Abstract
Direct measurements of swash zone bed shear stress obtained with a shear plate are presented and a new swash zone boundary layer model is introduced. The model considers boundary layer growth in a Lagrangian framework, in terms of fluid particle displacement and flow history. The model is based on the momentum integral analysis of the smooth, flat-plate boundary layer and predicts the observed onshore asymmetry in bed shear stress. Friction coefficients back-calculated from the measured bed shear stress display strong temporal variation. In the absence of bore-induced turbulence, the uprush friction coefficient is predicted well by the smooth, turbulent boundary layer model. Early in the backwash phase, the friction coefficient may be better predicted by a laminar boundary layer model. Measurements suggest that the backwash boundary layer transitions from laminar to turbulent. It is proposed that the backwash boundary layer may remain laminar for a longer period than expected due a favorable pressure gradient.
Published Version
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