Abstract
Abstract. River floods are usually featured by a downstream flattening discharge peak whereas a downstream increasing discharge peak is observed at a rate exceeding the tributary discharge during highly silt-laden floods (hyperconcentrated floods) in China's Yellow River. It entails a great challenge in the downstream flood defence and the underlying mechanisms need to be unravelled. Previous study on this issue only focuses on one possible mechanism, while the present work aims to reveal the relative importance of bed roughness change and bed erosion in the hyperconcentrated flood. Using a newly developed fully coupled morphodynamic model, we have conducted a numerical study for the 2004 hyperconcentrated flood in the Xiaolangdi-Jiahetan reach of the Lower Yellow River. In order to focus on the physical mechanism and to reduce uncertainty from low-resolution topography data, the numerical modeling was carried out in a schematized 1-D channel of constant width. The basic understanding that bed roughness decreases with concentration at moderate concentrations (e.g. several 10 s to 100 s g L−1) was incorporated by a simple power-law relation between Manning roughness coefficient and sediment concentration. The feedback between the bed deformation and the turbid flow, however, was fully accounted for, in the constituting equations as well as in the numerical solutions. The model successfully reproduced the downstream flood peak increase for the 2004 flood when considering the hyperconcentration-induced bed roughness reduction. As the hyperconcentration lags shortly behind the flood peak, later parts of the flood wave may experience less friction and overtake the wave front, leading to the discharge increase. In comparison, bed erosion is much less important to the discharge increase, at least for hyperconcentrated flood of moderate sediment concentration.
Highlights
The Yellow River, the second longest river in China, is famous for the high sediment load in its middle and lower reaches
The effect of bed roughness reduction is considered by a power law relation between the sediment concentration and Manning roughness
The 2004 hyperconcentrated flood with moderate concentrations is modeled in a 1-D schematic Xiaolangdi-Jiahetan reach of the Lower Yellow River
Summary
The Yellow River, the second longest river in China, is famous for the high sediment load in its middle and lower reaches. Huayuankou hydrological stations (in the Lower Yellow River), which are approximately 125.8 km apart In this period, the average increasing rate is as high as 50 %, with floods of moderate concentrations (i.e., 100 to 400 kg m−3) mostly conveyed inside the main channel. The average increasing rate is as high as 50 %, with floods of moderate concentrations (i.e., 100 to 400 kg m−3) mostly conveyed inside the main channel This considerable increase in the peak discharge greatly increases the flood risk in the lower reach, along with the severe sedimentation due to runoff reduction. Focusing on the floods mostly conveyed inside the main channel, this paper aims to reveal the relative importance of bed roughness change and bed erosion to the peak discharge increase. A simple power-law relation is used to represent a decreasing roughness with concentration
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