River Plume Modulation by Infragravity Wave Forcing.

We present measurements at the mouth of Río Maipo, a small river that discharges into an energetic surfzone on the coast of Chile, which document for the first time the plume response to infragravity (IG) wave forcing. We find that inlet dynamics are strongly modulated by IG waves; inlet discharge velocity varies by 20% in the IG band relative to peak ebb velocity. Drone imaging and measurements of surfzone salinity show that the plume edge is forced offshore and onshore, modifying plume width by 20%–50% at IG frequencies. We conclude that the observed modulation results from surfzone forcing at IG frequencies consistent with a moving break point; increased (decreased) wave amplitude generates stronger (weaker) wave‐driven circulation and forces the plume onshore (offshore). Although river water is often trapped in the surfzone, these dynamics may enhance mixing and cross‐shore dispersion of river‐borne materials. Plain Language Summary: Small coastal rivers often discharge directly into the surfzone, where the fate of river‐borne materials such as sediment, nutrients and contaminants is determined by the combined effects of buoyant plume processes, wave‐driven dynamics and turbulence. In this work we present field measurements at the mouth of Río Maipo, a small river that discharges into a very energetic surfzone on the coast of central Chile, documenting the response of the plume to infragravity (IG) wave forcing (timescales of 30–300 s) for the first time. We observe persistent velocity and salinity oscillations at IG frequencies when freshwater is leaving the estuary, and that the plume edge is moved onshore and offshore at these frequencies, in response to forcing in the wave breaking region. This work suggests that the IG forcing may influence the dispersion of freshwater and river‐borne materials in small river systems subjected to strong wave forcing. Key Points: Infragravity (IG) motions modulate the dynamics of a small river plume in the surfzoneOutflow velocity, wave breaking intensity and plume salinity all show variability at IG frequenciesThe river plume expands and contracts by 20%–50% at IG frequencies [ABSTRACT FROM AUTHOR]