The New Orleans Levees 3

[electricpete]

Does anyone have thoughts or links regarding the technical details of what caused the Levees to fail and how they can be repaired?

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[Prepakt1]

Levees repairs.

Probably best repair is to plug the gap with barges filled with rock and scuttle as the main anchors.You can then fill any remaining gaps by rock and place rock around the barge and grout both the barge and surrounding rock .
Prepakt

 

[dmoler]

Could be an environmental nightmare to scuttle the ships unless the barges are prepared accordingly, which takes some effort.

 

[jdonville]

According to what I have heard from an Army Corps guy on NPR, the storm surge in Lake Ponchatrane (?) topped the floodwalls on top of the failed levees (think broad-crested weir). There was apparently no "failure" other than the walls were not high enough.

The water falling over the floodwalls, however, scoured the floodwall foundations on the city side of the levee enough that the water pressure behind the floodwalls was enough to topple the walls. Finally, the scour from the wall of water unleashed by the breach in the floodwalls eroded the levee through.

Reportedly the floodwalls were designed to handle water levels due to "only" a Category-3 hurricane.

Like many others, I suspect that the disruption in the US and global economy may warrant a re-examination (with respect to cost-to-benefit) of the design criteria for the water management and flood protection of the whole New Orleans and environs area.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[cvg]

The question that should be asked is - if the walls were 17.5 feet high and the predicted storm surge was to be 20 - 25 feet high - why wasn't the mandatory evacuation sounded sooner and enforced? It seemed obvious to me at the time that the city would be inundated. In addition, why didn't the Corps start reinforcing some of the flood walls to prevent what actually happened? And why didn't the Army mobilize troops last Saturday or Sunday in advance?

 

[BigH]

I'm not in the US now so I haven't heard the details you are discussing - but I do remember Wuhan China several years ago - they actually broke the Yangtse flood walls to try to save the city. Wrecked havoc in the farm lands but did save it. I remember what a Prof said when I was at school. To paraphrase: "No matter what you do or try, Mother Nature and Water is still going to get you!" Bangladesh is a prime example of flooding during monsoon.

 

[DRC1]

There are excellent details of what happened at the ENR website (

http://www.enr.com)

Apparently the Corp has wanted 2.5 Billion to make the 350 miles of walls hurricane resistant. This year their budget for the city was cut by 70 million. Pumps had a capacity of 25,000 cfs, inflow from breeches exceeded 1,000,000cfs Sinkin stone barges is being considered. (at this point with the city underwater, I don't think it is possible to do further enviornmetal damage. Boh Brothers will drive sheets at some locations.
At this point I think they would be better off to drive a celluler dam outbord of the break so that once it is pumped down, they can repair the levy. My question is once they pump down, will tey rebuild the levy and the city below lake level, or will they fill the site to an elevation above lake level?

 

[dmoler]

Wow, looking at the news of the true extent of this, a gas plume from a barge is the least of their worries.

Regarding cvg comments on enforcing a mandatory evacuation, many people are poor and cant really just 'get up and leave' that easily. The governement should have provided busses or some other way to get them out before the hurricane. The hurricane center predicted the category 4 level and where it would go right after it went past here in Fort Lauderdale, and the feds still did not prepare ahead of time. Quite unbelievable.

 

[GeoPaveTraffic]

I agree with dmoler, the lack of planning before this event is increadable. It is very sad that a country such as ours is not able to take care of our own. This was not only a predictable and quanitifable risk, it was predicted. The lack of resources to get everyone out of the City prior to the storm is a clear indication of the lack of planning on both a local and federal level. Also, the pace of the responce since the levees failed is unbelivably slow.

This should show everyone the difference between real planning and lip service. Most governments in this country are very very good at giving lip service the problem on the moment, but when it comes to real planning and setting aside resources to cope with problems, they fail completely. I don't know if this is a reflection of our socity as a whole or just the way our elected officals behave due to the short election cycle that puts them in office.

 

[dgillette]

DRC1 asks an interesting question about filling the area to or above sea level before the city is rebuilt.

I can think of a couple possibilities. There are probably others.

1. The money is promised for rebuilding of levees high enough to prevent reoccurrence - $100 Gig??? People rebuild at existing grade. Maybe my yet-to-be-born grandchildren will get jobs designing the new levees.

2. Money is found for only modest levees. Poor people rebuild at present elevation. Government agencies, corporations, and wealthier people, acting individually and jointly, buy nearby properties to use as borrow areas to build up their own sites. Borrow areas become wetlands. City becomes smaller, and therefore very expensive to live in.

 

[electricpete]

So as Jeff said and the ENR link confirms, the water overflowed the Levee which resulted in erosion at the base of the Levee which resulted in failure.

As I understand it, the entire Levee is built to the same height and there is no control over the location where it will overflow and no ability to predict it.

So, knowing that the Levee might possibly be exposed to levels beyond it's capacity, why wouldn't we have designed the Levee with strategically placed overflow "notches" at perhaps a level of 1' below the top. The Levee could be reinforced in this area with some structure to guide the water away from the Levee to minimize erosion.

Presumably with this type of setup we might have topping of the Levy slightly sooner, but when the surge subsides and the Lake drops below the level of the notch, the water stops flowing.

It seems to me a necessary design feature. A parallel situation is a pressure vessel whose purpose is to contain pressure. If the design pressure is approached, a relief valve actuates which reduces pressure... momentarily disrupting the function of the pressure vessel but preserving it's ability to function after the overpressure condition is relieved.

Of course the ideal solution now is to build the entire Levee greater than the category 5. But I am looking in retrospect at the Engineers that were designed the resources for the full category 5 and knew there was a possibility the level might exceed the top. Wouldn't notches have been a good low-cost design feature?

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[electricpete]

Typo in my last paragraph. Let me repeat it with correction:

"Of course the ideal solution now is to build the entire Levee greater than the category 5. But I am looking in retrospect at the Engineers that were denied the resources to build against the full category 5 and knew there was a possibility the level might exceed the top. Wouldn't notches have been a good low-cost design feature? "

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[electricpete]

Also note the flow rate through the notches would be much less than the flow rate through a breached Levee.

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[richardarmstrong]

Won't other weak spots soon become apparent once they start pumping down the water and the pressure is no longer equalized?

 

[dgillette]

BigH wrote a few days back about breaching of levees on the Yangste to protect Wuhan. In the new issue of US News and World Report that arrived yesterday, there is a one-pager about the 1927 flood in Lousiana and Mississippi, which aided the political careers of Herbert Hoover (who was an important and effective part of recovery efforts) and Huey The Kingfish Long. There, they (i.e., COE?) dynamited a levee just south of New Orleans to give the water some place to go so the city would not be flooded, but many of the flooded-out residents never did get the compensation they were promised.

Flooding near Wuhan should now be a thing of the past with Three Gorges Dam nearly finished upstream. Saw it a few weeks ago, and it is IMPRESSIVE (in ways both good and bad). As our small cruise ship went through the locks, I realized that the locks alone were bigger than any project I've ever worked on and probably ever will.

 

[electricpete]

Won't other weak spots soon become apparent once they start pumping down the water and the pressure is no longer equalized?

Ummm. Could be. Just heard on CNN something about another break 8pm cst 9/12.

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[geochem1st]

Guys,

You are thinking like engineers. As a geologist, I see this as a never ending race, the levees will never be high enough. As Lake Pont. fills in with sediment the lake level will continuously rise, requiring the levees to be made higher and higher.....

 

[Dirtguy4587]

I also hear that New Orleans actually remains sinking slightly. Is this true? I know the whole area is essentially a bog - muskeg like (or at least thats how I envision it), but I am unfamilliar with the ground conditions in the Gulf Coast areas.

 

[dgillette]

It's a bit of an exaggeration to call it a bog, but I believe you're correct about continuing settlement in some areas. The soils there are mostly deltaic, therefore fine-grained with high void ratio. Any new construction loads would cause consolidation, as would pumping of groundwater. Even if primary consolidation is effectively over, there could be some secondary consolidation that is still significant. There might also be some effect from oxidation of organics that are exposed to air by lowering of the water table.

 

[DRC1]

Geochemist1:

As the lake fills with sediment, the bottom of lake Pot. (I couldn't spell it if I wanted to.) will rise. The surface of the lake will always be at about the level of the Gulf. Eventually you would get a bog. The top of the lake will not rise except for tides, storm surges and local fluxuations known as seeches