Thin viscous debris flows are common and dangerous

and their mean velocities are the most important one of their dynamic parameters.It is very important to be able to simply and accurately calculate the mean velocity of thin viscous debris flows.There are still some problems in application of the empirical formulas currently available for the mean velocity of thin viscous debris flows.Based on analysis of the relation of the volume concentration vesus velocity and resistance of the thin viscous debris flows from a series of field observation data

an empirical formula has been proposed to calculate the mean velocity of the thin viscous debris flows.The empirical formula is applicable to various types of flow gullies

and general rapid thin viscous debris flows;but not slow ones because the calculated velocities are larger than the observed ones.The formula is simple and stable in practical calculation

and its results are very close to those obtained with other methods.It is also applicable to the thin viscous debris flow upstream the piled fan;but the calculated velocity of the piled fan is larger than the real one;the further down stream the piled fan is the location

the larger the velocity difference is.