Wednesday, August 31, 2005
New Orleans: Facing The Inevitable
The local authorities will surely point fingers at the feds. Part of this situation is the human inability to understand how such a situation can happen so suddenly. But people will have to overcome their amazement and concentrate on the priority of relocating people into functioning communities. I am not suggesting stopping trying to fix the problem; I am just pointing out that it will be a long time before the city can be stabilized and the process of restoring water and utilities can even begin.
The reason why I say this is that a levee break like the 17th Street Canal breach is not easy to repair. It appears that the levee was scoured by overflow. That means that as water came over the top, it dug out a pit at the foot of the wall on the dry side. Eventually the pit undermined the levee and it collapsed. The talk of simply dumping sandbags in there is fiction, and the break is expanding. Yesterday it was reported as 200 feet; today it has been reported as 300 feet. As the water flows through the gap, it is digging away at the foundations of the levee on each side of the break and undermining additional sections.
The tidal flow will keep sloshing more water in and out of there, so this is not going to be simple. Lest I be accused of a conspiracy against New Orleans, here are some articles explaining what I am saying. The best is this SFgate article which explains a lot about how levee breaks in other places have been fixed and what is required. Access is the most important factor, and the 17th Street levee is not very accessible to the type of barges or trucks required to bring in the material required. Here's another article explaining what is known about the 17th Street Canal break.
Several important points are the tidal flow and the lack of accessibility. The first priority is to shore up the ends. You don't have to use boulders. You could use manufactured steel and concrete caissons, but you have to have some way to get them there. As the SFgate article points out, the usual way to do this is to use barges or build an access road. Dropping huge and heavy stuff in from the air requires specialized helicopters. Nor can you carefully place the shoring materials this way. If they try to bombard this thing from the air they may well end up knocking down more of the levee. And just dropping materials in the middle won't work either. Even if you can get heavy enough materials in the middle to hold, all that does is divert the water toward the ends of the break which expands the breach.
They can try to sandbag the ends of the levee breach to stop the expansion, but it is going to be very hard to prevent more scouring as long as much water is running through the gap. That scouring will tend to undermine the piles of sandbags as it digs pits around the edges. You are working against time so the lack of access makes this a much harder problem.They may have to block the water flow in the canal itself to slow this down enough to repair the breach. It may well be a few weeks until they even have a workable plan. I am sure they will keep trying, but it is not a simple matter.
A couple of quotes from the articles. The State:
Until engineers can repair breaks in the huge levees that separate New Orleans from Lake Pontchartrain, the city will essentially be an arm of the Gulf of Mexico, subject to the ebb and flow of the tides.and:
And because the tidal pull weakens the levees and widens the breaks, experts said Tuesday, that will make it all the more difficult to repair them.
• One challenge was that the narrow canal is not accessible by barge, in part because a newly built low bridge and hurricane barrier sits 700 feet down the canal toward the lake end.SFgate:
“We can’t get at it,” he said.
• Another problem is that whatever is done to block the breach also must not block the canal itself, because that would impede the pumping of the floodwaters.
Engineers who have patched broken barriers in California say it could take weeks -- even months -- before the levees damaged from Hurricane Katrina are repaired and for floodwaters to be pumped out.and:
Maurice Roos, chief hydrologist for the state Department of Water Resources, said it usually took two to four weeks to fill large levee breaks and stop the flow of water, but that's just part of the problem.Once they even get close to repairing the 17th Street break there are other breaks. Under the best of circumstances the city of New Orleans continuously pumps water out of its lowlying areas, so this flooding is not likely to improve. During the period of fixing the levees, which may well take months, any severe storms may worsen the situation quite dramatically.
"The question is how fast can you drain the area that's flooded,'' Roos said. "The longer the water is there, the worse the damage.''
Nationally we must turn our attention to the hundreds of thousands of people who have been uprooted. They need places to live with schools, hospitals and municipal services.
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