Infragravity response to variable wave forcing in the nearshore

Analysis of field observations of sea-surface elevation reveals the role of time-varying incident forcing and bound wave release mechanisms on the generation of infragravity waves on a barred beach. Observations of infragravity response were obtained during sea breeze and swell periods with wave height varying from 0.2 to 1.2 m and wave period from 3 to 16 s. The observations show a stronger infragravity response to longer period incident swell than to short period wind-sea. During the peak of the sea breeze, short period wind-sea is conducive to long wave generation by breakpoint forcing. Under swell-dominated conditions, the frequency and wave number of incoming bound waves associated with wave groups may satisfy the linear dispersion relationship, leading to resonant forcing of a free wave that appears to be released from the wave group. The transition between long wave generation due to bound wave release and time-varying breakpoint forcing agrees with the surf beat similarity parameter. Key Points The infragravity response is stronger to swell than wind-sea forcing Breakpoint forcing is dominant long-wave generation mechanism during sea breeze Bound wave release mechanism is only present during swell-dominated period