Surf and swash zone hydrodynamics forced by oblique, monochromatic waves in a wave basin with a smooth, impermeable beach

Abstract

Beaches are coastal features that offer economic benefits, ecosystem services, and natural barriers against flooding. Sustainable management of beaches requires a firm understanding of hydrodynamics and sediment transport processes. This experimental study reports on surf and swash zone hydrodynamics, forced by oblique, monochromatic waves with heights of 0.15 m, 0.20 m, and 0.25 m and periods of 2.0 s and 2.5 s, approaching a smooth impermeable beach with three different offshore incident angles (5°, 10°, 15°). Fluid velocities and bed shear stress were separated into cross-shore and alongshore directions. The data were analyzed using a triple decomposition isolating the time-averaged flow from the wave-driven flow and turbulence in each wave cycle. The velocities showed variations for different wave incident angles and cross-shore positions. The cross-shore velocities were skewed-asymmetric with steep wave fronts and velocity magnitudes reaching nearly 0.5 m/s. The cross-shore bed shear stress magnitude peaked during the start of each wave cycle reaching up to 18 N/m². Alongshore flows remained relatively uniform during the wave cycle, with typical velocities up to 0.5 m/s. There was a non-monotonic variation of alongshore velocities with wave angle, with waves forced at 10° often exhibiting the largest alongshore velocities compared to waves forced at 15° These variations were likely due to variations in wave breaking location relative to fixed in situ sensors. Alongshore bed shear stress magnitudes peaked at the beginning of the wave cycle with values between 0.8 and 4.5 N/m² depending on forcing conditions. The alongshore flow and bed shear stress were dominant during flow reversal when cross-shore velocities were near zero, even for small wave angles. These findings highlight the potential importance of alongshore processes on resultant bed shear stress and sediment and solute transport.