English: Mean flow-velocity vectors in the undertow under plunging waves, as measured in a laboratory wave flume. Below the wave trough, the mean velocities are directed offshore. The beach slope is 1:20; note the vertical scale is distorted relative to the horizontal scale.
Figure 8 in:
A. Okayasu, T. Shibayama, and N. Mimura (1986) "Velocity field under plunging waves". In: B. L. Edge (Ed.), Proceedings of the 20th International Conference on Coastal Engineering, Taipei, vol. 1, ASCE, pp. 660–674.