We focus here on a particular (and deadly)
type of levee failure that is characteristic of California rivers and displays the following
characteristics:
-associated with landside seepage and sand boils, often at considerable distance from toe,
during floods;
-failure at up to one day after flood peak;
-boils at levee toe enlarging in the last hour before failure to large sand spouts;
-loss of levee toe by sinking or sliding followed by levee subsidence and overtopping.
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California's Central Valley and delta region is
a nice fit to this geographic pattern of levee failure. (Is the delta region really a
"delta"? Not exactly in the original "shape" sense of the word, coined by Herodotus when he
realized that the form of the Nile delta was a giant greek letter delta. But
geomorphically the place is in fact a delta, that is an accumulation of flat sediments on the
edge of a basin, in this case the hill-pinched upper estuary of San Francisco Bay. Which
means that it shares much of the history and stratigraphy of other deltas and estuaries of the
world, and has conditions that match the pattern we shall adduce here.)
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Interesting similarity: Genesis 1 states that
Eden (Sumerian for "plain") is sited at the meeting point of four rivers. The Gihon
(present-day Karun) winds through Cush (Kasibites, Iran) and the Pishon now is a dry
wadi from Havilah (now Arabia.) Today the junction is a vast marsh a hundred miles
inland of the Persian Gulf, inhabited by the Shiite marshland Arabs, scene of the ancient
cities of Ur, Uruk, and Eridu, and more recently the site of the Gulf War.
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A classic geographic urban pattern beginning
with the ancient Sumerian city of Ur (of the Chaldees, of biblical fame) and carrying on to
Tokyo, San Jose California, Galveston Texas etc. involves urbanization of lower floodplains,
diversion and diking of rivers, and (especially in recent times) extraction of groundwater
leading to subsidence. River and tidal surfaces end up well above ground levels in cities,
creating problems of both underground flooding (affecting underground space and destabilizing
the ground) and surface flooding of the depressed urban basin arising from levee breaches in
susceptible areas upstream.
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Certain patterns of subsurface geology and
seepage coincide with this history, as demonstrated in the 1940s by Fiske for the Mississippi
River and elaborated by other research on levee failures and flow slides conducted by the
Corps of Engineers over the years. Holocene evolution of lower valleys in California is
somewhat different than in the Mississippi because of smaller watersheds, tailings deposition,
and tectonically defined coasts. Nonetheless mid-Holocene river evolution still gives rise
to a classic fine-grained topstratum with effective thickness of 20 ft. or so. Depending on
local conditions, high upward seepage pressures with associated instability may develop at
relatively shallow depths beneath the toe (and throughout the landside floodplain) during or
shortly (up to 36 hours) after high water.
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The Sacramento Riverconsidered
longitudinally: Pleistocene gravel channels are found 25 feet below the ground in the
floodplain. The gravel was deposited in fast moving braided streams when the sea was 300 feet
lower (until about 7000 years ago). When sea level stabilized, a delta formed and the rivers
switched to a meandering pattern, blanketing the gravels with silt.
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Notice the similarity to the stratigraphic
section of the Holocene deposits in the lower Tigris-Euphrates basin
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A longitudinal profile of a 20 mile stretch
of the Feather River centering on the Arboga failure at Country Club Road. Notice that at both
the Country Club Road and Linda areas (where notable levee failures occurred in 1997 and 1986
respectively, each causing on the order of 1/2 billion dollars in damage) the levee is built
on a silt (and tailings) layer (brown color) which is roughly as thick as the levee is high.
Beneath this silt and tailings is the coarser and more pervious gravel from the Pleistocene.
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Claim: The failure behavior of the levees during
flooding demonstrates that the upwardly fining gravel layers (Laguna Formation?) at 20-30 ft
beneatht the floodplain (which is deeper than usually explored in California levee safety
investigations) play a critical role in failure. High seepage pressures and ample flow
delivery capacity is brought to the levee toe behavior and beyond.
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Supporting evidence: this is the only way that
sand boils and collapse features could occur hundreds or thousands of feet on the land side of
failure-prone levees. Here is one (discovered by my 11 year old son Kevin) being photographed
for KPIX a few hundred feet downstream from the Arboga collapse of January 1997 (video
available). Ours were the first footprints in this orchard, many weeks after the failure.
animation/enlargement
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Implication: The repressurized gravel layer
pushes the potentials toward the landside and up into the sensitive foundation area. Seepage
conditions in flood time are not as often assumed in (a), but as shown in (b). The
gradients increase progressively with time. When they exceed 0.5 sand boils become active and
the toe foundation and lower slope become mushy and unstable, effectively forming quicksand.
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In anticipating failure D, the fatal combination
occurs where the levee is built over the floodplain, and is built on a silty cap (covering
old gravel), that is relatively thin. These areas are marked by low ground and high
groundwater, C.
Hence diagnosis of areas with acute failure risk is based on recognition of (a) levee
configuration on flood plain, (b) history of seepage symptoms, (c) details of Holocene and
man-made stratigraphy.
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Consider point (a): If you knew about the 1955
and 1986 failures, could you suggest where a future failure might be?
Notice how the levee extends well out into the floodplain (yellow) at the failure
locations. This is a legacy of the "levee wars" of the late 19th century. Land owners hoped to
push the river over into the other county. The situation became so unfriendly that levees
were sabotaged by dynamiting.
These same areas have been subject to well-documented, flood-induced seepage (which damages
orchards) for decades.
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We can see point (c) most dramatically in
another flood-prone river, the Pajaro River.
Note how the 1995 failure, shown on the news clip to the right, occurs at a
point where the county line (1890 river channel) crosses the river and the topstratum is
accordingly thin and vulnerable to seepage. Note that the floodplain at the failure area is at
50 ft above sea level.
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Consider this: 1995 the U.S. Army Corps of
Engineers planned to reinforce the levee at Country Club Drive. Nonetheless, laws required
that the work could not begin on the levee repair until an artificial pond was constructed
for environmental "mitigation" purposes.
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Here is visual comparison between the 1986 and
1997 levee failures and the location of pits dug in the floodplain near the levees. Both
photographs are the same scale.
Artificial pits were present at both failure sites as shown in the photographs (same scale).
Pits are 1000 to 2000 ft. away from the failure points, in line with old channels that pass
beneath the levees.
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General observation: areas of high risk oftenappear to correspond to areas that are
otherwise suitable for preventive strategies emphasizing targeted land use controls, floodway
purchase options, insurance, and controlled failure.
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Without these long-term measures flood prone
lands will continue under heavy development pressure (as here, where the site of the 1955
levee failure (28 fatalities) at Yuba City is for sale to developers. The levee has not been
improved.)
Note that flood insurance is not required, the site being outside the levee and therefore
protected against flooding. Ironically, the history in the Yuba City-Marysville area
demostrates that the return period for extremely dangerous flooding arising from levee failure
is close to 15 years. (1955, 1986, 1997)
Nonetheless, vast tracts of land are scheduled for development in these high-risk areas.
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It appears likely that the discrimination of high risk areas (most susceptible 10-20%
of levee reaches) is feasible with good reliability. Also, the proposed model should allow
for improved risk-stage relationships in this type of fluvial environrment
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Short-term or flood-fighting techniques
suggested by these findings:
A stategy of pressure relief in susceptible levee reaches;
Identification of susceptible top strata (pressure, stratigraphy, defects, etc)
Improved and economical pressure relief installation and monitoring;
Legal and O&M issues;
Longer term possibilities: identification of criteria for land use strategies (floodway
easements, etc)
Note that cutoffs and shallow toe drains will not be effective for the condition which we
claim to exist in the Central Valley of California. Deep drains and berms are more effective.
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Quiz: now that you have learned all this, can
you tell why the pattern of levee failures of January 1997 is the way it is?
Would it help if you knew that the failures here (and at the Pajaro River in 1995) all took
place where the floodplain elveation was about 50 feet above sea level.
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