Large eddy simulation (combined with the mixture model) and laboratory experiment were used to investigate the impact of emergent and rigid vegetation on the dynamics of downslope gravity currents in stratified environments. The reliability of the numerical model was assessed with the corresponding laboratory measurements. The results show that the vegetation cylinders lead to severe lateral non-uniformity of the current front, causing more evident lobe and cleft structures. In stratified environments, the smaller driving force leads to less propagating velocity until the current separates from the slope. The transition point (from acceleration to deceleration phases) of current velocity appears earlier as the vegetation becomes denser. The peak value of the bulk entrainment coefficient $E_{buik}$ is inversely proportional to the vegetation density, while the final converged value of $E_{buik}$ is proportional to the vegetation density. Vegetation patches make the degree of fluctuation of the instantaneous entrainment coefficient $E_{inst}$ more intense, and even negative values appear locally, indicating that the gravity current is detrained into the ambient fluid. The velocity profiles of gravity current develop multi-peak patterns in stratified environments due to fingering intrusive patterns. Our analysis reveals that as the vegetation density increases, the generated wakes behind vegetation cylinders increase local entrainment and mixing, causing the density of current flow from vegetation to decrease and reach the neutral buoyancy layer of ambient fluids earlier, finally leading to a smaller separation depth.

This study experimentally investigated the impacts of rough and porous (RP) bed and sedimentation processes on the coherent structures, turbulence intermittency, and anisotropy of saline and turbidity currents. The results reveal that the local current concentration responds immediately (saline current) or languidly (turbidity current) to turbulence bursting events. Inside the dense current, the turbulent momentum fluxes in the streamwise (u′u′¯) and vertical (w′w′¯) directions transfer downstream and downward, which favor the sweep events. Inside the ambient water, u′u′¯ and w′w′¯ transfer upstream and upward, contributing to the formation of ejection events. At the current–ambient water interface, u′u′¯ and w′w′¯ do not tend to transfer in particular directions resulting in almost equal quantities of sweep and ejection events. The Gram–Charlier series expansion is strictly applicable to probability density functions (PDFs) of the sweep and ejection events but not suitable ideally for PDFs of the outward and inward interaction events. The primary anisotropy invariant map (AIM) of gravity currents starts from the two-component plain strain limit (near the bed). It is followed by the three-dimensional isotropy (inside the dense current and ambient water) and the axisymmetric contraction limit (current–ambient water interface). Finally, it ends in two-dimensional isotropy (near the free surface). This AIM is sensitive to the RP boundary and the sedimentation processes. Along the streamwise direction, the RP boundary causes alternations between the anisotropic and isotropic turbulence, but the arranged pattern of the rough units determines the period of this alternation.

A three-dimensional numerical model was established with OpenFOAM-5.x to investigate plume characteristics under windless and rainless weather conditions. The large eddy simulation was applied, combined with a modified solver for solving governing equations with the Boussinesq approximation in a single rotating frame. The relationship between plume characteristics (e.g., gradient Richardson number and maximum plume width) and quantified parameters (e.g., rotation period, shelf slope, and reduced gravity) was analyzed progressively. The results show the model can reproduce the change in plume types and instability found in the laboratory experiments. With the increase in the rotation period, river plumes change from a surface-advected type to a bottom-attached type. The outline of the plume bulge accurately delineates the external region where the gradient Richardson number is less than 0.25, as well as the region near the wall. When the shelf slope approaches 0, the offshore movement becomes stronger while the alongshore coastal current comes into being with a delay associated with the slope and the rotation period. Compared with the extremely gentle slope case and the steep slope case, the maximum width in the gentle slope case changes significantly at about 1.5 rotation periods. Greater reduced gravity does promote offshore propagation, especially near the surface.

This study employed large eddy simulation coupled with a mixture model to investigate downslope gravity currents (GCs) flowing through a cylindrical pile in the two-layer stratified environment. Sixteen simulation cases were performed, covering a range of ambient stratified strength and pile submergence ratios. Results demonstrate that ambient stratification determines the fate of GCs head upon impinging the pycnocline, leading to underflow-dominated, interflow-dominated, or interflow morphological patterns with increasing ambient stratified strength. The pile disrupts the coherence of GCs head, promoting dilution of the concentration core, and enhancing the vertical redistribution of dense fluid from the underflow into interflow. The vortex-strain structures of GCs inherently transition from lateral bands to longitudinal stripes but are reorganized by the ambient stratification into vortex-stripes intensified, weakened, or suppressed regimes and also adjusted by the pile into a strain-dominated pattern, particularly upstream of the emergent configuration. Acting as a localized energy sink, the pile shortens the dominance period of kinetic energy and enhances the dissipated energy of GCs. The potential-kinetic energy reversal is modulated by internal waves (IWs): locked IWs facilitate bidirectional energy exchange with GCs under weak ambient stratification, whereas launched IWs cause unidirectional energy loss of GCs under moderate and strong ambient stratification. Irreversible mixing quantified by the background potential energy is maximized for the case with a shallow-submerged pile configuration, which optimally balances wake production and wall shear-induced turbulence. The pile reduces the peak friction velocity at GCs head but enhances it downstream via wake effects, with these influences being moderated by ambient stratification. This work provides critical insight into the coupled dynamics of GCs with stratified environments and pile obstacles, offering a valuable foundation for assessing seabed scouring potential and optimizing the design of offshore infrastructures.