Background On 4 August 2016, a debris flow event occurred in Zechawa Gully, Jiuzhai Valley, Sichuan province. During this debris flow event, the pedestrian walkways were buried, and the only scenic road from Nuorilang Waterfall to Long Lake was blocked, causing serious impact on the scenic area. The rainfall data from precipitation stations showed that the maximum 1 h rainfall and 24 h cumulative rainfall on 4 August 2016 were 10.0 mm and 17.5 mm respectively, which were far below the rainfall intensity of the study area at once in 10-year, and the scale of the debris flow disaster was inconsistent with rainfall.

Methods To clarify the cause of the debris flow disaster, field investigations were conducted, and the debris flow peak discharge was obtained by the cross-section survey method, rain-flood method and dam breach analysis.

Results The debris flow peak discharges at the two typical mud mark cross sections estimated by the cross-section survey method were 33.29 m³/s and 36.69 m³/s respectively, which were similar to the debris flow peak discharge (32.73 m³/s) at once in 20-year period. The debris flow peak discharges calculated by dam breach analysis varied in 36.5 m³/s to 43.6 m³/s, which were similar with the peak discharge obtained by the cross-section survey method. The total volume of the debris flow material calculated by the equation recommended by Specification of Geological Investigation for Debris Flow Stabilization was 1.48×10⁴ m³, which was consistent with the field measured value 1.39×10⁴ m³, and further supported the calculated debris flow peak discharge obtained from the cross-section survey method and dam breach analysis.

Conculsions Based on our research results, the debris flow on 4 August 2016 was caused by dam breach, and the scale of the debris flow was equivalent to that at once in 20-year. The destruction of check dam in the channel has produced an amplification effect on the peak discharge, aggravating the debris flow disaster. Affected by the Ms 7.0 Jiuzhaigou earthquake recently, more than 20×10⁴ m³ of loose material are active for debris flow events in the future, and the frequency and scale of debris flow disasters in Zechawa Gully will increase in the future. The original design standard of the debris flow mitigation countermeasures for once in 20-year was low, and the engineering countermeasures with the design standard of 50-year period should be taken after the Ms 7.0 Jiuzhaigou earthquake, and concrete materials should be used to construct control projects (such as check dams) to reduce the risk of breach.