Evaluation of the Economic Loss Caused by Zhouqu Debris Flow

[Objective] The aim of this study was to evaluate the economic loss caused by Zhouqu debris flow. [Method] After the large debris flows happened on August 7, 2010 in Zhouqu, Gansu Province, we collected data at the first time after the disaster, and then built an assessment model to estimate the potential economic losses. [Result] The total loss reached 16.57×10^2 million Yuan, in which indirect economic loss was up to 2.42×10^2 million yaun while the actual direct economic loss was around 14.15×10^2 million Yuan. [Conclusion] The proportional coefficient method is a rapid and efficient method for evaluating the indirect loss caused by disasters.

Prediction of Debris-flow Danger Area by Combining Hydrological and Inundation Simulation Methods

Debris flows have caused serious human casualties and economic losses in the regions strongly affected by the Ms8.0 Wenchuan earthquake of 2oo8. Debris flow mitigation and risk assessment is a key issue for reconstruction. The existing methods of inundation simulation are based on historical disasters and have no power of prediction. The rain- flood method can not yield detailed flow hydrograph and does not meet the need of inundation simulation. In this paper, the process of water flow was studied by using the Arc-SCS model combined with hydraulic method, and then the debris flow runoff process was calculated using the empirical formula combining the result from Arc-SCS. The peak discharge and runoff duration served as input of inundation simulation. Then, the dangerous area is predicted using kinematic wave method and Manning equation. Taking the debris flow in Huashiban gully in Beichuan County, Sichuan Province, China on 24 Sep. 2oo8 as example, the peak discharge of water flow and debris flow were calculated as 35.52 m3·s-1 and 215.66 m3·s-, with error of 4.15% compared to the measured values. The simulated area of debris-flow deposition was 161,500 m2, vs. the measured area of 144,097 m2, in error of 81.75%. The simulated maximum depth was 12.3 m, consistent with the real maximum depth between lO and 15 m according to the field survey. The minor error is mainly due to the flow impact on buildings and variations in cross-section configuration. The present methodology can be applied to predict debrisflow magnitude and evaluate its risk in other watersheds inthe earthquake area.

The Critical Rainfall Characteristics for Torrents and Debris Flows in the Wenchuan Earthquake Stricken Area

Critical rainfall assessment is a very important tool for hazard management of torrents and debris flows in mountainous areas. The Wenchuan Earthquake 2008 caused huge casualties and property damages in the earthquake-stricken area,which also generated large quantities of loose solid materials and increased occurrence probabilities of debris flows. There is an urgent need to quantify the critical rainfall distribution in the area so that better hazard management could be planned and if real time rainfall forecast is available,torrent and debris flow early-warning could be issued in advance. This study is based on 49-year observations (1954-2003) of up to 678 torrent and debris flow events. Detailed contour maps of 1 hour and 24 hour critical rainfalls have been generated (Due to the data limitation,there was insufficient 10 minute critical rainfall to make its contour map). Generally,the contour maps from 1 hour and 24 hours have similar patterns. Three zones with low,medium and high critical rainfalls have been identified. The characteristics of the critical rainfall zones are linked with the local vegetation cover and land forms. Further studies and observations are needed to validate the finding and improve the contour maps.

Comparison between FLO-2D and Debris-2D on the Application of Assessment of Granular Debris Flow Hazards with Case Study

Numerical simulation has been widely applied to the assessment of debris flow hazards. In East Asia and especially Taiwan, the most widely used numerical programs are FLO-2D and Debris-aD. Although these two programs are applied to the same engineering tasks, they are different in many aspects. These two programs were compared according to their fundamental theories, input and output data, computational algorithms and results. Using both programs, the simulations of a real debris flow with abundant granular material induced by landslides at Xinfa village in southern Taiwan are performed for comparison. The simulation results show that Debris- 2D gives better assessment in hazard area delineating and flow depth predicting. Therefore, Debris-2D is better for simulation of granular debris flows.

Risk Assessment of Highways Affected by Debris Flows in Wenchuan Earthquake Area

Since the Wenchuan earthquake in China on May 12th, 2008, highways in earthquake-affected areas have been frequently interrupted by debris flows. We analyzed the hazard effect modes and damage processes along highways and developed three key indexes, scale of debris flows, deposits on highways and river blockage, to describe quantitatively the highway disasters. By combining the empirical methods and the actual terrain conditions, we proposed new methods to determine the value of hazard indexes. In addition, we used the economic value and resistance of highway as vulnerability assessment indexes, then determined the specific subindexes for the subgrade, bridges and culverts, and developed a way for the quantified vulnerability zoning. Moreover, we proposed the assessment and mapping methods for highway risk. The risk is described into 5 grades： extremely low risk, low risk, middle risk, high risk and extremely high risk. We applied these methods in a case study carried out on provincial highway S3o3 from Yingxiu Town to Wolong Town, in Wenchuan County. Analysis of debris flow risk for the whole highway, showed that the total length of highway in extremely low risk area was 28.26 km, 4.83 km in low risk area, 8.0 km in middle risk area, 3.65 km in high risk area, and 3.06 km in extremely high risk area. The assessment results are consistent with the field survey data which reflected the disaster situation. This risk method can be used objectively to evaluate the debris-flow risk along highways, and is useful for highway reconstruction in mountainous areas suffering from active debris flows.

Debris Flow Susceptibility Assessment Using a Probabilistic Approach:A Case Study in the Longchi Area, Sichuan Province, China

The Longchi area with the city of Dujiangyan, in the Sichuan province of China, is composed of Permian stone and diorites and Triassic sandstones and mudstones intercalated with slates. An abundance of loose co-seismic materials were present on the slopes after the May 12, 2008 Wenchuan earthquake, which in later years served as source material for rainfall-induced debris flows or shallow landslides. A total of 48 debris flows, all triggered by heavy rainfall on 13th August 20l0, are described in this paper. Field investigation, supported by remote sensing image interpretation, was conducted to interpret the co-seismic landslides in the debris flow gullies. Specific characteristics of the study area such as slope, aspect, elevation, channel gradient, lithology, and gully density were selected for the evaluation of debris flow susceptibility. A score was given to all the debris flow gullies based on the probability of debris flow occurrence for the selected factors. In order to get the contribution of the different factors, principal component analyses were applied. A comprehensive score was obtained for the 48 debris flow gullies which enabled us to make a susceptibility map for debris flows with three classes. Twenty-two gullies have a high susceptibility, twenty gullies show a moderate susceptibility and six gullies have a low susceptibility for debris flows.

The Optimal Cross-section Design of the “Trapezoid-V” Shaped Drainage Canal of Viscous Debris Flow

Debris flow drainage canal is one of the most widely used engineering measures to prevent and manage debris flow hazards.The shape and the sizes of the cross-section are important parameters when design debris flow drainage canal.Therefore,how to design the appropriate shape and sizes of the cross-section so that the drainage canal can have the optimal drainage capacity is very important and few researched at home and abroad.This study was conducted to analyze the hydraulic condition of a Trapezoid-V shaped drainage canal and optimize its cross-section.By assuming characteristic sizes of the cross-section,the paper deduced the configuration parameter of the cross-section of a Trapezoid-V shaped debris flow drainage canal.By theory analysis,it indicates that the optimal configuration parameter is only related to the side slope coefficient and the bottom transverse slope coefficient.For this study,the Heishui Gully,a first-order tributary of the lower Jinsha River,was used as an example to design the optimal cross-section of the drainage canal of debris flow.

Slope Debris Flows in the Wenchuan Earthquake Area

Avalanches and landslides,induced by the Wenchuan Earthquake on May 12,2008,resulted in a lot of disaggregated,solid material on slopes that could be readily mobilized as source material for debris flows.Rainstorms triggered numerous slope debris flows with great damage to highways and rivers over the subsequent two years.Slope debris flows(as opposed to channelized debris flows) are defined as phenomena in which high-concentration mixtures of debris and water flow down slopes for short distances to highways and river banks.Based on field investigations and measurements of 19 slope debris flows,their main characteristics and potential mitigation strategies were studied.High rainfall intensity is the main triggering factor.Critical rainfall intensities for simultaneous occurrence of single,several and numerous slope debris flow events were 20 mm/day,30mm/day,and 90 mm/day,respectively.Field investigations also revealed that slope debris flows consist of high concentrations of cobbles,boulders and gravel.They are two-phase debris flows.The liquid phase plays the role of lubrication instead of transporting medium.Solid particles collide with each other and consume a lot of energy.The velocities of slope debris flows are very low,and their transport distances are only several tens of meters.Slope debris flows may be controlled by construction of drainage systems and by reforestation.

Combined Impacts of Antecedent Earthquakes and Droughts on Disastrous Debris Flows

This paper describes a study on the combined impacts of antecedent earthquakes and droughts on disastrous debris flows.This is a novel attempt in quantifying such impacts using the effective peak acceleration(EPA)(to represent earthquakes) and standardized precipitation index(SPI)(to represent droughts).The study is based on the analysis of 116 disastrous debris flow events occurred in China's Mainland in the last 100 years covering a wide spectrum of climate types and landforms.It has been found that the combined impacts from earthquakes and droughts on disastrous debris flows do exist and vary from low to very high according to different climate conditions and terrains.The impacts from earthquakes increase with the increased terrain relief,and the impacts from droughts are strongest in semi-humid climate condition(with reduced impacts in humid and semi-arid /arid climate conditions).Hypothetical explanations on the study discoveries have been proposed.This study reveals the possible reasons for the disastrous debris flow distributions around the world and has significant implications in paleo-climate-seismicanalysis and disastrous debris flow risk management.

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.

Discrete Element Modeling of Debris Avalanche Impact on Retaining Walls

In China,gravity retaining walls are widely used as protection structures against rockfalls,debris flows and debris avalanches along the roads in mountainous areas.In this paper,the Discrete Element Method(DEM) has been used to investigate the impact of granular avalanches and debris flows on retaining walls.The debris is modeled as two dimensional circular disks that interact through frictional sliding contacts.The basic equations that control the deformation and motion of the particles are introduced.A series of numerical experiments were conducted on an idealized debris slide impacting a retaining wall.The parametric study has been performed to examine the influences of slope geometry,travel distance of the sliding mass,wall position,and surface friction on the impact force exerted on the wall.Results show that:1) the force achieves its maximum value when slope angle is equal to 60°,as it varies from 30° to 75°;2) an approximate linear relationship between the impact force and the storage area length is determined.

Monitoring and Recognition of Debris Flow Infrasonic Signals

Low frequency infrasonic waves are emitted during the formation and movement of debris flows, which are detectable in a radius of several kilometers, thereby to serve as the precondition for their remote monitoring.However, false message often arises from the simple mechanics of alarms under the ambient noise interference.To improve the accuracy of infrasound monitoring for early-warning against debris flows, it is necessary to analyze the monitor information to identify in them the infrasonic signals characteristic of debris flows.Therefore, a large amount of debris flow infrasound and ambient noises have been collected from different sources for analysis to sum up their frequency spectra, sound pressures, waveforms, time duration and other correlated characteristics so as to specify the key characteristic parameters for different sound sources in completing the development of the recognition system of debris flow infrasonic signals for identifying their possible existence in the monitor signals.The recognition performance of the system has been verified by simulating tests and long-term in-situ monitoring of debris flows in Jiangjia Gully,Dongchuan, China to be of high accuracy and applicability.The recognition system can provide the local government and residents with accurate precautionary information about debris flows in preparation for disaster mitigation and minimizing the loss of life and property.

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 earthquake- triggered 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× 10^4 m2 in area, and these accounted only for 5o% of the total area; 84% of the landslide volumes were less than 60×10^4 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 ×10^4m2 and 30 ×10^4 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 = 0.9231）; 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 = 0.8421Vprel-0972 （R2 = 0.9821） （Vpre is the peak discharge ofpre-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.

Comparison of satellite-estimated and model-forecasted rainfall data during a deadly debris-flow event in Zhouqu, Northwest China

The data of several rainfall products, including those estimated from satellite measurements and those forecasted via numerical weather modeling, for a severe debris-flow event in Zhouqu, Northwest China, are compared and analyzed in this paper. The satellite products, including CPC MORPHing technique（CMORPH）, TMPA-RT, and PERSIANN are all near-real-time retrieved with high temporal and spatial resolutions. The numerical weather model used in this paper for precipitation forecasting is WRF. The results show that all three satellite products can basically reproduce the rainfall pattern, distribution, timing, scale, and extreme values of the event, compared with gauge data. Their temporal and spatial correlation coefficients with gauge data are as high as about 0.6, which is statistically significant at 0.01 level. The performance of the forecasted results modeled with different spatial resolutions are not as good as the satellite-estimated results, although their correlation coefficients are still statistically significant at 0.05 level. From the total rainfall and extreme value time series for the domain, it is clear that, from the grid-to-grid perspective, the passive microwave-based CMORPH and TRMM products are more accurate than the infrared-based PERSIANN, while PERSIANN performs very well from the general point of view, especially when considering the whole domain or the whole convective precipitation system. The forecasted data — especially the highest resolution model domain data — are able to represent the total or mean precipitation very well in the research domain, while for extreme values the errors are large. This study suggests that satellite-retrieved and model-forecasted rainfall data are a useful complement to gauge data, especially for areas without gauge stations and areas not covered by weather radars.

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 dose 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 flowtriggering 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.

Catastrophic Debris Flows on July 10^(th) 2013 along the Min River in Areas Seriously-hit by the Wenchuan Earthquake

Over 240 debris flows occurred in hill-slopes, gullies （ indicated those with single-channel） and watersheds （indicated those with tributaries and channels） on July 10th 2013 in the Wenchuan county, and caused 29 casualties and about 633×10^6 USD losses. This work aimed to analyze characteristics, hazards and causes of these events and explore mitigating measures based on field investigation and remote sensing images interpretation. The debris flows contained clay content of 0.1%~3.56%, having densities of 1.72-2.14 t/m^3, velocities of 5.0-m.7 m/s, discharges of 335-2353 m^3/s and sediment yields of 0.10-1.26×10^6 m^3, and also numerously occurred in large watersheds with the area over lo km^2. Large debris flows formed 3 hazard-chains in slopes, gullies, watersheds and rivers, which all evolved in dammed lakes and outburst flood, and 26 dammed lakes and lO newly ones were generated along the rivers of Min and Yuzi. The remarkable spatial difference of loose solid materials accumulation and intense rainfall, with the cumulative of about or more than 150 mm and the hourly of over 16mm, caused debris flows in the sections from Yingxiu to Miansi and Gengda. The damages on buildings, reconstructions, highways,factories and hydro power station originated from the impacting, scouring, burying of debris flows, the submerging of dammed lake and the scouring of outburst flood, and the huge losses came from the ruinous destructions of control engineering works of debris flows as well as the irrational location and low- resistant capabilities of reconstructions. For hazards mitigating of debris flows in long term, the feasible measures for short term, including risk-reassessing of foregone and potential hazard sites, regional alarming system establishing and integrated control in disastrous sites, and middle-long term, including improving reconstruction standard, rationally disposing river channel bed rise and selecting appropriate reconstruction time and plans, were strongly suggested.

Monitoring and Warning of Landslides and Debris Flows Using an Optical Fiber Sensor Technology

Landslides and debris flows are typical geo-hazards which occur in hilly or mountainous regions. Debris flows may result from landslides. Geotechnical instrumentation plays an important role in monitoring and warning of landslides and resulted debris flows. Traditional technologies for monitoring landslides and debris flows have certain limitations. The new optical fiber sensors presented in this paper can overcome those limitations. This paper presents two new optical fiber sensor systems: one is the Fiber Bragg Grating (FBG)-based in-place inclinometer for monitoring landslides and the other is the FBG-based column-net system for monitoring debris flows. This paper presents the calibration results of FBG-based in-place inclinometers in laboratory. It is found that the calibration results are in good agreement with theoretical results. Both the FBG-based in-place inclinometers and the FBG-based column-net system have been installed at a site in Weijiagou valley, Beichuan County, Sichuan Province of China. Some preliminary results have been obtained and reported in the paper. The advantages of the FBG monitoring systems and their potential applications are also presented.

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.

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 rainfall.It was found that the characteristic rainfall corresponds to the daily rainfall of 90% cumulative probability by analyzing the basin's daily rainfall histogram.The result provides a simple and useful method for estimating a debris-flow warning rainfall threshold from the daily rainfall distribution.It was applied to estimate the debris-flow warning rainfall threshold for the Subaohe basin,a watershed in the 2008 Wenchuan earthquake zone with many physical characteristics similar to those of the Jiangjia basin.