Dammit Jim, I'm a farmer not a hydrologist!
But I know why levees break.
Blew holes.
Here's how it works, and what may well happen to New Orleans, below the fold:
I live and farm in a creek bottom a half mile from the Missouri River. In the Great Flood of 1993 water was all we could we see in three directions for over 15 weeks and for all of that time the only way in or out of my place was in a canoe. Except in a very few instances, the big river levees were not overtopped. They blew out. One of them is a few hundred yards from one corner of our farm. I'll never forget the night it blew. It sounded like a very large bomb, except that it lasted for about a full minute, followed by peculiar, loud cracking sounds. The cracking sounds, we learned at daybreak, were giant trees snapping like twigs -- some of them hundreds of years old and over 25 feet in circumference. When the cracking sounds ceased we could hear a sound like a 12 lane freeway as if one were standing about two blocks away, only there are no paved roads in that direction for many miles. That was the water rampaging through the newly blown hole in the levee.
Water always moves. When it comes up against an earthen barrier its movement shifts from straight on or alongside the levee and is transformed into a swirling motion. Any variation in the levee -- a dip, a swerve, a slight indentation -- catches the swirling currents far below the top of the levee itself. The water swirls down there like a giant very efficient drill, weakening the levee from within. When it goes, it can do so quite suddenly; it "blows" a hole in itself and thus the name "blew holes," sometimes called "scour holes." These weaknesses are dastardly hard to identify before the water finds them first and by then it's way too late to do much about it.
Once the water on the high side starts rushing into the low side, there isn't anything any people, machines, or technology can do about it at the point of the breach until the two sides reach equilibrium. The little 12 foot tall creek levee that blew near our place was about 8 feet across at the moment of the break. The next day it was forty yards across, and the water poured through it with the force of the Colorado River in the Grand Canyon. The day after that it was a hundred yards across pouring through with such force that we couldn't go "upstream" through it in a boat with twin-80hp Mercury outboards. You don't stop water that is moving like that. Once the low side is near the level of the high side, the velocity lessens and then, and only then, can repairs begin. In our case, as long as the Missouri River was up past flood stage, there was a nearly inexhaustible supply of water on the high side. There was nothing we or anyone could do save to watch the fields fill with water, get our tetanus shots and try to enjoy all the hundreds of great blue herons who took up residence here circling overhead like pterodactyls and feasting on the millions of fish swimming around in our hayfields, now 14 feet underwater.
I know nothing of the particulars of Lake Ponchartrain, nor can I guess when the breaches there will reach equilibrium. Unless they can break a hole that would drain more water in a different direction, water will continue to pour through that levee break into the city no matter if the pumps are working or not and regardless of how many 14,000 pound sandbags they can drop on top of it.
I can't speak to the political or economic machinations that caused New Orleans to cease its levee strengthening programs before they were completed. But 12 years ago I learned firsthand how levees work and how they fail, and I suspect that huge break near New Orleans happened similarly to what I described above. If Bush is responsible for redirecting money to his so-called war on terror that could have saved that levee, then he is responsible for the months and years of pain and misery wrought on Louisiana in this immediate aftermath of this most terrible storm, too.
When the water finally does quit flowing in, and after they are able to repair the breach and pump it all back out, and after they clear the millions of tons of mud, silt, and debris, they will find many of the wooden structures there thoroughly compromised and fit only for demolition.
Worst will be many of the oldest buildings built on old and crumbly stone or brick and mortar foundations. The old mortar will be saturated and mealy and in some instances washed out completely, leaving the foundations weakened beyond repair in many cases. That's what happened here. The flooded old buildings that were restorable after the flood were the oldest of the old, many barns and a few homes, that were built entirely of wood and set on stone pillars. Some of those "floated" a bit and could be jacked or moved back into place, but they survived. Most of the "newer" mortared brick and stone foundation buildings, save for some of the double walled cinder block commercial buildings, didn't make it.
Lots of things were different about our great flood. Except for a few catastrophic levee breaks that washed away whole buildings in a few seconds, most of us watched the water rise over many days and weeks. Some days it would creep up only a few inches, some days it would even drop a little, and other days it would rise a foot or two, just fast enough to watch it creep up the blacktop on the road where we banked our canoe each morning. But it took a long time; time enough for those whose homes and business were in harm's way to move their stuff to higher ground and watch in slow motion horror as their homes and farms were inundated and their lives were changed forever.
It's different in New Orleans. People are watching the water rise by several feet per day. And the mucky, smelly backwater that flooded us here in the heartland was nothing compared to what will flush itself into the poisonous soup filling a major city like New Orleans.
Life here returned to mostly normal a few years after the great floods of '93 and the nearly as great flood of '95. Bottomland farmers hired huge Cat tractors with 7 foot plowshares to turn under the six to 12 feet of sand that topped and nearly ruined the millions of acres of the world's most fertile soil. Today, the corn and beans and wheat are back where they were before in most places. I don't mean to underestimate the toil and expense and hardship we endured, but it will pale in comparison to rebuilding a major metropolitan area.
I love New Orleans. I've been there quite a few times and know people who grew up there and few who still lived there until this week. I fear it will never be the same again. Ever.
And that's just New Orleans. We are just now beginning to grasp the scope of the devastation in Mississippi. All this while our president golfs and plays guitar and pats himself on the back for the great strides for what he calls democracy, but smells like civil war, is making in Iraq. We must remember this and remind our friends and neighbors of it when he finally stands on some muddy rubble and tries to take credit for calling us together as a nation to mourn the loss of our Big Easy.