INFLUENCE OF LONG WAVES AND WAVE GROUPS ON SWASH ZONE SEDIMENT TRANSPORT AND CROSS-SHORE BEACH PROFILE EVOLUTION

There are only a few detailed measurements of the cross-shore variation in the net sediment transport and beach evolution for single or multiple swash events, and no data showing the influence of long waves and wave groups on swash zone morphology. Novel laboratory experiments and numerical modeling have been performed to study the influence of long waves and bichromatic wave groups on sediment transport and beach morphodynamics in the swash zone. Due to complex processes, difficulties in measuring, and very significant difficulties in isolating the morphodynamic processes induced by long waves and wave groups on natural beaches, a laboratory study was designed to measure in very high detail the bathymetric evolution of model sand beaches under monochromatic waves, long wave and short wave composites (free long waves), and bichromatic wave groups (forced long waves). Net sediment transport, Q(x), and beach morphology changes under the monochromatic waves were analyzed and compared to conditions with and without the free long waves, and then compared with the bichromatic wave groups. A range of wave conditions, e.g., high energy, moderate energy, and low energy waves, were used to obtain beach evolution ranging from accretionary to erosive, and including intermediate beach states. Hydrodynamics parameters, e.g., instantaneous water depths, wave amplitudes, run-up and rundown, were also measured to study and test a sediment transport model for the swash zone, based on modifying the energetic-bedload based sediment transport equations with suspended sediment. The experimental data clearly demonstrate that for the monochromatic wave conditions, beach evolution develops erosion for high steepness waves and accretion for lower steepness waves. The model beach profile evolutions are similar to natural beaches, and form and develop bars and berms over time. Adding a free long wave to the short wave in the composite wave results in changes to the overall trend of erosion/accretion of the beach profile, but the net transport pattern does not change significantly. The short wave strongly dominates beach behavior and the net transport rate, instead of the free long wave in the composite wave. The free long wave, however, carries more water and sediment onshore, leading to an increase in shoreline motion and wave run-up further landward. The long wave influences the structure and position of the swash bar/berm, which generally tends to move onshore and forms a larger swash bar/berm for higher long wave amplitudes. The free long wave also increases overall onshore sediment transport, and reduces offshore transport for erosive conditions. The long wave tends to protect the beach face and enhances onshore transport for accretive conditions, especially in the swash zone. In contrast, for bichromatic wave groups having the same mean energy flux as their corresponding monochromatic wave, the influence on sediment transports is generally offshore in both the surf and swash zone instead of onshore. The swash berm is, however, formed further landward compared with the berm of the corresponding monochromatic wave. The sediment transport patterns (erosion or accretion) generated by the bichromatic wave group or corresponding monochromatic wave are similar, but differ in magnitude. The numerical model, starting in the inner surf zone to reduce the effect of poor breaker description in the non-linear shallow water equations, can produce a good match between observed data and the modeled hydrodynamics parameters in the SZ. The sediment transport model shows the important role of suspended sediment in the swash zone. In contrast with the observed data, energetic-based bed-load models predict offshore sediment transport for most wave conditions because of negative skewness. The modified sediment transport model, with added suspended sediment terms and optimized coefficients, produces a good match between model results and observed data for each wave condition, especially for low frequency monochromatic waves. The optimized coefficient set corresponding to particular monochromatic wave conditions can be used to predict the net sediment transport quite well for some composite wave conditions. Overall, the same optimized coefficient sets can be applied to predict the correct overall trend of net transport for most composite wave conditions. However, the predicted net transport for the bichromatic wave groups does not match well with the overall net transport patterns. There is no set of single transport coefficients that can be used to predict sediment transport for all wave conditions. This suggests that the present sediment transport models cannot predict evolution correctly, even for conditions which represent only perturbation from those for which they were calibrated