Field Observations and Numerical Modeling of Infragravity Waves

Field observations and numerical modeling of infragravity (IG) waves were conducted in support of the expansion of a major seaport. Measurements show that the overall energy level of IG waves is similar between stations deployed outside and inside the port, while spectra of IG waves show a clear peak of ~0.01 Hz at the inside-port stations and IG spectra at the outside-port stations are very broad-banded and somewhat white-noise-like, without a predominant peak frequency. Roughly linear dependence of long-wave amplitude on short-wave amplitude is observed from the field data. Long waves are also observed to be very directionally scattered, with seaward-propagating components dominant over contributions from the shoreward propagating components. In the numerical model, a non-standard approach, by employing a customized filter layer in the surf zone to mimic the short-wave breaking and run-up processes, is used to reduce the resolution requirement and thus the computational burden by an order of magnitude. Numerical model results are in good agreement with measured long-wave amplitude and compare favorably with the measured long-wave spectra. Furthermore, the directional properties of long waves from the numerical model are also confirmed by the analysis of measurement data.