Cross-shore modelling of multiple nearshore bars at a decadal scale

This paper presents a numerical model designed to simulate subaqueous cross-shore profile behavior, including response of feeder mounds and barred systems. The present model development builds on the semi-empirical model proposed by Larson et al. (2013), designed to simulate the evolution of longshore bars exposed to incident waves, as well as the exchange of material between the bar and the berm region. Here, efforts are made to expand the theory for the evolution of a single-bar to a 2-bar system, where the volumes of the individual bars (inner and outer) and their responses are modeled. In order to investigate the predictive capacity of the model, this exploratory numerical tool is first calibrated and validated against data from Duck, North Carolina, USA, where 2 bars typically appear (inner and outer). Field data derived from nearshore sand placement projects (Silver Strand State Park, California, and Cocoa Beach, Florida, USA), involving the construction of artificial longshore bars, are also employed to test the model in complex situations with diverse wave climates and typical beach profile shapes. The study presented in this paper shows that the equilibrium-based model is skilled at predicting the time-varying volume of the outer bar (ε = 0.39; NMSE = 0.24), suggesting that this morphological feature is strongly influenced by offshore wave forcing in a predictable, equilibrium-forced manner. Model skill was lower (ε = 0.51; NMSE = 0.29) when predicting the inner bar evolution at Duck, remaining questions about the predictability and the equilibrium-driven cross-shore behavior of more transient features. Model prediction of the evolution of feeder mounds (artificial bars) proved to be also successful through description of hypothetical bars characterized by zero equilibrium bar volume, leading to a good agreement with the field observations. Overall, the potential for using rather simple models to quantitatively reproduce the main trends of cross-shore volume changes in bars in a time perspective from years to decades has been demonstrated.