Double-thresholds in scour–fill processes and some implications in channel adjustment

Through an analysis of data collected from the Yellow River and its tributaries on the Loess Plateau of China, the phenomenon of double-thresholds in scour–fill processes of wide-range water-sediment two-phase flows has been shown. Thresholds located in non-hyperconcentrated flows may be called the lower threshold, and that in hyperconcentrated flows the upper threshold. This double-threshold phenomenon leads to complicated sediment transport behavior of heavily sediment-laden rivers. With an increase in suspended sediment concentration, the channel sediment delivery ratio increases initially and becomes higher than 1, followed by a decrease and finally becomes lower than 1 again. lled by the double-thresholds in the scour–fill processes, channel adjustment of the lower Yellow River is non-linear and complex. When the suspended concentrations were lower than the lower threshold or higher than the upper threshold, scour or bed downcutting was the dominant channel-forming process. Channel shape tends to be narrower and deeper, and the channel thalweg became more sinuous. When the suspended concentrations lay between the lower and upper thresholds, deposition of sediment was the dominant channel-forming process; channel shape tended to be shallower and wider, and channel thalweg became less sinuous.