Tidal modulation of infragravity wave dynamics on a reflective barred beach

Abstract

Nearshore dynamics of infragravity (IG) waves on a barred beach with a steep foreshore (~1:10) is discussed. One-day nearshore wave measurements were conducted under a low-energetic offshore wave condition (wave height: ~0.5 m). Notably, IG waves were modulated in antiphase with the tidal water level. The observed data suggested that IG waves significantly developed over the shoaling zone as bound waves at low tide, whereas the breakpoint-forcing excited the dynamic wave setup in the swash zone at high tide. The tidal modulation mechanisms were investigated using a one-dimensional nonhydrostatic wave model, SWASH, which successfully reproduced the observed wave fields over tidal cycles. The analyses of energy transfer processes based on the model results revealed that the sandbar played a key role in modulating IG waves. Incident short waves did not break over the sandbar during high tides and excited bound IG waves dominantly on the shoreward of the sandbar. Subsequently, the IG waves lost approximately 40% of their energy even on the steep foreshore due to the breakpoint-forcing by the wave groups which were unbroken over the sandbar. In contrast, decreased water depth during low tides intensified energy transfer to IG waves on the seaward of the sandbar, and short wave attenuation over the sandbar resulted in nearly full reflection of IG waves from the shore. This study highlights the strong variability of IG wave dynamics on a barred beach profile.