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Safety of Dams: Flood and Earthquake Criteria (1985)
Commission on Engineering and Technical Systems (CETS)

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APPENDIX A Design Criteria in Use for Dams Relative to Hazards of Extreme Floods CONTENTS PART 1 FEDERAL AGENCIES Ad Hoc Interagency Committee on Dam Safety Bureau of Reclamation . Federal Energy Regulatory Commission. Forest Service Interagency Committee on Dam Safety National Weather Service . Soil Conservation Service . Tennessee Valley Authority U.S. Army Corps of Engineers (for Corps Projects) U.S. Army Corps of Engineers (for National Dam Inspection Program) U.S. Nuclear Regulatory Commission PART 2 STATE AGENCIES RESPONSIBLE FOR DAM SAFETY Alaska . Arizona Arkansas . California Colorado . .118 .118 .120 .121 .123 .124 .125 .126 .128 .130 .132 115 .134 .134 .136 .137 .138

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116 Georgia Hawaii Illinois. Indiana Kansas Louisiana Maine . Michigan. ~ ,. . . . MISSISSIpp} Missouri Nebraska . New jersey New Mexico . New York. North Carolina North Dakota Ohio Pennsylvania South Carolina. South Dakota Texas Utah Virginia Washington . West Virginia Appendix A .138 1Qa PART 3 OTHER GOVERNMENTAL AGENCIES City of Los Angeles, California, Department of Water and Power . East Bay Municipal Utility District, California Salt River Project, Arizona Santee Cooper (South Carolina Public Service Authority) . PART 4 TECHNICAL SOCIETIES American Society of Civil Engineers International Committee on Large Dams U.S. Committee on Large Dams PART 5 FIRMS IN UNITED STATES Acres American, Inc., Buffalo, New York Alabama Power Co., Birmingham, Alabama - .141 .143 .144 .144 .144 .145 .146 .146 .146 .147 .149 .150 .150 .151 .153 .153 .153 .153 .154 .156 .156 .1S7 .158 .1S8 .lS9 .lS9 .160 .161 .163 .164

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Appendix A R.W. Beck and Associates, Seattle, Washington Central Maine Power Co., Augusta, Maine . Duke Power Co., Charlotte, North Carolina Charles T. Main, Inc., Boston, Massachusetts Planning Research Corporation, Denver, Colorado . Yankee Atomic Electric Co., Framingham, Massachusetts PART 6 OTHER ENTITIES IN UNITED STATES lllinois Association of Lake Communities PART 7 FOREIGN COUNTRIES The Institution of Civil Engineers, London, England 117 . . · · .164 .164 .165 .165 .165 .166 .168 .169

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118 PART 1 FEDERAL AGENCIES Appendix A Ad Hoc Interagency Committee on Dam Safety of the Federal Coordinating Council for Science, Engineering, and Technology This group, a forerunner of the present ICODS, issued "Federal Guide- lines for Dam Safety," dated June 25, 1979. The following is extracted from those guidelines: The selection of the design flood should be based on an evaluation of the relative risks and consequences of flooding, under both present and future conditions. Higher risks may have to be accepted for some existing structures because of irreconcilable conditions. When flooding could cause significant hazards to life or major property damage, the flood selected for design should have virtually no chance of being exceeded. If lesser hazards are involved, a smaller flood may be se- lected for design. However, all dams should be designed to withstand a relatively large flood without failure even when there is apparently no downstream hazard involved under present conditions of development. Bureau of Reclamation, U.S. Department of the Interior (From letter dated June 6, 1984) The following is extracted from a description of the Bureau of Reclama- tion's practices relating to floods and earthquakes: The PMF (Probable Maximum Flood) is a hypothetical flood for a selected location on a given stream whose magnitude is such that there is virtually no chance of its being exceeded. It is estimated by combining the most critical meteorologic and hydrologic conditions considered reasonably possible for the particular location under consideration. The term PMF has been adopted by the Bureau which brings us in line with terminology used by all other Federal agencies. Many past Bureau publications use MPF (Maximum Probable Flood) which has the same definition and usage as the PMF. Bureau of Reclamation procedures estimate the PMF by evaluating the runoff from the most critical of the following situations: 1. A probable maximum storm in conjunction with severe, but not un- common, antecedent conditions. 2. A probable maximum storm for the season of heavy snowmelt, in conjunction with a major snowmelt flood somewhat smaller than the proba- ble maximum. 3. A probable maximum snowmelt flood in conjunction with a major rainstorm less severe than the probable maximum storm for that season.

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Appendix A 119 All of the Bureau reservoirs are designed to accommodate an IDF (Inflow Design Flood) and an MDE (Maximum Design Earthquake). The IDF and the MDE are defined as the flood and the earthquake, respectively, which control the design of a specific dam and its related features. The evaluation of the protection level is essential for formulating alterna- tives to solve the problem. This evaluation will result in one of three general cases from which to select loading conditions. Case A Maximum Loading Conditions This would be the case where the level and proximity of the downstream hazard make it clear at the outset of the problem that the consequences of dam failure in terms of potential loss of life or property damage would be unacceptable regardless of how remote the chance of failure may be. Thus, the loading conditions for the various alternatives are established at the maximum level (MCE, PMF, etc.) . Case B Loading Conditions Determined by Economic Analysis This would be the case where the level and/or remoteness of the down- stream hazard are such that it is apparent (or becomes apparent) that incre- mental impact of dam failure would not significantly change the potential for loss of life or other nonmonetary factors, and that an economic analysis in which the costs and benefits of reducing the hazard becomes the primary consideration. Case C Loading Conditions as a Parameter in the Ultimate Decision Making Process This case is one where the incremental consequences of dam failure (with or without consideration of warning or other nonstructural modifications) do not clearly indicate that the dam falls under Case A or Case B. Compari- son of alternatives for this case would include the economic comparison as for Case B. but would require a more comprehensive assessment of the incremental effects of dam failure on potential for loss of life (with and without warning system) as well as the incremental effects socially, environ- mentally, and politically for each alternative and load level. Additional Considerations for Existing Dams It is desirable that existing dams meet the Bureau's basic IDF criteria for proposed dams. Therefore, a reevaluation of an existing dam with respect to selecting and accommodating the IDF should be based on the same basic criteria. The reevaluation should be performed in a systematic manner tak- ing into account present conditions at the dam, reservoir, and downstream flood plain. Present or anticipated conditions may reduce or increase re- quirements related to selection and accommodation of the IDF. Perfor-

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120 AppendLY A mance information for the dam and operation history of the reservoir may reduce uncertainties that were conservatively accounted for in the original design. Likewise, land use pattern around the reservoir rim and downstream from the dam may now be well established. It is recognized that for some existing dams where hazardous conditions prevail, there is the potential, if accomplished in a very cautious manner, for selection of an IDF of lesser magnitude than the PMF; this may be justified because of irreconcilable conditions that have developed since construction. However, any relaxation of established criteria is undertaken with extreme caution on a case-by-case basis after the consequences of dam failure have been evaluated and quanti- fied. Federal Energy Regulatory Commission (FERC) (From letter dated dune 12, 1984) The following is extracted from material submitted by FERC: The criteria presented herein apply to both the review of designs by Com- mission staff prior to licensing and review of licensed projects by indepen- dent consultants under Part 12 of the Commission's regulations. The adequacy of new and existing projects for extreme flood conditions is evaluated by considering the hazarc! potential which would result from failure of the project works during flood flows. If structural failure would present a hazard to human life or cause significant property damage, the project is evaluated as to its ability to withstand the loading or overtopping which may occur from a flood up to the probable maximum. If structural failure would not present a hazard to human life or cause significant prop- erty damage, a spillway design flood of lesser magnitude than the probable maximum flood would be acceptable provided that the basis for the finding that structural failure would not present a hazard to human life is signifi- cantly documented. As a result of the publications of Hydrometeorological Reports Nos. S1 (Schreiner and Riedel, 1978) and 52 (Hansen et al., 1982), the Commission staff has adopted guidelines Shown below] for evaluating the spillway adequacy of all licensed and exempted projects located east of the 105th meridian. (1) For existing structures where a reasonable determination of the Prob- able Maximum Precipitation (PMP) has not previously been made using suitable methods and data such as contained in HMR No. 33 (Riede} et al., 1956) or derived from specific meteorologic studies, or the PMF has not been properly determined, the ability of the project structures to withstand the loading or overtopping which may occur from the PMF must be reevaluated using HMR Nos. 51 and 52.

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Appendix A 121 (2) For existing structures where a reasonable determination of the PMP has previously been made, a PMF has been properly determined, and the project structures can withstand the loading or overtopping imposed by that PMF, the reevaluation of the adequacy of the spillway using HMR Nos. 51 and 52 is not required. Generally no PMF studies will be repeated solely because of the publication of HMR Nos. 51 and 52. However, there is no objection to using the two reports for necessary PMF studies for any water retaining structure. (3) For all unconstructed projects and for those projects where any pro- posed or required modification will significantly affect the stability of water impounding project structures, the adequacy of the project spillway must be evaluated using: (a) HMR Nos. 51 and 52, or (b) specific basin studies where the project lies in the stippled areas on Figures 18 through 47 of HMR No. 51. Forest Service, U.S. Department of Agriculture (From letter dated May 23, 1984) The following is extracted from material submitted by the Forest Service: Hazard-PotentiaZ Assessment The hazard class (see Definitions) is based on the potential damage that can be anticipated in the event of dam failure. Potential damage is to be assessed under clear weather conditions with normal base inflow to the reservoir anti the water surface at the elevation of the uncontrolled spillway crest. Hydrologic Criteria Select a spillway design flood based on an evaluation of the potential risk and consequences of flooding under both present and future conditions. The flood selected for design of spillways should have virtually no chance of being exceeded when failure could pose a hazard to life or cause significant property damage. The spillway capacity and/or storage capacity shall safely handle the design flood without failure. Where a spillway design flood range is shown in Table A-1, select the magnitude commensurate with the involved risk. It is recognized that failure of some dams with a relatively small reservoir capacity may have little influence on the potential damage anticipated dur- ing the spillway design flood event. Exceptions to the recommended spillway design flood magnitude may be permissible for some structures. Requests for an exception must include sufficient documentation to demonstrate that economic loss and/or the po-

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122 Appendix A TABLE A-1 Recommended Spillway Design Flood Hazard Size Potential Class Spillway Design Flood High A B C D Moderate A B C A B C Low PMF PMF PMF to PMF 100 yr to 1/2 PMF PMF 1/2 PMF to PMF 100 yr to 1/2 PMF /2 PMF to PMF 100 yr to 1/2 PMF 50 yr to 100 yr tential for loss of life resulting from dam failure during occurrence of the proposed spillway design floor] would be essentially the same as would occur without a dam failure. The Regional Director of Engineering must approve exceptions to the recommended spillway design flood. When documenta- tion is not available to support an exception, use the recommended spillway design flood criteria shown in Table A-1. Definitions 1. Administrative. The classification of a project for administrative pur- poses, based on height and storage. a. Class A Projects. Dams that are 100 feet high or more, or that impound SO,OOO acre-feet or more of water. b. Class B Projects. Dams that are 40 to 99 feet high, or that impound 1,000 to 49,999 acre-feet of water. c. Class C Projects. Dams that are 2S to 39 feet high, or that impound 50 to 999 acre-feet of water. d. Class D Projects. Dams that are less than 25 feet high and that impound less than 50 acre-feet of water. The inclusion of structures less than 6 feet high or impounding less than 15 acre-feet of water is op- tional with the approving officer. 2. Hazard Potential. The classification of a dam based on the potential for loss of life or damage in the event of a structural failure under clear weather conditions with normal base inflow to the reservoir and the water surface at the elevation of the uncontrolled spillway crest. a. Lou) Hazard. Dams built in undeveloped areas where failure would result in minor economic loss, damage would be limited to undeveloped or agricultural lands, and improvements are not planned in the forseeable future. Loss of life would be unlikely.

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Appendix A 123 b. Moderate Hazard. Dams built in areas where failure would result in appreciable economic loss, with damage limited to improvements, such as commercial and industrial structures, public utilities, and transportation systems, and serious environmental damage. No urban development ant] no more than a small number of habitable structures are involved. Loss of life would be unlikely. c. High Hazard. Dams built in areas where failure would likely result in loss of life or where economic loss would be excessive; generally, areas or urban- or community-type developments that have more than a small number of habitable structures. Interagency Committee on Dam Safety (ICODS) (From draft of proposed "Federal Guidelines for Selecting and Accommodating Inflow Design Floods for Dams" prepared by a working group and submitted to the Chairman of ICODS by letter dated October 11, 1983) The following is extracted from the draft guidelines: Selecting an IDF for the hydrologic safety design of a dam requires bal- ancing the likelihood of failure by overtopping against the consequences of dam failure. Consequences of failure include the loss of life and social, environmental, and economic impacts. The inability to accurately define flood probabilities for rare events, and to accurately assess the potential loss of life and economic impact of failure when it would occur, dictate use of procedures which provide some latitude to meet site-specific conditions in selecting the IDF. The PMF should be adopted as the IDF in those situations where conse- quences attributable to dam failure from overtopping are unacceptable. The determination of unacceptability exists when the area affected is evalu- ated and factors indicate loss of human life, extensive property and environ- mental damage, or serious social impact may be expected as a result of dam failure. A flood less than the PMF may be adopted as the IDF in those situations where the consequences of dam failure are acceptable. Acceptable conse- quences exist when evaluation of the area affected and factors in section F.1.c. twhich material relates to evaluating impacts of dam failure] show one of the following conditions: · There are no permanent human habitations, or commercial or indus- trial development, nor are such habitations, or commercial or industrial developments projected to occur within the potential hazard area in the foreseeable future and transient population is not expected to be affected.

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124 Appendix A · There are only a few permanent human habitations within the poten- tial hazard area that would be impacted by failure of the dam and there would be no significant increase in the hazard resulting from the occurrence of floods larger than the proposes] IDF up to the PMF. An example is where impoundment storage is small and failure would not add appreciable vol- ume to the outflow hydrograph, and, consequently, the downstream inun- dation would be essentially the same with or without failure of the dam. The consequences of dam failure would not be acceptable if the hazard to these habitations was increased appreciably by the failure flood wave or level of inundation, e.g., the case where failure of a storage reservoir would acid appreciably to the outflow hydrograph. In addition to the conditions listed in section F. l .c. Which material relates to evaluating impacts of dam failure], the selectee] magnitude of the IDF should be based on the following special considerations: · Dams which provide vital community services such as municipal water supply or energy may require a high degree of protection against failure to ensure those services are continued during and following extreme flood conditions when alternate services are unavailable. O Dams should be designed to not less than some minimum standard to reduce the risk of loss of benefits during the life of the project; to hold OHM costs to a reasonable level; to maintain public confidence in agencies respon- sible for dam design, construction, and operation; and to be in compliance with local, State, or other regulations applicable to the facility. National Weather Service (NWS), National Oceanic and Atmospheric Administration, U.S. Department of Commerce (From letter dated June 1, 1984) The following is extracted from material submitted by the NWS: Although the agency is not directly involved with dams and design criteria for dams, the National Weather Service has furnished extensive material on Probable Maximum Precipitation estimates and the techniques for develop- ingsuch estimates, which provide the bases for the most conservative criteria for spillway design. The PMP has been defined as "the theoretically greatest depth of precipitation for a given duration that is physically possible over a given size storm area at a particular geographical location at a certain time of year." From this definition, theoretically the PMP has zero probability of actual occurrence. A report (Riedel, 1. T., and Shreiner, L. C. 1980) compares the greatest known storm rainfall depths with generalized PMP estimates for the

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Appendix A United States east of the 105th meridian and west of the Continental Divide. This was done for rainfall depths averaged over six area sizes (10, 200, 1000, 5000, 10,000, and 20,000 mi2) each for five durations (6, 12, 24, 48, and 72 fir) covering the eastern United States. This gives comparisons for 30 combi- nations of area sizes and durations. The western states comparisons are more difficult to make, so only six combinations were made. These combinations were: for 10 mi2 and durations of 6 and 24 hours; for 500 mi2 and durations of 24 and 48 hours; and for 1000 mi2 and durations of 24 and 48 hours. For the eastern United States there were the following number of inci- dents (from the 30 combinations of area size and duration) where the rainfall was within the indicated percent of the PMP: Percent of PMP equaled or exceeded 70 80 90 No. of incidents 160 49 4 For the western states from only six combinations of area size and duration the number of incidents were: Percent of PMP equaled or exceeded No. of incidents 125 70 80 90 16 5 0 Another comparison shows that for the eastern states there were 170 separate storms which had depths exceeding 50 % of PMP for at least one area size and duration. The comparable number for the western states is 66. It should be noted that both the number of storms and storm incidents are directly related to the number of area and duration combinations compared. Soil Conservation Service, U.S. Department of Agriculture (From letter dated May 21, 1984, Criteria presented in Technical Release No. 60, "Earth Dams and Reservoirs," revised August 1981) SCS has established three classes of dams as follows: Class (a) Dams located in rural or agricultural areas where failure may damage farm buildings, agricultural land, or township and country roads. Class (b) Dams located in predominantly rural or agricultural areas where failure may damage isolated homes, main highways or minor rail- roads or cause interruption of use or service of relatively important public utilities. Class (c) Dams located where failure may cause loss of life, serious damage to homes, industrial and commercial buildings, important public utilities, main highways, or railroads.

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164 Appendix A Design spillway to pass 10,000-year flood with no reservoir surcharge, all gates in operation, no power turbines in use; - Route flood through drawn down reservoir, if drawdown will always be accomplished by time of flood (e.g. snowmelt flood); - Verify that MPF (Maximum Probable Flood) can be handled without major damage or loss of life, through the use of freeboard for storage and/or fuse plug spillways, or other emergency spillways. Alabama Power Co., Birmingham, Alabama (From letter dated July 18, 1984) Alabama Power Company supplied information on hydrologic studies now under way of eleven projects in the Coosa and Taliapoosa river basins. In the PMF determinations the company is transposing two actual storm rainfall patterns, the Yankeetown, Florida, storm of September 1950 and the Elba, Alabama, storm of March 1929, adjusted in accord with Hydromet practice, in lieu of using PMP estimates from the U.S. Weather Service. It is the company's position that such use of transposed and adjusted rainfalls will come closer to depicting actual conditions to be expected in the basin during such intense storms. Company's projects must meet FERC standards. R.W. Beck and Associates, Seattle, Washington (From letter dated June 12, 1984) The following is quoted from the firm's letter: Beck generally has followed the U.S. Army Corps of Engineers (COE) criteria for severe hydrologic events by developing the Probable Maximum Flood (PMF) from the Probable Maximum Precipitation (PMP) and apply- ing COE hazard criteria to select the Spillway Design Flood (SDF). Most State and Federal agencies have accepted the Corps approach as being con- servative, and only in special circumstances involving unimportant struc- tures where substantial savings can be realized in analysis and engineering are simplified methodologies employed by Beck. Central Maine Power Company, Augusta, Maine (From letter dated July 31, 1984) The Central Maine Power Company has supplied data sheets pertaining to structural analyses for five of its hydroelectric power projects. The analy- ses were made by Charles T. Main, Inc. The data sheets are not explicit in regard to hydrologic criteria used but do indicate that a "probable maximum

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Appendix A 165 flood" was used in the structural analyses. Company's projects are subject to FERC regulations. Duke Power Company, Charlotte, North Carolina (From letter dated July 19, 1984) Information supplied by Duke Power Company indicates that its stan- dards for dams are comprised of the regulations of the Federal Energy Regulatory Commission supplemented by standards and criteria issued by a number of federal and state agencies. Charles T. Main, Inc., Boston, Massachusetts (From information furnished by Llewellyn L. Cross, June 18, 1984) In serving Main's various clients, who are scattered about the world, all of the standard hydrologic techniques are employed. In the U.S. and other areas where the Probable Maximum Flood is man- dated as the design standard, the applicable Hydrometeorological Reports are used. Where these are not available, a hydrometeorological approach using precipitable water and clew points is taken. Storm transposition and maximization techniques are also employed. Unit hydrographs are derived from historically appropriate flood events where the data are available. In cases of no records, unit hydrographs are developed from the physical characteristics of the basin. Diversion floods are computed using statistical methods adapted to site- specific situations. In many instances, for projects in remote areas having no data, storm models appropriate to the catchment are developed using meteorological methods and parameters. These models are then maximized for rainfall intensity and duration and critically sited on the project catchment. For many cases, the spillway design flood has been the result of snow melt and this has resulted in the development of necessarily crude models relating snow melt to incremental melt temperature. Planning Research Corporation (PRC), Denver, Colorado (From letter dated dune 19, 1984) The following is extracted from a description of the hydrologic criteria used by PRC: We normally follow the generally accepted design criteria that, if the failure of a water storage dam could result in loss of life or substantial loss of

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166 Appendix A property, the dam and spillway should be sized to safely pass the Probable Maximum Flood (PMF). For projects where loss of life or substantial prop- erty loss will not be a consequence of a dam failure, then a lesser flood is used as the Inflow Design Flood (IDF) . The size of the IDF is site specific for each project, but we never use anything less than the 100-year event. In the United States, the magnitude of the project IDF is almost always set by regulation (State Engineers Office or some other State or Federal Agency). Overseas, however, the decision with regard to the magnitude of the IDF is the responsibility of the engineer. We always present our recom- mendation to our client, discuss it with him and reach agreement at an early stage of the project. The majority of our projects include major dams to supply water to large irrigation or hydropower developments anal, therefore, we normally use the PMF as the Inflow Design Floocl. At times, we believe it is in the public's best interest to take a different approach to establishing the project inflow design flood. In some instances, the routed PMF outflows from the project spillway are so great that signifi- cant damage will take place as a result of those outflows even without the occurrence of a dam failure. Also, if one considers the incremental down- stream flood hazard resulting from a slam break, compared to an existing condition during the same flood event, the additional flooding, and there- fore flood damage, may prove to be insignificant. If a review of the proposed project features and downstream topographic conditions indicates that a dam failure would result in insignificant incremental damages, then we might propose that a dam break analysis be performed, and that consider- ation be given to designing for an IDF which is smaller than the PMF, thus attempting to optimize project cost and risk. One must use caution in consid- ering the use of this approach, however, because the results of a clam break analysis are highly dependent on assumptions made concerning the time of failure, the mode of failure and the downstream topographic conditions. For example, I know of an instance where a 25-foot high dam resulted in a 70-foot high downstream flood wave. This occurred because the valley downstream was relatively narrow and heavily wooded, resulting in debris dams being formed downstream during the flooding, and those dams re- sulted in temporary ponding and then failed suddenly. Yankee Atomic Electric Co., Framingham, Massachusetts A company representative has made available a report dated April 1984, titled "Probability of Extreme Rainfalls and the Effect on the Harriman Dam" and an early draft of the same report, dated March 1984, titled "Probability of Failure of Harriman Dam due to Overtopping." These re-

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Appendix A ports describe studies of a 60-year-old hydroelectric power project in Ver- mont in the upper Deerfield River basin, which is upstream of the site of the Yankee atomic power development. As part of the study of safety of the atomic power installation, the Nuclear Regulatory Commission has re- quired an assessment of the failure potential of the upstream dam. The studies of the flood-producing potentials of the 200-square-mile drainage area of Harriman Dam had three aspects of considerable perti- nence to the present effort of the Committee on Criteria for Dam Safety: (1) the range in the estimates for probable maximum precipitation (PMP) over the area, (2) the use of what was termed the "unconditional probability approach" in developing estimates of average frequency of return for ex- tremely large rainfalls, and (3) the development of estimates of probability of dam failure by overtopping with various confidence levels. The 24-hour, 200-square-mile PMP estimates ranged from 14.3 inches to over 22 inches. The "unconditional probability approach" is described in the following quotation from the April 1984 report: "In the unconditional probability approach, no a priori assumption was made concerning the mathematical form of the statistical distribution. In its simplest sense, the probability of exceeding a particular rainfall depth at a point of interest is estimated by multiplying the annual frequency of the events of such depth occurring anywhere within a large zone of interest times the probability that that event will occur directly over a specific point of interest. The former annual frequency can be calculate`] from the historical records. The latter probability of the event occurring over a specific location can be estimates] simply as the ratio of the average storm area in which a depth is equaled or exceeded to the total area of the large zone of interest." 167 In applying this approach, the annual frequencies of 24-hour rainfalls equaling or exceeding various depths above 6 inches over any 200-square- mile area within each of a number of geographical zones were developed from historical recorcls. A total of seven zones were used (ranging in total area from 36,783 square miles to 249,372 square miles), and each zone contained the 200-square-mile area upstream from Harriman Dam. The frequencies for occurrence over any 200-square mile area within each geo- graphical zone were converted to estimated probabilities for occurrence over the drainage area above Harriman Dam by simple ratios of the target areas involved. Thus a rainfall with annual frequency of 0.01 over any 200- square-mile area within the largest 249,372-square-mile zone would have an estimated annual probability of occurrence over the drainage area of Harri- man Dam of 0.01 x 200/249,372 = 0.000008, or, to put this in terms in common use, the 100-year rainfall for any 200-square-mile area in the zone

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168 Appendix A becomes the 125,000-year rainfall for the area upstream from the dam. This conversion is based on these assumptions: 1. The approximately 100-year period in New England for which results of depth-area-duration studies for all major storms are available is repre- sentative of long time averages. 2. The geographic zones used are meteorologically homogeneous. 3. Occurrence of a major rainfall over a specific target area is a random chance event. By the "unconditional probability approach," the annual probabilities of the PMP estimates for the drainage area of Harriman Dam were assessed as follows: 24-hour PMP 14.3" 22+" Annual Probability 3.5 x 10-5 2.2 x 10-7 The Yankee Atomic Electric Company's report states that the Nuclear Regulatory Commission generally has accepted, as a basis for design, seismic hazard curves with annual probabilities of 10-3 to 1O-4 and implies that hydrologic design events with similar probabilities should be reasonable bases for design. PART 6 OTHER ENTITIES IN UNITED STATES Illinois Association of Lake Communities (From letter dated July 19, 1984) The President, Illinois Association of Lake Communities, stated that he was writing on behalf of the communities of the association and other mu- nicipal dam operators within the state whose dams have been inspected under the National Dam Inspection Program of the U.S. Army Corps of Engineers and found to have inadequate spillway capacity under the criteria used for that program. He protested any requirement that operators of dams, for which construction permits were originally issued and which are being operated and maintained in a safe, reliable manner, be required to meet new dam safety criteria. He emphasized the costs of upgrading such dams, stated such costs could mean potential bankruptcy for home owner associations, and suggested it would be senseless and unrealistic to require spillway designs for 26" of rain in a six-hour period. A separate communication of same date from a law firm representing the

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Appendix A 169 Association (McDermott, Will & Emory) questions the legality of requiring application of PMF flood criteria to existing dams. The following bases of argument were presented. a. Retroactive application of PMF criteria for existing dams would be a violation of the constitutional rights of the dam owners. b. The classification of a dam as "high hazard" based only on the location of the dam is a "conclusive and irrebuttable presumption" that is violative of due process rights of the owners. c. A system of regulation of dams not based on the actual condition of existing dams is not reasonably related to the purpose of protecting citizens from unsafe dams. d. The application of the PMF standard to an existing dam is a taking of property without compensations. PART 7 FOREIGN COUNTRIES The Institution of Civil Engineers, London In Great Britain, dam safety is entrusted to individual members of a statutory pane} of engineers determined by the government to be qualified to design and inspect impoundments. After appointment as a "panel engineer," the individual may be hired by dam owners to design and inspect dams to meet statutory requirements. Each such panel engineer is personally respon- sible for the safety of the dams he is hired to supervise, and no mandatory standards are imposed by the government. However, to assist the panel engineers in meeting their individual responsibilities, the Institution of Civil Engineers in 1978 published a report of the Institution's Working Party on Floods and Reservoir Safety, uncler the title "Floods and Reservoir Safety: An Engineering Guide." Extracts from Chapter 2, "Reservoir Flood Protection Standards," of that guide follow: Protection standards must resolve acceptably the conflicting claims of safety and economy. Although it is now considered possible to design a spillway for the total protection of a dam against overtopping, there is the clear possibility that a smaller spillway built at less expense would survive several generations without any disaster or damage occurring. However, it is not simply a matter of economic judgment . As the Institution's 1973 state- ment on social responsibilities states, the civil engineer should recognize the many factors which may defy expression in direct money values, particu- larly those which arise from effects on a community's way of life. A crucial question when considering flood protection is the combination

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172 Appendix A of circumstances that may arise in progressively rarer events. Three main factors have to be defined: (a) initial reservoir level; (b) floodinflow; (c) concurrent wind speed. Despite continually improving techniques for defining flood hydro- graphs, wave run-up and flood routing, there is no indication that the engi- neer can do other than make separately reasoned assumptions on the levels at which the three factors listed above should be set. In Table A-16 are set out the standards which are appropriate for the wide variety and scale of dams coverer! by British safety legislation. To apply them it is necessary to route the appropriate dam design flood inflow using the corresponding initial reservoir condition and to obtain two levels, one being the theoretical flood surcharge level and the other being the total surcharge level; the latter includes the appropriate allowance for wave run-up caused by the wind speed given in Table A-16 (or the minimum wave surcharge if that is greater), this wave surcharge allowance being sufficient to prevent overtopping reaching quantities that would hazard a dam crest. Although Table A-16 may appear complex at first sight, it is designed to take account of those factors which are weighed together by panel engineers during dam inspections. Its main intentions are to ensure that, where a community could be endangered by a dam, the risk of any failure caused by a flood is virtually eliminated, but in other cases to keep expenditure to a scale justified by the risk. Category A dams. It is considered that public opinion will not accept conscious design for a specific threat to a community, even though it tolerates to an extent both random and accidental loss of life. Consequently, no dam above a village or town should be designed knowingly with a definite chance of a disastrous breach due to the under-provision of spillway capacity. A community defies definition in a few words; it is considered that inspection of any valley will soon reveal whether the presence of a hamlet, school or other social group means that a dam at its head should be in category A. Road and rail traffic caught in a valley flood would only accidentally be involved and would not by itself justify category A. A more difficult situation exists where an occasional camp site exists in the holiday season alongside a reser- voired river; if, for example, this is in regular use by school parties it could well justify a community rating, but if it is frequented by a few unrelated short-stay individuals it need not.

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Appendix A 173 Category B dams. Category B(i) is intended to refer to inhabitants of isolated houses and, for example, to treatment plant operators in a works immediately below a dam. (These situations lend themselves to taking mea- sures to buy out the property or to arrange flood escape routes where appro- priate.) Category B(ii) refers to extensive damage, including erosion of agricultural soils ant] the severing of main road or rail communications. Category C dams. Category C covers situations with negligible risk to human life and so includes flood-threatened areas that are inhabited only spasmodically, e.g., footpaths across the flood plain and playing fields. In addition this category covers loss of livestock and crops. Category D dams. Many small reservoirs with low earth dams may cause no real problem, except that of replacement, if they wash out. These special cases, many of which are ornamental lakes kept full for aesthetic reasons, are given a separate category. A flood intense enough to cause failure of a dam would create some damage even if the valley was still in its natural state; the additional damage caused by the release of stored water may well be insignificant if the lake is small. So where the amount stored would add no more than 10% to the volume or peak of the flood it is recommended that the spillway need not pass more than the outflow from the 150 year flood (or 0.2 PMF if that is calculated more readily). The point of reference for calculating whether the dam is significant or not can be taken as the first site below the dam at which some feature of value exists (e. g., a mill or road bridge). The 1000 year flood hydrograph applicable to that catchment prior to dam construction can be used for making this 10 % sensitivity test. Economic considerations. Some reservoirs pose no threat to life but their loss would have severe economic consequences. Providing that all the losses caused by a failure can be met by remedial works and compensation pay- ments, the sizing of the spillway and freeboard is a matter of locating the economic optimum. Provision is made in Table A-16 for the use of an economic standard as an alternative. The strength of the least-cost method is its ability to reduce the arbitrary choice of standards which may have costly implications. However, the most economic solution over the long term may not be one that the owner can finance in the short term. Indeed the economic study itself may be expensive (although this need not always be so). The economics of the situa- tion can be self-evident when, for example, a water treatment works is sited

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174 Appendix A immediately below a dam and the loss of its output would have grave eco- nomic consequences for inclustrial consumers. Even for those cases where the failure of a new dam would not pose a serious threat to existing property, the additional cost of providing protection against the Probable Maximum Flood may be relatively small and it may be prudent to do so in order not to limit future development below the dam. After an economic study the pane} engineer should be free to adopt safer flood control works than the nominal minimum solution if his appreciation of the extra costs of greater protection so indicates. Table A-16 contains an important qualification that the alterna- tive economic standard should not be allowec! to produce a result that in- volves more risk of overtopping than the minimum standard.

Representative terms from entire chapter:

design flood