PIV measurements of the bottom boundary layer under nonlinear surface waves

• Detailed measurements of the bottom boundary layer under nonlinear waves
• Significant deformation of velocity in the wave bottom boundary layer
• Boundary layer growth rate is related to the duration of wave front and back.
• Observations of vortex tubes under free surface waves

Sediment in the nearshore is largely mobilized in the wave bottom boundary layer (wbbl) hereby emphasizing the importance of this relatively thin layer to nearshore morphology. This paper presents a laboratory experiment where hydrodynamic properties of the wbbl were quantified by measuring flow velocities using Particle Image Velocimetry. The bottom of the wave flume was rigid with a single bar profile. The measurements consisted of the velocities of the wbbl with a resolution in the order of 10 points in time and space for skewed and asymmetric waves. In the wbbl there was significant transformation of velocity skewness and asymmetry. Negative wbbl streaming was generated by asymmetric waves. Boundary layer development and generation of vortex tubes were observed. The velocity measurements included the turbulent components resulting in quantification of turbulent stresses that were of similar magnitude as the viscous stresses.