Abstract: Since the operation of Xiaolangdi Reservoir, significant changes have occurred in the conditions of water and sediment entering the lower Yellow mainstream. Due to the impact of clear water discharge from the reservoir, noticeable bed sediment coarsening has occurred downstream of the reservoir. To investigate the impact of bed sediment gradation adjustment on flood routing in this mainstream section, this study first develops a one-dimensional water-sediment coupling mathematical model suitable for the section. And the model is validated using the observed floods occurring from Apr. to Oct. 2020 in the reach of Huayuankou to Sunkou. Then, we design four sets of bed sediment gradations as the initial bed sediment conditions for numerical simulations, based on bed sediment coarsening characteristics over this reach, and taking bed sediment coarsening at Huayuankou as a representative process. Finally, the validated model is used for simulations and the impact of bed sediment gradation adjustment on flood routing is examined. The simulations show the adjustment has an influence on the discharge, water level, sediment concentration, flood routing time, and scour and deposition in the lower Yellow mainstream. Generally, the influence on the river discharge is relatively small, while more sensitive are the responses of water level, sediment concentration, flood routing time, and scour and deposition to the changes in bed sediment gradation. We predict that when the median diameter (d50) of bed sediment increases from 0.076 mm (Gradation 1) to 0.242 mm (Gradation 4), water level at the Jiahetan cross-section will rise by 0.89 m, and scour and deposition in the Huayuankou-Sunkou reach will change from a scour volume of 67.9 million m3 to a deposition volume of 5.9 million m3, indicating a reduction of 109% in the scour rate. The findings of this study help manage the lower Yellow River channel under new water-sediment conditions.

Key words: 1D water-sediment coupling model, bed sand coarsening, flood routing, scour and deposition change, lower Yellow River