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Summary of Present Practices on Dam Safely Standards INVENTORY OF CURRENT PRACTICES In preparation for this study, inquiries regarding current practices relat- ing to safety provisions for the hazards to dams from extreme floods and earthquakes were directed to the federal agencies most concerned with dams, the appropriate unit of each state government, several private engi- neering firms with worldwide prominence in the dam design field, other professional organizations with interests in dam safety, and a cross section of utility firms and other organizations that own dams. Also, there have been reviews of a number of standards or policy statements, issued by technical societies relating to the safety of dams against extreme floods and earthquake hazards. Responses to these inquiries and the pertinent actions of the techni- cal societies are summarized in Appendixes A and B. The data from 10 federal organizations, 35 state and local agencies, 9 private firms, and4 professional engineering societies provide a comprehen- sive overview of current practices in the United States and, to a great extent, in foreign countries. Because U.S. engineering firms are active in engineer- ing for dams in other countries and because U.S. engineers play a prominent role in such organizations as the International Commission on Large Dams (ICOLD), many practices followed in the United States, particularly those of major federal dam-building agencies, have been adopted in other coun- tries. However, there is considerable variation in the criteria adopted in the United States for evaluating the ability of dams to withstand extreme floods, especially in criteria for the smaller, less-hazardous dams. 15
16 CLASSIFICATION OF DAMS SAFETY OF DAMS Basic to all safety standards relating to hydrologic and seismic events are systems for classifying dams according to the probable damages caused by dam failure. As indicated by material in Appendixes A and B. there is consid- erable variety in the classification systems that have been adopted, and this variety often makes difficult any precise comparisons between criteria used by different agencies. Most systems for classifying dams specifically utilize dam height, volume of water impounded, and character of the development in the relevant downstream area as parameters in regard to probable effects of dam failure. The classifications used by the U. S. Army Corps of Engineers in the National Dam Inspection Program are typical of such systems, and for ease of refer- ence, the tables used in that system are shown below (Table 3-1~. Although the committee has not specifically recommended a system for classifying hazard potentials, usage of the terms "low," "significant," and "high" in this report when referring to hazards generally conforms to Table 3-1, with the term "intermediate" being used interchangeably with "significant." A number of federal and state agencies (e.g., U.S. Forest Service, Alaska, Illinois, South Carolina, and Virginian have adopted a classification system TABLE 3-1 Terms for Classifying Hazard Potentials Category Impoundment (ac-ft) Height of Dam (ft) Size of dama Small 50 to 1,000 25 to 40 Intermediate 1,000 to 50,000 40 to 100 Large Over 50,000 Over 100 Loss of Life (Extent Category of Development) Economic Loss Hazard potential classification Low None expected (no perma- Minimal (undeveloped to occa- nent structures for signal structures or agriculture) human habitation) Significant Few (no urban develop- Appreciable (notable agriculture, meets and no more than industry, or structures) a small number of inhab- itable structures) High More than few Excessive (extensive community, industry, or agriculture) aCriterion that places project in largest category governs.
Summary of Present Practices 17 either identical to or essentially the same as that displayed below. Other agencies and states (e.g., U.S. Soil Conservation Service, Arizona, Kansas, Missouri, and Pennsylvania) have systems that are similar but use the prod- uct of dam height (in feet) and storage volume (in acre-feet) as a size crite- rion. A number of states (e.g., Arizona, Arkansas, Georgia, Kansas, North Carolina, North Dakota, and South Carolina) have four or more categories of size. Arkansas appears to base its dam classification entirely on storage capacities and drainage areas, while Georgia and North Dakota utilize only height of dam and reservoir storage in their systems. New Jersey has three categories based on "hazard potential" and one labeled "small dams." The classification systems of the Tennessee Valley Authority, the State of Utah, and the Institution of Civil Engineers of the United Kingdom in general refer only to the level of hazards that would be created by failure of the dam. In some systems for classifying dams, overall evaluation of the factors affecting downstream hazards is implied, but criteria for such evaluation are not set out. The following types of classification criteria, described in Appen- dix A, are such systems: · "loading conditions" as used by the U.S. Bureau of Reclamation, · "functional design standards" as used by the U.S. Army Corps of Engi- neers, and · "security standards" as user] by the U.S. Committee on Large Dams (1970~. While it appears that many of the differences in dam classification systems are the result of arbitrary choices of regulatory authorities, it also appears that most of the classification systems have been structured to meet the perceives] needs of the issuing agency or state government. SPILLWAY CAPACITY CRITERIA Table 3-2 shows all the spillway capacity criteria as stated in agency standards in terms of either design rainfall or design floods reported to be in current use by the entities responding to the committee's inquiries. Criteria based on estimates of probable maximum precipitation (PMP), and proba- ble maximum flood (PMF) are widely used. In fact there is some indication that corresponding values (e.g., 0.50 PMP ant} 0.50 PMF) are used more or less interchangeably by some engineers. The mixer! criteria are listed by the Soil Conservation Service and by West Virginia. The California Division of Safety of Dams allows use of the one-in-1,000-year flood as the required minimum flood for spillway design for low hazard! dams. Michigan's criteria call for use of a 200-year flood. Those two are the only references to any
18 SAFETY OF DAMS TABLE 3-2 Spillway Capacity Criteria Reported to Be in Current Use by Various Agencies Deterministic Mixed Criteria Criteria Probabilistic Criteria C"te7~a specifying rainfalls PMP Peso + 0.40 (PMP-P~oo) 0.90 PMP Plot + 0.26 (PMP-P~oo) 0.80 PMP Pioo+ 0.12 (PMP-P~oo) 0.75 PMP Peso + 0.06 (PMP-P~oo3 0.50 PMP 0.45 PMP 0.40 PMP 0.33 PMP 0.30 PMP 0.25 PMP 0.225 PMP 0.20 PMP 0.10 PMP Cr?tena spec~fyingiloods PMF 0.75 PMF 0.50 PMF 0.40 PMF 0.30 PMF 0.25 PMF 0.20 PMF 2.25 Ploo 1.50 Ploo Ploo P50 Plo 10,000 1,000 200 150 100 50 NOTES: This is simply a listing of reported criteria. Position of entries in adjacent columns does not imply any relationship. PMP, probable maximum precipitation; PMF, probable maximum flood; P (with subscripts), precipitation having average return period in years indicated by subscript; 10,000 year, etc., flood having indicated aver- age return period. average return period greater than 100 years in any criteria in use in the United States. The 10,000-, 1,000-, and 150-year frequency floods are listed in the criteria of the Institution of Civil Engineers, London. Table 3-3 gives an approximate comparison (based on the classifications used for the National Dam Inspection Program) of the various criteria more fully described in Appendix A. As noted above, differences in systems for classifying dams make precise general comparison of this type difficult. Although not set out in many published criteria, there appears to be growing use of dam safety evaluation procedures based on estimating effects in rele- vant downstream areas of a dam being overtopped] and failing during vari-
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Summary of Present Practices 23 ous size floods. By such procedures, the dam safety evaluation flood selection is based on these estimated effects. Thus, fixed criteria, such as illustrated in Tables 3-2 and 3-3, are not used. This inventory of current practices in providing dam safety cluring ex- treme floods shows considerable diversity in approach by various federal, state, and local government agencies, professional societies, and privately owned firms. There is a fair consensus on the spillway requirements for large, high-hazard dams. But the results of the inventory show widespread uncertainty as to what might be appropriate hydrologic criteria for safety of other classes of dams. From study of the inventory results, the following observations can be made: · Use of PMP estimates for evaluating spillway capacity requirements for large, high-hazard dams predominates, although a number of state agencies have indicated that their standards do not require that such dams pass the full estimated PMF based on the PMP. · The influence of the practices of the principal federal dam-building agencies is evident in the majority of the standards for large, high-hazard dams, but the practices of those agencies have had less effect on current state standards for small dams in less hazardous situations. · Apparently as a result of the National Dam Inspection Program for nonfederal dams carried out by the Corps of Engineers in the 1977-1981 period, several state dam safety agencies have adopted the spillway capacity criteria used in those inspections. · Several states have adapted the standards used by the Soil Conservation Service for the design of the tens of thousands of smaller dams constructed under that agency's programs. · Current practices include use of arbitrary criteria (such as 150 percent of the 100-year flood, fractions of the PMF, and combinations of the PMF with probability based floods) for which there is no apparent scientific rationale. · Practices of the major federal dam-building agencies for large, high- hazard dams have been adopted by most U.S. companies owning dams and by U.S. engineering firms designing dams for domestic and foreign clients. (The regulations of the Federal Energy Regulatory Commission have re- quired such standards for licensed hydroelectric projects. ~ · It appears that only three agencies (the Federal Energy Regulatory Commission, the Mississippi Department of Natural Resources, and the New York State Department of Environmental Conservation) have issued explicit standards for existing dams that differ from the requirements for new dams. (However, other responses did not specifically state whether different standards were applicable to existing dams.)
24 CRITERIA FOR EARTHQUAKE EFFECTS SAFETY OF DAMS Table 3-4 shows a summary of current practices in evaluating safety of dams against earthquakes as specifically reported in response to the commit- tee's requests for information. Since many of the responses gave no specific information in regard] to a number of the practices shown, this table should be used with caution, as some of the agencies may be actually using more of the approaches to dam safety than indicated. However, even though it may be not completely reliable, Table 3-4 does give some indication of the proba- ble extent of use of the various techniques for analysis of earthquake effects on dams. Seismic Zones of the United States Seismic zone maps of the United States (AIgermissen, 1969; AIgermissen and Perkins, 1976) are used by most federal and state agencies as basic references when deciding if any seismic factors should be considered in dam design and if special investigations are required. Since such maps are incor- porated in most building cocles, they are often employed in selecting design criteria for buildings and ancillary structures and systems at dams. The zone maps, in part, are developed on the basis of historic seismicity, such as shown in Figure 3-1 . The historic earthquake record indicates that damaging earth- quakes occur throughout the United States. Figure 3-2 shows the Algermis- sen (1969) seismic zone map, as it appears in the Uniform Building Code, 1979 Edition. The Soil Conservation Service, the Corps of Engineers, and some states, when dynamic response analyses are not required, employ seismic zones for determining the minimum seismic coefficients for pseudostatic analyses. The following Corps of Engineers criteria (giving coefficients to be multi- plied by weight of structure to determine estimated horizontal earthquake loadings) are typical: Seismic Zone o 2 3 4 Minimum Coefficient (x g) o 0.05 0.10 0.15 0.20
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Summary of Present Practices ,25° ,20° ,,5° ,~o° 305° ~oo° A-- ~~'r~rl-~1 95° 90° 85° 80° 75° 70° 65° 4501~ ~n° 25° _... ..~.. 1 \ ~/ ~ SEISMIC RISK MAP OF THE UNITED STATES '~_ ZONE 0 No damage. r2~F _ ZONE 1 Minor damage; distant earthquakes may cause damage to structures with tunda. mental periods greater than 1.0 second. corresponds to intensisios V and Vl of the M, M. ~ Scale. ZONE 2 Moderatadamago;correspondsto intensity Vl1 of the M.M.. Scale. ZONE 3 Major damage; corresponds to intensity All I and higher of the M.M.. Scale. ZONE 4 Those areas within Zone No. 3 determined by the proximity to certain major fault sVstoms. ·Modifiod Mercatal tntonsitv Scaloof 1931 \~~_:L~ 1 1 \ ma_ ~~~ _ . 80° 75° so ~ 1 Do 1 o5o 1 ooo 950 900 85° 29 45° 40° 35° Boo 25° FIGURE 3-2 Seismic risk map of the United States. Source: Reproduced from the Uniform Building Code, 1979 (1982) (1985) edition, ~ 1979 (1982) (1985), with permission of the pub- l~sher, the International Conference of Building Officials. Earthquake Intensity and Magnitude Scales Both the Modified Mercalli intensity scale (Table 3-5) and the Richter magnitude scale are in use to describe earthquakes, although clam safety criteria usually refer to the Richter scale. The Richter magnitude scale describes the size of the earthquake; that is, it describes the seismic energy released by a fault rupture as well as the size of the area affected by strong ground shaking. Thus, Richter magnitudes are not directly comparable with Mercalli intensity ratings that vary over the affected area. The approximate relationships between Richter magnitudes and areas in square miles affected by various levels of peak accelerations, as developed for the western United States, are shown in Table 3-6. Seismologists actually have defined four types of magnitude measures or scales: Me, Ms. mb, and Mw, which are based on the recorded amplitudes of local waves, surface waves, body waves, and very long period waves, respec- tively; these waves become prominent on seismograms at different distances from the earthquake source. The Richter magnitudes commonly reported in the news media are actually Mr for earthquakes relatively close to the seis- mograph and Ms for earthquakes at greater distances from the seismograph.
30 TABLE 3-5 Modified Mercalli Intensity Ratings SAFETY OF DAMS MMI Condensed Description I Not felt II Felt indoors by few, especially on upper floors III Felt indoors by several IV Felt indoors by many, outdoors by few V Felt indoors by practically all, outdoors by many VI Felt by all, damage slight in poorly built buildings VII Damage negligible in buildings of good design and construction, slight to moderate in well-built ordinary buildings, considerable in poorly built or badly designed buildings VIII Damage slight in structures built especially to withstand earthquakes, considerable in ordinary substantial buildings IX Damage considerable in structures built especially to withstand earthquakes X Many specially designed structures destroyed XI Few, if any, structures remaining standing XII Complete destruction The commonly used Richter magnitude (based on Me for smaller earth- quakes and Ms for large earthquakes) does not have the same numerical value as the mb and Mw; hence these differences should be taken into ac- count. Seismic Design Terminology The following terms in reference to ground motions at the dam site, or to earthquakes causing those motions, are presently used by various govern- ment agencies ant! other entities with respect to seismic design criteria: DBE design basis earthquake (Planning Research Corporation) DE- design earthquake (R. W. Beck & Associates) EDBE economic design basis earthquake (USER, California) MCE maximum credible earthquake (most agencies) MCGM maximum credible ground motion (USER, California) MDE maximum design earthquake (ICODS) MEmaximum earthquake (FERC) OBE operational basis earthquake (NRC, USACE, TVA) PMA probable maximum acceleration (Missouri) SSEsafe shutdown earthquake (NRC3 Definitions of these terms as used by various entities follow. Maximum credible earthquake (MCE) is defined by the U. S. Army Corps of Engineers
Summary of Present Practices TABLE 3-6 Richter Magnitudes and Square-Mile Areas as Affected by Peak Accelerations 31 Peak Acceleration Earthquake Magnitude (Richter) (percent of acceleration (area in square miles) of gravity equalled or exceeded) 5 6 7 8 5 400 3,600 13,000 50,000 10 90 1,600 7,500 30,000 20 - 150 2,500 14,000 30 - 300 6,000 40 - 3,000 50 - - as, "the earthquake that would cause the most severe vibratory ground motion or foundation dislocation capable of being produced at the site under the currently known tectonic framework" (emphasis supplied); by the U.S. Bureau of Reclamation as, "at a specific seismic source the maximum earth- quake that appears capable of occurring in the presently known tectonic framework. It is a rational and believable event that is in accord with all known geologic and seismologic facts"; by the Tennessee Valley Authority as, "the earthquake associated with specific seismotectonic structures, source areas, or provinces that would cause the most severe vibratory ground mo- tion or foundation dislocation capable of being produced at the site under the currently known tectonic framework"; and by the Interagency Commit- tee on Dam Safety as, "the hypothetical earthquake from a given source that could produce the severest vibratory ground motion at the dam." The term maximum credible ground motion (MCGM) or equivalent ter- minology is used by the U.S. Bureau of Reclamation, California Division of Safety of Dams, and others; however, all entities do not define MCGM in the same way for use in dam safety analysis. For such purpose, MCGM may be described by several of the following sets of data: · peak ground acceleration · peek ground velocity · duration of strong shaking · response spectrum · time history (a recording of ground acceleration versus time) The draft paper entitled "Proposed Federal Guidelines for Earthquake Analysis and Design of Dams," developed by an interagency task group of the Interagency Committee on Dam Safety (ICODS), uses the term maximum
32 SAFETY OF DAMS design earthquake to describe the earthquake selected for design analysis of a project after considering earthquake potential of the dam site and the poten- tial losses from failure of the dam. The term maximum earthquake used by the Federal Energy Regulatory Commission, apparently generally refers to the same type event as the term maximum credible earthquake. Planning Research Corporation has defined design basis earthquake (DBE) as, "the largest earthquake which would be expecter] to occur once during the expected life of the project." R. W. Beck & Associates used design earthquake (DE) in the design of Swan Lake arch dam in Alaska, and define it as the "largest earthquake that would be expected to occur during the economic life of the dam (recurrence interval of once in 100 years)." "Largest earthquake" implies the "earth- quake producing the greatest loading on the structure." The U.S. Bureau of Reclamation defines the economic design basis earth- quake (EDBE) as that earthquake under the loacling from which "the proj- ect facilities not critical to the retention or release of the reservoir would be designed to sustain the earthquake with repairable damage." The degree of damage that wouicI be acceptable could be based on an economic analysis or an estimate of the cost of the repair versus the initial cost to repair the damage. Operating basis earthquake is definer! by the U.S. Army Corps of Engi- neers as, "the maximum level of ground motion that can be expected to occur at the site during the economic life of the project, usually 100 years"; by the Tennessee Valley Authority as, "the earthquake for which the dam is de- signed to resist and remain operational"; and by the Nuclear Regulatory Commission as, "that earthquake which, considering the regional and local geology and seismology and specific characteristics of local subsurface mate- rial, could reasonably be expected to affect the plant site during the operat- ing life of the plant." Regulations proposed by the State of Missouri specify a fraction of proba- ble maximum acceleration (PMA) as the design acceleration for various stages of design and different classes of dams, defining PMA as the "probable maximum acceleration of bedrock determined by the seismic zones" used by the U.S. Army Corps of Engineers. The Nuclear Regulatory Commission defines safe shutdown earthquake (SSE) as that earthquake based on an evaluation of the maximum earth- quake potential considering the regional and local geology and specific char- acteristics of local surface material. Maximum credible and safe shutdown earthquakes are used to evaluate the safety of dams; however, some damage to the facility cluring such an earthquake is acceptable, provided there is no release of reservoir water. All
Summary of Present Practices 33 respondents said they determine such earthquakes for dams using determi- nistic approaches, except Acres American, Inc., which develops both proba- bilistic and deterministic approaches and adopts the most severe earthquake found by either approach. Operating basis, design, design basis, and eco- nomic design basis earthquakes are used for dynamic analyses. Probabilistic methods sometimes are used to develop these earthquake estimates. Minimal damage is expected from these earthquakes. Seismic Criteria for Pseudostatic Stability Analysis Before the mid-1960s only the pseudostatic methods of stability analysis were used for dams in seismic areas. The seismic load was assumed to be a sustained horizontal force acting on the dam in the most critical direction. Depending upon the size of the dam and the seismic risk, the seismic force was assumed to range from 0.05 to 0.15 times the weight of the structure. For larger dams the U.S. Bureau of Reclamation combined horizontal acceleration effects with a vertical component, which was 50 percent of the horizontal acceleration; the assumed directions of the two components were those most unfavorable to structural stability. Most large foreign dams adopted similar criteria. For example, Bhakra Dam in India, a 740-foot- high concrete gravity structure located about 180 kilometers from the epi- center of the Richter magnitude (M) 8.6 Kangra earthquake of 1905, was designed for a lateral force coefficient of 0.15 and a vertical force coefficient of 0.075. The U.S. Army Corps of Engineers still requires the use of seismic coefficients for sliding and stability analyses of concrete dams and struc- tures. Hydrodynamic pressures also were taken into account by similar methods in some cases. Dynamic Response Analyses Analysis of the dynamic response of a dam to specified earthquake ground motion, when it is located in seismic zones 3 or 4 (and under some conditions, in zone 2), is now part of the dam safety criteria of most federal agencies. Only three states specifically stated that dynamic response analyses were used; others stated they user] standards of federal agencies and, thus, implied use of such analyses. It is surmised that states that have relatively new dam safety programs and/or do not have mapped active faults within their boundaries are using pseudostatic methods or are not considering earth- quake loadings. The dynamic response analyses may be two- or three-dimensional and employ the finite element technique. Depending on the size and type of dam, the foundation characteristics, and the severity of the design earth-
34 SAFETY OF DAMS quake, the analysis may consider deformations of the foundations and abut- ments as well as the structure. For embankment dams, the principal objectives of a dynamic analysis are assessment of liquefaction potential of susceptible materials and determina- tion of permanent deformations and the potential for cracking. For concrete dams, the dynamic response analyses determine the instantaneous total (dy- namic plus static) stresses at both faces of the dam and at designated loca- tions. The possible effects of fault movement on the dam are included as appro- priate.
