Bound and free infragravity waves in the nearshore zone under breaking and nonbreaking conditions

Abstract

Recordings of near‐bottom pressure obtained at three positions in a gently sloping multiple‐bar system were analyzed with a bispectral technique to quantify the relative importance of forced motions to the total infragravity (0.004–0.04 Hz) wave field under breaking and nonbreaking conditions, both on a temporal and spatial scale. The temporal variations in the ratio of bound to total infragravity energyEbnd/Eigwere well associated with the local relative wave heightHss/h(whereHssis the significant sea‐swell wave height andhis water depth).Ebnd/Eigincreased with higherHssvalues, reaching maximum values of up to 0.8 at the onset of short‐wave breaking. With an increasing intensity of breaking the coupling between incident and infragravity waves reduced to negligible values in a saturated sea‐swell wave field. The strong phase coupling at the breakpoint with a phase relationship close to the relationship predicted by bound‐wave theory strongly supports the hypothesis that forced waves are the main source of free infragravity motions. In the cross‐shore direction,Ebndwas largest at the breakpoint. Wave breaking was associated with a rapid decrease in the ratioEbnd/Eigin the onshore direction and, consequently, with a rapid increase in the contribution of free infragravity energy to the total infragravity field. The data set indicated that free infragravity energy may also be generated in the absence of breaking waves; the reason for this is not understood.