Experimental Investigation of the Influence of a Sloped Bottom on the Behavior of Inclined Dense Jets

Inclined dense jets are widely used in outfall systems, yet their behavior over sloped bottoms remains insufficiently understood. This study investigated the influence of sloped bottom on jet dynamics and mixing through 36 experiments using Laser Induced Fluorescence (LIF), with discharge angles of 30°, 45°, and 60°, bottom slopes of 0°, 1°, 3°, and 5°, and densimetric Froude numbers of 17.3, 22.1, and 28.1. The results show that jet trajectories are unaffected by the sloped bottom before the return point; however, after impingement, the impact-point location, local dilution, and lateral spreading all increase with increasing slope. Flow visualization showed a strong ring-shaped vortex at impingement, followed by Kelvin–Helmholtz vortices, which became more pronounced on sloped bottoms. The energy spectra along the bottom follow Kolmogorov’s − 5/3 law. The variance and energy spectra analyses consistently showed that shear-induced turbulence is significantly enhanced over the sloped bottom, leading to higher fluctuations and turbulence energy and thereby enhancing mixing. The length of mixing zone and spreading layer thickness also increased with slope angle. The dilution enhancement over the sloped bottom is more pronounced for jets with a higher ratio of the initial momentum flux to the buoyancy flux. These findings clarify slope–momentum–buoyancy and jet–bottom interactions, provide practical guidance for outfall design, and highlight the importance of accounting for seabed slope and bottom conditions to optimize mixing efficiency.