A study on the forecast calculating method of the density of rainfall debris flow in Southwestern of China

Density is the most important parameter of debris flow. This work looks into the typical debris flow and mountain torrent occurred in Longmen Mountains in the past 20 years and compares the calculation results of the density using various methods. It shows that the mean deviations of result are in the range of 20–30%, and the static model is only suitable to the occurred debris flow event but not the forecast. Based on the analysis of typical rainfall debris flow, the influence factors of density are revealed; it shows that the density is controlled by the form conditions and the soil-water coupling process of debris flow. A function is introduced to describe the activity of material source, which defined by volume and relative elevation of materials, and the watershed area. The forecast model is established by the relationship between the density and its influence factors. When the new method is applied to the Longmen mountain area and the upper basin of the Minjiang River, although there are still few discretization errors, the results still can explain nearly 95% of the evolution trend, which shows that the new method is not compromised by regional differences.