Is New Orleans destined to become the new American Venice, with permanently flooded streets?
That's one possible future for the devastated city, according to a Alan Zundel, a Brigham Young University civil engineer who has modeled the New Orleans situation.
Last week, Lake Pontchartrain spilled into the Crescent City when levees failed under storm surges generated by Hurricane Katrina. The city suffered immense damage, while estimates of the dead now number in the thousands.
But disaster may have been inevitable sooner or later, according to experts interviewed by the Deseret Morning News. New Orleans is below sea level and has been settling since long before the hurricane.
"Now the decision needs to be made as to where they rebuild," said Zundel, associate professor of civil engineering at BYU.
Should the city be rebuilt where it is? Should structures be on pontoons or fill? Should New Orleans be moved to the north, where it would be less vulnerable?
One possibility planners may consider, he said, is whether New Orleans should be turned into an American Venice "with canals everywhere."
According to Zundel, engineers in the New Orleans area have known for some time that if a Category 3 or larger hurricane "were to hit the city in any type of direct fashion, there would be wide-scale problems."
The main difficulty, he said, is that "the city is below sea level and settling very rapidly. . . . They don't even do surveys anymore (tracking the city's downward motion); they just install monitoring stations."
If a survey were carried out, it would be out of date in a year, he added.
"The whole city is on the Mississippi River delta," which is made of loose, poorly consolidated soil washed down the great river. But with levees to hold back the water, the river doesn't flow there anymore and no new soil is deposited on that part of the delta.
Soil under the city "continues to consolidate and settle." New Orleans is sinking as the loose material settles. New Orleans is now one of the few places in the United States, like Death Valley, that are below sea level.
"It keeps getting lower and lower, and the problem gets worse and worse," he said.
Normally, huge levees hold back the water from Lake Pontchartrain and the Gulf of Mexico. But if a storm should cause either to overtop a levee, "then the city becomes a swimming pool," Zundel said.
"The levees are holding the water in. . . . And that's even if the levees hold." With levee failure, the city fills with water. The water, sludge and chemicals remain until the levees can be patched and the mess can be pumped out.
New Orleans has been there for 200 years, and the levee system was developed over time. Changing the levees would be a challenging project.
In Utah, levees are like the dikes along the Provo River in Heber Valley, or beside a reservoir — relatively small structures. "But along the Mississippi River, their levees are potentially 100 feet high and a quarter of a mile across. They're massive structures," Zundel said.
Levees are "very successful when floods are small in scale or moderate in scale," said Marek Matyjasik, associate professor of geosciences at Weber State University.
But in extraordinary events like huge hurricanes, he said, "the levees are more likely to fail, because they're not designed to sustain extreme water pressure."
He stressed that he does not know any of the details about the New Orleans levee failure, other than news reports. But sometimes, he said, levees fail because they are overtopped with water, causing them to erode, or because uncontrollable leaks burrow through the levee material.
When a levee starts to leak, it can create "an artificial river inside the levee," he said. "Then it's almost impossible to stop."
The flow can happen even if there's water on both sides, when water pressure is higher on one side. Water eats through the levee's fill in a process called piping, and as it continues the opening grows. Material inside the levee, often sand or other fill, erodes more swiftly, he said.
The process is hard to monitor, according to Matyjasik. "We don't see the first symptoms on the surface, because the damage is more likely to be created somewhere at the base of the levee.
"As the process continues the flow velocity increases, and it's more difficult to stop it."
Eventually the levee can fail. Sometimes water knocks over or rotates a section. At times, a section of levee can slide away, he said.
"The sea surge of 20 or 25 feet creates enormous pressure. . . . What I heard in the news is those levees were not necessarily designed to sustain a hurricane of Category 5."
Tom Cova, associate professor of geography at the University of Utah, noted that New Orleans has levees both along the gulf and Lake Pontchartrain, and the levee broke.
"They've always had pumping stations because they have to deal with this (flooding) after heavy rain," he said. "They have a huge, sophisticated pumping situation."
This time, of course, the lake poured into the city and pumps were helpless. As soon as the breach is patched, he said, "they can pump the streets dry."
But the Army Corps of Engineers needs to plug the holes somehow, he said in an interview this past week.
Meanwhile, bodies floating in New Orleans' streets pose public health threats. Officials are "worried about cholera and typhoid," Cova said.
New Orleans isn't the only city that was built in a dangerous zone, he noted. "We have development in hazard areas all around the country." Utah's Wasatch Fault, which runs through Salt Lake City, is likely to experience a disaster because of a large earthquake, he said.
Everyone knew that someday a hurricane would score a direct hit on New Orleans, he said. "Someday the Wasatch Fault is going to have an earthquake that kills thousands of people. . . . If it happens in a hundred years, there will be that many more millions of people living here."