Experimental study on the near-bed flow characteristics of alluvial channel with seepage

This paper aims to analyze the turbulent structure of flows over beds subject to downward seepage under clear-water conditions. For this purpose, laboratory experiments were carried out in a straight rectangular channel 17.20 m long and 1.00 m wide. A sandy bed with median grain size d500.50 mm and sediment gradation g1.65 (i.e., slightly non-uniform sediment) was used for the channel bed. 3D instantaneous velocities of water were measured with an Acoustic Doppler Velocimeter (ADV) at the working test section. In the vicinity of the bed surface with seepage, measurements revealed that the flow longitudinal velocities (velocities in x direction) were higher than those in case of a bed without seepage. Also, the variations of the Reynolds shear stresses increased for bed with seepage, indicating a higher exchange of flow energy towards the boundary and vice versa. Therefore, seepage processes influence the turbulence intensity with prominent magnitude in streamwise and vertical directions. The paper also focuses on the third-order moment (skewness) and the kurtosis of velocities fluctuations and the governance of sweep events within the near-bed flow in case of seepage was also observed.