Why cannot sediment transport be accurately predicted

Sediment transport is a geophysical phenomenon that sediment particles are driven to move in streamwise and vertical directions by the corresponding forces. Almost all existing formulae of sediment transport were derived based on the assumption that sediment transport can be fully expressed by streamwise parameters like velocity or boundary shear stress etc., whilst the vertical parameters are not included, like the variation of water depth (pressure) over time and space, vertical velocity or seepage. This paper investigates the effect of vertical motion on sediment transport, it was found that the vertical motion can be well represented by a vertical velocity and the upward velocity increases particles' mobility, and the downward motion increases its stability. Decelerating flows can promote the upward flow and vice versa, this is why severe erosion always occurs in decelerating flows. New equations were developed to express the influence of vertical motion on sediment transport, in which streamwise and vertical parameters are included. A reasonably good agreement between the measured and predicted sediment transport was achieved.