Enhanced understanding of flow structure at a river confluence is essential for predictions of sediment transport and morphological evolution. To date, however, the confluent flow structure of a reservoir and tributary carrying high sediment loads has remained poorly understood, and may be vertically layered sharply, featuring subaqueous sediment-laden flow, i.e., turbidity currents underneath subaerial clear water. Here a recently established 2D double layer-averaged model, able to resolve the formation, propagation, and recession of turbidity currents, is used to investigate a series of idealized laboratory-scale cases and a prototype case study of the Guxian Reservoir on the Yellow River, China. Four primary patterns of the stable, vertically layered flow structure at a reservoir-tributary confluence are identified: 1) single layers of sediment-laden inflow in both the main channel and tributary, sustained by sufficient vertical mixing; 2) a double layer in the main channel and a single layer of sediment-laden inflow in the tributary, when the sediment-laden flow in the tributary suffices to block intrusion of flow in the main channel; 3) a single layer of sediment-laden inflow in the main channel and a double layer in the tributary, induced by the intrusion of sediment-laden flow from the main channel into clear-water flow with small discharge in the tributary; and 4) double layers in both the main channel and tributary, which may be further divided into three subpatterns, as turbidity current exists in both the main channel and tributary, or in the main channel (tributary) intruding into the tributary (main channel). In response to unsteady discharge and sediment inputs from upstream, the vertically layered flow structure evolves in time, and may fall into one of the patterns identified above. Although bed deformation in the long term may modify the confluent flow, the vertically layered flow pattern remains so far as the present cases are concerned. The findings have implications for sediment transport and morphological evolution at a reservoir–tributary confluence, for which further studies are suggested to inform the optimization of reservoir operation schemes to mitigate capacity loss caused by sedimentation.
Fluvial flows carrying high sediment loads may plunge into reservoirs to form turbidity currents. However, the effects of tributary inflows on reservoir turbidity currents have remained poorly understood to date. Here a 2D double layer-averaged model is used to investigate a series of laboratory-scale numerical cases. By probing into the hydro-sediment-morphodynamic processes, we find that tributary location and inflow conditions have distinct effects on the formation and propagation of reservoir turbidity currents, and lead to complicated flow dynamics and bed deformation at the confluence. Two flow exchange patterns are generated at the confluence: turbidity current intrusion from the main channel into the tributary; and highly concentrated, sediment-laden flow plunging from the tributary into the turbidity current in the main channel. Tributary sediment-laden inflow may cause the stable plunge point to migrate downstream and is conducive to propagation of the turbidity current, whilst the opposite holds in the case of clear-water inflow from the tributary. Tributary inflow leads to a lower sediment flushing efficiency as compared to its counterpart without a tributary. Yet a high sediment concentration in the tributary may reinforce turbidity current in the reservoir, thereby increasing sediment flushing efficiency.Around the confluence, the planar distributions of velocity and bed shear stress of the turbidity current resemble their counterparts in confluence flows carrying low sediment loads or clear water. Yet, the bed exhibits aggradation near the confluence due to the turbidity current, in contrast to pure scour in a river confluence with a low sediment load. Appropriate account of tributary effects is required in studies of reservoir turbidity currents, and for devising strategies for long-term maintenance of reservoir capacity. 3 KEYWORDS reservoir; turbidity current; tributary; sediment flushing efficiency; double layer-averaged model Highlights Tributary inflow may cause the stable plunge point of reservoir turbidity current to migrate either upstream or downstream and modify its propagation. Tributary inflow may lead to lower sediment flushing efficiency by reservoir turbidity current. Tributary discharge and sediment concentration may lead to disparate bed deformation at confluence.
<p>Fluvial flow with high sediment load may plunge into the reservoir to form turbidity current, which may feature strong interaction with inflow from a tributary. However, to date, the understanding of confluent flow structure with high sediment load has remained poor. Here, a computational fluid dynamic software, Flow-3D, is applied to resolve such flows for a series of cases at laboratory-scale by solving unsteady, 3D Reynolds-averaged Navier-Stokes and sediment transport equations, based on finite difference method with structured meshes. The 3D results are compared with those due to a recently established 2D double layer-averaged shallow water hydro-sediment-morphodynamic model. One distinctive flow structure pattern is generated at the confluence with the intrusion of reservoir turbidity current from the main channel into the tributary. Apparent bed aggradation occurs inside the tributary mouth after a long-term hydro-sediment-morphodynamic process. The present finding has a more profound influence on sediment transport and morphological evolution at a reservoir&#8211;tributary confluence with high sediment load.</p>
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