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It’s not necessarily “just overtopping”


 

A quick note on what’s happening in New Orleans, which looks like it could be fairly harmless — except for the way those floodwaters are overtopping the Industrial Canal levee near the Lower Ninth Ward. Here’s a picture from Tulane University geologist Stephen Nelson, who gave me a tour of Katrina flood damage in 2006:

See that trench along the floodwall? It’s important to understand something: that’s the “dry” side; the flooding was happening on the other side. But as it splashes over the top of the wall, it lands, with some force, on the “dry” side and starts eroding a trench, quite rapidly. And if the floodwall isn’t dug down deep enough, that erosion can combine with the pressure from the other side to tip part of the floodwall over. It’s quite clear that happened along some of the levee breaches in 2005 (there were several breaches, cause by  different failures in different locations). That’s why the footage of water splashing over the top of the Industrial Canal floodwall is causing a lot of worry right now.


 

It’s not necessarily “just overtopping”

  1. Thanks for this. Interesting.

    Found out that, following Katrina, the Army Corps of Engineers added reinforcing and ‘t-walls’ to the levees. Any over-topping water falls onto a concrete pad–not on earth as before–so undermining no longer happens. Live views of the storm here:

    http://www.wwltv.com/video/?nvid=57429&live=yes

  2. True in many places, but not all, Mike:

    “The wall under attack from the storm waters is one of the old I-walls that was still standing after Katrina, and so it was not
    replaced with stronger T-walls, which are shaped like an inverted T.”
    http://thelede.blogs.nytimes.com/2008/09/01/tracking-gustavs-landfall/?scp=1&sq=i-walls&st=cse

    What struck me when I visited is how complex and often seemingly piecemeal the flood protection system is…

  3. In situations like these, people often forget about basic things like physics. During the Red River flood (affecting MN and MB), many people in MN made a basic mistake. They found their basements flooded. Their natural reaction was to remove the water. They did. And then, the water pressure on the exterior of their basements being greater than the zero pressure inside, their basements collapsed. Along with their houses. Sounds like a lot of things that happened in NOLA then and now.

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