Hydrodynamics of surf and swash on natural beaches

The hydrodynamics of surf and swash on natural beaches are presented. Wave height evolution in the surf zone is investigated with data from ten predominantly swell dominated, micro-tidal, sandy beaches. Wave height evolution is presented in terms of wave height-to-water depth ratios (γ=H/h), and both a conventional time-averaged analysis (γrms) and a new wave-by-wave analysis (γw) are performed. Data clearly show unsaturated surf conditions are typical for these beaches, with γ values increasing with increasing offshore wave height (Ho). Values of γ increased slowly in the deeper water depths of the surf zone and increased rapidly in the shallower water depths, with the change in the rate of increase of γ occurring at h/Ho=0.5. No dependence of γ on either absolute or relative beach slope was observed. The skewness of the distributions of γw is consistent with waves that are not depth limited. For the observed unsaturated surf, the terminal bore height at the shoreline is shown to be approximated by Hb≅0.12Ho. Two wave height transformation models are investigated: a monochromatic model and a random wave model. Two different wave transformation regimes are observed in the monochromatic wave model results: a regime termed over-dissipative with concave up wave height transformation curves following breaking and a regime termed under-dissipative with concave down wave height transformation curves following breaking. The boundary between these two regimes has constant γ values immediately following breaking, which is characteristic of saturated and depth limited surf. A relative beach slope parameter, βγ, is derived to distinguish between these regimes. This can also be regarded as a new surf similarity parameter that has a different power relationship for wave steepness when compared to the conventional parameter. In all monochromatic model results, however, values of γ are shown to increase rapidly in the shallow water depths of the surf zone, and in no cases is a constant value of γ observed close to the shoreline. In comparison, the random wave model predicts only under-dissipative conditions, with γ values always increasing shoreward. For all model-data comparisons, the value of the model fitting parameter, B, needs to be increased to obtain more accurate model predictions, indicating that the basic model assumptions do not generate sufficient dissipation to accurately model surf in the laboratory or on natural beaches. The distributions of observed wave heights are compared to theoretical Rayleigh and normal distributions and found to be more normally distributed. The observed wave height distribution is also found to become narrower as depth decreases. The kinematics of the swash zone are investigated using optical remote sensing techniques to examine the flow asymmetry at the surf-swash boundary. This is examined in relation to new solutions to the non-linear shallow water equations where the flow asymmetry is parameterised using a variable, k, that represents the relative duration of inflow into the swash zone. For natural swash it is shown that there is significant variability in k, with a range of -