Effects of water and sediment discharge conditions on variation in sand bar morphology

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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 rivers are 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 rotational degradation/aggradation caused by an imbalance between water and sediment transport capacity on variation in alternate bar morphology are investigated in uniform and non-uniform sediment beds. Effect of grain sorting on the variation is also discussed. Results are summarized as follows :1.In case that the degradation progresses under the condition of small-water discharge near the double-low bar formation regime, the wavelength of alternate bars increases gradually because the areas with little sediment transport develop. When the degradat … More ion progresses further, both of the wavelength and height increase because low watercourse develops.2.In case that the degradation progresses under the large-water discharge near the no-bar formation regime, the wavelength and height decrease because the development of alternate bars is restricted. In particular, the alternate bar development is unstable in the non-uniform sediment bed.3.In case that the aggradation progresses under the large-water discharge near the no-bar formation regime, the wave length increases gradually because the development of alternate bars is promoted.4.The increase and the decrease of wavelength and height of alternate bars in consequence of the degradation are remarkable in the non-uniform sediment bed.5.Not only bed slope but also its changing rate affects variations in bar morphology.6.The migration velocity of bars decreases with degradation, and that increases with aggradation. The decrease of migration velocity relates to immobilization of pools and the formation of a low-watercourse. Less