Tidally discontinuous ocean forcing in bar‐built estuaries: The interaction of tides, infragravity motions, and frictional control

Abstract

Abstract Shallow, bar‐built estuaries on wave‐dominated coasts in Mediterranean climates experience an intermittent connection to the ocean. In the presence of low streamflow, their inlets may completely close as a result of nearshore sand transport, but even in the open condition, these inlets remain constricted. Extensive field measurements in the highly salt‐stratified Pescadero estuary in northern California show that the shallow mouth causes these estuaries to experience discontinuous tidal forcing. While the ocean and estuary are fully connected with near‐equal water levels, tidal velocities are slow but infragravity motions in the nearshore induce large velocity oscillations within the estuary. As the ocean tide falls, infragravity forcing is cut off, because the estuarine mouth is perched above the low tide ocean water level, and ebbing velocities are set by bed friction. Observations reveal this oscillation between ocean‐forced and frictionally controlled conditions characterizes and sets estuarine hydrodynamics. Additional wave setup of the lagoon emphasizes the dependence of these estuaries on nearshore ocean conditions, but the diurnal or semidiurnal retreat of the ocean below the mouth cuts off this nearshore influence so it too is tidally varying. Here we present detailed observations and a framework for understanding hydrodynamics in small, shallow bar‐built estuaries.