Detailed Measurements of Boundary Layer Flow under Swash with High-resolution PIV

Abstract

A laboratory study under swash flow on a steep slope was designed to investigate the characteristics of bottom boundary layer. By careful setup of the PIV system and optimizing the traditional cross-correlation algorithm, the velocity fields within this thin boundary layer of an order of 1 mm were successfully resolved over ten points. The spatially and temporally resolved near-bed velocity profiles had higher resolution than previous studies and were expected to make reliable insights into the boundary layer structure, including the mean velocities, turbulence intensity and bed shear stress. Results suggested that mean velocities and turbulence intensity in uprush are larger than that in the backwash. The uprush flow is associated with the bore-generated and bed-generated turbulence, while the backwash flow is supposed to be dominated by bed-generated turbulence, and the high turbulence intensity in the final stage of backwash is supposed to be related to high velocity gradient. The bed shear stress estimated by fitting the logarithmic profiles to the measured velocity profiles in the uprush was larger than that in the backwash. The detailed measurement data can be used to develop and verify numerical models for boundary layer studies under swash.