Morphodynamics of mobile dunes with downward seepage

The experimental study examines the flow dynamics and sediment transport behavior of mobile dune-shaped bedforms induced by downward seepage. The longitudinal velocities in the stream flow direction, Reynolds shear stresses (RSS), energy budget, and transitional probabilities of turbulence bursting events were analyzed without and with downward seepage. Application of downward seepage discharge significantly altered flow patterns, increasing streamwise velocities and magnitude of RSS on the gradually rising face and reducing them at the crest and trailing section of the bedform. Downward seepage intensifies turbulence dissipation and diffusion at the steep slip side of the dune due to enhanced circulation, which encourages scour hole formation. However, in the proximity of the bed at the initial region on the gradually rising bed surface and leeward sections of the mobile bed features, turbulent production surges substantially with seepage. With seepage, the anisotropy invariant map shows a shift in patterns of turbulence anisotropy from two-dimensional (2D) to one-dimensional at the initial and middle sections, while at the crest and leeward side sections, 2D anisotropy. Enhanced transition probabilities of outward interaction and sweep events at the initial sections on the gradually rising bed surface and leeward side intensify under seepage conditions, increasing vortex strength and promoting erosion and sediment mobilization under seepage conditions. Scour depth on the leeward side section of the dune intensifies over time both under no-seepage and seepage, with greater scour observed under seepage conditions. Sediment transport rates were also significantly higher under seepage than under no seepage conditions.