Jiangjia Ravine Research Articles

The Erosion Pattern and Hidden Momentum in Debris‐Flow Surges Revealed by Simple Hydraulic Jump Equations

AbstractThe erosion‐deposition propagation of granular avalanches is prevalent and may increase their destructiveness. However, this process has rarely been reported for debris flows on gentle slopes, and the contribution of momentum hidden under the surge front to debris‐flow destructiveness is ambiguous. Therefore, the momentum carried by the apparent surge front is often used to indicate debris‐flow destructiveness. In this study, the erosion‐deposition propagation is confirmed by surge‐depth hydrographs measured at the Jiangjia Ravine (Yunnan Province, China). Based on simple hydraulic jump equations, the eroded deposition depth of surge flow is quantified, and the erosion pattern can be divided into two patterns (shallow and deep erosion). For surge flows with erosion‐deposition propagation, significant downward erosion potential is confirmed, and debris‐flow surge erosion is considered the deep erosion. The total momentum carried by surge flow is further quantified by two Froude numbers (surge‐front and rearward Froude numbers) and verified through the field observation of surge flows. The total momentum of surge flow not only originates from the apparent surge front, but also includes the momentum within the eroded deposition layer. This study provides a theoretical approach for quantifying the upper limit of erosion depth and revealing the destructiveness of debris‐flow surges. A perspective on the importance of substrate deposition for debris‐flow erosion on gentle slopes is emphasized, as this approach can improve the reliability of debris‐flow risk assessment.

Deriving Debris‐Flow Dynamics From Real‐Time Impact‐Force Measurements

AbstractUnderstanding the impact forces exerted by debris flows is limited by a lack of direct field measurements and validated numerical models. In this study, we use real‐time impact‐force measurements and field observations of debris flows recorded by a sensor network in Jiangjia Ravine, China, to quantify the impact‐force distribution of natural debris flows. We observed one debris flow event during and after a storm on 25 August 2004, including 42 short‐duration surges and seven long‐duration surges, and impact‐force signals were successfully recorded for 38 surges. Our observed debris flows comprise high‐viscosity laminar flows with high sediment concentration and frequent solid‐to‐solid interactions. We identified a large magnitude (up to 1 kN), high‐frequency (greater than 1 Hz) fluctuating component of the impact force that we interpret as solid particle impact on the sensors. The variability of particle impact forces increases with the mean impact force. Our results show that a log‐logistic distribution can describe the probability density distribution of impact forces. Solid‐dominated surges and fluid‐dominated intersurge flows have similar impact‐force distributions, but surges usually have heavy tails. We created a dimensionless number to describe the impact force and correlated it against existing dimensionless parameters. Finally, we develop a simple particle impact model to understand the relationship between flow dynamics and the impact force inside debris flows that could be applied to improve debris‐flow flume experiments and design debris‐flow hazard mitigation measures.

Visco‐Collisional Scaling Law of Flow Resistance and Its Application in Debris‐Flow Mobility

AbstractFlow resistance against gravity‐driven forces is a key factor controlling debris‐flow mobility, which is an important parameter for hazard risk assessment. In practice, Manning's formula is widely used for debris‐flow mobility analysis. However, this formula depends on the boundary conditions (channel roughness) and neglects the physical mechanisms of debris flow. Based on the systematic analysis of the field observation data of 93 debris‐flow events at Jiangjia Ravine (Yunnan Province, China), this study investigates the dynamic mechanisms and sources of flow resistance of debris flow. As the flows tend to be liquefied, the fluid viscous effect and particle collisions are the main sources of flow resistance. Flow resistance can be described by a visco‐collisional scaling law. Under fixed channel boundary conditions, this law is further incorporated into Manning's formula, bridging the gap between the resistance model based on physical mechanisms and the empirical formula of flow resistance. The modified Manning's coefficient is closely related to flow regimes, giving Manning's coefficient an explicit physical meaning. The modified Manning's coefficient provides a more reasonable basis for the mobility analysis and risk assessment of debris flow.

Analysis on the dynamic characteristics of debris flow in Jiangjia Ravine, China

Dynamic characteristics determine the mobility of debris flow and are also key to hazard risk assessment. However, the dynamic process of natural debris flow is very complex. Based on the systematic analysis of the field observation data of 93 debris-flow events at Jiangjia Ravine (Yunnan, China), this study attempts to investigate the dynamic mechanisms and sources of flow resistance of debris flow. The Jiangjia Ravine debris flows are almost completely liquefied, indicating that grain contact friction plays a negligible role. Flow regime analysis shows that the flow regimes of the Jiangjia Ravine debris flows varie from viscous to inertial. The fluid viscous effect and particle collisions may be the main sources of flow resistance.

Slope erosion induced by surges of debris flow: insights from field experiments

We conducted field observations and experiments to explore debris flow dynamics, sediment transportation and slope erosion at an active natural debris flow gully in the headwaters of Jiangjia Ravine (Dongchuan region, Southwest China). In this region, the hillslopes were heavily jointed, weathered and sparsely vegetated, providing continuous and rich sediment supplies for initiating debris flows. The debris flow propagated in the channel as a sequence of surges, with periodical changes of flow flux, velocity, water content, and viscosity as controlled mainly by the conditions of erodible sediments and water supplies from the upstream. The water content of bank sediments ranged from 5 to 8%, while it was 16 to 26% for debris surges in the channel. The particle size distribution of sediments on the alluvial fan followed the Weibull’s cumulative distribution and the mean size was in the range of 2 ~ 4 mm. The coarse particles were primarily elongated or prismoidal and aspect ratios followed well a normal distribution with the mean value of 0.4. The angular particles entrained in dense viscous debris flow surges could effectively abrade and groove the channel bed and banks, increasing the intensity of slope erosion. The incised slope had a sequence of terraced depositional layers on both banks. The layer thickness decreased as the erosion depth moved deeper into the stratum where hard bed soil/rock layers existed. The water-soil mixture of debris flow exhibited a clear shear-thinning behavior with its viscosity decreasing gradually with the increase of shear rate following the widely accepted power-law model. The dense viscous debris flow can facilitate the transportation of coarse gravels in channel and contribute to slope erosion.

Formation Conditions And Debris Flow Regime In Jiangjia Ravine, Yunnan, China – Applicability Of Russian Methodology

The requirements of the debris flows’ parameters assessments vary from country to country. They are based on different theoretical and empirical constructions and are validated by data from different regions. This makes difficult comparison of the reported results on estimated debris flows activity and extent. The Russian normative documents for the debris flows’ parameters calculations are based on empirically-measured parameters in wide range of geological and climatic conditions at the territory of former USSR, but still not cover all the possible conditions of debris flow formation. An attempt was made to check applicability of the Russian empirical constructions for the conditions of the debris flows formation in Yunnan, China, where unique long-term dataset of debris flows characteristics is collected by the Dongchuan Debris Flow Observation and Research Station. The results show, that in general the accepted in Russia methodology of calculation of the parameters of debris flows of certain probability corresponded well to the observed in Dongchuan debris flows characteristics. Some discrepancies (in the average debris flow depth) can be explained by unknown exact return period of the actually observed debris flows. This allowed to conclude that the presently adopted empirical dependencies based on country-wide (USSR) empirical data can be extrapolated up to the monsoon climate and geological conditions of Yunnan province.

Bayesian learning of Gaussian mixture model for calculating debris flow exceedance probability

ABSTRACT Probabilistic modelling of debris flow data provides useful information for quantitative risk assessment, such as exceedance probabilities (EPs) of debris flow quantities. This task can defy many classical statistical models because debris flow data are frequently collected over years or even decades, and the nonuniformity of the nature and the complex physical mechanism of debris flows lead to multimodal distribution characteristics of observational data. This paper proposes a Bayesian framework for learning Gaussian mixture model (GMM) of debris flow quantities (e.g. total discharge Q total and maximum impact pressure P max) and calculating their EPs for risk-informed decision making. GMM provides great flexibility to fit observation data, but are intrinsically unidentifiable due to the label switching. These computational difficulties are addressed using Random Gibbs Sampling and Bridge Sampling in the proposed framework, allowing incorporating the statistical uncertainty in GMM parameters into EP estimation. Equations are derived for the proposed approach and are illustrated using Q total and P max data at Jiangjia Ravine, China. Results show that the proposed approach identifies a bivariate GMM of Q total and P max reflecting the multimodal characteristics of the observed data and quantifies the statistical uncertainty of GMM parameters. Incorporating the statistical uncertainty into EP estimation provides robust estimates.

Reliability-based design for debris flow barriers

In the European Union since 2010, the design of any type of structures must comply with EN-1997 Geotechnical Design (CEN 2004) (EC7) referring to engineering projects in the rock mechanics field. However, the design of debris flow countermeasures in compliance with EC7 requirements is not feasible: EC7 uses partial safety factors for design calculations, but safety factors are not provided for phenomena such as debris flows and rock falls. Consequently, how EC7 can be applied to the design of debris flow barriers is not clear, although the basic philosophy of reliability-based design (RBD), as defined in EN1990 (CEN 2002) and applicable to geotechnical applications, may be a suitable approach. However, there is insufficient understanding of interactions between debris flows and structures to support RBD application to debris flow barrier design, as full-scale experimental data are very limited and difficult to obtain. Laboratory data are available but they are governed by scale effects that limit their usefulness for full-scale problems. The article describes an analysis, using the first-order reliability method (FORM), of two different datasets, one obtained through laboratory experiments and the other reflecting historical debris flow events in the Jiangjia Ravine (China). Statistical analysis of laboratory data enabled a definition of the statistical distributions of the parameters that primarily influence debris flow and barrier interactions. These statistical distributions were then compared to the field data to explore the links between flume experiments and full-scale problems. This paper reports a first attempt to apply RBD to debris flow countermeasures, showing how the choice of the target probability of failure influences the barrier design resistance value. An analysis of the factors governing debris flows highlights the applicability and limitations of EN1990 and EN1997 in the design of these rock engineering structures.

Statistical and probabilistic analyses of impact pressure and discharge of debris flow from 139 events during 1961 and 2000 at Jiangjia Ravine, China

Debris flows often cause catastrophic damage to communities in the downstream area, by direct impact and deposition. Theoretical predictions of impact pressure and volume of discharge, however, still remain very challenging, mainly due to inadequate understanding of the complex problems and limited field data at the local scale. In this study, the maximum impact pressure (Pmax) and total discharge (Qtotal) of 139 debris flow events that occurred during 1961 and 2000 in the “debris museum” of China (i.e., the Jiangjia Ravine) are reported and interpreted with statistical tests and probabilistic analyses. Four common probabilistic models (Normal, Lognormal, Weibull and Gamma distributions) are used to simulate the distributions of Pmax and Qtotal. The level of fitting of each model is assessed by performing two quantity-based statistic goodness-of-fit tests (Chi-square and Kolmogorov–Smirnov tests). The field data show that during the period from 1961 to 2000, the maximum values of Pmax and Qtotal are 744kPa and 1,751,537m3, respectively. It is suggested by the goodness-of-fit tests that the Weibull distribution is the only model (among the four probabilistic models) that is able to capture the distributions of Pmax and Qtotal of both surge and continuous flows. Using the verified Weibull distributions and Gaussian copula approach, univariate and bivariate exceedance probability charts considering Pmax and Qtotal are developed. Regression models between Pmax and Qtotal are also established.

Study on Activity and Feature of Sediment Delivery of Debris-Flow in the Jiangjia Ravine

Combined with recent observation data, the paper conclude that the debris flow break-out in high frequency in Jiangjia Ravine, and they belong to rainstorm type, which concentrated in the rainy season, especially during Jun to Aug. As a typical gully of viscous debris flow, the debris flow move with the bed process and obvious feature of surge. The intermittent flow possesses features of high intensity of delivery and long intermittent period. There have obvious relevance between the amount of sediment delivery and the surges of debris-flow break-out at the same year.

Landslides & debris flows formation from gravelly soil surface erosion and particle losses in Jiangjia Ravine

Gravelly soils are made up of gravel, sand, silt and clay. They are widely used in engineering applications such as rock-fill dams with clay cores, which are the main researches at present. The strength and mechanical properties of the gravelly soils are affected by the content of coarse grain, fine particles, and their adhesive states. These Properties can be verified by laboratory unconsolidatedundrained triaxial tests with grain size less than 5 mm and by large scale direct-shear tests with original grain content. Fine particles of the loose gravelly slopes are released under rainfalls, alternated the structures and mechanical properties, even affected the slope stability. There are a series of large scale direct-shear tests with different coarse grain contents to study the influence of fine particles releasing and migration, results showed the strength behavior of the gravelly soils were affected by the coarse grain content (P (5) ) and the inflection coarse grain contents. In order to study the erosion features of the gravelly soil slopes on rainfall conditions and the slopes stability alteration, we had carried out one sort of artificial rainfall local and model experiments, the runoff sediment contents were monitored during the experiments. Result showed that the shapes of the slopes surface transformed periodically, runoff sediment contents were divided into five phases according to the experiment phenomena, runoff sediment contents maintained downtrend during the rain time and the downtrend was obviously interpreted by one descend belt no matter the rainfall intensity and the slope angels. Particle size analysis released the deposit on the slope surface lost almost all of the clay, most of the silt and sand after the experiments, this meant the fine particles releasing, migration and accumulation process on condition of rainfall resulted in the instability factor of the slopes even induced landslide or debris flow.

Debris flow warning threshold based on antecedent rainfall: A case study in Jiangjia Ravine, Yunnan, China

Debris flows in Jiangjia Ravine in Yunnan province, China are not only triggered by intense storms but also by short-duration and low-intensity rainfalls. This reflects the significance of antecedent rainfall. This paper tries to find the debris flow-triggering threshold by considering antecedent rainfall through a case study in Jiangjia Ravine. From 23 debris flow events, the I-D (Intensity-Duration) threshold was found, which is very close to the line of 95th percentile regression line of rainfall events, representing that 95% of rainfalls can potentially induce debris flows and reflects the limitation of I-D threshold application in this area. Taking into account the effect of antecedent rainfall, the debris flow-triggering threshold for rainfall quantity and intensity is statistically and empirically derived. The relationships can be used in debris flow warning system as key thresholds. Coupling with the rainfall characteristics in this area, new thresholds are proposed as triggering and warning thresholds.

An empirical formula for suspended sediment delivery ratio of main river after confluence of debris flow

In order to calculate the suspended sediment discharge of the flood debris flows into the main river, a small scale flume test was designed to simulate the process of confluence of Jiangjia Ravine and Xiangjiang River in Yunnan province, China. By test observation and data analysis, suspended sediment discharge of Debris flow after its entry into the main river was found to have a close relation with the bulk density, the confluence angle of the Debris flow and the main river, the ratio between per unit width discharge of Debris flow and main river. Based on the measured and simulated results, and statistical analysis, an empirical formula was proposed for the suspended SDR (Sediment Delivery Ratio) of the main river after the confluence of Debris flow. Compared with the observed results of Debris flow in 2009, the error between the data calculated by the empirical formula and the monitored data is only about 10%.

Influence of Flow Width on Mean Velocity of Debris Flows in Wide Open Channel

Debris flow in a wide open downstream channel has a significant transverse velocity component that strongly influences its mean longitudinal velocity. Investigation of observation data of debris-flow surges at Jiangjia Ravine in China shows the dependency of Manning resistance of debris flows on the ratio of flow width to depth. Regression fit reveals a power function relationship between the Manning resistance coefficient and the width-to-depth ratio. This derives a new formula of mean velocity incorporating the influence of flow width. The result indicates that the width-to-depth ratio can be viewed as a kind of shape roughness analogous to grain roughness in the Darcy-Weisbach resistance coefficient expression.

Discussion of “Mean Velocity of Mudflows and Debris Flows” by Pierre Y. Julien and Anna Paris

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

Closure to “Mean Velocity of Mudflows and Debris Flows” by Pierre Y. Julien and Anna Paris

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

Rainfall, landslide and debris flow intergrowth relationship in Jiangjia Ravine

Jiangjia Ravine is a world-famous debris flow valley in Dongchuan, Yunnan Province, China. Every year large numbers of landslides and collapses happened and caused enormous damages to people’s properties and lives. With longtime observation and testing in Jiangjia Ravine we had found out one kind of special landslide which had the characteristics of landslide and collapse. Landslide and collapse supplied sufficient materials for debris flow. When a debris flow broke out, some kind of intergrowth existed among rainfall, landslide and debris flow. In order to study the intergrowth and some key parameters, we carried out artificial rainfall landslide tests and model experiments to observe the phenomena such as collapse, surface slide and surface flow. By observing the experimental phenomena and monitoring water contents, the transformation process among landslide deposits and debris flow under the condition of rainfall had been analyzed. Research results revealed the relationship of this kind of intergrowth among rainfall, landslide and debris flow in Jiangjia Ravine. Meanwhile, it was found that this kind of intergrowth relationship existed only when the moisture content was in a certain range. That is, the critical state seemed to be existed in the transformation process.

Real-time measurement and preliminary analysis of debris-flow impact force at Jiangjia Ravine, China

Impact forces associated with major debris flows (Jiangjia Ravine, China, August 25, 2004) were recorded in real time by a system consisting of three strain sensors installed at different flow depths. This provides the first real-time and long-duration record of impact forces associated with debris flows. A comprehensive approach including low-pass filtering and moving average methods were used to preprocess the recorded signals. The upper limit of impact frequency in the debris flows was estimated at 188.66 Hz under the assumption that only coarse grains cause effective impact loadings. Thus, a low-pass filter with a 200 Hz cut-off frequency was needed to denoise the original data in order to extract the impact force. Then the moving average method was applied to separate long-term and random components of the filtered data. These were interpreted as, respectively, the fluid pressure and grain impact loading. It was found that the peak grain impacts at different depths were non-synchronous within the debris flows. The impact loadings were far greater than, and not proportional to the fluid pressures. Analysis of the impact force of 38 debris flow surges gives an empirical value for the ratio of the hydrodynamic pressure to the momentum flow density, i.e. the product of debrisflow density and mean velocity square, which provides a very valuable basis for understanding debris flow dynamics and designing debris flow management systems. Copyright (C) 2011 John Wiley & Sons, Ltd.

Experimental analysis of shear strength of undisturbed soil in leucaena forest in Jiangjia Ravine, Yunnan, China

Five leucaena trees of similar age were chosen in Jiangjia Ravine of Dongchuan, Yunnan Province, China, near which the soil samples were collected by digging profiles 2m in depth and 1m in width. In each section, soil samples at different depths were taken for direct shear experiments to determine the root amount and mechanical composition. It is found that the cohesion and internal friction angle of the undisturbed soil are related to the root amount, depth, clay content and breccias content. Cohesion correlates negatively with root content, a finding that differs from that of other researchers. In addition, internal friction angle correlates positively with all these factors.

Annual risk assessment on high-frequency debris-flow fans

An empirical model of debris-flow risk assessment is developed to estimate annual loss ratio on high-frequency debris-flow fans where more than one hazard events occur every year. Based on observations of debris flows in Jiangjia Ravine, Yunnan Province, China, it is found that Gamma distribution is more appropriate for describing the annual frequency than the exponential distribution, which is supposed to be a good description of the low-frequency cases. Further analyses reveal that the two parameters of Gamma distribution can be explained, respectively, as the number of factors which dominate debris-flow occurrence and one-third of the mean annual frequency. Given the expectation of loss ratio is unchanged for each event we deduced a simple relationship between the expectations of one-event and annual loss ratios. Combined with the Gamma model, an equation is proposed to calculate the expectation of the annual loss ratio, which can be also used to assess the potential risk of fans formed by high-frequency debris flows.