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OCR for page 97
9
Proposed Hydrologic Criteria
BASIS FOR PROPOSALS
The selection of level of protection against extreme floods for a specific
dam, like many design choices in engineering, is basically a problem in
allocation of resources. However, as noted in Chapter 5, none of the cur-
rently available approaches to evaluating the safety of a dam against such
floods provides a fully satisfactory method for this allocation. The determi-
nistic approach, with its concentration on probable maximum events, does
not directly consider problems of resource allocation. The probabilistic and
risk-based approaches require estimates of probable frequencies of extreme
flood events. Estimates of this type based on currently available data and
techniques do not inspire high levels of confidence. Also, as noted in prior
chapters, an analysis using the risk-based approaches may be nullified by
changes in downstream development and other factors included in a risk-
based analysis.
In recognition of these problems, the proposals set out in the following
paragraphs seek to strike reasonable balances between what is theoretically
desirable and what is practical based on current technologies.
SAFETY EVALUATION FLOOD
The committee recommends the adoption of the term safety evaluation
flood (SEE) to designate the maximum flood for which the capability of the
dam to withstand extreme floods without failure is to be determined. Such
97
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98
SAFETY OF DAMS
usage will avoid the incongruity of using the term "spillway design flood" in
connection with investigations of an existing dam when no design is contem-
plated. It also avoids the implication that only the spillway is involved in
establishing the capability of a dam to withstand floods.
The selection of an appropriate SEF for a specific dam should consider
that, as new information is collected, estimates of flood magnitudes and
frequencies tend to change. As noted elsewhere, increases in estimates of
probable maximum precipitation (PMP) have caused increases in estimates
of probable maximum floods (PMF) in large sections of the country. Similar
increases sometimes occur in estimates of magnitudes of floods of given
frequencies as more data become available for frequency analyses.
PROPOSED CRITERIA FOR NEW HIGH-HAZARD DAMS
As noted in Chapter 7, there is a general tendency to impose a higher
standard of safety on new developments that create new risks than is re-
quired of existing developments. There are reasons why higher standards
might be imposed upon proposed dams as opposed to existing dams. First,
for an existing dam the option of not building a dam in the first place is no
longer available. A dam has been built, and all those living downstream of
the dam are already exposed to some risk of dam break. Moreover, intention-
ally removing the dam to eliminate any possibility of breaching is usually not
a tenable option because such removal would (1) increase the frequency of
downstream flooding, (2) squander a valuable economic resource in which
many may have invested, and (3) deprive many individuals of such benefits
as recreation, irrigation, and water supply on which they have come to
depend. Such constraints are not involved when a dam is proposed. Also,
decisions in design for a new dam based solely on economic analysis without
regard to who bears costs and risks could violate principles of equity. Such
considerations indicate that the design of a new dam for a location upstream
from an urban-type development, which would introduce potential for ex-
cessive damages and loss of life in the event of dam failure, should incorpo-
rate the maximum reasonable level of protection against failure during
extreme floods, unless it can be shown that the failure of the dam during such
floods would not increase the potential for loss of life and damages down-
stream.
Although legal considerations do not point to a specific basis for design of a
new dam, court decisions have emphasized the need for dam designs to meet
standards of reasonableness and prudence and to provide for reasonably
foreseeable risks.
All things considered, the PMF provides the best criteria currently avail-
able to meet the standards of reasonableness mentioned above. Further-
more, although some may question justification for such conservative
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Proposed Hydrologic Criteria
approach, the dam engineering profession has more confidence in the ade-
quacy of the PMF criteria for major dams than in any other criteria that have
been advanced. Hence, retention of the PMF criteria for design of spillways
for new dams in high-hazard locations is generally recommended. However,
we note that there may be instances when a smaller SEF is appropriate. In
spillway design, the concern should be with the incremental damages associ-
ated with dam failure cluring an extraordinary flood. The failure of a small
dam during a PMF event may have a negligible impact on the downstream
hydrograph some distance from the dam. In such instances, SEFs smaller
than the PMF would be appropriate. Likewise, it may be the case that
downstream areas would already be flooded and evacuation activities com-
pleted before a PMF would overtop some reservoirs, resulting in dam
breach. This is again an instance in which SEFs smaller than the PMF are
appropriate based upon an incremental hazard analysis.
It is the committee's recommendation that for proposed high-hazarc]
clams, the PMF should serve as the SEF unless risk analyses that examine the
incremental impact of overtopping and dam failure during an extraordinary
flood demonstrate that little or nothing is gained by such a high standard. In
such instances, smaller SEFs should be aclopted. A reasonable SEF would be
the smallest value that ensures that a dam breach results in no significant
increase in potentials for loss of life or major property damage.
99
CRITERIA FOR EXISTING HIGH-HAZARD DAMS
Prescribing an appropriate safety evaluation floor] for an existing dam
where failure could result in significant loss of life or property damage in
downstream areas should involve a number of considerations. One ap-
proach, which some agencies have considered in past years, is to require that
all existing clams in such high-hazard situations be capable of passing current
PMF estimates. Such a requirement raises problems because of the follow-
~ng:
· In some instances, little additional safety would be provicled by modify-
ing a project to pass the most recent estimate of the PMF.
· For some existing dams it would be extraordinarily costly to modify the
project to accommodate the full PMF.
· It is rather general practice in some other fields of endeavor not to
require that a facility, designee] to meet one safety standard or criteria, be
retrofitted or modified to meet newly adopted criteria unless the existing
facility is currently judged, given new evidence, to expose the public to
unacceptably large and immediate risks.
These factors suggest that some criterion in addition to the PMF should be
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100
SAFETY OF DAMS
considered for the SEF for at least some existing dams. Whether an existing
dam should be subjected to the same safety criteria as proposed new dams
becomes a fundamental question in public policy, of balancing risks among
various interests, as mentioned in Chapter 2.
If the PMF is ruled out, the following types of alternatives may be consid-
ered as bases for a safety evaluation for an existing dam:
· a flood having some selected estimated annual probability or average
return period;
· a project-specific evaluation based on trial analyses of effects in down-
stream areas of potential dam failures during floods of various magnitudes
(this risk analysis approach is discussed further below); and
· a flood that is some arbitrary fraction of a PMF (or perhaps derived
from a fraction of the PMP) or of a flood having some estimated probability.
As noted in Chapters 4 and 5, although arbitrary criteria of the last listed
type are in use by some agencies, there are significant disadvantages to use of
criteria based upon fractions of a PMF or fractions of a flood of certain
estimated probability or combinations of such floods.
Probability-based criteria, as noted earlier, offer advantages in any type
of risk-cost analyses or comparisons of various types of risks. However, our
limited abilities to reliably assign probabilities to rare floods have restricted
the usefulness of the probability approach. As noted in Chapter 2, even if it
were possible to accurately predict the probabilities of all sizes of floods at a
dam site, there would still be the problem of selecting an appropriate basis
for testing dam safety. There is little in the way of general guidance or
precept for directly choosing an SEF for a high-hazard dam based on esti-
mated frequencies. California specifies the 1,000-year return period flood
for testing small dams in remote farm areas. The Institution of Civil Engi-
neers in the United Kingdom recommends use of the 10,000-year return
period flood or one-half PMF (whichever is larger) for design of a dam where
failure of the dam will endanger a community but rare overtopping is tolera-
ble.
From the above it is apparent that there is not one universally satisfactory
approach to establishing spillway capacity criteria for existing high-hazard
dams. There are some dams where the additional damage and loss of life
caused by a dam failure due to overtopping may justify protection for the full
probable maximum flood. Other situations may indicate that protection
against the PMF is desirable but compromise on such items as freeboard
allowance could be tolerated. At present, the best-attainable flood fre-
quency estimates for streams in the United States cannot be used directly to
determine spillway capacity requirements with confidence that future expe-
rience will not greatly change the estimated probabilities. However, as dis-
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Proposed Hydrologic Criteria
101
cussed below, even crude extrapolations of flood frequency curves may give
satisfactory bases for comparisons of alternative modification plans by use of
risk analysis procedures. Thus, the use of risk-based analysis should be con-
sid~ered in safety evaluation of any existing high-hazarc] dam for which the
PMF is not required.
As suggested in Chapter 7, for existing dams the primary need in an
evaluation may be to estimate the probable safety of a dam over a relatively
short time in the future. Appendix D outlines procedures for estimating
probabilities that events will occur within definite time periods once esti-
mates of the annual probabilities or average return periods are established.
As indicated by the above, the committee considers that there is no single,
universally correct approach to evaluating the safety of all existing high-
hazard dams against extreme floods. The characteristics of each such dam,
its drainage basin, the purposes served by the project, the area that wounds be
affected by dam failure and the development in that area should be consid~-
ered~ in arriving at an appropriate SEF for the project. As a preliminary
guide to such consideration the following sequence of activities is suggested
freer an "Yictina ream that in ~xnr~ted to remain in mace for an indefinitely long
· ~ ^ ~ ~— _~ rev ~ ~——t~ ~ ~ ~ ~
time:
· Develop the estimated PMF for the site. (It is considered that an esti-
mate of the probable maximum flood potential of the watershed should be
available for every dam classed a high hazard.)
· If it is reasonably probable that the dam wounds fad! if overtopped and
the incremental impact (marginal damages and potential loss of life) clearly
would~ be of such magnitude that potential for overtopping must be elimi-
nated insofar as reasonably possible, adopt the PMF as the SEF and proceed
to develop any needed remedial measures to assure that the SEF may be
safely passed with normal allowances for freeboard, etc. (In some situations
encroachment on the normal freeboard allowance by the SEF may be con-
sidered as acceptable.)
· If the dam would be overtopped! and probably fail during a PMF but it
is not clear that remedial work to permit safe passage of the PMF is justified,
determine the magnitude of the following floods:
(1) The maximum flood that can be passed by existing project works with
little or no danger of dam failure.
(2) The minimum flood for which failure of the dam would cause no
significant increase in downstream damages under present and foreseeable
future conditions.
If the flood determined by (1) is larger than that determined by (2),
consider the consequences of dam failure and loss of project services at a
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102
SAFETY OF DAMS
probable frequency indicated by flood (1~. If it were judged that such risks
can be tolerated, no remedial work to provide further safety against extreme
floods would be indicated.
· If the flood as determined in (2) above is larger than that determined by
(1) or if it is considered that the consequences of dam failures caused by a
flood such as determined by (1) are unacceptable, proceed with a risk-based
analysis such as discussed in Chapter 5 to develop further bases for decisions
on remedial work.
CRITERIA FOR INTERMEDIATE- AND LOW-HAZARD DAMS
Safety evaluations for intermediate- and low-hazard dams are primarily
concerned with the economic effects of their potential failures. However, a
continuing problem in such evaluations is the actual or potential develop-
ment of the area downstream from the dam after the dam is constructed and
the consequent change in the hazard ratings for the project. For this reason
any agency having responsibility for protecting the public interest in dams
should require periodic critical review of the hazard ratings for dams previ-
ously rated as intermediate and low hazard.
It is noted that dams having intermediate- and low-hazard ratings do not
occupy a prominent position in the programs of the agencies requesting this
study. Hence, no specific recommendations for safety criteria for these
classes of dams are presented. However, as noted in Chapter 5, some stan-
dardization among the agencies concerned with such dams in regard to
classification and safety criteria would be desirable and is encouraged.
RISK-BASED ANALYSES
Risk-based analysis when used to determine spillway capacity require-
ments provides the opportunity to weigh objectively the relative merits of
alternative modifications embodying either variation in scope or variation
in design concept. The difficulties associated with this approach relate to (1)
uncertainty associated with the probability assigned to floods in excess of the
100-year average return period and (2) the inability to place monetary val-
ues on such intangible considerations as the loss of life. The difficulties
associated with the uncertainties in assigning probabilities for remote flood
events can be partly overcome by performing sensitivity studies as part of the
risk-based analysis. Also, the potential for loss of life can be quantified in a
risk-based analysis but this loss of life aspect should not be combined with
economic considerations.
Despite the foregoing difficulties, the committee endorses the basic con-
cept of the risk-based method for some purposes. The method appears espe-
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Proposed Hydrologic Criteria
cially appropriate for examining alternative procedures for upgrading
spillway capacities of existing significant and high-hazard dams.
At this time the committee would caution against strict adoption of a
"benefit minus cost" rule or a "benefit over cost'' rule. The reasons are
severalfold, but relate to the uncertainty in assigning probabilities and the
obvious inability to quantify many of the broad social issues encountered,
which can range from the possible loss of life to environmental concerns.
Instead, the committee recommends initial ranking of the alternatives by
the average annual value of the tangible costs. The latter should be the sum
of incremental downstream or upstream damage caused by project opera-
tion and/or failure, cost of the proposed modifications, damage sustained by
the dam and appurtenant structures, and cost of interrupted dam services.
This initial analysis should be accompanied by a descriptive appraisal of the
other, i.e., nonquantifiable (intangible), considerations that need to be
brought to the attention of the decision makers and the public at large.
The foregoing procedures inherently involve the following consider-
ations. First, the absolute value of the economic calculations could differ
appreciably as procedures for determining the probabilities for flow be-
tween the 100-year and the PMF levels are varied. This explains the commit-
tee's reluctance to utilize specific benefit-cost rules. Second, although the
relative cost positions in a ranking of alternative mollification schemes may
be affected by the frequencies assigned to extremely rare events, sensitivity
studies should bring out this dependence and provide basis for judgments
among the schemes. For the present, the committee recommends develop-
ment of the frequency curves for average return periods in excess of 100 years
in accordance with procedures described in Appendix E.
103
HAZARD CLASSIFICATIONS FOR DAMS
Dams are often categorized as high, medium, or low hazard, depending
upon the potentials for loss of life and property damage existing down-
stream. Such hazard classifications are extremely useful for identifying dams
whose failure due to earthquakes, to floods, or to structural, piping, or
foundation problems could cause major losses. However, in contrast to dam
failure due to other causes, dam failure due to extreme floods only occurs
during flood events. Thus, the important dimension in terms of spillway
design is the incremental loss of life or property damage that would result
from dam failure when the river is already in flood. In many situations, low-
lying buildings, camp sites and picnic areas, and residences in the flood-
plain, will have been flooded and evacuated before peak flow rates are
reached and a large dam might fill and its emergency spillway fail. Thus,
one should not confuse the loss of life and property damages likely to occur
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104
SAFETY OF DAMS
from a "sunny day" dam failure, perhaps due to an earthquake, with the
incremental aspect of the loss of life and property damage due to dam failure
caused by an inadequate spillway during a major flood.
The considerable differences in hazard classifications in use by various
agencies are discussed in Chapter 5. However, as such classifications are not
of major importance in the programs of the agencies requesting this study, no
specific classification table for categorizing dam hazards is proposed. It is
suggested, however, that existing standards should be improved.
IMPACT OF PROPOSALS
Federal Implications
Part of the committee's charge was to comment on how its suggested
methods and criteria would impact federal costs. However, not enough
information is available to the committee to allow other than a few com-
ments based on value judgments.
The suggested criteria for design of new dams appear to be generally in
line with present application. Accordingly, no significant fiscal impact
should result from these recommendations. However, if as advocated by the
committee, it is no longer required that all existing dams meet current crite-
ria for design of new dams, significant savings in costs of rehabitating exist-
ing dams should be achieved.
Where risk-based analyses are utilized, it is quite clear that the costs of
analysis and design could increase significantly. However, it is believed that
application of this method will lead to significant reduction in modification
costs in certain cases. Therefore, the overall federal cost should be reduced.
Nonfederal Implications
This report has been prepared in response to a specific request submitted
by two federal agencies and with primary focus on federal projects. As the
economic analyses for most federal dam projects attempt to evaluate all
project-related costs, regardless of who bears such costs, the use of risk-based
cost analyses is pertinent to selection of levels of risk that can be tolerated at
such projects. Past congressional actions in compensating damages resulting
from failures of federal dams reinforce this view. Hence, the committee
advocates use of risk-based analyses for federal dams in the face of acknowI-
edged need for added research and development on this method of analysis.
Similarly, the nonfederal owner is confronted by the obvious trend toward
full liability for damages, as demonstrated by recent court decisions exhibit-
ing strict liability concepts. However, there are major differences between
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Proposed Hydrologic Criteria 105
the federal government and most nonfederal dam owners in their capabili-
ties to sustain a major loss resulting from a dam failure. Accordingly, at this
time, consideration of the public interest may cause these federal and state
agencies that regulate dams in the interest of public safety to delay in adopt-
ing the risk-based analysis methods, although some state agencies already
permit such practices. In the meantime, nonfederal owners, regulators, and
designers will need to keep abreast of potential research findings relating to
risk-based methods. Until the risk analysis approach can be extended to the
nonfederal field, the potential overall savings to society that it appears to
offer will not be realized.
Representative terms from entire chapter:
existing dams
