A numerical model of open-channel flow over a rough bed was established based on double-averaged NS(DANS)equations. The turbulence was simulated by a modified Spalart-Allmaras(S-A)turbulence model, and the form-induced stress was calculated with a sub-layer-scale eddy viscosity model. The results show that the form-induced stress within the roughness layer cannot be ignored, and it decreased rapidly with the distance above the top of the roughness layer. The form-induced stress accounted for the dispersion of the velocity profiles and caused a slight reduction of the mean velocity. It also significantly decreased the Reynolds stress at the top of the roughness layer. In addition, this study shows that the velocity profile, the Reynolds stress profile, and the form-induced stress profile are affected by the distribution of the porosity. A smoother profile of porosity has less impact on the flow and turbulence characteristics.