Storm- and tide-induced infragravity dynamics at an
intermediate-to-dissipative microtidal beach
Abstract
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 swash characteristics with good skill (Willmott index
of agreement D = 0.61-0.79). IGW at Sabaudia beach evolve exclusively as
bound long waves (BLW) growing across the shoaling region for both mild
and intense wave climate. BLW release occurs still in the inner shoaling
zone and, thus, may not be strictly connected with wave breaking
inception, as usually assumed. Furthermore, values of the dimensionless
bed slope βH at which transition from steep- to
mild-slope behaviour occurs are higher that traditional threshold.
Finally, although the small tide (0.4 m between MHWS and MLWS; 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.