Effects of grouped suspended sediment transport on channel evolution in the Lower Yellow River

The operation of the Xiaolangdi Reservoir greatly changed the transport process of grouped suspended sediment in the Lower Yellow River (LYR), accompanied with the discrepancy in channel evolution of different reaches. Based on the measured hydrological data from 1999 to 2018, the transport characteristics of different grouped suspended sediment during flood seasons have been analyzed, and moreover an empirical function for calculating the cumulative channel evolution volume of each reach has been established. Results show that: ① Sediment discharge of each grain-size dramatically decreased after the reservoir operation, among which the sediment discharge of medium grain-size decreased most by 85%;from the spatial perspective, the sediment discharge of fine grain-size seldom increased along the channel, while that of medium and coarse grain-sizes increased to the stations of Lijin and Aishan respectively. ② The riverbed in the lower reaches became more coarsely grained owing to the continuous channel degradation, which resulted in the inadequate supply of fine and medium grain-size sediments to the suspended sediment in the braided and transitional reaches. While the mildly coarsened riverbed in the meandering reach could still supply the medium grain-size sediment to the suspended load. ③ Channel undercutting generally occurred in the LYR, accompanied by different channel adjustment characteristics in these reaches, which resultantly exerted different effects on the flow condition. A close correlation has been found between cumulative channel erosion and post-flood geomorphic coefficient in the last year and incoming sediment coefficients of the specified grain-sizes during flood seasons, which can reasonably account for the effects of channel boundary condition and non-equilibrium transport of grouped suspended sediment. Additionally, the verification process of the proposed empirical equation shows that the calculated values agreed with the measured channel evolution process.