Model Simulation Studies of Hurricane Isabel in Chesapeake Bay

1. Model Simulation Studies of Hurricane Isabel in Chesapeake Bay Jian Shen Virginia Institute of Marine Sciences College of William and Mary

2. Isabel: The 100-Year Storm !

3. Background of Storm Surge Modeling • Numerical models have been successfully applied to simulate and predict tide and storm surge in coastal seas • SLOSH (Sea, Lake, and Overland Surges for Hurricanes • ADCIRC (Advanced CirculationModel) • Impact of the storm surge at any particular location is sensitive to meteorological and topographic parameters • Inundation is crucial for disaster planning • Prediction of flooding areas depends on model grid resolution

4. New Challenges for Numerical Modeling • More high resolution terrain data are available • LIDAR (LIght Detection And Ranging) • More real-time observation data are available • Surface elevation • Vertical velocity profile • Wave • Real-time simulation vs. prediction • Rescue • Inundation • How to integrate high resolution terrain and real-time observation data into models ?

5. VIMS Real-Time Observation System

6. Current Observation at Gloucester Point

7. Airborne LIDAR Data • LIDAR (LIght Detection And Ranging) • Infrared laser ranger provides distance to ground • Differential GPS provides aircraft location • Inertial reference system provides aircraft orientation • Post-processing provides x,y,z coordinates of ground surface • Construction of Digital Elevation Models (DEMs)

8. DTMS 3 Sec (80-90 m) DTM Vertical Res. 1 m 10 DTM Vertical Res. 0.3 m

9. USGS 10m DEM LIDAR 10m DEM Elevation (feet) Example of LIDAR Data (Miami)

10. Model Domain Representation • Small domain is inadequate for storm simulation • Coarse grid is inadequate to resolve irregular shoreline and small topographic features in estuary • Structured grid is difficult to represent complex bathymetric in estuary • Unstructured grid has advantage of storm surge modeling • Use nested grids • Place fine grid in the areas of interest and coarse grid in the remaining large areas

11. Model Requirements • Must resolve complex shoreline • Must resolve land features • coastal ridge, dam, inlet, and river • small scale on the order of meters • Must cover large modeling domain • Must be computationally efficient

12. Example of Unstructured Grid (Miami)

13. Example of Nested Model Grids

14. Unstructured 3D Model (UnTRIM) • “UnTRIM” incorporates an Unstructured grid into TRIM model (Tidal, Residual, Intertidal Mudflat), originally developed by Vincenzo Casulli • It simulates three-dimensional hydrodynamic and transport processes • It uses an orthogonal unstructured grid • It conserves mass locally as well as globally • It uses Eulerian-Langangian transport scheme • It employs semi-implicit finite difference and finite volume method- very efficient computationally • It is capable of simulating wet-dry processes

15. Grid Structure • Use polygons to represent a prototype estuary (3-, 4-, 5-sides) • Better fitting complicated geometry in estuarine and coastal environment • Using orthogonal grid simplifies the numerical algorithm

16. Model Simulation Studies • Study the accuracy of model prediction of Isabel forced by a stationary, circular wind model • Compare model prediction with and without simulating inundation • Study influence of open boundary condition specification on surge simulation • still boundary condition vs. inverse pressure adjust boundary condition • Study influence of model domain size on surge prediction • Study influence of wind field on model prediction • Study influence of hurricane on transport

17. Model Grids Surface elements =121,338 Surface elements =239,541

18. Grid layouts at York and James Rivers

19. Model Calibration • Calibrate model for tide forced by 9 tidal constituentsM2, S2, K1, O1, Q1, K2, N2, M4, and M6 • Model was run for 3 months and the results of the last 29 days were used for computing tidal harmonics • Time step = 5 min.

20. Tidal Simulation (M2 tide)

21. Tidal Simulation (K1)

22. Observation Stations

23. Tidal Constituents Comparison (Amplitude) Amplitude is in m Observations are based on 1992 data

24. Tidal Constituents Comparison (phase) Phase is in degree

25. Wind Model (Myers and Malkin, 1961) • r = the distance from the storm center • p(r) = pressure, pa = central pressure, • = the inflow angle across circular isobar toward the storm center V is the wind speed, f = Coriolis parameter, ks and kn are friction coefficients.

26. Example of Wind Field

27. Isabel Simulation ResultsWith and Without Simulating inundation

28. Comparison of Model ResultsWith and Without Simulating Inundation

29. Comparison of Model ResultsWith and Without Simulating Inundation

30. Test Influence of Open Boundary Condition • Apply inverse pressure adjustment at BC • Run large domain model (east coast) and apply time series output from large domain model to force small domain model

31. Large Domain Model Simulation

32. Influence of Open Boundary Condition Inverse Pressure Adjustment Nested Grids

33. Influence of Wind Field

34. Wind Field Analysis

35. Comparison of Using Different Wind Fields

36. Current Simulation at Gloucester Point Model Wind Field Modified Wind Field

37. Conclusions • Unstructured model is very efficient in simulation tide and storm surge • Open boundary condition specification influences the surge prediction • Wind field is critical in the accurate simulation of storm surge

38. Thanks !