Boulders are sometimes placed in rivers to enhance an ecological function by making scour hole and downstream deposition. At first, flow structure around single large hemispherical roughness was investigated in emerged and submerged conditions. The effect of large roughness on the flow variation was larger for submerged cases than for emerged cases although emerged cases have lower velocity. Then bed deformation was observed around a large spherical roughness put in the movable sand bed. Roughness was moved upstream and sank because of the front scouring due to a horse-shoe vortex. The deformed bed configuration with sunken spherical roughness attained a stable situation. The maximum scour depth was almost 0.5 - 0.6 of the roughness diameter. The scoured volume became larger in the side-placement cases than the center-placed cases.
