Runout Length Research Articles

Assessing the effect of lithological setting, block characteristics and slope topography on the runout length of rockfalls in the Alps and on the island of La Réunion

Abstract. In four study areas within different lithological settings and rockfall activity, lidar data were applied for a morphometric analysis of block sizes, block shapes and talus cone characteristics. This information was used to investigate the dependencies between block size, block shape and lithology on the one hand and runout distances on the other hand. In our study, we were able to show that lithology seems to have an influence on block size and shape and that gravitational sorting did not occur on all of the studied debris cones but that other parameters apparently control the runout length of boulders. Such a parameter seems to be the block shape, as it plays the role of a moderating parameter in two of the four study sites, while we could not confirm this for our other study sites. We also investigated the influence of terrain parameters such as slope inclination, profile curvature and roughness. The derived roughness values show a clear difference between the four study sites and seem to be a good proxy for block size distribution on the talus cones and thus could be used in further studies to analyse a larger sample of block size distribution on talus cones with different lithologies.

On gravity currents confined by porous boundaries in containers of general cross-sections

We consider an inertial (large Reynolds number) gravity current (GC) released from a lock of length $$x_0$$ and height $$h_0$$ into an ambient fluid of height $$Hh_0$$ which propagates along a channel with permeable floor and/or walls. The cross section (CS) of the container is defined by the general $$-f_1 (z) \le y \le f_2 (z)$$ for $$0 \le z \le Hh_0$$ , while the top and the bottom are at $$z = Hh_0$$ and $$z=0$$ (z is the physical coordinate measured from the bottom of the channel). We use Boussinesq assumption and formulate the shallow-water equations of motion. These equations include a parameter $$\lambda$$ which reflects the ratio between the typical time of propagation of the current over a length $$x_0$$ to the typical time of the drop of the interface due to porous boundary over a height $$h_0$$ . To solve the problem we employ the method of finite-differences. It is demonstrated that initially, the effect of a porosity of the floor or the walls on the distance of propagation of the GC is insignificant, but the “slumping” stage is absent. During the advanced stages, the porosity slows down the current and decreasing the effective distance of propagation. We illustrate the methodology for flow in symmetric channels with typical power-law CSs $$f_1(z) = f_2(z) = 0.5(c+bz^{\alpha })$$ , where $$b,c, \alpha$$ are non-negative constants and show that for the identical characteristics of the porous material, the presence of the porous floor has more significant effect on the velocity of the propagation of the current than the presence of the permeable walls in all tested forms of containers. Special attention is given to triangular CS channels $$f_{1}(z)=f_{2}(z) = bz$$ with permeable walls. We show that in such containers the volume of the current decreases like $$\exp \left( -\sqrt{1+\frac{1}{b^2}}\lambda t\right)$$ (where t is time) and for $$Hh_0 \gg 1$$ its run-out length can be expressed by a compact analytical formula.

Debris Flow Numerical Simulation Model Comparison with Field Events in Kuala Kubu Baru and Lentang, Malaysia

Debris flow is one of Malaysia’s natural disasters which could cause casualties and serious infrastructure damage. Precisely predicting the factors associated with the occurrence of debris flow such as the run-out lengths, velocities and thickness of alluvial deposits can greatly mitigate damage, minimize or even avoid the aftermaths of the occurrence. Applying numerical simulation models explaining debris flow deposition would be a valuable tool in forecasting potential debris flow activity and providing criteria for designing protective measures. Comprehensive studies of available records of past debris flow events from relevant sources and site investigations have been carried out in order to assemble field information for the particular debris flow event in Malaysia. A number of calamitous debris flow events occurred in Malaysia have been closely observed and studied. The well-documented events, i.e. Lentang and Kuala Kubu Baru debris flow disasters occurred on 2nd November 2004 and 10th November 2003, respectively were simulated using the Kanako 2D (Ver.2.04) simulation model software. The results obtained from the numerical simulation model were compared with the real events in order to evaluate their predictive capabilities. The results showed an accuracy of more than 93% was obtained from the simulation model as compared to the real in-situ measurements. A positive simulation result will become a valuable method to predict potential debris flow hazard behavior of the same type and characteristics.

Preliminary report of a catastrophic landslide that occurred in Gokseong County, South Jeolla Province, South Korea, on August 7, 2020

We report a catastrophic landslide occurred in Gokseong County, South Korea, on August 7, 2020. Torrential rain, with a cumulative rainfall of more than 250 mm for the preceding 3 days, contributed as a trigger or a primer to an initiation of slope failure and ensuing evolution to a debris flow. The runout length, area, and volume of the Gokseong landslide are estimated to be approximately 678 m, 4.63 × 104 m2, and 2–5 × 104 m3, respectively. This Gokseong landslide, with its long runout, has caused severe damage to the village located downstream with a total of five fatalities and five buried houses. This study not only reports the damaged area, rainfall characteristics, and triggering factors but also presents the terrain changes obtained through a field survey using unmanned aerial vehicles (UAVs) conducted 5 days after the event and the normalized difference vegetation index (NDVI) using Sentinel-2 data.

Collapse of conical granular columns

This paper experimentally analyzes the collapse of conical granular columns made out of non-cohesive granular materials. Dimensionless run-out lengths were found, as a function of the aspect ratio a, similar to those occurring in the collapse of cylindrical columns, although in the case of conical columns of high aspect ration, at the end of the collapse, a layer of grains that rotates around the cone axis appears and it affects the final profiles of the mounds.

Different aspects of slope failures considering large deformation: application of smoothed particle hydrodynamics (SPH)

Understanding the insight of slope collapse is necessary for effective hazard mitigation policy. The conventional method may not be able to capture the actual failure mechanics as these methods are based on simplified assumptions of the predetermined slip surface. Highlighting this real engineering problem, an attempt is made in this research to simulate different aspects of slope collapse numerically. Lagrangian particle-based continuum model, namely, smoothed particle hydrodynamics (SPH) has been used to develop a three-dimensional numerical slope model to understand the failure mechanics of unstable slopes. Both homogeneous and non-homogeneous slope models have been simulated, and the time history of naturally occurring failure planes has been tracked. A distinct slip surface has been seen from displacement contour of particles, which portray the innate response of a collapsed slope. The effect of soil properties on the response of slope surface has also been evaluated quantitatively by the different combination of non-homogeneous slope models. Afterwards, two conventional remedial measures have been simulated, and significant reduction of run-out length is seen, which can put a spotlight on future mitigation strategy in geotechnical hazard.

Rock Avalanche-Generated Sediment Mass Flows: Definitions and Hazard

Rock avalanches can trigger destructive associated hazards following the initial collapse and fragmentation of a rock slope failure. One of these associated hazards occurs when the material derived from the initial collapse of the source zone impacts and mobilizes a mass flow composed of sediment from along the travel path. These mass flows can be grouped into radial impact areas that occur on relatively flat, open terrain (typically a floodplain), and more linear impact areas that occur in channelized terrain. Rock avalanche-generated sediment mass flows are an important consideration because they can significantly increase the area impacted by an event, thereby increasing the hazard area, especially in valley bottoms where there are likely more elements at risk. Existing runout prediction methods do not consistently account for the increase in the impact area from rock avalanche-generated sediment mass flows. Thus, there is a need for a simple data-supported method for estimating the extent of mass flow impacts resulting from an initial rock avalanche event with sediments along the potential travel path. This paper presents data from 32 rock avalanches and 23 rock avalanche-generated sediment mass flows from around the world, described using a consistent set of quantitative and qualitative attributes. A wide range of mass flow impacts were observed, with the sediment mass flow impact area or runout length exceeding the impact of the coarse, rocky debris in some cases. The area and length impacted by the coarse, rocky debris is estimated using multiple linear regressions considering the event volume and topographic features. The sediment mass flow dataset is used as input to develop an exponential distribution of the area or runout length of the sediment mass flow over that of the coarse, rocky debris. A decision tree framework is presented for estimating the extent of potential rock avalanches and potential rock avalanche-generated sediment mass flows for hazard and risk analysis, which is demonstrated by comparing the stochastic empirical predictions to those from numerical runout modelling.

Isolated buoyant convection in a two-layered porous medium with an inclined permeability jump

Abstract

Assessing the protective role of alpine forests against rockfall at regional scale

Worldwide, mountain forests represent a significant factor in reducing rockfall risk over long periods of time on large potential disposition areas. While the economic value of technical protection measures against rockfall can be clearly determined and their benefits indicated, there is no general consensus on the quantification of the protective effect of forests. Experience shows that wherever there is forest, the implementation of technical measures to reduce risk of rockfall might often be dispensable or cheaper, and large deforestations (e.g. after windthrows, forest fires, clear-cuts) often show an increased incidence of rockfall events. This study focussed on how the protective effect of a forest against rockfall can be quantified on an alpine transregional scale. We therefore estimated the runout length, in terms of the angle of reach, of 700 individual rockfall trajectories from 39 release areas from Austria, Germany, Italy and Slovenia. All recorded rockfall events passed through forests which were classified either as coppice forests or, according to the CORINE classification of land cover, as mixed, coniferous or broadleaved dominated high forest stands. For each individual rockfall trajectory, we measured the forest structural parameters stem number, basal area, top height, ratio of shrub to high forest and share of coniferous trees. To quantify the protective effect of forests on rockfall, a hazard reduction factor is introduced, defined as the ratio between an expected angle of reach without forest and the back-calculated forest-influenced angles of reach. The results show that forests significantly reduce the runout length of rockfall. The highest reduction was observed for mixed high forest stands, while the lowest hazard reduction was observed for high forest stands dominated either by coniferous or broadleaved tree species. This implies that as soon as one tree species dominates, the risk reduction factor becomes lower. Coppice forests showed the lowest variability in hazard reduction. Hazard reduction due to forests increases, on average, by 7% for an increase in the stem number by 100 stems per hectare. The proposed concept allows a global view of the effectiveness of protective forests against rockfall processes and thus enable to value forest ecosystem services for future transregional assessments on a European level. Based on our results, general cost–benefit considerations of nature-based solutions against rockfall, such as protective forests as well as first-order evaluations of rockfall hazard reduction effects of silvicultural measures within the different forest types, can be supported.

Developing nationwide avalanche terrain maps for Norway

Snow avalanches are a significant natural hazard in Norway. One method to manage the backcountry avalanche hazard is through detailed mapping of avalanche terrain. Avalanche terrain can be mapped using a variety of methods, including using the Avalanche Terrain Exposure Scale (ATES); however, manual classification of terrain using ATES is time consuming. This study has developed and compared a fully automated algorithm to provide ATES mapping for all of Norway. Our new algorithm is based on the technical model for ATES mapping. This model has specific terrain-based thresholds that can be applied for automated terrain-based modeling. Our algorithm expands on prior work by including the potential release area (PRA) model to identify and calculate the likelihood of an avalanche releasing from a start zone. We also use the raster-based TauDEM-model to determine the avalanche runout length. The final product is a 10-m resolution ATES map. We compared this nationwide ATES map with areas that have been manually mapped by avalanche experts, and find that the automated approach yields similar and reliable results. In addition to comparing mapped areas, we also examine manually mapped linear routes and compare these with the automated mapped ATES areas. Our results suggest that for open terrain, the vast majority of the manually classified tracks are predominantly in the same ATES class as our algorithm. For forested areas, we get mixed results, which can be attributed to a lack of suitable vegetation data at an appropriate scale. Despite this limitation, the current ATES algorithm and resulting spatial data are already valuable as a large portion (~ 70%) of the Norwegian backcountry terrain is above tree line. The automated algorithm is also useful to ensure consistent manual classification across different regions in Norway, or globally, and will permit greater reproducibility and easier updating of mapping for the future.

Large wood inhibits debris flow runout in forested southeast Alaska

AbstractDue to their potentially long runout, debris flows are a major hazard and an important geomorphic process in mountainous environments. Understanding runout is therefore essential to minimize risk in the near‐term and interpret the pace and pattern of debris flow erosion and deposition over geomorphic timescales. Many debris flows occur in forested landscapes where they mobilize large volumes of large woody debris (LWD) in addition to sediment, but few studies have quantitatively documented the effects of LWD on runout. Here, we analyze recent and historic debris flows in southeast Alaska, a mountainous, forested system with minimal human alteration. Sixteen debris flows near Sitka triggered on August 18, 2015 or more recently had volumes of 80 to 25 000 m3 and limited mobility compared to a global compilation of similarly‐sized debris flows. Their deposits inundated 31% of the planimetric area, and their runout lengths were 48% of that predicted by the global dataset. Depositional slopes were 6°–26°, and mobility index, defined as the ratio of horizontal runout to vertical elevation change, ranged from 1.2 to 3, further indicating low mobility. In the broader southeast Alaskan region consisting of Chichagof and Baranof Islands, remote sensing‐based analysis of 1061 historic debris flows showed that mobility index decreased from 2.3–2.5 to 1.4–1.8 as average forest age increased from 0 to 416 years. We therefore interpret that the presence of LWD within a debris flow and standing trees, stumps, and logs in the deposition zone inhibit runout, primarily through granular phenomena such as jamming due to force chains. Calibration of debris flow runout models should therefore incorporate the ecologic as well as geologic setting, and feedbacks between debris flows and vegetation likely control the transport of sediment and organic material through steep, forested catchments over geomorphic time. © 2020 John Wiley & Sons, Ltd.

Experiments on the sedimentation front in steady particle-driven gravity currents

We present an experimental investigation of steady particle-driven gravity currents with Reynolds numbers in the range 500–1600, and with the ratio of the initial current speed to the fall speed of the particles, S = u 0 /u fall , in the range 5 < S < 160. We identify three regimes: (i) For S < 10, the particles settle close to the source at a velocity corresponding to their fall speed, consistent with the observation of sedimenting fronts in classical settling column experiments. (ii) In the range 10 < S < 40, a steady gravity current develops within the tank. The experiments show that the depth of the gravity current gradually decreases away from the source and dye added to the source liquid appears above the gravity current along its entire length, suggesting that there is a sedimentation front, so that the volume and momentum fluxes of the current gradually decrease with distance from the source. We find that as S increases, the descent speed of the sedimentation front decreases relative to the fall speed of the particles, and the run-out length of the gravity current increases. We note that the density of the interstitial fluid corresponds to the density of the ambient fluid, so that any reduction in buoyancy of the gravity current is attributed to the sedimentation of particles on the floor of the tank and we do not observe lofting of the interstitial fluid. (iii) For 40 < S < 160, the gravity currents reach the end of our experimental tank and we no longer observe a sedimentation front. For these experiments, it appears that the entrainment at the top of the current begins to match the sedimentation and so the current depth does not change significantly over the scale of the tank, but a larger scale experimental system would be needed to explore the full run-out behaviour for these larger values of S. For the intermediate case, 10 < S < 40, we develop a model for the conservation of volume, momentum and buoyancy fluxes in the current, accounting for the sedimentation front and the release of fluid at the top surface of the gravity current, and we compare this with our new experimental data.

Subduction of an extinct rift and its role in the formation of submarine landslides in NW South America

Abstract On the eastern margin of the Panama Basin, the Nazca oceanic plate converges towards the continental plate of South America at approximately 53 mm a −1 . Subduction processes are accompanied by the presence of anomalous bathymetric elements including the Sandra Ridge. This east–west-orientated ridge is catalogued as an aborted rift derived from a magmatic spreading axis that was active between 12 and 9 Ma. Seismic activity within this structure is considered evidence of fault reactivation and tectonism. Once the structure reached the subduction trench several submarine landslides were triggered. Run-out lengths of these submarine landslides are perpendicular to the convergence of the structure with some units spreading and forming a wide fan that reaches tens of kilometres to the north and south of the trench. The area affected by the three main landslides varies between 130 and 300 km 2 approximately, with relatively superficial earthquakes (&lt;33 km) and with magnitudes that reach up to M w 7.2. The morphology of the landslides suggests a retrogressive nature with younger events proximal to shore. This paper presents estimates of the age of these landslides and discusses sources of uncertainty regarding these times of occurrence.

The Rocky‐Like Behavior of Cometary Landslides on 67P/Churyumov‐Gerasimenko

AbstractLandslides have been identified on several solar system bodies, and different mechanisms have been proposed to explain their runout length. We analyze images from the Rosetta mission and report the global characterization of such features on comet 67P/Churyumov‐Gerasimenko's surface. By assuming the height to runout length as an approximation for the friction coefficient of landslide material, we find that on comet 67P, this ratio falls between 0.50 and 0.97. Such unexpected high values reveal a rocky‐type mechanical behavior that is much more akin to Earth dry landslides than to icy satellites' mass movements. This behavior indicates that 67P and likely comets in general are characterized by consolidated materials possibly rejecting the idea that they are fluffy aggregates. The variability of the runout length among 67P landslides can be attributed to the different volatile content located in the top few meters of the cometary crust, which can drive the mass movement.

Rock avalanche runout prediction using stochastic analysis of a regional dataset

Rock avalanches involve extremely rapid, flow-like movement of fragmented rock with extreme destructive potential. With increasing development pressures in mountainous regions, there is a need for simple, stochastic estimates of runout distances to aid in hazard assessments and prioritize sites for more detailed investigation. To support the development of an empirical predictive tool, a systematic method was used to describe 49 rock avalanches in the Canadian Cordillera, which had been documented in the literature but not previously compiled into a regional inventory. These cases were described using measured or estimated numerical values for volume, fall height, runout length, total impacted area and the ratio of total impacted area over runout length (referred to as mean path width), and qualitative descriptions of the topographic confinement, substrate material and source geology. Linear regressions were fit to the data, with qualitative attributes treated as indicator variables. A strong relationship was found for runout distance predicted from volume, fall height and lateral confinement. A second relationship was found for mean path width over runout length predicted from volume. These relationships were converted to survival functions to estimate the runout exceedance probability and the mean path width exceedance probability. These survival functions were implemented in a computer tool, called the Probabilistic Runout Estimator—Rock Avalanche (PRE-RA), which can be used to estimate spatial impact probability ranges for rock avalanche runout. An application of this tool was demonstrated using two recent Canadian rock avalanches, which were not used in the dataset to estimate the statistical relationships.

Landslides on Charon

We investigated five large landslides identified in the Serenity Chasma region of Charon. The identification of these landslides involved a search for these features in images taken by cameras onboard the New Horizons spacecraft. Various landslide properties were analyzed based on their morphologies using a digital terrain model of the region. We found that landslides are confined to the walls of the large normal fault scarps that make up Serenity Chasma. Based on extensive landslide runout lengths (L) relative to their drop heights (H), we classified these features as long-runout landslides. By analyzing their geometries, we estimated the friction coefficients of the landslide material (H/L) to be between 0.15 to 0.31 and the runout efficiencies (L/H) to be between 3.2 and 6.8. We also estimated that the specific energy released during landslide motion ranged from 0.8 to 1.3 kJ kg−1. These amounts of energy were too low to have generated significant melt around landslide particles.

Analysis of Empirical Relationships to Estimate the Runout Length of Fine-Grained Materials Flows

Analysis of Empirical Relationships to Estimate the Runout Length of Fine-Grained Materials Flows

Landslides on Ceres: Inferences Into Ice Content and Layering in the Upper Crust

AbstractWe analyze landslides on Ceres using several quantitative approaches to constrain the composition and structure of the top few kilometers of Ceres' crust. We focus on a subset of archetypal landslides classified morphologically as thick, steep‐snouted “type 1” (T1) flows and thin spatulate “type 2” (T2) flows (Schmidt et al., 2017, https://doi.org/10.1038/ngeo2936) to explore the landslides' mechanical properties. Our results confirm earlier observations showing that T1 landslides are typically found poleward of 70° latitude and T2 mostly equatorward of 70° latitude. Measurements of landslide drop height and runout length imply effective friction coefficients lower than common friction coefficients in any of Ceres' identified or suggested non‐ice surface materials, including saturated clays. Our measurements of the volume and area of landslide scars suggest that T1 landslides can fail to greater depths than T2 for a given scar area, consistent with depth‐limited failure in T2 landslides. These results are consistent with a layer of lower shear strength material overlying a stronger layer in Ceres' outer shell at low to middle latitudes and a single layer without an overlying weak layer at polar latitudes. Combining these observations with known constraints on Ceres' near‐surface composition, we propose that Ceres' crust at low to middle latitudes consists of a topmost layer with an ice content in excess of the spectral and elemental detection depths, thins out at high latitudes, and overlies a stronger and more ice‐rich layer.

Transition of a Hyperpycnal Flow Into a Saline Turbidity Current Due to Differential Diffusivities

AbstractWe provide a new perspective on the transition of hyperpycnal flows into saline turbidity currents, which permits longer runout lengths than might be otherwise expected. This mechanism relies on the differential turbulent diffusion of salt and sediment, and in contrast to ambient saltwater entrainment it enables the salinification of the freshwater current without diluting the sediment concentration field by a corresponding amount. The freshness‐to‐sediment ratio is introduced in order to quantify the transition process. The results of high‐resolution simulations provide estimates for the transition distance of hyperpycnal model flows into saline turbidity currents.

Collapse of granular–cohesive soil mixtures on a horizontal plane

The collapse test with granular or cohesive materials known as ‘slump test’ is a simple, small-scale experiment. It can be used to study the rheology of soil masses involved in flow slides and to calibrate the numerical models used for analyzing the propagation of soil flows. This study presents the results of an extensive, experimental program carried out with this type of test, using different amounts of granular and cohesive soil fractions with different percentages of water. The experiments aim at evaluating how the mixture composition influences the flow behavior. In particular, the runout length and the profile of final deposits can be correlated to the composition of a mixture and its behavior. Moreover, the time taken to complete the collapse and the slumping velocity are also taken into account. The results of 80 tests are considered all together to formulate different predictive models based on regression between the geometrical and kinematic features of the slump test. Different sets of predictors, i.e., the material composition, the initial diameter and height of the sample, are included in these regression models. The accuracy of the models is then evaluated, based on some performance coefficients.