Background There were many debris flow gullies along the Mangkang county section of the Jinsha River. The frequent debris flow activities had destroyed the roads along the river, and even blocked the river to form a dammed lake, which seriously threatens the normal life of people in this area. In order to clearly understand the formation law and disaster-causing characteristics of debris flows in this area, this study selects the representative Chutigang gully as the research object, and we conducted detailed field investigation and analysis for providing theoretical reference for the prevention and control of debris flow disasters in this region.

Methods In this study, the peak discharge of the latest debris flow was calculated by using the cross-section survey method: firstly, a typical channel section was selected, and its geometric parameters, channel longitudinal gradient and debris flow's mud mark high were measured, secondly these parameters were substituted into the Manning formula to complete the calculation. The outbreak scales of debris flows under different rainfall frequencies were calculated by using the rain-flood method, and the calculation results were substituted into two empirical formulas of river blocking to analyze the river blocking probability by debris flows under different rainfall frequencies.

Results After field investigation, the solid source reserve of Chutigang gully was 625.1×10⁴ m³, among which solid source of slope erosion was 576.8×10⁴ m³. The peak discharge of the latest debris flow activity was 122.92 m³/s, and its outbreak scale was close to the magnitude of once in 50-year period. According to two empirical calculation formulas about river blocking by debris flow, it was known that when a debris flow with a magnitude of once in 100-year period, it may cause serious blockage of the river, and a once in 50-year period debris flow may cause minor blockages in the river. It should be alerted to the risk of debris flow blocking the main river to form a dammed lake.

Conclusions According to the field investigation, most of the solid source of Chutigang gully was located in the middle and lower reaches of the watershed, which was prone to cause blockage-collapse phenomenon and expands the magnitude of debris flow. Based on the disaster characteristics of debris flow in Chutigang gully, a "Reinforcement+Blocking" disaster mitigation project was proposed, of which the check-dams group were mainly arranged in the middle and lower reaches of the watershed, they were used to reduce the accumulation of loose solid materials from both sides of the hillslope, and to reduce the probability of the channel being blocked. Silt-trap dam can reduce the debris flow peaks discharge and thereby mitigate the degree of erosion on the downstream channel. The treatment project can provide reference for the management of debris flow disasters in this region.