What May Have Occurred Had Hurricane Ivan

Made Landfall Within the Tampa Bay Region?

R.H. Weisberg and L. Zheng

Tampa Bay Chapter of AMS 6/6/06

What is a storm surge?

Abnormal sea level elevations (or depressions) caused by winds and atmospheric pressure. The components are:

1. Coastal set up (down) by the along shore wind stress.In deep water, the Earth’s rotation causes a water to move at a right angle

the wind stress. This sets up a sea level slope against the coast and an alongshore current in geostrophic balance. With the current limited by friction the sea level set up is less than a meter.

2. Coastal set up (down) by atmospheric pressure.Atmospheric pressure operates like an inverted barometer. Each mbar of

pressure drop (increase) raises (lowers) sea level by 1 cm. The largest hurricanes with pressure drops of 100 mbar can cause a 1m surge by this mechanism.

3. Coastal set up (down) by the across shore wind stress.In shallow water, and because of friction, the wind stress drives water

downwind and piles it up against the coastline. The resulting sea surface slope (tending to balance the across shore wind stress) is the largest contributor to coastal storm surge and can exceed several m.

Other Factors

4. Coastal geometry.By varying fetch and direction relative to a hurricane the embayment

geometry is very important, as are the water depths and land elevations.

5. Continental shelf width.In shallow water the sea surface slope required to balance the across

shelf wind stress is inversely proportional to water depth. Hence wide, shallow shelves are prone to larger storm surges.

6. Tides.Water level will be higher (lower) at high (low) tide. Since tides in Tampa

Bay are about plus and minus 1.5’ this is small relative to the storm surge.

7. Water density.By being lighter, warmer water in summer stands higher than colder

water in winter. This can amount to about 1’.

8. Waves.Waves are additive to surge. Theoretically a solitary wave can be 1.8

times the water depth. While this is not naturally realized, waves can have a huge impact. Imagine the surf zone on a very rough day displaced to Gulf Blvd.

Why should we study the potential for Tampa Bay

storm surges?

1. The Southeast U.S. and the Gulf of Mexico are regularly impacted by hurricanes.2. Whereas Tampa Bay has not had a major hurricane hit since 1921 it seems inevitable that one will occur again.3. In the meanwhile population has grown and coastal development has burgeoned.4. Since the region is low lying the potential for property damage and loss of life is severe.

Inundation based on a 5-foot uniform sea level rise

Inundation based on a 20-foot uniform sea level rise

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.

The Model

We use the Finite Volume Coastal Ocean Model (FVCOM) of Chen at al. (2003). The FVCOM attributes are:

1) An unstructured triangle grid for representing complex coastal geometry.

2) Three-dimensional, primitive equations, with flow dependent turbulence closure.

3) Finite-differences for mass, momentum, heat, and salt conservation, plus computational efficiency.

4) Provision for flooding and drying land.

Overall Model Domain and Grid

A Zoom View of the Tampa Bay Region

Merged Bathymetry and Topography

Wind and Pressure Distributions for a Prototypical Hurricane (Holland, 1980)

Hurricane Ivan Simulations for the Tampa Bay Region

The Ivan track (red dots) and the tracks (black dots) used in our 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

Details of the Indian Rocks Beach Landfall Experiment.

1) The entire Tampa Bay region.2) Zoom views of the Pinellas beaches, Old Tampa Bay, and

Hillsborough Bay.3) Two methods of display are used:

• Surge relative to mean sea level• Inundation relative to land

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.

MLWMSL

1 ft

Seawall and road levels

4 ft5 ft

9 ft11 ft

Old building code

New building code

Meters and Feet 1m = 3.28 ft 3m = 9.84 ft 6m = 19.68 ft

Surge elevation relative to mean sea level (left) and wind speed and direction (right) 6 hrs before landfall

Surge elevation relative to mean sea level (left) and wind speed and direction (right) 3 hrs before landfall

Surge elevation relative to mean sea level (left) and wind speed and direction (right) at landfall

Surge elevation relative to mean sea level (left) and wind speed and direction (right) 1 hr after landfall

Surge elevation relative to mean sea level (left) and wind speed and direction (right) 2 hrs after landfall

Animations of surge height relative to mean sea level (left) and winds (right)

Surge elevation relative to land elevation (left) and wind speed and direction (right) 6 hrs before landfall

Surge elevation relative to land elevation (left) and wind speed and direction (right) 3 hrs before landfall

Surge elevation relative to land elevation (left) and wind speed and direction (right) at landfall

Surge elevation relative to land elevation (left) and wind speed and direction (right) 1 hr after landfall

Surge elevation relative to land elevation (left) and wind speed and direction (right) 2 hrs after landfall

Animations of surge height relative to land elevation (left) and winds (right)

Zoom views for the Pinellas beaches of inundation relative to land elevation

Highest surge relative to land elevation for this sub-region

Animation of surge relative to land elevation for this sub-region

Zoom views for Old Tampa Bay of inundation relative to land elevation

Highest surge relative to land elevation for NE St. Petersburg

Highest surge relative to land elevation for upper Old Tampa Bay

Animation of surge relative to land elevation for this sub-region

Zoom views for Hillsborough Bay of inundation relative to land elevation

Highest surge relative to land elevation for downtown Tampa

Highest surge relative to land elevation for Apollo Beach

Animation of surge relative to land elevation for this sub-region

What might have happened if the landfall point occurred elsewhere?

Tarpon SpringsBayport

Cedar KeysSarasota

Time series of surge height sampled at selected locations

Time series of surge height sampled at four bay bridges

ENCORE

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

Key

9m=30’

Time series of surge height sampled at four bay bridges

Summary1) We previously showed (Weisberg and Zheng, 2006a) the sensitivity of storm surge

in the Tampa Bay region to landfall location and direction and speed of approach, in addition to intensity (category) and size (radius to maximum winds), and

2) We applied these concepts (Weisberg and Zheng, 2006b) to explain why Hurricane Charley caused only a minor surge in Charlotte Harbor despite category 4 intensity.

3) Here we explored the surge for an Ivan-like storm, the results of which would have been catastrophic. In addition to the surge are also the effects of waves that add to the surge height and repeatedly batter structures.

4) We also explored a worst case scenario for Hillsborough Bay, a cat. 5 storm paralleling the bay axis, displaced by a radius to maximum winds to the northwest.

5) The bottom line is the potential for hurricane storm surge damage in the greater Tampa Bay region is enormous, almost unimaginable. While we have been fortunate in not having a direct hit since 1921, future planning must take these findings very seriously. Three recommendations are:

• Citizens should heed emergency management advisories• Improved contingency planning is needed for the aftermath of a major hit

since lines of communication (roads, rail, bridges, airports could all be damaged or destroyed) under a bad-enough storm.

• Future rezoning decisions should take these findings into consideration.

Acknowledgments

This work was supported by the Office of Naval Research, grants # N00014-05-1-0483 and N00014-02-1-0972, the second of which is for the Southeast Atlantic Coastal Ocean Observing System (SEACOOS). Changsheng Chen (UMassD) kindly shared the FVCOM code. This is also part 1 of a collaboration with USGS colleague A. Sallenger who is computing the wave field that could have resulted from the Ivan winds.