A Debris-flow Simulation Model for the Evaluation of Protection Structures

Debris flow is the flow of a solid-fluid mixture and in this investigation it is treated as the flow of a continuum in routing. A numerical model is proposed describing debris flow including erosion and deposition processes with suitable boundary con-ditions. The numerical model is applied to evaluate the effects of protection structures against debris flow caused by heavy rainfall on the Shen-Mu Stream of Nantou County located in central Taiwan. Simulation results indicated that the proposed model can offer useful pre-planning guidelines for engineers.     

Characteristics of a drainage channel with staggered indented sills for controlling debris flows

The characteristics of a new type of drainage channel with staggered indented sills for controlling debris flows were studied. The intermediate fluid in the non-viscous debris flow exhibited a helical movement, whereas the fluid near the sidewall had a stop-start movement pattern; the viscous debris flow exhibited a stable structure between the indented sills. The experimental results indicate that the mean velocity of the debris flow increased with increasing channel gradients, and the debris flow velocity was slightly affected by the angle of the sills. The average velocity of the non-viscous debris flow increased in the range of(0.5–1.5) interval between the indented sills, whereas the average velocity of the viscous debris flow increased initially and then decreased in the range of(0.75–1.25) interval between the indented sills. The depth of the non-viscous debris flow tended to gradually increase as the channel gradients increased, whereas the depth of the viscous debris flow gradually decreased as the channel gradients increased. When the discharge of the debris flow was constant, the angle and the interval between the indented sills had a slight effect on the depth of the viscous debris flow, whereas the depth of the non-viscous debris flow exhibited a different trend, as the sill angles and intervals were varied.     

Types and Causes of Debris Flow Damage to Drainage Channels in the Wenchuan Earthquake Area

Debris flows are among the most common geological disasters in China,and have been particularly frequent in Sichuan Province since the Wenchuan earthquake on 12 May 2008.The construction of debris flow drainage channels is a countermeasure used to distribute debris flow fans,and these channels play a critical role in the mitigation and prevention of damage resulting from debris flows.Under field conditions,the useful life of drainage channels can be greatly shortened as a result of strong abrasions to the drainage structure caused by the debris flow.Field investigations have shown that the types of damage to drainage channels include（a） erosion caused by hyper-concentrated silt flow,（b） impact fractures and foundation scour at the groundsills of the drainage channel,（c） destruction of the drainage channel outlet,and（d） destruction of the drainage channel caused by debris flow abrasion.In addition,based on the destruction of the drainage channel during the debris flow drainage process,a new type of drainage channel with energy dissipation components was proposed and applied in a steep,narrow gully for debris flow mitigation.Moreover,design and engineering repair recommendations for drainage channels are provided as a reference for repairing the damage to the channel.The results can provide an important reference for the effective repair and optimal design of drainage channels.     

Curvature derived from LiDAR digital elevation models as simple indicators of debris-flow susceptibility

To mitigate the damage caused by debris flows resulting from heavy precipitation and to aid in evacuation plan preparation, areas at risk should be mapped on a scale appropriate for affected individuals and communities. We tested the effectiveness of simply identifying debris-flow hazards through automated derivation of surface curvatures using LiDAR digital elevation models. We achieved useful correspondence between plan curvatures and areas of existing debris-flow damage in two localities in Japan using the analysis of digital elevation models(DEMs). We found that plan curvatures derived from 10 m DEMs may be useful to indicate areas that are susceptible to debris flow in mountainous areas. In residential areas located on gentle sloping debris flow fans, the greatest damage to houses was found to be located in the elongated depressions that are connected to mountain stream valleys. Plan curvaturederived from 5 m DEM was the most sensitive indicators for susceptibility to debris flows.     

Characteristic rainfall for warning of debris flows

A characteristic rainfall is introduced to overcome the difficulties encountered in determining a critical rainfall value for triggering debris flow.The characteristic value is defined as the rainfall at which debris-flow occurrence probability shows a rapid increase,and can be used as a warning rainfall threshold for debris flows.Investigation of recorded debris flows and 24-hour rainfall data at Jiangjia basin,Yunnan Province,in southwestern China,demonstrates the existence of such a characteristic rainfa...     

Numerical simulation on a tremendous debris flow caused by Typhoon Morakot in the Jiaopu Stream,Taiwan

In August 2009,Typhoon Morakot brought a large amount of rainfall with both high intensity and long duration to a vast area of Taiwan.Unfortunately,this resulted in a catastrophic landslide in Hsiaolin Village,Taiwan.Meanwhile,large amounts of landslides were formed in the Jiaopu Stream watershed near the southeast part of the Hsiaolin Village.The Hsiaolin Village access road（Provincial Highway No.21 and Bridge No.8） was completely destroyed by the landslide and consequent debris flow.The major scope of this study is to apply a debris flow model to simulate the disaster caused by the debris flow that occurred in the Jiaopu Stream during Typhoon Morakot.According to the interviews with local residents,this study applied the destruction time of Bridge No.8 and Chen＇s house to verify the numerical debris flow model.By the spatial rainfall distributions information,the numerical simulations of the debris flow are conducted in two stages.In the first stage（before the landslide-dam failure）,the elevation of the debris flow and the corresponding potential damages toward residential properties were investigated.In the second stage（after the landslidedam failure）,comparisons of simulation results between the longitudinal and cross profiles of the Jiaopu Stream were performed using topographic maps and satellite imagery.In summary,applications of the adopted numerical debris flow model have shown positive impact on supporting better understanding of the occurrence and movement of debris flow processes.     

Analysis on the dynamical process of Donghekou rockslide-debris flow triggered by 5.12 Wenchuan earthquake

Among the geo-hazards caused by the great Wenchuan Earthquake, the rapid and long runout rockslide-debris flow is of primary concern due to the large volume of displaced material and the resultant catastrophic impacts to the landscape and socioeconomic structure. In order to analyze the dynamical process of this kind of geo-hazard, the Donghekou rockslide-debris flow is given as an example in this paper. This event, which killed 780 people, initiated at an elevation of 1300 m with a total long run-out distance of more than 2400 m. The slide mass is mainly composed of dolomite limestone and siliceous limestone of Sinian system, together with carbon slate and phyllite of Cambrian. During the processes from slide initiation to the final cessation of slide movement, five dynamic stages took place, here identified as the initiation stage, the acceleration of movement stage, the air-blast effect stage, the impact and redirection stage and the long runout slidematerial accumulation stage. Field investigations indicate that due to the effects of the earthquake, the dynamics of the Donghekou rockslide-debris flow are apparently controlled by geologic and tectonic conditions, the local geomorphological aspects of the terrain, and the microstructural and macroscopic mechanical properties of rocks which compose the slide mass. These three main factors which dictate the Donghekou rockslide-debris flow dynamics are discussed in detail in this paper, and significant results of field investigations and tests of materials are presented. The above dynamical processes are analyzed in this paper, and some useful conclusions have been gained.     

Characteristics of earthquake-triggered landslides and post-earthquake debris flows in Beichuan County

Field investigations and aerial photography after the earthquake of May 12,2008 show a large number of geo-hazards in the zone of extreme earthquake effects.In particular,landslides and debris flows,the geo-hazards that most threaten post-disaster reconstruction,are widely distributed.We describe the characteristics of these geo-hazards in Beichuan County using high-resolution remote sensing of landslide distribution,and the relationships between the area and volume of landslides and the peak-discharges of debris flows both pre-and post-earthquake.The results show:1) The concentration(defined as the number of landslide sources per unit area:Lc) of earthquaketriggered landslides is inversely correlated with distance from the earthquake(DF) fault.The relationship is described by the following equation:Lc = 3.2264exp(-0.0831DF)(R2 = 0.9246);2) 87 % of the earthquake-triggered landslides were less than 15×104 m2 in area,and these accounted only for 50% of the total area;84% of the landslide volumes were less than 60×104 m3,and these accounted only for 50% of the total volume.The probability densities of the area and volume distributions are correlated:landslide abundance increases with landslide area and volume up to maximum values of 5 × 104 m2 and 30 × 104 m3,respectively,and then decreases exponentially.3) The area(AL) and volume(VL) of earthquake-triggered landslides are correlated as described with the following equation:VL=6.5138AL1.0227(R2 = 0.9131);4) Characteristics of the debris flows changed after the earthquake because of the large amount of landslide material deposited in the gullies.Consequently,debris flow peak-discharge increased following the earthquake as described with the following equation:Vpost = 0.8421Vpre1.0972(R2 = 0.9821)(Vpre is the peak discharge of pre-earthquake flows and the Vpost is the peak discharge of post-earthquake flows).We obtained the distribution of the landslides based on the above analyses,as well as the magnitude of both the landslides and the post-earthquake debris flows.The results can be useful for guiding post-disaster reconstruction and recovery efforts,and for the future mitigation of these geo-hazards.However,the equations presented are not recommended for use in site-specific designs.Rather,we recommend their use for mapping regional seismic landslide hazards or for the preliminary,rapid screening of sites.     

Empirical-statistical models based on remote sensing for estimating the volume of landslides induced by the Wenchuan earthquake

The Wenchuan Ms 8.0 earthquake on May 12, 2008 induced a huge number of landslides. The distribution and volume of the landslides are very important for assessing risks and understanding the landslide - debris flow - barrier lake - bursts flood disaster chain. The number and the area of landslides in a wide region can be easily obtained by remote sensing technique, while the volume is relatively difficult to obtain because it requires some detailed geometric information of slope failure surface and sub-surface. Different empirical models for estimating landslide volume were discussed based on the data of 107 landslides in the earthquake-stricken area. The volume data of these landslides were collected by field survey. Their areas were obtained by interpreting remote sensing images while their apparent friction coefficients and height were extracted from the images unifying DEM (digital elevation model). By analyzing the relationships between the volume and the area, apparent friction coefficients, and the height, two models were established, one for the adaptation of a magnitude scale landslide events in a wide range of region, another for the adaptation in a small scope. The correlation coefficients (R2) are 0.7977 and 0.8913, respectively. The results estimated by the two models agree well with the measurement data.     

Initiation and development of water film by seepage

When water seeps upwards through a saturated soil layer,the soil layer may become instability and water films occur and develop.Water film serves as a natural sliding surface because of its very small friction.Accordingly,debris flow may happen.To investigate this phenomenon,a pseudothree-phase media is presented first.Then discontinuity method is used to analyze the expansion velocity of water film.Finally,perturbation method is used to analyze the case that a water flow is forced to seep upwards through the soil layer while the movement of the skeleton may be neglected relative to that of water.The theoretical evolutions of pore pressure gradient,effective stress,water velocity,the porosity and the eroded fine grains are obtained.It can be seen clearly that with the erosion and redeposited of fine grains,permeability at some positions in the soil layer becomes smaller and smaller and,the pore pressure gradient becomes bigger and bigger,while the effective stress becomes smaller and smaller.When the effective stress equals zero,e.f.liquefaction,the water film occurs.It is shown also that once a water film occurs,it will be expanded in a speed of U(t)(1-ε).     

Study on the Rainfall and Aftershock Threshold for Debris Flow of Post-earthquake

Due to the special condition of provenance and disaster environment after 5·12 Earthquake, the probability and conditions of the occurrence of gully debris flow change greatly after the event, which make it difficult to prevent disaster effectively. In this study the hydrological model of ground water table in loose sediment is established. According to infinite slope theory, the safety factor of deposits is defined as the ratio of resistance force to driving force. The starting condition of post-earthquake...     

Debris flows risk analysis and direct loss estimation: the case study of Valtellina di Tirano,Italy

andslide risk analysis is one of the primary studies providing essential instructions to the subsequent risk management process. The quantification of tangible and intangible potential losses is a critical step because it provides essential data upon which judgments can be made and policy can be formulated. This study aims at quantifying direct economic losses from debris flows at a medium scale in the study area in Italian Central Alps. Available hazard maps were the main inputs of this study. These maps were overlaid with information concerning elements at risk and their economic value. Then, a combination of both market and construction values was used to obtain estimates of future economic losses. As a result, two direct economic risk maps were prepared together with risk curves, useful to summarize expected monetary damage against the respective hazard probability. Afterwards, a qualitative risk map derived using a risk matrix officially provided by the set of laws issued by the regional government, was prepared. The results delimit areas of high economic as well as strategic importance which might be affected by debris flows in the future. Aside from limitations and inaccuracies inherently included in risk analysis process, identification of high risk areas allows local authorities to focus their attention on the “hot-spots”, where important consequences may arise and local （large） scale analysis needs to be performed with more precise cost-effectiveness ratio. The risk maps can be also used by the local authorities to increase population’s adaptive capacity in the disaster prevention process.     

Shaking table tests and numerical simulation of dynamic properties of underground structures

It is considered that the damage of the underground structures caused by earthquakes is minor for a long time. However, the catastrophic damages induced by several recent earthquakes ( e. g. Kobe earthquake in 1995) revealed that the study on the dynamic properties of the underground structures is indispensable. The dynamic behavior and damage mechanism of underground structure are analyzed by using shaking table tests ( both shallow- and deep-buried) and numerical simulation ( 3D FEM) including horizontal and vertical input motions, individually and simultaneously. From the results, the underground structure collapsed due to strong horizontal forces although vertical deformation is not negligible. The vertical excitation increases the response of structure, especially the stress and shear stress at the upper section; the soil influenced the property of soil-structure system. In the same excitation, the response in shallow-buried test is larger than deep case. Both overburden and vertical earthquake play important roles in the response of structure and those are two critical aspects in the design of the large-span underground structures, such as subway stations.     

Shaking table tests and numerical simulation of dynamic properties of underground structures

It is considered thai the damage of the underground structures caused by earthquakes is minor for a long time. However, the catastrophic damages induced by several recent earthquakes （e. g. Kobe earthquake in 1995 ） revealed that the study on the dynamic properties of the underground structures is indispensable. The dynamic behavior and damage mechanism of underground structure are analyzed by using shaking table tests （ both shallow-and deep-buried） and numerical simulation （3D FEM） including horizontal and vertical input motions, individually and simultaneously. From the results, the underground structure collapsed due to strong horizontal forces although vertical deformation is not negligible. The vertical excitation increases the response of structure, especially the stress and shear stress at the upper section; the soil influenced the property of soilstructure system. In the same excitation, the response in shallow-buried test is larger than deep case. Both overburden and vertical earthquake play important roles in the response of structure and those are two critical aspects in the design of the large-span underground structures, such as subway stations.     

Damming of large river by debris flow: Dynamic process and particle composition

The frequency and extent of debris flows have increased tremendously due to the extreme weather and the Wenchuan earthquake on May 12, 2008. Previous studies focused on the debris flow from gullies damming the mountain streams. In this paper, an equation for the run-out distance of debris flow in the main river is proposed based on the dynamic equation of debris flow at different slopes given by Takahashi. By undertaking field investigations and flume experiments, a new calculation method of the volume of debris flow damming large river is obtained. Using the percolation theory and the renormalization group theory it was deduced that the large particles should comprise more than 50% for forming a stable debris flow dam. Hence, the criteria of damming large river by debris flow is presented in terms of run-out distance and grain composition which was then validated through the event of damming river by debris flow at Gaojia gully, the upper reaches of the Minjiang River, Sichuan, China, on July 3, 2011.     

基于Prophet-RF模型的GNSS高程坐标时间序列预测分析

针对GNSS高程坐标时间序列非平稳性与非线性等特点,在深入分析Prophet模型与随机森林(ran-dom forest,RF)模型特性的基础上,构建了Prophet-RF组合预测模型,解决了Prophet模型对时间序列非线性部分预测能力较弱的缺陷,且该组合模型具有较强的鲁棒性.本文选用BJFS站高程方向的连续观测数据...     

基于GIS的地下水流有限差数值模拟参数自动提取研究

本文根据地下水流有限差模拟有关参数的空间分布特点,将其划分为点状、线状与面状空间分布参数三种类型。利用GIS的空间分析功能,研究基于GIS的点状、线状与面状空间分布参数自动提取的技术路线。通过点、线、面三者之间空间位置关系的拓扑分析,确定地下水开采井、水位动态监测井、线状与面状水系、模拟区域边界、面状参数分布区与模型空间离散格网之间的空间位置关系。经过矢量数据与栅格数据互相转换,实现地下水流有限差数值模拟有关参数的自动提取。结合地下水流有限差分数值模拟有关参数数据文件的结构,将基于GIS提取的栅格结构数据文件自动转换成模型所需结构的数据文件,达到模型参数数据文件的自动组织与提高地下水流数值模拟的效率目的。     

“等待时间”强余震预测方法的改进与应用——以尼泊尔地震为例

在对"等待时间法"进行残差结构分析的基础上提出基于加权最小二乘拟合的模型改进方法,并将其应用于尼泊尔地震的强余震预测。结果表明,由模型误差转换至时间误差具有ln10·Δt倍的缩放效果,选择主震后3d内的余震样本进行强余震预测结果较为可信。实际震例表明,加权最小二乘模型可以有效降低模型预测误差,缩小预测值置信区间,从而提高预测的实用性。     

Succession Features and Dynamic Simulation of Subalpine Forest in the Gongga Mountain, China

The Gongga Mountain of eastern Tibet Plateau is a representative of the alpine regions with high peaks and deep valleys. Climate change over the last thousand years has controlled the dynamics of glacier and debris flow occurrence, which resulted in substantial changes in the mountainous environment. The authors surveyed the community structure of primary forests in Gongga Mountain and forest successign processes in woodland plots. The changing features in the subalpine environment are discussed in this paper. Tree species and sizes between the glacier shrinking areas and debris flow fans in Hailuogou Valley are compared. The pioneer species that settle in debris flow fans and the glacier shrinking areas are Salix spp. and Populus purdomii. Abies fabri and Picea brachytyla are the climax tree species. The succession process of primary vegetation in Hailuogou (2700 ～ 3200 m) can be divided into four stages:     

Fine-resolution simulation of surface current and sea ice in the Arctic Mediterranean Seas

A fine-resolution model is developed for ocean circulation simulation in the National Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG), Chinese Academy of Sciences, and is applied to simulate surface current and sea ice variations in the Arctic Mediterranean Seas. A dynamic sea ice model in elastic-viscous-plastic rheology and a thermodynamic sea ice model are employed. A 200-year simulation is performed and a dimatological average of a 10-year period (141st-150th) is presented with focus on sea ice concentration and surface current variations in the Arctic Mediterranean Seas. The model is able to simulate well the East Greenland Current, Beaufort Gyre and the Transpolar Drift, but the simulated West Spitsbergen Current is small and weak. In the March climatology, the sea ice coverage can be simulated well except for a bit more ice in east of Spitsbergen Island. The result is also good for the September scenario except for less ice concentration east of Greenland and greater ice concentration near the ice margin. The extra ice east of Spitsbergen Island is caused by sea ice current convergence forced by atmospheric wind stress.