When Floodgates Fail
To Hold Back Water

For more than a century, the low-lying city of New Orleans has depended on a sprawling network of earthen levees to protect it from flooding, not only from heavy rains but also from massive Lake Pontchartrain and the mighty Mississippi River.

Hurricane Katrina is testing that system -- built to sustain up to a category-three hurricane -- as never before. Katrina made landfall Monday as a powerful category-four storm.

Part of at least one levee -- along New Orleans's 17th Street Canal -- has failed, and the central city was filling with water. Despite the bleak picture, J. David Rogers, chairman of geological engineering at the University of Missouri-Rolla, said it appears New Orleans has so far avoided a "catastrophic" failure of the levee system, which he said is one so large that it would have filled the city with water much more quickly.

New Orleans Mayor Ray Nagin said yesterday that roughly 80% of the city was under water, in some parts as deep as 20 feet. Col. Jeff Smith of the Louisiana National Guard said at a news conference in Baton Rouge that officials are considering a plan to lift large shipping containers into the 17th Street Canal levee breach to temporarily halt the flow of water and buy rescuers some time. The repair attempt late yesterday failed. A second rupture, on what is known as the Industrial Canal, is apparently less of a concern.

Pumps and canals were sending some water from the city back into the adjoining lake, but water continued to rise in many sections of the city yesterday. Other local officials said water had receded from some areas of Kenner and Metairie, two neighborhoods west of New Orleans that were extensively flooded Monday.

Meanwhile, hundreds of residents may have remained stranded on rooftops and in attics in the heavily submerged and mostly low-income Ninth Ward area.

New Orleans is a shallow saucer that sits mostly below sea level, sandwiched between enormous Lake Pontchartrain to the north and the Mississippi to the south. The shallow, 40-mile-wide lake is more than twice the city's size. The Gulf of Mexico is roughly 50 miles away. As a result, the city is extremely vulnerable to flooding and depends on an aging system comprising 125 miles of levees, along with drainage canals and pumping stations, to push water back into the lake. In the flood-control business, New Orleans is known as The Big Pumper, rather than The Big Easy.

Levees typically began as natural structures, created by the silt deposited by large rivers like the Mississippi when they periodically overflow their banks. The formal levee system in New Orleans dates to about 1890, when the Orleans Levee District was created. But especially after the catastrophic Mississippi River flood of 1927, most levees in the U.S. were fortified to make them stronger.

Levees may resemble earthen dams, but are not as strong. Unlike dams, which are considered "permanent" structures, many levees are engineered to withstand strong flood pressure for only a few days at a time, said Mr. Rogers. That could call into question the durability of the levee running the full length of the northern edge of New Orleans, where the waters of Lake Pontchartrain are lapping near the top.

Levees built by the federal Army Corps of Engineers, which took on a major flood-control role after the 1927 deluge, are typically engineered to high standards and are quite strong. Most levees along the main Mississippi River channel are federally constructed. But in some areas, including around Lake Pontchartrain, some levees were built privately or by local governments and may not have the same degree of engineering.

"Those are the kind that break," said Mr. Rogers, noting that in a 1993 Mississippi River flood in the Midwest, only 17 of 79 levees that failed were federally built.

It is rare for floodwaters to "overtop" levees, as they usually succumb first to "under seepage," in which water seeps underneath from the river side and emerges on the land side. Sometimes the severe pressure causes soil within the levee to liquefy in a phenomenon known as "sand boils," which often precede structural failure.

Improvements to New Orleans's levee system have been somewhat piecemeal because such work is very expensive. For example, designs have been completed to reinforce the pumping stations with walls to prevent the backflow of water into the city during heavy storms. But as of less than a year ago, just one of three major drainage canals had received such reinforcements. Levee district officials couldn't be reached yesterday to provide an update.

One bit of good news is that New Orleans doesn't appear to face a threat of being surrounded by even more water than it has outside its levees right now. By yesterday, the water level in the Mississippi had dropped about 11 feet since Monday, as the storm surge that had pushed upriver from the Gulf of Mexico, temporarily reversing the river's course, receded. Yesterday, the water level in the stretch of river that runs through the city was down to a level of about 4.28 feet, well below the flood level of 17 feet and low even under normal circumstances, hydrology experts said.

While the rain pummeled the Gulf region and continued to fall as Katrina cut its way through Tennessee, Kentucky, and Ohio yesterday, it isn't likely to swell the river's water levels much. While the Mississippi drains water from about 41% of the U.S., stretching from the Appalachians to the Continental Divide, it is so large that only massive, extended precipitation produces swells at its delta, and usually they aren't very big. Major flooding along the upper Mississippi in 1993 produced only a small rise in the river in southern Louisiana, as most of the extra water was absorbed by the river along the way, hydrologists say.

The rain from Katrina "is going to be a very small amount compared to what the Mississippi can carry," said Frank Richards, a hydrometeorologist with the National Weather Service in Silver Spring, Md.

The rain that Katrina dumped on Louisiana and Mississippi on Monday won't affect the river's water levels because many of the rivers in the area flow directly into the Gulf. Rain that Katrina deposits in Tennessee and farther north isn't likely to reach New Orleans for at least two weeks, Mr. Richards said.

Parts of Kentucky, Ohio, and other areas where Katrina was headed have been unusually dry, meaning less runoff from the storm, Mr. Richards said.

Likewise, the water in Lake Pontchartrain isn't likely to rise, said Richard Keim, assistant professor at the School of Renewable Natural Resources at Louisiana State University in Baton Rouge. The fresh water source that feeds it is only about as large as the lake itself, so while much rain has fallen in the area, not much will end up in the lake, says Mr. Keim. The larger question is when the storm surge will recede into the Gulf, the lake's other water source. Lake Pontchartrain has only two narrow, winding outlets to the Gulf, so it is unclear how long it will take them to empty.

-- Kris Hudson contributed to this article.

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