A High-Resolution Model of the Apalachee Bay

1. A High-Resolution Model of the Apalachee Bay Table 1. Estimates of maximum storm tide 2. Model Description The FVCOM, a time-dependent, three-dimensional, primitive equation, unstructured grid numerical ocean model, is configured for the Apalachee Bay region from 85˚ 40’W to 83˚ 2’W and 29˚ 10’N to 30˚ 15’N (Figure 1). Topography of the region is derived from the 3 arc-second (~90 m) resolution gridded database of the National Geophysical Data Center (NGDC) . The FVCOM Apalachee Bay model is nested into the coarser Gulf of Mexico model which provides boundary conditions at the lateral boundaries. The FVCOM model is forced at the surface by the wind fields constructed for the time period July 8 0:00 UTC – July 11 0:00 UTC, 2005, by applying an objective gridding method (Morey et al., 2005) to combine data from the NOAA AOML Hurricane Research Division Wind Analyses (H*Wind fields) and the National Centers for Environmental Prediction Reanalysis II (NCEPR2) winds. Dmitry Dukhovskoy, Eric Chassignet, and Steven Morey Center for Ocean-Atmospheric Prediction Studies The Florida State University ddmitry@coaps.fsu.edu 1.Introduction A high-resolution storm surge model of Apalachee Bay in the northeastern Gulf of Mexico is developed using an unstructured grid finite-volume coastal ocean model (FVCOM) with wetting and drying capabilities. In this study, the model is nested into a 1/60˚ Gulf of Mexico model based on the Navy Coastal Ocean Model (NCOM) of Morey et al. (2006) (Figure 1). The model is applied to the case of Hurricane Dennis (July 2005). Hurricane Dennis made landfall at Santa Rosa Island and caused unexpected serious inundation of the coastal zone of Apalachee Bay, including underpredicted flooding in the town of St. Marks approximately 275 km to the east of the landfall location (FDEP, 2006) that has yet to be adequately explained. Accurate resolution of complicated geometry of the coastal region and waterways in the model reveals substantial spatial variability in the amplitude and timing of the maximum water level and processes responsible for the unanticipated high storm tide in the area. Model experiments suggest that excessive flooding in the coastal zone during Dennis was caused by additive effects of coincident high tides, wave setup, and a propagating shelf wave that added to the locally wind generated surge. In the future, the FVCOM can be applied in 3-D configuration to estuaries like Apalachicola Bay and nested into the FSU Big Bend model that is under development as part of the NGI project. Figure 5. Simulated time series of sea level (storm surge), added to tidal predictions at St. Marks. Figure 1. (a) The domain of the coarse-resolution NCOM described in Morey et al. (2006). The domain of the high-resolution 4. Local flooding of St. Marks The town of St. Marks is located approximately 10 km up the St. Marks River, 275 km east of the Hurricane Dennis storm track, and experienced only modest (minimal tropical storm strength) winds. However, HWM’s indicate a maximum storm tide at the town of 2.83 m (Figure 3). To further investigate the causes of the flooding of this town and the surrounding areas, two additional model experiments are performed (Figure 4). In the first experiment, the model is forced by winds only with radiation open boundary conditions. This eliminates the incoming shelf wave from the simulation. In the second experiment, the model is forced at the lateral open boundaries with the barotropic shelf wave from the Gulf of Mexico model, but with no local wind forcing. The purpose of these experiments is to examine the relative contributions of the large-scale sea level anomalies and local wind-driven surge to the water level rise within the Wakulla and St. Marks River system. From the realistic model run (Figure 4 a-c), water levels in the St. Marks area began to rise on July 10 shortly after 13:00 UTC and reached the maximum of 2.06 m up the Wakulla River at 21:00 UTC. The model experiment with lateral open boundary forcing (long shelf wave, Figure 4 d-f) reveals wave retardation in the St. Marks river channel. The shelf wave generates a smaller wave with a positive sea level anomaly that propagates up the river (Figure 4e). By 21:00 UTC (Figure 4f) the shelf wave crest has passed the area, but the crest propagates slower in the river and the crest is just passing the St. Marks / Wakulla confluence resulting in the maximum storm surge of 0.75 m in the area at that time. The model experiment forced by winds only (“Wnd” in Figure 10) predicts a noticeably smaller surge (~1.4 m) in the area with much less inundation compared to the realistically forced experiment. The model experiments suggest that during Hurricane Dennis, the timing of the maximum surge at St. Marks was due to the arrival of the sea level anomaly from the long shelf wave (which was retarded as it traveled upriver) coincident with the maximum wind-driven local surge. Unfortunately for the residents of St. Marks, high tide occurred nearly at the same time (21:22 UTC) as the maximum storm surge (Figure 5) and added ~0.50 m to the storm tide. model encompasses the Apalachee Bay region (small box) and is nested in the coarse model. The Hurricane Dennis track is shown with a black line with storm locations every 12 hours (UTC). (b) Map of Apalachee Bay region. The -20 m, 0 m, and 5 m levels are contoured. (c) Unstructured mesh of the Apalachee Bay region. Figure 3. Hurricane Dennis coastal High Water Marks in Apalachee Bay (from FEMA, 2005). Values are relative to the NAVD 88 datum. 3. Simulated Storm Surge and Estimated Storm Tide The Apalachee Bay model predicts significant inundation of the coastal zone of Apalachee Bay, particularly near the Apalachicola River mouth, the back of Ochlockonee Bay, Bald Point, and in the Wakulla - St. Marks River area (Figure 2). The surge model forced only by wind stress and perturbations in the barotropic flow at the lateral boundaries predicts the sea level rise due to the locally wind-driven surge and the remotely generated sea level anomalies propagating into the region, but does not take into account other factors that may contribute to the sea level response to the storm such as wave setup, inverse barometer effect, and tides. This precludes the validation of the model results with many observations, which are representative of the storm tide and include all these effects on the local sea level. An estimate of these other factors contributing to the sea level during the storm is obtained using empirical and theoretical methods (Dukhovskoy and Morey, 2008). These estimates are added to the simulated storm surge (Table 1) to compare model results with High Water Marks (HMS) collected during post-storm surveys (FEMA, 2005) (Figure 3). Acknowledgments This work was supported by funding through the NOAA Applied Research Center grant to COAPS. NCEP_Reanalysis 2 data obtained from the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their Web site at http://www.cdc.noaa.gov/. Sea level and wind observations are from the NOAA National Data Buoy Center. Mark Bourassa (COAPS FSU) prepared the gridded wind fields used in this study. References Dukhovskoy, D.S. and S.L. Morey, Simulation and analysis of storm surge in the northeastern Gulf of Mexico during Hurricane Dennis, Estuarine, Coastal and Shelf Science, in review, 2008. FDEP, Hurricane Dennis and Hurricane Katrina. Final Report on 2005 Hurricane Season Impacts to Northwest Florida, pp. 116, Florida Department of Environmental Protection Division of Water Resource Management Bureau of Beaches and Coastal Systems, Florida, 2006. FEMA, Hazard Mitigation Technical Assistance Program Contract N0. EMW-2000-CO-0247, Task Orders 403 & 405, Hurricane Dennis Rapid Response Florida Coastal High Water Mark (CHWM) Collection, FEMA-1595-DR-FL, Final Report, December 19, 2005. Federal Emergency Management Agency, Region IV, Atlanta, GA, 2005. Morey, S.L., S. Baig, M.A. Bourassa, D.S. Dukhovskoy, and J.J.O'Brien, Remote forcing contribution to storm-induced sea level rise during Hurricane Dennis, Geophys. Res. Lett., 33 (19), L19603, 2006. Morey, S.L., M.A. Bourassa, X.J. Davis, and J.J. O'Brien, Remotely sensed winds for episodic forcing of ocean models. J. Geophys. Res., 110 (C10024), doi: 10.1029/2004JC002338, 2005. Figure 4. Simulated storm surge (m) in the Shell Point - St. Marks River region for different model experiments including: wind and the shelf wave (a), (b), (c); shelf wave only (d), (e), (f); wind only (g), (h), (i). Wind vectors are shown by gray arrows and black arrows represent the depth-averaged water velocity. The values and location of the maximum simulated sea level elevation for each experiment are indicated. Note that a different scale is used for sea surface elevation for each experiment. Figure 2. Maximum storm surge simulated in the Apalachee Bay FVCOM simulation. Black contours are isobaths, and the coastline at MSL is marked by the grey curve. Red stars indicate locations of: 1 – Apalachicola; 2 – Dog Island; 3 – Bald Point; 4 – Shell Point; 5 – St. Marks River entrance; 6 – St. Marks / Wakulla rivers.