Deposition morphology in large-scale laboratory stony debris flows

Predicting debris flow runout is of major importance for hazard mitigation. Apart from topography and volume, runout distance and area depends on debris flow composition and rheology, but how is poorly understood. We experimentally investigated effects of composition on debris flow runout, depositional mechanisms, and deposit geometry. The small‐scale experimental debris flows were largely similar to natural debris flows in terms of flow behavior, deposit morphology, grain size sorting, channel width‐depth ratio, and runout. Deposit geometry (lobe thickness and width) in our experimental debris flows is largely determined by composition, while the effects of initial conditions of topography (i.e., outflow plain slope and channel slope and width) and volume are negligible. We find a clear optimum in the relations of runout with coarse‐material fraction and clay fraction. Increasing coarse‐material concentration leads to larger runout. However, excess coarse material results in a large accumulation of coarse debris at the flow front and enhances diffusivity, increasing frontal friction and decreasing runout. Increasing clay content initially enhances runout, but too much clay leads to very viscous flows, reducing runout. Runout increases with channel slope and width, outflow plain slope, debris flow volume, and water fraction. These results imply that debris flow runout depends at least as much on composition as on topography. This study improves understanding of the effects of debris flow composition on runout and may aid future debris flow hazard assessments.

Debris flows in mountainous regions are of great concern because they present a serious threat to the residents and infrastructures in downstream areas as a result of their long runout distances. However, the mechanisms of debris-flow runout are still unclear because of many factors influencing debris-flow mobility. This study focuses on two major factors, namely, particle size and slurry viscosities. A series of experimental model tests were conducted in a rectangular inclined flume connected to a final horizontal plain. Results reveal that the debris-flow mobility is significantly influenced by the slurry viscosities. The runout distance initially increases and then decreases as the slurry viscosities increase. Also, runout distance for debris flows contain large particle sizes is longer than that of debris flows contain small particle sizes. The depositional widths are almost unchanged in the experimental test, which suggests that the debris flows are constrained by the fixed channel boundaries. The results of this study can improve the understanding the behavior and the deposition features of debris flows.

The gravitational debris flow, such as the agent forming alluvial cones in the mouths of mountain canyons, could bring about devastating disaster to downstream structures in mountainous areas. In the present study, a series of model tests were conducted on the sand and the ceramsite to systematically explore the interaction between debris flow and baffles. During the runout process, the impact force exerted by debris flow was measured by dynamometers. The runout distance, velocity of the flow, and flow depth were monitored by a video camera and a high-speed camera in a real time. The dynamic interaction under different particle sizes of dry granular materials, slop angles, and baffle configurations was simulated. Experimental results show that the smaller size material is favorable for the frictional energy dissipation during the sliding process, giving rise to the smaller runout distance. The present findings provide important references for the debris flow control engineering.

Our aim is to determine the run-out distance of the debris flow that is crucial in the assessment, prevention and control of the debris flow hazard. Based on the variation characteristic of debris flow velocity in the alluvial fan, this paper proposes the calculation method of the velocity attenuation coefficient of the debris flow. By defining the velocity attenuation coefficient and deducing its calculating formula, this paper puts forward a new method to determine the run-out distance of the debris flow based on the velocity attenuation coefficient, and Gangou debris flow in Luding County, Sichuan Province is selected as a case for calculation and verification. Having 10 m as its measuring spacing, this paper measured 19 sections at the alluvial fan of the Gangou debris flow (among them, 11 sets of data are valid). And based on the measurement, this paper analyzes the characteristic of the velocity attenuation and calculates its velocity attenuation coefficient after the 2005 debris flow. The study indicates that when the velocity of Gangou debris flow at the alluvial fan is greater than 12 % of the initial velocity (at the mouth of gully), the attenuation is quite remarkable. But when the velocity at the alluvial fan is less than 12 % of the initial velocity, the attenuation is quite slow. Besides, when Gangou debris flow rushes out of the gully mouth (the initial velocity is 10 m/s) and when it attenuates to the 32 time, its velocity is less than 0.1 m/s, the debris flow is considered to stop flowing, and the run-out distance of Gangou debris flow is calculated to be 320 m. But the present alluvial fan of Gangou debris flow is measured to be 285 m in length, and the calculated run-out distance is 320 m, which is 35 m longer than its present length. This means when the debris flow runs out in 2005, it blocked up the main river (Dadu River) in some extent. And this finding is generally in accordance with that from the field survey. The findings can be of theoretical and practical significance in the debris flow hazard assessment, as well as its prevention and mitigation.

Geophysical granular flows, such as snow avalanches and debris flows, represent a great hazard to man and infrastructures. These natural phenomena are characterized by the rapid flow of a granular solid phase, embedded in an ambient fluid. Since such events occur with little warning, in addition to investigating the triggering conditions, it is of crucial interest to predict their propagation and run-out distances. The main purpose of the present research is to better understand the flow dynamics of dry granular flows, in the presence of no-slip bottom boundary conditions, and predict their propagation by using a depth-averaged approach. In this case, a stratification of different flow regimes, such as quasi-static and dense collisional, occurs and makes the utilization of the classical single layer depth-averaged models insufficient, because of important uncertainties about the velocity and shear stress distributions along the flow depth. In the first part of the present work, an experimental-numerical study on dam-break flows of dry granular material is reported. It has been found that a modification to the formula, proposed in the Savage-Hutter model for calculating the earth-pressure coefficient, leads to an improved agreement with experimental data, in presence of no-slip bottom conditions. In the second part, an experimental study on steady state velocity profiles of dry granular flows is reported. The granular material, used in this experimental research, was Ottawa sand (ASTM C-778 20/30). The velocity profiles at the side walls and free surfaces have been obtained, through granular PIV techniques. The measurements are in accordance with other experimental works on different granular materials and suggest the occurrence of a rheological stratification along the flow depth, in case of no-slip bottom condition. In order to better describe such a complex flow dynamics, a two-layer depth-averaged model has been proposed. The dynamics of the two layers, ideally corresponding to dense-collisional and quasi-static regimes, have been considered independently. As well, mass exchanges between the layers have calculated through a physically based closure equation. The well-known mathematical issue of the hyperbolicity loss in two-layer models has been carefully addressed. In order to overcome it, a local modification of source terms of the original mathematical model has been proposed. Such a treatment consists of introducing an extra resistance at the interface, if necessary, to avoid the hyperbolicity loss. This approach has been found to be robust and yields reasonable results, as long as the asymptotic steady state solution and boundary conditions fulfil the hyperbolicity requirements of the original model. Through comparisons with experimental data, the two-layer approach turns out to be very promising to describe the complex flow dynamics in case of no-slip bottom boundary conditions, although a more detailed description of the basal shear stress seems to be required. In the third part of the present dissertation, the same two-layer approach has been used in order to rewrite the mathematical model in a curvilinear coordinates system, attached to the interface between the two layers. The model equations have been obtained, by using the same approach proposed in Tai and Kuo (2008). In the model derivation, the physically negligible quantities are identified in a more rational way than the previous model, by means of a scaling approximation.

Exploring debris flow deposit morphology in river valleys: Insights from physical modeling experiments

The fluid dynamics of stony debris flows generated in two small tributaries adjacent to each other and flowing into a main receiving channel was analyzed experimentally at a laboratory scale. The analysis on the propagation along the tributaries and deposition in the main channel provide information about sediment‐water mobility, dangerous damming, and potential hazard. Debris flows were generated by releasing a preset water discharge over an erodible layer of saturated gravels material. As a consequence, the debris flow sediment concentration varied accordingly to the entrainment rate which, in turn, was strongly controlled by the tributary slope. The data collected by acoustic level sensors, pore fluid pressure transducers, and a load cell were used to characterize the evolution of bulk density and solid concentration of the sediment‐water mixture. These two parameters were relevant to assess the stony debris flow mobility which contributes to determine the shape of sediment deposits in the main channel. The detailed bed topography surveys carried out in the main channel at the end of each experiment provided information on the morphology of these deposits and on the interplay of adjacent confluences. The influences of confluence angle, tributary slopes, and triggering conditions have been investigated, for a total of 18 different configurations. Within the investigated range of parameters, the slope angle was the parameter that mainly influences the stony debris flow mobility while, for adjacent confluences, the degree of obstruction within the receiving channel was strongly influenced by the triggering scenario.

Abstract. This study compares documented debris flow runout distances with numerical simulations in the Yosemite Valley of California, USA, where about 15% of historical events of slope instability can be classified as debris flows and debris slides (Wieczorek and Snyder, 2004). To model debris flows in the Yosemite Valley, we selected six streams with evidence of historical debris flows; three of the debris flow deposits have single channels, and the other three split their pattern in the fan area into two or more channels. From field observations all of the debris flows involved coarse material, with only very small clay content. We applied the one dimensional DAN (Dynamic ANalysis) model (Hungr, 1995) and the two-dimensional FLO-2D model (O'Brien et al., 1993) to predict and compare the runout distance and the velocity of the debris flows observed in the study area. As a first step, we calibrated the parameters for the two softwares through the back analysis of three debris- flows channels using a trial-and-error procedure starting with values suggested in the literature. In the second step we applied the selected values to the other channels, in order to evaluate their predictive capabilities. After parameter calibration using three debris flows we obtained results similar to field observations We also obtained a good agreement between the two models for velocities. Both models are strongly influenced by topography: we used the 30 m cell size DTM available for the study area, that is probably not accurate enough for a highly detailed analysis, but it can be sufficient for a first screening.

A mechanical model for phase separation in debris flow

Debris flow fan affects the river profile and landscape evolution. The propagation of multiple debris flows along a river can cause inundation and breaching risk, which can be exemplified by the Min River after the Wenchuan earthquake, Sichuan province, China. In this work, large flume tests were conducted to examine the interactions between debris flows and water current with the fan geometry, momentum, runout distance, deposited width, the relative water level upstream and dominated stress. The results reveal that stony flow commonly travels at a high speed and forms a long rectangle shape fan, while the muddy flow generally travels at a low speed and forms a fan-shaped depositional area. The stony flow can block a river even when the momentum is close to the water current; the muddy flow can block a river when the momentum is lower than that of water current. In case of complete river damming, the relative water level upstream indicates that the inundation risk from the muddy flow damming river would be higher than the inundation risk of stony flow. The diversion ratio of muddy flow decreases as damming ratio. Comparison of dimensionless numbers reveals that stony flow is dominated by grain collision stress combined with turbulent mixing stress, while the muddy flow is dominated by viscous shear stress over friction stress. The fan geometry, damming ratio, diversion ratio, and the dominated stress all together indicate that stony flow strongly interacts with water current while the muddy flow does not. The results can be helpful for understanding the physical interactions between water current and various debris flows, and debris flow dynamics at the channel confluence area.

Debris flows are frequently occurring natural processes in geologically complex terrains of the Indian Himalayas. Debris flow runout modelling leading to hazard assessment is essential for planning, designing, and execution of mitigation measures. In the present context, debris flow hazard assessment has been carried out for the Tangni debris flow in Garhwal Himalayas, India. Runout modelling was carried out using a Voellmy model-based 3D numerical simulation for estimation of flow intensity parameters such as runout distance, flow velocity, height and pressure along the propagation path. For calibration of the model inputs, back analysis of Tangni debris flow event that occurred in 2013, with its known runout length and deposition volume as the criteria, has been conducted. The best calibrated values of frictional parameters are obtained at μ = 0.10 and ξ = 400 m/s2. Using the best calibrated values of frictional parameters, hazard assessment was carried out for two potential release areas separately, with different initial volumes, and in combination to derive the probable runout distance along with other flow intensity parameters for different scenarios that may happen in the future. It has been observed that this debris flow scenario including the combination of two potential release areas will block the Alaknanda River, forming a landslide dam with a probable height of 6 m. Debris flow runout modelling-based hazard assessment will be helpful in determining quantitative information on flow intensity parameters, where complete data on past events are generally not available or were not possible to capture for the Indian Himalayan region.

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

The dynamic process and sensitivity in the debris transportation process are important topics in the study of rapid landslides and debris flows. The motion process of debris flow is recognised as a dynamic interaction between the original moving material and the entrained basal topsoil shearing along their non-slip contact surface. In this paper, we employed numerical modelling to clarify the erosional effects, and use statistical method to compare the influence degree of rheological parameters on the runout behavior. The simulated results show that unconfined flows typically exhibit a wide range of entrainment and deposition volume, whereas confined flows yield moderate volumes of entrainment and small volumes of deposition. The basal topography is an important factor in influencing the debris transportation and deposition processes. The velocity, runout distance, and deposition area and volume are more sensitive to underlying surface conditions, particles of debris, pore water pressure; and can affect the debris intensity, disaster area and scale to a large extent. Conversely, the velocity, runout distance, and deposition area and volume, are less sensitive to the source volume of debris flows, and can only affect the velocity to a little extent. The proposed suggestions can be utilized conveniently in general geotechnical engineering practices.

Rock avalanches events pose significant concerns in mountainous regions characterised by deep and narrow valleys. This has not deterred the ongoing development in these areas, where population settlements and infrastructure continue to expand, becoming increasingly susceptible to these risks. Cima del Simano instability in the Swiss Alps, located in the narrow Blenio Valley, is a deep-seated rockslide which could trigger such events in the future. A previous work outlined several scenarios for the rockslide failure defined by a specific area, volume (ranging from 2.30x105 m3 to 4.30x106 m3), and susceptibility to happen. Given the proximity of a major road and several villages on both sides of the Valley, there is a real need to evaluate the potential runout distance in the event of rupture and propagation of the different failure scenarios.  Literature often presents two distinct approaches for estimating the runout distance and the impacted area, both based on the retrospective analysis of historical landslide occurrence. The first approach is through empirical equations linking volumes of failure V and Fahrböschung angles f (tanf=aV^(-b), with a and b two empirical parameters to determine). The second approach consists in numerically simulating the flow propagation by means of dedicated software and by applying specific rheological models.  This present work suggests applying both those techniques to evaluate the area that would be affected in the case of a rock avalanche at Cima del Simano, triggered by one of the suggested scenarios. We evaluated the runout distance for different angles f estimated based on the empirical relationship, and Dan3D for simulating the propagation applying the Voellmy rheology. Four simulations were conducted by varying the friction coefficient μ [-] and the parameter of turbulence ξ [m.s-2] in order to assess the minimal and maximal possible propagation in terms of runout distance L and lateral spreading based on domain of validity of those parameters according to literature.  The distances L obtained empirically are longer than the ones from the simulations. This can be explained by the frontal confinement of the flow slowing down the propagation. The study is completed by an evaluation for each scenario of the probability of exceeding a certain distance L using existing statistical models for f variations.  Additionally, the numerical simulations highlight the areas in gullies where debris are deposited during the flow propagation. Those areas can be sources for subsequent debris flow events. In a second step, we conducted an analysis of areas susceptible to debris flow with Flow-R and compared them with former debris flow events for validation.  This study aligns with risk management to assist in making informed decisions regarding the evacuation plan in the event of a rupture and propagation of an important volume at Cima del Simano. 

ABSTRACTConcrete piers located in steep mountainous regions are highly susceptible to damage from debris flows. Existing studies often oversimplify debris flows as particle flows or equivalent fluids, neglecting their multiphase characteristics. In this paper, a three‐dimensional numerical model of debris flow‐bridge piers interaction is established based on the coupled CFD‐DEM approach. The Hertz–Mindlin (no‐slip) model and the Navier–Stokes equations are employed to describe the behavior of particle contacts and the fluid phase, respectively. An interface program compiled using user‐defined functions (UDFs) facilitates the transfer of information (drag, buoyancy, and viscous forces) between the fluid and particles. The accuracy of the model is validated by the simulation results of single‐particle settlement and underwater granular collapse. This study conducts a detailed investigation into the interaction mechanisms between the debris flow—bridge piers, considering the cross‐sectional shape of the piers and the solid volume fraction. It has been discovered that the shape of the bridge piers significantly influences the behavior of debris flows and the separation points. A power function effectively describes the relationship between the solid volume fraction and the peak impact force coefficient, Cd. Simplifying debris flows to dry granular flows or equivalent fluids may lead to an underestimation of the impact pressure.