STUDY ON THE EMPIRICAL EQUATION OF VELOCITY OF VISCOUS DEBRIS FLOWS IN CHANNEL

Velocity of debris flow is the most important parameter in the dynamics parameters of debris flows. Velocities of debris flows are quite different at different areas: high velocities of debris flows were found in low resistance areas, low velocities of debris flows were found in high resistance areas. The asymmetric coefficients (here is the ratio of mid-value grain size D₅₀ and grain size of less than 10% D₁₀ of the sediment in debris flow) of debris flows are quite different: debris flows with high velocities have large asymmetric coefficients; debris flows with low velocities have small asymmetric coefficients. The asymmetric coefficients of debris flows could be used to classify resistance characteristics of debris flows. An empirical equation of mean velocity of viscous debris flow in channel was got by analyzed a few series field observation data of channelized debris flows:

U=1.1(gR)^1/2S^1/3(D₅₀/D₁₀)^1/4

in which U=mean velocity of viscous debris flow in channel (m/s); g=acceleration of gravity (9.81m/s²); R=hydraulic radius of debris flow (m); S=slope of the channel; D₅₀= mid-value grain size of the sediment in debris flow (mm); D₁₀ = grain size of less than 10% of the sediment in debris flow (mm).

The empirical equation is good for calculating velocities of viscous debris flows both in high resistance and low resistance areas. It is also good consistent for the measuring velocities of others series field observation data of debris flows. The Froude number (Fr) of flow is the factor of flow status: supercritical flow or subcritical flow. Ordinary viscous debris flows are supercritical flows (Fr>1), minorities are subcritical flows (Fr<1), and few are slow-motion (here Fr<0.33) debris flows which have too large densities. The empirical equation is excellent for calculating the velocity of ordinary supercritical viscous debris flow, but it is bad for calculating the velocity of the slow-motion flow of debris flow, and it is gentle large for calculating the velocity of subcritical viscous debris flow. In the evaluation and prevention of debris flows, the empirical equation could be used for calculating velocity of viscous debris flow in the channel. It is gentle large for calculating velocity of viscous debris flow on the debris flow fans.