Catastrophic debris flows triggered by a 4 July 2013 rainfall in Shimian,SW China: formation mechanism,disaster characteristics and the lessons learned

On 4 July 2013, three catastrophic debris flows occurred in the Hougou, Majingzi, and Xiongjia gullies in Shimian county and produced debris dams and river blockages, resulting in serious casualties and huge economic loss. Though debris flows have been identified prior to the catastrophic events, their magnitudes and destructive power were far beyond early recognition and hazard assessment. Our primary objective for this study was to explore the formation mechanism and typical characteristics and to summarize the lessons learned from these disastrous events in order to avoid the repeat of such disasters in the future. Based on field investigation and imagery interpretation of remote sensing carried out following the catastrophic events, four conclusions were drawn: (1) The catastrophic debris flows were initiated from surface-water runoff, and the triggering factor was attributed to the local intensive rainfall with an hourly intensity of more than 46.7 mm. (2) Entrainment was the most important sediment-supplying method for the debris flow occurrence, and the source materials transported by debris flows from the three gullies were estimated to be about 97?×?10⁴ m³ in volume altogether. (3) As surface-water runoff eroded and entrained hillslope and channel materials persistently, debris flows were characterized by intensive incision at upper or middle reaches and significant magnification effect in flow discharge and volume downstream. Corresponding peak discharge surveyed at the outlets of the Hougou, Majingzi, and Xiongjia gullies was estimated up to 751.0 m³/s, 870.1 m³/s, and 758.7 m³/s, respectively. (4) Debris flows that occurred from the three gullies all belonged to viscous ones and the bulk densities were calculated more than 1.80 g/cm³, indicating a huge carrying capacity and destructive impacting power. In addition, the lessons learned from the catastrophic events were summarized, including recognition and assessment on debris flow hazard and utilization of deposition fan. In this paper, prevention suggestions on debris flow prone valleys with high-vegetation coverage and low occurrence frequency were also put forward. The results of this study contribute to a better understanding on the initiation mechanism, dynamic characteristics, and disaster mitigation of debris flows initiated from intense rainfall and surface-water runoff in mountainous areas.     

Improving mass-wasting inventories by incorporating debris flow topographic signatures

Debris flows are a prevalent and destructive mass-wasting type in many mountainous regions throughout the world, yet the recent identification of a debris flow topographic signature has not been incorporated into landslide inventories. We have detected this signature in a digital elevation model of the mountainous Oconaluftee River basin of the southern Appalachians, USA, where we have conducted mass-wasting inventories. We evaluate the applicability of this topographic signature in debris flow mapping efforts using inventories created by semiautomated classification of topographic derivative and vegetation index maps. Debris flow detection was increased by 12 % when the inventory was limited to the portion of the landscape that exhibits the debris flow topographic signature. The extent of drainages with this topographic signature, which have areas of 6 to 35 km², is corroborated by analyses of channel form, knickpoint and bedrock distributions, and hypsometry. This mass-wasting inventory technique provides a more focused approach to statistically characterize the land surface, which resulted in increased inventory proficiency across a landscape with an extensive and relatively well-documented debris flow history.     

Initiation process of debris flows on different slopes due to surface flow and trigger-specific strategies for mitigating post-earthquake in old Beichuan County, China

The 12 May 2008 Wenchuan earthquake (Ms 8.0) in China, produced an estimated volume of 28 × 10⁸ m³ loosened material, which led to debris flows after the earthquake. Debris flows are the dominant mountain hazards, and serious threat to lives, properties, buildings, traffic, and post-earthquake reconstruction in the earthquake-hit areas. It is very important to understand the debris flow initiation processes and characteristics, for designing debris flow mitigation. The main objective of this article is to examine the different debris flow initiation processes in order to identify suitable mitigation strategies. Three types of debris flow initiation processes were identified (designated as Types A, B, and C) by field survey and experiments. In “A” type initiation, the debris flow forms as a result of dam failure in the process of rill erosion, slope failure, landslide dam, or dam failure. This type of debris flow occurs at the slope of 10 ± 2°, with a high bulk density, and several surges following dam failure. “B” type initiation is the result of a gradual increase in headward down cutting, bank and lateral erosion, and then large amount of loose material interfusion into water flow, which increases the bulk density, and forms the debris flow. This type of debris flow occurs mainly on slopes of 15 ± 3° without surges. “C” type debris flow results from slope failures by surface flow, infiltration, loose material crack, slope failure, and fluidization. This type of debris flow occurs mainly on slopes of 21 ± 4°, and has several surges of debris flow following slope failure, and a high bulk density. To minimize the hazards from debris flows in areas affected by the Wenchuan earthquake, the erosion control measures, such as the construction of grid dams, slope failure control measures, the construction of storage sediment dams, and the drainage measures, such as construction of drainage ditches are proposed. Based on our results, it is recommend that the control measures should be chosen based on the debris flow initiation type, which affects the peak discharge, bulk density and the discharge process. The mitigation strategies discussed in this paper are based on experimental simulations of the debris flows in the Weijia, Huashiban, and Xijia gullies of old Beichuan city. The results are useful for post-disaster reconstruction and recovery, as well as for preventing similar geohazards in the future.     

Characteristics of spatial distribution of debris flow and the effect of their sediment yield in main downstream of Jinsha River,China

Debris flow hazard posts a big threat to the main downstream of Jinsha River where a number of huge power stations are under construction. The characteristics of spatial distribution of debris flows and the effect of their sediment yield on the reservoir areas have been studied. An automatic recognition module was developed to extract the geometry of debris flow channels from remote sensing data. Spatial distribution pattern of debris flows is obtained through combining the inventory database and multi-source remote sensing investigation. The distribution of debris flows has high dependency on the various factors including geology, geomorphology, climate, hydrology and human economic activities. The debris flows distributed in the study area are characterized by group and pair distribution, uncompleted deposition fans, highly controlled by faults and tectonic activities, spatial variation between left bank and right bank, and different subdivisions. The sediment yield caused by debris flow activates is evaluated using multi-year observation data from numerous observation stations. Quantitative studies have been performed on the relationship between the sediment yield and the debris flow area. A relatively fix ratio of 2.6 (×10⁴ t/km²) has been found in different subdivisions of main downstream area which shows that the source of sediment discharged into Jinsha River primarily come from debris flow activities. Another ratio is evaluated to represent the transforming possibility of debris flow materials to bed-sediment load and suspended-sediment load in the river. Based on these findings, the potential effect of sediment yield caused by debris flows on reservoir areas is discussed. The zonation map shows the different effect of debris flow sediment on different dam site area which shows a good agreement with variation of debris flow spatial distribution.     

Debris flows in the Lushan earthquake area: formation characteristics,rainfall conditions,and evolutionary tendency

Debris flows often occur in the mountainous watersheds of earthquake-affected areas, and in the Lushan earthquake area of southwestern China, they have become a significant hazard. In this study, the influencing factors and spatial distribution of debris flows were analyzed through a review of their occurrence history. Debris flows are mainly distributed in the northwestern part of the study area, which hosts the greatest density of active faults. The debris flows are generally formed by the ‘progressive bulking’ effect in channels, and deep incision, lateral erosion, and blockage breaking are common processes that amplify the magnitude of such debris flows. Rainfall thresholds for different types of debris flow were proposed to explain the spatial differences between debris-flow regions, and the temporal variations of those thresholds highlighted how the rainfall conditions required for the occurrence of debris flows have changed. Natural vegetation recovery, reduction in the availability of solid material, and artificial debris-flow control projects play important roles in raising the threshold of the rainfall conditions required for triggering debris flows.

     

An empirical approach for evaluation of the potential of debris flow occurrence in mountainous areas

Debris flows can occur relatively suddenly and quickly in mountainous areas, resulting in major structural damage and loss of life. The establishment of a model to evaluate the occurrence probability of debris flows in mountainous areas is therefore of great value. The influence factors of debris flows are very complex; they can basically be divided into background factors and triggering factors. Background factors include the mechanical characteristics of geo-materials, topography and landscape, and soil vegetation; and triggering factors include hydrological and rainfall conditions, and human activities. By assessing the dynamic characteristics of debris flows in mountainous areas, some important influence factors are selected here for analysis of their impacts on the occurrence probability of debris flow. A mathematical model for evaluation of the occurrence probability of debris flows is presented and combined with probability analysis. Matlab software is used for the numerical implementation of the forecasting model, and the influences of rainfall, lithology and terrain conditions on the occurrence probability of debris flows are analyzed. Finally, the presented model is applied to forecast the occurrence probability of debris flows in the mountainous area around Qingping Town; the simulation results show that many loose landslide deposits and heavy rainfall are the key factors likely to trigger debris flows in this region.     

Recent catastrophic debris flows in Chile: Geological hazard,climatic relationships and human response

Debris flows are an important type of geological hazard in Chile, affecting cities, towns and rural areas throughout the country despite the variation in climate regimes. In this summary paper, recent debris flows in the cities of Antofagasta and Santiago, in northern and central Chile, and in a rural area near Lake Ranco in central-southern Chile in 1991, 1993 and 2004, respectively, are reviewed. Triggering factors for flow occurrence are identified and different approaches to debris flow hazard assessment and the effects of debris flows on people and the environment are discussed. Furthermore, the relationships between debris flow occurrence and climatic anomalies such as El Niño episodes are analysed. A clear pattern of debris flow generation associated with El Niño events is found for Antofagasta and Santiago. The risk related to debris flows in Chile is of increasing importance because of the continuous expansion of cities to hazardous areas such as alluvial fans. The results show that hazard assessment based on several factors is essential for the implementation of proper prevention and mitigation measures for future debris flow events in the country.     

Magnitude–frequency relations in debris flow

Debris flow occurs frequently in mountainous regions in China. Because of the difficulties involved in predicting and catching live debris flows, an assessment of the potential for debris flow is crucial in hazard mitigation. Magnitude–frequency (MF) relations are of special significance in such assessments. In previous studies, MF relations have been inferred by analyzing environmental factors and historical records and using empirical relations. This paper is concerned with the derivation of MF relations at regional and valley scales, using a large database of statistics. At the regional scale, it is represented by the distribution of the valley area, because the area is often taken to indicate the potential magnitude of debris flow. Statistics from over 5,000 debris flow valleys in various provinces in China show that a power law holds for the distribution, i.e., p(A) ∼ A ⁻ⁿ , where p(A) is the percentage of valleys with area A and n varies with region and thus describes regional differences. At the valley scale, a case study focusing on Jiangjia Gully (JJG) was conducted, and the MF relations derived from it were expressed by the distributions of discharge and runoff (i.e., the total volume) of living debris flows observed over the last 40 years. The distributions can be expressed as exponential functions where the exponents vary with the events. These MF relations provide not only a potential quantitative reference for practical purposes but also hint at the intrinsic properties of the debris flow.     

Debris flow susceptibility assessment by GIS and information value model in a large-scale region, Sichuan Province (China)

Debris flow susceptibility assessment is the premise of risk assessment. In this paper, Sichuan Province is chosen as a study area, where debris flow disasters happen frequently. Information value model is applied to calculate the information values of seven environmental factors, namely elevation, slope, aspect, flow accumulation, vegetation coverage, soil type and land-use type. Geographic information system technology is used to analyze the comprehensive information values so as to determine the debris flow susceptibility. The results show that the northeast, the central and the south of Sichuan are the most hazardous regions, which display a zonal distribution feature from the southeast to the south. From the validation results, 7.53 % of the total area suffers from high susceptibility and 19.97 % suffers from very high susceptibility. However, 80 % of the debris flows are concentrated in two regions. The actual occurrence ratios of debris flows of the high-susceptibility and very high-susceptibility areas are 4.95 and 2.14, respectively.     

四川美姑河牛牛坝水电站库区泥石流成因分析

牛牛坝水电站是美姑河流域“一库五级开发方案”的龙头电站。牛牛坝库区为泥石流多发段。泥石流具有规模较大、分布范围广、发育密度大(1.33条/km)、发生频率高、致灾作用强烈的特点。泥石流灾害是库区重大地质灾害之一。通过系统分析工程区陡峻的地貌条件、发育的区域构造、软硬相间的岩层、广泛分布的堆积体、降雨集中的气候条件及不合理的人类活动等因素对泥石流的影响,揭示了美姑河牛牛坝水电站库区泥石流的成因。     

Debris flows of 28 June 2014 near the Arshan village (Siberia,Republic of Buryatia,Russia)

On 28 June 2014, high intensity rainfall resulted in seven simultaneous debris flows going down the slopes of the Tunka Ridge in the vicinity of Arshan village, which is a balneological and alpine resort (51° 54′ 31″ N, 102° 25′ 44″ E). The debris flows caused one life loss and several injuries, 112 buildings were damaged, and 15 were completely destroyed. The total volume of the transported deposits amounted to 3?×?10⁶ m³. Debris flows’ formation began with the failure of weak sediments in the hanging cirques. Similar phenomena had not been recorded in the study area for over 40 years. The article presents a complete picture of the event and analysis of geological, geomorphological, and meteorological conditions for debris flows formation, for which extreme local rainfall was the major cause.     

Recognition, classification and mechanical description of debris flows

Various types of flow or mass movement involving water and sediments occur on steep slopes in mountainous areas. Among them, debris flows are peculiar events during which a large volume of a highly concentrated viscous water-debris mixture flows through a stream channel. Throughout the world these phenomena cause considerable damage but remain poorly understood although a basic knowledge is already available concerning their recognition and propagation.

Firstly, a synthesis of the useful practical criteria of recognition is proposed. Debris flows must be seen as intermediate phenomena between hyperconcentrated flows (intense bed load transport) and landslides separated from them by sharp transitions of some characteristics (celerity, deposit nature and flow type). Two parameters, solid fraction and material type, thought to be appropriate for a sound and practical classification, are brought out, and the corresponding complete classification of flow and mass movements in mountain areas is presented. Two extreme debris flow types are thus distinguished: muddy debris flows and granular debris flows. A critical review of recent advances in debris flow dynamic is then proposed. It is pointed out that adequate work must be carried out in the field of non-Newtonian fluid mechanics. In particular, one fundamental rheological property of debris flow materials is the yield stress, which explains thick deposits on steep slopes and can be inferred from field measurements. Furthermore it can be used to estimate viscous dissipation within the bulk during flow. Relevant models predicting muddy debris flow dynamics are already available whereas further progress is needed concerning granular flows.     

Impacts of small woody debris on slurrying,persistence, and propagation in a low-gradient channel of the Dongyuege debris flow in Nu River,Southwest China

Debris flows occurring in well-vegetated alpine areas usually contain a range of sizes of woody debris. Large woody debris (LWD), which has a retaining effect on further transportation of debris downstream, is mainly distributed in upstream reaches, and the amount of small woody debris (SWD) deriving from LWD increases dramatically midstream and downstream. The Dongyuege (DYG) bouldery debris flow with a sandy-matrix took place in a wildwood area, causing 96 deaths and its clay-sized fraction does not contain typical clay minerals. However, its total travel distance and runout distance in a low-gradient reach (between 2° and 5°) upstream of the depositional fan apex reached 11 km and 3.3 km, respectively. The abundant SWD in the DYG debris flow might have played a crucial role in slurrying, persistence, and the long runout over the low gradient. To understand why this debris flow extended so far, slurrying experiments, pore water escape experiments, and excess pore pressure experiments were performed. Crude debris (CD) collected from the DYG debris flow deposit was used throughout the experiments, the tested materials of which are separated into CD-containing SWD with a maximum grain size (MGS?=?2 mm), purified debris (PD) without SWD with a MGS of 2 mm, and SWD <?2 mm in diameter. In the five slurrying experiments with PD-SWD-water mixtures, as the SWD content was elevated from 0.0 to 2.0 wt%, the current velocity of escaping pore water decreased uniformly from 17.2 to 0.9 mm/s. When the SWD content was 1.0 wt% or greater, the mixtures can be considered as one-phase flows of viscous fluids. The six pairs of pore water escape experiments based on the slurries remolded with CD and PD, respectively showed that the time needed for pore water to escape from the CD slurries was much greater than those from their PD counterparts. Also, measured was the dissipation rate of the relative excess pore pressure of CD and PD slurries of various densities and volumes, which showed that most of the rates of the PD-slurries were always greater than CD-slurries. Overall, the results show that SWD has a strong influence on the slurrying of the DYG debris without typical clay minerals found in other debris flows, and SWD helps to sustain the high excess pore pressure in the interior of the debris flow mass which resulted in the extended travel distance over such a low gradient. SWD favors the formation and stability of one-phase water-debris mixtures because of its large specific surface area and low density, which makes it able to absorb fine particles and able to be suspended in slurry flows over long timescales. In well-vegetated mountainous areas, SWD should be taken into account in the assessment of debris-flow hazards.     

Design and performance of a novel multi-function debris flow mitigation system in Wenjia Gully,Sichuan

The post-earthquake debris flows in the Wenjia Gully led to the exposure of the shortcomings in the design of the original conventional debris flow mitigation system. A predicament for the Wenjia mitigation system is a large amount of loose material (est. 50 × 10⁶ m³) that has been deposited in the gully by the co-seismic landslide, providing abundant source material for debris flows under saturation. A novel design solution for the replacement mitigation system was proposed and constructed, and has exhibited excellent performance and resilience in subsequent debris flows. The design was governed by the three-phase philosophy of controlling water, sediment, and erosion. An Early Warning System (EWS) for debris flow that uses real-time field data was developed; it issues alerts based on the probabilistic and empirical correlations between rainfall and debris flows. This two-fold solution reduces energy of the debris flow by combining different mitigation measures while minimizing the impact through event forecasting and rapid public information sharing. Declines in the number and size of debris flows in the gully, with increased corresponding rainfall thresholds and mean rainfall intensity-duration (I-D) thresholds, indicate the high efficacy of the new mitigation system and a lowered debris flow susceptibility. This paper reports the design of the mitigation system and analyzes the characteristics of rainfall and debris flow events that occurred before and after implementation of the system; it evaluates the effectiveness of one of the most advanced debris flow mitigation systems in China.     

Submersion simulation in a typical debris flow watershed of Jianzhuangchuan catchment,Loess Plateau

Debris flow is one of the most serious and frequent geological disasters that occur in the Loess Plateau. The outbreak of a debris flow is sudden, ferocious, swift, and destructive. The characteristics and mechanism of debris flow were explored in this study via survey, numerical simulation, and simulation analysis in a Loess Plateau area (Huangling County, Shaanxi Province, China). Numerical models and formulas corresponding to the occurrence and movement mechanism were established based on the HEC-RAS, HEC-GeoRAS, and SWAT results. The range of debris flow deposition was determined through capturing the debris flow free surface. A hydrological model and critical rainfall threshold were determined in order to provide technical support for debris flow forecasting in the Loess Plateau. The results suggest that 10-year floods do not submerge any portion of the basin. One village area was affected by the 100-year flood (total area of 0.648 km²) while four villages areas were submerged by the 1000-year flood (total area of 1.39 km²). The method presented here may provide a reliable scientific basis for mitigating loss due to debris flow hazards.     

Summer rainstorm associated with a debris flow in the Amarilla gully affecting the international Agua Negra Pass (30°20′S), Argentina

The Central-West region of Argentina was seriously affected by a series of convective summer storms on January–February of 2013 generating many debris flows and rockfall in the Central Andes mountain regions. In particular, the unreported 8th February event caused the sad death of a 10-year-old child being completely ignored by society and local authorities. Despite this, meteorological conditions associated with this event and further episodes were rarely measured and determined mainly due to scarce meteorological stations in Andean mountain areas. In this paper, meteorological data from CMORPH algorithm and measurements of surrounding gauges were analyzed for estimating the triggering precipitation value of this event. As well, the particular debris flow channeled into the main branch of the Amarilla gully in the Agua Negra valley was geomorphologically described. The amount of precipitation associated with this debris flow was 5.5 and 13.2 mm accumulated previous to the event. This violent debris flow was generated in a talus zone in a periglacial environment located just below a covered rock glacier. However, the influence of the permafrost thawing in this process is not feasible. The altitude of the 0 °C isotherm was lower during the previous days of the event, and no monitoring on permafrost is available for this area. The volume of removed mass was estimated in 5 × 10⁴ m³, and the mean velocity was 35 km/h. Boulders of 4 m diameter were found in the source area, while the deposit is up to 75% sandy with clasts that hardly exceed 10 cm in the alluvial fan distal part. Herein the main objective is to advice about the probable catastrophic impact of similar events in the future. These findings could be useful for hazard remediation, mitigation, and prevention plans for the Agua Negra international pass under construction.     

北京山区泥石流灾害预警方法研究

北京山区泥石流灾害较为频繁,总体以暴雨型沟谷泥石流为主,受地形、地貌、地质、降雨以及松散物类型等因素的影响比较明显。在调查分析北京山区泥石流灾害发育特征的基础上,从泥石流的形成和启动条件入手,对泥石流灾害的预警方法进行研究与探讨。     

The formation and development of debris flows in large watersheds after the 2008 Wenchuan Earthquake

The Wenchuan earthquake has caused abundance of loose materials supplies for debris flows. Many debris flows have occurred in watersheds in area beyond 20 km², presenting characteristics differing from those in small watersheds. The debris flows yearly frequency decreases exponentially, and the average debris flow magnitude increases linearly with watershed size. The rainfall thresholds for debris flows in large watersheds were expressed as I?=?14.7 D ^?0.79 (2 h?<?D?<?56 h), which is considerably higher than those in small watersheds as I?=?4.4 D ^?0.70 (2 h?<?D?<?37 h). A case study is conducted in Ergou, 39.4 km² in area, to illustrate the formation and development processes of debris flows in large watersheds. A debris flow develops in a large watershed only when the rainfall was high enough to trigger the wide-spread failures and erosions on slope and realize the confluence in the watershed. The debris flow was supplied by the widely distributed failures dominated by rill erosions (14 in 22 sources in this case). The intermittent supplying increased the size and duration of debris flow. While the landslide dam failures provided most amounts for debris flows (57 % of the total amount), and amplified the discharge suddenly. During these processes, the debris flow velocity and density increased as well. The similar processes were observed in other large watersheds, indicating this case is representative.     

泥石流的二维数学模型

泥石流是在重力作用下,由砂粒石块和水等组成的固液混合物,是一种发生于山区的复杂的地质灾害现象。泥石流主要是由暴雨诱发引起的,它沿着复杂的三维地形高速流动,具有流体流动的特性。为了模拟泥石流的运动规律,预测降雨诱发的泥石流的到达距离和泛滥范围,减少和避免泥石流引起的灾害,把泥石和雨水组成的固液混合物假定为遵循均匀、连续、不可压缩的、非定常的牛顿流体运动规律。基于质量守恒方程和Naiver-stokes方程,采用深度积分方法,推导出了一个模拟泥石流运动的二维数学模型。所有方程式可用有限差分法来求解。结合GIS,该模型可用于预测泥石流的流动距离和泛滥范围,以及泛滥范围内的危险房屋和路段,也可以用于泥石流灾害的风险性分析。     

Large volcanic landslide and debris avalanche deposit at Meru,Tanzania

Meru volcano is located within the Northern Tanzanian Divergence Zone where the east branch of the East African Rift splits into several branches. The 4565-m-high Meru volcano is breached on the east flank by a horseshoe-shaped scar following a major collapse associated with the Momella debris avalanche approximately 9000 years ago. Remote sensing combined with detailed field mapping allowed the characterisation of the Momella debris avalanche deposit, structure, and texture. Hummocks, ridges, lineaments, lobes, grabens and shear zones are observed on the surface of the deposit. The most common facies observed are the mixed facies with indurated and shattered outcrops and the matrix facies. The collapse involved a volume of 20 ± 2 km³ with a deposit that spread over an area of 1250 km², up to the base of Kilimanjaro. Based on field evidence, we suggest that water played a key role in the deformation, facies formation, avalanche emplacement and mobility of the entire deposit but to a lesser extent south of Ngurodoto complex. The deformation and emplacement of the avalanche were accommodated by both extension and shearing on a water-fluidised basal layer.