Abstract: The vertical distribution formulas of suspended sediments’ particle size and gradation in turbulent flow region are derived respectively based on the Rouse formula and the formula of sediment particle settling velocities in turbulent flow. The former directly quantifies the vertical distribution law of sediment particles being “fine in the upper and coarse in the lower”, while the latter can be used to calculate the suspended sediment’s gradation in different water layers and the total gradation in vertical direction. The formula of particle size distribution is applied to a practical example to calculate the suspended sediment’s gradation in different water layers. Results suggest that the gradation is obviously in the law of “fine in the upper and coarse in the lower”. The farther away from the bed the water layer, the larger the proportion of fine particles, the more uniform the particle size distribution. In consideration of the influence of suspension height, the maximum suspension height of suspended sediments with the largest particle size is taken as the demarcation below which the gradation is in equal width, and above which the gradation distributes narrowly in the lower whereas widely in the upper.

Key words: gradation of suspended sediment, vertical distribution formula, Rouse formula, relative maximum suspension height, turbulent flow region