Wave- and Tide-Induced Infragravity Dynamics at an Intermediate-To-Dissipative Microtidal Beach

Numerical simulations of a range of wave climates and tide conditions made with the model XBeach are exploited to study propagation, evolution, dissipation, and reflection patterns of infragravity waves (IGW) at the intermediate-to-dissipative beach of Sabaudia (Tyrrhenian sea, Italy). On the basis of a novel calibration process performed using field swash properties collected by a nearby monitoring station, the model reproduces average run-up characteristics with good skill (Willmott index of agreement of 0.61 for R2%). IGW at Sabaudia beach evolves exclusively as bound long waves growing across the shoaling region for both mild and intense wave climates. Furthermore, values of the dimensionless bed slope βH at which transition from steep-to mild-slope behavior occurs are higher than the threshold commonly assumed in the literature. Finally, although the small tide (0.4 m between mean high water spring and mean high water spring; tide range/breaking wave amplitude ratios between 0.14 and 1.1) does not alter bulk IGW reflection significantly, low tide is effective in reducing onshore IGW fluxes and, ultimately, reflection coefficients R2 selectively for intermediate IG frequencies. Our study, for the first time, gives clear evidence that the tide has a role in determining frequency-dependent IGW dynamics and altering the dissipative state of a mild sloping beach also in a microtidal environment.