On the origin of infragravity waves in the surf zone of a dissipative multiple bar system

Time series of surface elevations and cross-shore currents were obtained at four positions in the multiple bar system of Terschelling, the Netherlands in order to investigate the origin of infragravity waves (frequency < 0.05 Hz) inside the surf zone. The long wave signal was split into time series of onshore and offshore propagating long waves. The highly negative cross-correlation at time lag 0 between the wave envelope and the incoming long waves outside the surf zone, and the observed time lags between successive measurement positions strongly favour the 'offshore forcing' of infragravity waves inside the surf zone. In other words, the infragravity waves inside the surf zone of the study area are released incoming bound long waves. The data suggests that this offshore forcing is not limited to situations with shore-normal incident waves ('leaky situations').