Choi, J.; Roh, M., and Hwang, H., 2018. Observing the laboratory interaction of undertow and nonlinear wave motion over barred and nonbarred beaches to determine beach profile evolution in the surf zone.A vertically two-dimensional laboratory experiment was performed with a movable bed to investigate the interaction of undertow and sawtooth-shaped nonlinear wave motion over the cross-shore evolution of barred (storm) and nonbarred (normal) beach profiles. The bottom elevations, free-surface motion, and flow velocity were measured under regular wave conditions, excluding infragravity waves, over a movable bed. The observations were conducted in the surf zone during alternate repeats of the two wave conditions until each quasiequilibrium state was reached. From the net sediment transport of the observation area, it was shown that the storm case was in an erosion state and that the normal case was in an accretion state. The measurements confirmed that the strong undertow, which is a dominant factor in the offshore migration mechanism, was developed in the erosion state but that the undertow in the accretion state was weaker. The measurements also showed that in the shoaling zone, the symmetry of wave motion of the storm-erosion case was larger than that of the normal-accretion case; however, in the surf zone, the symmetry of wave motion of the normal-accretion case was larger than that of the storm-erosion case. In addition, the asymmetry of wave motion of the storm-erosion case was larger than that of the normal-accretion case in both regions. The onshore migration effect of the sawtooth-shaped nonlinear wave motion, which was relatively weak in the accretion state, was more significant than that in the erosion state, because the undertow was not enough strong in the accretion state.