Mass transport induced by infragravity wave

Mass transport induced by group-forced subharmonic waves (infragravity waves) is investigated in the present study. A theoretical solution for subharmonic waves' kinematic contributions to fourth-order mass transport and drifting velocity has been proposed for any depth and bandwidth for the first time. This model is validated using particle-tracking simulations driven by the flow field generated by the SWASH numerical model. The subharmonic-induced mass transport solution is dependent on a weighted sum of the subharmonic velocity variance spectrum and velocity skewness bispectrum. For narrow-banded waves with long wave group relative to depth, the weightings become independent of spectral components, and the solution is recovered in the time domain. Two mechanisms contributing to mass transport were identified: a forward drift resulting from self-interaction similar to Stokes drift, and a depth-decaying backward drift induced by negative subharmonic velocity skewness due to the anti-phase coupling between subharmonics and wave groups. For narrow-banded waves the forward transport surpasses the backward transport for kh<0.72. For other waves, the critical kh for this phenomenon decreases as wave period, bandwidth and bed slope increases. At greater depths or steeper bed slopes, near-surface backward transport predominates over forward transport; at shallower depths or gentler slopes, forward transport is dominant throughout the water column. Although smaller than Stokes transport, the subharmonic wave-induced mass transport can affect the long-term trajectory of a particle. This study provides the first evidence and insight for the influences of group-forced subharmonics on vertically varying mass transport from ocean surface to seabed in coastal environments.