2010 8.14 Hongchun debris flow

The Hongchun catchment is located in the left bank of Minjiang River and adjacent to the 2008 Wenchuan earthquake's epicenter. It has a catchment area of 5.35 km2 and a height range from 880 m to 1700 m. The bedrocks mainly consist of highly fractured and weathered granitic rocks, Sinian pyroclastic rock, Carboniferous limestone and Triassic sandstone.

Hongchun was a debris-flow gully before the Wenchuan earthquake. Massive collapses, landslides, and debris flows were induced by the Wenchuan earthquake. The field survey showed that there were 40 new collapses or landslide initiation points. The largest landslide is located on the right bank of the upper reaches of the gully, with a width of 180 m, length of 300 m, thickness of 10 - 20 m, and total volume of about 36×104 m3. The total volume of loose solid materials in the gully caused by the earthquake was 380×104 m3.

    Figure 1 Map of coseismic landslides in Hongchun Gully

Figure 2  Postseismic debris flows fan driven by heavy rainfall in Hongchun Gully on August 13, 2010 (taken 14 August, 2010).

Before the debris flow occurred, the antecedent rainfall was monitored to be 162.1 mm. The triggering rainfall intensity of the debris flows was about 16.4 mm/h between 2 and 3 AM on August 14, 2010. Both the main gully and three branch gullies upstream contributed to the debris flow. The volumes of initial activated source are 11.2 × 104 m3, 3.9 × 104 m3 and 3.2 × 104 m3, respectively. A total volume of about 80.5 × 104 m3 was carried from the valley in this event and about 40 × 104 m3 material was flooded into the Minjiang River. The large amount of depositional material almost blocked the river and changed its course. The most apparent entrainment occurred in the center of main gully with an average height of 6 - 10 m and maximum depth more than 20 m.

Figure 3 Depositional fan of the Hongchun gully prior to (b) and after (a) the debris flow occurrence.

Figure 4 Debris flows on August 13, 2012 Hongchun Gully (aerial photograph from the Sichuan Geological Environment Monitoring Station).

Figure 5 The snapshots of the debris flow height contour of Hongchun debris flow at (a) t=0, (b) t = 200, (c) t = 800 and (d) the final entrainment depth contour.

The basal entrainment of debris flows play a significant role in its amplification and the final volume of deposition could be several to hundred times of its initial volume. After the event, many researchers carried out indoor experiments and numerical simulation about focusing on the entrainment process during the debris flow movement. Among them, Ouyang et al. (2015) proposed their model and simulated the debris flow process in Hongchun Gully, and the results are shown in Figures.