Search

Unlike strong earthquake-triggered or heavy rainfall-triggered landslides, silent large-scale landslides (SLL) occur without significant triggering factors and cause unexpected significant disaster risks and mass casualties. Understanding the initiation mechanism of SLLs is crucial for risk reduction. In this study, the mechanism of the Zhaobishan SLL was investigated, and the SLL was jointly controlled by weak-soil (fractured rock mass) and strong-water (abundant water replenishment) conditions under the impact of active tectonism and complex hydraulic properties. Strong tectonic uplift, high fault density, and historical earthquakes led to weak-soil conditions conducive to the Zhaobishan SLL. The combined effect of unique lithology, antiform, and cultivated land contributed to the water replenishment characteristics of extensive runoff confluence (3.16 times that of the landslide body) and supported long-distance groundwater replenishment, thereby forming strong-water conditions for the landslide. The amplified seepage amount caused the strength of the soil mass on the sliding surface to decrease to 0.4 times its initial strength, eventually triggering the Zhaobishan SLL, which occurred 4.6 days after the peak rainfall. Moreover, the landslide deposits have accumulated on the semi-diagenetic clay rock, thereby controlling the subsequent recurring debris flows in the Lengzi Gully. To reduce disaster risk of SLL in vulnerable mountainous regions, the water confluence area behind the main scarp of the landslides and the hysteresis characteristics between landslides and peak rainfall should be further considered, and recurring debris flows following massive landslides also should be focused. [ABSTRACT FROM AUTHOR]

A large-scale debris flow occurred in Tongzilin Gully in Puge County, Sichuan Province in southwestern China from 4:00 a.m. to 4:30 a.m. on August 8, 2017. Although early warning measures were taken before the occurrence of the debris flow, the disaster resulted in 25 fatalities, the destruction of 71 houses, and economic losses amounting to 160 million RMB. This study aimed to analyze the movement characteristics of the debris flow, reveal the cause and mechanism of the catastrophe, and assist in avoiding similar future disasters. To this end, we calculated the debris flow density, velocity, and peak discharge and analyzed the dynamic characteristics of the debris flow. The distributions of regional precipitation and solid materials were also determined to analyze the formation conditions. Moreover, the debris flow blockage and disaster amplification mechanisms were investigated through field surveys, remote sensing images, and parameter calculation. The results show that the debris flow was a low-frequency turbulent event with a peak flow rate of once in a hundred years and an average velocity of 6.34 m/s. The debris flow developed under unique conditions, corresponding to the combined action of natural and anthropogenic factors. First, the low-frequency debris flow was initiated by antecedent drought and short-term heavy rainfall. The antecedent drought increased the amount of loose materials in the watershed (141.5 × 104 m3). The antecedent drought and short-term heavy rainfall nearly doubled the scale of the debris flow. Second, coarse particles blocked road culverts in the study area, leading to overflow and exacerbating the disaster. Consequently, the affected area and scale of the debris flow have been enlarged by 25% and by two times, respectively. The study findings provide deeper insight into the initiation and development mechanisms of debris flow disasters in mountainous areas, which may help improve the monitoring, early warning, and forecasting system. [ABSTRACT FROM AUTHOR]

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 × 10 m 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 m/s, 870.1 m/s, and 758.7 m/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/cm, 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. [ABSTRACT FROM AUTHOR]