U.S. Army Corps of Engineers Report Addresses Controversy over Hurricane Isaac Flooding

By Scott Madere

isaac tmo 2012241Some called Hurricane Isaac a test of New Orleans’ state-of-the art $14.5 billion flood control system. If so, the verdict is that it performed well for New Orleans. But those in flooded communities outside the 133 miles of levees, floodwalls and barriers now protecting the Crescent City are left questioning whether the new hurricane protection system shielded New Orleans at the expense of those outside the system. Did New Orleans’ Hurricane and Storm Damage Risk Reduction System (HSDRRS) actually contribute to the flooding of communities surrounding the Greater New Orleans Metro Area like LaPlace, Lafitte, Mandeville, Madisonville, Slidell and Braithwaite? This question may have been answered in a report from the U.S. Army Corps of Engineers presented to the Coastal Protection and Restoration Authority of Louisiana (CPRA) at its meeting on Nov. 28 in Baton Rouge.

New Orleans District Commander Col. Ed Fleming introduced the 300-page report (available on the USACE web site) by summarizing three of its major findings:

   1.  All hurricanes and tropical storms are unique, and comparisons from one storm to the next are difficult.

   2.  The impact of Hurricane Isaac would have been similar with or without the presence of HSDRRS.

   3.  The results of the report are consistent with modeling conducted before HSDRRS was put in place.

The report uses data collected from Hurricane Isaac’s surge and aftermath to project whether or not the presence of HSDRRS contributed to increased levels of flooding in certain communities. The report summary states that “model simulations showed that any changes of water level due to the 2012 100-year HSDRRS system are 0.4 feet or less at communities outside the system.”

The report says the old system “would have displaced about the same amount of water as the new system and the HSDRSS could not have significantly influenced inundation at communities external to the system.”

The report goes on to attribute the cause of flooding to communities like LaPlace, Lafitte, Braithwaite and others to “intense and long duration storm surge due to long duration of tropical force winds, which, in some cases were aggravated by extreme local rainfall.”

Fleming noted in his presentation to CPRA that the forward motion of Hurricane Isaac was “very slow,” at 1/3 the speed of Katrina. This led to 45 hours of tropical force winds in southeast Louisiana, and some parishes in southeast Louisiana experienced even more severe flooding from Isaac than they experienced from Hurricane Katrina in 2005. Isaac’s track, slow forward motion, large maximum wind radius and high levels of rainfall caused it to be a much more dangerous storm than many expected from its Category 1 status on the Saffir-Simpson Hurricane Wind Scale. As a result, the Corps report on Isaac’s impact clearly states that the Saffir-Simpson Hurricane Wind Scale “should not be used as the sole predictor of inundation risk” for hurricanes and tropical storms.

82337-laplace-la-d4bd0Echoing the report was a point of action by CPRA later in the meeting, suggesting that the time has come to rethink how we predict the destructive potential of hurricanes and tropical storms. CPRA voted to approve a letter to the National Oceanic and Atmospheric Administration (NOAA) asking for alternate methods to accurately evaluate a hurricane’s speed, rainfall, size and other characteristics, which can convey a better idea of a storm’s combined magnitude. With a better evaluation system in place, authorities can more properly communicate to the public the risk associated with storms as well as encourage pre-storm preparations and evacuation.

CRCL agrees that the time has come to end our reliance on wind speed as the primary estimator of hurricane’s destructive impact on our coast. Tropical storms and hurricanes are complex systems, with damage occurring from not only wind, but storm surge, flooding and other events associated with natural disasters. We need to develop a more sophisticated way of communicating and understanding the threat of a hurricane or tropical storm. This means authorities should continue efforts to describe the destructive nature of these storms in terms of surge potential, size and location of impact, in addition to wind speed.

The big lesson we should learn as coastal citizens is to see more than a simple category rating when making our own evaluations about storms. We must also realize that our personal experiences with various categories of storms cannot translate from one storm to another. All storms are unique. As Isaac proved to many communities in southeast Louisiana, a category 1 storm can be just as or more destructive than a category 3. As a coastal population, we must strive to view each storm in its entirety, a unique set of conditions which can vary greatly but still combine for high levels of damage to our coast and communities.    

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30 November 2012, 19:00
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