Spatial features of debris flows and their rainfall thresholds in the Wenchuan earthquake-affected area

Abstract

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 km 2 , 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.

Formation-Evolutionary Mechanism Analysis and Impacts of Human Activities on the 20 August 2019 Clustered Debris Flows Event in Wenchuan County, Southwestern China

“…As shown in Figure 6, the formation and development of a catastrophic debris flow event is a result of the sufficient and continuous interaction of water and sediment. Indisputably, abundant antecedent rainfall provides hydrodynamic conditions for the formation of debris flow, triggering the movement of loose solid material (Tang et al, 2011b;Guo et al, 2016;Hu et al, 2016). Furthermore, the rapid flow ceaselessly incorporates a supply of new solid material through new landslides and collapses, slope-rill erosion, and channel-bed erosion.…”

Susceptibility Analysis of Geohazards in the Longmen Mountain Region after the Wenchuan Earthquake

“…Meteorological and hydrological factors affect the driving mechanism and spatial distribution of geohazards and include precipitation ( Figure 3 k) and Euclidean distance of river (EDR) ( Figure 3 l). The residual loose materials on the slope by the Wenchuan earthquake and the differences in the topographic gradient make the geohazards restricted by precipitation [ 65 ]. Liner erosion and the undercutting of a slope under high flow conditions may cause a slope to become unstable and induce the development of geohazards, and this effect is significantly correlated with the distance from the river [ 66 ].…”

The structure of a magnitude-frequency relation for debris flows conditioned by earthquake shock

“…The rate then decreases (Fan et al, 2018a), finally reverting to near preearthquake levels (Hovius et al, 2011;Marc et al, 2015). It has also been established that, in the aftermath of a strong earthquake, the rainfall threshold for debris flows is reduced (Shieh et al, 2009;Yu et al, 2014;Guo et al, 2016a;Guo et al, 2016b;Fan et al, 2019). This reduction persists for a number of years before reverting back (increasing) to near pre-earthquake levels (Yu et al, 2014;Zhang and Zhang, 2017;Domènech et al, 2019).…”