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September 23, 2005

Katrina the ‘Super Model’

Nicole Hemsoth

The first detailed computer model of Hurricane Katrina's storm surge shows a gargantuan, 15-foot dome of water forming in the Gulf of Mexico. Propelled westward by 140-mph winds, the surge slams into levees east of New Orleans and pours over them, flooding a large inhabited area.

Later, the model shows water flooding most of the rest of New Orleans from Lake Pontchartrain through levee breaches. Meanwhile, Katrina's giant wave continues its relentless northeast course, pushing a 30-foot wave over the Biloxi-Gulfport area in Mississippi.

The computer model on a Louisiana State University supercomputer was done with a program called AdCirc employed by the Army Corps of Engineers, the Federal Emergency Management Agency and other agencies to simulate storm surge dynamics.

Researchers from LSU and the Corps will study the computer simulation to help reconstruct what happened. Particular attention will be paid to which areas were flooded when, how levee and drainage systems performed — or failed to. They also will look for clues on how to engineer the landscape for the next hurricane.

AdCirc, developed by researchers at five universities, the Corps and the U.S. Navy, is considered the state-of-the-art for storm surge modeling. It creates a virtual landscape, including detailed data on ocean, lake and river depths, ground and levee heights relative to sea level. Using basic fluid dynamic equations, it shows how a wind-driven storm surge forms and moves across open water, then rises up and changes shape as it impacts land.

The Katrina model is a work in progress, according to Joannes Westerink, a University of Notre Dame engineering professor who is overseeing the simulation. For example, the current version shows extensive flooding in some suburban areas that did not occur. The error occurred because of incorrect data on the floodwalls and other features in AdCirc's virtual landscape that are now being corrected.

The model dramatically illustrates how over 36 hours Katrina's enormous surge built up against the fragile Mississippi delta, overwhelming New Orleans from the east and via Lake Pontchartrain to the north, then barreled into the Mississippi Gulf Coast.

The simulation also indicates Katrina pushed a substantial storm surge up the Mississippi River, rising close to the top of river levees in the New Orleans area and even overtopping them at one point in Algiers.

To the east, as the storm itself moves closer to New Orleans, the model shows northeasterly winds pushing a wall of water directly against one hurricane levee and over the top, flooding St. Bernard Parish and New Orleans' Lower Ninth Ward. Photographs and surveys show that levee took heavy damage in the storm.

The model shows Katrina's surge rising above hurricane levees that run toward the center of New Orleans, flooding large portions of the city and suburbs to the east.

As Katrina moves toward Mississippi, the model shows a north wind pushing water in Lake Pontchartrain against levees protecting central New Orleans and into drainage canals, then flooding much of the rest of the city through canal breaches. Then, the simulation's wave rises up 30 feet as it hits the Mississippi Coast, where it caused massive destruction.

The model may not resolve a key question facing Corps investigators and other researchers: how did the floodwall breaches form? Corps officials reportedly said the canal floodwalls were probably overtopped and their structures eroded from the inside. LSU researchers have cast doubt on that scenario, in part because some data indicate water wasn't high enough.

The model — which does not render the landscape at scales of less than about 300 feet, about half the size of some breaches — does not show water in canals overtopping levees. But it does show surge heights near the canals may have reached 12 feet or more — which could have been enough to overtop levees or floodwalls, which are built to withstand a maximum surge of 11.5 feet.

An animation of the model can be found at www.nd.edu/~adcirc/katrina.htm