What is a 100-year level of risk reduction?

The 100-year level of risk reduction means reduced risk from a storm surge that has a 1% chance of occurring or being 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.

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). Supercomputers then calculated the conditions that would result from these 152 theoretical storms. This data allowed the Corps to estimate the amount of surge and waves that would be produced by various storms in the greater New Orleans area.  The surge and wave data was then used as the basis for determining the structural specifications (height, elevation, etc) required for the Hurricane and Storm Damage Risk Reduction System (HSDRRS) to provide the 100-year level of risk reduction.

A number of factors – in addition to surge and wave data – were considered when determining the elevation or height of the structures. 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 HSDRRS structures.

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.)?

Beginning in 1972, weather forecasters used the Saffir-Simpson Hurricane Scale to describe the strength of hurricanes. This scale categorized a hurricane according to its maximum wind speed at any given time (Category 1 - 5), and storm surges were predicted based on that information alone.

Later research proved that wind speed alone cannot reliably describe the storm surge generated by a hurricane. Storm surge and associated flooding are dependent on a combination of the storm’s intensity, size, motion and barometric pressure, as well as the depth of the near-shore waters and local topographical features.  As a result, storm surge can be significantly different than the ranges suggested in the original 1972 Saffir-Simpson Hurricane Scale.

For example, the very large Hurricane Ike (with hurricane force winds extending as much as 125 miles from the storm center) made landfall in Texas in 2008 as a Category 2 hurricane and had peak storm surges of about 20 feet.  In contrast, tiny Hurricane Charley (with hurricane force winds extending only 25 miles from the storm center) struck Florida in 2004 as a Category 4 hurricane, but produced a peak storm surge of only about 7 feet.  These storm surges were substantially outside of the ranges suggested in the original 1972 Saffir-Simpson Hurricane Scale.

As a result of these findings and experiences, hurricane risk reduction planning is now based on a more comprehensive view of the storm and its characteristics, including size, strength, motion and track, all of which have a significant impact on storm surge.  Knowing the category of a hurricane is important to help understand the potential wind impacts, but it is does not provide information regarding the storm surge potential associated with the hurricane.