HURRICANE STORM TIDE ATLAS
(SEPTEMBER 1990)

1. INTRODUCTION
 

The purpose of the maps contained in this atlas are to reflect a worst case scenario of hurricane storm surge inundation (which includes the addition of an astronomical high tide). It should be noted that the data reflects only stillwater saltwater flooding. It is incumbent upon local emergency management officials to estimate the degree and extent of freshwater flooding, as well as to determine the magnitude of the waves that will accompany the surge.
 

The maps contained in this atlas summarize surge height estimates made using the SLOSH (Sea, Lake, and Overland Surges from Hurricanes) model. The model was developed by Chester Jelesnianski of the National Oceanic and Atmospheric Administration, National Weather Service. The storm surge computations and analysis were done by the Storm Surge group of the National Hurricane Center, headed by Brian Jarvinen.
 

The hurricane storm surge inundation information has been developed as part of the Cape Canaveral Hurricane Evacuation Study covering Flagler, Volusia, Brevard, and Indian River Counties, and is based on the National Hurricane Center's Cape Canaveral Basin Model.
 

2. HOW THE MAPS WERE DEVELOPED
 

The SLOSH model was used to develop data for various combinations of hurricane strength, wind speed, and direction of movement. Hurricane strength was modeled by use of the central pressure (defined as the difference between the ambient sea level pressure and the minimum value in the storm's center), the storm eye size, and the radius of maximum winds (using the five categories of hurricane intensity as depicted in the Saffir-Simpson Hurricane Scale). The modeling for each hurricane category was done using the mid-range wind speed for that category. Ten storm track headings (WSW, W, WNW, NW, NNW, N, NNE, NE, ENE, E) were selected as being representative of storm behavior in the Central Florida region, based on observations by forecasters at the National Hurricane Center. Additional inputs into the model included depths of water offshore, and the heights of the terrain and barriers onshore (all measurements were made relative to mean sea level).
 

To determine surge values the SLOSH model uses a telescoping polar grid as its unit of analysis. Use of the grid configuration allows for individual calculations per grid square which is beneficial in two ways: (1) provides increased resolution of the storm surge at the coastline and inside the harbors, bays and rivers, while decreasing the resolution in the deep water where detail is not as important; and (2) allows economy in computation. The disadvantage associated with this telescoping grid pattern is the lack of resolution at each end of the model because of the increased size of the grids. Where the grid size is approximately 0.11 square miles near the center (Brevard County), the grids on the outer edges (Flagler County) contain approximately 8.47 square miles, consequently, the information produced in the outer grids has a decreased degree of reliability.
 

Once surge heights have been determined for the appropriate grids, the maximum surge heights are plotted by storm track and hurricane category. These plots of maximum surge heights for a given hurricane category and storm track are referred to as Maximum Envelopes of Water (MEOW)s. The surge inundation limits displayed on the maps in this atlas reflect a further compositing of the MEOWs into Maximums of the Maximums (MOM)s. The MOMs represent the maximum surge expected to occur at any given location, regardless of the storm track or direction of the hurricane. The only variable is the intensity of the hurricane represented by category strength (1-5). The MOM surge heights which were furnished by the National Hurricane Center, as displayed in this atlas, include an upward adjustment to reflect surges occurring during an astronomical high tide.
 

3. HOW TO USE THE MAPS
 

In order to determine the depth of surge flooding at a particular location, the ground elevation at that location must be known. At the inland extent of depicted surge inundation, water depths may be shallow, even for Category 5 storms. Time/History points have been included on the Atlases at selected locations, to define surge elevations for the Category 1-,3-, and 5- hurricanes in these areas. The depth of surge, for a given hurricane category, at these locations can be determined by deducting the known ground elevation (using local survey data, referenced to the National Geodetic Vertical Datum--NGVD) from the respective hurricane category surge elevation. United States Geological Survey Quadrangle Sheets, or other appropriate topographic reference which is based on the same datum can also be used to determine ground elevation at a specific location, but the accuracy of these elevations will be limited to the precisions and tolerances associated with that map.
 

Regarding interpretation of the data, it is important to understand that the configuration (narrow or wide) and depth (bathymetry) of the ocean bottom will have a bearing on surge and wave heights. A narrow shelf, or one that drops steeply from the shoreline and subsequently produces deep water in close proximity to the shoreline, tends to produce a lower surge but a higher and more powerful wave. Those regions which have a long, gently sloping shelf and shallower normal water depths, can expect a higher surge but smaller waves. The reason this occurs is because a surge in deeper water can be dispersed down and out away from the hurricane. However, once that surge reaches a shallow gently sloping shelf it can no longer be dispersed away from the hurricane, consequently water "piles-up"as it is driven ashore by the wind stresses of the hurricane.
 

Because waves "roll"toward shore, their height is also a function of water depth. A wave is cylindrical and rolls toward shore with its strength and size dependent on the velocity of the wind driving it, the length of time the wind has blown, and the distance the wind has driven the wave across the ocean surface. Once the wave nears the shore, where the depth of water decreases, it slowed by frictional drag against the bottom. As a result, the wave form steepens, becomes higher, leans forward, and finally breaks. A wave will break when it reaches water which is only slightly deeper than its (the wave's) height. Where water maintains a depth of 10 to 20 feet close to shore, a wave will only break when it has almost reached land, thus expending its energy directly against the shore.

Back to Atlas