AbstractScale effects generated in the flow over nonlinear weirs have been studied on a limited number of occasions. Furthermore, the existing studies have been almost exclusively focused on the flows upstream of the weir, while criteria to minimize scale effects downstream of the weir remain to be examined. This study investigates the scale effects that arise in several aspects of the flow over a labyrinth weir and spillway with scale series based on three-dimensional (3D) computational fluid dynamics (CFD) simulations. Results showed that, overall, scale effects decreased with increasing Reynolds number. Changes in the position of the channel-dominant cross waves were observed as a function of the simulation scale and tended to diminish with decreasing scale factor. The flows on the spillway channel generally exhibited relatively small scale effects on the main flow properties (i.e., mean flow depths, mean velocities, and Froude numbers) for Reynolds numbers greater than 3×104. However, higher values (>5×104) would be required to minimize changes in the positions of the cross waves in the channel. Additionally, an upstream head over the weir crest of approximately 0.03 m was found to be sufficient to avoid scale effects in the prediction of the labyrinth weir rating curve. The limiting criteria estimated for the rating curve and the main flow properties on the spillway channel are closely correlated with available literature limits and may constitute valuable guidance for future practice.