Effect of river bed degradation and aggradation on transformation of alternate bar morphology

摘要

Many studies on alternate bars have been conducted under an equilibrium sediment supply condition which corresponds to the water discharge, and discussed phenomena in an equilibrium state. However, bar morphology in an actual river is not always formed under such condition. Therefore, it is important to investigate how the bar morphology formed by some water and sediment discharge condition varies by the other conditions. In this study, effects of river bed degradation or aggradation and water discharge on alternate bar transformation are investigated by means of flume tests using uniform and non-uniform sediment. Results are summarized as follows: (1) In the experiments, the river bed degradation rate of the non-uniform sediment bed is larger than that of the uniform one, and the aggradation rates of both sediment beds are almost the same. (2) The river bed degradation under the low-water discharge causes the low-watercourse formation, and that under the high-water discharge causes the restriction of alternate bar development. And, the aggradation under the high-water discharge causes the increase of the alternate bar wavelength. (3) The increase and the decrease of wavelength of alternate bars in consequence of the river bed degradation are remarkable in the non-uniform sediment bed due to grain sorting.