Abstract for the 18th Physics of Estuaries and Coastal Seas Conference, 2016

Surface wave processes in river plumes

Samuel Kastner1, Alexander Horner-Devine*2 and James Thomson3

Keywords: Surface waves, River plumes, Turbulence, Fronts.

Abstract Surface waves are often observed to break at fronts due to gradients in the surface water velocity. This effect is pronounced in river plumes, which are bounded by density fronts with strong surface velocity convergence. However, our understanding of the impact that the incident wave field may have on the plume dynamics and the impact of the plume on the wave field remains largely qualitative. In this work we present field observations from two large river plumes, the Columbia River plume in WA, USA and the Fraser River plume in BC, Canada. In both systems we made co-located wave, salinity, velocity and turbulence measurements from SWIFT (Surface Wave Instrument Float with Tracking) drifters, which transited the plume after being initially released close to the river mouth. Observations from the Columbia River plume document a loss of wave energy in the wind-wave band as waves propagate across the plume front. On a low wind day, wave energy spectra are similar inside and outside the plume (Figure 1a). On a windy day, however, the wind wave peak observed outside the plume in the incident wave field is eliminated in measurements inside the plume (Figure 2a), indicating that those waves did not pass the plume front. Indeed, visual observations (Figure 1c) and breaking rates (Qb) derived from video on the SWIFT drifters (Figure 1d) both confirm that breaking is elevated at the plume front. We estimate the turbulent kinetic energy dissipation �!"#$ due to wave breaking at the plume front based on the observed gradient in wave energy flux across the front. The wave estimates agrees well with the dissipation rate derived from the near-surface SWIFT turbulence measurements and are an order of magnitude higher than observed dissipation rates in the same region on the calm day under otherwise similar conditions. Together, these show that energy is effectively removed from the incident wave field in the wind-wave band due to wave breaking at the plume front and that the energy lost from the wave field is injected into the near- surface plume as turbulence. Simple estimates based on the change in salinity observed along the drifter tracks suggest that turbulence generated by this process may also contribute to plume mixing. Although the wave-breaking turbulence is typically confined to a less than 0.5m thick region near the water surface, strong downwelling at the plume front may carry surface turbulence down to regions of high density gradient where it can more effectively contribute to plume mixing.

outside plume outside plume inside plume inside plume 2 2 10 10

/Hz] (a) 24 May 2014 (b) 25 May 2014 2

0 0 10 10

−2 <−blocked−> −2 <−blocked−> 10 10 (c) SWIFT in the plume front Wave energy [m 0 0.1 0.2 0.3 0.4 0 0.1 0.2 0.3 0.4 Frequency [Hz] Frequency [Hz]

46.4 (d) SWIFT tracks 400 Q 46.3 b 0.00 200 0.03 46.2 0 0.05 CDIP 179 0.08 46.1 −200 Depth [m] 0.10

−400 0.13 46 −125 −124.8 −124.6 −124.4 −124.2 −124 −123.8 Figure 1: Comparison of surface waves inside and outside of the Columbia River plume on calm (24 May) and windy (25 May) days. Wave energy spectra on the calm (a) and windy (b) days reveal a significant reduction in energy at the wind-wave frequencies between the measurements outside and inside the plume. The reduction in wind-wave energy is attributed to intensified wave- breaking at the front, which is documented visually (c) and via breaking rates (Qb) derived from video on the SWIFT drifters (d). Figure from Thomson et al. (2014).

1 Department of Civil and Environmental Engineering, University of Washington, Seattle, [email protected] 2 * Presenter, Department of Civil and Environmental Engineering, University of Washington, Seattle, [email protected] 2 * Presenter, Department of Civil and Environmental Engineering, University of Washington, Seattle, [email protected] 3 Applied Physics Laboratory and Department of Civil and Environmental Engineering, University of Washington, Seattle, [email protected]

Data from the Fraser River plume provide a contrasting case to the Columbia River plume, which is somewhat anomalous due to its intensely energetic fronts. The wave field near the Fraser River mouth is primarily driven by local winds, since its location in the Strait of Georgia is sheltered from oceanic swell. Our data from a two-week field campaign in January 2016 captured a range of tidal and plume conditions, in addition to wind-wave conditions (Figure 2). During strong northwesterly winds plume water flows almost directly across the Strait, with the winds apparently arresting the plume’s northward Coriolis tendency. Evidence of this behaviour is observed in the long straight drift tracks in Figure 2a and 2b. Under these conditions waves propagating from the northwest are incident on a pronounced northerly plume front and we observed very energetic wave breaking at the front. During southeasterly winds, which are more common in the winter in this region, the plume turns to the right under the combined influence of the wind and Coriolis and the drifters may be blown across the plume front. Under these conditions waves were generated over a thin, strongly sheared plume and we see little evidence of intensification of wave activity at the front. Analysis of the Fraser data will evaluate the impact of the plume on the surface wave field, focusing in particular on the role of fronts, wind direction relative to the plume orientation and on subsurface velocity structure.

Figure 2: Drift tracks showing a) salinity and b) significant wave height for deployments in the Fraser River plume. Drift tracks represent a subset of the total measured drifts during the experiment.

References Thomson, J., A. R. Horner-Devine, S. Zippel, C. Rusch, and W. Geyer (2014), Wave breaking turbulence at the offshore front of the Columbia River Plume, Geophysical Research Letters, 41, doi:10.1002/2014GL062274.