Dam Safety Analysis

by Philip B. Williams, Ph.D., P.E.

The
consequence of failure at the Three Gorges Dam would rank as history’s
worst man-made disaster. More than 75 million people live downstream on
an intensively cultivated floodplain that provides much of China’s
food. It is therefore reasonable to expect that a key design criterion
for the project is ensuring that the risk of failure is kept extremely
low.

Because of the limited operating experience with
large dam projects of this type, and the disquieting number of safety
incidents that have threatened the integrity of large dams in the last
two decades, it is reasonable to expect that CYJV would use the best
state-of-the-art techniques to demonstrate that the design,
construction, operation and decommissioning of the project would keep
the risk of failure acceptably low.

Unfortunately, CYJV does not address the safety
issue either systematically or coherently. It provides no acceptable
risk criteria, no mapping of the area and population at risk, no
comprehensive risk assessment which identifies all the potential
failure modes, and no identification of fail-safe measures. Because
safety is not analyzed as a discrete topic, major failure mechanisms
and combinations of failure mechanisms are ignored. There are many such
possibilities; for example, a reservoir-induced earthquake that
initiates new landslides close to the dam; sabotage or military action
that disables spillway gates immediately before the flood season;
unanticipated delays in construction leading to the overtopping and
washing out of one of the cofferdams.*

Although CYJV discusses some safety issues, it
makes many major assumptions and gross underestimations about the dam’s
design which effectively put the probable risk of dam failure greater
than the risk of a 1000-year flood for which the project is designed.
Examples of the flaws in CYJV’s analysis are as follows:

• Underestimation of earthquake ground accelerations

One
of the most important structural design criteria for a dam is the
estimation of ground acceleration in the event of what is termed the
maximum credible earthquake (MCE). For the Three Gorges design, a 6.5
magnitude earthquake occurring on a fault 17 kilometres away is used
for structural analysis. There are substantial uncertainties in the
selection of the MCE and also in the prediction of ground accelerations
at various distances from the fault. CYJV uses a ground acceleration
factor only one-third the value that would be used in a reasonably
prudent design.¹ CYJV’s use of these values effectively
results in the most optimistic interpretation possible of likely ground
accelerations due to earthquakes.

• Inadequate analysis of reservoir-induced seismicity

CYJV
recognizes that the weight of the water in a large reservoir can
initiate earthquakes. However, in developing ground acceleration design
criteria, CYJV uses only historical records of earthquakes, which means
that the design accelerations selected are likely to be too low and/or
would occur more frequently than expected. In addition, there appears
to be substantial uncertainty about the movement of the most important
of these faults, since CYJV stresses the “need for careful assessment”² – an assessment that presumably has not yet been undertaken.

The treatment of reservoir-induced seismicity (RIS)³
is cursory and does not acknowledge the serious potential for
structural damage, property damage and loss of life downstream that
could occur. CYJV assumes that RIS occurs only on faults that are
presently proven active, and implies that only short lengths of long
faults close to the dam site would be activated. The length of faults
passing under the dam itself and the displacement that would occur if
these were activated are not identified. Therefore, it appears that the
dam design is based on the optimistic assumption that no movement would
occur on these faults, despite the experience with RIS elsewhere. For
example, the Koyna Dam in India initiated an earthquake (approximately
6.0 in magnitude) that seriously damaged the dam and killed 200 people
in an area that had not previously been seismically active.

• Inadequate analysis of structural stability

Apart
from the optimistic estimates of ground acceleration during earthquakes
and the fact that potential RIS is downplayed, it is clear that there
are substantial unresolved problems related to the structural design of
the dam which, if satisfactorily resolved, could add hundreds of
millions of dollars to the project cost.

For example, with higher, more realistic
assumptions for ground acceleration, the upstream face of the dam would
be subject to stresses which would almost certainly cause cracking. And
while CYJV recognizes that this would occur, it did not conduct the
necessary analysis of the dam to identify where cracking could occur
and what design modifications are needed.

Furthermore, CYJV failed to conduct a
comprehensive assessment of project operation management to analyze
possible failure modes such as rupture of the dam due to fault movement
underneath it, and the performance of the dam during an earthquake with
prior cracking.

Other examples of how CYJV has systematically downplayed the risk of dam failure are as follows:

• Underestimation of the risks caused by catastrophic landslides

In
1963, at the Vaiont Dam in Italy, a landslide in the reservoir
generated a flood wave that killed 4000 people. In the Three Gorges
region, major landslides occur every few years, disrupting navigation
and causing property damage and loss of life. CYJV states that the
Three Gorges Project would result in “no significant change in slope
stability,”⁴ which is highly questionable considering that
wide fluctuations in reservoir levels in the Three Gorges region are
highly likely to have a destabilizing effect on potential slide areas.

Although CYJV discusses the risk of landslides
in the reservoir, it did not investigate the effect of earthquakes,
including those induced by the reservoir itself, on activating
landslides in areas it has rated as stable. Nor did it evaluate the
impact of landslide waves on spillway gates at the time of rapid
drawdown* in the reservoir immediately prior to the flood season.

Finally, CYJV did not conduct a systematic
analysis of zones at risk from waves 20 to 50 metres high that could
result from individual landslides and could conceivably kill tens of
thousands of people living near the reservoir. Therefore, it appears
that the threat to people living around the reservoir and dowstream,
and the threat to safe operation of the dam, have been greatly
underestimated.

Apart from other optimistic assumptions, CYJV’s
discussion of impacts due to landslide-generated flood waves assumes
that all people living in the reservoir area would be relocated above
the 182-metre elevation mark, contrary to the resettlement plans
stating that only people living below the 162-metre elevation mark
would be relocated.

• Underestimation of risk of spillway failure

As
the world’s largest hydroelectric dam on the world’s third longest
river, the Three Gorges Project incorporates many experimental
technological innovations. One such experiment is the construction of
the world’s largest submerged spillway bays. Each of the 27 spillway
units has a capacity equivalent to the average flow of the Missouri
River in the United States. CYJV confidently asserts “there is no
reason to believe that these structures could not be successfully
designed, constructed and operated,” even though the discharge per unit
width is “well beyond proven world experience.” CYJV’s confidence in
the spillways is further undercut in the same paragraph with the
statement: “The feasibility of such a high unit discharge should be
reviewed during final design.”⁵

In
fact, operating experience with extremely large flows through such
spillways has not been good. At the Tarbela Dam (Pakistan), and the
Glen Canyon and Hoover Dams (U.S.), extremely high velocities and
pressures caused cavitation* and erosion which threatened the
structural integrity of the dam and necessitated serious and costly
repairs. Similarly, at the Three Gorges Dam there would be a high
possibility of failure.

Another questionable assumption is that the
“good and homogeneous quality” of the rock immediately downstream of
the dam would minimize scouring (erosion of the channel caused by the
river’s flow).⁶ Actual operating experience with this is
very limited, but there is a significant possibility that scouring
could threaten the structural integrity of the dam, as nearly occurred
at the Tarbela Dam. Once scouring begins it is very difficult to
correct and requires continual remedial measures which can add
significantly to operating costs.

• Failure to consider downstream effects of cofferdam failure

During
the construction of the project a series of temporary cofferdams would
be constructed across the river in order to divert its flow. The second
and third phase cofferdams would, at best, be capable of withstanding a
100-year flood and a 200-year flood, respectively. If larger floods
occur, these cofferdams could quickly wash out,* releasing a flood wave
that would overwhelm the Gezhouba Dam** and continue downstream to
overtop the Jingjiang Dyke, drowning hundreds of thousands of people.
CYJV estimates the probability of such a catastrophe to be about 1 in
20, which should be considered an unacceptably high risk.

• No provision for decommissioning of the dam

The
risk of dam failure increases with its age as construction materials
deteriorate, mechanical systems such as spillway outlet gates fail, and
the effects of a series of problems, such as corrosion, abrasion,
sedimentation, and downstream scouring, become intractable.

CYJV has calculated the costs and benefits of
the project over a 50-year period (for comparison’s sake, Chinese
culture has developed alongside the Yangtze River over some 4000
years). Regardless of the dam’s economic lifespan, CYJV should have
made provision in the feasibility study for decommissioning the project
in a way that would ensure the safety of those living downstream. The
costs of decommissioning should have been included in the cost-benefit
analysis.

Sources and Further Commentary

*A cofferdam is a temporary dam built across the
river to divert the river’s flow around the dam site while the
permanent dam structure is under construction. Plans include
construction of three cofferdams during three stages of the Three
Gorges Project construction.

*Drawdown refers to the release of water to lower the level of water in the reservoir.

*Cavitation: extremely high velocities cause negative pressure which can break off pieces of the spillway’s concrete surface.

*On a much smaller scale, such a cofferdam failure
occurred in 1986 at the Auburn Dam site on the American River (U.S.).
The cofferdam was washed out by a flood flow, only one-fifth of the
volume that would be released from the Three Gorges cofferdam, but
fortunately a disaster was averted by the Folsom reservoir downstream
which was able to contain the flood.

**The Gezhouba reservoir capacity is less than
one-half of a cubic kilometre (1/2 km3), a mere fraction of the Three
Gorges reservoir capacity.

Continue to Chapter 11

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Chapter 9