Particle Image Velocimetry Measurements within a Laboratory-Generated Swash Zone

A particle image velocimetry (PIV) technique is used to make vertically resolved two-dimensional measurements in swash zone flows, which are notoriously recalcitrant to quantitative measurement. The PIV implementation directs the light sheet into the measurement region from beneath the beach thus avoiding issues of free surface diffraction effects. Fluorescent particles and an optical filter are used to ensure that only particles, and not bubbles or free surface anomalies, are imaged. The spatially and temporally resolved velocity fields measured in a plunging and spilling wave-driven swash zone are used to investigate the boundary layer structure of the mean and turbulent quantities as well as the phase evolution of the bed shear stress, near-bed turbulent kinetic energy, and the dissipation. Results suggest that vertical structure in spilling and plunging wave forced swash zones are similar. The uprush phase is dominated by bore-generated and bore-advected turbulence, which evolves analogously to grid turbulence, while the downrush phase is ultimately dominated by boundary layer generated turbulence, which compares well near-bed with classic flat plate boundary layer theory.