Response of Bed Morphology to Variation of Discharge Ratios of a Channel Confluence

Discharges of the tributaries change in a large range by time sometimes in a plain rivernet, leading to a large various dicharge ratio of a channel confluence. Bed morphology and mass transport at the confluence respond to the large variation of inflows from the two tributaries. This study designed a test with an alternant discharge ratio, and the flow structure and response of the bed morphology were investigated before and after the discharge ratio was changed. The discharges of the main channel and the tributary were 0.358 m³/s and 0.240 m³/s, and lasted for about 24 hours until the bed morphology is unchanged; three-dimensional velocity at 14 cross sections and the bed morphology were measured. Then, the discharges of these two channels were exchanged and lasted for the same time, and velocity and morphology were measured again. Turbulence characteristics such as Reynolds shear stress and turbulent kinetic energy were analyzed. The results show that: (1) the exchanged discharge has caused a secondary scouring of the sand bar within the flow separation zone,which made the maximum height of the sand bar drop by about 10% and made the sand bar move downstream; (2) larger sand pool area was observed within the flow deflection zone and the deepest terrain decreased about 11%; (3) the maximum velocity zone migrated downstream, and the velocity decreased by about 14% due to the enlarged area of this zone; (4) due to the alternant discharge ratio, another shallow sand pool and another short sand bar formed downstream of the sand bar within the separation zone, where higher lateral velocity, lower vertical velocity, higher turbulent kinetic energy and lower Reynolds shear stress were observed. All these results indicate that the topographic changes of the confluence rivers in the plain river network area caused by the change of discharge conditions and the interaction between them.