Characteristics of a bidirectional position of natural sediment on a two-dimensional river bed

The initial position of the sediment particles on a bed is among the important factors that influence the incipient motion of sediments. In this paper, the concepts of relative protrusion degree and bidirectional hiding-exposure degree are proposed to understand the random arrangement characteristics of natural sediment on a two-dimensional bed more precisely. For this purpose, a series of measurement tests were done. Three sets of pebbles, with different size distributions, were randomly placed into a narrow glass groove, and the surface particles were made nearly horizontal. Subsequently, crucial factors such as the exposure degree and protrusion degree of pebbles were investigated using a vernier caliper with an electric display. Finally, through the analysis of the measured data, the distribution laws of the initial position of the particles were obtained. The statistical analysis reveals that the values of the relative protrusion degree of natural sediment range from 0 to 1, and their probability density distribution follows a negative skewed distribution; the values of the relative exposure degree of natural sediment range from 0 to 0.5, while their probability density distribution follows a normal distribution approximately; the values of bidirectional hiding-exposure degree range from 0 to 1.5, and their probability density distribution follows a normal distribution approximately. Additionally, with an increase of the standard deviation for pebbles, the deviation coefficient of the relative protrusion degree as well as the mean value of the bidirectional hiding-exposure degree increases gradually; however, the mean value of the relative exposure degree decreases moderately.