Theoretical analysis on influencing factors of optimal channel form for bed load transport in alluvial rivers
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Abstract:
The optimal channel of rivers dominated by bed load is determined by many factors. In addition to hydrodynamic conditions, it is also closely related to water and sediment inflows and boundary conditions. Since the rivers on the alluvial plains in the United States and India transport overloaded sediment via wide and shallow channels, some foreign scholars believe that wide and shallow channels are ideal for transporting overloaded sediment. However, the sediment transport capacity of the channel is inversely proportional to the width-depth ratio of the channel in the lower reaches of the Yellow River. The reduction of the width-depth ratio can improve the maximum sediment transport capacity of the channel. The relationship between the sediment transport capacity and the optimal section of rivers in different regions manifests different or even opposite relationships, and the channel boundary conditions may be a major factor. When the scouring resistance of the river bank is high, the transverse deformation of the river channel is greatly constrained, and the corresponding channel form is relatively narrow and deep. On the contrary, the river channel is subjected to relatively weak horizontal constraints and is prone to widening. To theoretically determine the influence of different factors on the optimal channel form for alluvial rivers and the corresponding minimum gradient, the methods of hydraulic radius segmentation and the river automatic adjustment variational are used to derive the relationship between bed load sediment transport rate and channel form. By taking the isosceles trapezoid as the cross-section, the relative roughness of the river bank to the river bottom, the bank slope angle, the flow discharge, the sediment transport rate, the median particle size, and the roughness of the river bottom are analyzed to ascertain how they affect the optimal channel form and the minimum gradient, respectively. The results show that the optimal channel form and minimum gradient decline with the increase of relative roughness of river-bank to river-bottom, and rise with the increase of bank slope angle. The increase of flow discharge or river-bottom roughness will cause the minimum gradient to decrease and the optimal channel form to become narrow and deep. The increase in sediment transport rate or median particle size will increase the minimum gradient, but their effects on the evolution of the optimal channel form are different since the increase of sediment transport rate will yield a wide and shallow optimal channel form while the increase of median particle size will make the optimal channel form narrow and deep. This study only conducts a quantitative analysis of the response of the optimal channel form of rivers dominated by bed load and the corresponding minimum gradient to multiple factors of hydrodynamics and river boundary. The response of the sediment transport capacity of the sediment-laden river dominated by suspended load transport to these factors needs further investigation.
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