Wave-induced nearshore currents at a tri-cuspate beach in the UKCRF

Abstract

This paper describes an investigation into the surf-zone hydrodynamics at a fixed tri-cuspate beach installed in the UK Coastal Research Facility (UKCRF). Measurements are presented of the wave height field, horizontal wave-induced current patterns and detailed structure of rip and meandering longshore currents. For waves normal to the beach, the flow patterns consist of repeated primary circulation cells driven by longshore currents feeding into seaward rip currents. Each rip current is a narrow time-varying jet. From the measurements, the rip currents reached a maximum non-dimensional speed of [Formula: see text] (below the threshold of a stopping current), where |V| is the magnitude of current velocity, g is acceleration due to gravity and h is still water depth. The vertical profile through a typical rip current is reasonably uniform above a thin wave-induced oscillatory boundary layer, indicating that the rip current is predominantly a two-dimensional flow feature. Within the surf-zone, peak seaward-directed velocities occur at the top of the bed boundary layer, and incorporate the undertow in addition to the rip current. For oblique regular waves, a stable meandering longshore current forms, of magnitude above 0·5 m/s (corresponding to [Formula: see text], with a current-induced logarithmic near-bed profile. The data should be useful in validating numerical models of nearshore wave–current interaction.