Discrimination of channel patterns for gravel-and sand-bed rivers

摘要

Sand-bed rivers differ from gravel-bed rivers in both hydraulic and sediment transport terms and they also differ in terms of channel pattern. Based on data from around 200 alluvial rivers in several countries, a systematic comparison of flow shear stress and stream power has been made between sand- and gravel-bed rivers. The formation of channel pattern can be regarded as the outcome of sediment transport and channel bank constraints In an alluvial river. To overcome the difficulty of quantifying channel bank constraints, channel width/ depth ratio and a dimensionless channel width index are used to reflect it indirectly. The sediment transport characteristics are expressed in terms of flow shear stress, stream power and the Shields stress tau* = dS/RD50 or its simplified form of dS/D-50, where S is channel slope, d is water depth and D-50 is median size of bed sediment. By plotting the relationships between these hydraulic indices and the indices expressing batik constraint in a two-dimensional plane, meandering and braided channel can be divided by one line and sand- and gravel-bed channels by another. These lines are regarded as channel pattern thresholds. The result shows that on the whole, the diagrams using unit water volume stream power and dimensionless shear stress discriminate channel patterns are better than those using shear stress and specific stream power.