The unsteady flow structures induced by Karman vortices and horseshoe vortices around a rectangular column are examined using 3-D numerical computations. The non-linear κ−ε model with the effect of the strain and rotation parameters is adopted as the turbulence model. Applicability of the turbulence model to the Karman vortex shedding in the 2-D flow field is firstly investigated through the comparison of the calculated results with the experimental ones conducted in the closed water section by Lyn (1992) and the model constants are adjusted. Then, the 3-D computations of unsteady flows in open channels are carried out focusing on the Karman vortex and the horseshoe vortex generated around a cylinder. Effects of the depth on the 3-D flow structures in open channels are also discussed through the comparison of the numerical results under the different conditions of water depth.