Flow and bedform dynamics in an alluvial channel with downward seepage

In this paper, we report the findings from an experimental study on a parabolic cross-sectional sand bed channel with uniform fine sand under no seepage and downward seepage conditions. Through experiments, we observed that an alluvial channel, which remained at threshold condition of sediment movement during the no seepage experiment, started transporting sediments with the application of seepage in the downward direction. Shields stress of the threshold channel increased significantly from its critical value after the application of downward seepage, which led to the deformation of the cross-sectional shape and consequent development of the bedforms. The role of turbulence in the development of bedforms has also been analyzed. Measures of turbulent statistics show that the time-mean velocities and Reynolds stresses were increased significantly with the application of downward seepage. Under the action of seepage conditions, increase in the flux of streamwise turbulent kinetic energy in the streamwise direction was observed in the region close to the channel boundary. Also, quadrant analysis exhibited an increase in the contributions from all the bursting events and the thickness of the sweep dominated zone in near-bed region after the application of downward seepage. A bedform tracking tool has been used to evaluate the variability in the geometry of bedforms. We have classified these developing bedforms as current ripples and linguoid ripples according to their evolution with time under the downward seepage condition. It has been further observed that the variation in Shields stress and corresponding bedform geometry reached an equilibrium state in the presence of downward seepage when the experiments were run over a longer period of time (24-31 h).