Surface-layer Armoring of the Jingjiang Reach after the Three Gorges Project Operation

Streams generally in a seriously unsaturated state, and the riverbed armoring occurred in the downstream of the Three Gorges Project (TGP), leading to the change of grain size distribution and bed surface resistance because of the operation of TGP. Firstly, the relationship between bed shear stress, flow and sediment regime of gravel-sand bed and sandy bed was investigated by using the measured data of Jingjiang Reach in the downstream of the TGP. Then, the theoretical formula of the active layer thickness in the Upper Jingjiang Reach is derived by considering the comprehensive influences of sediment motion probability and the effects of hiding and exposure between sediment particles on the surface-layer armoring thickness; and based on the analysis of the measured data in the Lower Jingjiang Reach, the empirical pick-up function of suspended sediment and surface-layer sediment in Yangtze river was obtained. Moreover, from the perspective of the exchange theory and statistical probability of sediment mixtures, the relationships between the grain size distribution of surface-layer armoring thickness, flow and sediment regime in the early stage of the riverbed armoring process in the Lower Jingjiang Reach is proposed. The results show that the grain size distribution of the Upper Jingjiang Reach was obviously armored with the change of flow and sediment regime year by year. In 2000, the bed sediment grain size range is from 0.062 mm to 0.5mm, and the median diameter D₅₀= 0.181mm; However, the bed sediment grain size range of the Upper Jingjiang Reach is from 0.25 mm to 64mm, and the median diameter D₅₀= 0.335mm in 2014; And the surface-layer shear stress changes of gravel-sand bed is also more obvious than that of sand bed along with the riverbed armoring process. Through validation with the measured data of Jingjiang Reach in the downstream of TGP, the formula developed in this study is proved to be reasonable and has a higher calculation accuracy, and it contributes to full understanding of sediment transport on both sand-bed and gravel-sand bed channels.