The Impact of Episodic Events on Nearshore-Offshore Transport in the Great Lakes:
Hydrodynamic Modeling Program

D. Schwab and D. Beletsky

This proposal is written in response to the NSF/OCE and NOAA/COP Announcement of Opportunity for Coastal Studies in the Great Lakes. It is part of a multidisciplinary program on the impact of episodic events on the coastal ecosystem in the Great Lakes. This proposal focuses on the hydrodynamics of episodic events. The purpose of this proposal is to create a numerical modeling effort (in close cooperation with the observational program of Saylor et al., meteorological modeling of Roebber, sediment transport modeling of Bedford and McDonald, and ecological modeling of Chen) to identify, quantify, and develop prediction tools for the primary physical processes responsible for nearshore-offshore transport of biogeochemically important materials in the Great Lakes, and in Lake Michigan in particular. The program is designed to test the following hypothesis: that the forced, two-gyre vorticity wave response of the lake to episodic wind events, occasionally modified by stratification, is a major mechanism for nearshore-offshore transport in the Great Lakes. The recurrent springtime appearance of an extensive turbidity plume in southern Lake Michigan (Eadie et al. 1996) provides a unique opportunity to examine the two-gyre vorticity wave hypothesis during a period when a large volume of suspended material can act as a natural tracer of nearshore-offshore circulation patterns. The program will study this phenomena in Lake Michigan during winter and spring transition period (thermal bar), in order to be able to compare cross-margin transport generated by purely barotropic processes in the winter to transport later in the spring when baroclinic processes may be more important. The specific objectives of this proposal are:
To identify and quantify the physical processes generating nearshore- offshore transport of biogeochemically important materials in the Great Lakes, a wind wave model, and a lake-scale hydrodynamic circulation model (the Great Lakes version of the Princeton Ocean Model) coupled with an ice model will be applied to Lake Michigan for selected periods in 1992-1997 during which the springtime turbidity plume has been observed, and for the program's field years. Model bathymetry (2km grid) will be based on the new, high resolution bathymetric grids recently released by the National Geophysic Data Center. Some experiments will also be carried out with higher (1 km) horizontal resolution, to study how improved horizontal resolution will influence velocity field and eventually sediment resuspension and transport during episodes of strong wind forcing. For the Lake Michigan study, from 20 to 30 vertical levels will be used. For these simulations, meteorological data from National Weather Service surface observing stations and two mid-lake weather buoys will be used to synthesize overwater momentum flux and heat flux fields to drive the model. Several scenario testing numerical experiments will also be carried out to determine the role of ice, thermal effects, local bathymetry, and mesoscale atmospheric variability on the timing and magnitude of the plume events. Model results will be tested against Eulerian and Lagrangian current measurements, and surface currents and waves measurements from an HF radar observations in southern Lake Michigan, surface temperature observations from the Great Lakes CoastWatch, and routine ice observations for the Great Lakes from the National Ice Center. To refine existing model parameterizations, model results will be compared to observational data from ship surveys, current meters, and thermistor arrays deployed during the field years.
In collaboration with other components of this program the hydrodynamic model will be coupled with sediment transport and lower food web models in order to assess the impact of nearshore-offshore transport on sedimentary and biological processes. Overall, the program is designed to provide the most comprehensive insight into the hydrodynamics of cross-margin transport of biogeochemically important materials ever accomplished on the Great Lakes.