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. Plain Language Summary: Infragravity waves (IGW) are long‐period oscillations of the water level, usually underlying common short waves. They are relevant to a number of coastal processes, like beach inundation and sand mobilization. A set of storm conditions and tidal levels is here simulated with a computer to observe how IGW evolves and dissipates at the beach of Sabaudia (Italy), characterized by low slopes and a small tidal excursion. It is found that IGW at Sabaudia develop and intensifies by gaining energy from short wind waves as they travel toward the shore. IGW are eventually set free to propagate where short waves start to break, and then stop growing or begin to decay due to the increasing dissipation they meet when they travel in low waters. The presence of IGW at Sabaudia is dominant, especially during intense storms. We have also seen that the variation of the tidal level at Sabaudia is capable of changing how much of the IGW energy is able to reach the shore, mainly for an intermediate range of IGW wave periods. We suggest that tide should be taken into account when evaluating beach inundation due to waves, even on coasts when the tidal range is small. Key Points: Storms and tide are simulated with XBeach through an innovative calibration approach, to study infragravity waves at Sabaudia (Italy)Bound long waves grow in the shoaling zone and stop growing/decay in the surf zone, with no shoreline dissipationAlbeit small, tidal excursion significantly alters onshore fluxes and reflection coefficients for intermediate IG frequencies [ABSTRACT FROM AUTHOR]