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2
Extreme Floods and
Earthquakes The Nature
of the Problem
Other chapters of this report describe the technical methods that have
evolved to estimate the magnitude of extreme floods and earthquakes, the
limitations of the methods, and some possible improvements in the methods.
In this chapter an attempt is made to take a broader look at the problems of
coping with extreme floods and earthquakes at dams from the viewpoint of
society as a whole. In so doing, it will be shown why there are no absolute
solutions to these types of problems.
DESIGN OBJECTIVES
Dams are designed and constructed to withstand various natural forces
and events that have occurred in the past or may be expected to occur in the
future. A vital part of the process of designing these structures, which gener-
ally are expected to serve society for 100 years or more, is to anticipate the
future vagaries of nature that may result in floods or earthquakes that would
cause the dam to fail (i.e., be breached or collapse).
Some people have the mistaken impression that dams, especially govern-
ment-built flood control dams, are designed and operated to protect prop-
erty and residents downstream against all floods that could conceivably
occur. This is not usually true. Extreme events could overwhelm the flood
storage capacity of even large reservoirs. When such extreme floods occur,
spillways pass on the large inflows, possibly leading to downstream flood-
ing. To set this matter in perspective, it is important to understand that the
primary purpose of a dam spillway is to protect the dam itself from failing
8
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Extreme Floods and Earthquakes
9
due to breaching or overtopping. A properly functioning spillway protects
the dam by passing excess flood waters downstream, thereby limiting the
amount of water held behind the dam. Thus an extraordinarily large flood
may pass over the spillway and cause damages downstream possibly ap-
proaching those that would have occurred if the dam had not been built.
However, the failure of the dam could produce flood rates and damages
greater than would have been experienced if the dam had not been built.
While either exceptionally severe earthquakes or extremely large floods
could cause a clam to fail, in this chapter the main attention is devoted to
floods. The reason for this is simply that essentially al' dams are exposed to
the threat (and reality) of floods, whereas a much smaller fraction of existing
dams may be subjected to significant earthquake forces (i.e., those located in
active seismic zones). Also, the structural characteristics of many clams pro-
vide them with an inherent capacity to withstand earthquake forces, while
protecting a clam against failure from overtopping can only be achieved in
most cases by providing specific flood handling facilities (i.e., spillways).
Experience indicates that rare and large-magnitude precipitation events
can produce flows of water with which most clam structures cannot cope—
except to pass them downstream. When considering such floods, it would be
very desirable to be able to predict how frequently extreme events of specific
magnitude might be expected to occur. Some argue that only after develop-
ing a satisfactory response to this problem can one rationally determine the
degree of protection from floods that should be provider! in dam structures.
Surprising as it may seem to some, uncler criteria in current use to deter-
mine capacity built into dam structures to withstand or pass floods, it is
possible for a flood event to exceed the clam's capacity to resist it. While terms
such as unlikely, rare, or low probability are used to describe these kinds of
events, it should be understood that what is being described is an event that
can occur. The problemfaced by designers of dams and members of the body
politic who use, pay for, or are affected by these structures is to determine
"how much protection should be provided for the dam," considering these
events can but may not occur during the life of the dam. It is not possible to
provide absolute safety against all hazards and especially from events pro-
duced by "mother nature." The objective should be to balance the benefits of
making dams safer against the cost of the increased safety and to reduce any
risks to acceptable proportions.
Objectives for either design or safety evaluations of clams relating to ex-
treme floods ant] earthquakes can be considered in two broad categories,
namely, (1) those relating to economic efficiency and (2) those relating to
equity. The economic efficiency objectives encourage maximizing the excess
of project benefits over project costs. Equity objectives seek appropriate
balance between competing interests of such parties as the dam owner, those
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10 SAFETY OF DAMS
who benefit from the dam, and those who would be harmed if the dam were
to fail. Since the magnitude and timing of future floods and earthquakes are
indeterminate, direct determination of optimum measures to attain the
economic objectives is not possible. For the same reason, simple answers to
problems of equity among those affected by a dam are not available.
The probable maximum flood (PMF) has become a standard design crite-
rion for flood protection of major dams over the past decades. The concept
that equity requires that a dam impose no additional potential for damage or
loss of life in downstream areas if such addition can be avoider! is usually
cited as the reason for this use of the PMF. Economic efficiency is not usually
cited as a basis for such choice, although for large, high-hazard dams, eco-
nomic considerations, if properly evaluated, possibly could lead to use of the
PMF. The PMF is first of all a hypothetical flood based upon a set of assump-
tions that attempt to define the maximum flood potential for the particular
site. The calculation of the PMF is based on a combination of facts, theory,
and professional judgments. The methods user] to calculate a PMF are not
standardized, at least not to the extent that a set of indivicluals with the
knowledge and expertise to make such calculations would independently
arrive at identical evaluations. The discrepancies arise primarily from the
technical, scientific, and moral issues underlying the professional judgments
of the estimators as well as the lack of a quantitative definition of exactly
what a PMF represents. Moral issues are involved because a dam owner may
make economic decisions involving risks to others without the input or con-
sideration of those at risk.
While it may be unsettling to accept the fact that one's ability to make
estimates of the PMF is less than one might like, it would be remiss to suggest
otherwise. In attempts to reassure the body politic that the level of flood for
which the dam is designed is reasonable, there may have been erroneously
perpetuated a myth of absolute safety by describing the PMF as one ". . .
where its magnitude is such that there is virtually no chance of its being
exceeded" (Bureau of Reclamation, 1981a,b), or ". . . (a flood that) . . .
would have a return period approaching infinity and a probability of occur-
rence in any particular year, approaching zero" (Wall, 1974~. Such state-
ments suggest that the ability to predict future extreme floods is greater than
that which actually exists and leads to unrealistic expectations on the part of
the public. In adjudicating disputes involving claims of liability for flood
damages, the courts have relied on criteria like "foreseeability" and the
"appearance of certainty" to reach results that fall within the mainstream of
legal analysis. However, from the perspective of the engineering profession,
such concepts are of questionable merit since they do not necessarily comport
with modern interpretations of probabilistic and statistical relationships.
No universal answers are available to questions on the degree of protection
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Extreme Floods and Earthquakes
that should be provided for a new dam if the PMF is not the appropriate
estimate of the worst possible flood that may occur, or on the actions that
should be undertaken at an existing dam if new information suggests that the
PMF for that structure was underestimated.
11
WHAT SHOULD SAFETY COST?
The responsibility for the general welfare requires that when government
considers a level of protection (or safety) for its facilities, it must simultane-
ously consider the cost of providing that level of safety. It is faced with
choosing whether an additional investment in reducing the risk to those who
would be directly affected is of greater benefit to society than would be
obtained by expending those funds for some other activity. In theory, it
would be possible to provide extraordinarily safe dams, e.g., by providing
spillway capacities equal to five or more times the PMF. Most would agree
that such gross conservatism is unwarranted. Although not directly compa-
rable, few of us would accept the Plea of buying or operating automobiles
that were built like army tanks even though such machines would reduce the
likelihood of beinginjurec] or killed in an auto accident. For most individuals
the cost and inconvenience of such protection would be viewed as excessive.
We recognize differences in personal reactions to an imposed risk (e.g., the
risk arising from a dam upstream from our residence) and a voluntary risk, as
that imposed by our ownership of an automobile.
The way society approaches the question of risk has been dominated by
feasible or practical concerns. Government by its own actions and by using
its authority to regulate the behavior of inclivicluals has generally satisfied
the public desire for increased safety. It has not, however, provided explicit
target levels for acceptable risk as matters of public policy. It is obvious that
these questions can only be resolved through the political process.
What inclividuals or groups ought to demand in terms of "safety" is not
entirely a technical or scientific issue. On the one hand, individuals make
judgments about their participation in "voluntary" risks and may influence
the riskiness of various goods and services by their willingness to pay for more
(or less) safety. On the other hand, society is called upon to provide various
goods and services for collections of individuals where, acting as the agent
for these collective interests, government has the obligation to decide what
level of risk to accept for these ("involuntary" risk) situations. In its struggles
to resolve these dilemmas, government is required to consider, evaluate, and
then choose among alternative courses of action that satisfy its responsibility
to individuals directly affected and simultaneously the "general welfare" of
society. It is unlikely that government can develop an ubiquitous risk policy
with respect to all activities, because the character and consequences of the
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12
SAFETY OF DAMS
risks imposed are so different. In this sense, government cannot be expected
to behave differently than individuals. Devising appropriate policies for
assessment and management of risk has become a dominant dilemma of this
decade.
REASONABLE CARE AND PRUDENCE IN DAM DESIGN
For the reasons discussed above, selecting the amount of protection from
floods or earthquake resistance that should be included in the design of a
clam is in the final analysis a matter of judgment. In order to determine
whether such judgments reflect reasonable care and prudence (i.e., the
exercise of good judgment ant] common sense) depends upon an understand-
ing of what these criteria mean.
The floods consiclered here result from natural precipitation without hu-
man influence or intervention. Thus, an important premise is that certain
floods are ". . . so extraordinary and devoid of human agency that reason-
able care would not avoid the consequences . . ." and are sometimes re-
ferred to as an act of God (New Columbia Encyclopedia, 1975~. While the
phrase, act of God, may imply to some the idea of divine intervention, it also
conveys important secular concepts. First, it suggests that certain natural
forces may result in events of such enormous force or consequence that no
reasonable persons would plan or conduct their lives in ways that anticipate
such events. Second, such acts or events occur only rarely so infrequently
that one intuitively assumes that the risk of such an event has little if anything
to do with the reality of day-to-day affairs.
The understancling of nature has improved and expanded through the
sciences, and engineers have been successful in applying this knowledge to
the construction of facilities that provide society with some degree of protec-
tion from natural events. Thus, the hydrological, meteorological, and geo-
logical sciences have improved our understanding of extreme floods and
earthquakes and provided some tools that enable us to predict limiting mag-
nitudes for such events.
The tools for predicting floods are based upon two different scientific
principles: historical observation and causality. An historical record of pre-
cipitation and runoff events (if sufficiently long) permits the use of the laws
of probability and statistics to estimate the risk of floods of various magni-
tudes. Regardless of the accuracy of these predictive tools, they do not offer
any guidance on the level of floor! risk that is appropriate for any dam. That
is, even if accurate predictions of the probabilities of all sizes of floods were
possible, it still would be necessary to decide whether spillways of dams
should be built to withstand the flood that arises once in a thousand years, in
ten thousand years, in a million years, etc.
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Extreme Floods and Earthquakes
13
Another method of defining the flood potential at a site involves construct-
ing a hypothetical but plausible storm (probable maximum precipitation)
that is assumed to occur over a particular drainage basin where a dam exists
or is contemplated. From the present knowledge of the meteorology of such
storms, the geological and hydrological characteristics of the drainage ba-
sin, and their interrelations, it is possible to estimate a flood (probable maxi-
mum flood- PMF) resulting from this storm. This method of flood
estimation, which is in general use, also presents a number of difficulties.
First, inasmuch as the method is hypothetical, it is difficult to estimate the
risk or probability of such a flood actually occurring. Second, if larger storms
are observed sometime in the future (i.e., larger than the estimated probable
maximum precipitation (PMP) used to calculate the PMF), these will in turn
result in a bigger estimate of the probable maximum flood. (Flood estimates
based on probability studies also tend to increase as more streamflow data
become available.) Such an increase in probable maximum flood estimate
challenges the adequacy of the existing spillway. Since this method does not
provide an estimate of the risk for the original PMF, it cannot be used to
determine how much additional protection would be obtained from ex-
panding the spillways to handle a larger PMF. Obviously, the risk of dam
failure due to floods will be reducer] by a design that permits larger floods to
pass, but such decisions must also meet the test of reasonable care and
prudence.
What constitutes reasonable care and prudence in selecting the magni-
tude of a flood for which a dam should be designed? There appears to be no
completely satisfactory answer to this question leastwise one that would
satisfy everyone. Those who would be directly affected by the possibility of a
dam failure would surely choose to make the dam as floodproof as possible.
Yet it is doubtful whether these individuals would be willing to pay the costs
required to decrease the risk of the dam failing if the risk of failure were
already relatively low (Thaler and Gould, 1982~.
The current procedures used for selecting the spillway design flood (SDF)
attempt to delimit reasonable care by acknowledging that the level of pro-
tection provided should reflect consideration of the hazard potential of the
dam (viz., loss of human life, property damage, dam services, opportunity
costs) . The PMF, in spite of the fact that it is a hypothetical event of unknown
risk or probability, appears to meet a standard of reasonable care, as demon-
strated by the performance of dams over the past five decades. On the other
hand, since the spillways of many existing dams are inadequate by PMF
standards but have survived in spite of this inadequacy, it is legitimate to
question whether this standard is higher than may be required. It is axio-
matic that excess protection, i.e., that capacity provided at oftentimes con-
siderable cost but that is never used is rarely challenged as unreasonable.
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14 SAFETY OF DAMS
However, if a dam should fail, one can be assured that a careful inquiry will
be made to determine whether the designers used reasonable care in select-
ing a design flood. While accountability for dam designers is essential, it is
obvious that the ambiguity imbedded in the requirement to exercise reason-
able care leads designers to act more conservatively in selecting a spillway
design flood.
While balancing risks and costs is the ideal, this balancing cannot be
accomplished with confidence at this time. In particular, the probability or
average recurrence intervals of extreme floods and earthquakes can only be
estimated approximately. While moving toward the ideal of balancing,
some recommendations are needed to answer current concerns for the design
of new dams and the retrofitting of existing dams.
The PMF is a concept that has prevented dam failures throughout the
world. To a lesser extent perhaps, the similar concept of a limiting earth-
quake magnitude (the maximum credible earthquake MCE) has provided
a basis for preventing dam failures resulting from seismic events. To date,
these notions have proven highly conservative, since few natural events have
challenged them. The concepts have great usefulness in the design of new
high-hazard dams. Even here, once the PMF and MCE have been estimated
and' the dam designed, there ought to be exploration of the cost of meeting
somewhat different design levels. For example, if only a tiny addition to the
cost of building the dam would be required to design to a higher standard,
this greater standard makes sense. Similarly, if the cost of designing to the
PMF and MCE are very large in relation to a slightly less stringent design,
careful consideration must be given to whether the more stringent design is
needed.
For dams that pose no threat to life, a balancing of the risks of property
damage and loss of dam services against the costs of greater dam safety is
appropriate. Such balancing is reasonable, since the relevant floods would
be sufficiently frequent that their probabilities could be estimated with
confidence.
For dams that involve a small risk to life, balancing is similarly appropri-
ate, although the risk to even a small number of people should call forth
somewhat greater safety than the case of only-risk to property. Such low-
hazard dams provide opportunities for research on balancing and also pro-
vide test cases for implementing the technique of balancing. The methods
developed for these cases may help bring the technique of balancing into the
design of high-hazard dams.
Representative terms from entire chapter:
reasonable care
