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
1
2
3
4
5
mph
74-95
96-110
111-130
131-155
>155
knots
64-82
83-95
96-113
113-135
>135
m/s
33-43
44-49
50-59
60-70
>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.
New building code
Old building code
9 ft
Seawall and road levels
11 ft
5 ft 4 ft
1 ft
MSL
MLW
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 Springs
Bayport
Cedar Keys
Sarasota
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
Summary
1)
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 N0001402-1-0972, 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.