River Plume Modulation by Infragravity Wave Forcing

Abstract

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.

Infragravity waves and cross-shore motion–a conceptual study

“…As already mentioned, IG waves are connected to a wide range of nearshore processes, and the results in this paper are in line with recent observations of horizontal movement of freshwater plumes (Flores et al, 2022) and also with wave-by-wave motions of tracer particles in the nearshore (Bjørnestad et al, 2021).…”

“…IG waves in the surf zone explain the well-known surf beats observed in (Munk, 1949;Tucker, 1950), and are connected to various nearshore and coastal processes such as rip currents, run-up and overtopping, as well as beach and dune erosion (Russell, 1993;van Thiel de Vries et al, 2008;Roeber and Bricker, 2015;Castelle et al, 2016;Bertin et al, 2018). It is also known that IG waves are crucial for the movement of sandbars along the beach (Aagaard and Greenwood, 2008), and recent field measurements have revealed the importance of IG waves for several nearshore transport phenomena such as river plume oscillations (Flores et al, 2022), movement of particle tracers (Bjørnestad et al, 2021), and transport of suspended sediment (Mendes et al, 2020).…”

Sustained increase in suspended sediments near global river deltas over the past two decades

“…In addition, waves may also confine river plumes nearshore (Supplementary Fig. 10c) 45 . Other factors, such as the strength and direction of the wind [46][47][48] , could also modulate the horizontal extent of river plumes and obscure the relation between SSC magnitude and RPA.…”