Studies on the Process of Development of Alternate Bars

Alternate bars play a major role in stream channel processes and are very interesting phenomena in rivers. It has a large influence on the safety of river courses and the use of river water. The mechanism and the process of development of the alternate bar, which may be regarded as one of the most important subjects on the meso scale river bed configuration, are investigated in this paper. One reason for this study is that very few confirmative studies have been carried out by means of detailed experiments. Conducting the experiments under widely changed conditions, particularly that of the channel width, we clarified in detail the characteristic changes of the wave length and the wave height of the alternate bar and the channel bed variations in the development process. The development process of the alternate bar proved to be divided into three phases: in the first phase, the bar edge has become clear and the wave length has almost developed, in the second, the bar increases in the wave height to an equilibrium value and the third is a kind of the equilibrium state of the bar geometry. A longitudinally averaged bed shows a Quonset-hut shaped cross section in the second phase, the height of which increases corresponding to the wave height. This is due to lateral sediment transport caused by the meandering characteristics of flow over the alternate bar. By using the results, we proposed an equation for the development time of the alternate bar, and moreover, experimentally verified the effectiveness of parallel groyne works for the control of the bar geometry.