Fluvial hydraulics of hyperconcentrated floods in Chinese rivers

摘要

Hyperconcentrated floods, with sediment concentrations higher than 200 kg/m(3), occur frequently in the Yellow River and its tributaries on the Loess Plateau. This paper studies the fluvial hydraulics of hyperconcentrated floods by statistical analysis and comparison with low sediment concentration floods. The fluvial process induced by hyperconcentrated floods is extremely rapid. The river morphology may be altered more at a faster rate by one hyperconcentrated flood than by low sediment concentration floods over a decade. The vertical sediment concentration distribution in hyperconcentrated floods is homogeneous. The Darcy-Weisbach coefficient of hyperconcentrated floods varies with the Reynolds number in the same way as normal open channel flows but a representative viscosity eta + 2R/3U tau(B), is used to replace the viscosity, eta. If the concentration is not extremely high and the Reynolds number is larger than 2000, the flow is turbulent and the Darcy-Weisbach coefficient for the hyperconcentrated floods is almost the same as low sediment concentration floods. Serious channel erosion, which is referred to as 'ripping up the bottom' in Chinese, occurs in narrow-deep channels during hyperconcentrated floods. However, in wide-shallow channels, hyperconcentrated floods may result in serious sedimentation. Moreover, a hyperconcentrated flood may cause the channel to become narrower and deeper, thus, reducing the flood stage by more than I m if the flood event lasts longer than one day. The fluvial process during hyperconcentrated floods also changes the propagation of flood waves. Successive waves may catch up with and overlap the first wave, thus, increasing the peak discharge of the flood wave during flood propagation along the river course.