Critical Tractive Force of Very Large Gravels on Mountainous River-Bed

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In this study, at first, the characteristic of fluid force acting on three types of gravel model was investigated experimentally with respect to both the time averaged and flactuating values of the drag and lift. Secondly, the process of the escaping phenomena was considered theoretically and an equation for the critical tractive force of an isolated gravel protoruding from sediment bed with an arbitrary protrusion hight.The Summary of the Results of this Research is as Follows.(1) The velocity at the hight of the top of gravel is most pertinent as the representative velocity of the fluid force.(2) The rate of the time-averaged value of lift to that of drag is not affected by relative depth but greatly varies with the change in protrusion rate.(3) The characteristics of drag and lift coefficients of sphere and short columns (circler and elliptical cross sections) were quantitatively clarified.(4) The probability density of the fluctuation of drag and lift is well approximated by the Gauss's normal distribution function.(5) The relation between the standard deviation of the fluid force fluctuation and the protrusion rate was also clarified quantitatively.(6) In the theoretical consideration, the behavior of a spherical gravel which rolls up a slope under the effect of turbulent flow was formulated and solved by the use of some assumptions for fluid force.(7) The deduced equation for the critical tractive force includes the pick-up rate as one of the para meters, and therefore the physical meaning of the value given by the equation is distinct.(8) The Validity of the Equation was Confirmed Experimantally.