A method to develop the input parameter database for site-specific debris flow hazard prediction under extreme rainfall

The predictive hazard analysis at a detailed scale for debris flow runout analysis can be improved significantly through reliable estimation of the input parameters. In this study, a method for database establishment of input parameters at a site-specific scale was laid out for the predictive-based debris flow hazard assessment under extreme rainfall. The adoption of the DAN-3D code necessitated the estimation of three main input parameters: initial volume, bulk basal frictional angle, and growth rate. The initial volume was assessed using a 3D coupled finite element seepage and limit equilibrium-based slope stability analysis. An artificial neural network-based model was developed using 27 debris flow events for predicting the basal bulk frictional angle and consisted of eight factors: plan curvature, profile curvature, percentage of fine content, D₅₀, initial unit weight, initial volume, relative relief ratio, and channel length. Finally, the growth rate was estimated using the previously assessed initial volume, soil depth, and the approximate runout length. The proposed method was validated by application to the Raemian slope in the Woomyeon mountain region, Seoul, for the extreme rainfall event of 27 July 2011. The analysis yielded a final volume of 53,067.9 m³, a velocity upon arrival on the road of 26.81 m/s, and an approximately 0.5-m debris thickness concentrated near the Raemian apartments. The comparison of the predicted debris flow path and debris flow velocity with the actual event demonstrates good similarity and provides a conservative estimate of the volume. This study therefore illustrates the importance of an input parameter database in providing a reliable debris flow runout hazard assessment.     

国内外泥石流活动关键指标估算方法之比较

泥石流流体容重、流速、冲击力是表征泥石流活动特征众多指标中的三个关键指标.本文在分析国内外对这些指标评估现状的基础上,总结了应用较普遍的估算公式,对比了不同公式的特点及其适用性.比较结果认为:由于泥石流组成和运动状态的复杂性以及影响因素的多样性,对于这三个指标,目前的估算公式都不具有普适性.在当前的认知水平下,建立基于各个地区泥石流特点的指标估算经验公式是最为实用的途径.在各个指标经验公式的建立方法上,考虑各个粒组的多变量容重统计分析方法相对较为合理;泥石流流速估算公式的建立途径在我国、前苏联和欧美国家之间有显著差异,前二者基于曼宁公式,后者基于强迫涡流公式、以弯道超高为主要参数;泥石流冲击力的估算方法国内外都以动量理论为基础,区别主要体现在经验系数取值上,巨石冲击力的计算则都考虑了拦挡建筑物的特点.     

基于BP神经网络的泥石流平均流速预测

泥石流平均流速是泥石流防治工程中不可缺少的重要参数,准确地预测泥石流平均流速对于泥石流防治工程的设计是至关重要的。将BP神经网络应用于泥石流平均流速的预测:将泥石流平均流速的影响因素--泥沙平均粒径、泥深、沟床比降和泥石流密度作为BP神经网络的输入单元,通过对云南东川蒋家沟泥石流观测数据的训练与预测建立了泥石流平均流速的BP神经网络预测模型。将预测结果与东川公式和曼宁修正公式的计算结果进行对比:曼宁修正公式和东川公式预测结果最大误差分别为27%和7.3%,BP神经网络的预测结果最大误差仅为3.2%,BP神经网络的预测精度是最高的,可见此方法对泥石流平均流速预测具有适用性和准确性。最后应用此方法预测了乌东德水电站近坝库区内的3条泥石流的平均流速分别为12.8 m/s、11.3 m/s和13.0 m/s,为库区泥石流防治工程提供了可靠的参考数据。     

Substantiation of debris flow velocity from super-elevation: a numerical approach

The use of super-elevations that a forced vortex flow leaves on the valley walls of a curved flume is a plausible approach toward estimating debris flow velocities in earthquake-induced geo-hazard studies. The centrifugal force of a speeding flow is responsible for a higher flow depth on the outer bend. However, in reality, a flow is not steady, and only the highest flow-marks are left at the outer and inner bends of the flow, which can lead to an inaccurate estimation of the actual velocity. Seeing the real scenario of the field, a series of numerical flume tests using smoothed particle hydrodynamics (SPH) is conducted to validate the estimation of debris flow velocities from flow-marks. Velocities estimated from flow-marks are lower than real velocities near the source region, but they converge to real velocities as the distance to the source increases. Based on several simulations, a best-fit line is proposed for adjusting debris flow velocity from mud-marks, and it is used to estimate flow velocities of the well-documented debris event called “Shiraito river debris flow,” which happened near the rim of the Hakone Crater, Kanagawa Prefecture, Japan, ensuing from the 1923 Great Kanto earthquake.     

Susceptibility analysis of large-scale debris flows based on combination weighting and extension methods

Susceptibility is an important issue in debris flow analysis. In this paper, 26 large-scale debris flow catchments located in the Wudongde Dam site were investigated. Seven major factors, namely, loose material volume per square kilometer, loose material supply length ratio, average gradient of the main channel, average hill slope, drainage density, curvature of the main channel, and poor vegetation area ratio, were selected for debris flow susceptibility analysis. Geographic information system, global positioning system, and remote sensing, collectively known as 3S technologies, were used to determine major factors. Weights of major factors affecting debris flow susceptibility were determined. This paper applied the combination weighting method, which considers both the preference of the engineers for major factors and the objective major factor information by using analytic hierarchy process and entropy method. Combination weights of major factors for the investigated 26 debris flow catchments are 0.20, 0.12, 0.20, 0.10, 0.08, 0.19, and 0.11, respectively. Combination weights follow the order of loose material volume per square kilometer = average gradient of the main channel > curvature of the main channel > loose material supply length ratio > poor vegetation area ratio > average hill slope > drainage density. This paper applied extension theory, which is used to solve incompatibility and contradiction problems, to determine susceptibility. Susceptibility results show that the susceptibility of 4 debris flow catchments are very low, 13 are low, 8 are moderate, and 1 is high. Assessment results exhibit consistency with the activity analysis.     

Estimation and analysis of the surface velocity field of mountain glaciers in Muztag Ata using satellite SAR data

Muztag Ata is a mountain known as the “Father of glaciers” in the west of China. However, its glacier flow pattern has never been studied. In this paper, the velocity of mountain glaciers on Muztag Ata is mapped by two methods (InSAR and offset-tacking) using SAR data. This map provides a detailed view of the features of glacier motion on Muztag Ata. A special effort was also made to assess the accuracy of the glacier velocity estimates. To ensure the accuracy of the derived results, the validation of the results was conducted over two glaciers (Saliymek Glacier and Kuksay Glacier). The two methods yielded similar results that agree well with each other. The patterns of glacier velocity on Muztag Ata suggest that mountain topography, westerly winds and the monsoon have a strong influence. Our results show that motion of glaciers in the southern and western regions of Muztag Ata is faster than that in the northern and eastern regions. Eleven glaciers were identified, along which glacier velocities exhibited distinctive behavior in terms of the spatial variability of the glacier motion. The occurrence of the local flow maxima and minima at consistent locations over different parts of different glaciers suggests that the subglacial topography, glacier size and glacier orientation affect the overall flow patterns. The velocities are very low in the surface debris cover of some glaciers, which suggests debris cover has an impact on glacier motion.     

Regional debris flow susceptibility analysis based on principal component analysis and self-organizing map: a case study in Southwest China

A method was developed to analyze the susceptibilities of 541 regional basins affected by debris flows at the Wudongde Dam site in southwest China. Determining susceptibility requires information on source material quantity and occurrence frequency. However, the large number of debris flows can hinder the individual field investigation in a each small basin. Factors that may trigger debris flows can be identified using remotely sensed interpretation information. Susceptibility analysis can then be conducted based on these factors. In this study, SPOT5 satellite imagery, digital elevation models (DEM), a lithology distribution map, and rainfall monitoring data were used to identify 12 debris flow trigger factors: basin relief ratio, slope gradient in the initiation zone, drainage density, downslope curvature of the main channel, vegetation coverage, main channel aspect, topographic wetness index, Melton’s ruggedness number, lithology, annual rainfall, form factor, and cross-slope curvature of the transportation zone. Principal component analysis was used to obtain the eight principal components of these factors that contribute to susceptibility results. Then, a self-organizing map method was adopted to analyze the principal components, which resulted in a debris flow susceptibility classification. Field validation of 26 debris flow basins was used to evaluate the errors of the susceptibility classification, as well as assess the causes of such errors. The study found that principle component analysis and self-organizing map methodologies are good predictors of basin susceptibility to debris flows.     

基于CFX的江口沟泥石流危险区范围预测模拟

拟建猴子岩水电站移民安置点位于江口沟泥石流堆积扇上,通过现场调查泥石流形成条件和运动特征,获得了1958年发生的50 a一遇泥石流危险区范围,根据雨洪法计算确定了泥石流的相关运动学参数。使用基于有限体积法的CFX软件,选择Bingham流变模型对江口沟泥石流流动过程的液面分布情况和速度场进行了三维数值模拟,得出了泥石流危险区范围和速度场分布情况。模拟结果显示,江口沟50 a一遇泥石流流过堆积区的平均速度为5.76 m/s,其中最大速度为13.59 m/s。数值模拟计算得出的危险范围较1958年扩大,是因为早期泥石流堆积物将原有地面特别是沟道附近地面抬高,使得现状条件下泥石流更容易向两侧漫流泛滥而扩大。上述结果为泥石流防治工程设计及危险区范围划定提供了一种新的方法,对工程实践具有重要的指导意义。     

Effects of the morphology of sediment-transporting channels on the erosion and deposition of debris flows

A series experiments are conducted to investigate the effects of streambed profile on the erosion and deposition of debris flows. It is found that straight channel can increase the run out of debris flows by 10–25%, compared to that of surfaces without channels, and that travel distance was positively correlated with the hydraulic radius of the channel. In addition, the presence of straight channels caused the volume of debris flow deposition to become normally distributed with respect to travel distance. In the case of curved channels, increases in the sinuosity index resulted in significant blockage and obstruction. In the deposition zone, the maximum deposition volume for a channel with a comparatively low sinuosity index (1.05) was <?50% of the minimum deposition volume for a straight channel. Furthermore, the channel curvature affected not only the positions of deposition peaks along the travel distance but also the debris flow magnitudes in each unit interval (0.5 m). This study demonstrates the effects of differences in channel morphology on the erosional and depositional processes of gully debris flows. These findings are of significant importance for guiding debris flow risk assessment and for the restoration and reconstruction of downstream regions.     

Upper-mantle structure under the Arabian peninsula

The phase velocity of Rayleigh waves across the Arabian Peninsula has been measured for profiles SHI-HLW, JER-SHI in the northern part and SHI-AAE in the southern part of the peninsula. The phase velocities for the Arabian stable shelf are lower than those for the Canadian shield and higher than those for the Gulf coastal plain of the United States. Shear-velocity models obtained from the inversion of these experimental data show a pronounced low-velocity channel for S-waves which is found throughout the region with a velocity of S in the channel of 4.25–4.45 km/sec. The top of the channel is at 100–140 km depth. This structure differs from those under the Canadian shield and the Gulf Coast. Crustal thickness is 35 ± 8 km.     

Modelling of turbidity currents on Navy Submarine Fan, California Continental Borderland

Several Holocene turbidites can be correlated across much of Navy Fan through more than 100 sediment core localities. The uppermost muddy turbidite unit is mapped throughout the northern half of the fan; its volume, grain-size distribution and the maximum height of deposition on the basin slopes are known. These parameters can be related to the precise channel morphology and mesotopography revealed by deep-tow surveys. Thus there is sufficient information to estimate detailed flow characteristics for this turbidity current as it moved from fan valley to distal basin plain. On the upper fan, the gradient and the increasing downstream width of the channel and only limited flow overspill suggest that the flow had a Froude number close to 1.0. The sediment associated with the channel indicates friction velocities of about 0.06 m s^?1 and flow velocities of about 0.75 m s^?1. Using this flow velocity and channel dimensions, sediment concentration (～2×10^?3) and discharge are estimated, and from a knowledge of the total volume of sediment deposited, the flow duration is estimated to be from 2 to 9 days. It is shown that the estimates of Froude number, drag coefficient, and sediment concentration are not likely to vary by more than a factor of 2. On the mid-fan, the flow was much thicker than the height of the surface relief of the fan and it spread rapidly. The cross-flow slope, determined from the horizontal extent of turbidite sediment, is used to estimate flow velocity, which is confirmed by consideration of both sediment grain size and rate of deposition. This again allows sediment concentration and discharge to be estimated. The requirements of flow continuity, entrainment of water during flow expansion, and observed sediment deposition provide checks on all these estimates, and provide an integrated picture of the evolution of the flow. The flow characteristics of this muddy turbidity current are well constrained compared to those for more sand-rich late Pleistocene and early Holocene turbidity currents on the fan.     

Evaluation of approaches to calculate debris-flow parameters for hazard assessment

Many different runout prediction methods can be applied to estimate the mobility of future debris flows during hazard assessment. The present article reviews the empirical, analytical, simple flow routing and numerical techniques. All these techniques were applied to back-calculate a debris flow, which occurred in 1982 at La Guingueta catchment, in the Eastern Pyrenees. A sensitivity analysis of input parameters was carried out, while special attention was paid to the influence of rheological parameters. We used the Voellmy fluid rheology for our analytical and numerical modelling, since this flow resistance law coincided best with field observations. The simulation results indicated that the “basal” friction coefficients rather affect the runout distance, while the “turbulence” terms mainly influence flow velocity. A comparison of the velocity computed on the fan showed that the analytical model calculated values similar to the numerical ones. The values of our rheological parameters calibrated at La Guingueta agree with data back-calculated for other debris flows. Empirical relationships represent another method to estimate total runout distance. The results confirmed that they contain an important uncertainty and they are strictly valid only for the conditions, which were the basis for their development. With regards to the simple flow routing algorithm, this methods could satisfactorily simulate the total area affected by the 1982 debris flow, but it was not able to directly calculate total runout distance and velocity. Finally, a suggestion on how different runout prediction methods can be applied to generate debris-flow hazard maps is presented. Taking into account the definition of hazard and intensity, the best choice would be to divide the resulting hazard maps into two types: “final hazard maps” and “preliminary hazard maps”. Only the use of numerical models provided final hazard maps, because they could incorporate different event magnitudes and they supplied output-values for intensity calculation. In contrast, empirical relationships and flow routing algorithms, or a combination of both, could be applied to create preliminary hazard maps. The present study only focussed on runout prediction methods. Other necessary tasks to complete the hazard assessment can be looked up in the “Guidelines for landslide susceptibility, hazard and risk zoning” included in this Special Issue.     

Conditioned Simulations of Random Velocity Fields

In petroleum exploration and production, it is essential to have good estimations of the uncertainties on the reserves. Uncertainties on the velocity model used during the data processing are of major importance in this estimation. The generation of several velocity fields gives access to a quantified estimation of the uncertainties due to the velocity model inversion. The use of statistical methods helps in generating several important, equiprobable velocity fields, matching all the available velocity information. This paper presents an efficient simulation algorithm to generate instantaneous velocity fields, constrained by the distribution of values measured at the wells, and calibrated by the stacking velocities, taken as root-mean-square velocities. The simulations also match the covariance model given for the instantaneous velocity fields. The method is developed in a simple one-layer case with constant velocity, and then extended to more realistic situations. Finally, a real data application is shown, using data provided by ENI–Agip Division, and the efficiency of the proposed simulation method is discussed.     

Prediction of the run-out distance of the debris flow based on the velocity attenuation coefficient

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.     

泥石流泥痕深度的计算研究

泥石流泥痕深度是一个通过勘测调查获取的重要参数,它直接关系到泥石流运动流速、冲击力等特征参数的定量计算。但实际调查中却往往因为时间久远、沟道冲淤变化等原因致使泥痕痕迹难以准确判断,甚至无法判断而造成误判。鉴于此,本文试图通过泥石流流量、流速等参数间的数量关系推算出泥石流泥痕深度的计算方法,并以工程实例验证其具有一定的实用性,可为相关研究与设计提供参考。     

弯道超高法在泥石流流速计算中的应用——以四川省汶川县G317国道磨子沟泥石流为例

选取四川省汶川至马尔康公路线的磨子沟作为试验对象,采用弯道超高法推算其泥石流流速。进而可以计算出该次泥石流的平均流量,为公路的改建工程提供设计依据。经过实地调查,用此方法计算磨子沟2005年8月17日暴发的泥石流流速为4.4m/s。同其他流速方法计算的结果相比较,前者值偏大,且偏向于工程安全值。弯道超高法可以解决泥石流观测的局限性和泥石流发生的不可重复性。此方法简便易行,对泥石流防治工程具有一定的实用价值。     

泥石流对桥墩冲击力的试验研究

泥石流冲毁桥墩是桥梁在遭受泥石流冲击时的常见破坏形式。为了研究泥石流对桥墩的冲击力大小,通过调整黏土、沙、石子、水的不同含量,配置不同流变特性、不同密度的泥石流,使用所配置的原料在泥石流槽内对两种形状（圆形、方形）的桥墩缩尺模型进行冲击,综合考察了流变特性、流速、桥墩形状以及冲击力的关系。试验表明：试验配置的泥石流原料流变特性差异明显,且可以用简单的选择流变仪测得,用牛顿流体或宾汉体描述。泥石流的流速可用曼宁公式求得,而公式中的糙率系数与泥石流黏度满足幂函数关系。相同工况下,不同形状桥墩所受的冲击力差异明显,方形桥墩阻力系数普遍大于圆形桥墩。使用非牛顿流体雷诺数（Re）可以综合反映流变特性和流速,因此,圆墩的阻力系数可表达为Re的函数,而方墩则没有明显关系。为方便工程应用,可根据黏性泥石流、稀性泥石流对圆墩的阻力系数分别为2.3、0.9,对方墩分别为2.6、1.9进行选用。     

滩地植被化明渠中横向水平射流特性试验研究

运用Micro ADV测速系统,对3种工况下滩地植被化的复式断面横流环境中三维圆形水平射流的特性进行了试验研究.试验测得的射流典型横断面速度场及湍动能的成果表明,射流与来流的相互作用表现出明显的分叉现象,其分叉半角为5.71°;复式断面二次流具有加速射流分叉的作用,滩地植被具有减缓射流分叉的效应.对射流出口平面的速度场及湍动能的分析表明,射流轨迹曲线的弯曲程度、射流中心线的位置主要取决于射流与来流流速比.流动横断面流速和湍动能特性表明,射流在整个流场中是各向异性的.     

基于沟床宽度与颗粒粒径的泥石流精细化预报模型

泥石流形成区沟床宽度和颗粒粒径对沟床起动型泥石流的发生影响很大,在强烈地震影响区内显得尤为突出,但目前的泥石流预报中还没考虑到这两个因素,无法准确预测强震区泥石流的发生.在泥石流10 min和1 h精细化预报模型基础上,通过现场调查群发泥石流事件,结合汶川地震强烈影响区泥石流的演化特点,引入了泥石流形成区沟道宽度和颗粒粒径的影响,建立了改进的精细化泥石流10 min和1 h预报模型,并在贵州望谟打易和四川德昌群发泥石流、汶川地震强烈影响区的文家沟多次泥石流事件中获得了很好的验证结果,得出泥石流形成区的颗粒粒径代表泥石流的地质因子,泥石流形成区沟床宽度代表泥石流的地形因子之一,这2个因子在泥石流发生中的作用都非常重要;改进的精细化10 min和1 h预报模型以及临界值,可以用于强烈地震区和一般的泥石流预报.      

A new finite element formulation for one-dimensional analysis of elastic-plastic materials

It is well accepted that severe numerical difficulties arise when using the conventional finite element displacement method to analyse incompressible, or nearly incompressible, solids. These effects are caused by the kinematic constraints imposed on the nodal velocities by the constant volume condition. In elastic-plastic analysis, these effects are due to a conflict between the plastic flow rule and the finite element discretization. Although several methods have been proposed to cope with this problem, none has been based on the appropriate choice of displacement interpolation to minimise the constraints. In this paper, a new displacement interpolation, which is able to reduce the imposed constraints, is adopted. Comparisons of the results with those from a conventional linear displacement interpolation are made for predictions of cylindrical and spherical cavity expansion limit pressures in elastic-plastic solids. This study suggests that the proposed displacement interpolation is preferable to the conventional one in the elastic-plastic finite element analysis of one dimensional-axisymmetric problems which involve nearly incompressible material behaviour.