Importance of bed roughness in transversal variability of the flow patterns and bed shear stress due to secondary currents

Laboratory experiments were carried out in order to study the influence of the bed roughness on secondary circulations and lateral flow variability in a straight open channel. Flow field and bed shear stress were measured using an acoustic doppler velocimeter and a particular type of the Preston tube, respectively. All experiments attest presence of noticeable lateral variations of the flow characteristics due to the secondary currents. The observations also reveal that the lateral variations in the experiments with larger bed roughness are more intense. This implies the formation of a more stable mechanism for the maintenance of the cellular pattern in the experiments with larger bed roughness elements. Regarding formation of the secondary currents, application of double averaging method (averaging of the time averaged turbulence parameters within a thin spatial slab parallel to the bed) is also discussed. It was found that, to properly consider the lateral variations of the flow characteristics, spatial averaging should be implemented among the measured data at different spanwise locations. As such secondary current enhancements can also affect more complex and natural flow like river flows, it can also be recommended to examine the importance of such phenomenon in those areas respecting double averaging method.