Probability of Individual Grain Movement and Threshold Condition

Abstract

A new criterion for the experimental characterization of the threshold of motion of sediment as bed load is introduced and this criterion is implemented in an experimental investigation of the role of sediment packing density on bed behavior near the threshold of motion. The new criterion, which may be interpreted as the probability of individual grain movement, accounts for the statistical nature of sediment movement in turbulent flow and the time scale of the flow. The threshold of motion is specified by a fixed value of the probability. The criterion represents an objective, quantitative and consistent approach to the threshold condition for sediment of uniform size, shape, and density. The experimental investigation demonstrates that this criterion leads to results that are comparable to those of other approaches, for nearly isolated grains; however, it is found that a threshold criterion based upon the probability of grain movement can yield relatively active sediment beds where the level of activity is strongly dependent upon sediment packing density.