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5
Design Flood Estimates:
Methods and Critique
TYPES OF APPROACHES
As indicated in Chapter 3, most current criteria for spillway adequacy can
be placed in two general classes: (1) criteria baser] on probable maximum
precipitation (PMP) estimates or probable maximum flood (PMF) estimates
derived from PMP estimates ant] (2) criteria based on either floods or rain-
falIs having specifier] probabilities or average return periods. A third ap-
proach to sizing spillways by analysis of risks to downstream areas from dam
failures has come into use, and a variation of this approach, involving eco-
nomic analyses of risks and costs of prevention, is also in use. The PMF is
considered to define the upper range of flood potential at a site. Because of
the methods user] in developing safety evaluation flood estimates, the crite-
ria based on PMP and PMF estimates are termec! the deterministic approach,
while those based on rainfalls or floods of specified frequencies are labeler]
the probabilistic approach. These approaches to establishing appropriate
design flood estimates are discussed in this chapter, and further details rele-
vant to the deterministic and probabilistic methods are presented in Appen-
dixes C and D. Details of the risk analysis approach, along with examples of
its use, are presented in Appendix E.
DETERMINISTIC APPROACH
Summary
In estimating a PMF or any design flood by deterministic methods, several
tools of meteorology, hydrology, and hydrologic engineering are employed
44
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Design Flood Estimates
45
to synthesize a hydrograph of inflow into the reservoir and to model or
simulate the movement of the flood through the reservoir and past the dam.
The various steps in such analysis, all of which have been discussed in the
NRC's report, Safety of Existing Dams: Evaluation and Improvement,
Chapter 4 (1983), are generally as follows:
1. dividing drainage area into subareas, if necessary;
2. deriving runoff model;
3. determining PMP using criteria contained in NOAA Hydrometeoro-
logical Report series;
4. arranging PMP increments into logical storm rainfall pattern;
5. estimating for each time interval the losses from rainfall due to such
actions as surface detention and infiltration within the watershed;
6. deducting losses from rainfall to estimate rainfall excess values for
each time interval;
7. applying rainfall excess values to a runoff model of each subarea of
the basin;
8. adding to storm runoff hydrograph allowances for base flow of
stream, runoff from prior storms, etc., to obtain the synthesized flood hydro-
graph for each subarea;
9. routing of flood for each subarea to point of interest;
10. routing the inflow through the reservoir storage, outlets, and spill-
ways to obtain estimates of storage elevations, discharges at the dam, tailwa-
ter elevations, etc., that describe the passage of the flood through the
reservoir. (This is essentially a process of accounting for volumes of water in
inflow, storage, and outflow through the flood period. If there are several
reservoirs in the watershed, the reservoir routing is repeated from the upper-
most to the most downstream reservoir, in turn.)
If the routing shows that the dam would be overtopped and if it is assumed
such overtopping will cause the dam to fail, the resulting flood wave may be
routed through the downstream valley to give a basis for assessment of
damages.
Probable Maximum Precipitation (PMP) Evaluation
Of the factors that have an influence on the magnitude of the probable
maximum flood (PMF), the intensity and duration of rainfall are the most
important; hence, considerable discussion of such rainfall estimates follows
here and in Appendix C. The rainfall that produces the PMF is termed the
probable maximum precipitation (PMP). Its definition (which goes back to
the early 1950s) is "the theoretically greatest depth of precipitation for a
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46
SAFETY OF DAMS
given duration that is physically possible over a given size storm area at a
particular geographical location at a certain time of the year." Several other
definitions have been given for PMP, but, in any case, PMP should be termed
an estimate, as there are yet unknowns and unmeasured atmospheric pa-
rameters that are important to extreme rain storms.
In the literature prior to about 1950, the term maximum possible precipi-
tation (MPP) was used for estimates of most extreme rainfall. This was
changed to probable maximum precipitation because the latter term implies
a somewhat less extreme and absolute estimate and reflects the uncertainties
involved in estimating maximum precipitation potentials. In practice, both
MPP and PMP have essentially the same meaning.
PMP estimates usually greatly exceed the rainfall amounts most people
experience. Table 5-1 lists current PMP estimates for the Washington, D.C.,
area as read from charts developed by Schreiner and Riedel (1978~. By
contrast, the maximum 24-hour point rainfall that has been recorded in 114
years of record at Washington, D.C., is 7.13 inches. However, within 125
miles, near Tyro, Virginia, 27 inches of rain fell in 12 hours during a storm
occurring august 19-20, 1969.
In Chapter 4 a brief history is given of the development of the concepts
involved in PMP determinations. Such determinations are described in some
detail in Appendix C and summarized sequentially as follows:
1. Study major rain storms to determine maximum areal rainfalls and
ascertain, as well as possible, the meteorological factors important to the
rainfall.
2. Transpose the major storms within topographically and meteorologi-
cally homogeneous regions. (Adjust the storm rainfall by multiplying it by
the ratio of an index of maximum atmospheric moisture in transposed loca-
tion to that where the storm occurred.)
TABLE 5-1 PMP Estimates (to Nearest 0.5 Inch) for Vicinity of
Washington, D.C.
Area Time Duration (h)
(sq mi) 6 12 24 48 72
10 27.5 32.S 36 40 42
200 19.5 23 27 31 32
1,000 14 18 22 25 26
5,000 8.5 12 15 19 19.5
10,000 6.5 9.5 12.5 15.5 17
SOURCE: Schreiner and Riedel (1978~.
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Design Floor! Estimates
3. Adjust the rainfall (for each transposed storm) by the ratio of maxi-
mum atmospheric moisture in place of occurrence to that which existed
cluring the storm.
4. Smoothly envelop the resulting rainfall values durationally, areally,
and if generalized PMP is being developed, regionally. Explanations should
be given for discontinuities.
47
The resulting rainfall values may be regarded as PMP if sufficient storms
were available to justify the assumption that the maximum rainfall potential
has, in fact, been assessed. Appendix C describes some procedures that give
the user a fee} for how conservative the PMP is. One shows the ratios of PMP
depths for a 10-square-mile area and 24-hour duration to the point rainfall
depths for 100-year average return period and 24-hour duration at the same
locations. These ratios range between 2 and 6 in the United States. Another
study shows that 90 known experienced storm rainfall depths are equal to or
greater than 50 percent of PMP, both rainfall categories being for 24 hours
and 10 square miles. Many more storms exceed 50 percent PMP for several
combinations of durations and area sizes (Riedel and Schreiner, 1980~. Fig-
ure 5-1 shows a comparison of generalized PMP for 200 square miles and 24
hours determined in 1947 with that in 1978 for the same region (United
States east of the 105th meridian). This comparison shows a general increase
in PMP estimates over the 31-year period. About 65 percent of the region
shows an increase between 10 and 30 percent. From such comparisons it can
be concluded that the PMP estimates for the United States from generalized
charts determined by the National Weather Service are most likely on the
low side when evaluated with reference to the accepted definition of PMP
and that future increases in PMP estimates are probable. However, such
future increases in estimates are likely to be incrementally less, in general,
than those of the past during the period that the science of hydrometeorology
was rapidly developing and the data base on past extreme events was being
accumulated. Also, studies to redetermine probable maximum precipitation
estimates for areas in the Tennessee Valley made in 1978 resulted in some
lower estimates than those developed for the same areas in 1968; then more
intensive studies sometimes can result in lower estimates even if more data
became available.
PMP estimates developed as indicated have been widely (but not univer-
sally) accepted as the appropriate basis for design of spillways for large dams
where failure of the structure by overtopping cannot be tolerated. The
increases noted in PMP have posed difficult questions as to what should be
clone with spillways at existing dams or those already under construction,
where the spillways were adequate under previous criteria but would not be
adequate with the revised PMP estimates.
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48
SAFETY OF DAMS
1 In 1.30
1.00
4~r 1~- ~':~:C
. .
1 / ( —~ ~ 1.30
_ ~—__
~ —1
--1
1'''' t: ~ ~ ~ ~ ~~
.30
.2~1 s7 \` ~ STATUTE MILES
:s ~ 100 0 100 200 300
- 1 Do 0 1 00 200 300 400
KILOMETERS
A- - -_1_~_: 'I
FIGURE 5-1 Comparison of generalized PMP estimates for 24 hours and 200 square miles
made in 1978 with those made in 1947. (Ratios shown are 1978 values/1947 values.) Sources:
The IS78 data are from Schreiner and Riedel (1978). The 1947 data are from U.S. Weather
Bureau (1947).
Antecedent Conditions
An important consideration in assessing the impact of any extreme flood
on a project is the spectrum of antecedent conditions. These conditions
include soil moisture, snowpack and water content where applicable, ex-
pected reservoir levels, state of vegetation, intended use or uses of the proj-
ect, probability of preceding and subsequent precipitation events, and
ambient temperatures.
It is generally recognized that even though a hypothetical flood, such as
the PMF, is an extreme event to be adopted as a basis for design, it should be
conceived in hydrologic and meteorological reasonableness. The antecedent
conditions, such as expected reservoir levels, existing snowpack, and soil
moisture, are considered in the context of the causative event of the primary
flood. For example, if the extreme floods for moderate to large basins result
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Design Flood Estimates 49
from tropical storms that occur from late summer through fall and highest
reservoir levels always result from spring snowmelt floods, the two events are
not usually combined. In another instance, the test of reasonableness might
indicate that in mountainous country, major floods for small basins could
result from a heavy rain producing convective thunderstorms, but not of the
PMP magnitude, that could occur concurrently with the seasonal snowmelt-
generated peak runoff. Generally, it can be considered meteorologically
unlikely for the maximum snowmelt runoff flood to occur in combination
with any extreme precipitation event. In each situation, it is desirable to
evaluate meteorologic reasonableness of the criteria rather than to apply
arbitrary rules.
Soil moisture and the state of the vegetation affect loss rates and basin
runoff characteristics. These vary seasonally and should be evaluated to
determine appropriate conditions for occurrence prior to the PMF. Anteced-
ent (and subsequent) storm conditions can have an impact on the adequacy
of any reservoir design.
The effects of the antecedent (or subsequent) storm are related to storm
magnitude, area covered, season of occurrence, dry interval between
events, and geographic region of the primary rainfall event. It is not feasible
to establish general criteria for such conditions. Each situation should be
carefully evaluated to assure that assumed conditions are sufficiently con-
servative but not atypical of the region.
The basic purpose for which the project is designed governs the rules for
reservoir operations. Customarily this rule curve is the end-product of study
of annual inflow patterns, the schedule of need for releases, the use of these
releases, and any site-specific restrictions imposed on reservoir discharges
and reservoir levels by downstream channel capacities and riparian inter-
ests. These are additional items that must be weighed in importance, and
from this analysis is obtained a reservoir level to be used as the initial point in
the routing of the design flood and the antecedent storm through the storage
and spillway facilities of the site. Usually to avoid increasing downstream
flood damages, any assumed reservoir operation plan should include the
requirement that no operating procedure will increase the peak reservoir
outflow over the peak natural inflow unless specific flood easements to
accommodate excess flow downstream have been obtained. As an example,
drawdown of reservoir storage in anticipation of a tropical event, which
ultimately tracks away from the project catchment, could produce down-
stream flows in excess of the inflow that did occur. If this excess flow ex-
ceeded channel capacity, it could lead to downstream property damage and
claims. Often, however, the orderly drawdown of reservoir storage in those
climates where the annual flood event is a snowmelt case, is an accepted
practice. This acceptance is based on the fact that the seasonal snowmelt
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50
SAFETY OF DAMS
runoff volume may be closely estimated with present-day prediction tech-
niques.
Reservoir Routing
The ability of any reservoir to release water downstream is determined by
the project components. Usually these components are comprised of some
combination of spillway gates, flashboards, stanchion bays, low-level out-
lets, and turbines if the project generates power. In preparation for routing
the design flood through the project storage and these facilities, rating curves
of discharge versus head must be computed for each applicable component
and for the proposed combination of discharge facilities to be used.
As a case in point, generally a project designed for flood control will limit
releases to the bankful channel capacity downstream up to the point where
the inflow has occupied the allocated flood storage volume. From that point
on, the typical emergency spillway operates so that the discharge is a func-
tion of the head on the spillway, the incremental flood storage usually being
fairly small.
In those cases where flood control is not the primary purpose of a project, a
gated spillway is usually operated when the inflows exceed the capacity of
outlet works, including hydropower turbines. This gate operation is usually
carried out so that the reservoir level remains constant, thus the outflow is
matched with the inflow up to the point of maximum gate operation. Some
regulatory bodies require a gate capacity such that a specific percentage of
the PMF can be accommodated with one gate inoperable without the dam
being overtopped.
The routing of the PMF inflow hydrograph through the available reser-
voir storage and the spillway facilities of the project utilizes some variation of
the volumetric conservation equation:
I - 0 = AS
where
I = reservoir inflow
0 = the outflow or discharge
AS = change of storage in reservoir
The mechanics of applying and solving this basic equation are given in the
standard hydrologic texts and will not be described herein.
The preceding discussion is a simplified explanation of what can be very
complex operational studies of effect of extreme storm rainfall on a basin and
a reservoir. In a typical safety evaluation, judgment is required on many
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Design Flood Estimates
51
factors. Experienced hydrologic engineers may differ to such an extent that
their assessments of project safety will be affected. Some may tend to adopt
the most critical value for each parameter involved and, thus, tend to make
the PMF estimate and routing excessively improbable. Others may tend to
regard each of the aspects as independent of the PMP and other factors and
adopt average or medium values for each factor. Detailed studies of each
situation may provide a guide to selection of the most logical array of choices
for that project, but it is not practicable to attempt to formulate rules for
such choices at all projects.
THE PROBABILISTIC APPROACH
As indicated in Chapter 3, safety evaluation floods for small dams, where
no serious hazards would exist downstream in the event of breaching, are
usually based on rainfall-runoff probability estimates. As discussed in Chap-
ter 4, the occurrence of floods in some basins far larger than could have been
predicted by probability studies of prior stream records has discouraged the
use of probabilistic methods for estimating extremely rare floods. However,
the state of California uses estimates of floods with average return periods of
1,000 years as minimum floods for evaluating safety of low-hazard dams. In
contrast to practices in the United States, the criteria of the Institution of
Civil Engineers call for use of estimates of floods with average return periods
of 10,000 years in the British Isles. Past experience has indicated that esti-
mates of magnitudes of very rare floods developed by probabilistic methods
are even more likely to change as additional basic data become available
than flood estimates developed by deterministic methods discussed earlier in
this chapter.
Some of the basic principles of probability studies are discussed in Appen-
dix D. Flood-frequency analyses as discussed in Appendix D produce esti-
mated instantaneous peak flows with no estimate of flood volume. A
complete hydrograph is needed to perform a flood routing through the
reservoir of a given dam. Such a flood hydrograph can be synthesized by
utilizing an observed hydrograph from a major historical flood and increas-
ing the ordinates of that hydrograph by the ratio of the peak flow determined
by frequency analysis to the observed peak flow.
Rainfall frequency data can provide another way of synthesizing a flood
hydrograph with desired estimated frequency of occurrence. Such a hydro-
graph can be developed utilizing an appropriate rainfall-runoff model and
rainfall frequency data such as available from NOAA Atlas 2 (Miller et al.,
1973).
Once a hydrograph of inflow into the reservoir, representing the safety
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52
SAFETY OF DAMS
evaluation flood, is obtained, the routing and analyses procedures are essen-
tially as described for the deterministic approach.
THE RISK ANALYSIS APPROACH
Methods of evaluating dam safety by analysis of effects of hypothetical
dam failures on downstream areas and on project benefits and costs have
been advanced in recent years. These methods do not depend on adoption in
advance of specific bases or criteria for dam safety evaluation floods but
depend upon site-specific analyses to select the flood appropriate to the
safety evaluation. Two types of analysis are in use: (1) those that evaluate
only the hydraulic effects of dam failures and (2) those that go further and
make an economic analysis to determine the design that has minimum total
cost. Appendix E discusses and gives an example of the latter approach.
Through computerized modeling of floods and dam break inundation
mapping, the safety evaluation flood can be selected at the flood peak level
where downstream flood damages would not be increased by the overtop-
pingof the dam. In other words, through an iterative trial-and-error compu-
tation process, the spillway is sized so that all significant downstream flood
damages, from spillway releases and other sources, will have occurred be-
fore the dam fails by overtopping. This approach is allowed, as an alterna-
tive to selecting a design flood from a generalized chart, by the U.S. Bureau
of Reclamation and by several state dam safety programs including Arizona,
Colorado, Georgia, North Carolina, Pennsylvania, and South Carolina.
Pacific Gas and Electric Company, a private utility firm, also uses this
approach to evaluate existing dams. This alternative is also proposed in draft
guidelines prepared by a working group of ICODS, the Interagency Com-
mittee on Dam Safety (1983) . One of the limitations of this general approach
of evaluating only hydraulic effects is the possibility that subsequent down-
stream development will encroach on the dam-break inundation area and
thus change the conditions which determine that dam failure would cause
no further significant damages. Another limitation is that potential damage
to project structures and the value of project services (e. g., water supply) are
not reflected in the analysis.
A more complex version of this approach is to estimate the dollar cost of
damage, loss of services provided by the dam, and construction costs of
several design alternatives; to estimate the probability of failure for each
alternative; and to select the final design at the lowest risk-cost combination.
No dollar value is assigned to human lives; in some cases, downstream warn-
ing systems and evacuation plans have been relied on to avoid putting hu-
man lives into the risk-cost analysis.
The quantitative risk-cost analysis approach has been applied to very few
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Design Floorl Estimates
dams and is such a recent development that it can barely be called "current
practice." However, its use in selecting design standards can be expected to
increase in coming years. Risk analysis procedures have been defined by the
Bureau of Reclamation (1981a) for internal use. The Interagency Commit-
tee on Dam Safety (1983) encourages site-specific breach-routing studies as
part of the hazard assessment for proposed dams and suggests that risk-based
analysis may be a basis for decision on selection of the safety evaluation flood
at particular existing dams.
53
CRITIQUE
Some of the deficiencies or limitations observed in currently used criteria
and procedures relating to provisions for safety of dams from extreme floods
are inherent to any attempt to deal with the random-chance nature of rare
floods. This discussion is not intended necessarily as criticism of the criteria
or procedures nor of those groups who use them. The comments herein are
based on the array of criteria and practices in current use and may not always
be applicable to the programs of the Corps of Engineers and the Bureau of
Reclamation.
The goal of dam safety is to limit the risks from dam failures to acceptable
levels. Probability of failure is controlled partly by design standards and
partly by quality of design, construction, inspection, operation, and mainte-
nance. Ideally, hazard, failure probability, and acceptable damage would
be quantified for the site-specific conditions of each individual existing or
proposed dam in order to establish site-specific standards for achieving this
goal. With few exceptions, current practices do not involve quantification of
these three critical elements for each dam.
Instead, the most widespread current practice is to classify dams in three
broad, not well-defined, qualitative damage potential categories (i.e., high,
intermediate, and low hazard) and to somewhat arbitrarily assign one of
three or four grades or ranges of design standards to each dam depending on
its height, storage capacity, and qualitative hazard rating. Current practice
treats all of the elements needed for selecting design standards in a general-
ized way; thus, the appropriateness of the design standards as applied to
individual dams is generally unknown.
In defense of this current general practice, it must be recognized that most
of the scores of federal and state regulatory agencies each have hundreds to
thousands of dams under their jurisdictions. Given their limited resources,
as a practical matter, they must use a generalized system of assigning design
standards according to generalized hazard and size classifications, at least as
an interim step until more detailed site-specific studies can be made. How-
ever, the wide range of hazard versus size versus design standards among the
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54
SAFETY OF DAMS
various agencies (see Table 3-3 and Appendix A) reflects a lack of uniformity
even within the generalized current practice.
This lack of uniformity in dam classification and safety design standards
appears to result from three main factors: (1) lack of interagency and inter-
governmental communication, (2) variations in engineering judgment in
selecting the generalized standards, and (3) variations in public policy atti-
tudes at the times the standards were selected. In any case, a critique of
present practices must point out that, though a generalized approach to
selecting design standards is justified as a practical interim step, there is a
need for more uniformity among the various federal and state agencies in
establishing size and hazard definitions and correlative design standards.
Dam Classification Systems
Even if we recognize the need for generalized hazard versus size versus
design criteria classifications, the almost universally used high-, intermedi-
ate-, and low-hazard classes are not well defined. Qualitative definitions for
these terms, such as those used by the Soil Conservation Service and Corps of
Engineers, are followed by most federal and state agencies.
Some examples of the lack of uniformity in defining "hazard" among the
many regulatory agencies are as follows:
· One federal agency estimates damage potential (i.e., hazard class) as-
suming "sunny day" failure, while another federal agency assumes failure
only during "floods" as a basis for its hazard classification.
· In defining high-hazard dams, agencies use such terms as "probable"
loss of life, "possible" loss of life, "rural" and "urban" houses downstream,
and one agency says that more than 10 houses in a dam-break floodwave
places the dam in a high-hazard category. One state agency says a dam is
high hazard if there is potential "extensive" loss of life, significant hazard if
the dam would endanger "few" lives; another defines a dam as high hazard if
there would be "substantial" loss of life, intermediate hazard if a "few" lives
would be lost. In contrast, some agencies consider the probable loss of one
human life as a high-hazard condition.
· No agencies define the dollar value of "extensive," "significant," or
"minor" economic losses in their hazard classes.
An attempt to quantify these hazard definitions was made by the North
Carolina Dam Safety Program in 1980 and 1982. Questionnaires asking
respondents to quantify the minimum values for each hazard class were
distributed among the program staff and among participants at a Southeast-
ern Regional Dam Safety Conference. The results (heretofore unpublished)
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Design Flood Estimates
TABLE 5-2 Boundaries for Hazard Classes
55
Mean Values of Opinions
Hazard Probable Loss
Classification of Life Economic Lossa
Low 0 Oto $30,000
Significant 0 $30,000 to $200,000
High 1 ormoreb Greater than $200,000
aIncludes downstream damages, but not cost of dam or value of services provided by reservoir.
bStrong consensus that loss of one life defines high hazard.
reflect an extremely wide range of opinions but indicate the following me-
dian opinions from the 46 individual respondents for quantifying the bound-
aries on hazard classes (Table 5-2~.
Undoubtedly, others will disagree with these evaluations, but such an
effort toward more specific hazard definitions could be a step toward a more
uniform approach to setting generalized standards.
Another weakness in current practice is that, generally, downstream haz-
ards are only roughly estimated through judgment based only on visual
inspection. This practice is a reflection of limited resources rather than
technology, however, and it is reasonable to believe that essentially all prac-
titioners recognize the desirability of inundation mapping through breach-
routing methods.
Spillway Capacity Criteria
As shown in Chapter 3, there has been general agreement, with some
exceptions, that the spillways of large, high-hazard dams should be able to
pass the probable maximum flood without the dam being overtopped. All
federal agencies agree with this standard. Only a few states indicated that
smaller floods are used as criteria for spillway capacity at such dams. One
other type of exception sometimes encountered involves concrete dams on
solid rock foundations. Indicated overtopping of such a dam during the
probable maximum flood may be permitted by some agencies, if the rock at
the toe of dam is judged able to withstand the hydraulic forces imposed and
the stability of the dam would not be compromised otherwise. For smaller
dams and those with lower hazard ratings, there is much greater divergence
in views concerning appropriate spillway capacity requirements. As noted
in Chapter 4, the rationale for some of the spillway capacity criteria in use
seems questionable, particularly the criteria based on an arbitrary percent-
age of the probable maximum flood or an arbitrary percentage of a flood of
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56
SAFETY OF DAMS
specified probability or criteria that combine the probabilistic and determi-
nistic approaches. The problem with such a criterion, based on an arbitrary
percentage of a derived flood or on arbitrary combination of floods devel-
oped from differing concepts, is that it permits no direct evaluation of the
relative degree of safety provided.
While regional differences in climate, geography, development, etc.,
could justify some of the differences in spillway capacity criteria, it appears
that not all the criteria could be efficient in limiting risks of dam failures to
acceptable limits or in protecting the public interest. Efforts to secure more
uniform approaches to specifying spillway capacity should be encouraged,
but such effort is considered beyond the scope of this report. The newly
established Association of State Dam Safety Officials may wish to consider
action toward such a goal.
Some differences among agencies have been noted in practices followed in
developing probable maximum flood estimates from probable maximum
precipitation values. These differences relate to assumptions regarding ante-
cedent rainfalls, initial reservoir levels, arrangement of precipitation values,
runoff models, etc.
The committee has found general agreement in the following observa-
tions regarding current spillway capacity criteria:
· Interpretations of data from past storms and storm mode} concepts are
required to make estimates of PMP.
· As shown by past experience, PMP estimates can change as more data
become available; thus, the PMP estimate cannot be regarded as a fixed
criterion, but confidence in the estimates should rise with successive PMP
estimates for a given locality.
· The probability that rainfall will equal or exceed current PMP esti-
mates is indeterminate but probably not uniform for projects in different
parts of the country.
· In order that judgments can be made on appropriate allocation of
resources, it would be desirable to be able to express spillway design flood
criteria in terms of annual probabilities.
· As has been found previously, statistical studies of data from past floods
may serve to indicate the minimum spillway capacities that should be con-
sidered but generally do not provide reliable basis for spillway design if there
are significant or high hazards downstream in the event of dam failure.
· As a dam owner or as a regulator of dams in the interest of public safety,
a government agency should seek to achieve a proper balance between costs
to improve dam safety and risks to the public.
· It is appropriate that dam safety criteria recognize differences in conse-
quences of failure.
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Design Flood Estimates
· It is good public policy to require management plans (such as warning
systems, evacuation plans, and operating rules) to reduce hazards (princi-
pally to lives) of dam failures, but the long-term use of such plans should not
be a substitute for work to remedy serious safety deficiencies. (The commit-
tee notes that maintenance over long time periods of an effective emergency
management system to avert loss of life from failure of a dam would require
continuing cledicated efforts and support to maintain and operate hard-
ware, to train and inform emergency action personnel and the public, to
establish and maintain institutional arrangements, ant] to upgrade the sys-
tem to meet changes in the area to be served.)
· Even though some problems with current PMP estimates have been
noted, such estimates still offer the bases on which the engineering profession
has the most confidence for sizing spillways of new, large dams in the high-
hazard category.
· Each existing large, high-hazard dam having a spillway that fails to
meet current PMF criteria should be considered separately. It does not seem
appropriate to adopt fixer] rules for such situations. Each study should con-
sider how deficient the project is under current criteria and the relationship
of the allocated spillway capacity to other floor] criteria. If the deficiency
relates to change in safety evaluation criteria (such as an increase in PMP
estimates), the reasons for such change and their relationship to the project
in question should be critically examined.
57
Risk-Cost Analyses
As described earlier in this chapter, the risk analysis approach has pro-
vided a significant trend toward improved assessments and toward selecting
more rational, site-specific spillway evaluation standards within the last few
years. Though risk-cost analyses may appear to represent the most desirable
approach to the goal of dam safety (i.e., in quantifying hazard, failure
probability, and acceptable damage) at this time, this method has certain
important problem areas or limitations that the user needs to consider. An
ICODS critique of the risk-cost analysis method mentions the following
points.
· Estimates of the probability of exceedance of extreme hydrologic events
are imprecise, whereas the total costs associated with different alternatives
may be sensitive to these estimates.
· Those factors which cannot be measured in economic terms such as loss
of human life, social losses, and environmental impacts are more difficult to
reflect in the risk analysis but may be the most important in making deci-
sions.
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58 SAFETY OF DAMS
· Results of a quantitative risk-based analysis reflect probable annual
. . ~ . , . . , . . . ~ . · . ..
costs but a dam failure may result in a single catastrophic toss from wn~cn the
owner and many others may not recover; thus the relevancy of the analysis to
the interests of the parties involved may be questionable.
Additional problem areas noted in risk-cost analyses are as follows:
· Future development below a dam is usually unpredictable and may
invalidate the risk-cost determination and the safety evaluation flood se-
lected for present (or inaccurately predicted future) conditions.
· There may be a tendency to rely on downstream warning systems to
eliminate loss of human lives from the analyses and thus to determine the
lowest risk-cost design standard. However, the real effectiveness of a down-
stream warning system may be questionable and reliance on such systems
may give a false sense of security to design engineers, dam owners, ant!
residents below a dam.
· Risk analysis places heavy emphasis on hydraulic evaluations of un-
usual situations at dams and downstream. However, the reliability of flood-
and dam-break-routing models has not been sufficiently determined; the
models have been checked against only a few actual dam-break floods. Yet
the accuracy of modeling flood and dam-break inundation areas is often
critical in a risk-based analysis. One unknown in even the best dam-break-
routing models is the "rate of breach development" to assign during the
modeled failure; the estimate of damages from relatively small reservoir
dam failures is often extremely sensitive to rate of failure (rate of reservoir
release), particularly for "sunny day" failure conditions. Also, in quantify-
ing downstream damage predictions (hazard) in risk-based analyses, the
water depth/velocity and debris load required to damage various structures
are sometimes uncertain. This can have a critical effect on the accuracy of
the computed lowest risk-cost.
· The depth and duration of overtopping that various dams can with-
stand without failure are unknown. It may be desirable economically, and
physically safe, to allow some overtopping of existing dams in a risk-based
analysis, but there are no reliable data on tolerable limits. This has dramatic
economic implications nationwide that will not be resolved by risk-based
analyses.
· Although a risk-based analysis may not be expensive if compared to
probable costs of remedial measures to improve dam safety, for some dam
owners such an analysis may not appear to be economically feasible for
smaller (though high- or intermediate-hazard) dams. Further, some regula-
tory agencies may not have the financial and technical resources to conduct
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Design Flood Estimates
risk-based analyses on tens of thousands of dams in order to set appropriate
site-specific design standards.
59
The above-discussed areas of potential problems in application of risk
analysis technique do not, necessarily, detract from the usefulness of this
type approach, as long as they are recognized and provided for. In fact, it is
expected that, with more experience and research, these limitations may be
minimized. Other factors that make risk-based analysis an attractive tech-
nique are as follows:
· Risk-based analyses, as presently performed, generally are not in-
tended to replace appropriately conservative design standards. Rather, risk-
based analyses provide additional information to decision makers to help
them decide how limited funds can best be allocates] to reduce risks.
· Risk-based analyses are not intended to provide a sole basis for making
decisions. They only provide a portion of the information needed.
· By performing sensitivity studies, many of the problems with perform-
ing a risk-based analysis can be minimizer] and the results bounclecI.
· The process of performing a risk-based analysis often uncovers factors
or sensitivity relationships that might otherwise not be identifiecI.
· Those factors that cannot be measured in economic terms, such as loss
of human life, can be accounted for in separate risk-based analyses and given
the appropriate weight (as implied in Appendix E).
Overview
This critique of current practices has focused on three levels of sophistica-
tion in setting standards: (1) the widespread generalized approach, relying
largely on judgment to assess hazard and selecting design standards based on
loosely defined categories; (2) using site-specific dam-break-routing studies
to better define hazard and to select a spillway design flood without quanti-
fying risks and costs; and (3) risk-based analysis, which extends the second
category by attempting to quantify all of the significant variables in selecting
standards. Some of the main strengths and deficiencies associated with each
of these three levels have been discussed. Two other deficiencies must be
pointed out in a broad overview of current practice. First, about one-half of
the states either have no standards for nonfederal dams or have seriously
inadequate implementation of standards (Tschantz, 1983, 1984~. Since
there are well over 60,000 nonfederal dams in this country, this current
practice (really, lack of applying any standards) has serious national implica-
tions regarding the achievement of safety goals for dams. Second, most
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60
SAFETY OF DAMS
standard-setting efforts have been focused (as does this report) on large,
high-hazard, federally owned dams where it is clear that very high standards
must be applied and where public investment in the dams and their services
is very important. However, smaller nonfederal dams (U.S. Army Corps of
Engineers, 1982) pose the greatest aggregate of risks nationally, and their
wide range of sizes and hazards requires a wide range of design standards.
Focus on the various aspects of the PMP or PMF for large, high-hazard dams
has tended to detract from the need for developing appropriate standards for
tens of thousands of smaller, yet very important dams.
In summary, design standards based on size ranges and general hazard
classifications of dams are a necessary evil from a regulatory or administra-
tive standpoint, but the diverse and ambiguous definitions within this pre-
dominant current practice reflect a serious lack of uniformity among federal
end state agencies in applying this approach. The generalized hazard defini-
tions are vague, and the appropriateness of the design standards applied to
the size/hazard ranges is generally unknown, leaving the appropriateness of
the generalized standards at specific dams even more in doubt than they
reasonably could be. Exceptions exist for very large, very high-hazard dams
for which there is a clear consensus that something like the probable maxi-
mum flood is the appropriate inflow design flood or is most likely the best of
the available alternatives for such applications. However, this design flood is
not necessarily appropriate for the thousands of existing smaller (yet high-
hazard) dams. Where site-specific studies are economically feasible, selec-
tion of the design standard] for each dam through dam-break routing studies
without placing a dollar value on predicted damages is definitely a tempo-
rary improvement over the generalized standards but is limited by the possi-
bility of future downstream development. Finally, risk-based analysis,
which attempts to quantitatively balance the total cost of alternative design
standards against probability of failure, has its own limitations in its present
state of development, but can provide information useful to decisions involv-
ing making dams safe from extreme floods.
Representative terms from entire chapter:
design flood
