Scientists working for the U.S. Army Corps of Engineers have taken many of the lessons learned during their exhaustive analysis of the impact of Hurricane Katrina on the New Orleans flood defenses and have created a system for calculating the probability of storm flooding, almost on a lot-by-lot basis, in each drainage basin of the city.

The Corps plans to use the tool to help citizens, planners and policy makers understand the risks present in various locations, and identify areas where improvements to the flood defenses will have the greatest risk-reduction benefits.

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  • Powerful New Tool Is Key To Reducing Risk
  • Although it is now optimized for New Orleans and storm surge hazards, the same approach could be applied to other locations with different hazard mitigation infrastructure systems. The process scientifically weighs the probability that a hazardous situation will develop, against the reliability of the performance of the protective infrastructure against the load. Ultimately, the process factors in the consequences of failure to develop a quantified measure of risk. If engineers find a way to increase the reliability of the protective system in any given area, the calculated risk to life and property in that location will be driven down.

    It is important to take the final step of evaluating the risk-reduction potential of potential projects. In lightly populated areas, decreasing the chances of flooding by improving levees, for instance, may not actually do much to reduce risk, since there are few lives and little improved property at stake. But in a dense urban area, the same investment in improvements could have a dramatic impact on economic health and public safety.

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    Storm Surge and Hurricane Categories -- The new research defines the way storm surge intensity is correlated to hurricane wind speed and atmospheric pressure, and the diameter of the storm. According to Bruce Ebersole, the hurricane surge and wave analysis co-leader on IPET, Hurricane Camille, a Category 5 hurricane with extremely low pressure and high wind speeds but a relatively small, 25-mile diameter, had a 24.6 ft (7.49-meter) storm surge. Katrina, with less intense Category 3 winds at landfall, nevertheless was twice the diameter and roared ashore with a 28 ft (8.53 meter) surge, based on the maximum reliable high water marks found. (Image: Army Corps of Engineers)
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    Strike Zone�By modeling 152 possible storms of a wide range of destructive potential and path, including those in the historical record, researchers have concluded that because of many factors, including high altitude wind patterns, water temperature and sea bottom characteristics, the central Gulf coast has a significantly greater likelihood of being struck by major hurricanes than other areas of the Gulf. (Image: Army Corps of Engineers)
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    The Odds�Risk is often related to the probability of a particular event happening in a given year. You may have a 10% chance, or 1-in-10 chance of catching the flu in a given year, but a 1-in-100, or 1% chance of having your car stolen in the same period. The Corps' current risk mitigation efforts to prevent storm flooding in New Orleans are aimed at reducing the probability of storm flooding in the city in any given year to the 1-in-100, or 1% level. (Image: Army Corps of Engineers)
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    A Mixed System�New Orleans� system of levees, gates and floodwalls have many elevations, based upon perceived threat at various locations. Levees on the Mississippi River banks are highest, while those on the Gulf Intracoastal Waterway and shore of Lake Pontchartrain are significantly lower.

    After intensive study and modeling of storms and storm surge in the wake of Katrina, scientists have now reevaluated the perceived threats at various locations, and also have calculated the likely performance of the collection structures and pumping facilities that make up the system under all storm surge scenarios. They now feel they know how to calculate the probability that storm surge and overtopping will cause flooding in various basins of the city, and have the tools to help systematically target improvements to reduce the probability of flooding in any given area. (Image: Army Corps of Engineers)

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    Pre-K Flood Risk�With the system as it existed before Hurricane Katrina, using the new methods, scientists calculate the basin of the Gentilly Neighborhood had a 1% chance of flooding in any given year, much of it to a depth of 8 ft or more. (Image: Army Corps of Engineers)
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    Risk Reduction�In that 1% chance event, the calculated depth is now half a foot less than before Katrina, leading to a reduction in the estimated risk of property losses of 4.6% and, in a situation where there were no evacuations, an 89% decrease in estimated risk of fatalities. (Image: Army Corps of Engineers)



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    Lakeview�A similar 1%-chance-of-flood risk map of the Lakeview basin as it was prior to Katrina shows near total, deep inundation. (Image: Army Corps of Engineers)

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    Lakeview Today�Storm gates and floodwall improvements since the storm have reduced flood probability more substantially in Lakeview than Gentilly, the scientists calculate. One reason given for the disparity between the two neighborhoods is that the defenses on the east side of Gentilly still have substantial vulnerability to overtopping or structural failure along the Inner Harbor Navigational Canal. If engineers strengthen defenses there, the calculated flood exposure will go down. (Image: Army Corps of Engineers)



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    The Gains�Lakeview�s risk reduction reflects the elimination of vulnerabilities since Hurricane Katrina, mostly through the construction of temporary storm gates that will close off the outfall canals that bracket the neighborhood, in the event of threatening storms. (Image: Army Corps of Engineers)