Introduction

Storm surge is the abnormal rise in water level caused by wind and pressure forces of a hurricane. Storm surge produces most of the flood damage and drowning associated with tropical storms. A numerical storm surge model has been applied to the Savannah/Hilton Head and Brunswick/Jacksonville Basin areas. The model calculates sea, lake and overland surges from hurricanes and has the acronym "SLOSH."

The output of SLOSH-model provides heights of storm surge for various combinations of hurricane strength, forward speed of storm, and direction of storm. Strength is modeled by use of the central pressure and storm eye size using the five categories of storm intensity.

Factors Affecting Surge Height

The elevation reached by the storm surge within a coastal basin depends upon the meteorological parameters of the hurricane and the physical characteristics existing within the basin. The meteorological parameters affecting the height of the storm surge include the intensity of the hurricane, measured by the storm-center sea-level pressure, track (path) of the storm, forward speed, and radius of maximum winds. Due to the complementary effects of forward motion and the counterclockwise rotation of the wind field, highest surges from a hurricane usually occur on the northeast quadrant of the storm's track. This radius of maximum winds, which is measured from the center of the hurricane eye to the location of the highest wind speeds within the storm, can vary from as little as 4 miles to as much as 50 miles or greater. Peak storm surge may vary drastically within a relatively short distance along the coastline depending on the radius of maximum winds and the point of hurricane eye landfall. The physical characteristics of a basin that influence the surge heights include the basin bathymetry (water depths), roughness of the continental shelf, configuration of the coastline, and natural or man-made barriers. A wide, gentle sloping continental shelf or a large bay may produce particularly large storm surges.

Total Flood Elevation

Other factors that contribute to the total water height are the initial water levels within the basin at the time the hurricane strikes and wave effects. A significant component of the total surge elevation is the height of the astronomic tide when the storm arrives at the coast. There are two high tides and two low tides each day along the Georgia Coast and the diurnal range is about 7 feet. Thus relative to mean sea level, total water levels could be about 3.5 feet higher if the storm arrives at high tide and 3.5 lower at low tide.

Waves breaking near the shore cause a transport of water shoreward. When there is an increase in wave height water cannot flow back to the sea as rapidly as it came in. This phenomenon, known as "wave setup", increases the water level along the beachfront. Waves will break and dissipate their energy in shallow water. Therefore, a relatively steep offshore beach slope allows large ocean waves to get closer to the shore before breaking and usually promotes larger waves. Wave setup is primarily a concern near the beachfront because waves are generally not transmitted inland of the coastline even if the beach has been overtopped.

Wave Effect

The SLOSH model does not provide data concerning the additional heights of waves generated on top of the still-water storm surge. Generally, waves do not add significantly to the area flooded and have little effect on the number of people that will be required to evacuate. Wave phenomena under hurricane conditions are not well understood, but it is believed that maximum wave heights occur near the time of landfall. Immediately along the coastline of very large sounds and estuaries, waves can increase the expected still-water depth by one-third or more. Due to the presence of barriers such as structures, dunes, or vegetation, the waves break and dissipate a tremendous amount of energy within a few hundred yards of the coastline. Buildings within that zone that are not specifically designed to withstand the forces of wave action are often heavily damaged or destroyed.

For evacuation planning purposes, it is perhaps more important to consider potential wave effects for less than sustained tropical storm winds. If wave heights above theoretical still-water levels exceed the elevations of roads, bridges, or other critical areas near the coastline, evacuation could be curtailed sooner than expected, increasing the pre-landfall hazards distance. Evacuation planners should be aware that low-lying sections of highway could be subject to some wave action and over-wash prior to the arrival of sustained tropical storm winds, especially with the coincidental occurrence of astronomical high tide.

Freshwater Flooding

Amounts and arrival times of rainfall associated with hurricanes are highly unpredictable. For most hurricanes, rainfall begins near the time of arrival of sustained tropical storm winds and generally reaches maximum rainfall rates as the center passes by. Unrelated weather systems in advance of the hurricane can also contribute significant rainfall amounts within a basin. The 100-year floodplain boundaries for each county are shown on the National Flood Insurance Rate Maps (FIRM) which are published by the Federal Emergency Management Agency (FEMA)