A catastrophic debris flow in the Wenchuan Earthquake area, July 2013: characteristics, formation, and risk reduction

In the Wenchuan Earthquake area, many co-seismic landslides formed blocking-dams in debris flow channels. This blocking and bursting of landslide dams amplifies the debris flow scale and results in severe catastrophes. The catastrophic debris flow that occurred in Qipan gully (Wenchuan, Southwest China) on July 11, 2013 was caused by intense rainfall and upstream cascading bursting of landslide dams. To gain an understanding of the processes of dam bursting and subsequent debris flow scale amplification effect, we attempted to estimate the bursting debris flow peak discharges along the main gully and analyzed the scale amplification process. The results showed that the antecedent and triggering rainfalls for 11 July debris flow event were 88.0 mm and 21.6 mm, respectively. The event highlights the fact that lower rainfall intensity can trigger debris flows after the earthquake. Calculations of the debris flow peak discharge showed that the peak discharges after the dams-bursting were 1.17-1.69 times greater than the upstream peak discharge. The peak discharge at the gully outlet reached 2553 m(3)/s which was amplified by 4.76 times in comparison with the initial peak discharge in the upstream. To mitigate debris flow disasters, a new drainage channel with a trapezoidal V-shaped cross section was proposed. The characteristic lengths (hi and h2) under optimal hydraulic conditions were calculated as 4.50 m and 0.90 m, respectively.