**Understanding the Destructive Flooding Associated with Hurricanes**

### 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)

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 three-dimensional, 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.

### 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 wind-stress 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 quasi-one-dimensional, two dimensional, or three-dimensional numerical schemes. The simplest, which is superficially described here, is a quasi-one-dimensional model which is a steady-state 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 open-ocean coast. Thus, it is labeled as quasi-one-dimensional. (Diagram above shows various setup components contributing to the hurricane surge on the shore)

Such a simple model uses the onshore and along shore wind-stress 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 one-dimensional 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 wind-stress components on the water surface and the effects of the Coriolis force to integrate numerically the one-dimensional hydrodynamic equations of motion and continuity.

The bathystrophic theory on which hurricane surge prediction is based, represents an approximation to the complete storm-generation 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 open-coast 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

Pararas-Carayannis, 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.

#### See also

#### Hurricane Iniki in Hawaii- September 11, 1992

#### Hurricane Imagery – Tropical Cyclones

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