Experimental investigation of dense-fluid evolution in a trench under propagating solitary waves

• Combined LIF-PIV experiments captured dense-fluid evolution under solitary waves. • Three governing factors were examined: density, wave nonlinearity, and trench width. • An upstream gravity current was identified under high density contrast. • Lee-wall blocking in narrow trenches increased dense-fluid trapping. • Vorticity analysis quantified CW vortex intensity across all experimental cases. A laboratory study was conducted to investigate dense-fluid transport in a two-layer stratified system confined within a submarine trench under solitary-wave forcing. Synchronized Laser-Induced Fluorescence and Particle Image Velocimetry were used to visualize dense-fluid motion and quantify the velocity field. The results show that the maximum transport height of the dense-fluid layer generally decreases with increasing density contrast and decreasing trench width. While stronger wave nonlinearity tends to enhance vertical transport, this trend is reversed in narrow trenches where vortex trapping suppresses outward transport. Across the experimental cases, the maximum normalized transport height in the high-density cases was reduced by approximately 50–60% relative to the corresponding near-homogeneous baselines. An upstream-propagating gravity current developed along the trench bed under conditions of higher density contrast. Quantitative tracking of the upstream front showed that stronger wave nonlinearity promoted a faster initial advance of the gravity-current front, while higher density contrast supported more persistent upstream propagation. In wide-trench cases with lower density contrast, the front tended to level off at later times, likely due to vortex-induced re-entrainment. Vorticity analysis further showed that stronger wave forcing enhanced the intensity of the primary clockwise vortex, whereas trench width governed the spatial extent and confinement of the vortex core. These results demonstrate that dense-fluid transport in a wave-forced trench is governed by the combined effects of density contrast, wave nonlinearity, and trench geometry.