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

Micro accelerometers are crucial tools for monitoring and providing early warning of geological disasters. This study focuses on the potential debris flow in Wangjiayuan, Fang County, Hubei Province, aiming to explore the relationship between clay content and debris flow motion characteristics. Physical model experiments were designed with varying clay content to simulate debris flow motion, and micro accelerometers were employed to monitor and record vibration acceleration changes during debris flow initiation and motion. Frequency spectra were generated using Fourier transform to analyze the motion characteristics in the frequency domain. The results showed that the predominant period of debris flow initiation ranged from 15 Hz to 29 Hz, while the period during motion in the middle section of the flow area ranged from 36 Hz to 54 Hz. Higher clay particle content was associated with a decrease in both the frequency range and mean of the predominant period, as well as a reduction in the Fourier amplitude extremum. These findings provide a foundation for monitoring and predicting debris flows, and the vibration signal characteristics of debris flow initiation and motion can be used to infer the clay content, offering insights into early warning systems for such geological hazards.

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.

The authors predicted mudflow and debris flow velocities with 350 field and laboratory measurements and obtained a slight trend for V=u to increase with h=d50, and the ratio of V=u is approximately 10 and rarely exceeds 30. The logarithmic relationship of turbulent resistance agrees reasonably well with the measurements. Good results also are obtained with the Manning-Strickler approach. The dispersive stress equation also compares well with the measurements, but only when h=d50 < 50. For individual series data such as Hashimoto and Puerco, there is a trend for V=u to increase with h=d50. But for other individual series data, such as Davies, Rickenmann, Paris, St. Helens, Nevado del Ruiz, Wanglin, Wang, and Wenhai, there is a trend for V=u to decrease with h=d50 (Fig. 1 of the original paper). The discussers have some field data that also show the same phenomena, which are presented in Fig. 1. The database includes a total of 119 flow velocity measurements, in which each point includes flow depth, density (volume concentration), median grain diameter d50, grain size of less than 10% d10, and slope. The data of Jiangjia were obtained from the field measurements in Jiangjia Ravine, Yunnan, China, in 1999; the data of Hunshui were obtained from the field measurements in Hunshui Ravine, Yunnan, China, in 1976–1978; and the data of Liuwan were obtained from the field measurements in Liuwan Ravine, Gansu, China, in 1963–1964. Debris flow is a gravity flow, and the gravity force plays an important role in the movement of debris flow. So debris flow moves on a large slope and deposits on a small slope. Fei and Su (2004) showed that the main driving force of debris flow is provided by particles, not by water. When the volume concentration C 1⁄4 0:27 (or density ρ 1⁄4 1:46 g=cm3), the particle driving force is equal to the water driving force in debris flow. They defined that the minimum density of debris flow is 1:46 g=cm3, and 1:46–1:80 g=cm3 is the range of less viscous debris flow. Therefore, the minimum density of viscous debris flow is 1:80 g=cm3 (C 1⁄4 0:47) (Fei and Su 2004; Yu 2008a). Two types of shear stresses describe these two kinds of debris flows: (1) the viscous stress for viscous debris flow, and (2) the turbulent stress for less viscous debris flow (Fei and Su 2004). As the driving force of debris flow is provided primarily by particles, the coarse particle plays an important role in the movement of debris flow, especially for viscous debris flow with a large volume concentration. So the larger particle diameter, the larger is the velocity. There is a trend for V to increase with d50, and there is a trend for V=u to increase with d50=h. Two empirical equations of mean velocity of viscous debris flow described this relationship (Wu et al. 1993; Yu 2001): V u 1⁄4 27:57 d50 h 0:245 ð1Þ

Depositional characteristics of debris flows in a rectangular channel with an abrupt change in slope

For houses situated in mountainous regions, evacuation routes during sediment disasters are limited. Thus, in the event of heavy rainfall, it may be difficult for inhabitants to evacuate from disaster areas. Therefore, it is important to understand the risk distribution before debris flow disasters in order to determine safe evacuation planning. Debris flow simulations are useful for the determination of hazard and risk mapping. When considering debris flow behavior and conducting debris flow simulations, there are two critical factors with respect to the debris flow flooding and deposition processes, namely, the scale of debris flow and landform conditions. The focus of this study was on various resolution digital elevation model (DEM) landform data in Japan. Simulations were performed for a residential area in Hiroshima, which was subject to debris flow events in 2014. Based on a comparison of the simulation results obtained using DEM data with a mesh resolution of 5 m, as sourced from the Geospatial Information Authority of Japan, and the results using high-resolution light detection and ranging (LiDAR) DEM data, the houses can be considered to describe the influence area and high risk region situated near the valley exit, in addition to locally dangerous areas and movement of debris flows on roads. To achieve a detailed hazard and risk mapping and determine safe evacuation routes and shelters, the application of high-resolution LiDAR DEM data and consideration of houses is critical.KeywordsDebris flowSimulationHazard and risk mappingDigital elevation model (DEM)

On 8 May 2016, eight debris flows occurred in Taining County, Fujian Province, China. Thirty-six people died as a result of the largest catastrophic debris flow (DF3), which occurred in the Luankeng gully and has attracted the attention from scholars all over the world. A field investigation was conducted, and site measurements were obtained to identify the processes of debris flow formation, movement, and deposition. The precipitation seepage was analyzed by numerical simulation with the Geo-Studio software. The mechanism that transforms the colluvium deposits in hollow regions (CDHs) into debris flows was studied by a stability analysis of the CDH in the initiation zone. The results showed the following: (1) the CDH instability was triggered by the combined effects of rainfall and flow concentration at the end of the hollow, and the CDH slide occurred along the interface between the bedrock and hollow, which transformed into a debris flow; (2) the debris flow discharge was amplified 6.5 times due to blockage by boulders distributed in the gully; and (3) the confluent area of the hollow region and the CDH slope gradient and thickness impart significant impact on the CDH stability. The CDH will experience failure when the slope gradient and thickness exceed the critical values of 30.5° and 1.25 m, respectively. The confluent area of the hollow may supply as much as four times the normal amount of water, which may trigger a debris flow during heavy rainfall.

During the initiation and movement of debris flow, low frequency infrasound is generated and emitted with characteristics such as strong penetration, low attenuation, and a high propagation speed that is faster than the debris flow. Monitoring infrasound has been used in debris flow detection and early warning systems; however, although the current infrasound-based warning system of debris flow can provide an alarm when debris flow occurs, it cannot locate and track the movement of debris flow in real time, and cannot determine the arrival time of debris flow to its potential victims. Thus, the potential applications of low frequency infrasound in debris flow disaster prevention and reduction have not yet been fully explored. In this study, we constructed an optimal triangular monitoring array based on factors such as the terrain of the monitoring area and characteristics of infrasound propagation. Combined with the time-delay estimation method, we determined and established an acoustic source localization model based on the time difference of arrival. Furthermore, with the help of a GIS platform, a visualized localization and tracking system for debris flow movement was developed to achieve real-time monitoring of debris flow. The performance of the system on acoustic source localization was validated using a simulation test and long-term field monitoring of debris flow at Jiangjia gully in Dongchuan, Yunnan province, China. Both the simulation and field test results showed that the system has high localization accuracy and strong real-time response. The results of the monitoring system could provide more accurate warning information of debris flow to local government and residents, allowing them to take appropriate mitigation measures well in advance to reduce the loss of life and property caused by debris flows.

Gravitational mass movements, such as debris and mud flows, are among the most destructive natural hazards, leading to substantial fatalities and extensive economic damage worldwide. Improving our understanding and modeling of these processes is essential for developing effective risk management and early warning strategies. When debris and mud flows pass through a curved channel, centrifugal forces may cause a difference in flow height between the inner and outer banks of the channel. This height difference, known as superelevation, can be described using analytical models that establish a relationship between the superelevation height and the flow velocity.Analytical models often employ a forced vortex approach, incorporating parameters such as the cross-sectional slope of the flow surface, flow width, and bend radius. These models, however, rely on assumptions such as a linear flow surface between mud deposits on the banks, a rectangular cross-section, and neglect both complex rheological behaviors and solid-fluid interactions. As a result, an empirically determined correction factor is required within the formula. The absence of a clear mechanical rationale for this correction factor presents challenges, as it is currently derived only through field investigations and laboratory experiments.This study presents an enhancement to the existing forced vortex approach by incorporating insights from numerical modeling. A coupled SPH-DEM numerical model is employed, where DEM particles represent coarse solid particles, and SPH accounts for the fluid phase, comprising fines and water. The SPH-DEM coupling is based on the no-slip interaction model, with simulations performed using a GPU-based solver to ensure enhanced computational efficiency. To validate the approach, a parametric test is conducted, initially back-calculating laboratory-scale experiments. The study further involves varying the water content in debris and mud flows to examine its impact on flow behavior and superelevation. Larger water contents lead to an increased superelevation angle. Results from the parametric test reveal a clear correlation between water content and the flow surface shape in curved channels. Specifically, mud flows are characterized by convex upward surface shapes, whereas more granular debris flows typically exhibit concave downward shapes.The distribution of material within the cross-section of the flow is governed by the equilibrium between boundary forces and centrifugal forces acting on the flow, which directly influences the superelevation. Numerical investigations are conducted to determine a correction factor and assess the extent to which inertial effects contribute to this correction factor for different material mixtures. Furthermore, we demonstrate that the effect of the flow surface shape is significant and is currently only accounted for by the empirical correction factor. This study offers new physical insights for the back-calculation of debris flow velocities in curved sections marked by mud deposits. Large-scale SPH-DEM simulations of a real debris flow event at Illgraben (Switzerland) are performed, showing good agreement with field data and its potential for further real-scale modeling.

Debris flow is a rapid flow which could lead to severe flooding with catastrophic consequences such as damage to properties and loss of human lives. It is important to study the movement of debris flow. Since during a debris flow process, the erosion and deposition processes are important, the no entrainment assumption is not acceptable. In this study, first we considered the debris flow as equivalent fluid and adopted the depth-averaged govern equations to simulate the movements and evolution of river bed. Secondly, the set of partial differential equations was solved numerically by means of explicit staggered leap-frog scheme that is accurate in space and time. The grid of difference scheme was derived from GIS raster data. Then the simulation results can be displayed by GIS and easily used to form the hazard maps. Finally, the numerical model coupled with GIS is applied to simulate the debris flow occurred on Oct. 20th, 2010, in Amamioshima City, Japan. The simulation reproduces the movement, erosion and deposition. The results are shown to be consistent with the field investigation.

With the rapid development of the Chinese econ- omy, many important building projects are planned on debris fans. Because of their location, these new con- structions will be threatened by debris flows. Thus, it is important to study debris flow deposition in different conditions. Twenty debris fans under different experi- mental conditions are considered in order to examine the geometric similarity of debris flow deposition. The experimental results show that the cross-section, pro- file, and plane configuration of debris fans all have par - abolic distributions. The quadratic term coefficients (a 1 , a 2 ) of the debris fan cross-section and profile define the average deposition slope in the longitudinal and trans- verse directions. For the plane configuration, -b 3 /2a 3 is the degree of deviation of the maximum downstream deposition width and c 3 is the ratio of the deposition width at the outlet to the maximum deposition width.

&amp;lt;p&amp;gt;Debris flow is one of the most hazardous events that occur in all mountain regions. &amp;amp;#160;Direct debris flow damage includes loss of human life, destruction of houses and facilities, damage to roads, rail lines and pipelines, vehicle accidents, and many other losses that are difficult to quantify. In July 2015, in the valley of the Barsemdara River (Gorno-Badakhshan Autonomous Region, Tajikistan), plenty of debris flows were observed. As a result, residential areas, social facilities, and infrastructure in Barsem village and neighboring settlements were destroyed and flooded. Besides, debris flow deposits blocked the Gunt River with the subsequent formation of a dammed lake with a maximum volume of 4.0 million m&amp;lt;sup&amp;gt;3&amp;lt;/sup&amp;gt;.&amp;amp;#160;&amp;lt;br&amp;gt;The aim of this study was to obtain hydrographs of debris flow waves in the source and detailed zoning of the Barsemdara river valley. For the debris flow source, we applied transport-shift model. Equations of this model were developed by Yu.B. Vinogradov basing on Chemolgan experiments of artificial debris flows descending. Previously, the model characteristics were compared with the observational data of the Chemolgan experiments, and the results were found to be satisfactory [Vinogradova, Vinogradov, 2017]. Based on the equations, a computer program was created in the programming language Python. Besides, we improved the model by adding flow velocity calculations, and eventually it became possible to obtain hydrographs. To investigate quantitative characteristics of the debris flow in the river valley we implied a two-dimensional (2D) model called FLO-2D PRO. It is based on the numerical methods for solving the system of Saint-Venant equations. Besides, in this model, it is assumed that debris flows move like a Bingham fluid (viscoplastic fluid) [O'Brien et al., 1993]. The input information for modeling was digital elevation model (DEM) and previously obtained hydrographs. The output information included flow depth, velocity distribution and hazard level of the territory. The results of the study will be reported.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;1. &amp;amp;#160; &amp;amp;#160;Vinogradova T.A., Vinogradov A.Y. The Experimental Debris Flows in the Chemolgan River Basin // Natural Hazards. &amp;amp;#8211; 2017. &amp;amp;#8211; V. 88. &amp;amp;#8211; P. 189-198.&amp;lt;br&amp;gt;2. &amp;amp;#160; &amp;amp;#160;O'Brien J. S., Julien P.Y., Fullerton W.T. Two-dimensional water flood and mudflow simulation //Journal of hydraulic engineering. &amp;amp;#8211; 1993. &amp;amp;#8211; V. 119, No 2. &amp;amp;#8211; P. 244-261.&amp;lt;/p&amp;gt;

Evaluating the flood discharge and deposition process of a debris flow is important for risk assessment, management, and design of possible supporting works for geo-hazard mitigation. The movement and deposition process of a typical debris flow gully in southern Gansu province, China, was simulated using the Soil Conservation Service-curve number (SCS-CN) approach and a two-dimensional finite model (FLO-2D PRO model) coupled with geographic information systems. Runoff volumes and depths were obtained by the use of the SCS-CN model using different precipitations and different intervals. The deposition, velocity, impact force, and influence zone of the debris flow were simulated with the FLO-2D PRO model based on the results of the SCS-CN method. Simulation results for a storm that occurred on 12 October 2010 suggest a maximum flow velocity of 23.1 m/s, a maximum deposition depth of 27.9 m, and a hazard zone of about 0.414 km2. These results were consistent with measured results from the documented debris flow. Verification demonstrated that model results could be used to help predict disaster-causing debris flows, thus helping to protect the lives, property, and economy of the local population.

Debris flows represent major hazards in most mountainous regions of the world where they repeatedly result in disasters. In order to protect people and infrastructure against future debris flows, many debris flow catchments have been artificially intervened by employing various mitigation measures, including civil engineering works. However, the commonly adapted engineering measures, such as check dams, are not effective for every debris flow catchment, and the failure of such measures even causes more damage, e.g. the Sanyanyu debris flow catchment in Zhouqu, China, killed 1756 people. In order to research the effectiveness of engineering strategies and explore much more effective mitigation works for debris flows in the mountainous regions, we took the Bailong River catchment of Southern Gansu of China as study area, with special emphasis on Sanyanyu debris flow catchment (with civil engineering works) and Goulinping debris flow catchment (without civil engineering works), and comparatively analysed the two catchments. The comparative results show that both catchments have similar material source, geomorphological/environmental and climatic conditions, however, vegetation cover and rock hardness are poorer in Goulinping than in Sanyanyu, the catchment that underwent larger-scale debris flows, suggesting that the mitigation measures had been applied in Sanyanyu catchment were inappropriate. Subsequently, we simulated the effectiveness of controlling debris flow peak discharge with check dams at the lower part of Sanyanyu and Goulinping catchment using the Kanako simulator, and summarised argument based on the hypothesis and facts from positive and negative aspects. We draw the conclusion that it is not reasonable to build check dams in the two catchments and instead, drainage channels should be primarily considered for reducing debris flow hazards in such densely populated areas. Finally, we undertook detailed field investigations and experiments on the native plants in the region, and found that the ecological mitigation measure with planting Robinia Pseudoacacia on the debris flow deposits is an effective method to alleviate debris flow hazards. It is concluded that channel works combined with ecological measures are the preferable approaches to minimize the debris flow damage in debris flow catchments characterised with high mountains, concentrated rainfalls and active neotectonic movement.

Subaqueous debris flows undergo various flow transformations, involving dilution and stripping of surface materials, penetration of ambient water into the flow interior, and detachment and disintegration of hydroplaning flow fronts. The surface transformation is a self-limiting process because the products of the process, such as an overriding suspended-sediment cloud or an armor of gravel at the flow front, inhibit effective working of the process. The degree of flow transformation therefore depends largely on whether a debris flow hydroplanes or not. For a subaqueous debris flow to hydroplane, its densiometric Froude number should be larger than 0.4, and the time scale of pore-pressure decay should be larger than the duration of a debris flow. In addition, a debris flow should be devoid of an extremely permeable girth of openwork gravel around the flow head because high pressures cannot be sustained underneath the gravelly material. Detailed sedimentological measurements and estimation of flow properties for three debris-flow beds in the Miocene fan deltas in SE Korea suggests that only a pebbly debris flow with a muddy (impermeable) matrix hydroplaned. On the other hand, bouldery debris flows are interpreted to have not hydroplaned irrespective of the nature of matrix. Nonhydroplaning debris flows were subject mainly to surface transformation and were outrun by surface-transformed suspended-sediment flows and debris-fall blocks after the flows entered a base-of-slope setting. Deposits of nonhydroplaning debris flows therefore overlie deposits of turbidity currents and debris falls. In the case of a hydroplaning debris flow, large chunks of debris could be detached from the flow front repetitively to form a series of small-volume flows that proceeded in front of the host debris flow. The preceding flows were promptly diluted to produce voluminous suspended-sediment clouds and were outrun by the faster-moving host debris flow. A deposit from a hydroplaning debris flow is therefore associated with thick and extensive turbiditic deposits that may either underlie or overlie the host debris-flow deposit. The turbiditic deposits associated with a hydroplaning debris flow are distinguished from those of a nonhydroplaning debris flow in that the former contain abundant gravel clasts and chunks of poorly sorted and clast-rich debris that cannot be suspended by the surface transformation process but were more likely derived from the detached fronts of a hydroplaning debris flow. These differences in sediment volume and grain size of turbiditic deposits and the stacking pattern of related debrites and turbidites provide a clue to the behavior of subaqueous debris flows.

An estimation method for debris flow potential is proposed to evaluate the possibility of the occurrence of rain-triggered debris flows. Sakurajima volcano has repeatedly erupted (Vulcanian type) and has continuously emitted volcanic ash at the Minamidake summit crater or Showa crater east of the summit since 1955, and debris flows have frequently occurred at rates of 10 to 111 events per year. Ground deformation associated with debris flows along the Arimura River were analyzed for the period from 2009 to 2016. Downward tilt (10–450 nrad) in the direction of the river and extensional strain (3–138 nstrain) were detected during occurrence of the debris flows. The tilt and strain changes were modeled using a point load caused by debris flow deposition beside a sabo dam. Depositional weights of individual debris flow events were estimated to range from 6 to 276 kt. The total weight of the debris flows was 2,154 kt, which is approximately 5% of the total weight of volcanic ash ejected from the craters during the study period. Debris flow potential (DFP) was defined as the difference in the volcanic ash deposits along the upper stream of the river (5% of the total) and the lower stream of the river, and the temporal change of the debris flow potential was investigated. When the debris flow potential reached a level of 0.4 Mt resulting from an increase in eruptive activity, debris flows frequently occurred or large debris flows were induced during rainy seasons. The concept of debris flow potential was applied to volcanoes in Indonesia as lahar potential. After the 2010 eruption at Merapi volcano, lahar potential, perhaps, quasi-exponentially decays during the dormant period. The lahar potential of Sinabung volcano complicatedly varies because of long-term eruptivity beginning in 2014.