I guess the title just about says it all. What exactly were the forces that caused these levees to break?
(My apologies if this has been addressed somewhere in one of the other gazillion New Orleans threads; it’s easy to miss things in all the discussion.)
Water. Lots and lots and lots and lots and lots and lots and lots and lots of water.
I think. I could be wrong.
You need to add: moving swiftly in a direction 90 degrees to the vertical w/respect to the levees.
Well, you don’t really need to. One apocalyptic buttload of water just sitting there would probably do the trick.
The fact that the water was being delivered by hurricane probably had something to do with it.
AFAIk, the levees were designed to withstand a cat 3 storm and as it was a cat 4 that hit it wasn’t really a surprise that the town was flooded.
Dr Jeff Masters at Weather Underground explains:
So is it just the amount of extra rain associated with a big hurricane, or do the winds themselves damage the levee’s superstructure.
I followed the floodwall breaches pretty closely, and I don’t recall there being an actual overtopping. The floodwall was holding, then a section broke open. Whether this is what actually happened is easy to determine after the fact, because overtopping would happen where the floodwall top is lowest, whereas a breach could happen anywhere. I do know that the water was clearly flowing through a discrete length of the wall and was held back at areas immediately adjacent to the breach. That is very suggestive of a failed wall, not an overtopping.
President Bush said, “I don’t think anyone anticipated the levees being breached.” He may be technically right, if you distinguish a breach from an overtopping. I am not up on the conventions of floodwall design, but it seems to me that the wall should have stood up to any level of water that didn’t overtop it. There’s no structural reason to build a wall higher than the level of water it’s expected to resist (adding height doesn’t improve the strength of the lower part.)
I think there are going to be some very heated recriminations over the floodwall breaches. If it was a case of overtopping, then the only criticism that could be made is that the wall wasn’t designed high enough. That was a known issue. But if it actually failed, then you start to question the structural engineering, workmanship and maintenance. Plus, you have to look at the safety of every other floodwall of similar design. Also, it seems that part of what contributed to the present chaos is that the day of the storm, it appeared as though New Orleans would not be flooded, because the water was lower than the wall, which resulted in certain decisions being made about where to send aid and rescue teams. Then, when it did flood, the authorities had to change direction and take care of New Orleans. (There are apparently many bigger mistakes the authorities have made, but this is still a factor.)
By the way, I’m not surprised the Army Corps of Engineers is characterizing the failure as an overtopping. They’re the ones who built the floodwalls, so if there was in fact a breach, it’s going to be their nuts on the block.
IANAEngineer but I think there would be more to it than that the wall must merely hold back calm water of any level that does not overtop it. I am willing to bet the flood wall would do that just fine. However, when you add in the pummeling it likely takes from waves and winds driven by a powerful hurricane not to mention the dynamics of fast moving currents perhaps digging at lower parts of the wall it is a whole new ballgame.
I do not understand the bit in the quote earlier in the thread where the expert says the water overtopped the wall which then undermined it. Huh? Water going over the top eats at the bottom? I’m missing something there.
I also do not understand building the wall for only a Category 3 hurricane. So they did an analysis that showed a Category 4 hits “only” every 70 years or roughly once in everyone who lives there lifetime. They know the city is sinking and below sea level. They know that this sinking city is practically surrounded by water higher than the city is. They know that Mother Nature throws a curve ball at us often enough to not take her for granted. Despite all of that they build a system insufficient for the tsak at hand? I understand there are cost considerations…cars could be built more like tanks and be safer for passengers but at some point the cost becomes prohibitive versus the risks. Yet in the case of New Orleans the cost savings for building a less capable flood wall/levee system almost certainly pales in comparison to the damage caused to the city. Add to that the economic repercussions the whole country feels from this. Add to that the current $10.5 billion in an aid package Congress is putting through. And last but certainly not least is the human factor in loss of life, pain, misery and suffering to which a cost can’t really be applied.
So with all of that was saving even a few billion dollars when building the wall really such a smart choice? Honestly I have no idea what ir costs to build those walls and at 325 miles it can’t be cheap but I’d be shocked if the cost is higher than whatever the final toll is from this disaster. Some may say it is only with the benefit of hindsight we can say that but I think looking at it all it is reasonable to see that something like this would happen someday (as mentioned generally at least once in everyone’s lifetime [assuming an “average” lifespan]).
I believe I read somewhere that the levees in question are earthen dams, perhaps with some concrete reinforcement. Therefore overtopping would cause the walls to erode, weakening them until they breached.
The Scientific American article linked above is very informative. Note its subtitle, “A major hurricane could swamp New Orleans under 20 feet of water, killing thousands. Human activities along the Mississippi River have dramatically increased the risk, and now only massive reengineering of southeastern Louisiana can save the city”, was eerily prescient.
During the flood of 1993, the problems around St. Louis were mostly caused by breaching, not overtopping. And that was a “calm” flood – just rising water, not strom driven.
According to the New Orleans Times-Picayune, the US Army Corps of Engineers knew these levee breaks would eventually happen. They had plans to shore them up years ago, but the funding was cut by 80%. It never got done.
A hurricane creates something called a storm surge, or ground swell. The center of the storm pushes down on the water, forcing a huge wrinkle of water away from itself. Usually, we see this on the sea shore, but this time it happened on the lake.
The undermining happens on the dry side of the levee when the overtopping water washes away the earth at the base of the levee. That weakens the whole structure.
The San Jose Mercury News published a nice graphic on how the Canal walls were breached, on Sept. 1. However the site mercurynews.com requires registration, and it may be that the graphics won’t be on the site, so I’ll just give a description.
| |
..| |..
/ |____| \
The verticals show the concrete canal walls. The dots would be the surface of the built-up earth which supports to walls, with a gentler slope than the /\ indicate. The water was about at the top of the walls when the surge(s) came in from Lake Ponchartrain, and cascaded over at least one side.
Enough water came over so that its weight and momentum gullied the earthworks. Water on a slope cuts downward in a way which undermines the earth immediately upstream of the gully, advancing the groove upwards until it meets something like the wall. As the heavy cascade continued, it would cut downward until it’s past the foot of the wall. As the top of the wall is fairly level, this process is wide enough so that the foot is undermined for, say, 100 feet, and the pressure of the water in the canal pushes out on the concrete wall.
The diagram also appeared to show the walls were footed with inverted-Tees several feet below the ground, and that the bottom of the canal was earth. If this is the case, rather than having a concrete bottom tied to the walls with enough reinforcing steel, the undermining from the gullying would allow water to start to seep out down&sidewards, which would remove the soil there and start opening up a nice outlet for the pressure of the water to force out enough soil to allow all the water out.
In either case, the support of the concrete walls is now insufficient for them to stand along that stretch.
Ref: The Merc gives their sources as the Army Corps of Engineers and the U.S. Geological Survey.
Other levees have different construction.