Matthew P. Barnes

FIELD OBSERVATIONS OF INSTANTANEOUS CROSS-SHORE FREE SURFACE PROFILES AND FLOW DEPTHS IN THE SWASH ZONE

Observations of instantaneous and spatial mean water surface slopes and flow depths in the surf and swash zone are presented. With the assumption of hydrostatic pressure and small shear stress at the free surface, the surface slope is equivalent to the horizontal pressure gradient which is directly proportional to the total fluid acceleration. In the swash, the water surface slope is observed to be positive (dipping seaward/rising landward) and the total fluid acceleration to be negative for the majority of the uprush and the entire backwash, contrary to some recent suggestions. Intantaneous swash depth profiles show that the uprush lens is weakly convex in shape, consistent with previous work and friction effects at the run-up tip. In addition, a simple method is presented for obtaining rapid inter-swash bed level measurements and hence the cross-shore variation in net total sediment flux for individual swash events. Copyright ASCE 2006.

Direct Bed Shear Stress Measurements in Bore-Driven Swash and Swash Interactions

Bore-driven swash and swash interactions are investigated experimentally. Direct bed shear stress measurements suggest bore/swash backwash interactions act to interrupt backwash (seaward) sediment transport. Catch up and absorption causes landward directed bed shear force (enhanced by impact forces onto the bed) of a short duration that does not follow the measured flow velocity trend. A single swash interaction is likely to restrict seaward direct transport and/or suspend sediment in large volumes but not directly contribute to the landward advection of sediment. Bore-driven swash hydrodynamics for a single swash event were predicted well using a non-linear shallow water wave equation model. In the mid-swash region measured uprush bed shear stress was predicted well using the drag law estimate with constant friction factor Cf [approximate]0.02. A smaller or time-varying Cf is required to accurately predict measured backwash bed shear stress.

Lagrangian modelling and direct bed shear stress measurement in the swash zone

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.