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 determine the role of ice in timing and magnitude of the plume events
To determine whether the plume occurrence represents a response to the aggregate effects
of a season of individual storm events, an episodic response to a single large storm event
or a complex interaction between the low-frequency (seasonal) preconditioning of the lake
and a single storm event that occurs at a critical time
To determine the importance of mesoscale atmospheric dynamics on the development of
the plume
To determine the role of local bathymetry in the separation/meandering of the plume
To determine the influence of thermal effects on the dynamics of the plume
To refine the ice and circulation models using the results of an extensive observation
program
To link the ice-circulation model and the Lake Michigan wind wave prediction model
with a sediment resuspension/transport model in order to quantify the cross-isobath
transport of resuspended material in the lake
To link the ice-circulation model with a nutrient and lower food web model in order to
investigate the impact of nearshore-offshore transport during episodic events on
biological processes in the lake.
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.