What May Have Occurred Had Hurricane Ivan Made Landfall Within the Tampa Bay Region?

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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
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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 Earths 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.
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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.
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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.
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Inundation based on a 5-foot uniform sea level
rise
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Inundation based on a 20-foot uniform sea level
rise
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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.
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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.

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Overall Model Domain and Grid
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A Zoom View of the Tampa Bay Region
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Merged Bathymetry and Topography
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Wind and Pressure Distributions for a
Prototypical Hurricane (Holland, 1980)
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Hurricane Ivan Simulations for the Tampa Bay
Region
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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
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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 gt155 gt135 gt70
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• Details of the Indian Rocks Beach Landfall
  Experiment.
• The entire Tampa Bay region.
• Zoom views of the Pinellas beaches, Old Tampa
  Bay, and Hillsborough Bay.
• Two methods of display are used
• Surge relative to mean sea level
• Inundation relative to land

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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
Seawall and road levels
11 ft
9 ft
4 ft
Meters and Feet 1m 3.28 ft 3m 9.84 ft
6m 19.68 ft
5 ft
MSL
1 ft
MLW
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Surge elevation relative to mean sea level (left)
and wind speed and direction (right) 6 hrs before
landfall
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Surge elevation relative to mean sea level (left)
and wind speed and direction (right) 3 hrs before
landfall
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Surge elevation relative to mean sea level (left)
and wind speed and direction (right) at landfall
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Surge elevation relative to mean sea level (left)
and wind speed and direction (right) 1 hr after
landfall
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Surge elevation relative to mean sea level (left)
and wind speed and direction (right) 2 hrs after
landfall
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Animations of surge height relative to mean sea
level (left) and winds (right)
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Surge elevation relative to land elevation (left)
and wind speed and direction (right) 6 hrs before
landfall
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Surge elevation relative to land elevation (left)
and wind speed and direction (right) 3 hrs before
landfall
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Surge elevation relative to land elevation (left)
and wind speed and direction (right) at landfall
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Surge elevation relative to land elevation (left)
and wind speed and direction (right) 1 hr after
landfall
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Surge elevation relative to land elevation (left)
and wind speed and direction (right) 2 hrs after
landfall
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Animations of surge height relative to land
elevation (left) and winds (right)
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Zoom views for the Pinellas beaches of inundation
relative to land elevation
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Highest surge relative to land elevation for this
sub-region
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Animation of surge relative to land elevation for
this sub-region
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Zoom views for Old Tampa Bay of inundation
relative to land elevation
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Highest surge relative to land elevation for NE
St. Petersburg
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Highest surge relative to land elevation for
upper Old Tampa Bay
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Animation of surge relative to land elevation for
this sub-region
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Zoom views for Hillsborough Bay of inundation
relative to land elevation
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Highest surge relative to land elevation for
downtown Tampa
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Highest surge relative to land elevation for
Apollo Beach
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Animation of surge relative to land elevation for
this sub-region
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What might have happened if the landfall point
occurred elsewhere? Tarpon Springs Bayport Cedar
Keys Sarasota
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Time series of surge height sampled at selected
locations
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Time series of surge height sampled at four bay
bridges
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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.
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A really bad case for Hillsborough Bay
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Surge relative to land elevation (Pinellas
beaches)
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Surge relative to land elevation (old Tampa Bay)
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Surge relative to land elevation (Hillsborough
Bay)
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Animation of surge relative to land elevation
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Time series of surge height sampled at selected
locations
Key
9m30
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Time series of surge height sampled at four bay
bridges
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• Summary
• 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
• 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.
• 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.
• 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.
• 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.

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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.