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... Catastrophic debris flow events after the Wenchuan earthquake and the triggering rainfall threshold model Rainfall characteristics of catastrophic debris flow events after the Wenchuan earthquake Debris flows often occur as the result of a combination of large amounts of accumulated rainfall and high rainfall intensity (Caine 1980;Wieczorek 1987;Tan and Han 1992;Cui et al. 2008;Qu et al. 2015; Dahlquist and West 2019). There are several catastrophic debris flows that occurred in the study area after the Wenchuan earthquake, such as the "14 August 2010" event, "21 August 2011" event, "10 July 2013" event, and "20 August 2019" event ( Fig. 9 and Table 3). ...
... Previous studies have proposed regional rainfall intensity duration thresholds for triggering debris flows in different regions (Jibson 1989;Tan and Han 1992;Tang and Liang 2008;Chen and Hawkins 2009;Guo et al. 2016). Although the rainfall threshold for triggering debris flows varies from region to region, it is still useful for the prediction of regional debris flows (Floris et al. 2010;Campbell et al. 2020). ...
... Since there are no detailed records of debris flow events and rainfall data for the study area before the Wenchuan earthquake, the critical cumulative rainfall of debris flow in the Longmenshan earthquake region is 80-100 mm, and the hourly rainfall intensity is 30-50 mm according to Tan and Han (1992). Compared with the pre-earthquake triggering rainfall intensity, the hourly rainfall intensity (24 mm/h) of the 2019 debris flows is about 1/2 to 4/5 of the pre-earthquake level. ...
Enhanced debris flow activity observed after the 2008 Wenchuan earthquake, Sichuan Province, SW China, is still intense more than a decade since the earthquake. A heavy rainstorm on 20 August 2019 caused catastrophic debris flows in the epicentral area of the earthquake. By remote sensing imagery interpretation and field investigation, 13 debris flows were identified. An empirical rainfall threshold I-D model (intensity–duration) of post-seismic debris flows is established. Changing rainfall thresholds, characteristics, and initiation mechanisms of the 2019 debris flows were analyzed, which are different from the events that occurred in previous years. We find that the 2019 debris flows were mainly initiated by concentrated runoff erosion of sediments stored in tributary channels of catchments, with little contribution from the coseismic landslides and new hillslope failures. Differently, the major sediment supply of debris flows in previous years was mainly from the reactivation of coseismic landslide deposits. By analyzing the triggering rainfall of all recorded debris flows after the Wenchuan earthquake, we find that the rainfall threshold in 2019 is about 1/2–4/5 of the pre-earthquake value, and the I-D model we built can predict the 2019 debris flows successfully. Considering that there are still a large number of coseismic landslide deposits in the mountain catchments and the rainfall thresholds are slowly recovering, the post-seismic debris flows may last for a long period.
... Table 4 lists the mean maximal 10 min, 1 h, 6 h, and 24 h rainfall amounts before the earthquake. Tan and Han (1992) found that debris flows in the region of the Longmenshan Mountains (which includes the four study areas) can be triggered by rainfall intensities ranging from 30 to 50 mm/h. This hourly rainfall amount is similar to the MAMH in each study area because the MAMH in the four areas is no b55 mm. ...
... According to Tan and Han (1992) and the Handbook of Hydrology for Sichuan Province, the MAMH is 55 mm in Beichuan and Dujiangyan and 50 mm in Qingping. In areas around Yingxiu, local MAMH ranges from 25 to 45 mm, i.e., from downstream areas of the Min River to upstream areas. ...
... As mentioned in other research (Tang et al., 2009, Guo et al., 2016, the rainfall threshold of post-quake debris flows decreases considerably with time. Based on the local MAMH map, the research of Tan and Han (1992), and the I-D expressions for the four study areas, we found that the ratio between the coefficient of the I-D threshold and the MAMH is about 30%, indicating that post-quake debris flows from 2008 to 2013 were triggered by a lower threshold than before the earthquake. As the devastating earthquake generates a sufficient sediment supply in the form of coseismic landslides Tang et al., 2012), it is of considerable importance to analyze the coefficient of the I-D threshold and the sediment supply conditions in the four study areas. ...
Debris flows in the Wenchuan seismic region have caused human casualties and severe damage to local infrastructure. Consequently, the triggering rainfall threshold and erosion capability of post-quake debris flows has become an important research topic worldwide. In this study, we analyze five years of rainstorms and debris flow data from four typical earthquake-hit regions in order to examine the local rainfall intensity–duration (I-D) thresholds and debris supply conditions. It was found that debris flow events in the four seismic areas exhibited different I-D thresholds, related to local mean annual hourly precipitation and debris flow supply conditions. The I-D thresholds, normalized by mean annual maximum hourly rainfall, illustrate that post-quake rainfall thresholds were reduced by at least 30% compared to pre-quake levels. Regression analysis revealed a clear linear relationship between the debris supply condition and the empirical coefficient, α, of the I-D equation. This means that rainfall thresholds of post-quake debris flows in different areas are distinctive and are strongly affected by sediment volume. Different relationships between the entrainment rate and the debris volume per watershed area and its product with the channel gradient illustrate that stream sediments in Yingxiu and Dujiangyan are more eroded, and that local debris flows might persist over a shorter time than in Qingping and Beichuan in the future. Finally, debris flows in the studied area exhibit no tendency of reduction in erosion competence entrainment rate, as found in Taiwan, which might be indicative of a higher entrainment rate persisting for a longer time.
... Shieh et al. (2009), Wu et al. (1990), and Tan and Han (1992) used the 1 h or 10 min rainfall intensity and cumulative precipitation to predict the triggering of debris flows. The 10-min intensity was used only for the runoffinduced mechanism (Wu et al. 1990, andTan andHan 1992). ...
... Shieh et al. (2009), Wu et al. (1990), and Tan and Han (1992) used the 1 h or 10 min rainfall intensity and cumulative precipitation to predict the triggering of debris flows. The 10-min intensity was used only for the runoffinduced mechanism (Wu et al. 1990, andTan andHan 1992). The 1-h intensity was used not only for the runoffinduced mechanism, but also for the shallow landslide mechanism (Shieh et al. 2009;Tan and Han 1992). ...
... The 10-min intensity was used only for the runoffinduced mechanism (Wu et al. 1990, andTan andHan 1992). The 1-h intensity was used not only for the runoffinduced mechanism, but also for the shallow landslide mechanism (Shieh et al. 2009;Tan and Han 1992). We used the 1-h rainfall intensity in our study. ...
Three groups of factors, topography, geology and hydrology have influence on the formation of gully-type debris flows triggered by shallow landslides. In this paper, a single representative factor (T factor) for the topography, and a single representative factor (R factor) for the rainfall (hydrology) are proposed, which can be used to define threshold values for debris flow formation. This study was carried out in the Dayi area, Guizhou Province, China. During a heavy rainfall event on June 5 and 6, 2011, 37 gully-type debris flows caused by shallow landslides were triggered. In some catchments no such debris flows were triggered even though these catchments were in the vicinity of gullies with debris flows. The triggering mechanism for gully-type debris flows is the transport of sediment provided by shallow landslides into the gully. We isolated and analyzed the influence of the topography on the formation of debris flows in gullies under almost identical hydrological and geological conditions and propose a T factor as a topographical indicator which is a combination of the catchment surface area, the ratio of the catchment area with a slope sensitive to trigger debris flows, and the average gradient of the drainage channel in the catchment. Additionally an R factor is proposed as a rainfall indicator which is a combination of the rainfall in 1 h before the debris flow was triggered, the cumulative rainfall before the debris flow was triggered, and the annual rainfall. Higher T factor values and higher R factor values are generally related to higher probabilities of debris flow formation. The primary probability factor P, which is the combination of T and R, gives an indication of the probability of debris flow formation. The T factor was successfully validated in debris flow gullies with the same initiation mechanism in the Cida River catchment, Sichuan, China.
... But the relationship between debris flow occurrence and rainfall characteristics in the study area, either in empirical equations or in physical interactions of loose materials, is still unclear. Few studies have evaluated the regional relationship between ID thresholds and debris flow occurrence in Wenchuan earthquake-stricken area prior to the earthquake using empirical models, e.g., the rainfall intensity and cumulative rainfall (Tan 1990;Tan and Han 1992). After the Wenchuan earthquake, the critical ID thresholds for the triggering of debris flows have decreased, but these thresholds are hard to quantify precisely. ...
... The rainfall intensities necessary to initiate debris flows in the Yingxiu and Longchi area are poorly known because no significant debris flow events were recorded prior to the Wenchuan earthquake. Tan and Han (1992) pointed out that the critical rainfall intensity in the Longmenshan area is greater than 30-50 mm/h, with a total rainfall of at least 80-100 mm. Comparing the characteristics of the triggering rainfall with the thresholds reported by Tan and Han (1992), the results indicate that only the minimum critical rainfall intensities have reduced to as low as one half of the pre-earthquake figures. ...
... Tan and Han (1992) pointed out that the critical rainfall intensity in the Longmenshan area is greater than 30-50 mm/h, with a total rainfall of at least 80-100 mm. Comparing the characteristics of the triggering rainfall with the thresholds reported by Tan and Han (1992), the results indicate that only the minimum critical rainfall intensities have reduced to as low as one half of the pre-earthquake figures. ...
The Wenchuan earthquake-stricken area is frequently hit by heavy rainfall, which often triggers sediment-related disasters, such as shallow landslides, debris flows, and related natural events, sometimes causing tremendous damage to lives, property, infrastructure, and environment. The assessment of the rainfall thresholds for debris flow occurrence is very important in order to improve forecasting and for risk management. In the context of the Wenchuan earthquake-stricken area, however, the rainfall thresholds for triggering debris flows are not well understood. With the aim of defining the critical rainfall thresholds for this area, a detailed analysis of the main rainstorm events was carried out. This paper presents 11 rainfall events that induced debris flows which occurred between 2008 and 2012 after the Wenchuan earthquake. The rainfall thresholds were defined in terms of mean rainfall intensity I, rainfall duration D, and normalized using the mean annual precipitation (MAP). An ID threshold and a normalized I
MAPD threshold graph could be set up for the Wenchuan earthquake-stricken area which forms the lower boundary of the domain with debris flow-triggering rainfall events. The rainfall threshold curves obtained for the study area were compared with the local, regional, and global curves proposed by various authors. The results suggest that debris flow initiation in the study area almost requires a higher amount of rainfall and greater intensity than elsewhere. The comparison of rainfall intensity prior to and after the earthquake clearly indicates that the critical rainfall intensity necessary to trigger debris flows decreased after the earthquake. Rainfall thresholds presented in this paper are generalized, so that they can be used in debris flow warning systems in areas with the same geology as the Wenchuan earthquake-stricken area.
... The threshold of rainfall necessary to initiate debris flows in the Yingxiu area is poorly known, though Tang et al. (2011b) reported that small debris flows in branches had already been initiated by more than 60 mm of cumulative rainfall before 02:00 am on 14 August. Previous studies (Tan and Han 1992;Lan et al. 2003) have reported rainfall that initiates debris flows with cumulative rainfall of near 100 mm in southwestern China. Tang et al. (2012b) suggested the expression I = 25.962 ...
... However, how much antecedent rainfall is actually required to initiate debris flows can be modeled. According to the rainfall characteristics and previous studies (Tan and Han 1992;Lan et al. 2003) in a neighboring region, three cumulative rainfall values (R1 = 106 mm, R2 = 60 mm and R3 = 38 mm; Fig. 3) are proposed in this study to generate debris flows. The rainfall particles are distributed averagely in the study area, including the debris source materials, which are generated in the first step. ...
Since the 12 May 2008 Wenchuan earthquake, numerous catastrophic debris flows have occurred in the Wenchuan earthquake-stricken zones. In particular, on 14 August 2010, long-duration, low-intensity rainfall triggered widespread debris flows at the epicenter of the Wenchuan earthquake. These flows caused serious casualties and property losses. In this study, a novel approach combining a soil-water mixing model and a depth-integrated particle method is applied to the analysis of the post-seismic debris flows in the epicentral area. The presented approach makes use of satellite images of the debris flow in the affected area. It is assumed that debris source materials are primarily generated from slope failure during the earthquake. Debris flows are initiated after different amounts of cumulative rainfall according to diffusion governing equations. The debris flow disaster is investigated in terms of volume, concentration, discharge, velocity, deposition thickness and affected area by setting the cumulative rainfall, Manning coefficient and diffusion coefficient to 38 mm, 0.1 and 0.004 m2 s−1, respectively. Although the thickness and volume of debris source materials are underestimated in this study, the numerical results, including the volume concentration, velocity, discharge and the affected area are in good agreement with the actual observations/measurements of the debris flow events. Adopting a simple and efficient numerical model, systematic analysis of the entire debris flow generation process not only contributes to understanding the mechanism of initiation, transportation and deposition, but is also very useful in designing effective protection structures according to the distribution characteristics of the main parameters. Additionally, the coupling effect of multiple debris flows is discussed.
... The threshold of rainfall necessary to initiate debris flows in the Yingxiu area is poorly known, though Tang et al. (2011b) reported that small debris flows in branches had already been initiated by more than 60 mm of cumulative rainfall before 02:00 am on 14 August. Previous studies (Tan and Han 1992;Lan et al. 2003) have reported rainfall that initiates debris flows with cumulative rainfall of near 100 mm in southwestern China. Tang et al. (2012b) suggested the expression I = 25.962 ...
... However, how much antecedent rainfall is actually required to initiate debris flows can be modeled. According to the rainfall characteristics and previous studies (Tan and Han 1992;Lan et al. 2003) in a neighboring region, three cumulative rainfall values (R1 = 106 mm, R2 = 60 mm and R3 = 38 mm; Fig. 3) are proposed in this study to generate debris flows. The rainfall particles are distributed averagely in the study area, including the debris source materials, which are generated in the first step. ...
The volume of debris flows occurred in mountainous areas is mainly affected by the volume of debris materials deposited at the valley bottom. Quantitative evaluation of debris materials prior to debris flow hazards is important to predict and prevent hazards. At midnight on 7th August 2010, two catastrophic debris flows were triggered by the torrential rain from two valleys in the northern part of Zhouqu City, NW China, resulting in 1765 fatalities and huge economic losses. In the present study, a depth-integrated particle method is adopted to simulate the debris materials, based on 2.5 m resolution satellite images. In the simulation scheme, the materials are modeled as dry granular solids, and they travel down from the slopes and are deposited at the valley bottom. The spatial distributions of the debris materials are investigated in terms of location, volume and thickness. Simulation results show good agreement with post-disaster satellite images and field observation data. Additionally, the effect of the spatial distributions of the debris materials on subsequent debris flows is also evaluated. It is found that the spatial distributions of the debris materials strongly influence affected area, runout distance and flow discharge. This study might be useful in hazard assessments prior to debris flow hazards by investigating diverse scenarios in which the debris materials are unknown.
... Short-duration high-intensity rainfall is the main triggering factor for the gully type debris flows (Shieh et al. 2009;Wu et al. 1990;Tan and Han 1992). Yu et al. (2014) used the 1h rainfall intensity and cumulative precipitation in the period before the triggering of the debris flow to describe the critical rainfall index for runoff induced debris flows. in which R* is the critical rainfall index (mm); B is the cumulative precipitation, until the start of the debris flow (mm); I is the amount of rainfall in the hour before the start of the debris flow (mm); K in Eq. 3 (see: Shieh et al. 2009;Wu et al. 1990;Tan and Han 1992) is an indicator for the role of rainfall intensity. ...
... Short-duration high-intensity rainfall is the main triggering factor for the gully type debris flows (Shieh et al. 2009;Wu et al. 1990;Tan and Han 1992). Yu et al. (2014) used the 1h rainfall intensity and cumulative precipitation in the period before the triggering of the debris flow to describe the critical rainfall index for runoff induced debris flows. in which R* is the critical rainfall index (mm); B is the cumulative precipitation, until the start of the debris flow (mm); I is the amount of rainfall in the hour before the start of the debris flow (mm); K in Eq. 3 (see: Shieh et al. 2009;Wu et al. 1990;Tan and Han 1992) is an indicator for the role of rainfall intensity. Yu et al. (2013Yu et al. ( , 2014 pointed that the coefficient K is 12.5 for the 1 h prediction model. ...
Debris flows are triggered every year in the Jiangjia Gully, Yunnan Province, China. High-intensity short duration rainfall was the main triggering factor for these gully type debris flows which are triggered by a runoff induced mechanism. The 10-min rainfall intensity is considered as the trigger. A revised prediction model with factors related to hydrology (rainfall) is introduced for this kind of gully type debris flows and applied to the Jiangjia Gully. A 10-min rainfall coefficient intensity is proposed based on the observational data of Jiangjia Gully, and a minimum 10-min rainfall intensity for triggering debris flow is presented. The Jiangjia data were used also to assess critical boundaries between three probability classes of debris flow initiation. The model was successfully validated in debris flow gullies with the same initiation mechanism in other areas of southwest China. The generic character of the model is explained by the fact that its factors are partly based on the initiation mechanisms and not only on the statistical analyses of a unique variety of local factors. The research provides a new way to predict the occurrence of debris flows initiated by a runoff induced mechanism.
... The recovery tendency of rainfall conditions required for debris flows is shown by the lowest variability of rainfall conditions (Fig. 7). In general, the initiation of debris flows depends on factors, such as precipitation, material source, and topography (Takahashi 1991). For a given watershed, the topography remains almost the same for several years. ...
... The inset table in Fig. 10 shows that the clay content of the samples in 2013 was visually lower than those in 2008, indicating that the fine particles were lost from the material sources. The D50 was also clearly higher than that in 2008, indicating that as the loose materials in the fluvial system became coarser each year, the hydraulic conditions required for debris flow initiation increased, based on the runoff thresholds for debris flows (Takahashi 1991;Cui 1992), and the debris flows became mainly transitional or semi-viscous. ...
The Ms 8.0 Wenchuan earthquake greatly altered the threshold for rainfall-triggered debris flows in the affected areas. It is of both scientific and practical significance to determine the rainfall thresholds. This study examines one of the regions most prone to debris flows to analyze the characteristics of rainfall that caused debris flows, and to explore local rainfall thresholds. We applied the relation between rainfall intensity and duration, peak intensity and event amount, and other single factor approaches. Comparison of effectiveness and accuracy indicates that the event rainfall is the most sensitive factor for forecasting. Analysis of the annual rainfall thresholds showed that the rainfall conditions required for debris flows have increased on the continent during the past 6 years. Besides the rainfall fluctuations over the past few years, material changes were the primary reason for threshold variability. Recovery of vegetation plays an important role in reducing potential loose material that supplies volume for debris flows. Natural solidification, decrease of the potential erosion depth, and surface coarsening make it more difficult to initiate a debris flow, and ultimately increased rainfall conditions required. The change in rainfall thresholds can be predicted and verified for the entire earthquake-affected region.
... However, some studies (Tan and Han 1992;Zhang et al. 2011a, b) focused on classifying the probability degrees of geological hazards. Tan and Han (1992) proposed seven critical precipitation intervals for rainstorm-induced debris flows in Sichuan province, China. ...
... However, some studies (Tan and Han 1992;Zhang et al. 2011a, b) focused on classifying the probability degrees of geological hazards. Tan and Han (1992) proposed seven critical precipitation intervals for rainstorm-induced debris flows in Sichuan province, China. Zhang et al. (2011a) classified the probability grades of landslides as low, moderate, and high, according to the values of effective precipitation in Zhejiang province, China. ...
An empirical minimum intensity-duration (I-D) condition of continuous rainfall for mudflows triggering is proposed for Yan'an, northern Shaanxi, China, where rainfall-induced mudflows occur frequently. Adopting the quantile-regression method, a program was built using R programming language and statistical computing environment, and 175 continuous rainfall records were analyzed objectively. The 35th percentile regression line was determined as the minimum I-D condition of continuous rainfall that can potentially trigger mudflows within the duration interval of 24-336 h, where I is measured in millimeters per hour and D in hours. Comparison of the minimum I-D condition of continuous rainfall determined by this study with those rainfall I-D thresholds developed for the world and other regions reveals that lower rainfall intensity has the potential to trigger mudflows in the Yan'an district, which proves the necessity of defining regional minimum rainfall I-D conditions and site-specific operational rainfall I-D thresholds for mudflows triggering in Yan'an.
... Thresholds delineate a range of rainfall combinations from short duration, high intensity to longer duration, lower intensity, any of which can result in debris-flow activity (USGS, 2005). Threshold conditions change with time and region as sediment supply conditions vary and vegetation recovers (note: say from what) (Tan and Han, 1992). Numerous studies analyze the effect of rainfall in triggering debris flows (Tan and Han, 1992;Francesco et al., 2004;Deganutti et al., 2000). ...
... Threshold conditions change with time and region as sediment supply conditions vary and vegetation recovers (note: say from what) (Tan and Han, 1992). Numerous studies analyze the effect of rainfall in triggering debris flows (Tan and Han, 1992;Francesco et al., 2004;Deganutti et al., 2000). As the most available rainfall data are the 24hr precipitation value (from 8:00 am to 8:00 am the following day), in the research area, 24hr rainfall thresholds were used as the climate index factor. ...
Debris flow is a serious geologic hazard in China. It is estimated that nationally debris flows cause up to 2 billion RMB (250 million US$) in damages and 300-600 deaths and injuries annually. To mitigate debris flow hazards, it is necessary to map, model, and identify zones of debris flow hazards and vulnerability as to inform the local people about the potential risk with a geographic information system. This research presents a regional scale case study modeling debris flow risk (hazard and vulnerability) in Sichuan Province, Southwestern China. In this area, 3,290 debris flows have been identified and the spatial-temporal distribution and activity characteristics of them have been documented. Based on the available meteorological data, a Digital Elevation Model with the rate of 1:250,000 and a regional geological map, the 24-hr rainfall threshold (y) for debris flow occurrence is closely related (significant at 99% confidence level) to the index (x) defined using a geology factor (rock hardness: a) and a topographical factor (channel gradient: d) where y = 21 + 10200 / x, in which x = 2.7 × ea + 1000 ×d. The discipline is constructive in developing the rainfall threshold for debris flow activity in remote mountainous areas that lack data. For a given watershed, a four-level debris flow hazard map is developed by comparing the rainfall threshold to the design rainfall intensities with 50-, 20-, and 5-year average recurrence intervals, respectively. The degree of debris flow vulnerability is determined by the watershed socio-economic conditions. A four-class debris flow risk map, at the final phase of the research, is generated by combining debris flow hazards and vulnerability. With the debris flow risk assessment, the Sichuan Province is classified into the slight, moderate, severe and very severe regions, which accounts for 36%, 19%, 20% and 25% of total area respectively.
... In this study, our investigation illustrates that the debris flow was induced by a strong rainstorm with short duration, the hourly rainfall intensity that triggered the debris flow was 12.60 mm, and the accumulative rainfall was 18.40 mm (Fig. 11). However, previous study has indicated that the rainfall intensity and accumulated rainfall for triggering debris flow in the Longmenshan area are greater than 30-50 mm/h and at least 80-100 mm, respectively (Tan and Hen 1992). This indicates that although 15 years have passed since the Wenchuan earthquake, the triggering rainfall conditions for debris flow initiation are still lower than the pre-earthquake level, i.e., the similar rainfall conditions in the later period may still trigger large-scale debris flow in our study area. ...
Catastrophic debris flow triggered by a June 26, 2023 rainstorm suggests the debris flow is still active 15 years after the Wenchuan seismic Abstract The Wenchuan earthquake strongly disturbed the earth surface and induced sufficient loose debris in the mountain basin, which supplies sufficient solid materials for debris flow initiation. Recently, a catastrophic debris flow was induced by strong rainfall on June 26, 2023 in the Banzi basin. Inconsistent with the first few years of the Wenchuan earthquake, the hillslope landslide sediment supply for debris flow occurrence is limited after 15 years of the Wenchuan earthquake, and the materials supplied for debris flow in the later period are mainly obtained from alluvium deposited along the channel. Simultaneously, this debris flow was triggered by flash flood through strongly erosion channel materials. However, although 15 years have passed since the Wenchuan earthquake, the triggering rainfall conditions have still not returned to pre-earthquake level. This debris flow first initiated in the upper branches of the basin and then gradually transported downstream at an average discharge of 806.99 m 3 /s, and approximately 82.80 × 10 4 m 3 solid materials were transported out the Banzi basin to form a debris flow fan with length of 120 m and width of 260 m in the Minjiang River, which poses a serious threat to the resettled population and destroy the reconstruction infrastructures. Importantly, there are still sufficient solid materials deposited in the basin, this could result in debris flow activity continuing for an unpredictable time. Therefore, implementation early warning, prevention, and mitigation measures in this basin are still important for debris flow disaster management in the later period of Wenchuan earthquake.
... Moderate clay particle content (3-10%) is conducive to the initiation of debris flow, while low (< 2.5%) or high (> 10%) clay particle content is challenging to generate debris flow ). Tan and Han (1992) found that the rain patterns of SDFs triggered by rainfall in the Panxi region were divided into concentrated and forward types. Based on the "S" line of accumulated rainfall, the induced rain types were further divided into four types: A, B, C, and D. Each type of rainfall had a significant difference in the degree of triggering debris flow, with A and B types of rainfall being the most likely to induce debris flow. ...
Slope debris flow (SDF) is a common geological disaster with complex formation processes and strong destructive forces causing significant casualties and economic losses in mountainous areas worldwide. Experimental research and models of the trigger mechanism of SDF are the key scientific issues as they provide the basis for studying technologies for the prevention, mitigation, prediction, and forecasting of these disasters. This paper summarizes the methods of data collection, analysis, and status of recent experimental research on the trigger mechanism and models of SDF under the action of artificial rainfall. The main progress and theoretical achievements related to the SDF are discussed in terms of the experimental parameter settings, the mechanism of water–soil coupling action, and the start-up model of SDF. On this basis, the suggestions for experimental research on the mechanism and models of triggers for debris flows are proposed. First, future experiments on debris flow triggering should increase the similarities between rainfall patterns and loose soil characteristics. Second, the mechanism research of SDF is needed on the changes in the physical and mechanical characteristics of soil and the response to debris flow triggers under enhanced rainfall. Third, the parameters of the debris flow trigger model should be simplified, and the model’s applicability should be improved with artificial intelligence. Through these efforts, the debris flow trigger test under artificial rainfall should be developed and refined, and the microscopic and multi-factor correlations of water–soil coupling should be applied to reveal the debris flow trigger mechanism in greater detail and establish a more applicable model of debris flow triggering.
... mm, and the maximum recorded daily maximum is 323.4 mm (Liu et al., 2009). According to Tan and Hen (1992), in the Longmenshan area the intensity of a rainfall that can initiate debris flows must be greater than 30-50 mm/h, with a total rainfall amount of at least 80-100 mm (Adegbe et al., 2013). ...
This research is devoted to Tangjiashan Lake, a quake landslide-dammed lake, situated in Sichuan Province, China, which was formed by a landslide triggered by the Wenchuan Earthquake on 12 May 2008. A STREAM_2D two-dimensional hydrodynamic model of Russia was applied to simulate the process of two flood scenarios: 1, lake dam outbreak, and 2, dam overtopping. An artificial dam outbreak was made after the earthquake
to lower the water level of the lake in 2008, which led to a great flood with a maximum water discharge of more than 6400 m3/s. The negative impact of the flood was reduced by a timely evacuation of the population. Flood hazards still remain in the event of new landslides into the lake and
lake dam overtopping (Scenario 2), in which case a maximum water discharge at the dam crest would reach 5000 m3 /s, placing the population of Shabacun and Shilingzi villages in the zone of flood impact.
... Maximum triggering rainfall intensity and inducing rainfall for debris flows is 38.9 mm/h and 140.8 mm. The mean and minimum rainfall condition required for debris flows are significantly lower than the 30-50 mm/h intensity and 80-100 mm/day daily rainfall required before the earthquake (Tan and Han 1992). Major debris flow events group-occurring in both slope gullies and large watersheds were recorded on rainstorms such as 24 Sep 2008, 13 Aug 2010, 3 July 2011, and 18 Aug 2012. ...
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.
... Debris flows have occurred in more than 800 streams since the earthquake (Cui et al., 2011). Moreover, the rainfall thresholds for debris flows have decreased remarkably (Tang and Liang, 2008;Zhou et al., 2012), much lower than those before the earthquake, which were about 30-50 mm h − 1 and 80-100 mm d − 1 (Tan and Han, 1992). ...
... Bell (2000) developed a critical precipitation coefficient for the Durban region of South Africa by analysing the relationship between mean annual rainfall and landslide occurrence [2]. Tan Wanpei (1994) furthered the research of critical precipitation value for valley-shaped debris flows and landslide initiation by applying statistical analysis of debris flow and landslide events in the Panzhihua-Xichang region [3]. Based on landslide and rainfall data of the Three Gorges region, Ding Jixin (2004) depicted quantificationally the susceptibility of rainfall-triggered landslides by analysing the landslide-rainfall relationship from 3 perspectives, namely the relationship between landslides and rainfall amount, rainstorms, and duration [4]. ...
Landslide forecasting for small and medium sized regions is taken as the subject matter. Liangshan Yi Autonomous Prefecture in Sichuan Province (China) as the study area. Geographic Information System (GIS) and Doppler Weather Radar (DWR) were used as the technologies. Eight influencing elements including slope gradient, stratigraphic lithology, land use, amount of precipitation, intensity of hourly precipitation, and proximity to the nearest fault, river, and road are the predictors. Information Model (IM), Fuzzy Mathematics (FM), and Extenics are the theoretical support in this paper, which builds a landslide forecasting model for small and medium sized regions and develops a GIS and DWR based short-term and impending landslide forecasting system on the ArcGIS 9.3 Platform for Liangshan Prefecture. The system provides a seamless rolling landslide forecast for Liangshan Prefecture, with a 1 h interval and a 3 h forecast period. The simulation results indicate that the system serves well in landslide forecasting for small and medium sized regions, and is thus applicable in the hazard forecasting practice at prefecture scale.
... The debris flow of the Taoguan Watershed, at semi-drought region, was triggered by the cumulative rainfall of 148.1 mm and the hourly of 18.6 mm. The cumulative rainfall was significantly greater than the threshold before 2008 with 80-100 mm, but the hourly rainfall for triggering debris flow only accounted for about 30%~50% of the threshold before 2008 with 30~50 mm (Tan and Hen 1992). It was concluded that the catastrophic debris flows on July 10 th concentrated in the areas and watersheds with the cumulative rainfall and hourly rainfall was about or more than 150 mm and 16.0 mm, respectively. ...
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 633×106 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/m3, velocities of 5.0∼12.7 m/s, discharges of 335∼2353 m3/s and sediment yields of 0.10∼1.26×106 m3, and also numerously occurred in large watersheds with the area over 10 km2. 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 10 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.
... The intensity of rainfall necessary to initiate debris flows in the Yingxiu catchment is poorly known and also was not the main interest of this research. Previous studies in the Longmenshan area Tan and Hen (1992) have reported rainfall that initiates debris flows to have intensities greater than 30-50 mm/h with a total rainfall of at least 80-100 mm. Lan et al. (2003) examined rainfall amount 110 mm/day as the rainfall thresholds for the occurrence of past landslide events in the Xiaojiang watershed, southwestern China. ...
In many parts of the world debris flows are one of the most dangerous of all mass wasting events. Mountainous areas with high slope instability, high seismic activities and extreme rainfall condition are the main triggering factors. The Monday, May 12, 2008, mega-earthquake of magnitude 8.0 that struck the Wenchuan area, Northwestern Sichuan province in China was catastrophic. This event, led to co-seismic landslides and subsequent rainfall induced debris flow in Yingxiu catchment on August 14 th , 2010. The catchment has a very steep topography, an area of 5.35 km 2 and a channel length of 3.55 km. The aim of this research is to model the post seismic debris flow, Parameterize and calibrate the event. Two main initiation zones were identified based on susceptibility assessed from geomorhological mapping and formed the bases for input in the FLO-2D model. 161, 350 m 3 (64.54%) of the debris flow volume was modeled with FLO-2D in a manner consistent within the limit of the data available. FLO-2D model do not incorporate entrainment of materials in the transport zone. Thus, limitation to the production of the total deposits volume on the debris flow fan. The model was parameterized and the result shows that, Sediment concentration and the coefficient of friction were the two main parameters that affected the velocity of debris flow, area of inundation and the impact force respectively. Finally, the debris flow was calibrated using a back analysis of the debris flow event of 2010.
... In the earlier debris flow prediction, most prediction models were built by means of the investigation of relationship between rainfall and debris flow on the basis of the processing of rainfall data (Kenneth, 1987;Tan and Han, 1992;Chen et al., 2007;Shieh et al., 2009). With the deep-going research of debris flow prediction and the innovative development of data obtaining technologies, environmental factors are paid more attention. ...
The destruction of mountain landform by forest fire often leads to the outbreak of catastrophic debris flow. In March 2020, a huge forest fire broke out in Xiangjiao Gully, seriously damaging the mountain landform and leading to the outbreak of a catastrophic debris flow on July 5th, 2021. To mitigate such disasters more scientifically, this paper analyzes the disaster characteristics of the “7·05” catastrophic debris flow and studies the cause mechanism and mitigation countermeasures. The results show that the debris flow density ranges from 1.831 g/cm3 to 1.930 g/cm3, belonging to viscous debris flow. The velocity of each section point is between 7.22 and 9.78 m/s, with huge kinetic energy. Forest fire, torrential rainfall, and gully terrain conditions combined to cause the outbreak of the debris flow. The rainfall intensity of the maximum rainfall point reached 77.84 mm, corresponding to a recurrence period of more than 200 years. The forest fire severely damaged the landform, and the high-intensity burned area reached 59.97%, which led to a surge in loose materials and wood debris. Combined with steep terrain, channel erosion and the blocking-bursting effect are prominent, leading to the scale amplification of debris flow. The peak flow of the upstream section D1 and the downstream section D6 are 308.21 m3/s and 759.08 m3/s, with the scale expanded by 2.46 times. To mitigate the harm of Xiangjiao Gully debris flow, this paper deduced the determination method of the optimal hydraulic section of a Rectangle-V-shaped drainage canal under the limited conditions. The calculated characteristic parameter of the section is m = 11.75, h2 = 0.60 m.
Rainfall and run-off prediction capabilities for flash flood need improvement in the high porn regions of mountain hazard, those caused massive distraction. At some point, these events result in a massive loss of life and property. This research demonstrates the application of soil conservation service (SCS), a hydrological model used to simulate the relationship between rainfall and run-off. The data cover a 62-year observation period of torrential rains and debris flows, which number up to thirty flow events, all occurring in the mountainous regions close to the Tieli watershed area. Rainfall hydrographs were generated for each of the said torrential rain cases and can be used as markers for flood warnings for critical rainfalls lasting from 1 to 24 h. The results show that the SCS model can successfully define the rainfall–run-off-water level relationship of small-medium-size catchments wherein the calculated run-off values show good agreement with measured discharges. Moreover, from the relationship of the water level and SCS calculated run-off is similar to the relationship obtained using measured run-offs. From this relationship, critical water levels that would correspond to the occurrence of a possibly devastating flood can be determined to provide for an early warning measure for the nearby regions.
Landslide susceptibility assessment is of great significance for the disaster prediction and prevention. At present, most studies used statistical methods by the influence factors of landslide distribution, or based on physical models to determine the assessment result, the research of these methods was mainly focused on the gully scale. At the same time, these methods did not focus on the specific principle of material storage. In this paper, the surface erosion index, being the integral of the hypsometric curve, is adopted to explore the landslides distribution characteristic in different tributaries of the gully. Firstly, 81 tributaries of JJG are taken from DEM with 10 m grid cells, and the hypsometric curves are used to characterize their evolution stages; five stages are identified by the evolution index (EI, the integral of the hypsometric curves) and most tributaries are in relative youth stage with EI between 0.5 and 0.6. Then 906 landslides are interpreted from Quickbird satellite image of 0.61 m resolution. It is found that LD (LD = landslides number in a tributary/ the tributary area) increases exponentially with EI, while LAp (LAp = landslides area in a tributary/the tributary area) fluctuates with EI, meaning that landslides are inclined to occur in tributaries with EI between 0.5 and 0.6, and thus these tributaries are the main material sources supplying for debris flows.
Along the Duwen freeway in the Wenchuan earthquake area, there are two main types of debris flow channels, that is, wide-gentle channel and narrow-steep channel. The wide-gentle channel has the feature of gentle longitudinal slope, large mean width and catchment area. Whereas the narrow-steep one generally shows opposite features with narrow longitudinal slope, small mean width and catchment area. During the heavy rainfall, these channels are subject to large-scale debris flows due to huge amount of earthquake-induced collapsing and sliding materials. With field investigation on the formation conditions and development characteristics in the Taoguan and the Mozi gullies, we analysed the resulting impact, burying and blockage to the Duwen freeway and the Minjiang River. The sediment characteristic in the above-mentioned types of debris flow gully was intensively studied. Finally, prevention measures of the debris flow and effectiveness analysis of the treatment were proposed based on the features of the two types of channels.
The 2008 Wenchuan earthquake in Sichuan, China triggered numerous landslides in the stricken area. The loose landslide materials retained on mountain slopes and in gullies are prone to reactivation and may transform into debris flows in the rainy season. Nine years after the Wenchuan earthquake, debris flows are still active in Gaojiagou Ravine 16.5 km north of the epicentre. On 14 August 2010, 3 July 2011, 13 July 2013, and 6 July 2016, four large-scale debris flows were triggered by heavy storms in Gaojiagou Ravine. The four debris flows blocked the Minjiang River twice and caused severe damage to nearby villages and reconstruction sites. Several questions arise from these repeated debris flows. Did the level of triggering rainfall change in the four debris flow events? How did the initiation mechanism evolve over time? What are the differences in the runout characteristics in the four events? How did the check dams constructed in 2012 function in the subsequent debris flows? In this paper, we evaluated the loose deposit volumes in Gaojiagou Ravine before and after each of the four debris flows and the runout volumes, and analysed the triggering rainfall intensities, initiation mechanisms and runout characteristics of the four debris flows. The rainfall threshold for the debris flows in Gaojiagou Ravine increased over time, and the initiation mechanisms evolved from landslides to channel-bed failure, and subsequently to channel-bank erosion. The mobility of the debris flows decreased from 2010 to 2016 as the initiation positions moved lower and the particle size of the runout materials became coarser.
In recent years, the increasing frequency of debris flow demands enhanced effectiveness and efficiency of warning systems. Effective warning systems are essential not only from an economic point of view but are also considered as a frontline approach to alleviate hazards. Currently, the key issues are the imbalance between the limited lifespan of equipment, the relatively long period between the recurrences of such hazards, and the wide range of critical rainfall that trigger these disasters. This paper attempts to provide a stepwise multi-parameter debris flow warning system after taking into account the shortcomings observed in other warning systems. The whole system is divided into five stages. Different warning levels can be issued based on the critical rainfall thresholds. Monitoring starts when early warning is issued and it continues with debris flow near warning, triggering warning, movement warning and hazard warning stages. For early warning, historical archives of earthquake and drought are used to choose a debris flow-susceptible site for further monitoring. Secondly, weather forecasts provide an alert of possible near warning. Hazardous precipitation, model calculation and debris flow initiation tests, pore pressure sensors and water content sensors are combined to check the critical rainfall and to publically announce a triggering warning. In the final two stages, equipment such as rainfall gauges, flow stage sensors, vibration sensors, low sound sensors and infrasound meters are used to assess movement processes and issue hazard warnings. In addition to these warnings, community-based knowledge and information is also obtained and discussed in detail. The proposed stepwise, multi-parameter debris flow monitoring and warning system has been applied in Aizi valley China which continuously monitors the debris flow activities.
Wenchuan earthquake occurred 7 years ago, and more than 100 landslides and rock avalanches were triggered by the earthquake in Gaochuan river watershed, in Southwest China, including the Daguangbao landslide, which is the largest one triggered by Wenchuan earthquake, having a volume around 1.15 billion m3. The massive landslide deposits provided uncountable material source for debris flow in the study area, resulting in 25 debris flow events occurred in the past 6 years (2009–2014). The purpose of this paper is to analyze the variability in the rainfall threshold for debris flow after the earthquake in the study area. Results suggest that the critical rainfall threshold is closely related to the rainfall intensity (I) and the antecedent accumulated precipitation (P). There was a sudden drop of the rainfall threshold when the earthquake happened. During the 6 years, the rainfall threshold value kept increasing gradually in a linear trend. However, it is now still much lower than the original rainfall threshold prior to the earthquake. The threshold is expected to reach the original level around 2022 based on the analysis of existing data. Our study indicates that the increase of rainfall threshold is related to the decrease of sediments. And there are still lots of loose deposits in the gullies. Thus, the debris flow can still happen in a high possibility in the next few years.
The southwestern region of China, including Yunnan Province, Sichuan Province, Guizhou Province and Chongqing City, is the area most severely affected by debris flows and landslides in China. There are about 15,000 known debris flow and landslide disaster sites, and they pose severe hazards for the local residents and their properties each year. Disaster prediction is the most important measure to minimize casualties from these hazards. This paper outlines a disaster prediction model for debris flows and landslides using statistical methods, and sets up a disaster prediction system using GIS techniques, based on the regional environmental background (including topography, geology, earthquakes, land-use and rainfall), data on debris flows and landslides, and historical disasters records. This system provides a disaster forecast for 12 hours and 24 hours in the future based on rainfall forecasting results using numerical weather prediction by the China Meteorological Administration, and observed data on rainfall from weather stations in the study area. The prediction system was tested by the China Meteorological Administration in the rainy season of 2005. The forecasting result had a high veracity, but the method requires improved spatial analysis of rainfall observed data.
There are three groups of factors related to topography, geology and hydrology which have influence on the formation of gully type debris flows. In this study, a single representative factor (G-factor) for the topography is proposed, which can be used to give threshold values for debris flow formation. The study was carried out in the upper reaches of Minjiang River, which is located in the Wenchuan earthquake area. During some rainfall events after the earthquake, some gully type debris flows were triggered in some catchments. In some catchments no debris flows were triggered even though these catchments were in the vicinity of gullies with triggered debris flows. We isolated and analyzed the influence of the topography on the formation of debris flows because during these rainfall events the hydrological conditions were almost the same in the investigated gullies. We could also neutralize the influence of the geological factors by selecting groups of neighboring catchments located in areas with the same geological characteristics. A new G-factor is proposed in this study as a topographical indicator G, in which F0 is the form factor of formation area, J0 is the average slope of formation area, A0 is the area of formation area, and A* is the unit area (=1 km2). Higher G-factor values are generally related to higher probabilities of debris flow formation. The roles of G-factor are analyzed in other areas such as Zhouqu, Gansu, and Chenyoulan River Watershed, Taiwan. It shows that the higher G-factor values, the higher probabilities of debris flow formation are in generally. In the typical areas of Minjian River areas, it is very high probabilities of debris flow formation when G≥0.21. The probabilities of debris flow formation is very low when G≤0.14.
A rainstorm triggered debris flow hazard occurred in Hongchun valley, Yingxiu town, epicenter of the Wenchuan earthquake on August 14, 2010. This event transported huge material sediment to the watercourse of the Minjiang River and produced a natural dam, resulting in flash flood in the new Yingxiu town. The flood claimed 13 lives, with a further 59 listed as missing. About 8000 local residents were rapidly evacuated from their homes. The debris flow resulted from the coaction of the rainfall and earthquake. Therefore, it is significant for better understanding of development characters of debris flows to conduct the research on the initiation and movement process in high seismic intensity area. This paper studies characteristics of the initiation conditions and depositional fans of the debris flow based on field reconnaissance and interpretations of aerial photographs, focusing on loose sediment supply in source areas of debris flows. On the basis of the above analysis, the initiation and deposition process of debris flows are discussed. The catastrophic event indicates that the areas impacted by the Wenchuan earthquake become more prone to debris flow occurrences in the future. So it is important to assess debris flow risk and take measures to enhance hazard monitoring and offer early warnings for debris flows in the earthquake areas. The control work is also necessary for debris flow hazard mitigation.
The 2008 Wenchuan earthquake greatly altered the thresholds for rainfall-triggered debris flows within the affected area. Debris flows were widely and densely distributed, and they exhibited a range of differing local rainfall thresholds. This study looked at 518 debris flow events that occurred post-earthquake in order to analyze their spatial characteristics. The duration D (in h) and the average rainfall intensity I (in mm/h) that triggered the debris flows were determined for 252 of these events in order to analyze the spatial features of rainfall thresholds. Results show that 49 % of debris flows occurred in the highest-intensity seismic zone, 58 % occurred within 10 km of active faults, and 49 % occurred in areas with humid climate. Rainfall thresholds in these three regions were persistently lower than others. Moreover, debris flows were most frequent in watersheds smaller than 5 km2, and rainfall thresholds tended to decrease with a decrease in watershed size. Given the abundant loose materials available throughout the study area, 11 extreme debris flow-prone sub-regions were selected to illustrate the spatial features of rainfall thresholds in relation to local climate conditions. The lowest and highest I–D thresholds in the sub-regions examined were I = 5.94D
−0.70 (2 < D < 53) and I = 21.4D
−0.58 (3 < D < 50), respectively. The lowest and mean rainfall intensities needed to trigger debris flows were power-related with the local maximum 1- and 24-h rainfall. By normalizing the rainfall intensity (I) by mean annual precipitation (MAP), the I
MAP–D thresholds were determined. Normalized results showed that the lowest and highest I
MAP–D thresholds were I
MAP
= 0.0034D
−0.55 (2 < D < 53) and I
MAP
= 0.0090D
−0.40 (3 < D < 51), respectively. Such results are useful for debris flow forecasting based on empirical rainfall thresholds and have implications for hazard and risk assessment in this region.
For the giant debris flow caused by heavy rainfall in Taoguan gully, in Wenchuan earthquake area on July 10th, 2013. Field investigation and experiments were made to analyze the loose materials, rainfall, and terrain conditions of the Taoguan gully, and study the process of startup flow and deposition of the debris flow. The result shows that the damage of the rock mass triggered by earthquake in the gully is the root cause of such frequent debris flow after the earthquake. Its mechanism is actually the startup and superposition of loose materials from collapse and landslides, and deposits on slopes and in the gully. From the development trend, the Taoguan debris flow is now on a high-frequency developmental stage, and the possibility of breakout of large scale debris flows still exists.
An adaptive control scheme is developed to study the generalized adaptive chaos synchronization with uncertain chaotic parameters behavior between two identical chaotic dynamic systems. This generalized adaptive chaos synchronization controller is designed based on Lyapunov stability theory and an analytic expression of the adaptive controller with its update laws of uncertain chaotic parameters is shown. The generalized adaptive synchronization with uncertain parameters between two identical new Lorenz-Stenflo systems is taken as three examples to show the effectiveness of the proposed method. The numerical simulations are shown to verify the results.
Many debris flows were triggered within and also outside the Dayi area of the Guizhou Province, China, during a rainstorm in 2011. High-intensity short-duration rainfall was the main triggering factor for these gully-type debris flows which are probably triggered by a runoff-induced mechanism. A revised prediction model was introduced for this kind of gully-type debris flows with factors related to topography, geology, and hydrology (rainfall) and applied to the Wangmo River catchment. Regarding the geological factor, the “soft lithology” and “loose sediments” in the channel were added to the list of the average firmness coefficient for the lithology. Also, the chemical weathering was taken into account for the revised geological factor. Concerning the hydrological factor, a coefficient of variation of rainfall was introduced for the normalization of the rainfall factor. The prediction model for debris flows proposed in this paper delivered three classes of the probability of debris flow occurrence. The model was successfully validated in debris flow gullies with the same initiation mechanism in other areas of southwest China. The generic character of the model is explained by the fact that its factors are partly based on the initiation mechanisms and not only on the statistical analyses of a unique variety of local factors. The research provides a new way to predict the occurrence of debris flows initiated by a runoff-induced mechanism.
Many debris flows were triggered in the Chenyulan River Watershed in Taiwan in a rainstorm caused by the Typhoon Toraji. There are 117 gullies with a significant steep topography in the catchment. During this Typhoon, debris flows were initiated in 43 of these gullies, while in 34 gullies, it was not certain whether they have occurred. High-intensity short-duration rainfall was the main triggering factor for these gully type debris flows which are probably entrained by a “fire hose” mechanism. Previous research identified 47 factors related to topography, geology, and hydrology, which may play a role in the formation of gully type debris flows. For a better understanding of the probability of the formation of debris flows, it is proposed to represent the factors related to topography, geology, and hydrology by one single factor. In addition to the existing topographic and geological factor, a normalized critical rainfall factor is suggested with an effective cumulative precipitation and a maximum hourly rainfall intensity. In this paper, a formation model for debris flows is proposed, which combines these topographic, geological, and hydraulic factors. A relationship of these factors with a triggering threshold is proposed. The model produces a good assessment of the probability of occurrence of debris flows in the study area. The model may be used for the prediction of debris flows in other areas because it is mostly based on the initiation mechanisms and not only on the statistical analyses of a unique variety of local factors. The research provides a new and exciting way to study the occurrence of debris flows initiated by a “fire hose” mechanism.
Susceptibility is an important issue in debris flow analysis. In this paper, 26 large-scale debris flow catchments located in the Wudongde Dam site were investigated. Seven major factors, namely, loose material volume per square kilometer, loose material supply length ratio, average gradient of the main channel, average hill slope, drainage density, curvature of the main channel, and poor vegetation area ratio, were selected for debris flow susceptibility analysis. Geographic information system, global positioning system, and remote sensing, collectively known as 3S technologies, were used to determine major factors. Weights of major factors affecting debris flow susceptibility were determined. This paper applied the combination weighting method, which considers both the preference of the engineers for major factors and the objective major factor information by using analytic hierarchy process and entropy method. Combination weights of major factors for the investigated 26 debris flow catchments are 0.20, 0.12, 0.20, 0.10, 0.08, 0.19, and 0.11, respectively. Combination weights follow the order of loose material volume per square kilometer = average gradient of the main channel > curvature of the main channel > loose material supply length ratio > poor vegetation area ratio > average hill slope > drainage density. This paper applied extension theory, which is used to solve incompatibility and contradiction problems, to determine susceptibility. Susceptibility results show that the susceptibility of 4 debris flow catchments are very low, 13 are low, 8 are moderate, and 1 is high. Assessment results exhibit consistency with the activity analysis.
In the Wenchuan area in SW China, an abundance of loose co-seismic landslide debris was present on the slopes after the Wenchuan earthquake, which in later years served as source material for rainfall-induced debris flows or shallow landslides. Slopes composed of Cambrian sandstones and siltstones intercalated with slates appeared to be most susceptible to co-seismic landsliding. A total of 20 debris flows are described in this paper; all were triggered by heavy rainfall on 13th of August 2010. Field reconnaissance and measurements, supported by aerial photo interpretation, were conducted to identify the locations and morphological characteristics of the debris flow gullies in order to obtain information about surface area and volume of landslides and the debris flows. The debris flows in the study area were initiated by two processes: a) run-off erosion on co-seismic landslide material, and concentrated erosion of landslide debris in steep channels; b) new landslides that transform into debris flows. The volume of debris flow deposits on individual fans varies by many orders of magnitude. The smallest deposit has a volume of from 5760 to 3.1 million m³. A comparison of the measured volumes, deposited on the fan with the volumes of debris stored in the catchment shows the huge potential for future debris flow activity. Whilst there is a weakly significant positive correlation between these two volumes, no significant statistical correlation could be established between volumes of debris flow deposits and other morphometric parameters of the catchment.
The Wenchuan earthquake of May 12, 2008 produced large amounts of loose material (landslide debris) that are still present
on the steep slopes and in the gullies. This loose material creates an important hazard as strong rainfall can cause the development
of devastating debris flows that will endanger the resettled population and destroy the result of reconstruction efforts.
On 14 August 2010, a total of 21 debris flows were triggered by heavy rainfall around the town of Yingxue, located near the
epicenter of the Wenchuan earthquake. One of these debris flows produced a debris dam, which then changed the course of the
river and resulted in the flooding of the newly reconstructed Yinxue town. Prior to this catastrophic event, debris flow hazard
had been recognized in the region, but its potential for such widespread and devastating impacts was not fully appreciated.
Our primary objective for this study was to analyze the characteristics of the triggering rainfall and the sediment supply
conditions leading to this event. Our field observations show that even small debris flow catchment areas have caused widespread
sediment deposition on the existing fans. It is concluded that the whole of the area shaken by the Wenchuan earthquake is
more susceptible to debris flows, initiated by localized heavy rainfall, than had been assumed earlier. The results of this
study contribute to a better understanding of the conditions leading to catastrophic debris flow events in the earthquake-hit
area. This is essential for the implementation of proper early warning, prevention, and mitigation measures as well as a better
land use planning in this 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.
Neuropsychological evidence has highlighted the role of the anterior temporal lobes in the processing of conceptual knowledge. That putative role is only beginning to be investigated with fMRI as methodological advances are able to compensate for well-known susceptibility artifacts that affect the quality of the BOLD signal. In this article, we described differential BOLD activation for pictures of animals and manipulable objects in the anterior temporal lobes, consistent with previous neuropsychological findings. Furthermore, we found that the pattern of BOLD signal in the anterior temporal lobes is qualitatively different from that in the fusiform gyri. The latter regions are activated to different extents but always above baseline by images of the preferred and of the nonpreferred categories, whereas the anterior temporal lobes tend to be activated by images of the preferred category and deactivated (BOLD below baseline) by images of the nonpreferred category. In our experimental design, we also manipulated the decision that participants made over stimuli from the different semantic categories. We found that in the right temporal pole, the BOLD signal shows some evidence of being modulated by the task that participants were asked to perform, whereas BOLD activity in more posterior regions (e.g., the fusiform gyri) is not modulated by the task. These results reconcile the fMRI literature with the neuropsychological findings of deficits for animals after damage to the right temporal pole and suggest that anterior and posterior regions within the temporal lobes involved in object processing perform qualitatively different computations.
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