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Hurricane & Storm Damage Risk Reduction |
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100-Year Level of Protection
What is a 100-year level of protection?
One hundred year level of protection actually means reducing risk from a storm surge that has a 1% chance of being equaled or exceeded in any given year.
The 1% chance is based on the combined chances of a storm of a certain size and intensity (pressure) following a certain track. Different combinations of size, intensity and track can result in a 100-year surge event.
View the 100-Year Design Map >>
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How was the 100-year surge event calculated?
Experts used computers to generate models of 152 different hurricanes with a wide variety of paths, forward speeds, rainfall volumes, intensities, and physical size (radius). Powerful supercomputers calculated the conditions that would result from these theoretical storms. These data allowed the Corps to estimate the amount of surge and waves that would be produced by various storms and use this information as the basis for determining the structural specifications required for the Hurricane and Storm Damage Risk Reduction System (HSDRRS) to provide a 100-year level of protection.
The elevation or height of the structures being designed and built considered a number of other factors besides the surge and wave levels. For example, expected sea level rise, settlement and subsidence of structures, and possible increases in storm severity or frequencies were all factored in to the final design of the structures. |
![IPET Models](https://webarchive.library.unt.edu/web/20121020104809im_/http://www.mvn.usace.army.mil/hps/images/IPETmodelstest.jpg)
IPET model storm tracks |
Why does the Corps describe the Hurricane and Storm Damage Risk Reduction System (HSDRRS) in terms of 100-year levels rather than in terms of hurricane category (1, 2, etc.)?
Weather forecasters typically use the Saffir-Simpson scale to describe hurricanes. The Saffir-Simpson scale labels a hurricane according to its wind speed at any given time (Category 1, 2, etc.) and predicts storm surge based on that alone.
Over the last quarter-century, hurricane surge has been assumed to be primarily function of maximum storm wind speed. More recent research has shown, however, that wind speed alone cannot reliably describe surge. For example:
Saffir-Simpson Scale
Category |
Sustained Winds |
Storm Surge |
1 |
74 - 95 mph |
4 - 5 ft |
2 |
96 - 110 mph |
6 - 8 ft |
3 |
111 - 130 mph |
9 - 12 ft |
4 |
131 - 155 mph |
13 - 18 ft |
5 |
155+ |
18+ ft |
- According to the Saffir-Simpson Scale, Hurricane Katrina was a Category 3 storm at landfall. Category 3 storms are described on this scale as generating storm surge 9-12 feet above normal, yet Katrina generated nearly 20 feet of surge.
- Hurricane Katrina produced 5 more feet of storm surge than did Hurricane Camille, which was a Category 5 storm at landfall according to Saffir-Simpson.
As a result of these findings, hurricane risk reduction planning is now based on a more comprehensive view of the storm and its characteristics, including size, strength, and track, all of which have a significant impact on storm surge. Knowing the category of a hurricane is important to understand how dangerous it is – especially to know how much wind damage it can cause. Knowing the category alone is not enough information to tell us the threat we have from the dangerous storm surge hurricanes produce. The larger of two hurricanes of equal intensity has more storm surge potential.
Contact
Webmaster
504-862-1606
webmaster-mvn@usace.army.mil
Updated 8/29/2011
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