Experimental study on bar formation in a scouring process

Bars are widely thought to be large sediment bodies formed under sediment supply. Nevertheless, little has been known on how they form in sediment scouring processes. In this study, we carried out a flume experiment to study the formation of bars without sediment supply. The experiment was divided into two stages. In the first stage, the discharge was successively increased; in each discharge step, the disturbance from upstream and downstream boundaries had little influence on the flow. We observed that no bars formed in this stage. In the second stage, we kept a small discharge at the flume inlet. In this stage, the bars emerged from the fluctuated bed topography with millimeter-scale bed forms by headcut initiated from the outlet. As the headcut migrated upstream, the accompanying undercut gradually forced the unconfined flow run into the low-elevation zone, lowering the water level and inducing the outcrop of regions free from the incision (i.e., bars). At the end of the experiment, a relatively stable topography formed under the joint effect of the upstream migrating headcut, the following undercut after the headcut and the lateral erosion on the emerged bars. The requirements for the formation of bars and the distinctive characteristics of the bars induced by headcut were investigated. This study shows that bars can form in a scouring process under appropriate conditions, and the headcut may be one of the precipitating factors for the formation of bars in natural rivers.