Turbulent characteristics and evolution of sheet flow in an alluvial channel with downward seepage

•Downward seepage results in sediment transport.•Bed shear stress and integral scales cause the evolution of sheet layer.•Empirical formulation was developed to predict the sheet flow rate.

Experimental investigation of the flow hydrodynamics and temporal changes in the cross-sectional profile of an alluvial channel has been studied in the present work. Experiments were carried out in a curvilinear cross-sectional shaped channel with no seepage and with a downward seepage condition to ascertain seepage effects on channel geometry and turbulent characteristics of flow. Measures of turbulent characteristics such as time-averaged near-bed velocities and Reynolds stresses were found to increase with the application of downward seepage. Stream power and the value of Shields parameter are increased under the action of downward seepage, causing bed particles to move in the form of a sheet layer. Integral scales of flow suggest that the size of eddies increases with the application of downward seepage, which is linked to the evolution of the sheet layer. Sheet development causes reduction in flow depth and rapid channel widening. Cross-sectional parabolic shape of the threshold channel is transformed into a trapezoidal shape with the presence of the sheet layer. With the passage of time (11 h), the channel attains another equilibrium geometrical state with the value of Shields stress around 0.074 from the no seepage value of 0.0399. An empirical equation for sheet flow rate is derived with the consideration of seepage in the downward direction.