On the occurrence of sandbars

Abstract

In channelized alluvial rivers, alternating sandbars are formed by alternating scouring and sedimentation in the downstream direction. The conditions for the occurrence of sandbars are obtained in the stability analysis when the ratio of channel width to depth, β, exceeds a critical value due to innate instability of bottom. However, neither the mechanism nor the conditions for the occurrence of sandbars are well understood because it is difficult to quantify bars before or early in the occurrence of that using conventional measurement methods. We conducted experiments in a laboratory flume with slope as a variable to obtain dense data on the geometry of the water surface, bottom surface, and flow depth. We then performed a wavenumber analysis of the measured data to verify the validity of the results estimated by the stability analysis. We calculated the temporal variation of the correlation coefficients between the depth–water surface rwl and depth–bottom surface rbl along the development stage of the sandbar since flow state could be estimated from the relationship between the flow depth and the water surface/bottom surface. The temporal variation of the correlation suggests that the development process of sandbars can be divided into three phases, and the flow state is different for each phase. We also found that the occurrence time of sandbars would be defined based on the intersection of the temporal variation of rwl and rbl. Furthermore, we show that the formation process of the sandbar is independent of Fr.