PIV Measurements of Turbulent Water Flows Over Fixed Low-Angle Compound Dunes Under Reversing Currents

This study investigates the influence of surface roughness, current intensity, and direction on tidal flows over asymmetric low-angle dunes using high-resolution particle image velocimetry in a laboratory flume. Experimental measurements reveal that, consistent with previous studies, low-angle dunes generate substantial turbulence through flow expansion and shear, despite inducing only intermittent flow separation. Enhanced surface roughness and current intensity significantly increase turbulence, leading to permanent flow separation, akin to that observed over angle-of-repose dunes. When the flow direction opposes the dune's morphological orientation, weaker turbulent stresses are observed; however, flow expansion at the crest generates sufficient turbulence that persists downstream, impacting the flow field over the next dune. Hydraulic roughness parameters estimated from single and double log-law velocity profiles offer a quantitative assessment of the variability of form roughness along the dunes during both ebb and flood tides. Ejection-sweep cycles dominate turbulent flow in both aligned and opposing flow conditions, albeit with distinct spatial distributions. This study highlights crucial aspects of the interaction between dune morphology and reversing tidal currents, demonstrating that asymmetric low-angle dunes can induce significant form roughness across the different tidal phases. These findings have important implications for flow resistance and sediment transport in tidal environments.