The role of meteorological forcing on the St. Johns River (Northeastern Florida)

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Abstract

Water surface elevations in the St. Johns River (Northeastern Florida) are simulated over a 122-day time period spanning June 1-September 30, 2005, which relates to a particularly active hurricane season for the Atlantic basin, and includes Hurricane Ophelia that significantly impacted the St. Johns River. The hydrodynamic model employed for calculating two-dimensional flows is the ADCIRC (Advanced Circulation Model for Oceanic, Coastal, and Estuarine Waters) numerical code. The region of interest is modeled using three variations of an unstructured, finite element mesh: (1) a large-scale computational domain that hones in on the St. Johns River from the Western North Atlantic Ocean, Gulf of Mexico, and Caribbean Sea; (2) a shelf-based subset of the large domain; (3) an inlet-based subset of the large domain. Numerical experiments are then conducted in order to examine the relative importance of three long-wave forcing mechanisms for the St. Johns River: (1) astronomic tides; (2) freshwater river inflows; (3) winds and pressure variations. Two major findings result from the various modeling approaches considered in this study, and are applicable in general (e.g., over the entire 122-day time period) and even more so for extreme storm events (e.g., Hurricane Ophelia): (1) meteorological forcing for the St. Johns River is equal to or greater than that of astronomic tides and generally supersedes the impact of freshwater river inflows, while pressure variations provide minimal impact; (2) water Surface elevations in the St. Johns River are dependent upon the remote effects caused by winds occurring in the deep ocean, in addition to local wind effects. During periods of calm weather through the 122-day time period, water surface elevations in the St. Johns River were generally tidal in response, with amplitudes exceeding 1 m at the mouth and diminishing to less than 10 cm 150 km upriver. Considering an extreme storm event, the timing of Hurricane Ophelia occurred during the neap phase of the tidal cycle and at the mouth of the St. Johns River, the wind-driven storm Surge was near equal to the tidal component, each contributing about 0.5 m to the overall water surface elevation. However, 150 km upriver, meteorological forcing dominated, as over 90% of the total water Surface elevation was driven by winds and pressures. The simulation results replicate these behaviors well. As a supplement, it is shown that applying a hydrograph boundary condition, generated by a large domain, to a localized domain is highly beneficial towards accounting for the remote wind forcing. (C) 2009 Elsevier B.V. All rights reserved.