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Understanding Wind/Wave Forcing of the St. Johns River Scott C. Hagen1, Yuji Funaksohi2, and Andrew Cox (shagen@mail.ucf.edu) 3

1U. of Central Florida, 2NOAA/NOS/Office of Coast Survey, 3Oceanweather, Inc.

We will focus on the development and application of a finite element, two-dimensional, hydrodynamic St. Johns River model for the simulation of water surface elevations and depth- integrated velocities. Three domain variations will be discussed: 1) A model domain that incorporates the entire East Coast of the United States out to the 60°W meridian, Gulf of Mexico and Caribbean Sea, while honing in on the St. Johns River area (see Figure 1 below); 2) A sub- domain that is shelf-based; and 3) A sub-domain that is inlet-based. Results will be presented from numerous hydrodynamic simulations that altered the following four forcing mechanisms in order to determine their relative importance: 1) astronomical tides; 2) inflows from tributaries; 3) winds and pressures; and 4) surface waves. The Hurricane Floyd event of 1999 and a 122-day period spanning June 1 – September 30, 2005 will be highlighted.

Four main conclusions are reported. First, wind forcing for the St. Johns River is equal to or greater than that of astronomic tides and generally supersedes the impact of inflows, while pressure variations have a minimal impact. Second, water levels inside the St. Johns River depend on the wind forcing in the deep ocean; however, if one applies an elevation hydrograph boundary condition from a large-scale domain model to a local-scale domain model the results are highly accurate. Third, wind-induced surface waves generate an approximately 10 – 15 percent higher peak storm tide for Hurricane Floyd. Finally, while a carefully calibrated model can reproduce local and even regional storm tide hydrographs, the wind surface drag is spatially and temporally dependent and the drag formulation requires significant research effort.

Figure 1. Spatial discretization of the Western Atlantic, with insets of the St. Johns River region from its inlet near Mayport (a) to its upstream limit at Lake George (d).

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