St. John's River Operational Forecast System (SJROFS)

The Environmental Fluid Dynamics Code (EFDC) (Hamrick, 1992a; 1992b) has been used to perform operational nowcasts and forecasts for the Lower St. Johns River. The physics of the EFDC model and many aspects of the computational scheme are equivalent to the widely used Blumberg-Mellor model (POM). The EFDC model solves the three-dimensional, vertically hydrostatic, free surface, turbulent averaged equations of motions for a variable density fluid.

An orthogonal, boundary-fitted, structured grid extending upstream from the Atlantic Ocean near Mayport to Buffalo Bluff (Figure 1) was applied. The model grid is based on a transformed 188 x 105 rectangular computational grid containing 2,210 water cells. Horizontal cell sizes, irrespective of direction, range from 81–2,040 m. The model grid generally does not extend up the tributaries to the head of tide, and the model application does not allow for flooding and drying of grid cells. There are six stretched, sigma vertical layers.

Figure 1. Boundary-fitted model grid of the lower St. Johns River and observation locations of water level and current (Click on the map to enlarge).

For the nowcasts, water levels along the open ocean boundary are determined using a slight correction factor of 1.05 applied to real-time water level data at Mayport, Florida NOAA water level gauge. Salinity along the open ocean boundary is set equal to 35 psu at the surface and 36 psu at the bottom. Wind data available from the NOAA Mayport station is applied to all the grid cells in the model. Real-time river discharges along six of the main tributaries entering the St. Johns River are downloaded from the USGS as lateral inflows. SJROFS make a one-hour nowcast every hour. For the forecasts, along the open ocean boundary, water levels are specified as a superposition of the tide predictions from Mayport and the subtidal water level forecasts at Fernandina Beach, Florida. The latter are made available by the National Weather Service (NWS) as output from their Extratropical Storm Surge Model. River discharges at the same station as nowcast are currently persisted from the latest observations. Forecasts of surface wind and air pressure from the NAM-12 model are interpolated onto EFDC model grid as surface forcing. Thirty-six hour forecasts are made four times a day.