Numerical modeling of infragravity wave response during DELILAH

Abstract

The nearshore circulation model SHORECIRC is applied to simulate the infragravity wave conditions at two intervals during the 1990 DELILAH campaign at Duck, N. C. The model has been extended to solve the energy equation for random short waves (on the timescale of the wave groups) which acts as a wave driver for the long‐wave model. To simulate the wave conditions an algorithm has been derived which synthesizes a directional offshore wavefield from measured data including the bound, directional low‐frequency components. The model‐data comparison shows that the mean short wave transformation and mean longshore current are predicted well. The structure of the infragravity spectrum as measured in the nearshore array is correctly represented, albeit that the energy levels are underpredicted in most locations. The 2‐D frequency longshore‐wavenumber spectra show that the infragravity wave energy consists mostly of edge waves, and that the computations qualitatively resemble the measurements. It is shown that for 1 day, bar‐trapped edge waves are present. The model is also used to show that the influence of the longshore variability is small for this particular beach and that the directional spreading in the offshore wave field is critical in predicting the nearshore infragravity wave conditions correctly. Edge waves which propagate against the current are generated by the directionally broad short‐wave forcing and to a lesser extent by back‐scattering. The nonlinear terms are relatively unimportant for the prediction of the infragravity wave climate. The cross‐shore gradient of the onshore‐directed radiation stress is the most important of the forcing terms.