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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.