The Turbulence Responsible for Horizontal Surfzone Dispersion

University Of California-San Diego Scripps Inst Of Oceanography, La Jolla CA

Investigators

Abstract

Improved understanding of the surfzone (within a few 100 m of the shoreline) horizontal turbulence that causes material dispersion is the goal of this project. Poor water quality at many beaches poses health risks to beach goers and negatively impacts local economies. Recent research has indicated that surfzone horizontal eddies, which have length scales greater than the water depth, mix and disperse surfzone tracers such as pollution. Although dispersion by surfzone eddies is critical to the dilution, as well as the cross-shore exchange of material (including larvae), how to parameterize the surfzone eddy diffusivity in computer models is unknown. Exactly how the Eulerian properties of the turbulence (the eddy velocity, and the eddy space and time scales) are related to surfzone Lagrangian statistics (i.e. the dispersion and eddy diffusivity) is unknown. Also not completely understood is how the surfzone Eulerian eddy properties are related to the incident wave field. By analyzing existing observations and performing model experiments, the link between Eulerian eddy properties and dispersion will be illuminated. Intellectual Merit: The first objective of this study is to examine surfzone horizontal eddy properties in existing observations. As dispersion depends critically on the velocity c, eddy length L, and eddy time scales T, and in particular the parameter alpha = cT/L, surfzone values will be determined from space- and time-lagged velocity autocorrelation functions. First, they will be calculated for SandyDuck observations (Huntington Beach 2006 and Imperial Beach 2009 will also be considered to the extent possible) and observed surfzone eddy fields will then be characterized. Small values of alpha indicate that the eddy field changes very quickly in time and results in qualitatively different dispersion than if the eddies are long lived (large alpha). How the observed eddy properties depend on the incident wave field will be investigated and whether alongshore currents change the nature of the eddy field determined. Also, the dependence of surfzone eddies on cross-shore position will also be calculated. In particular, as surfzone eddies are only generated within the surfzone, whether or not the surfzone eddy field is different within and seaward of the surfzone will be examined. The second objective is to perform the same type of analysis done for the observations on wave resolving and wave averaged numerical simulations of observed (e.g. SandyDuck) conditions. Do the models reproduce the observed eddy velocities, length and time scales even though wave averaged and wave resolving velocity frequency-wavenumber spectra can differ significantly? This is an important consideration as dispersion depends on the bulk properties of the frequency-wavenumber spectra rather than its detailed shape. Model simulations will also allow the calculation of very reliable Lagrangian statistics as virtual drifters will be added to the model. Eulerian and Lagrangian eddy statistics will then be compared and the relevance of an existing theory which relates Eulerian to Lagrangian statistics, developed for isotropic homogeneous turbulence, for the surfzone will be determined. If this theory is applicable, the surfzone eddy diffusivity can then be inferred without explicit Lagrangian statistics as long as the Eulerian eddy statistics are known. Broader Impacts: At United States beaches in 2008, 7% of beach water samples exceed national health standards and there were over 20,000 closure and advisory days. Pollution from urban runoff often drains directly onto beaches and poses significant health risks to swimmers. Management decisions could benefit from better understanding of surfzone turbulence and dispersion. As part of educational outreach efforts, the PI will collaborate with I Love A Clean San Diego (ILACSD), a nonprofit environmental group, and assist them with the development of their teaching materials and education program. Additionally, the PI will provide guest presentations at selected ILACSD programs. Partnering with ILACSD is going to have a large impact as they have provided environmental education to 30,000 high school students (45% Title 1 or underserved) in total and they work closely with resource managers throughout San Diego.