Impacts of bottom slope gradient on regional sediment settlement and transport in a vortex settling basin

The mechanisms by which the bottom slope gradient of a vortex settling basin (VSB) governs its internal flow field and sediment transport remain inadequately understood, posing a significant obstacle to performance-driven structural optimization. This study was therefore undertaken to systematically elucidate the causal relationships between slope configuration, hydrodynamic characteristics, and sediment behavior within a VSB.An integrated experimental and numerical methodology was employed. Systematic flow field measurements were conducted through laboratory experiments. Complementing the physical tests, a sophisticated three-dimensional numerical model was developed by coupling the Reynolds stress model (RSM) for turbulence closure, the volume of fluid (VOF) method for air-water interface tracking, and the discrete phase model (DPM) for sediment particle trajectory simulation. Three representative bottom slope gradients—1:5, 1:7.5, and 1:10—were systematically analyzed to investigate flow field characteristics and sediment transport dynamics.The analysis revealed that a gentler slope fundamentally alters the flow-sediment system. The lateral offset of the air-entraining vortex toward the non-suspended plate side increased significantly with slope reduction, with the offset distance rising from 0.011 m to 0.021 m, accompanied by intensified vortex core oscillations. Concurrently, the secondary flow structure on the suspended plate side stabilized under milder slopes. The time-averaged radial velocity exhibited fluctuations exceeding a twofold increase on the non-suspended plate side but remained relatively stable near the suspended plate. The overall time-averaged tangential velocity displayed an increasing trend with slope reduction, while the peak of the time-averaged axial velocity migrated toward the central axis; directional reversals in axial velocity were identified as a primary driver for secondary flow formation. Based on distinct sediment transport and deposition functions, two complementary functional zoning systems were established: circumferential zoning, dividing the basin into inner, middle, and outer rings, and sectoral zoning, comprising five zones—Efficient Sediment Transport zone, Disturbance zone under the suspended Plate, Turbulent Transition zone, Stable Deposition zone, and the inner ring. As the slope became gentler, the lateral sediment transport capacity weakened while vertical transport efficiency enhanced, leading to sediment accumulation increases of 38.7%-74.6% and 150%～300% times in the middle and outer rings, respectively. Sectorally, slope reduction triggered a distinct spatial reorganization: the inner ring expanded; the Efficient Sediment Transport and Stable Deposition zones contracted by 11.1%-22.2% and 6.3%-20.0%, respectively; the Disturbance zone under the suspended Plate expanded by 5.9%-29.4%; and the Turbulent Transition zone remained largely unchanged. This evolution promoted sediment deposition along sidewalls and increased its entrainment toward the water surface by secondary flows.This study successfully clarifies the governing influence of bottom slope gradient on VSB flow field and sediment transport, quantitatively delineating the coupled interactions within the "slope-flow-sediment" triad. The findings provide critical insights and a robust mechanistic framework for the rational optimization of VSB slope parameters, directly supporting the design of more efficient sediment management structures in hydraulic engineering projects.