Activity characteristics and risk assessment of debris flow in Mengboluo River basin, a tributary of Nujiang River

Mengboluo River is a first-level tributary on the left bank of Nujiang River. And it is representative in Nujiang River Basin due to its deep canyon landform, widespread sedimental -metamorphic soft rocks, and rich material sources formed by fault shear failure. Ten highly destructive debris flows on the right bank of Mengboluo River were selected as the study objects in this paper, and refined investigation results and Geographic Information System (GIS) analysis technology were adopted to systematically analyze topographic condition characteristics of the debris flows. Besides, provenance types and reserve information were finely classified. The research on dynamic characteristic parameters such as debris flow velocity, flood peak flow, and the total amount of primary solid matter was carried out. The authors took gully basin as the unit and selected the evaluation factors such as the dynamic reserve of material sources, the slope of the hillside, the basin area, the longitudinal slope of the gully, the length of the main gully, the vegetation coverage rate and the rainfall to complete the risk assessment of debris flow. The results showed that the total reserve of provenance in the 10 highly destructive debris flow gullies was 140.49×10⁴ m³, and the amount of material sources that can be activated was 38.96×10⁴ m³, including three major types of provenance, that is landslide accumulation, gully bed erosion, and slope surface erosion. Among them, gully bed erosion was the most developed, and the amount of provenance could be activated, reaching 20.34×10⁴ m³. The debris flows on the right bank of this basin all belonged to rare debris flows, through on-site analysis of provenance types. The overall peak flow of the debris flow in the area was relatively large, and the peak flow of the debris flow in Luziyuan Valley reached 108.92 m³/s. The total amount of solid matter at one time was generally large, and the total amount of solid matter at one time in Luziyuan Valley reached 24.35×10⁴ m³. The results of the risk assessment showed that there were 6 high-risk debris flows, 3 medium-risk debris flows and 1 low-risk debris flow in the basin. After field verification, the assessment results were highly consistent with the on-site conditions. The research results could provide scientific basis for the prevention and control of debris flows along Mengboluo River, a tributary of Nujiang River.