In the Meuse catchment in Belgium, the Auvelais lock nowadays allows for ECMT class Va ships (2000 tons, 110 m x 11.40 m). However, the present upper guard wall, which separates the upstream harbor from the river flow, is not well suited for class Va vessels because it is 110 m-long and its extremity is curved. It is planned to be modernized. To ease and secure the navigation, the new configuration should respect six criterions: it should (1) increase the space at the harbor entrance; (2) minimize the stream velocity in the lock axis; (3) reduce the transverse currents; (4) ensure a smooth velocity gradient distribution to minimize the forces and yawing moments exerted on the vessel at the harbor entrance; (5) reduce the flow contraction in the river channel to maintain the river flood discharge capacity; and (6) not be too expensive. A 1:50 physical model is used to analyse the velocity field in the upper harbor for several geometries and discharges. The present layout is compared to three solutions: (1) a 124 m-long straight solid wall; (2) a 124 m-long straight wall with 9 openings; and (3) a 124 m-long straight wall with 5 openings. The result reproducibility is satisfactorily checked. The velocity profiles show that solution (3) gives the best results according to the six criterions