Estimation of the velocity field induced by plunging breakers in the surf and swash zones

This study presents an investigation into the spatial and temporal evolution of the velocity field induced by plunging waves using the bubble image velocimetry (BIV) technique. The BIV velocity estimates are validated with both direct single-point measurements and a well-validated VOF-type numerical model. Firstly, BIV-derived time series of horizontal velocities are compared with single-point measurements, showing good agreement at two cross-shore locations on the impermeable slope in the swash and surf zones. The comparison includes a discussion on the uncertainty associated with both data sets. In order to evaluate the transient two-dimensional description of the flow field, a high-resolution VOF-type numerical model based on the Reynolds-averaged Navier–Stokes equations is used. A reliable estimation of the numerically derived surf zone velocity is established. In the swash zone, however, an overprediction of the offshore flow is identified, which may be ascribed to the single-phase nature of the numerical description, suggesting the importance of the dynamics of the air/water mixture for accurate modelling of this breaker type. The non-intrusive BIV technique was shown to be a good complementary tool to the numerical model in the estimation of velocity field induced by plunging waves in the laboratory. It is shown that the BIV technique is more suitable when the nature of the velocity field under the presence of an aerated flow is sought. This is relevant for hydrodynamic studies of plunging breakers when, due to air entrainment, the use of other measurement techniques or single-phase formulations in numerical models may provide uncertain results.