Abstract
Both global climate change and human activities are continuously impacting the abruptness and frequency of water-related natural disasters such as flash floods, debris flows, and landslides in mountainous areas, greatly threatening the safety of lives and properties. A recent rainfall-induced debris flow event happened on July 6, 2020 in the Chenghuangmiao Gully, in Sichuan Province, China, resulting in severe damage to buildings at the outlet. An integrated analysis of the consequence and triggering mechanism of this debris flow event was conducted with hydrologic information, topographic details, vegetation regimes, and drone aerial imagery. The result shows that the entire runout of the debris flow differs from that of common ones (debris flow and rainfall were highly related and synchronized), which happened 4 h after the stop of the rainfall. The hysteretic feature increases the difficulty of the prediction and warning of the debris flow due to lack of a responsible triggering mechanism. The hillslope surface is well covered by vegetation, hindering regular observation and cleaning up of long-term deposited wood and sediment debris. This effect increases the crypticity and abruptness of potential debris flows. With field evidence and analysis, it is speculated that long-term accumulative processes of dead wood sand sediment deposition formed a small-scale debris dam, and the continuous water release from the watershed led to dam breaching, subsequently triggering the initiation of the debris flow. Multiple steps distributing along the gully of an average slope of 15.65° contributed to the amplification of the debris flow once the breach of the upstream wood and sediment dam occurred. Along the gully, small-scale landslide scars can be observed, possibly amplifying the scale of the debris flow and disaster impact. This debris event gives a lesson of necessary demands of predicting and managing the risks of a low-frequency debris flow non-synchronized with rainfall events.
Highlights
In the mountainous region of Southwest China, water-related disasters such as flash floods and debris flows represent the most catastrophic hazards, leading to infrastructure damage, property loss, and fatality
The model is based on the concept of the geomorphologic instantaneous unit hydrograph (GIUH), which was initially developed by Rodriguez (Rodríguez-Iturbe and Valdes, 1979; Lee and Yen, 1997; Yen and Lee, 1997)
The needed geomorphologic features can be statistically obtained from the digital elevation model (DEM) through ArcGIS
Summary
In the mountainous region of Southwest China, water-related disasters such as flash floods and debris flows represent the most catastrophic hazards, leading to infrastructure damage, property loss, and fatality. Water-Related Disaster in Mountainous Areas due to both gravitational erosion and hydraulic erosion, becoming massive sources for debris flows (Yin et al, 2016; Gregoretti et al, 2018) Such physical processes can be amplified by earthquakes with large magnitude (Cui et al, 2011; Huang and Fan, 2013; Horton et al, 2019). An example is that both the frequency and amount of debris flows in and near the earthquake area significantly increased after the dominant 2008 Wenchuan earthquake, with magnitude Ms 8.0 as well as 10-year aftershocks (Chen et al, 2012; Tang et al, 2012; Zhang et al, 2014; Hu et al, 2019). Another factor to amplify the debris flow magnitude and increase the frequency is global climate change since extreme rainfall generating intense surface runoffs potentially triggers the initiation of debris flows (Stoffel et al, 2014)
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