Seepage Induced Morphodynamics of Alluvial Channels: Unraveling Dune Dynamics and Flow Characteristics

Experimental studies were conducted to analyze the alterations in flow behavior and turbulent characteristics over dune-shaped bed features in the absence and presence of downward seepage. Experiments were designed in two categories to understand the flow characteristics: (1) over fixed (immobile) dunes, and (2) dunes made of sand (mobile). In both the categories of experiments, measurements were taken for no seepage condition and the results were then compared when downward seepage discharge of 10% and 15% of the no seepage discharge were allowed through the porous sand bed placed on the flume bed. Instantaneous velocities were collected using Acoustic Doppler velocimeter (ADV) for no seepage and seepage conditions. Significant enhancement in streamwise velocities, RSS, turbulent intensities, turbulent kinetic energy (TKE) values, and bed shear stress was observed at the initial sections on the stoss side and the lee side sections under the influence of downward seepage over both fixed bed as well as mobile bed conditions. The Anisotropic Invariant Map (AIM) illustrates the prevailing 1D anisotropy in the initial and lee side sections of the dune under downward seepage condition. Quadrant and octant analyses show increase in sweep and ejection events in the near bed zone of the initial and lee side section of the dune under downward seepage conditions. Similar patterns of turbulent parameters were observed for the dunes under mobile conditions. However, at the middle sections and crest portion of the fixed bed dune, the magnitudes of turbulent parameters have been found decreasing along the depth of flow. Contradicting the flow and turbulence patterns observed at the crest portion of the fixed dune, the magnitudes of turbulent parameters increase significantly under seepage conditions in the near-bed region of the crest portion. The increase in the magnitudes in the near-bed region of the crest portion for the mobile bed experiments is due to an increase in the scour depth on the lee side section of the dune, owing to higher amount of sediment movement from lee side section of the dune. The increase in scour depth on the lee side section of the mobile dune under the influence of seepage generates higher magnitude of turbulence eddies, which reach the crest portion, leading to a rise in flow velocity and turbulence parameters in the near bed region at the crest portion of the mobile bed dunes, resulting in increased celerity of the dunes under seepage conditions.

Keywords: Two-dimensional dune; downward seepage; acoustic Doppler velocimeter; turbulence characteristics; anisotropy; bursting events; dune morphology.