Response of Sediment Discharge Ratio to Water-Sediment and Riverbed Boundary Conditions During Flood Events in the Lower Yellow River Since 2000

The sediment discharge ratio is greatly affected by water-sediment and riverbed boundary, which indicate the river's ability to transport sediment under specific conditions. This study explores how the sediment discharge ratio responds to water-sediment and riverbed boundary conditions in the Lower Yellow River (LYR) since the operation of the Xiaolangdi Reservoir began. It evaluates the spatial-temporal variations of water-sediment and riverbed boundary based on hydrologic data and topographic data from 2000 to 2023. Based on the sediment transport rate formula, a theoretical formula for the sediment discharge ratio during flood events is developed, which thoroughly accounts for the effects of riverbed boundary conditions, including median particle size of bed sediment, river gradient, and bankfull width-to-depth ratio. The results show that the sediment discharge ratio negatively correlates with the inflow sediment coefficient, median particle size of bed sediment, and bankfull width-to-depth ratio. In contrast, it positively correlates with the runoff variation coefficient and river gradient. Compared to only considering runoff and sediment conditions, including the riverbed boundary in the theoretical formula yields a better fit to the measured sediment discharge ratio data, demonstrating that the riverbed boundary is a significant factor affecting the sediment discharge ratio. Under the current boundary conditions, the Aishan to Lijin reach has the highest sediment transport capacity. To enhance the sediment transport capacity of the Tiexie to Lijin reach, it is recommended to narrow the river width above Gaocun and increase the bankfull width-to-depth ratio. The outcomes of this research offer crucial scientific insights into the sediment transport capacity of alluvial rivers subjected to variations in water-sediment and riverbed boundary conditions, thereby providing vital references for hydrological engineering and river management practices.