Hurricane
Surge Prediction
Understanding
the Destructive Flooding Associated with Hurricanes
George PararasCarayannis
(Excerpts
from a study prepared for the U.S. Nuclear Regulatory Commission
under contract with the U.S. Army, Corps of Engineers Coastal
Engineering Research Center, Washington, D.C., (Technical Memorandum
No. 50, May 1975.) (© 1975 George PararasCarayannis)
INTRODUCTION
Hurricanes are severe
tropical cyclones with winds spiraling inward toward a center
or eye of low pressure at speeds which may reach more than 150
miles per hour (130 knots). Although usually erratic and unpredictable,
hurricanes generally follow a westerly to northwesterly path
toward the gulf or Atlantic coasts causing abnormal water level
fluctuations. These water level fluctuations are called storm
surges and are caused by an atmospheric pressure field and wind
stress on the water surface, accompanying moving storm systems.
Specific factors which can combine to produce extreme water fluctuations
at a coast during the passage of a storm include: storm intensity,
size, path, and duration over water; atmospheric pressure variation;
speed of translation; winds and rainfall; bathymetry of the offshore
region; astronomical tides; initial water level rise; surface
waves and associated wave setup and runup. (Sattelite Photo of Hurricane Hugo)
Understanding
Hurricane Surge
Hurricane surge is
an oceanographic phenomenon and constitutes a greater hazard
to lives and coastal property than hurricane winds. Hurricane
surges have been estimated to account for 75 to 90 percent of
all deaths resulting from a hurricane. Surge inundation is also
responsible for extensive damage to coastal property. The factors
contributing to hurricane surge are briefly described in the
following sections below.
Historical Hurricanes: Historically, since 1900, hurricane
damages to coastal property have averaged more than $50 million
per year. The storm that hit Galveston
in 1900 resulted
in 6,000 deaths and the almost complete destruction of a large
part of the city.
The hurricane of 3
October 1949, devastated
the Yukatan Penisnsula, Honduras and Guatemala before stiking
Texas. Surge heights were 11 feet at Velasco, 8 feet at Matagorda,
9 feet at Anahuac, and 11.4 feet at Harrisburg in the Houston
Ship Channel.
Hurricane Camille, which struck the Mississippi
coast in 1969, killed 262 persons and caused damages of nearly
$1 billion. Maximum surge height of Camille was 31.65 feet, 25.4
feet at Pass Christian, Miss., and 20.4 feet at Biloxi, Miss.
Hurricane Carla, which struck the Texas coast
in 1961, killed 32 persons. The damage from Hurricane Carla was
more than $400 million. Maximum surge height was 12.4 feet at
Freeport Harbor.
Hurricane Betsy hit near New Orleans in 1965,
and was responsible for 81 deaths. Damages from Hurricane Betsy
were estimated to be $372 million. Hurricanes in the 70's, 80's
and 90's have been even more destructive in terms of damage,
but fortunately have not taken as many lives as the hurricanes
of the past. This is attributed to better preparedness and effective
systems of prediction and warning.
Estimating the Hurricane
Surge
Increasing pressure
for large coastal installations, such as power plants, and super
port terminals, and the increasing residential development of
the coastal zones, emphasized the need for more accurate estimates
of potential storm surge hazards.
The prediction of
storm surge resulting from the combined meteorologic, oceanic
and astronomic effects coincident with the arrival of a hurricane
at the coast is a very important problem, but rather a difficult
one to solve. It will not be attempted here to explain exactly
how the problem is solved. Only some basic concepts and components
which cumulatively contribute to hurricane surge will be presented.
The capability for
the prediction of hurricane surge is based primarily on the use
of analytic and mathematical models which estimate the interactions
between winds and ocean, also taking into account numerous other
factors.
An estimate of water
level fluctuation due to the passage of a hurricane is essential
for warning purposes and the planning and design of important
coastal structures. A hurricane is a threedimensional, dynamic
weather system with continually changing direction and wind speeds.
Thus, because of the ever changing dynamic conditions it becomes
a little more difficult to calculating accurately hurricane surge,
and total water flux energy, at different coastal locations in
the path of the hurricane.
To calculate hurricane
surge, the following meteorological parameters must be first
determined. These are the hurricane's central pressure index
(CPI), its peripheral pressure (pn), the radius to maximum winds
(R), the maximum gradient wind speed (Umax), the maximum wind
speed (UR), and the speed of hurricane translation (VF) (overall
speed of the system) . Additionally in situ conditions of astronomical
tide, Coriolis effects, coastal topography and existing ambient
wave conditions must be determined. Only then one can proceed
with the solution of the complex hydrodynamic equations of motion
and continuity that will allow determination of the time history
of expected changes in sea level associated with the hurricane,
at any given point.
The
Bathystrophic Storm Surge Model
A Brief Overview
There are many sophisticated
mathematical models which have been developed in recent years
that provide accurate three dimensional estimates of energy flux
and flooding that can be caused by a passing hurricane. All numerical
models, regardless of sophistication of methodology must use
the Bathystrophic Storm Tide Theory to estimate the rise of water
on the open coast taking into account the combined effects of
direct onshore and along shore windstress components on the
surface of the water, and the effects of the Coriolis force (bathystrophic
effect), and the different pressure effects.
Mathematical models
using the Bathystrophic Storm Tide Theory can be quasionedimensional,
two dimensional, or threedimensional numerical schemes. The
simplest, which is superficially described here, is a quasionedimensional
model which is a steadystate integration of the wind stresses
of the hurricane winds on the surface of the water from the edge
of the Continental Shelf to the shore, taking into consideration
some of the effects of bottom friction and the along shore flow
caused by the earth's rotation.
The bathystrophic
contribution to hurricane surge can be explained as follows:
In the northern hemisphere hurricane winds approaching the coast
have a counterclockwise motion. Because of the Coriolis effect,
the flow of water induced by the cyclonic winds will deflect
to the right, causing a rise in the water level. The bathystrophic
storm tide, therefore, is important in producing maximum surge
even when winds blow parallel to the coast. Coastal morphology
may also affect the extent of rise of water. However, in this
model the surge is calculated only along a single traverse line
at a time over the Continental Shelf for a straight openocean
coast. Thus, it is labeled as quasionedimensional. (Diagram above shows various
setup components contributing to the hurricane surge on the shore)
Such a simple model
uses the onshore and along shore windstress components of a
moving wind field over the Continental Shelf, and a frictional
component of bottom stress. The nonlinear storm surge is computed
at selected points along the traverse by integrating numerically
the onedimensional hydrodynamic equations of motion and continuity.
The hurricane surge
estimated by this simple model is a composite of water elevation
obtained from components of the astronomical tide, the atmospheric
pressure, the initial rise, the rises due to wind and bottom
friction stresses, and wave setup. The adjacent diagram portrays
graphically the contributions of each component to the total
hurricane surge estimate.
Obviously such a basic
model has its limitations. A hurricane is not stationary, and
as it moves towards the coast, the wind speeds may increase and
wind vectors will change direction changing frictional effects
on the water surface. Such changes cannot always be predicted
with accuracy to introduce them into the model.
*** For details on the
Bathystrophic Storm Surge Model go to the original report referenced
below.
SUMMARY AND CONCLUSIONS
The prediction of
storm surge resulting from the combined meteorologic, oceanic,
and astronomic effects coincident with the arrival of a hurricane
at the coast is important in warning the public and in the planning
and the design of important coastal structures. Increasing requirements
for large coastal installations, have required conservative criteria
in obtaining estimates of potential storm surges from hurricanes.
The present capability
for prediction of hurricane surge is based primarily on the use
of analytic and numerical models. The numerical model approach
summarized here, is based on the Bathystrophic Storm Tide Theory
and used to estimate the rise of water on the open coast by taking
into account the combined effects of direct onshore and along
shore windstress components on the water surface and the effects
of the Coriolis force to integrate numerically the onedimensional
hydrodynamic equations of motion and continuity.
The bathystrophic
theory on which hurricane surge prediction is based, represents
an approximation to the complete stormgeneration process. Therefore,
such a model of prediction is limited by a number of initial
conditions and assumptions. In most instances, the bathystrophic
approximation appears to give a reasonable estimate of the opencoast
surge; however, at times the surge estimate can be in error by
a factor of 2 or more. However, the accuracy of hurricane prediction
can be improved through calibration with known historical data
 something which was accomplished successfully by the author
more than 25 years ago. More recently developed numerical models
using a three dimensional approach, faster and more efficient
computers and more accurate weather data from satellites, have
greater potential for more accurate prediction. However, the
fundamental principles in the prediction of hurricane surge described
here, remain essentially the same
REFERENCE
PararasCarayannis,
George. Verification
Study of a Bathystrophic Storm Surge Model. U.S. Army, Corps of Engineers  Coastal Engineering
Research Center, Washington, D.C., Technical Memorandum No. 50,
May 1975.
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