Barred Beach Profile Equilibrium Investigated With A Process-Based Numerical Model

Investigating the equilibrium profile of a sandy beach is important to evaluate the beach response to natural forces. Empirical or semi-empirical relationships among beach states, wave climates and sediments properties have been established in various studies. In this study, detailed physical insights of hydrodynamics and sediment transport patterns on a quasi-equilibrium beach profile are provided using a process-based model. Model results show that during the beach profile transition to equilibrium, both the magnitude and the cross-shore gradient of sediment transport rate decrease. The total net sediment transport in surfzone is mainly affected by the balance/ imbalance between offshore-directed current-related suspended sediment transport and onshore-directed bedload transport, both of which showing a decreasing tendency during the transition to equilibrium. The former is due to the decreasing undertow velocity and sediment concentration, and the latter is due to the decrease of peak bedload transport rate near the intra-wave crest phase. The wave-related suspended sediment transport is relatively small compared to the other two transport components. Based on the entropy concept, a dimensionless parameter representing the spatial uniformity of wave energy dissipation rate is proposed as an indicator of the equilibrium degree of a beach profile. Results suggest that a quantitative relationship can be established between this parameter and the overall characteristics of sediment transport on a beach profile.