The Evolution and Effects of Infragravity Waves inside Low Inflow Estuaries with Shallow Inlets

Recent studies have shown that shallow tidal inlets located within the surf zone are strongly affected by infragravity waves. In deep water, energetic wave groups generate bound infragravity waves (0.004 - 0.04 Hz) through nonlinear difference interactions. As these groups approach shallow sloping beaches, the higher frequency waves break and infragravity waves may propagate as free waves. As a result, shallow inlets may act as a filter, transmitting infragravity waves into an estuary while blocking the higher frequency waves. In tidal inlets, wave-current interactions enhance infragravity wave propagation during flood tides and impede infragravity wave propagation during ebb tide, when currents and sea level disconnect the estuary from the ocean and block waves of all frequencies from entering. This study aims to assess how infragravity waves decay and affect flow inside the estuary, after the waves have passed the inlet. Over the past several years, velocity and pressure measurements have been taken at different locations along Los Peñasquitos Lagoon from December 2014 to the present. Los Peñasquitos Lagoon is a low-inflow, bar-built estuary in southern California. A shallow sill in the inlet causes frictional control and perched water levels during ebb tides. Complete closures occur seasonally due to wave-driven sediment accretion. The channel itself is shallow (less than 4m deep), winding, and approximately 2.5 km long. With our measurements, we examine the transformation of infragravity waves as they propagate at least 0.75 kilometers upstream. In addition, we compare the strength of infragravity velocity oscillations with tidal currents to determine the processes affected by infragravity waves and the conditions under which infragravity waves are important.