Empirical Relationships for Debris Flows

The assessment of the debris flow hazard potential has to rely on semi-quantitative methods. Due to the complexity of the debris-flow process, numerical simulation models of debris flows are still limited with regard to practical applications. Thus, an overview is given of empirical relationships that can be used to estimate the most important parameters of debris-flow behavior. In a possible procedure, an assessment of a maximum debris-flow volume may be followed by estimates of the peak discharge, the mean flow velocity, the total travel distance, and the runout distance on the fan. The applicability of several empirical equations is compared with available field and laboratory data, and scaling considerations are used to discuss the variability of the parameters over a large range of values. Some recommendations are made with regard to the application of the presented relationships by practicing engineers, apart from advocating field reconnaissance and searching for historic events wherever possible.     

束流型拦砂坝溢流口及其关键参数确定

为进一步优化拦砂坝溢流口体型,确定溢流口几何参数取值,通过物理模拟实验研究了不同拦砂坝溢流口体型条件下泥石流过坝的流态、坝后冲刷深度、消能率变化规律,并根据拦砂坝埋深设计标准与松散体水下临界平衡关系,探讨了溢流口收缩率的合理取值范围,结果表明:采用斜面或者大倾角的反弧型溢流口时,坝后泥石流落点与坝脚的距离近、冲刷深度大,而采用无倾角的反弧型溢流面时,不仅可以在一定程度上减小冲刷深度,而且大大增加了坝后泥石流落点与坝脚的距离;在泥石流规模与溢流面曲率半径相同情况下,适当增大侧向收缩率,有利于增强泥石流体与坝后动床之间的相互作用,提高泥石流通过坝后冲刷坑的消能率;当溢流口收缩率在0.2~0.6之间时,冲刷深度满足设计要求,且泥石流跌落点距坝脚较远,冲刷坑发展不会危及坝体安全。     

Faulting parameters of earthquakes in the New Madrid, Missouri, region

The advent of high-resolution digital seismic recording and advances in computer technology enable the combination of traditional regional seismic network observations with direct seismogram modeling to improve estimates of small earthquake faulting geometry, depth, and size. We illustrate a combined modeling approach using observations from three earthquakes that occurred within the environs of the New Madrid Seismic Zone: two Missouri earthquakes from September 26, 1990 and May 4, 1991; and the southern Illinois earthquake of February 5, 1994. We also re-examine the faulting geometry for two events from the 1960s that are inconsistent with the current estimate of the regional stress field. Based on direct modeling of the long-period seismograms associated with these events, we revise earlier estimates of the earthquake parameters for the March 3, 1963 and July 21, 1967 Missouri earthquakes. Comparing the new and revised results with existing earthquake mechanisms in the region, we find that tension-axes are generally aligned in a N-S to NW-SE direction, while the compression-axes trend in a NE to E direction. An interesting exception to this pattern are the March 3, 1963 and two nearby earthquakes that lie within a well-defined 30-km long left step in seismicity near New Madrid.     

Deep structure of the Nojima Fault, southwest Japan, estimated from borehole observations of fault-zone trapped waves

To estimate the deep structure of the southern part of the Nojima Fault, southwest Japan without the influence of near-surface structures, we analyzed the Love-wave-type fault-zone trapped waves (LTWs) recorded by a borehole seismometer at 1800 m depth. We examined the polarization, dispersion, and dominant frequency of the wavetrain following the direct S-wave in each seismogram to identify the LTW. We selected eight candidates for typical LTWs from 462 records. Because the duration of the LTW increases with hypocentral distance, we infer that the low velocity fault-zone of the Nojima Fault continues towards the seismogenic depth. In addition, since the duration of the LTW increases nonlinearly with hypocentral distance, we infer that the S-wave velocity of the fault-zone increases with depth. The location of events showing the LTW indicates that the fault-zone dips to the southeast at 75° and continues to a depth of approximately 10 km. We assumed a uniform low-velocity waveguide to estimate the average structure of the fault-zone. We estimated the average width, S-wave velocity, and Q_s of the fault-zone by comparing an analytical solution of the LTW with measured data. The average width, S-wave velocity, and Q_s of the fault-zone are 150 to 290 m, 2.5 to 3.2 km/s, and 40 to 90, respectively. Hence the fault-zone structure with a larger width and smaller velocity reduction than the fault-zone model estimated by previous surface observation is more suitable to represent the average fault-zone structure of the Nojima fault. The present study also indicated that the shallow layers and/or a shallow fault-zone structure drastically changes the characteristics of the LTW recorded at the surface, and therefore cause a discrepancy in the fault-zone model between the borehole observation and surface observation.     

A debris-flow alarm system for the Alpine Illgraben catchment: design and performance

We describe the development, implementation, and first analyses of the performance of a debris-flow warning system for the Illgraben catchment and debris fan area. The Illgraben catchment (9.5 km²), located in the Canton of Valais, Switzerland, in the Rhone River valley, is characterized by frequent and voluminous sediment transport and debris-flow activity, and is one of the most active debris-flow catchments in the Alps. It is the site of an instrumented debris-flow observation station in operation since the year 2000. The residents in Susten (municipality Leuk), tourists, and other land users, are exposed to a significant hazard. The warning system consists of four modules: community organizational planning (hazard awareness and preparedness), event detection and alerting, geomorphic catchment observation, and applied research to facilitate the development of an early warning system based on weather forecasting. The system presently provides automated alert signals near the active channel prior to (5–15 min) the arrival of a debris flow or flash flood at the uppermost frequently used channel crossing. It is intended to provide data to support decision-making for warning and evacuation, especially when unusually large debris flows are expected to leave the channel near populated areas. First-year results of the detection and alert module in comparison with the data from the independent debris-flow observation station are generally favorable. Twenty automated alerts (alarms) were issued, which triggered flashing lights and sirens at all major footpaths crossing the channel bed, for three debris flows and 16 flood flows. Only one false alarm was issued. The major difficulty we encountered is related to the variability and complexity of the events (e.g., events consisting of multiple surges) and can be largely solved by increasing the duration of the alarm. All of the alarms for hazardous events were produced by storms with a rainfall duration and intensity larger than the threshold for debris-flow activity that was defined in an earlier study, supporting our intention to investigate the use of rainfall forecasts to increase the time available for warning and implementation of active countermeasures.     

冀西北地区泥石流发育的环境因素遥感研究

文章介绍了利用Landsat—TM遥感图像对影响泥石流发育的环境因素所作的定量研究。该项研究以在冀西北1．8万km^2范围内解译出的339条(处)泥石流沟作为研究对象，在全面分析了该区影响泥石流发育的环境因素后，选择出植被盖度、岩石类别、沟床坡降、流域平面形态、地形地貌特征等5项因素，通过提取遥感图像特征进行分级细化和定量研究。采取大样本统计方法，得出每种因素在不同状态下与泥石流发育的关系，定量表示不同发育状态的环境因素对泥石流发育的影响。文章还对影响泥石流发育的所有环境因素进行了分类和评价，将9类因素归结为影响泥石流发育的物源因素和影响泥石流运动状态的因素2大类，建立了泥石流发育影响因素分类体系，并评价、排序了它们在泥石流孕育过程中的重要性。最后，总结出研究区最易发生泥石流的环境因素组合。     

Driftwood deposition from debris flows at slit-check dams and fans

Experience shows that debris flows containing large woody debris (driftwood) can be more damaging than debris flows without driftwood. In this study, the deposition process of debris flows carrying driftwood was investigated using numerical simulations and flume experiments. Debris-flow trapping due to driftwood jamming in a slit-check dam was also investigated. A numerical model was developed with an interacting combination of Eulerian expression of the debris flow and Lagrangian expression of the driftwood, in which the fluctuating coordinates and rotation of the driftwood were treated stochastically. The calculated shapes and thicknesses of a debris-flow fan and the positions and orientations of the deposited driftwood on a debris-flow fan were consistent with experimental flume results. The jamming of driftwood in a slit-check dam was evaluated based on geometry and probability. The simulated results of outflow discharge and the proportion of driftwood passed through the slit-check dam also agreed with the experimental results.     

Hydraulic geometry change of a large river: a case study of the upper Yellow River

Study on hydraulic geometry can reveal the response of river systems to basin attributes, and the trends in channel change. Based on 35 years of data collected at 10 hydrological stations (cross-sections), the hydraulic geometry relations between cross-sectional variables and discharge were established, and the derived parameters were analyzed. The channel behavior of the mountainous bedrock reach (from riverhead to Xunhua cross-section) and the alluvial reach (from Xunhua to Toudaoguai cross-sections) differed significantly over the past decades in response to dramatic water and sediment changes. The hydraulic geometry quasi-equilibrium was achieved through equally adjusting water depth and flow velocity in the mountainous bedrock reach; while primarily through flow velocity in the alluvial reach. The change rate of river width with varying discharge was relatively small in the whole upper Yellow River. Compared with the lower Yellow River and other rivers in the world, both similarity and differences existed, indicating the general adjustment direction of hydraulic quasi-equilibrium and also the importance of considering other influencing factors in hydraulic geometry studies. In addition, dams played an important role in affecting the changes of hydraulic geometry exponents.     

An estimation of discharge using mean velocity derived through Chiu’s velocity equation

To estimate discharge through the year (dry season and flood season), a stage–discharge curve derived through monitoring discharge in about 20 rivers or channels every year has been generally used. As revealed in many studies, however, the stage–discharge curve is inevitably affected by their hydraulic characteristics. This suggests that the use of a stage–discharge curve derived without considering hydraulic characteristics unique to a river or channel may produce significant errors in estimating discharge at not only low stage (during dry seasons) but also high stage (during flood seasons). In this study, the authors proposed a method to calculate the mean velocity and to estimate the discharge considering the hydraulic characteristics of a river or channel (e.g. the bed slope, wetted perimeter, width, kinematic viscosity, etc.); the method was developed using Chiu’s velocity equation. With the proposed method, it is possible to calculate a maximum velocity that is difficult to measure in an open channel, derive an entropy function representing the equilibrium of the channel, and thereby, estimate reliable discharge even in a flood season. To comparatively verify the utility of the proposed method, relations between the results of analysis using Manning’s and Chezy’s mean velocity equations and the values of measured discharge were addressed together. The results of analysis using lab data sets and measured data sets revealed that the proposed method was significantly more accurate in estimating discharge, even in flood seasons, when compared with the conventional method.     

Comparison of numerical models of two debris flows in the Cortina d’ Ampezzo area,Dolomites, Italy

The accurate prediction of runout distances, velocities and the knowledge of flow rheology can reduce the casualties and property damage produced by debris flows, providing a means to delineate hazard areas, to estimate hazard intensities for input into risk studies and to provide parameters for the design of protective measures. The application of most of models that describe the propagation and deposition of debris flow requires detailed topography, rheological and hydrological data that are not always available for the debris-flow hazard delineation and estimation. In the Cortina d’Ampezzo area, Eastern Dolomites, Italy, most of the slope instabilities are represented by debris flows; 325 debris-flow prone watersheds have been mapped in the geomorphological hazard map of this area. We compared the results of simulations of two well-documented debris flows in the Cortina d’Ampezzo area, carried on with two different single-phase, non-Newtonian models, the one-dimensional DAN-W and the two-dimensional FLO-2D, to test the possibility to simulate the dynamic behaviour of a debris flow with a model using a limited range of input parameters. FLO-2D model creates a more accurate representation of the hazard area in terms of flooded area, but the results in terms of runout distances and deposits thickness are similar to DAN-W results. Using DAN-W, the most appropriate rheology to describe the debris-flow behaviour is the Voellmy model. When detailed topographical, rheological and hydrological data are not available, DAN-W, which requires less detailed data, is a valuable tool to predict debris-flow hazard. Parameters obtained through back-analysis with both models can be applied to predict hazard in other areas characterized by similar geology, morphology and climate.     

Estimation of the 3-D velocity structure of the Sorachi–Yezo and Oshima Belts in the western part of Hokkaido, Japan

We adopted the seismic tomography technique to refine the three-dimensional velocity structure model of the western part of Hokkaido, Japan. Using the P-wave first arrival data listed by Japan Meteorological Agency from 2002 to 2005, we could estimate a 3-D inhomogeneous velocity structure model with a low velocity at a depth of 14 km beneath Asahikawa. The crustal structure near Sapporo was characterized by lateral velocity change toward the southern seaside. The low-velocity zone near Urakawa, proposed by previous research, was also clarified. In general, the present model showed lower-velocity values for most of the crustal layers in the area concerned. The results of this study were affected by less number of higher magnitude events (M?≥?0.5) in the central part of the area of interest. However, the perturbation results for comparatively shallow layers (6–50 km) were good in resolution. It was found that the source region of the Rumoi–Nanbu earthquake of December 14, 2004 was characterized by a low-velocity zone, located between high velocity zones. Such an inhomogeneous crustal structure might play an important role in the relatively high seismic activity in the Rumoi–Nanbu earthquake source region.     

烧房沟滑坡型泥石流工程治理及效果分析

国内对泥石流治理措施谷坊坝、格栅坝的作用和效果研究较多,而对滑坡型泥石流的综合治理措施和效果研究较少。2010.8.14强降雨过程导致汶川震区映秀镇烧房沟滑坡型泥石流暴发,通过分析其运动过程和现状沟道特征,结合滑坡堵点和沟道深切的特点,治理工程采用防堵防切综合工程结构形式:上游谷防群+中游抗滑桩、挡土板和肋板护脚护底+下游3座格栅坝+渡槽明洞跨越G213,并分析各分项工程作用:减弱龙头动能+控制LS01堵点、保护鞋尖+拦粗放细、顺畅排导。最后利用2010.8.14与2013.7.10的历史降雨量和固体参与量对比、工程治理前后动储量对比和各分项工程治理前后的沟道特点对比,验证了烧房沟滑坡型泥石流综合治理工程的效果良好。为今后震区滑坡型泥石流工程防治提供参考。     

Assessment of Risk due to Debris Flow Events: A Case Study in Central Taiwan

In this paper, based on the concepts of uncertainty and reliability analyses, a method used for assessment of risk due to debris flow events is proposed. First, procedures for obtaining the configuration of debris-flow fans are presented. Then, the parameters affecting the configuration of debris-flow fans are identified and their corresponding means and standard deviations are derived. Finally, the proposed method is applied to the Shih-Pa-Chung Creek in central Taiwan. The expected deposition thickness at any point in the deposition area is computed and then the contours of risk for the 50-year and 100-year events are constructed. Regarding the expected deposition thickness, it is found that the closer the distance from the canyon mouth, the larger the debris-flow thickness becomes. The results also show that the contours of risk are of the shape of an ellipse similar to the shape of deposition area, and the risk at a point decreases with increasing distance of that point from the canyon mouth. In addition, when the return period of rainfall event is fixed, the variation in risk decreases as the distance from the canyon mouth increases. For the assessment of risk due to debris flow events, the proposed method is recommended as an alternative to the existing methods, because the influence of all the uncertainty of the parameters is considered.     

Application of a long-range Terrestrial Laser Scanner to a detailed rockfall study at Vall de Núria (Eastern Pyrenees, Spain)

In this study we show the application of a long-range Terrestrial Laser Scanner (TLS) to a detailed rockfall study in a test zone at Vall de Núria, located in the Eastern Pyrenees. Data acquisition was carried out using TLS-Ilris3D, the new generation of reflector-less laser scanners with a high range, accuracy and velocity of measurements. Eight scans were performed at 3 stations to acquire coordinates of almost 4 million points. The results from the acquired data are a high accuracy Digital Elevation Model (DEM) and the reconstruction of the joint geometry. The former is used for inventory of rockfalls and for more accurate rockfall simulation (trajectories and velocities). The latter allows us to model the geometry and volume of the source area in recent rockfalls. Our findings suggest that TLS technology could be a tool of reference in rockfall studies in the near future.     

Debris-flow simulations on Cheekye River, British Columbia

Cheekye River fan is the best-studied fan complex in Canada. The desire to develop portions of the fan with urban housing triggered a series of studies to estimate debris-flow risk to future residents. A recent study (Jakob and Friele 2010) provided debris-flow frequency-volume and frequency-discharge data, spanning 20-year to 10,000-year return periods that form the basis for modeling of debris flows on Cheekye River. The numerical computer model FLO-2D was chosen as a modelling tool to predict likely flow paths and to estimate debris-flow intensities for a spectrum of debris-flow return periods. The model is calibrated with the so-called Garbage Dump debris flow that occurred some 900  years ago. Field evidence suggests that the Garbage Dump debris flow has a viscous flow phase that deposited a steep-sided debris plug high in organics in centre fan, which then deflected a low-viscosity afterflow that travelled to Squamish River with slowly diminishing flow depths. The realization of a two-phase flow led to a modelling approach in which the debris-flow hydrograph was split into a high viscosity and low viscosity phase that were modelled in chronologic sequence as two separate and independent modelling runs. A perfect simulation of the Garbage Dump debris flow with modelling is not possible because the exact topography at the time of the event is, to some degree, speculative. However, runout distance, debris deposition and deposit thickness are well known and serve as a good basis for calibration. Predictive analyses using the calibrated model parameters suggest that, under existing conditions, debris flows exceeding a 50-year return period are likely to avulse onto the southern fan sector, thereby damaging existing development and infrastructure. Debris flows of several thousand years return period would inundate large portions of the fan, sever Highway 99, CN Rail, and the Squamish Valley road and would impact existing housing development on the fan. These observations suggest a need for debris-flow mitigation for existing and future development alike.     

Earthquake slip estimation from the scarp geometry of Himalayan Frontal Thrust, western Himalaya: implications for seismic hazard assessment

Geometric and kinematic analyses of minor thrusts and folds, which record earthquakes between 1200 AD and 1700 AD, were performed for two trench sites (Rampur Ghanda and Ramnagar) located across the Himalayan Frontal Thrust (HFT) in the western Indian Himalaya. The present study aims to re-evaluate the slip estimate of these two trench sites by establishing a link between scarp geometry, displacements observed very close to the surface and slip at deeper levels. As geometry of the active thrust beneath the scarp is unknown, we develop a parametric study to understand the origin of the scarp surface and to estimate the influence of ramp dip. The shortening estimates of Rampur Ghanda trench by line length budget and distance–displacement (D–d) method show values of 23 and 10–15 %, respectively. The estimate inferred from the later method is less than the line length budget suggesting a small internal deformation. Ramnagar trench shows 12 % shortening by line length budget and 10–25 % by the D–d method suggesting a large internal deformation. A parametric study at the trenched fault zone of Rampur Ghanda shows a slip of 16 m beneath the trailing edge of the scarp, and it is sufficient to raise a 8-m-high scarp. This implies that the Rampur Ghanda scarp is balanced with a single event with 7.8-m-coseismic slip in the trenched fault zone at the toe of the scarp, 8–15 % mean deformation within the scarp and 16-m slip at depth along a 30° ramp for a pre-1400 earthquake event. A 16-m slip is the most robust estimate of the maximum slip for a single event reported previously by trench studies along the HFT in the western Indian Himalaya that occurred between 1200 AD and 1700 AD. However, the Ramnagar trenched fault zone shows a slip of 23 m, which is larger than both line length and D–d methods. It implies that a 13-m-high scarp and 23-m slip beneath the rigid block may be ascribed to multiple events. It is for the first time we report that in the south-eastern extent of the western Indian Himalaya, Ramnagar scarp consists of minimum two events (i) pre-1400 AD and (ii) unknown old events of different lateral extents with overlapping ruptures. If the more optimistic two seismic events scenario is followed, the rupture length would be at least 260 km and would lead to an earthquake greater than Mw 8.5.     

基于HBP本构模型的泥石流动力过程SPH数值模拟

本构模型是描述泥石流流变特性的关键,也是决定其动力过程数值模拟准确性的核心问题之一。泥石流流体属多相混合物,现有的研究已证实其存在剪切增稠或剪切变稀的现象,传统基于Bingham及Cross线性本构关系的数值模型难以准确描述泥石流流变特性。文中探讨了Bingham模型在低剪应变率下的数值发散问题,在光滑粒子流体动力学（SPH）方法框架上建立了整合Herschel-Bulkley-Papanastasiou(HBP）本构关系的稀性泥石流动力过程三维数值模型。相比传统基于浅水波假设的二维数值模型,所述方法从三维尺度建立SPH形式下的泥石流浆体纳维?斯托克斯方程并进行数值求解,可获取泥石流速度场时空分布及堆积形态,同时采用HBP本构关系描述泥石流流变特性,能在确保数值收敛的前提下反映泥石流流体在塑性屈服过渡段及大变形状态下应力?应变的非线性变化。为验证提出方法的合理性,结合小型模型槽实验观测进行了对比,结果表明数值模拟与实测结果基本吻合。     

Debris-flow hazards on tributary junction fans,Chitral, Hindu Kush Range,northern Pakistan

The Chitral district of northern Pakistan lies in the eastern Hindu Kush Range. The population in this high-relief mountainous terrain is restricted to tributary-junction fans in the Chitral valley. Proximity to steep valley slopes renders these fans prone to hydrogeomorphic hazards, including landslides, floods and debris flows.This paper focuses on debris-flow hazards on tributary-junction fans in Chitral. Using field observations, satellite-image analyses and a preliminary morphometry, the tributary-junction fans in the Chitral valley are classified into (1) discrete and (2) composite. The discrete fans are modern-day active landforms and include debris cones associated with ephemeral gullies, debris fans associated with ephemeral channels and alluvial fans formed by perennial streams. The composite fans are a collage of sediment deposits of widely different ages and formed by diverse alluvial-fan forming processes. These include fans formed predominantly during MIS-2/Holocene interglacial stages superimposed by modern-day alluvial and debris fans. Composite fans are turned into relict fans when entrenched by modern-day perennial streams. These deeply incised channels discharge their sediment load directly into the trunk river without significant spread on fan surface. In comparison, when associated with ephemeral streams, active debris fans develop directly at composite-fan surfaces. Major settlements in Chitral are located on composite fans, as they provide large tracts of leveled land with easy accesses to water from the tributary streams. These fan surfaces are relatively more stable, especially when they are entrenched by perennial streams (e.g., Chitral, Ayun, and Reshun). When associated with ephemeral streams (e.g., Snowghar) or a combination of ephemeral and perennial streams (e.g., Drosh), these fans are subject to frequent debris-flow hazards.Fans associated with ephemeral streams are prone to high-frequency (∼10 years return period) debris-flow hazards. By comparison, fans associated with perennial streams are impacted by debris-flow hazards during exceptionally large events with return periods of ∼30 years. This study has utility for quick debris-flow hazard assessment in high-relief mountainous regions, especially in arid- to semi-arid south-central Asia where hazard zonation maps are generally lacking.