Beach Profile Equilibrium and Patterns of Wave Decay and Energy Dissipation Across the Surf Zone Elucidated in a Large-Scale Laboratory Experiment

The widely accepted assumption that the equilibrium beach profile in the surf zone corresponds with uniform wave-energy dissipation per unit volume is directly examined in six cases from the large-scale SUPERTANK laboratory experiment. Under irregular waves, the pattern of wave-energy dissipation across a large portion of the surf zone became relatively uniform as the beach profile evolved toward equilibrium. Rates of wave-energy dissipation across a near-equilibrium profile calculated from wave decay in the surf zone support the prediction derived by Dean (1977). Substantially different equilibrium beach-profile shapes and wave-energy dissipation rates and patterns were generated for regular waves as compared to irregular waves of similar statistical significant wave height and spectral peak period. Large deviation of wave-energy dissipation from the equilibrium rate occurred at areas on the beach profile with active net cross-shore sediment transport and substantial sedimentation and erosion. The rate of wave-energy dissipation was greater at the main breaker line and in the swash zone, as compared to middle of the surf zone. Based on analysis of the SUPERTANK data, a simple equation is developed for predicting the height of irregular waves in the surf zone on an equilibrium profile. The decay in wave height is proportional to the water depth to the one-half power, as opposed to values of unity or greater derived previously for regular waves