A numerical technique for simulating turbulent flows in which the free surface is allowed to undergo arbitrarily large deformations and is subject only to a maximum slope limit is applied to turbulent open channel flow at a Reynolds number of approximately 3000 based on the surface velocity and depth. The test problem has been extensively studied in the literature and allows detailed comparisons to be made. It is found that the method is in general agreement with published results and can be used for a more extensive examination of turbulent fluid mechanics at a free surface.