Effects of regularly arrayed sphere roughness on flow resistance and turbulent characteristics in an open channel have been investigated over a completely rough bed with uniform glass beads, where the sphere diameter were 15mm and 30mm. Detailed spatial measurements of streamwise and vertical velocity fluctuations were conducted using Particle Image Velocimetry (PIV) in a vertical plane along the completely rough bed surface. Experimental results indicated significant degrees of spatially

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... r variation in the time averaged velocities, where the upflow velocity was much stronger than the downflow one and imhomogeneous turbulent characteristics were generated along the rough bed in case of the large ratio of the sphere diameter to flow depth. In addition, Reynolds shear stress and turbulent intensity showed minimum values at the ridge of roughness elements and maximum ones at the trough. These organized flow structure were caused by the vortex shedding generated by the roughness elements.