<p>An experimental study was performed for a flow in a channel with a local roughness. The influence of the relative rib height (ranging from 2 to 10 % of the channel height) on the turbulence characteristics and flow structuring in the channel near-wall zone was studied. The research was focused on the case of obstacles with a low height. We found that the rib height below 5.5 % induces a drastic change in fluctuation and spatial flow characteristics; this alters the flow pattern in the separation zone and the vortex generation mechanisms. The transverse size of vortex behind the rib with a relative height of 2 % is twice as big as the obstacle height. The characteristic peak in the spectrum that corresponds to the vortex shedding frequency with fully developed roughness tends to degenerate with a decrease in the obstacle height. Here the key factor in vortex generation is due to a vertical swing motion of the separation zone boundary.</p>