Effects of bed discordance on flow dynamics at open channel confluences

Although a discordance in bed elevation between mainstream and tributary channels is a common feature of most river confluences, this height differential is largely ignored in many models of junction flow dynamics. Here we present new laboratory flume data that demonstrate the significant influence of bed discordance on flow at an acute angled confluence. A difference in bed height between mainstream and tributary channels is shown to obliterate flow deflection at the bed and create a distortion of the mixing layer between the flows, resulting in fluid upwelling at the downstream junction corner. This upwelling is responsible for the absence of a flow separation zone near the bed at the downstream junction corner and the lack of a zone of marked flow acceleration in the postconfluence channel. These experiments reveal complex flow fields generated by bed discordance and suggest the need for major revisions to current models of river channel confluence flow dynamics and their application to sediment routing at these sites.