Abstract: Changes in the energy dissipation rate of a river reflect its ability to transport water and sediment and its patterns in bed morphology adjustment. This study explores the quantitative relationships of boundary resistance energy dissipation rate (BREDR) versus incoming water and sediment, cross-sectional morphology, and riverbed stability, from a perspective of energy dissipation, combined with statistical analysis of data measured in the lower Yellow reach of Huayuankou to Sunkou. We determine an efficient flood sediment transport threshold for this reach, based on two indicators: BREDR, and riverbed stability. The results show that the flow rate is the main factor of BREDR, and it is linearly related to the power of BREDR. As the flow increases above 2000 m3/s, its BREDR shows a trend of weakening first and then strengthening with the increase in sediment concentration. Generally, a river’s BREDR is negatively correlated with the width-to-depth ratio under high flow conditions. The correlation is low for the study reach of Huayuankou and Jiahetan where cross-sectional morphology is complex, while high at the cross sections of Gaocun and Sunkou with simple morphology. Under unsteady water and sediment conditions, the riverbed will mostly develop instability. Considering both bed morphological stability and the minimum of BREDR, our calculations reveal the flow rate threshold for efficient flood sediment transport in the study reach is roughly 1500 m3/s, with a sediment concentration of about 23 kg/m3.

Key words: lower Yellow River, boundary resistance energy dissipation, section shape, riverbed stability, high-efficient flood sediment transport