It is necessary to explore the measures to reduce the scale of the sandbar and thus to alleviate its blockage impact to main river flow.The influence of confluence angle on sandbar morphology at the confluence with hyperconcentrated tributary flow was investigated through physical model experiments.The confluence of upper Yellow River and Xiliugou River,one of the ten tributaries the hyperconcentrated floods from which resulted in most of the sandbar blocking events,was chosen as the simulated target.The model was designed following the similarity laws for hyperconcentrated river flow.The angles of 90°,60°,30°,and 15° were set as the confluence angles in the experiments.It was found that,with the increase of the confluence angle,the main stream blocking was enhanced.The flow velocity of the backwater area upstream of the tributary river mouth slowed down gradually and the water depth increased.When the confluence angle increased to 30° the recirculation zone was formed and its size was expanded gradually with the increase of the confluence angle,accompanied by the shifting of the high velocity zone towards the opposite bank.The scales of the sandbar in backwater area and recirculation zone both increased with the growing of the confluence angle.When the confluence angle was small,there was no sandbar formed in those two areas,but a sandbar adjoining to the opposite bank of the tributary river mouth would be formed.The scales of the opposite bank sandbar decreased with the increase of the confluence angle.The main channel immediately upstream of the river mouth was blocked by the sandbar in the backwater area,while the main channel immediately downstream of the river mouth was blocked by both the opposite bank sandbar and the sandbar in the recirculation zone.As a whole,the scale of the sandbars in the main channel was the smallest when the confluence angle was 30°.According to the analysis on the sandbar scale and the river bank stability,it is believed that under the current river channel and water-sediment conditions,the confluence angle of 30° is a relatively optimal angle which can reduce the sediment deposition at the confluence.
