THE RAINFALL PREDICTION MODEL OF DEBRIS FLOWS TRIGGERED BY RUNOFF MECHANISM

The channel width and particle size in the formation section of debris flow catchment are very important factors in the initiation of debris flows triggered by runoff mechanism. This phenomenon was often found in the debris flows in strong earthquake area. Up to now, there is no prediction model of debris flow involving these two factors in the same time. Field investigations were conducted for the group debris flow events of Dayi, Wangmo County, Guizhou Province, China, and the group debris flow events of Dechang, Sichuan Province, China. By analyzing the initiation mechanism of debris flows triggerd by runoff mechanism, the geological factor was expressed by the particle size in the formation section of debris flow catchment, and the topographical factor was added the channel width of the formation section of debris flow catchment. The new prediction model was obtained by comparing the triggering thresholds of debris flows of Jiangjia Gully and Dayi area：

P=RT^0.2/G^0.38 ............... (1)

T=JF(A/W2)0.2 ............... (2)

G=D/D₀ ................... (3)

R=(B+kI)/(R₀C_v) ...............(4)

in which P = prediction factor; Cr = critical value for the prediction of debris flows; T = dimensionless topographic factor of the formation section of the gully where the debris flows are initiated; A = area of formation section (m²); F = form factor of formation section; J = average slope of the channel of formation section; W = the channel width in formation section (m); G = dimensionless geological factor; D = particle size in the formation section (mm); D₀ = 2mm; R = dimensionless rainfall factor; R₀ = annual precipitation of the site (mm); C_v = coefficient of variation of 10 min; B = cumulative precipitation, until the start of the debris flow (mm); k = 12.5 (1 h model) or 8 (10 min model); I = amount of rainfall in the hour (1 h model) or in the 10 min (10 min model) before the start of the debris flow (mm).

The critical values are 0.24 and 0.33 (1 h model) or 0.055 and 0.072 (10 min model), which indicate the alarm class are warning and refuge. The validations were made successfully with the group debris flow events of Dechang, and with a series debris flows in Wenjia Gully, which in the Wenchuan earthquake area. The new prediction model of debris flows can be used for forecasting of the ordinary debris flows and debris flows in strong earthquake areas.