Abstract: Channelized ditches often occur on channel bars in a braided river, promoting the mobility of the bars and affecting the stability of river regime. Formation and development of these ditches is primarily a comprehensive response of the river to its volatile runoff. To explore the channeling sensitivity to river flow rates, a two-dimensional depth-integrated water-sediment-morphodynamic model is used in this work to study the erosion and deposition process of channel bars under different flow rates, and the formation and development of the ditches is reproduced numerically. Results show that the formation and development is closely related to the river’s characteristic flow rate. 1) When the river flow rate is small and the water cannot flow over the bar, the flow runs through the left and right branches and no ditch will be generated; 2) When the flow rate is in the medium range and the water just flows over the bar with a small depth on the bar, flow exchange between the two branches occurs through streams passing over the low-lying part of the bar bridge, and these streams will scour the bar and create channelized ditches; 3) When the flow rate is large and a major part of the channel bar is underwater, the flow exchange is weakened and no ditch can be generated. This study also shows that at different characteristic flow rates of the river, the location of ditch formation will swing. And the threshold of the characteristic flow rates for the ditch to form and develop depends, to some extent, on the planar shape of the channel bar. The results of this work would be useful for the study of the stability of channel bars in branched rivers.

Key words: braided channel, stability, channel bar, channelized ditch, characteristic flow rate, hydro-sediment-morphodynamic model, numerical study