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We are examining here some levee failures that occur
along the dangerous blue-tinted "levee failure belt" in California and elsewhere.
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We focus on three deadly failures north of Sacramento
near the Gold Rush city of Marysville
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Thirty years after the famous "Christmas Eve" failure
that flooded Yuba City, a similar failure happened in a similar geographic area on the other side
of the river.
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Photo of the 1986 Linda Levee after failure.
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None of the failures we are discussing involved
overtopping of the levee. In fact, the history of levee failures in the U.S. indicates 50-75
percent of failures occur when the river is below the crest of the levee. Eyewitnesses agreed on a
sequence of events like this.
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How exactly do they fail? One traditionally
conceived failure made is embankment seepage occasioned by seepage through the levee itself,
aggravated by those great scapegoats of levee failure, ground squirrels.
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But for decades levee failures have been associated
with sand boils which do not appear to be related to the levee itself.
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Studies by the Corps of Engineers on the Mississippi
indentified a process of failure that involved underseepage as a fundamental start of the failure
process.
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It wasn't the first time I had pondered the
destruction of levees. In the mid sixties as young engineer I had built a whole system of them in
Thailand. They looked very handsome in the dry season.
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Here is the village of Takhop where we lived-- in
the dry season.
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The rainy season was a different story; many of
my levees failed inundating our village. Fortunately the people were quite used to these sorts of
problems there, but I was left with a problem of how to do the job better.
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One day there was a festival and I was taking a
photo of my daughter standing on one of the levees
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Suddenly down the road a line of elephants
appeared, giving me a possible answer to the levee failure problem.
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We did appropriate tests of elephant soil
compaction capability (ultimately featured in the new Yorker Magazine as a kind of joke). We wrote a
paper on the matter. (Click here for
more on this).
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Meanwhile I found that studies by the California's
Department of Water Resources in yearss past had identified an entirely different problem --
seepage that killed orchard crops after floods. It seemed to identify exactly the same condition
that occurred in floods.
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Another clue was the apparent delay in the Linda
failure -- nearly a day a year the flood peak had passed.
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These ideas led me to conclude that the failure was
a foundation failure -- brought about by deep foundation seepage followed by piping out of a large
enough cavity to permit settlement of the levee.
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During the several days of trial questioning I
commented that another area that looked ripe for failure was down river a few miles at Arboga.
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This prediction proved to be true. The day after New
Years 1997 the levee broke at that very point.
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.causing the usual toll of death, property damage,
and misery.
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The break occurred just where history.
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.and geology suggested it should. See
http://www.stanford.edu/~meehan/flood/aeg.html for more on this.
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Leading to fame for a day.
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But I was not satified that I understood just what
had happened in the fialure process. Here we had more eye witness history and testimony, featuring
extensive boils.
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Some quite tiny.
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Others evolving into sinkholes
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Visible from the air.
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and on the ground.
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Interestingly similar patterns of boils were known
to develop in earthquakes as here in the Pajaro River during the Loma Prieta quake.
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They also suggested future levee failures -- as
shown at Pajaro. (Note the correspondence of the failure with the intersection of the county line
with the levee; this interesting coincidence is explored in
http://www.Stanford.edu/~meehan/parajo/index.html)
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Many people laughed at the claims by flood victims
that just before the failure snakes come out of the ground. People thought "Go tell that to your
bible class."
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The comparison to Pajaro raised some interesting
thoughts. Thinking laterally for a moment, we note that in earthquakes boils = liquefaction of soil
= buildings tipping in Japan.
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.in Taipei, and at almost any other earthquake
affecting an urban area in a lower river valley.
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Combining all these ideas suggests the following
sequence: rising water and foundation recharge.
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.leading to boils.
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.and finally to foundation bearing capacity failure.
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The traditional standard flownet suggesting low
uplift pressures at the toe
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Bearing capacity failures of levees following
earthquakes provide important insights
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.leading to a new model of flood failure.
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Concentration of upward seepage beneath the toe of a
sound, tight levee is the most dangerous.
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Computer modeling shows that complete liquefaction
occurs with upward gradients of less than 0.5 toe failure from loss of bearing capacity happens at
even lower gradients.
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As suggested previously the pressure in the gravel
driving the upward gradient will eventually reach somewhere between 50 and 100 percent of the river
level, the latter where the river accesses the gravel in its natural channel or through artificial
pits, which have been long and wisely banned.
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It was clear that the reptiles were able to
foretell the failure.
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Except where rich quarry owners are able to change
the rules. Or in the case of the 1997 failure where wetland restorationism prevails over
engineering principles.
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Toe liquefaction under 18 ft uplifet, 30 ft
non-stratified topstratum. See flowneta.wk4
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Pore pressures rise and fall beneath the toe of the
levee
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The Osaka earthquake destroyed many of the levees
that protect the city. Fortunately for the residnets the river level was low and there was no
catastrophic flooding.
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State studies after the 1997 flood called for levee
repair schemes that seemed unlikely to solve the problem. Certainly the type of slurry wall shown
here would do nothing to prevent the type of failure that happened at Marysville.
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This is a little better, but still of questionable
value.
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Comparable floods struck the Central Valley in the
last century
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Comparable floods struck the Central Valley in the
last century
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Mesopotamia
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Imperial Valley
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Santa Clara Valley
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I learned that you don't want to dig holes in the
ground on the water side of the levee.
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A lesson learned 35 years ago at LamPra Plerng
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And remembered today.
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Questions or Comments?