Free infragravity waves on the inner shelf: Observations and
Parameterizations at two Southern California beaches
Abstract
Co-located pressure and velocity observations in 10-15m depth are used
to estimate the relative contribution of bound and free infragravity
(IG) wave energy to the IG wave field. Shoreward and seaward going IG
waves are analyzed separately. At the Southern California sites,
shoreward propagating IG waves are dominated by free waves, with the
bound wave energy fraction <30% for moderate energy incident
sea-swell and <10% for low energy incident sea-swell. Only
the 5% of records with energetic long swell show primarily bound waves.
Consistent with bound IG wave theory, the energy scales as the square
(frequency integrated) sea-swell energy, with a higher correlation with
swell than sea energy. Seaward and shoreward free IG energy is strongly
tidally modulated. The ratio of free seaward to shoreward propagating IG
energy suggests between 50-100% of the energy radiated offshore is
trapped on the shelf seaward of 10-15m and redirected shoreward. Remote
sources of IG energy are small.
The observed linear
dependency of free seaward and shoreward IG energy on local sea-swell
wave energy and tide are parameterized with good skill (R2
~ 0.90). Free (random phase) and bound (phase-coupled)
IG waves are included in numerically simulated timeseries for shoreward
IG waves that are used to initialize (~ 10m depth) the
numerical nonlinear wave transformation SWASH. On the low slope study
beach, wave runup is only weakly influenced by free shoreward
propagating waves observed at the offshore boundary (foreshore slope =
0.02).