Scaling wave-plume interactions in two coastal river discharges

The dynamics of coastal rivers discharging into the surf zone have been the focus of several recent studies because of the disproportionate role of small rivers in transporting sediment, nutrients and pollutants into nearshore waters. The Maipo River, Chile, for example, passes through the city of Santiago and agricultural regions of central Chile before discharging into the Pacific, where the combined processes of an energetic surf zone and river plume dispersion determine the fate of river-borne material. Recent work by Kastner et al (submitted JGR) based on observations at the mouth of the Quinault River WA outlines a scaling analysis that predicts the fate of river water discharged into the surf zone in terms of river flow, incident wave energy, beach slope, channel bathymetry and tidal amplitude. In this work we use new in situ observations and aerial imaging from the Maipo River to generalize our current understanding of the dynamics of river plumes in the surf zone. The Maipo discharge is 4-10 times smaller than the Quinault and the beach slope is significantly gentler. As a result of the lower river discharge, salt water is observed to intrude well upstream in the Maipo estuary, whereas the high flows observed in the Quinault inhibited significant salinity intrusion. Additionally, the Quinault bathymetry is complex compared with the Maipo, and this complexity is amplified by tidal action. The comparison between these two river systems highlights the wide range of dynamical behaviors at the mouths of small rivers that result from the diversity of their morphologies and challenge efforts to generalize them. Preliminary results from aerial imaging confirm that the majority of the Maipo discharge is trapped in the surf zone (e.g. Figure 1), and also show that rip currents likely play an important role in exporting river water trapped in the surf zone to the inner shelf. We use a comparison between the two systems to test and extend the scaling proposed by Kastner et al.