HEC 25 - Tidal Hydrology, Hydraulics, and Scour at Bridges

Appendix C Hurricane Frequency

The frequency of hurricanes varies significantly along the Atlantic coastline. To assess the frequency of different hurricane categories, the coastline was divided into 50 nautical mile segments and the number of hurricane strikes (determined by the hurricane eye) were recorded. The maps seen in Figures C.1 - C.3 show approximate return periods along the East Coast for category 1 or greater, category 2 or greater, and category 3 or greater hurricanes, respectively. The number in parentheses is the landfall occurrences of hurricane eye for each 50 nautical mile segment during the 113-year period from 1886 to 1999. The return period was calculated by taking the total number of occurrences and dividing it by the 113 years of record. The maps only indicate the return frequency of the storm eye for each window. The maps do not take into account other factors such as the size of the storm, storm asymmetry, or whether the hurricane was land falling or land exiting. Along shore hurricanes are not included.

The maps show that the majority of hurricanes make landfall along the Florida and North Carolina coasts. The strongest hurricanes (category 3 or greater) are most frequent in south Florida and the Outer Banks of North Carolina. Category 3 storms are rare north of Virginia and the eye of a storm greater than category 3 has never made landfall north of Wilmington, North Carolina.

The return periods reflect the frequency with which the eye of a storm passes through a particular coastline segment. The storm eye is easy to track but is only one aspect of a storm that must be considered when determining its impact. The storm (and storm surge) may extend 50 or more miles from the center. The most intense winds of a hurricane are on the right side of the storm. Therefore, a storm that is heading west and for which the eye makes landfall near Miami may have a significant impact on the coast 50 to 100 miles to the north. For a storm that is heading north and makes landfall at the Outer Banks of North Carolina, the most intense winds would be east of the eye and over open water, reducing the impact on coastal areas. Factors such as these should be considered when using the maps. The correlation of occurrence and hurricane category is not a design tool but should be used as an indicator of the frequency with which one may expect the potential damage, flooding, and wind conditions in an area of interest.

Figure C.1. Return period of Category 1 or greater hurricanes along the Atlantic coast.

Figure C.2. Return period of Category 2 or greater hurricanes along the Atlantic coast.

Figure C.3. Return period of Category 3 or greater hurricanes along the Atlantic coast.

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