Velocity Fields in Near-Bottom and Boundary Layer Flows in Prebreaking Zone of a Solitary Wave Propagating over a 1:10 Slope

Abstract

The velocity characteristics of a solitary wave shoaling in the prebreaking zone and near the breaking point are investigated experimentally. The study focuses on the near-bottom and boundary layer flows on a 1:10 slope, with the incident wave steepness varying from 0.133 to 0.384. Both a flow visualization technique (FVT) with thin-layered dye as well as particle image velocimetry (PIV) with a high-speed camera were used. Results from FVT reveal that laminar boundary layer flow occurs not only in the prebreaking zone during the shoaling phases, but also in the postbreaking zone during the run-up and run-down phases. However, the laminar boundary layer disappears soon after breaking but before the run-up motion, and immediately after the flow separation followed by hydraulic jump during the later stage of the run-down motion. Results from the PIV measurement show that the maximum horizontal velocity appears under the wave crest and increases during the shoaling process. Flow reversal is observed after the passage of the wave crest. Three unique similarity profiles are then obtained for the velocity distributions in the acceleration phases and in the layers of flow reversal with overshooting, as well as for the maximum onshore and offshore velocities in the boundary layer. In addition, the nondimensional time for the beginning of flow reversal at each measuring section is found to be exponentially proportional to the dimensionless distance to the slope origin, but nearly independent of the incident wave steepness. This study also investigates the lead-time effects of the horizontal velocity at different heights in the boundary layer and at different measuring sections along the slope. The maximum value of the nondimensional lead time, occurring very close to the sloping bottom, decreases linearly with the dimensionless distance to the toe of the slope.