Abstract: In order to fully reflect the randomness of sediment’s incipient movement, we deduced a formula of incipient velocity suitable for both coarse and fine grains by using a sediment rolling initiation model based on the probability density distribution of dimensionless exposure degree of the grains. The stochastic characteristics of instantaneous flow velocity and sediment particle arrangement on a bed surface were also considered in this paper. Then, the incipient velocities calculated by the proposed formula were compared with measured data and those by several classical incipient velocity formulae. The results show that the incipient velocities calculated by the proposed formula are in more agreement with the measured data than those calculated by the classical formulae. Therefore, the proposed formula’s reliability and superiority in predicting incipient movement of coarse and fine sediment are testified.

Key words: sediment, incipient velocity, exposure degree, instantaneous velocity, probability statistics