Hydrodynamic Response of Channel Flow Confluence to the Tributary Floodplain Topography

Abstract River confluences with a compound‐channel tributary are common in large river networks, for example, the Yangtze River basin and the Amazon basin. They affect the confluence hydrodynamics, nutrient depletion and fish migration in the network, due to the seasonal exposure and submergence of the tributary floodplain. The corresponding influencing mechanisms are critical but poorly understood. This study investigated the hydrodynamic response of channel flow confluence to the tributary floodplain, that is, various widths and heights of the floodplain. Two kinds of secondary circulations were identified: One was due to the tributary streamline curvature, and the other produced by flow separation in the floodplain step. An increase in the floodplain size enhanced the strength of the secondary circulation in the compound tributary channel, but it disappeared near the junction because of the effects of the main channel. The decreased tributary cross‐sectional area amplified the flow momentum, resulting in a larger separation zone near the tributary‐side wall. The strength of floodplain‐induced secondary circulation increased as the floodplain width increased, and it moved toward the tributary‐side bank destroying the separation zone. Moreover, strong upwelling in the secondary circulation caused the rising water surface in the separation zone, which was supposed to be a region of falling water surface caused by energy loss and negative pressure. A conceptual model was proposed to summarize the hydrodynamics of confluence with different tributary floodplain topography. Our results provided a comprehensive knowledge of the confluence hydrodynamics, which have important implication for the flood management and ecological restoration of river basin.