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

Abstract

Catastrophic debris flows triggered by a 4 July 2013 rainfall in Shimian, SW China: formation mechanism, disaster characteristics and the lessons learned Abstract 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 46.7mm. (2) Entrainment was the most important sedimentsupplying 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 4 m 3 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 751.0m 3 /s, 870.1m 3 /s, and 758.7m 3 /s, respectively. (4) Debris flows that occurred from the three gullies all belonged to viscous ones and the bulk densities were calculated more than 1.80g/cm 3 , 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.

Preliminary investigation of the 20 August 2014 debris flows triggered by a severe rainstorm in Hiroshima City, Japan

“…García-Martínez and López 2005;Casagli et al 2006;Tang et al 2012;Cevaso et al 2014;Chen et al 2014;Ni et al 2014;Yang et al 2015). For example, in December 1999, extreme rainfall on the northern Venezuelan coast triggered a disastrous debris flow, which caused about 1,500 fatalities and the destruction of 23,000 houses (García-Martínez and López 2005).…”

Monitoring of the reconstruction process in a high mountainous area affected by a major earthquake and subsequent hazards

“…8c), as the magnitude of the 2010 debris flow exceeded the mitigation capacity. Such mitigation might create a false sense of security and possibly leading to more losses (Olugunorisa, 2009;Cigler, 2009). It might be not beneficial to start installing mitigation measures right after an earthquake, as the magnitude and frequency of mass-movement hazards might be too costly to mitigate.…”

“…The most commonly witnessed cases include landslides forming barrier lakes which later cause outburst floods Dong et al, 2011) or debris flows (Hu and Huang, 2017). Moreover the debris flows can result in river damming and riverbed rising (Ni et al, 2014;Tang et al, 2012;Fan et al, 2019b), causing floods. A comprehensive summary of postearthquake hazard chains was made by Fan et al (2019b).…”

Multiply factors driving continual post-wildfire debris flows with varied rainfall thresholds in the Reneyong Valley, southwestern China

“…The rainfall intensity recorded by the downstream rain gauge was 1 mm/h or less and the duration was quite short, which is in line with the memory of the local citizen in the downstream area who witnessed the flow, suggesting the rain gauge was working well; however, it seems impossible for a low-intensity short-duration rainfall to generate surface water runoff, let alone entrain sediment and trigger debris flows. In the Hengduan Mountains, isolated convective rainfall is common and has been found to be an important trigger of debris flows in this area (Tang et al, 2011;Ni et al, 2014); in addition, rainfall intensity has also been found to increase with elevation in the Jinsha Basin (Tan et al, 1994). Here, we speculate that the three post-wildfire debris flows in 2014 were triggered by isolated convective rainfall and that the rain gauges down slope can definitely record the triggering rainfall.…”