Estimations of Horizontal Dispersion in the Green Bay of Lake Michigan Using a Lagrangian Drifter Experiment and a Hydrodynamic Particle-Tracking Model

In the 1980s the International Joint Commission listed Lower Green Bay and the Fox River as an Area of Concern. Problems that needed priority attention included contaminated sediment, poor water quality, and lost or altered habitat. The transport of nutrients, sediments and contaminants plays an important role on the biogeochemistry, ecology and health of the bay. Local, state and federal agencies have studied the problems caused by excessive nutrients and contaminated sediments. In 1989 Gottlieb et al. deployed tracked drifters from six locations in the bay to map flow patterns. In 2015 Hamidi et al. examined the role of circulation and heat fluxes in the formation of stratification and hypoxia in the bay. Their hydrodynamic model calculates horizontal dispersion using the Smagorinsky turbulence closure model. In 2014 Nguyen et al. used a hydrodynamic model to examine circulation and exchange in the Saginaw Bay-Lake Huron system. Their model was tested against data from a Lagrangian drifter experiment. For this study drifters were deployed in the bay in 2014. Locations of drifters were tracked using GPS transmitters, and those locations were used to calculate one-point dispersion statistics. A Lagrangian particle-tracking model was used to simulate the drifter trajectories. Cross-bay and along-bay velocities were used to compute diffusivities in those directions for observed and simulated drifter tracks. Diffusivity values were estimated using statistical analysis of particles released from a point source in isotropic turbulent flow. The field-data validated diffusivities will be used to improve predictions of the transport and fate of nutrients and sediments in the bay.