Abstract: Since the operation of the Three Gorges dam (TGD), both bed material and suspended sediment load in the Jingjiang reach have become coarser. Certain vertical profiles of suspended sediment concentration (SSC) are characterized with a near-bed layer (with a thickness less than 10% of the water depth) of extraordinarily large concentration. Such profiles significantly deviate from the classical Rouse equation, and errors in the numerical simulations of them have not been fully understood yet. In this study, we collect the SSC data measured at the Shashi and Jianli hydrological stations before and after the dam operation, and estimate the profiles using the Rouse equation and Han equation for non-equilibrium conditions. Our analysis shows the vertical profiles have two new characteristics–coarser particles of depth-averaged diameters larger than 0.1 mm, and a longer tail or a near-bed layer of extraordinarily high concentration. We found the Rouse equation is not applicable to the SSC profiles of coarse diameters in the unsaturated sediment regime. And contribution to suspended load by this near-bed layer with the two new characteristics may be much larger than that of a common SSC profile, causing underestimation of the suspended load in the Jingjiang reach. Thus, it is essential to take measurement of the near-bed layer.

Key words: hydraulic engineering, unsaturated sediment transport, vertical distribution of suspended sediment, Jingjiang reach, heavy-tailed phenomenon