• Akan, Ç., S. Moghimi, H. T. Özkan-Haller, J. Osborne, and A. Kurapov, 2017: On the dynamics of the mouth of Columbia River: Results from a three-dimensional fully coupled wave-current interaction model. J. Geophys. Res. Oceans, 122, 52185236, https://doi.org/10.1002/2016JC012307.

    • Search Google Scholar
    • Export Citation
  • Battjes, J. A., and J. P. F. M. Janssen, 1978: Energy loss and set-up due to breaking of random waves. 16th Int. Conf. on Coastal Engineering, Hamburg, Germany, American Society of Civil Engineers, 32 pp., https://doi.org/10.1061/9780872621909.034.

  • Bowen, A. J., and R. A. Holman, 1989: Shear instabilities of the mean longshore current: 1. Theory. J. Geophys. Res., 94, 18 02318 030, https://doi.org/10.1029/JC094iC12p18023.

    • Search Google Scholar
    • Export Citation
  • Chen, F., and D. G. MacDonald, 2006: Role of mixing in the structure and evolution of a buoyant discharge plume. J. Geophys. Res., 111, C11002, https://doi.org/10.1029/2006JC003563.

    • Search Google Scholar
    • Export Citation
  • Clark, D. B., F. Feddersen, and R. T. Guza, 2010: Cross-shore surfzone tracer dispersion in an alongshore current. J. Geophys. Res., 115, C10035, https://doi.org/10.1029/2009JC005683.

    • Search Google Scholar
    • Export Citation
  • Clark, D. B., S. Elgar, and B. Raubenheimer, 2012: Vorticity generation by short-crested wave breaking. Geophys. Res. Lett., 39, L24604, https://doi.org/10.1029/2012GL054034.

    • Search Google Scholar
    • Export Citation
  • Dalrymple, R. A., 1975: A mechanism for rip current generation on an open coast. J. Geophys. Res., 80, 34853487, https://doi.org/10.1029/JC080i024p03485.

    • Search Google Scholar
    • Export Citation
  • Derakhti, M., J. Thomson, and J. T. Kirby, 2020: Sparse sampling of intermittent turbulence generated by breaking surface waves. J. Phys. Oceanogr., 50, 867885, https://doi.org/10.1175/JPO-D-19-0138.1.

    • Search Google Scholar
    • Export Citation
  • Feddersen, F., 2012a: Observations of the surfzone turbulent dissipation rate. J. Phys. Oceanogr., 42, 386399, https://doi.org/10.1175/JPO-D-11-082.1.

    • Search Google Scholar
    • Export Citation
  • Feddersen, F., 2012b: Scaling surf zone turbulence. Geophys. Res. Lett., 39, L18613, https://doi.org/10.1029/2012GL052970.

  • Feddersen, F., 2014: The generation of surfzone eddies in a strong alongshore current. J. Phys. Oceanogr., 44, 600617, https://doi.org/10.1175/JPO-D-13-051.1.

    • Search Google Scholar
    • Export Citation
  • Feddersen, F., and J. H. Trowbridge, 2005: The effect of wave breaking on surf-zone turbulence and alongshore currents: A modeling study. J. Phys. Oceanogr., 35, 21872203, https://doi.org/10.1175/JPO2800.1.

    • Search Google Scholar
    • Export Citation
  • Feddersen, F., J. H. Trowbridge, and A. J. Williams III, 2007: Vertical structure of dissipation in the nearshore. J. Phys. Oceanogr., 37, 17641777, https://doi.org/10.1175/JPO3098.1.

    • Search Google Scholar
    • Export Citation
  • Fisher, A. W., N. J. Nidzieko, M. E. Scully, R. J. Chant, E. J. Hunter, and P. L. F. Mazzini, 2018: Turbulent mixing in a far-field plume during the transition to upwelling conditions: Microstructure observations from an AUV. Geophys. Res. Lett., 45, 97659773, https://doi.org/10.1029/2018GL078543.

    • Search Google Scholar
    • Export Citation
  • Fong, D. A., and W. R. Geyer, 2001: Response of a river plume during an upwelling favorable wind event. J. Geophys. Res., 106, 10671084, https://doi.org/10.1029/2000JC900134.

    • Search Google Scholar
    • Export Citation
  • George, R., R. E. Flick, and R. T. Guza, 1994: Observations of turbulence in the surf zone. J. Geophys. Res., 99, 801810, https://doi.org/10.1029/93JC02717.

    • Search Google Scholar
    • Export Citation
  • Gerbi, G. P., J. H. Trowbridge, E. A. Terray, A. J. Plueddemann, and T. Kukulka, 2009: Observations of turbulence in the ocean surface boundary layer: Energetics and transport. J. Phys. Oceanogr., 39, 10771096, https://doi.org/10.1175/2008JPO4044.1.

    • Search Google Scholar
    • Export Citation
  • Gerbi, G. P., R. J. Chant, and J. L. Wilkin, 2013: Breaking surface wave effects on river plume dynamics during upwelling-favorable winds. J. Phys. Oceanogr., 43, 19591980, https://doi.org/10.1175/JPO-D-12-0185.1.

    • Search Google Scholar
    • Export Citation
  • Gerbi, G. P., S. E. Kastner, and G. Brett, 2015: The role of whitecapping in thickening the ocean surface boundary layer. J. Phys. Oceanogr., 45, 20062024, https://doi.org/10.1175/JPO-D-14-0234.1.

    • Search Google Scholar
    • Export Citation
  • Geyer, W. R., A. C. Lavery, M. E. Scully, and J. H. Trowbridge, 2010: Mixing by shear instability at high Reynolds number. Geophys. Res. Lett., 37, L22607, https://doi.org/10.1029/2010GL045272.

    • Search Google Scholar
    • Export Citation
  • Geyer, W. R., D. K. Ralston, and R. C. Holleman, 2017: Hydraulics and mixing in a laterally divergent channel of highly stratified estuary. J. Geophys. Res. Oceans, 122, 47434760, https://doi.org/10.1002/2016JC012455.

    • Search Google Scholar
    • Export Citation
  • Gregg, M. C., 2004: Small-scale processes in straits. Deep-Sea Res. II, 51, 489503, https://doi.org/10.1016/j.dsr2.2003.08.003.

  • Grimes, D. J., F. Feddersen, and N. Kumar, 2020: Tracer exchange across the stratified inner-shelf driven by transient rip-currents and diurnal surface heat fluxes. Geophys. Res. Lett., 47, e2019GL086501, https://doi.org/10.1029/2019GL086501.

    • Search Google Scholar
    • Export Citation
  • Haller, M. C., U. Putrevu, J. Oltman-Shay, and R. A. Dalrymple, 1999: Wave group forcing of low frequency surf zone motion. Coastal Eng. J., 41, 121136, https://doi.org/10.1142/S0578563499000085.

    • Search Google Scholar
    • Export Citation
  • Hally-Rosendahl, K., and F. Feddersen, 2016: Modeling surfzone to inner-shelf tracer exchange. J. Geophys. Res. Oceans, 121, 40074025, https://doi.org/10.1002/2015JC011530.

    • Search Google Scholar
    • Export Citation
  • Hally-Rosendahl, K., F. Feddersen, and R. T. Guza, 2014: Cross-shore tracer exchange between the surfzone and inner-shelf. J. Geophys. Res. Oceans, 119, 43674388, https://doi.org/10.1002/2013JC009722.

    • Search Google Scholar
    • Export Citation
  • Hetland, R. D., 2010: The effects of mixing and spreading on density in near-field river plumes. Dyn. Atmos. Oceans, 49, 3753, https://doi.org/10.1016/j.dynatmoce.2008.11.003.

    • Search Google Scholar
    • Export Citation
  • Hickey, B. M., and Coauthors, 2010: River influences on shelf ecosystems: Introduction and synthesis. J. Geophys. Res., 115, C00B17, https://doi.org/10.1029/2009JC005452.

    • Search Google Scholar
    • Export Citation
  • Horner-Devine, A. R., R. D. Hetland, and D. G. MacDonald, 2015: Transport and mixing in coastal river plumes. Annu. Rev. Fluid Mech., 47, 569594, https://doi.org/10.1146/annurev-fluid-010313-141408.

    • Search Google Scholar
    • Export Citation
  • Ivey, G. N., and J. Imberger, 1991: On the nature of turbulence in a stratified fluid. I: The energetics of mixing. J. Phys. Oceanogr., 21, 650658, https://doi.org/10.1175/1520-0485(1991)021<0650:OTNOTI>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Izett, J. G., and K. Fennel, 2018: Estimating the cross-shelf export of riverine materials: Part 2. Estimates of global freshwater and nutrient export. Global Biogeochem. Cycles, 32, 176186, https://doi.org/10.1002/2017GB005668.

    • Search Google Scholar
    • Export Citation
  • Jennings, W. C., S. Cunniff, K. Lewis, H. Deres, D. R. Reineman, J. Davis, and A. B. Boehm, 2020: Participatory science for coastal water quality: Freshwater plume mapping and volunteer retention in a randomized informational intervention. Environ. Sci.: Processes Impacts, 22, 918929, https://doi.org/10.1039/C9EM00571D.

    • Search Google Scholar
    • Export Citation
  • Jurisa, J. T., J. D. Nash, J. N. Moum, and L. F. Kilcher, 2016: Controls on turbulent mixing in a strongly stratified and sheared tidal river plume. J. Phys. Oceanogr., 46, 23732388, https://doi.org/10.1175/JPO-D-15-0156.1.

    • Search Google Scholar
    • Export Citation
  • Kakoulaki, G., D. MacDonald, and A. R. Horner-Devine, 2014: The role of wind in the near field and midfield of a river plume. Geophys. Res. Lett., 41, 51325138, https://doi.org/10.1002/2014GL060606.

    • Search Google Scholar
    • Export Citation
  • Kastner, S. E., A. R. Horner-Devine, and J. Thomson, 2018: The influence of wind and waves on spreading and mixing in the Fraser River plume. J. Geophys. Res. Oceans, 123, 68186840, https://doi.org/10.1029/2018JC013765.

    • Search Google Scholar
    • Export Citation
  • Kastner, S. E., A. R. Horner-Devine, and J. M. Thomson, 2019: A conceptual model of a river plume in the surf zone. J. Geophys. Res. Oceans, 124, 80608078, https://doi.org/10.1029/2019JC015510.

    • Search Google Scholar
    • Export Citation
  • Kilcher, L. F., J. D. Nash, and J. N. Moum, 2012: The role of turbulence stress divergence in decelerating a river plume. J. Geophys. Res., 117, C05032, https://doi.org/10.1029/2011JC007398.

    • Search Google Scholar
    • Export Citation
  • Kumar, N., and F. Feddersen, 2017a: The effect of stokes drift and transient rip currents on the inner shelf. Part I: No stratification. J. Phys. Oceanogr., 47, 227241, https://doi.org/10.1175/JPO-D-16-0076.1.

    • Search Google Scholar
    • Export Citation
  • Kumar, N., and F. Feddersen, 2017b: The effect of stokes drift and transient rip currents on the inner shelf. Part II: With stratification. J. Phys. Oceanogr., 47, 243260, https://doi.org/10.1175/JPO-D-16-0077.1.

    • Search Google Scholar
    • Export Citation
  • Lentz, S., 2004: The response of buoyant coastal plumes to upwelling-favorable winds. J. Phys. Oceanogr., 34, 24582469, https://doi.org/10.1175/JPO2647.1.

    • Search Google Scholar
    • Export Citation
  • Longuet-Higgins, M. S., and R. W. Stewart, 1962: Radiation stress and mass transport in gravity waves, with application to ‘surf beats.’ J. Fluid Mech., 13, 481504, https://doi.org/10.1017/S0022112062000877.

    • Search Google Scholar
    • Export Citation
  • Lund, B., H. C. Graber, P. O. G. Persson, M. Smith, M. Doble, J. Thomson, and P. Wadhams, 2018: Arctic sea ice drift measured by shipboard marine radar. J. Geophys. Res. Oceans, 123, 42984321, https://doi.org/10.1029/2018JC013769.

    • Search Google Scholar
    • Export Citation
  • MacCready, P., 2007: Estuarine adjustment. J. Phys. Oceanogr., 37, 21332145, https://doi.org/10.1175/JPO3082.1.

  • MacCready, P., W. R. Geyer, and H. Burchard, 2018: Estuarine exchange flow is related to mixing through the salinity variance budget. J. Phys. Oceanogr., 48, 13751384, https://doi.org/10.1175/JPO-D-17-0266.1.

    • Search Google Scholar
    • Export Citation
  • MacDonald, D. G., and W. R. Geyer, 2004: Turbulent energy production and entrainment at a highly stratified estuarine front. J. Geophys. Res., 109, C05004, https://doi.org/10.1029/2003JC002094.

    • Search Google Scholar
    • Export Citation
  • MacDonald, D. G., L. Goodman, and R. D. Hetland, 2007: Turbulent dissipation in a near-field river plume: A comparison of control volume and microstructure observations with a numerical model. J. Geophys. Res., 112, C07026, https://doi.org/10.1029/2006JC004075.

    • Search Google Scholar
    • Export Citation
  • MacMahan, J. H., A. J. H. M. Reniers, E. B. Thornton, and T. P. Stanton, 2004: Surf zone eddies coupled with rip current morphology. J. Geophys. Res., 109, C07004, https://doi.org/10.1029/2003JC002083.

    • Search Google Scholar
    • Export Citation
  • McCabe, R. M., B. M. Hickey, and P. MacCready, 2008: Observational estimates of entrainment and vertical salt flux in the interior of a spreading river plume. J. Geophys. Res., 113, C08027, https://doi.org/10.1029/2007JC004361.

    • Search Google Scholar
    • Export Citation
  • Moghimi, S., J. Thomson, T. Özkan-Haller, L. Umlauf, and S. Zippel, 2016: On the modeling of wave-enhanced turbulence nearshore. Ocean Modell., 103, 118132, https://doi.org/10.1016/j.ocemod.2015.11.004.

    • Search Google Scholar
    • Export Citation
  • Moulton, M., S. Elgar, B. Raubenheimer, J. C. Warner, and N. Kumar, 2017: Rip currents and alongshore flows in single channels dredged in the surf zone. J. Geophys. Res. Oceans, 122, 37993816, https://doi.org/10.1002/2016JC012222.

    • Search Google Scholar
    • Export Citation
  • Nash, J. D., and J. N. Moum, 2005: River plumes as a source of large-amplitude internal waves in the coastal ocean. Nature, 437, 400403, https://doi.org/10.1038/nature03936.

    • Search Google Scholar
    • Export Citation
  • Nash, J. D., L. F. Kilcher, and J. N. Moum, 2009: Structure and composition of a strongly stratified, tidally pulsed river plume. J. Geophys. Res., 114, C00B12, https://doi.org/10.1029/2008JC005036.

    • Search Google Scholar
    • Export Citation
  • Olabarrieta, M., W. R. Geyer, and N. Kumar, 2014: The role of morphology and wave-current interaction at tidal inlets: An idealized modeling analysis. J. Geophys. Res. Oceans, 119, 88188837, https://doi.org/10.1002/2014JC010191.

    • Search Google Scholar
    • Export Citation
  • Peregrine, D. H., 1998: Surf zone currents. Theor. Comput. Fluid Dyn., 10, 295309, https://doi.org/10.1007/s001620050065.

  • Raubenheimer, B., R. T. Guza, and S. Elgar, 1996: Wave transformation across the inner surf zone. J. Geophys. Res., 101, 25 58925 597, https://doi.org/10.1029/96JC02433.

    • Search Google Scholar
    • Export Citation
  • Rehmann, C. R., 2004: Scaling for the mixing efficiency of stratified grid turbulence. J. Hydraul. Res., 42, 3542, https://doi.org/10.1080/00221686.2004.9641181.

    • Search Google Scholar
    • Export Citation
  • Reniers, A. J. H. M., J. H. MacMahan, E. B. Thornton, T. P. Stanton, M. Henriquez, J. W. Brown, J. A. Brown, and E. Gallagher, 2009: Surf zone surface retention on a rip-channeled beach. J. Geophys. Res., 114, C10010, https://doi.org/10.1029/2008JC005153.

    • Search Google Scholar
    • Export Citation
  • Rodriguez, A. R., S. N. Giddings, and N. Kumar, 2018: Impacts of nearshore wave-current interaction on transport and mixing of small-scale buoyant plumes. Geophys. Res. Lett., 45, 83798389, https://doi.org/10.1029/2018GL078328.

    • Search Google Scholar
    • Export Citation
  • Spydell, M. S., and F. Feddersen, 2009: Lagrangian drifter dispersion in the surf zone: Directionally spread normally incident waves. J. Phys. Oceanogr., 39, 809830, https://doi.org/10.1175/2008JPO3892.1.

    • Search Google Scholar
    • Export Citation
  • Spydell, M. S., and F. Feddersen, 2012: A Lagrangian stochastic model of surf zone drifter dispersion. J. Geophys. Res., 117, C03041, https://doi.org/10.1029/2011JC007701.

    • Search Google Scholar
    • Export Citation
  • Spydell, M. S., F. Feddersen, R. T. Guza, and W. E. Schmidt, 2007: Observing surf-zone dispersion with drifters. J. Phys. Oceanogr., 37, 29202939, https://doi.org/10.1175/2007JPO3580.1.

    • Search Google Scholar
    • Export Citation
  • Stacey, M. T., and D. K. Ralston, 2005: The scaling and structure of the estuarine bottom boundary layer. J. Phys. Oceanogr., 35, 5571, https://doi.org/10.1175/JPO-2672.1.

    • Search Google Scholar
    • Export Citation
  • Stretch, D. D., J. W. Rottman, S. K. Venayagamoorthy, K. K. Nomura, and C. R. Rehmann, 2010: Mixing efficiency in decaying stably stratified turbulence. Dyn. Atmos. Oceans, 49, 2536, https://doi.org/10.1016/j.dynatmoce.2008.11.002.

    • Search Google Scholar
    • Export Citation
  • Terray, E. A., M. A. Donelan, Y. C. Agrawal, W. M. Drennan, K. K. Kahma, A. J. Williams, P. A. Hwang, and S. A. Kitaigorodskii, 1996: Estimates of kinetic energy dissipation under breaking waves. J. Phys. Oceanogr., 26, 792807, https://doi.org/10.1175/1520-0485(1996)026<0792:EOKEDU>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Thomson, J., 2012: Wave breaking dissipation observed with SWIFT drifters. J. Atmos. Oceanic Technol., 29, 18661882, https://doi.org/10.1175/JTECH-D-12-00018.1.

    • Search Google Scholar
    • Export Citation
  • 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. Geophys. Res. Lett., 41, 89878993, https://doi.org/10.1002/2014GL062274.

    • Search Google Scholar
    • Export Citation
  • Thomson, J., and Coauthors, 2019: A new version of the swift platform for waves, currents, and turbulence in the ocean surface layer. 2019 IEEE/OES Twelfth Current, Waves, and Turbulence Measurement and Applications Workshop, San Diego, CA, Institute of Electrical and Electronics Engineers, 1–7, https://doi.org/10.1109/CWTM43797.2019.8955299.

  • Thornton, E. B., and R. T. Guza, 1983: Transformation of wave height distribution. J. Geophys. Res., 88, 59255938, https://doi.org/10.1029/JC088iC10p05925.

    • Search Google Scholar
    • Export Citation
  • Thornton, E. B., and R. T. Guza, 1986: Surf zone longshore currents and random waves: Field data and models. J. Phys. Oceanogr., 16, 11651178, https://doi.org/10.1175/1520-0485(1986)016<1165:SZLCAR>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Thorpe, S. A., 1973: Turbulence in stably stratified fluids: A review of laboratory experiments. Bound.-Layer Meteor., 5, 95119, https://doi.org/10.1007/BF02188314.

    • Search Google Scholar
    • Export Citation
  • Turner, J. S., 1973: Buoyancy Effects in Fluids. Cambridge University Press, 368 pp.

  • White, H., 1980: A heteroskedasticity-consistent covariance matrix and a direct test for heteroskedasticity. Econometrica, 48, 817838, https://doi.org/10.2307/1912934.

    • Search Google Scholar
    • Export Citation
  • Wong, S. H. C., S. G. Monismith, and A. B. Boehm, 2013: Simple estimate of entrainment rate of pollutants from a coastal discharge into the surf zone. Environ. Sci. Technol., 47, 11 55411 561, https://doi.org/10.1021/es402492f.

    • Search Google Scholar
    • Export Citation
  • Wright, L. D., R. T. Guza, and A. D. Short, 1982: Dynamics of a high-energy dissipative surf zone. Mar. Geol., 45, 4162, https://doi.org/10.1016/0025-3227(82)90179-7.

    • Search Google Scholar
    • Export Citation
  • Zippel, S., and J. Thomson, 2015: Wave breaking and turbulence at a tidal inlet. J. Geophys. Res. Oceans, 120, 10161031, https://doi.org/10.1002/2014JC010025.

    • Search Google Scholar
    • Export Citation
  • Zippel, S., and J. Thomson, 2017: Surface wave breaking over sheared currents: Observations from the mouth of the Columbia River. J. Geophys. Res. Oceans, 122, 33113328, https://doi.org/10.1002/2016JC012498.

    • Search Google Scholar
    • Export Citation
All Time Past Year Past 30 Days
Abstract Views 465 465 22
Full Text Views 136 136 4
PDF Downloads 162 162 3

Observations of River Plume Mixing in the Surf Zone

S. E. KastneraDepartment of Environmental Science, Western Washington University, Bellingham, Washington

Search for other papers by S. E. Kastner in
Current site
Google Scholar
PubMed
Close
,
A. R. Horner-DevinebDepartment of Civil and Environmental Engineering, University of Washington, Seattle, Washington

Search for other papers by A. R. Horner-Devine in
Current site
Google Scholar
PubMed
Close
,
J. M. ThomsoncApplied Physics Laboratory, University of Washington, Seattle, Washington

Search for other papers by J. M. Thomson in
Current site
Google Scholar
PubMed
Close
, and
S. N. GiddingsdScripps Institution of Oceanography, University of California, San Diego, La Jolla, California

Search for other papers by S. N. Giddings in
Current site
Google Scholar
PubMed
Close
Restricted access

Abstract

We use salinity observations from drifters and moorings at the Quinault River mouth to investigate mixing and stratification in a surf-zone-trapped river plume. We quantify mixing based on the rate of change of salinity DS/Dt in the drifters’ quasi-Lagrangian reference frame. We estimate a constant value of the vertical eddy diffusivity of salt of Kz = (2.2 ± 0.6) × 10−3 m2 s−1, based on the relationship between vertically integrated DS/Dt and stratification, with values as high as 1 × 10−2 m2 s−1 when stratification is low. Mixing, quantified as DS/Dt, is directly correlated to surf-zone stratification, and is therefore modulated by changes in stratification caused by tidal variability in freshwater volume flux. High DS/Dt is observed when the near-surface stratification is high and salinity gradients are collocated with wave-breaking turbulence. We observe a transition from low stratification and low DS/Dt at low tidal stage to high stratification and high DS/Dt at high tidal stage. Observed wave-breaking turbulence does not change significantly with stratification, tidal stage, or offshore wave height; as a result, we observe no relationship between plume mixing and offshore wave height for the range of conditions sampled. Thus, plume mixing in the surf zone is altered by changes in stratification; these are due to tidal variability in freshwater flux from the river and not wave conditions, presumably because depth-limited wave breaking causes sufficient turbulence for mixing to occur during all observed conditions.

Significance Statement

River outflows are important sources of pollutants, sediment, and nutrients to the coastal ocean. Small rivers often meet large breaking waves in the surf zone close to shore, trapping river water and river-borne material near the beach. Such trapped material can influence coastal public health, beach morphology, and nearshore ecology. This study investigates how trapped fresh river water mixes with salty ocean water in the presence of large breaking waves by using high-resolution measurements of waves, salinity, and turbulence. We find that the surf zone is often fresh and stratified, which could have significant implications for the fate of riverine material. Wave breaking provides a constant source of turbulence, and the amount of mixing is limited by the degree of vertical salt stratification; more mixing occurs when stratification is higher.

© 2023 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Samuel E. Kastner, sam.kastner@wwu.edu

Abstract

We use salinity observations from drifters and moorings at the Quinault River mouth to investigate mixing and stratification in a surf-zone-trapped river plume. We quantify mixing based on the rate of change of salinity DS/Dt in the drifters’ quasi-Lagrangian reference frame. We estimate a constant value of the vertical eddy diffusivity of salt of Kz = (2.2 ± 0.6) × 10−3 m2 s−1, based on the relationship between vertically integrated DS/Dt and stratification, with values as high as 1 × 10−2 m2 s−1 when stratification is low. Mixing, quantified as DS/Dt, is directly correlated to surf-zone stratification, and is therefore modulated by changes in stratification caused by tidal variability in freshwater volume flux. High DS/Dt is observed when the near-surface stratification is high and salinity gradients are collocated with wave-breaking turbulence. We observe a transition from low stratification and low DS/Dt at low tidal stage to high stratification and high DS/Dt at high tidal stage. Observed wave-breaking turbulence does not change significantly with stratification, tidal stage, or offshore wave height; as a result, we observe no relationship between plume mixing and offshore wave height for the range of conditions sampled. Thus, plume mixing in the surf zone is altered by changes in stratification; these are due to tidal variability in freshwater flux from the river and not wave conditions, presumably because depth-limited wave breaking causes sufficient turbulence for mixing to occur during all observed conditions.

Significance Statement

River outflows are important sources of pollutants, sediment, and nutrients to the coastal ocean. Small rivers often meet large breaking waves in the surf zone close to shore, trapping river water and river-borne material near the beach. Such trapped material can influence coastal public health, beach morphology, and nearshore ecology. This study investigates how trapped fresh river water mixes with salty ocean water in the presence of large breaking waves by using high-resolution measurements of waves, salinity, and turbulence. We find that the surf zone is often fresh and stratified, which could have significant implications for the fate of riverine material. Wave breaking provides a constant source of turbulence, and the amount of mixing is limited by the degree of vertical salt stratification; more mixing occurs when stratification is higher.

© 2023 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Samuel E. Kastner, sam.kastner@wwu.edu
Save