivery to downstream sites where detrital sediments are often collected to represent upstream bedrock sources. To use detrital sediments as indicators of erosion, we need to understand potential sediment accumulation in flat glaciated reaches or behind rockfall barriers. This study investigates the stream channel in Garnet Canyon, a glaciated catchment located in the central Teton Range, to describe hillslope coupled channel morphology and the subsequent effects on sediment transport throughout the catchment. Stream cross-section surveys and sediment size measurements of the surface bedload were collected in the field within a glacially flattened segment of Garnet Canyon. Calculations of shear stress conditions allowed evaluation of the importance of mineral densities on potential grain entrainment. The length of the Garnet Canyon stream observed in this study was coupled with hillslope deposits. Critical shear stresses were sufficient to move gravel-sized sediments through all sections when calculated with quartz mineral density and through most sections when applying apatite mineral density. These results verify the application of detrital sediments to evaluate erosion rates or spatial bedrock sources because snowmelt stream flow efficiently moves entrained sediment past glacially reduced slopes and potential talus barriers.

Post-fire hillslope debris flows: Evidence of a distinct erosion process

After wildfire a hitherto unexplained erosion process that some authors have called ‘miniature debris flows on hillslopes’ and that leave behind levee-lined rills has been observed in some regions of the world. Despite the unusual proposition of debris flow on planar hillslopes, the process has not received much attention. The objectives of this study were to (1) accumulate observational evidence of Hillslope Debris Flows (HDF) as we have defined the process, to (2) understand their initiation process by conducting runoff experiments on hillslopes, to (3) propose a conceptual model of HDF, and to (4) contrast and classify HDF relative to other erosion and transport processes in the post-wildfire hillslope domain. HDF have been observed at relatively steep slope gradients (0.4–0.8), on a variety of geologies, and after fire of at least moderate severity and consist of a lobe of gravel- to cobble-sized material 0.2–1m wide that is pushed by runoff damming up behind it. During initiation, runoff moved individual particles that accumulated a small distance downslope until the accumulation of grains failed and formed the granular lobe of the HDF. HDF are a threshold process, and runoff rates of 0.5Ls−1 2Ls−1 were required for their initiation during the experiments. The conceptual model highlights HDF as a geomorphic process distinct from channel debris flows, because they occur on planar, unconfined hillslopes rather than confined channels. HDF can erode very coarse non-cohesive surface soil, which distinguishes them from rill erosion that have suspended and bedload transport. On a matrix of slope and grain size, HDF are enveloped between purely gravity-driven dry ravel, and mostly runoff driven bedload transport in rills.

Role of vegetation restoration in mitigating hillslope erosion and debris flows

After the 2008 Wenchuan earthquake, a large amount of loose landslide deposits was left in the stricken region. A heavy storm in 2010 turned numerous deposits into debris flows. After 2010, vegetation gradually recovered on the remnant soil deposits. During the wet seasons in 2013 and 2015, the number of debris flows decreased significantly compared with that in 2010. This paper investigates the role of vegetation in mitigating runoff induced hillslope erosion and debris flows. The study slope is near the epicentre of the Wenchuan earthquake. Three levels of vegetation restoration are considered: bare in 2010, middle level in 2013 and high level in 2015. Soil samples were taken from the landslide deposits to determine the root mass density, which is used to quantify erosion resistance parameters at different vegetation levels. Massive erosion is simulated in the 2010 case, which in turn develops into hillslope debris flows. Very little or no erosion is simulated in the 2013 and 2015 cases, which is in part attributed to the enhanced resistance to erosion by restored vegetation. A parametric study is conducted to further investigate the effectiveness of erosion reduction through revegetation.

Neogene terrestrial sedimentary environments of the Orava-Nowy Targ Basin: a case study of the Oravica River section near Čimhová, Slovakia

The Orava-Nowy Targ Basin is an intramontane depression located at the border of the Inner and Outer Carpathians. It is filled mostly with fine-grained terrestrial and fresh-water deposits of Neogene and Quaternary age, which record the structural evolution and denudation processes of the surrounding regions: Orava, Podhale, and the Tatra Mts. The Orava-Nowy Targ Basin fill was studied in the Oravica River section, which reveals 90 m thick Neogene deposits and allows observation of their lateral diversity. Fifteen lithofacies and seven facies associations distinguished here document the depositional evolution of the basin fill, starting from hill-slope debris flow through silt turbidite-bearing lake, to flood-dominated fluvial plain, swamp and alluvial fan deposits, as well as a pyroclastic event. The palaeorelief was low during deposition and the Orava-Nowy Targ Basin could have spread significantly more to the south.

Field and laboratory analysis of the runout characteristics of hillslope debris flows in Switzerland

Hillslope debris flows are unconfined flows that originate by shallow failures in unconsolidated material at steep slopes. In spite of their significant hazard for persons and infrastructure in mountainous regions, research on hillslope debris flows is rather scarce in comparison to other landslide types. This study focusses on the runout characteristics of hillslope debris flows applying two different approaches. First, detailed landslide inventories, which include field measurements of 548 slope failures that occurred during the last two decades in seven parts of Switzerland, were analysed. Second, laboratory tests were carried out to study the effect of the soil water content, grain-size distribution and mobilized volume on the runout behaviour of hillslope debris flows. Most of the failures in the field started as shallow translational slides at terrain slopes between 25° and 45° and involved volumes of some tens to a few hundred cubic meters. An analysis of the runout distance of 117 hillslope debris flows showed that they normally travelled some tens of meters, but sometimes the runout exceeded 300m. A positive relation between volume and runout distance and between volume and affected area was observed, although there is considerable scatter in the data. The affected area of 63 hillslope debris flows ranged from ~100 to ~1500m2. Based on the field data, a 7.5m long laboratory hillslope was designed with a geometrical scale factor of 20. A total of 75 runs with volumes from 4 to 20dm3, water contents from 18% to 38%, and four grain-size distributions were carried out. The laboratory tests revealed that water content is the dominant control, but also the clay content strongly influences the runout distance and the affected area. Even a small increase in water or clay content produces a considerably larger or smaller runout distance, respectively. In contrast, the influence of the volume on the runout was smaller, and a positive relation was observed between these two parameters. The field and laboratory results are in general agreement and consistent with the results of other studies. The results of this work improve the understanding of hillslope debris flows and may aid in the hazard assessments of these processes.

The Preliminary Study on Definition of Slope Debris Flow

So far, the definitions of slope debris flow are cinders and divergent, even, some scholars equate it with the hill slope debris flow/landslide-induced debris flow and other similar terms. This article summarizes research on the slope debris flow, from the definition and classification of debris flow, study on the classification of slope debris flow , analyzes the formation conditions, properties and characteristics of slope debris flow, at last, use the definition method of attribute and kind, tentatively give the preliminary definition of the slope debris flow .

Changes in runout distances of debris flows over time in the Wenchuan earthquake zone

A large number of debris flows occurred in the Wenchuan earthquake zone after the 12 May 2008 earthquake. The risks posed by these debris flows were rather high. An appropriate model is required to predict the possible runout distance and impacted area. This paper describes a study on the runout characteristics of the debris flows that occurred in the Wenchuan earthquake zone over the past four years. A total of 120 debris flows are analyzed. Separate multivariate regression models are established for the runout distances of hil l-slope debris flows and channelized debris flows. The control variables include type of debris flow, debris flow volume, and elevation difference . Comparison of the debris flows occurring before and after the earthquake shows that the runout distance increased after the earthquake due to sufficient material supply and increased mobility of the source materials. In addition, the runout distances of annual debris flow events in 2008, 2010 and 2011 are analyzed and compared. There is a tendency that the runout distance decreases over time due to the decreasing source material volume and possible changes of debris flow type. Comparison between the debris flows in the earthquake zone and the debris flows in Swiss Alps, Canada, Austria, and Japan shows that the former have a smaller mobility.

Classification of debris flow phenomena in the Faroe Islands

Landslides and debris flow phenomena in particular constitute a threat to human activities in the Faroe Islands. As a contribution to ongoing landslide risk management research, this paper proposes a classification scheme for debris flow phenomena in the Faroe Islands. The scheme, produced through a multidisciplinary study involving geomorphological fieldwork and qualitative collection of indigenous landslide knowledge, presents physical characteristics to classify debris flow phenomena into groups named with Faroese terms. The following landslide definitions are proposed. Brekku-skriðulop (English translation: hillslope debris flow): a very rapid to extremely rapid non-channelised downslope movement of remoulded colluvium and eventually fragmented rock, initiated as a debris slide in colluvial soil (type 1) or from rock fall (type 2). Áar-skriðulop (English translation: channelised debris flow): a very rapid to extremely rapid channelised downslope movement of remoulded colluvium and/or talus/fluvial material and eventually fragmented rock, initiated as a debris slide in colluvial soil (type 1), from rock fall (type 2) or from mobilisation of talus/fluvial material (type 3). The scheme, which is compatible with international landslide classification systems, significantly increases the knowledge of debris flow phenomena and promotes a consistent terminology of these within the Faroe Islands.

Fire-related debris flows in the Iberian Range, Spain

Debris flows occurred three weeks after a wildfire in August 1986 in the Najerilla River valley in the Iberian Range, northern Spain. The flows were triggered by a brief, intense rainstorm (approximately 25mmh−1 over 15min) in a small area with steep slopes covered by a thick colluvium of quartzite clasts. This storm resulted in the development of several unconfined hillslope debris flows and the formation of an alluvial fan at the mouth of the Pítare stream, which partially blocked the Najerilla River. We analysed the conditions that led to the development of the debris flows, and estimated the rainfall threshold for the debris flows to occur as well as the total volume of mobilised sediment. Four factors contributed to the debris flows: (i) the occurrence of a rainstorm three weeks after a wildfire, which had removed the plant cover from the soil; (ii) the steep slopes in the area (>30°), which were the most affected by debris flows; (iii) the presence of quartzite scarps on the hillslopes, which favoured the development of a ‘firehose effect’ involving channelised surface runoff; and (iv) the low plasticity index values of the fine material of the colluvium (indices of 7 to 8), which enabled rapid liquefaction. Estimates of rainfall intensity derived from the estimated peak flow in the Pítare stream suggests that around 80mm of rainfall fell in approximately 15min, although this is clearly an overestimated value given the high proportion of sediment load transported during the peak flow. Various equations estimated a rainfall-threshold of approximately 25mmh−1 considering a concentration time of 15min. The total sediment transported by the debris flows was 10,500m3 (15,750Mg, 6800Mgkm−2), and the Pítare stream alone transported a minimum of 4000m3 (6000Mg, 2500Mgkm−2). These results suggest that the rainfall threshold for initiating debris flows decreases following a wildfire, such that an ordinary rainstorm is able to trigger a severe erosion and sediment transport event. Given the absence of fresh landslide scars on the hillslopes, the origin of the fire-related debris flows in the Najerilla River valley appears to have been directly linked to increased rates of overland flow having a greater effect than infiltration for triggering debris flows.

Mechanisms and runout characteristics of the rainfall-triggered debris flow in Xiaojiagou in Sichuan Province, China

The 2008 Wenchuan earthquake induced a large number of landslides, and a vast amount of loose landslide materials deposited on steep hill slopes or in channels. Such loose materials can become sources of deadly debris flows once triggered by storms. On 13 August 2010, a storm swept Yingxiu and its vicinity, triggering a catastrophic debris flow with a volume of 1.17 million m3 in Xiaojiagou Ravine. The debris flow buried 1,100 m of road, blocked a river and formed a debris flow barrier lake. A detailed field study was conducted to understand the initiation mechanisms and runout characteristics of this debris flow. Two types of debris flows are identified, namely hill-slope debris flow and channelized debris flow. The hill-slope debris flow was triggered in the forms of firehose effect, rilling and landsliding, whereas the channelized debris flow was triggered in the form of channel-bed failure. This debris flow was a water–rock flow since most particles were gravel, cobble or larger rocks and the fraction of silt and clay was less than 2%. Grain contact friction, pore-pressure effects and inertial grain collision were the three most important physical interactions within the debris flow. Such interactions yielded a smaller runout distance (593 m) compared with those of mud–rock flows of similar size.

Centrifuge modelling of hillslope debris flow initiation

Physical modelling of hillslope debris flow initiation was undertaken using the geotechnical beam centrifuge at the University of Dundee. The tests were carried out on model slopes consisting of soils with known properties prepared with constant initial density and material thickness. The results showed that soils with a higher silt fraction can sustain a higher increase in pore water pressure and thus a greater reduction in effective stress before failure is induced. This suggests that sand rich soils with lower fines content are geotechnically more susceptible to slope failure and that observed higher spatial frequencies of debris flow on slopes with coarse grained bedrocks can be partially explained in terms of lower critical pore pressure failure thresholds amongst the sandier soil matrixes produced from such lithologies. Differences in rates of water ingress into the model soils required to trigger a critical rise in pore pressures during the tests provide insights into the varying synoptic conditions and antecedent rainfall characteristics that are likely to trigger debris flows in different soil types.

Measurements of hillslope debris flow impact pressure on obstacles

We present measurements of hillslope debris flow impact pressures on small obstacles. Two impact sensors have been installed in a real-scale experimental site where 50 m3 of water-saturated soil material are released from rest. Impact velocities vary between 2 and 13 m/s; flow heights between 0.3 and 1.0 m. The maximum impact pressures measured over 15 events represent between 2 and 50 times the equivalent static pressures. The measurements reveal that quadratic velocity-dependent formulas can be used to estimate impact pressures. Impact coefficients C are constant from front to tail and range between 0.4 < C < 0.8 according to the individual events. The pressure fluctuations to depend on the sensor size and are between 20% and 60% of the mean pressure values. Our results suggest that hazard guidelines for hillslope debris flows should be based on quadratic velocity-dependent formulas.