Ivan-like hurricane storm surge simulations for Tampa Bay, FL with 3-D and 2-D models R.H. Weisberg and L. Zheng Storm Surge Workshop St. Pete Beach 2/11/09
Potential for Tampa Bay area inundation is large due to land elevation and geometry Inundation based on uniform sea level rise of: 5 ft 20 ft
A house on the Waveland, MS coastline water-gutted from below.
Rte 90 bridge across Bay St. Louis, MS. All spans were knocked from their supports.
Destroyed homes in South Diamondhead, MS.
Hurricane Storm Surge Simulation Requirements 1. A high resolution, physics-based circulation model with flooding and drying capabilities. 2. A high resolution water depth (bathymetry) and land elevation data set on which to overlay the model. 3. Accurate enough wind and pressure fields to drive the model. Here we use the Finite Volume Coastal Ocean Model (FVCOM) of Chen et al. (2003).
Overall Model Domain and Grid
The Ivan track (red dots) and the tracks (black dots) used in this study (with landfalls as Sarasota, Indian Rocks Beach, Tarpon Springs, Bayport, and Cedar Keys.
Ivan Winds on approach and at Landfall While Ivan reached category 5 in the Caribbean it was a 4 upon approach and a 3 at landfall. Category mph knots m/s 1 74-95 64-82 33-43 2 96-110 83-95 44-49 3 111-130 96-113 50-59 4 131-155 113-135 60-70 5 >155 >135 >70
Surge elevation relative to mean sea level (left) and land elevation (middle), plus wind vectors on wind speed contours (right) 3 hours before and at IRB landfall (hrs. 27 and 30, respectively).
Maximum IRB landfall surge relative to land at sub-domains emphasizing St. Pete Be. (left), Old Tampa Bay (middle), and Hillsborough Bay (right).
Time series of surge height sampled at selected locations
Vertically Integrated Momentum Balance - ghζ τ s τ b R ρ 0 ρ 0 where: ζ is the sea level, H=h+ζ is the total water depth, τ is the surface wind stress, τ s b is the bottom friction stress, and R is the sum of the local and Coriolis accelerations, the advective accelerations, and the horizontal diffusion, each calculated separately before summation.
Wind Stress 3-D FVCOM C d 2-D IPET C d Bottom Stress 3-D FVCOM C z 2-D IPET C f Stress Parameterizations C d 10 3 τ C ρ V V s d a w w 1.2 Vw 11.0 ms -1 0.49 0.065 Vw 11.0 ms Vw 0.49 0.065 25 V w 25.0 ms -3 Cd (0.75 0.067 Vw ) 10 τ b C z ρ w V b k max [ln(1 σ kb V Cz 2 1 )H/z 0 ] H b Cf C f min 1 H 2 b -1-1,0.0025 25.0 ms -1
Parameter Examples Wind Stress For 50 ms -1 wind C d (3-D) is 2.11x10-3, whereas C d (IPET 2-D) is 4.1x10-3 Bottom Stress For the 3-D FVCOM used here, C z is capped at 0.005 For IPET Katrina ADCIRC C f (2-D) is 0.003 at 2 m depth, 0.007 at 1m depth, and larger for shallower depths. Hence, The IPET 2-D ADCIRC compensates for larger bottom stress by using larger wind stress.
Absolute (black) and percent (red) differences between 3-D and 2-D surges at four positions from the mouth to the head of the bay.
Conclusions 1. Local: Tampa Bay, FL is as vulnerable to hurricane storm surge inundation as was coastal Mississippi for H. Katrina. 2. General: A. Storm surge simulation is sensitive to model construction: i.e., 3-D, versus 2-D. The explanation is bottom stress. A 2-D model overestimates bottom stress, and hence underestimates surge. B. Calibration can mitigate this (e.g., the IPET, Katrina analyses are excellent), but forecasts without calibration data may be in significant error. C. Agencies (NOAA, FEMA, USACE) employ 2-D models. Our findings suggest the importance of 3-D. D. Studies are necessary to improve surface and bottom stress parameterizations.
Acknowledgments This work began with support by ONR, grant #s N00014-05-1-0483 and N00014-02-1-0972, and it continues with support from NOAA, grant # NA07NOS4730211. The second of these was for the Southeast Atlantic Coastal Ocean Observing System (SEACOOS), and the third is related to the Southeast Atlantic Coastal Ocean Observing Regional Association (SECOORA). Changsheng Chen (UMassD) kindly shared the FVCOM code.
Coupled Wave Effects To investigate the effects of waves on the combined surge and waves we (with Yong Huang) coupled an unstructured version of SWAN to the FVCOM and repeated the experiments. The following figures show: 1) significant wave height, radiation stress and wind stress at several times during the simulation, 2) surge heights with and without the wave coupling by radiation stress, and 3) surge height differences with and without the wave coupling. To the surge heights with wave coupling must also be added the wave amplitude (between approximately 0.5 to 0.7 times the wave height) to get the total vertical reach of water over the evolution of the storm.
Surge height with/without wave radiation stress.
Surge height difference with/without wave radiation stress.
Significant wave height, radiation stress, and wind stress
Significant wave height, radiation stress, and wind stress