Forced Versus Free Analysis of Infragravity Waves Across an Offshore Reef on the Coast of Oregon

Abstract On the Oregon coast, infragravity (IG) waves can be especially energetic and therefore are an important component of many nearshore processes. Herein, we present results from a field experiment conducted at Nye Beach on the central Oregon coast. A main goal of the field experiment was to characterize the IG transformation across an alongshore rock reef that lies approximately 1.5 km offshore, including the relative contributions of forced versus free IG components. The data analyzed herein are from the cross-shore array of bottom-mounted pressure sensors deployed during the experiment during the fall of 2022. Results demonstrate (somewhat surprisingly) that for all sensors both the bound and free IG components are strongly correlated with the local swell energy. In addition, the theoretical bound IG from nonlinear interactions agrees well with the measured bound IG energy extracted through a bispectral analysis. The theory agrees less well at the offshore gage on top of the rock reef -potentially pointing to the increased importance of either IG directionality or IG reflection at the reef.