Observation and measurement of the bottom boundary layer flow in the prebreaking zone of shoaling waves

In this paper, the characteristics of the bottom boundary layer flow induced by nonlinear, asymmetric shoaling waves, propagating over a smooth bed of 1115 uniform slope, is experimentally investigated. Flow visualization technique with thin-layered fluorescent dye was first used to observe the variation of the flow structure, and a laser Doppler velocimeter was then employed to measure the horizontal velocity, U. The bottom boundary layer flow is found to be laminar except within a small region near the breaking point. The vertical distribution of the phase-averaged velocity (U) at each phase is non-uniform, which is directly affected by the mean velocity, (U) over bar. The magnitude of (U) over bar increases from zero at the bottom to a local positive maximum at about z/deltacongruent to2.0 similar to 2.5 (where z is the height above the sloping bottom and delta is the Stokes layer thickness), then decreases gradually to zero at z/deltacongruent to6.0 similar to 7.0 approximately, and finally becomes negative as z/delta increases further. Moreover, as waves propagate towards shallower water, the rate of increase in the maximum onshore oscillating velocity component (U) over tilde (+) is greater than that of the offshore counterpart (U) over tilde (-) except near the breaking point. The free stream velocities in the profiles of the maximum onshore and offshore oscillating velocity components, (U) over tilde (+)(infinity) and (U) over tilde (-)(infinity) are found to appear at z/deltagreater than or equal to6.0. This implies that, if the Stokes layer thickness is used as a length scale, the non-dimensionalized boundary layer thickness remains constant in the pre-breaking zone. Although (U) over tilde (+)(infinity) is greater than (U) over tilde (+)(infinity) and the asymmetry of the maximum free stream velocities (i.e. (U) over tilde (+)(infinity) / (U) over tilde (=)(infinity)) increases with decrease of water depth, a universal similar profile can be established by plotting z/delta versus (\(U) over tilde (+) / (U) over tilde (+)(infinity)\) or (\(U) over tilde (=) / (U) over tilde (=)(infinity)\). The final non-dimensional profile is symmetric and unique for the distributions of the maximum onshore and offshore oscillating velocity components within the bottom boundary layer, which are induced by nonlinear, asymmetric shoaling waves crossing the pre-breaking zone. (C) 2002 Elsevier Science Ltd. All rights reserved.