Laboratory experiments are conducted to measure the free-surface profile and the velocity field of an open channel flow over a semicylinder weir. The streamwise velocities of the flow are measured using a laser Doppler anemometer. The numerical simulation of the flow case having similar conditions with the experiment is performed. The basic equations of the problem are solved using the finite-volume method. A grid convergence analysis is applied to ensure the discretization error keeping within an acceptable limit for the predicted velocities. In the numerical modeling, the volume of fluid method is used to compute the free surface of the flow that interacts with the semicylindrical structure. In the numerical analysis, standard k-epsilon, renormalization group k-epsilon, realizable k-epsilon, modified k-omega, shear stress transport, and Reynolds stress turbulence closure models are employed. The computed results for the velocity field and free-surface profile of the flow are compared with the experimental data. The comparisons of the experimental and numerical results show that the numerical simulation using the Reynolds stress turbulence model provides better predictions for the horizontal velocities than the other turbulence models used herein.