Laboratory measurements of 3-D flow patterns and turbulence in straight open channel with rough bed

Laboratory experiments have been carried out in order to study secondary circulation and flow variability in straight open channel flows over a rough bed. 3D flow velocities and turbulence characteristics were measured using a micro Acoustic Doppler Velocimeter. Two flow conditions were analyzed, with aspect ratios b/h (width over depth) of 8.5 and 6.3, respectively. These two cases cannot be classified in principle as narrow (b/h < 5) or wide (b/h > 10) channel flows, but exhibited well defined secondary flows in the whole cross section. Bed shear stresses and turbulence patterns were consistent with the secondary flow cellular circulation. These results contradict the general perception that secondary circulation cells die out at a distance of b > 2.5 h from the walls. The behavior was attributed to the bed roughness (k_s/h - 0.1) and to the difference in roughness between bed and walls, the latter being much smoother. The results allowed for quantification of hydrodynamic variability in terms of secondary velocities (5% of mean flow velocity), cores of high streamwise velocities (10% higher than surrounding flow) and bed shear stress (20% oscillation with respect to cross sectional averaged value). All these patterns are consistent with the very few results reported in the literature for similar roughness conditions. These results can be used to assess the background flow conditions existing in a straight channel with a plane bed, before the addition of pools and riffles as part of a restoration strategy for streams that have been channelized and do not present the flow variability needed to enhance habitat conditions.