For China Miéville

Understanding the levee breach: a brief discussion of how the 17th Street levee canal containment wall failed.

A reader who is a physical geographer has sent me an essay and photos explaining how and why levees fail:

Understanding the levee breach: a brief discussion of how the 17th Street levee canal containment wall failed.

The levee is earthen.  The canal containment wall rests on, and is partially embedded in the earthen levee.  This wall is formed by a series of preform concrete panels joined end-to-end.

The strength of the wall depends on the underlying support of the levee, and the strength of the attachments between individual panels.

Those are the Achilles heels of the wall, if you will.

When high water pressure due to high water levels, i.e., flood or surge, occurs, the following can occur.

One, water infiltrates the earthen levee.  When the earthwork is loaded with water, hydrologic pressures weaken the frictional forces within holding that earthwork together.  Enough water will cause the earthen levee to slump as loosened soils move apart.  Equally, the weakened earthen levee is more likely to be pushed or "blown out" by water pressure both from hydraulic pressure within the soil and from the flood water pushing  laterally against it.

Once weakened to the point of slump or failure, the earthen levee no longer supports the panel or panels above.  At this point, the panels are under the stress not only of water pressure pushing against them but also the force of gravity pulling down.

The preform concrete panels can withstand considerable pressure over their surface, and can withstand far more force than the attachments between the panels.

Bereft of support from below when the levee slumps or fails, the panels are bound to fail if sufficient water pressure pushes against them, and when they fail, they will almost certainly fail at the attachments -- individual  panels will not break, but will tear away from their sister panels because the joints are the weakest point.

This seemingly overlong description is actually grossly simplified and doesn't go into actual pressures, etc., but I offer it to you so you can understand what you are looking at when you view pictures of the breach and wonder why the break looks so "clean." 

Likewise, when others post to your site, I hope they can look at my composition and come to a basic understanding.

I hasten to add one final thing.  Where the panels fail is a matter of site and situation, and the result of one of the many amazing properties of water, namely its ability to find the weakest point in any formation, be it mountain or molehill.


Regarding rumored books when the levees went out, he adds:

. . .  a man in the dome who said to his interviewer that (I paraphrase
poorly here) "they come and blow up the levee ... in the night ... after the hurricane" ("who?" asked the interviewer). "The money men," he replied.
he went on to say how "(we) heard boom, and then ... it was the explosion..."

Ever been in a hurricane?  I have.  Fran, Sept 1996. 
Every transformer blowing sounded like a bomb.  Large trees snapping
sounding like the very fundaments were ripping asunder.  Snapping
hardly seems the word -- more like, I don't know. 

In the dark, after or during a traumatizing event, with nerves on edge,
loud noises can almost knock you off your chair.

No telling what all people heard that night after the storm, when the surge
was peaking and all hell was breaking loose in the vicinity of the breaches.

Look at the pictures again, and think about what it must sound like
when such large, reinforced concrete panels are ripped apart and literally
tossed or swept away.

Then, think about all the debris washing along in the rushing waters, banging
into things.

I'm sure these people heard many things, many loud booms.  Who knows what
all they heard? 

Now, I'm not saying that it's not possible in this country in this day and age
that persons or agencies with whatever agendas are not capable
of deliberately breaching levees.

In fact, it's often done legally and deliberately in flood events, to spare
property or cities downstream, for example, or conversely to spare
an upstream community at the exense of one downstream.

And, we don't have to go too far back in US history to find cases of
sabotaging dams and levees.  For instance, look at the Imperial Valley
history of water wars.