What may have happened here had Hurricane Ivan visited us instead of Pensacola?R.H. Weisberg and L. ZhengCollege of Marine Science University of South Florida

Greater Tampa Bay ACP Meeting5/26/2010

What is storm surge?

Abnormal sea level change caused by wind and pressure.

1. Along shore wind stress.A sea level slope balancing the Coriolis force of an alongshore current causes sea level to set up by ~ 1 - 3 ft. 2. Atmospheric pressure.The inverted barometer effect causes sea level to set up by ~ 1 - 3 ft. 3. Across shore wind stress.A sea level slope balancing friction causes sea level to set up by as much as 28 ft. This is the largest contributor to hurricane storm surge. Other factors:

4. Coastal geometry. 5. Continental shelf width. 6. Tides. 7. Water density. 8. Waves.Waves when added to the surge are the most destructive aspect of hurricane storm surge.

Rte 90 bridge across Bay St. Louis, MS. All spans were knocked from their supports.

Destroyed homes in South Diamondhead, MS.

Why should we study the potential for Tampa Bay storm surges?

1. The Southeast U.S. is regularly hit by hurricanes. 2. Whereas Tampa Bay has not had a major direct hurricane hit since 1921 it seems inevitable that one will again occur. 3. In the meanwhile population and coastal development have burgeoned. 4. Because of low land elevations, the potential for property damage and loss of life is severe.

Storm Tracks: 1851-2005 Figure provided by J. I. Virmani, 2007

Storm tracks by month Figure provided by J. I. Virmani, 2007

Storm Tracks: 1851-2006 270-28.50N, 820-830WFigure provided by J. I. Virmani, 2008

Inundation based on uniform sea level rise of:

5 ft 20 ftPotential for Tampa Bay area inundation is large due to land elevation and geometry

Hurricane Storm Surge Simulation Requirements1. 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.

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 LandfallWhile Ivan reached category 5 in the Caribbean it was a 4 upon approach and a 3 at landfall.Categorymphknotsm/s 174-9564-8233-43 296-11083-9544-49 3111-13096-11350-59 4131-155113-13560-70 5>155>135>70

Relative Elevations (Approximate)

Seawall height (and nominal street level): 5 above mean low water (MLW); 4 above mean sea level (MSL);

Finished floor heights: 9 and 11 above MLW for old and new building codes (8 and 10 above MSL (7 and 9 above MHW); hence a 2.5m (3m) surge would put water in an older (newer) home.MLWMSL1 ftSeawall and road levels4 ft5 ft9 ft11 ftOld building codeNew building codeMeters and Feet 1m = 3.28 ft 3m = 9.84 ft 6m = 19.68 ft

Surge height relative to mean sea level (left) and land elevation (middle), plus wind vectors and speeds (right) 3 hours before and at landfall.

Maximum surge heights relative to land elevations at St. Pete Be. (left), Old Tampa Bay (middle), and Hillsborough Bay (right).

2.75 - 3.0 m (msl) 9 - 10 ft (msl) 3 - 5 ft (le)3.75 - 4 m (msl)12 - 13 ft (msl) 6 - 8 ft (le)3.75 - 4 m (msl)12 - 13 ft (msl) 6 - 8 ft (le)4 4.25 m (msl)13 - 14 ft (msl) 7 - 9 ft (le)

Anim. of surge height relative to land elev. (L) and winds (R)

Time series of surge height sampled at selected locations

Are surges simulated with 3D models different from those simulated with 2D models?

Why bother: Agencies use 2D models (e.g., NOAA: SLOSH; FEMA and USACE: ADCIRC).

Whereas storm surge dynamics are 3D:

The problem is: 2D dynamics overestimate bottom stress and therefore underestimate surge height.

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.

Coupled Wave Effects

To investigate the effects of waves on the combined surge and waves we coupled a waves model (SWAN) to the circulation model (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

2.5 - 3 m 8 10 ft

Katrina damage to Long Beach MS, where surge heights were ~ 20 ft and waves were ~ 6 ft. Houses at elevations less than about 16 ft were destroyed.

Katrina damage to Pascagoula MS, where surge heights were ~ 14 ft and waves were ~ 4 ft.

ENCORE(Presented 6/06 at WFAMS and 7/07at TBRPC)

The worst case for Tampa Bay as a whole is not necessarily the worst case for any given position within the bay.

Now consider the inundation potential for downtown Tampa under a category 5 hurricane paralleling the bay axis and displaced northwest by a radius to maximum winds, such that the maximum winds are directed toward the head of Hillsborough Bay.

A really bad case for Hillsborough Bay

Surge relative to land elevation (Pinellas beaches)

Surge relative to land elevation (old Tampa Bay)

Surge relative to land elevation (Hillsborough Bay)

Animation of surge relative to land elevation

Time series of surge height sampled at selected locations

Conclusions

Local: Tampa Bay, FL is as vulnerable to hurricane storm surge inundation as was coastal Mississippi for H. Katrina.

2. General: Storm surge simulation is not a solved problem. It is sensitive to model construction: i.e., 3-D, versus 2-D and surface and bottom stresses, which remain uncertain.

Agencies (NOAA, FEMA, USACE) employ 2-D models. Our findings suggest the importance of 3-D.

Studies are necessary to improve surface and bottom stress parameterizations.

3. Finally: Residual risk is real, e.g., Katrina. Dont underestimate the importance of flood insurance, even if you are above the FEMA BFE, and heed emergency managers warnings!