Stony Debris Research Articles

Experimental study of stony debris flow and its feature importance with varying coarse grain and water content

Experimental study of stony debris flow and its feature importance with varying coarse grain and water content

An alternative approach for the sediment control of in-channel stony debris flows with an application to the case study of Ru Secco Creek (Venetian Dolomites, Northeast Italy)

Controlling sediment to reduce debris-flow hazard is generally approached using retention basins that can be closed or have an outlet structure, generally an open check dam. They are usually placed in mild slope zones that allow minimal works for the excavation and the foundation of the outlet structure if present. Recently, it has been shown that the detention of sediments can also be achieved in the high-sloping reaches of debris-flow channels using deposition areas, basins that are open on the downstream side. In this work, we propose an approach for controlling the sediment volume transported by debris flows consisting of a cascade of deposition areas and retention basins. We also include a framework for planning, sizing, and checking the works. Two scenarios are considered, corresponding to the maximum values of the debris-flow peak discharge and volume, respectively. Moreover, the presence or absence of boulders is also considered. For this purpose, a method that evaluates the clogging of a single open check dam as a function of the coarse fraction of the sediment volume is simply extended to the case of multiple dams and implemented in a routing model. The proposed approach is applied along Ru Secco Creek in northeast Italy to defend a resort area and a village hit by a high-magnitude debris flow in 2015. After a careful survey and study, a solution with a combination of deposition areas and retention basins is planned and sized. The validity and performance of the proposed solution are analyzed using debris-flow modeling for two scenarios, considering both the absence and presence of boulders. Most of the sediment volume transported by debris flows is trapped, and a small solid discharge flows downstream of the works.

Modelling stony debris flows involving culverted streams: the Abbadia San Salvatore case (Mt. Amiata, Italy)

Modelling stony debris flows involving culverted streams: the Abbadia San Salvatore case (Mt. Amiata, Italy)

Deposition areas: An effective solution for the reduction of the sediment volume transported by stony debris flows on the high‐sloping reach of channels incising fans and debris cones

SummaryThe main approach to managing the volume of sediment transported by stony debris flows routing along channels is through retention basins and open check dams, usually built in the lower reach just upstream of inhabited areas where slopes are gentler. In some cases, these measures are not sufficient to retain all the volume of sediment transported by debris flows. Works for trapping the sediments should also be placed in the upper reach of debris‐flow channels. In this area, where the channel bed is characterized by very high slopes and vertical variability over time, constructing and maintaining transversal embankments for sediment retention is difficult. In addition, they could also be susceptible to failure, potentially increasing the magnitude of the phenomenon instead of mitigating it. On the other hand, other types of works, such as solid‐body check dams or net barriers, have a decreasing efficiency over time in reducing the volume of sediment transported by debris flows. An alternative solution could be a retention basin open on the downstream side, that is, without the ending transversal embankment or berm. Therefore, it can be designated as a deposition area because the retention effect of the downstream embankment is missing, and sediment deposition only occurs due to the flatness of the basin. For sizing purposes, here, we derive two relationships between the deposition area and the volume of the deposit. These two relationships are derived from a physically based geometrical approach and empirical approach, respectively. After analyzing the morphology of the depositional processes that occurred in an existing deposition area during debris‐flow events, we test the validity of the two relationships. Overall, these can be used independently or in combination for designing a deposition area. At last, we introduce the necessary geomorphological conditions for the construction of the deposition area and guidance for its placement.

Tensile stress development and critical behavior of a flexible barrier

A DEM numerical approach is employed to illustrate the critical stress development of a three-supporting cable flexible barrier under a large scale stony debris movement impact. A proper linear parallel bond is applied to build the flexible barrier model. Critical state is presented by lowering the tensile resistance of the flexible barrier and steepening the channel slope. The tensile stress of the supporting cables is primarily studied. Results mainly indicate that the particle-structure contact is essential to the stress development. The bottom cable always exhibits larger tensile stress due to the massive collision and compressive force of particles and the tensile force exerted by the vertical cable-net joint system. On the other hand, the intermediate cable receives lower collision and compressive force, resulting in smaller tensile stress. The bottom cable indicating numerous direct particle-structure contact part performs brittle failure, which is analogous to the concentrated load feedbacks. The loading of the intermediate one reaches ductile failure, reflecting the desirable effect of dispersing loads. This study suggests that practical engineering should try to avoid massive boulder impact or reinforce the bottom cable in order to improve the hazard prevention design.

How are fine sediments described in sediment sheet flow?

Stony debris flow transits to sediment sheet flow when the river bed gradient becomes gentle. The sediment sheet flow consists of a water flow layer and a sediment moving layer. Fine sediments are expected to behave as a part of the fluid rather than a solid phase in the sediment moving layer. Further, it can be thought that a part of fine sediment can be suspended in the water flow layer. However, it was not possible to physically express whether the fine sediment behaves as a solid phase or a fluid phase in the numerical simulation model. Here we physically modeled fine sediment behavior in sediment sheet flow. We confirmed the applicability of the new model to describe the longitudinal deposited sediment gradient in flume experiments.

Laboratory investigation of the effects of grain size on the dynamics of debris flows: Measurement of pore fluid pressure in an open channel

The dynamics of debris flow depend on internal stress components, such as particle–particle stress, the stress exerted by pore water, and interactions between particles and pore water. Although dominant internal stress components depend on the grain size composition, the effects of grain size on the dynamics of debris flow are not fully understood. To investigate the effects of grain size on the dynamics of debris flows, pore fluid pressures were measured in an open channel experiment. In the experiment, monodisperse debris flows were triggered for five different grain sizes: 0.2, 0.8, 1.3, 2.2, and 2.9 mm. The pore fluid pressures in debris flows of 0.2 mm grains had larger excess pressures over the hydrostatic pressure, and were close to the total normal stress, while those of other grain sizes had smaller excess pressures and were relatively close to the hydrostatic pressure. Comparing the measured friction factors and theoretical ones for stony debris flows, particle–particle stress dominated in debris flows, except for 0.2 mm grains, and the measured excess pore pressures could be explained by the Reynolds stress of pore fluid due to shear by particles in laminar motion. By contrast, particle–particle stress did not dominate in debris flows of grain size 0.2 mm, and a large portion of the particles was in suspension affected by turbulence. These differences in flow dynamics may correspond to the flow transition from laminar to turbulent flow described by the threshold of relative flow depth, which is the ratio of the flow depth to grain size.

Validation and potential forecast use of a debris-flow rainfall threshold calibrated with the Backward Dynamical Approach

The aim of this work is to propose a way of validating a rainfall threshold for stony debris flow calibrated with the Backward Dynamical Approach (BDA), a physical-based approach that combines the debris flows' forcing and dynamics. Given the assumptions underlying the BDA, two different validation approaches are developed and then applied to a suitable study area for which a calibrated BDA threshold and a dataset related to a calibration-independent reference period are available. The first approach assesses the reliability of the threshold by applying the BDA to compute the rainfall conditions related to the debris flows that occurred. Instead, the second approach, thanks to the introduction of the concept of “potential debris flows”, considers all rainfall events, regardless of whether or not they are associated with a debris flow, providing a complete overview of the threshold reliability. In both approaches, the threshold's performances are quantified by employing the contingency table and skill scores. The results of the validations applied to the study area threshold highlight its satisfactory reliability. Moreover, the introduction of the potential debris flows concept has cleared the way to apply a BDA-based threshold in a predictive framework and to roughly estimate the solid volumes we may expect if a debris flow occurs.

Fauna and flight activity of bats in natural forests in the southern Brdy Mts. and the first information on the occurrence of Hypsugo savii in this area (Chiroptera: Vespertilionidae)

The bat fauna was studied at six localities of protected natural forest fragments in the southern part of the Brdy Mts. (south-western Bohemia), in the years 2019–2020. Netting and acoustic detection on line transects were used in order to document the species composition and flight activity of bats at the localities under study. Altogether, 24 individuals of seven bat species were netted. In total, 643 minutes of the presence of flying bats were registered within 56.5 transect hours. With use of these methods, at least 16 bat species were recorded. Of them, Myotis myotis, M. mystacinus / M. brandtii, M. nattereri, Eptesicus nilssonii, Pipistrellus pipistrellus, Plecotus auritus, and Barbastella barbastellus represent the constant species group with respect to their occurrence at the localities. Pipistrellus pipistrellus was the most frequent species (54%), with the highest relative flight activity at all localities. The highest intensity of flight activity of the bat community was observed in habitats with rocks, boulder accumulation, and open stony debris (14.17 min+/h). The highest bat species diversity was registered in habitats with the prevalence of herb-rich mixed beech forest stands (15 species, diversity index H’=2.02; equitability E=0.75). A series of calls of Hypsugo savii was registered in the Na skalách Nature Reserve on 8 May 2020. It represents the first finding of this species in the region of the Brdy Mts.

Desoria calderonis sp. nov., a new species of alpine cryophilic springtail (Collembola: Isotomidae) from the Apennines (Italy), with phylogenetic and ecological considerations

We describe and delimit with integrative taxonomy the new springtail species Desoria calderonis sp. nov. (Collembola: Isotomidae). This cryophilic species is strictly linked to the supraglacial stony debris of the isolated Calderone glacier (Central Apennines, Italy), one of the southernmost glaciers of Europe. Desoria calderonis sp. nov. could belong to the nivalis-complex, a group of European mountain species included in the violacea-group. Genetic analysis (COI mtDNA barcoding) confirms the morphological attribution to the genus Desoria Nicolet in Desor, 1841, but highlights that the genus, in its current definition, is polyphyletic. We specify the peculiar micro-habitat preferences and highlight the threat of extinction for this cryophilic species in the context of the ongoing climate change and subsequent risk of complete disappearance of the glacier.

Deposition morphology in large-scale laboratory stony debris flows

Debris flows affect vulnerable areas by unloading their destructive impact and depositing sediments in debris-flow lobes in the runout area. The debris flow deposition morphology characteristically displays a complex process and is of particular interest for hazard mapping, economic activities planning, construction of civil structures, and settlements in vulnerable areas. This laboratory study is based on large-scale stony debris flow experiments, focusing on the runout distance, mobility, and inundated area. The experiments were conducted under different initial and boundary conditions like particle size, solid volume fraction, and basal roughness. Our experimental results reveal that runout distance and lobe shape change dramatically under different solid volume fractions and are highly dependent on basal roughness. On the contrary, the particle size has a marginal effect on deposit morphology. The newly introduced dimensionless runout capacity allows the evaluation of the momentum transfer of the debris flows to the end of the flume into a runout distance. The experimental results show that the runout capacity is inversely proportional to the Froude number of the stony debris flow. We deduce a relationship for the runout distance from this concept, which reveals that flow velocity contributed to the runout distance on a par with the flow depth. A weak velocity can be counterbalanced by an increased front depth to achieve a similar runout distance. Furthermore, we evaluate flow mobility, deposited volume, as well as the inundated area, and compare our results with other experimental and field data. The presented data match existing hypermobility models in-between small-scale laboratory and field-scale events. Consequently, this study complements missing data to the previous dataset of runout distance, elevation difference, mobility, and inundated area, contributing to better understand debris flow dynamics.

Uncertainty analysis of a rainfall threshold estimate for stony debris flow based on the backward dynamical approach

Abstract. A rainfall threshold is a function of some rainfall quantities that provides the conditions beyond which the probability of debris-flow occurrence is considered significant. Many uncertainties may affect the thresholds calibration and, consequently, its robustness. This study aims to assess the uncertainty in the estimate of a rainfall threshold for stony debris flow based on the backward dynamical approach, an innovative method to compute the rainfall duration and averaged intensity strictly related to a measured debris flow. The uncertainty analysis is computed by performing two Monte Carlo cascade simulations: (i) to assess the variability in the event characteristics estimate due to the uncertainty in the backward dynamical approach parameters and data and (ii) to quantify the impact of this variability on the threshold calibration. The application of this procedure to a case study highlights that the variability in the event characteristics can be both low and high. Instead, the threshold coefficients have a low dispersion showing good robustness of the threshold estimate. Moreover, the results suggest that some event features are correlated with the variability of the rainfall event duration and intensity. The proposed method is suitable to analyse the uncertainty of other threshold calibration approaches.

Characterization of a landslide-triggered debris flow at a rainforest-covered mountain region in Brazil

Debris flows represent great hazard to humans due to their high destructive power. Understanding their hydrogeomorphic dynamics is fundamental in hazard assessment studies, especially in subtropical and tropical regions where debris flows have scarcely been studied when compared to other mass-wasting processes. Thus, this study aims at systematically analyzing the meteorological and geomorphological factors that characterize a landslide-triggered debris flow at the Pedra Branca catchment (Serra do Mar, Brazil), to quantify the debris flow’s magnitude, peak discharge and velocity. A magnitude comparison with empirical equations (Italian Alps, Taiwan, Serra do Mar) is also conducted. The meteorological analysis is based on satellite data and rain gauge measurements, while the geomorphological characterization is based on terrestrial and aerial investigations, with high spatial resolution. The results indicate that it was a large-sized stony debris flow, with a total magnitude of 120,195 m3, a peak discharge of 2146.7 m3 s−1 and a peak velocity of 26.5 m s−1. The debris flow was triggered by a 188-mm rainfall in 3 h (maximum intensity of 128 mm h−1), with an estimated return period of 15 to 20 years, which, combined with the intense accumulation of on-channel debris (ca. 37,000 m3), indicates that new high-magnitude debris flows in the catchment and the region are likely to occur within the next two decades. The knowledge of the potential frequency and magnitude (F–M) can support the creation of F–M relationships for Serra do Mar, a prerequisite for reliable hazard management and monitoring programs.

Биотопическое распределение пауков-кругопрядов (Aranei: Araneidae) в привершинной зоне гор Южного Сихотэ-Алиня

Until recently, the study of spiders in the mountainous conditions of the Primorsky Region of Russian Far East was carried out only with the example of ground dwelling spiders. The purpose of this work is to reveal the species composition and features of the biotopic distribution of orb-weaver spiders (Aranei: Araneidae) in the ridge-top zone of the Southern Sikhote-Alin mountains (Primorsky Region). The field material was collected from may to october in 2016 and 2017 at key sites located on different mountain ranges and their spurs in the altitude range from 1000 to 1560 m above sea level. Within each key sites, transect-sites with a size of 3 x 25 m were laid. The spider's collections were by the hands collecting and sweeping method. A total of 305 specimens of spiders belonging to 10 species from 7 genera were caught. Comparison of species lists of spiders was carried out by cluster analysis methods (unweighted pair group method with arithmetic mean, UPGMA) using the Jacquard coefficient. For automatic calculations data processing program Past v. 2.17 is used. The maximum diversity of spiders is observed among the herbage-reed meadows, which do not form a continuous belt of vegetation at altitudes of 1000–1400 m above sea level. seas. Seven species belonging to 6 genera were caught here. The minimal diversity is noted among the continuous open stony debris, devoid of vegetation. Here is the only species caught – Aculepeira matsudae. The species Plebs sachalinensis and Araniella yaginumai are most numerous and widespread in the ridge-top zone of the Southern Sikhote-Alin. The high abundance of Araniella yaginumai is achieved due to favorable conditions for this species in the herbage-reed grass lands. In all the studied biotopes in the ridge-top zone of the mountains the density of the orbweaver spiders is low and does not exceed 2.04 specimens per 10 m2.

Ecology of the cold-adapted species Nebria germari (Coleoptera: Carabidae): the role of supraglacial stony debris as refugium during the current interglacial period

In the current scenario of climate change, cold-adapted insects are among the most threatened organisms in high-altitude habitats of the Alps. Upslope shifts and changes in phenology are two of the most investigated responses to climate change, but there is an increasing interest in evaluating the presence of high-altitude landforms acting as refugia. Nebria germari Heer, 1837 (Coleoptera: Carabidae) is a hygrophilic and cold-adapted species that still exhibits large populations on supraglacial debris of the Eastern Alps. This work aims at describing the ecology and phenology of the populations living on supraglacial debris. To this end, we analysed the populations from three Dolomitic glaciers whose surfaces are partially covered by stony debris. We found that supraglacial debris is characterised by more stable colder and wetter conditions than the surrounding debris slopes and by a shorter snow-free period. The populations found on supraglacial debris were spring breeders, differently from those documented in the 1980s on Dolomitic high alpine grasslands, which were reported as autumn breeders. Currently, Nebria germari seems, therefore, to find a suitable habitat on supraglacial debris, where micrometeorological conditions are appropriate for its life-cycle and competition and predation are reduced.

Evolution of stony debris flows in laboratory experiments

The run out and destructive potential of gravitational multi-phase flows is largely determined by the mixture composition, the material properties of the solid particles and the fluid. One instrument to expand the understanding of the governing processes of flow is laboratory experiments. In this study, we concentrate experimentally on landslide-induced stony debris flows as a particular type of flow-like mass movement. We aim to observe different natural flow types for varying initial and boundary conditions. In a laboratory flume, 12.0 m long, 1.3 m wide and 0.3 m deep, we initiate stony debris flows and measured flow variables such as flow depth, mass, bulk density, front velocity and front shape for varying particle size, solid volume fraction and basal roughness. Our experimental results reveal that flow type and evolution changes significantly for different solid volume fractions, as well as for different basal roughness. The particle size had a noticeable effect on flow velocity and front shape. The smooth surface facilitated rapid, shallow, and turbulent flows. In contrast, experiments with rough beds showed relatively lower velocities and dense flow behaviour. Although the flow parameters covered only a small spectrum of the naturally possible parameter space, flow phenomena such as phase-separation, longitudinal sorting, steep front surges, or front overtopping were observed. We group our observed flows regarding the flow properties and classify them into common flow types: (1) debris flood (hyperconcentrated flow), (2) debris flow, and (3) non-liquefied debris flow. To compare our results with natural events and other experimental results, we analyse the data with several dimensionless numbers. The flows were generally dominated by grain collision on the smooth surface. Naturally, frictional forces gain more influence on the rough surface but did not overrule collisional forces. Viscous forces played only a minor role in our experiments, due to the lack of highly viscous fluid. Overall, we infer, that our well-controlled experiments mimicked natural stony debris flow and give new profound insights into the causal relationship of how the initial and boundary conditions affect the flow evolution.

Experimental Study of Structural Strategy on Debris Brake Performance

We performed an experimental study to investigate the effects of structural strategy for improving the debris brake performance. The stony debris flow was generated by releasing water discharge from a tank that contained gravel material. The efficiency of debris brake, debris accumulation pattern and its position parameters was used to evaluate the performance of the debris brake structure. The influences of debris brake slope, opening size and nonuniformity in deck opening size parameters that have not been studied before were investigated for different debris flow conditions. The results showed that the slope of debris brake was the most important factor on debris brake performance. The suitable slope was found to be a function of the debris flow velocity, and it can increase debris brake efficiency up to 10%. The analysis of the experimental data suggests the optimum opening size ratio (i.e., the ratio of opening size to median diameter of debris) smaller than or equal to one. Experimental data indicated that the nonuniformity in opening size of the debris brake cannot improve the debris brake efficiency.

The role of the phase shift of fine particles on debris flow behavior: an numerical simulation for a debris flow in Illgraben, Switzerland

Physically based numerical simulation models have been developed to predict hazard area relating to debris flows. Since fine sediments are expected to behave as a part of the fluid rather than solid phase in stony debris flows, several models have recently included this process of the phase shift from solid to fluid in the context of fine sediment. However, models have not been fully tested regarding the ability to reproduce a variety of debris flow characteristics. We therefore tested (i) applicability of a numerical simulation model for describing debris flow characteristics and (ii) the effect of phase shift of fine sediment on debris flow behaviors. Herein we applied a numerical simulation model to a well-documented dataset from the Illgraben debris flow observation station in Switzerland. Based on the stony debris flow concept, we physically modeled effects of the phase shift of sediment on transport capacity and flow resistance. We successfully reproduced the observed bulk density, erosion and deposition patterns, front velocity, and erosion rate, although we had to tune the ratio of fine sediment that behaves as a fluid. Considering the effects of the phase shift of sediments, we conclude that physically based numerical simulation models can describe a variety of debris flow behaviors.

Model-based approach for design and performance evaluation of works controlling stony debris flows with an application to a case study at Rovina di Cancia (Venetian Dolomites, Northeast Italy)

This paper proposes a methodology for the design and performance evaluation of debris-flow control works based on modeling the phenomenon and its interaction with the works. The reliability of such an approach depends on the trustworthy reproduction of both the phenomenon and its interaction with the structures: the former is provided by simulating all the physical processes concerning the phenomenon (runoff generation, initiation and propagation of a solid-liquid mixture) by models tested against field measurements; the latter by upgrading the propagation model for considering the non-erosive surfaces of the works overflowed or hit by debris flows and computing the forces exerted on them. This methodology is applied to a case study on the Rovina di Cancia channel (northeast Italy), frequently affected by stony debris flows. Longitudinal and transversal works are planned in a reach of the channel subject to high erosion because of its high slope and the supply of water discharge by the Bus de Diau Creek tributary. The works consist of a sectional dam at the end of the reach and in moving there the mouth of the Bus de Diau Creek. Both of them contribute to decrease bed erosion along the reach and to reduce the peak discharge and the transported sediment volume. The proposed methodology is used in the design phase to explore different geometries of the dam opening and choose its transversal position through the analysis of flow (velocity and free surface), whereas, in the evaluation phase, to estimate the reduction in the bed erosion, peak discharge and transported sediment volume provided by the works (i.e. the performance evaluation). Finally, the plot of the forces acting on the dam breakers and bottom over time allows their static dimensioning.

A new method for evaluating stony debris flow rainfall thresholds: the Backward Dynamical Approach

Debris flow rainfall thresholds aim to provide a level of rainfall duration and average intensity above which the probability of a debris flow occurrence is significant. Estimating reliable thresholds for use in early warning systems has proved to be a challenging task. In fact the methodologies currently available in the literature are not entirely satisfactory since they provide thresholds unlikely low. The goal of the present research is exploring new paths aimed at improving the reliability of rainfall thresholds. A possible weak point of the literature approaches is the way the duration and the average intensity pertaining to a debris flow is determined. Up to now, these values are evaluated using only the characteristics of the hyetograph associated to a debris-flow event. In the present paper, we propose a new methodology based on volumetric relations deriving from a simplified description of the dynamic of a stony debris flow: by using these relations, from a measure of the deposited volume it is possible to estimate backwardly the volume of rain that caused the deposition; then, from this last value and the knowledge of the relevant hyetograph, it is possible to reconstruct the duration and the average intensity. Application of this new technique to a sample study area allowed us to prove the feasibility of the method and, to some extent, its capabilities: with respect to a classical literature method, the new approach produces an higher threshold and a smaller characteristic duration scales. Finally, strengths and weakness of the method have been evaluated thoroughly.