Safety of Dams: Flood and Earthquake Criteria (1985)

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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

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

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

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.

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-

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.

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-

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.

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.

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

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.

126 Appendix A Minimum criteria for spillway design to prevent overtopping of dams are given in Table 2-5 of Technical Release No. 60, which is reproduced below. Much less demanding criteria for small low hazard potential dams (hav- ing effective heights of 35 feet or less and for which the product of the storage in acre-feet times the effective height in feet is less than 3,000) are given in SCS Practice Standard 378 for Ponds which provides for minimum spillway design storms having 10-year to 50-year frequencies. SCS does not differentiate between new dams and existing dams in its criteria. Tennessee Valley Authority (From letter dated June 7, 1984) TVA uses a hazard classification for structures described as follows: High Hazard The high hazard classification includes structures whose failure during floods would likely cause serious social or economic toss. Unless specific studies show otherwise, structures 100 feet or more in height or with 50,000 acre-feet or more of total capacity at maximum flood levels shall arbitrarily be classified as high hazard. Medium Hazard The medium hazard classification includes structures whosefailure dur- ing floods would cause significant but not serious social or economic loss. When a higher hazard situation is not evident, structures over 25 feet but less than 100 feet in height or with total capacity at maximum flood levels greater than 5,000 acre-feet but less than 50,000 acre-feet shall arbitrarily be classified as medium hazard unless specific studies show otherwise. Low Hazard The low hazard classification includes any structure whosefailure during floods would likely cause only minor social or economic loss. Structures not in the high hazard or medium hazard classifications defined above shall be classified as low hazard when neither existing nor prospective future condi- tions indicate that a higher hazard situation is to be expected. TVA's guidelines provide that high hazard structures will be tested with the probable maximum flood, medium hazard structures with the TVA maximum probable flood and low hazard structures with a design flood "appropriate to the economic life and planned purpose of the structure." The probable maximum flood and the TVA maximum probable flood determi- nations are to be based upon combinations of hydrologic factors which are selected to prevent unrealistic combinations of hydrologic conditions.

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128 Appendix A Probable Maximum Floods are based on probable maximum precipita- tion (PMP) defined as "that rainfall over a particular basin which has vi- tually no risk of being exceeded." PMP estimates come "from National Weather Service studies applicable to watersheds in the Tennessee Valley." TVA Maximum Probable Floods used for the design of the older TVA dams were based on maximum observed runoff rate diagrams and maximum observed storms. Currently such floods are based upon TVA maximum prob- able precipitation estimates defined "as that magnitude of rainfall over a particular basin which is equivalent to maximum storms that have been observed within regions of similar meteorological character. Storm rainfall amounts are based upon 'TVA precipitation' from the National Weather Service studies applicable to watershed in the Tennessee Valley." Guidelines for other factors affecting the development of probable maxi- mum and TVA maximum probable floods generally call for average or me- dian conditions observed during maximum past floods. A "decision tree," illustrating the TVA approach to hydrologic design is shown in Figure A-1. U.S. Army Corps of Engineers (For Corps Projects) (From letter dated May 24, 1984) (Spillway and freeboard criteria are from Engineer Circular 1110-2-27, dated August 1, 1966) The following is extracted from material submitted by the Corps: The Corps has established four functional design standards for new dams designed by the Corps as follows: Standard 1: Design dam and spillway large enough to assure that the dam will not be overtopped by floods up to probable maximum categories. Standard 2: Design the dam and appurtenances so that the structure can be overtopped without failing and, insofar as practicable, without suffering serious damage. Standard 3: Design the dam and appurtenances in such manner as to assure that breaching of the structure from overtopping would occur at a relatively gradual rate, such that the rate and magnitude of increases in flood stages downstream would be within acceptable limits, and that dam- age to the dam itself would be located where it could be most economically repaired. Standard 4: Keep the dam low enough and storage impoundments small enough that no serious hazard would exist downstream in the event of

Appendix A BASIC DATA STRUCTU RE Significant 1 Minor Loss . | Loss Serious _ Loss 1 HIGH MEDIUM LOW HAZAR D HAZAR D HAZAR D ...._.... _ . PROBABLE MAX IMUM MAXIMUM FLOOD PROBABLE FLOOD 1 1 INITIAL LEVELS, OPERATING PROCEDURE, SILT ACCUMULATIONS DISCHARGE CAPACITY WIND-WAVE CONSIDERATIONS I . ~ I . r I . . I . NO OVER- OVER- NO OVER- OVER- TOPF ING TOPPING TOPFING TOPPING 1 1 1 HYDROLOGICALLY DESIGNED STRUCTURE FIGURE A-1 Decision tree of TVA. 129 ~ 1 PMP TVA Lesser Rain Max. Prob. Rain . LESSE R F LOOD of ~ OVER- | TOPP I NG TOPP I l\l G | l , ~ ~1~ Failure I failure , I . . . breaching, and so that repairs to the dam would be relatively inexpensive and simple to accomplish. In application of these standards most Corps of Engineers new dams are designed to pass the PMF with full freeboard; the exceptions being run-of- river developments, diversion dams, and small dams with small impounding capacities and Tow downstream hazard potentials. The Corps uses the same hydrologic safety criteria in design of new dams and in analyzing and upgrading existing dams. However, because of the limited resources available at any one time, the Corps uses a decision tree involving consideration of relative existing project capabilities in setting priorities for such upgrading.

130 Appendix A U.S. Army Corps of Engineers (for National Dam Inspection Program) (From ER 1110-2-106 and ETL 110-2-234) The following is extracted from material submitted by the Corps: In cooperation with other federal agencies, state agencies and other groups and individuals knowledgeable about dam safety matters, the Corps of Engineers developed a pamphlet, "Recommended Guidelines for the Safety Inspection of Dams," to guide the inspection of nonfederal dams authorized in 1972 by P.L. 92-367. The Corps' recommended guidelines provide for classifying dams by height of dam and storage impounded and, also, by hazard potentials in the downstream areas in the event of failure of the dams. These provisions (as modified by a change in September 1979) are described in the following: Size. The classification for size based on the height of the dam and storage capacity should be in accordance with Table A-2. The height of the dam is established with respect to the maximum storage potential measured from the natural bed of the stream or watercourse at the downstream toe of the barrier, or if it is not across a stream or watercourse, the height from the lowest elevation of the outside limit of the barrier, to the maximum water storage elevation. For the purpose of determining project size, the maximum storage elevation may be considered equal to the top of dam elevation. Size classification may be determined by either storage or height, whichever gives the larger size category. Hazard Potential. The classification for potential hazards should be in accordance with Table A-3. The hazards pertain to potential loss of human life or property damage in the area downstream of the dam in event of failure or misoperation of the dam or appurtenant facilities. Dams conforming to criteria for the low hazard potential category generally will be located in rural or agricultural areas where failure may damage farm buildings, lim- ited agricultural land, or township and country roads. Significant hazard TABLE A-2 Size Classification Impoundment Category Storage(ac-ft) Height (ft) Small 1,000 and 50 40 and 25 Intermediate 1,000 and 50,000 40 and 100 Large 50,000 100

Appendix A TABLE A-3 Hazard Potential Classification Loss of Life Category (extent of development) Economic Loss 131 Low None expected (no Minimal (undeveloped to permanent structures occasional structures or for human habitation) agriculture) Significant Few (no urban Appreciable (notable developments and no agriculture, industry, more than a small or structures) number of inhabitable structures) High More than few Excessive (extensive community, industry, or agriculture) potential category structures will be those located in predominantly rural or agricultural areas where failure may damage isolated homes, secondary highways or minor railroads or cause interruption of use or service of rela- tively important public utilities. Dams in the high hazard potential category will be those located where failure may cause serious damage to homes, extensive agricultural, industrial and commercial facilities, important pub- lic utilities, main highways, or railroads. The Corps issued the following supplementary guidelines regarding cias- sifying dams as unsafe: A finding that a dam will not safely pass the flood indicated in the Recom- mended Guidelines does not necessarily indicate that the dam should be classified as unsafe. The degree of inadequacy of the spillway to pass the appropriate flood and the probable adverse impacts of dam failure because of overtopping must be considered in making such classification. The follow- ing criteria have been selected which indicate when spillway capacity is so seriously inadequate that a project must be classified as unsafe. All of the following conditions must prevail before designating a dam unsafe: dam. a. There is high hazard to loss of life from large flows downstream of the b. Dam failure resulting from overtopping would significantly increase the hazard to loss of life downstream from the dam from that which would exist just before overtopping failure. c. The spillway is not capable of passing one-half of the probable maxi- mum flood without overtopping the dam and causing failure.

132 Appendix A TABLE A-4 Hydrologic Evaluation Guidelines: Recommended Spillway Design Floods Hazard Size Spillway Design Flood (SDF)a Low Small 50- to 100-yr frequency Intermediate 100-yr to i/2 PMF Large i/z PMF to PMF Significant Small 100-yr to i/z PMF Intermediate i/2 PMF to PMF Large PMF High Small i/e PMF to PMF Intermediate PMF Large PMF aThe recommended design floods in this column represent the magnitude of the spillway design flood (SDF), which is intended to represent the largest flood that need be considered in the evaluation of a given project, regardless of whether a spillway is provided; i.e., a given project should be capable of safely passing the appropriate SDF. Where a range of SDF is indicated, the magnitude that most closely relates to the involved risk should be selected. 100-yr = 100-Year Exceedance Interval. The flood magnitude expected to be exceeded, on the average, once in 100 years. It may also be expressed as an exceedance frequency with a one percent chance of being exceeded in any given year. PMF = Probable Maximum Flood. The flood that may be expected from the most severe combination of critical meteorologic and hydrologic conditions that are reasonably possible in the region. The PMF is derived from probable maximum precipitation (PMP), which information is generally available from the National Weather Service, NOAA. Most federal agencies apply reduction factors to the PMP when appropriate. Reductions may be applied because rainfall isohyetals are unlikely to conform to the exact shape of the drainage basin and/or the storm is not likely to center exactly over the drainage basin. In some cases local topography will cause changes from the generalized PMP values; therefore, it may be advisable to contact federal construction agencies to obtain the prevailing practice in specific areas. U.S. Nuclear Regulatory Commission (NRC) (From letter dated June 8, 1984) The following is extracted from material furnished by NRC: Although the Nuclear Regulatory Commission (NRC) by itself does not plan, design, construct or operate dams, the NRC does regulate dams whose failure could result in a radiological risk to public health and safety. By virtue of this regulatory responsibility, which is described in the Code of Federal Regulations, the NRC has developed guidelines and design criteria for ad-

Appendix A 133 dressing flood and earthquake hazards, which applicants for permits and licenses to operate nuclear facilities are required to meet. The regulations and criteria are primarily related to the design and con- struction of nuclear power plant structures, systems, and components. The Nuclear Regulatory Commission is also involved with the regulation of em- bankment retention systems for uranium mill tailings where the radiological risk to the public health and safety is considerably less than it is with nuclear power plants. In recognition of this reduced risk, less stringent flooding and earthquake design criteria have been considered for special site conditions (small dams built in isolated areas), where the dam failure would neither jeopardize human life nor create damage to property or the environment beyond the sponsor's legal liabilities and financial capabilities. Nuclear power plants should be designed to prevent the loss of capability for cold shutdown and maintenance thereof resulting from the most severe flood conditions that can reasonably be predicted to occur at a site as a result of severe hydrometeorological conditions, seismic activity, or both. The conditions resulting from the worst site-related flood probable at the nuclear power plant (e.g., PMF, seismically induced flood, seiche, surge, severe local precipitation) with attendant wind-generated wave activity constitute the design basis flood conditions that safety-related structures, systems, and components are designed to withstand. There will always be some catchment area contributing runoff into the [uranium mill] tailing retention system. This may vary from the area of the system itself to a substantial area incorporating the drainage area of streams entering the valley across which a retention dam is constructed. Substantial runoff volumes and flows can result from heavy precipitation or snowmelt over relatively small catchment areas. The maximum runoff used in the designing is usually called the Spillway Design Flood (SDF), representing the largest flood that needs to be ana- lyzed, regardless of whether or not a spillway is provided. The magnitude of the SDF (flood volume, peak, flow, etc.) as adopted in the United States for the past 30 years is equal to that of the Probable Maximum Flood at the site of the dam. For smaller retention dams built on isolated streams in areas where failure would neither jeopardize human life nor create damage to property or the environment beyond the sponsor's legal liabilities and financial capabilities, less conservative flood design criteria may be used in the design. However, the selection of the design flood needs to be at least compatible with the guidelines set for the by the Corps of Engineers ["Recommended Guidelines for Safety Inspection of Dams". If decant or other reclaim systems have not been designed specifically to

134 Appendix A pass the design flood, other measures need to be taken. Those other measures may be one or a combination of the following: a. Storing the whole volume of flood runoff. Sufficient freeboard should always be available to provide the necessary storage capacity without over- topping the dam. b. Providing a spillway or diversion channels to convey runoff water safely past the clam. Because of the toxic nature of the impounded material, a. is preferred. PART 2 STATE AGENCIES RESPONSIBLE FOR DAM SAFETY Alaska (From letter dated May 16, 1984) Alaska is now in the process of preparing legislation and developing crite- ria for review of plans for dams and inspections and relies heavily on Corps of Engineers criteria. Hydrologic evaluation criteria for spillways currently used are as follows: Height of dam (ft) or volume 10' to 40' 40' to 100' 100' impounded (A.F.) 50 to 1000 1,000 to 50,000 50,000 Low hazard 50-100 yr freq. 100 yr to i/z PMF 1/2 PMF to PMF Significant hazard 100 yr to ]/2 PMF to PMF PMF ]/2 PMF High hazard its PMF to PMF PMF PMF Arizona (From letter dated June 7, 1984) The Arizona Department of Water Resources is completing a revision of its "Guidelines for the Determination of Spillway Capacity Requirements." Extracts from the revised draft guidelines, which have been in use for some time, are shown in Table A-5. Size Classification Dams are classified into small, medium and large sizes. A numerical rating procedure, based on the descriptive characteristics of height and

Appendix A TABLE A-5 Hazard Potential Classification 135 Loss of Life Category (extent of development) Economic Loss Low None expected (no Minimal (undeveloped to permanent structures occasional structures or for human habitation) agriculture) Significant Few (no urban Appreciable~notable developments and no agriculture, industry, more than a small or other structures) number of inhabitable structures) High More than few Excessive (extensive community, industry, or agriculture) reservoir capacity, has been developed to determine the dam size classifica- tion. Height is measured from the lowest elevation of the outside limit of the dam (usually the downstream toe) to the spillway crest, or top of spillway gates if so equipped. For dams with no spillway, the height is measured to the crest of the dam. Capacity, in acre-feet, is measured to the spillway crest or top of the spillway gates, if so equipped. For dams with no spillway, capacity is mea- sured to the clam crest. The categories and corresponding rating factors are shown in Table A-6. A numerical rating is computed for each dam by adding the corresponding rating factors for each of the two categories. For example, a dam that is 65 feet in height and has a reservoir capacity of 22,000 acre-feet would have a rating of (3 +4 =7). Small dams have a rating in the range 0-2, medium dams in the range 3-7, and large dams, 8 or greater. All new dams, existing dams that are being enlarged or improved, and dams being reevaluated for safety may have spillways of lesser capacity than that outlined by Table A-7 as discussed below. A spillway capacity less than outlined above will be acceptable, where the owner (or his engineer) can demonstrate to the department that the incre- mental damages due to failure of the dam are insignificant and will not cause loss of life. The analysis shall be based upon the dam failure caused by a flood that just exceeds the routing capacity of the reservoir. The result shall be compared to the pre-failure conditions such as the spillway discharge and any reasonable rainfall runoff occurring between the dam site and the pointers) of interest below the dam. The burden of proof rests with the owner.

136 TABLE A-6 Size Classification Appendix A Category Rating Factor Category Rating Factor Height (ft) Reservoir Capacity (ac-ft) 6-24 0 2S-39 1 2 3 .......... 4 ....... 5 40-59........................ 60-79.. 80-99.. 100+ ... 15-499 ..... 500-999 ..... 1,000-2,999 ... 3,000-9,999 ... 10,000-24,999 .. 25.000+ ... · · ·0 . . .1 ...2 ...3 ...4 · · ·5 TABLE A-7 Spillway Capacity Requirements: Recommended Spillway Design Floods Size Inflow Design Flood Hazard Category Designation Magnitude Low Small 100 yr Medium 100 yr to i/2 PMF Large i/' PMF Significant Small 100 yr to i/o PMF Medium i/2 PMF Large i/ PMF to PMF High Small 1/2 PMF Medium i/2 PMF to PMF Large PMF Arkansas (From letter dated May 14, 1984) The following is extracted from "Rules Governing the Arkansas Dam Safety Program": The spillway capacity must be capable of passing the spillway design flood (SDF) without endangering the safety of the dams. The spillway design must include sufficient capacity and freeboard to prevent overtopping of the dam, have sufficient strength to prevent structural failure, and an adequate en- ergy-dissipating device at the outlet. The following minimums will apply (PMF = probable maximum flood The flood that may be expected from the most severe combination of critical meteorologic and hydrologic condi- tions that are reasonably possible in the region.~:

Appendix A 137 Equal to or Greater Than Spillway Class Storage Capacity Drainage Area Design Flood 1 10,000 (ac-ft) 10 sq. mi. 0.75 PMF 2 5,000 (ac-ft) 1000 act 0.50 PMF 3 1,000 (ac-ft) 100 act 0.25 PMF 4 20 (ac-ft) 0 100 year California (From letter dated dune 1, 1984) The following is quoted from a letter from the Chief, Division of Safety of Dams: In response to your May 4, 1984 letter, we are outlining our approach to hydrologic and earthquake related safety criteria and standards. As a matter of policy we do not publish standards or criteria so the information provided herein has been compiled from several internal documents specifically to answer your letter. Hydrology—spillway capacity. The basic requirement is stated: "The size and type of dam and its vulnerability to failure because of an inadequate spillway shall be considered in the selection of the magnitude of the spillway design flood, and consequently the spillway capacity." The minimum design flood required is a one in 1000 year flood and the maximum is a probable maximum flood as derived from the probable maxi- mum precipitation determined from Hydrometeorological Report No. 36 (U.S. Weather Bureau, 1961a) or Technical Paper No. 38 (U.S. Weather Bureau, 1960) as appropriate for the drainage area. The return period for the flood is selected by using a rating system that considers (1) the reservoir capacity, (2) dam height, (3) estimated number of people that would have to be evacuated in anticipation of dam failure, and (4) potential downstream damage. The system is such that only remote farm dams qualify for the one in 1000 year floods. Typically probable maximum floods are required for dams that impound 1000 acre feet or more, are at least 50 feet high, the estimated evacuation is at least 1000 people, and the damage potential is $20,000,000 or greater. The scale for floods between the 1000 year and probable maximum is continuous; so a dam with a slightly lower rating in one of the four factors than the example would require a statistical flood equal to about 90 percent of a probable maximum flood. New embankment dams must pass the spillway design with a minimum of 1~/z feet of resiclual freeboard above the maximum reservoir stage. Addi-

138 Appendix A tional freeboard] is required for severe wave conditions. Residual freeboard for new concrete dams is based on the ability of abutment and foundation to resist damage from overpour. Existing dams must only safely pass the spill- way design flood. The Department of Water Resources as owner of 16 dams uses probable maximum floods for all dams "except for low diversion dams and other small dams impounding relatively insignificant quantities as compared to the volume of flood flows." These dams must also conform to the Division of Safety of Dam requirements. Colorado (From letter dated June 6, 1984) Colorado's spillway capacity criteria have been stated as follows: The inflow design flood for a reservoir "generally] is the probable maxi- mum flood. However, it may be smaller than a probable maximum flood provided it can be shown that the incremental damages due to failure of the dam are insignificant and will not cause loss of life. The analysis shall be based upon the dam failure caused by a flood which just exceeds the routing capacity of the reservoir. This result shall be com- pared to the pre-failure conditions such as the spillway discharge and any reasonable rainfall runoff occurring between the dam site and the points of interest below the dam. A minor dam situated in a remote area, where loss of life or property damage is not envisioned, will not require an incremental damage analysis. However, the minimum size spillway must safely pass the 100-year flood. In order to ensure the safety of the dam embankment during the IDF, all new dams, and enlargements of existing dams, shall have spillways which can safely pass the inflow design flood and have a minimum of one foot of residual freeboard exclusive of camber. Existing dams shall be able to pass the IDF safely. In the design of the spillway, all new dams shall have a minimum normal water level freeboard of not less than five feet ant] existing dams may have a minimum of three feet of freeboard, if it can be shown that the structure will be safe. Exceptions will be on a case by case basis. Georgia (From letter dated June 5, 1984) Dam safety criteria were specified by the original 1978 Georgia State Dams Act. These criteria have been amencled by legislation in 1982 and 1984. Current criteria for spillway capacity are shown below.

Appendix A 139 Spillway Requirements* 1) Small Dams (height < 25 ft and max. storage < 500 acre-feet)—25 % of PMP 2) Medium Dams (25 ft < height < 35 ft or 500 acre-feet < storage < 1000 acre-feet) 33 % of PMP 3) Large Dams (35 ft < height < 100 ft or 1000 acre-feet < storage 50,000 acre-feet) 50 % of PMP 4) Very Large Dams (100 ft < height or 50,000 acre-feet < storage)- 100% of PMP < *Baser] on visual inspection and detailed hydrologic and hydraulic evalu- ation, including documentation of competent design and construction pro- cedures, up to a 10 percent lower requirement (22.5, 30, 45, 90) can be accepted, at the discretion of the director, providecl the project is in an acceptable state of maintenance. The design storm may also be reducecl if the applicant's engineer can successfully demonstrate to the director by engineering analysis that the dam is sufficient to protect against probable loss of human life downstream at a lesser design storm. Hawaii (From letter dates] May 31, 1984) State does not have an authorized dam safety program. Illinois (From letter dated May 1984) Hydrologic requirements have been summarized by a state official as follows: Class I (High Hazard Potential) Dams All dams in this hazard potential classification shall hold and pass the following floods: Large Dams PMF Intermediate Dams PMF Small Dams—I/2 PMF to PMF Should dams in this hazard] potential classification be designed to have emer- gency spillways, the principal spillway shall pass at least the entire 100-year flood before the emergency spillway functions, unless special site conditions justify variations. Class II (Moderate Hazard Potential) Dams All dams in this hazard potential classification shall hold ant] pass the following floods:

140 Appendix A Large Dams PMF Intermediate Dams its PMF to PMF Small Dams 100-yr to ~/z PMF Should dams in this hazard potential classification be <resigned to have emer- gency spillways, the principal spillway shall pass at least the entire 50-year flood before the emergency spillway functions, unless special site conditions justify variations. Class III (Low Hazard Potential) Dams M1 dams in this hazard poten- tial classification shall hold and pass the following floods: Large Dams l/2 PMF to PMF Intermediate Dams 100 yr to ~/~ PMF Small Dams 100 yr Should dams in this hazard potential classification be designed to have emer- gency spillways, the principal spillway shall pass the 25-year flood before the emergency spillway functions, unless special site conditions justify varia- tions. The following is quoted from letter of the Chief, Illinois Bureau of Re- source Management: We are very interested in the study now being conducted by the National Research Council. In fact, we have postponed enforcement actions that involve inadequate spillway capacity related to the probable maximum flood pencling completion of your research effort, unless there is a definite, immediate safety hazard. I hope that your evaluation of the various criteria used by the respondents will inclucle a definitive statement of the appropri- ate standards which are discerned as being reasonable by the National Re- search Council. Such a statement would provide a positive benefit to the respondents in assessing their own standards and perhaps aid in the develop- ment of more uniform standards nationwide. Indiana (From pamphlets supplied by Edwin B. Vician) By a 1945 law the Indiana Flood Control and Water Commission was established. By a 1961 law the Commission was granted authority over dams and authorized to issue rules, regulations and standards for maintenance and operation. However, the data furnished do not include any criteria for dams.

Appendix A 141 Kansas (From letter dated dune 12, 1984) The following is extracted from information furnished by the Chief Engi- neer-Director of the Kansas State Board of Agriculture: Structures built in Kansas are predominately earthen enbankments lo- cated in a rural or semi-rural setting. In order to assist the designer and to provide an acceptable level of consistency of design for this area, we have adopted Engineering Guide Nos. 1 and 2 by reference in our rules and regulations. These standards and design criteria were based upon many years of expe- rience and we fee! they are both adequate and practical for conditions in Kansas. Table No. 2, page 11, of Engineering Guide No. 1 (reproduced below) outlines the use of variable probable maximum precipitation based upon hazard classification of structure. In addition, please note the required mini- mum spillway dimensions. We may also require more extensive safety mea- sures to be incorporated into the design if the magnitude and location of the structure warrants consideration beyond perimeters set forth in Table No. 2. These guidelines provide the general minimum hydrologic requirements of the Division of Water Resources for the design and construction of earthfil1 dams. They are not intended] to constitute a text for design and construction. Final determination of the acceptability of design and adequacy of the plans and specifications will be made on an individual basis. The following list of definitions relates to the data shown on the Table No. 2 mentioned above: Effective Height of Dam the difference in elevation between the crest of the emergency spillway and the original streambed on the centerline of the dam. Effective Storage the volume of the reservoir below the crest of the emergency spillway. Size Factor the product of the Effective Height of Dam (in feet) and the Effective Storage (in acre-feet). Size of Dams— (1) Those dams whose effective height is less than 2S feet; effective stor- age is less than 50 acre-feet; and size factor is less than 1,250. (2) Those dams whose effective storage is greater than 50 acre-feet; and size factor is between 1,250 and 3,000. (3) Those dams whose effective storage is greater than 50 acre-feet; and size factor is between 3,000 and 30,000.

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AppendLx A 143 (4) Those dams whose effective storage is greater than 50 acre-feet, and size factor is greater than 30,000. Hazard Classes of Dams— Class (a) Low Hazard dams located in rural or agricultural areas where failure may damage farm buildings, limited agricultural land, or county, township and private roads. Class (b) Significant Hazard dams located in predominantly rural or agricultural areas where failure may endanger few lives, damage isolated homes, secondary highways or minor railroads or cause interruption of use or service of relatively important public utilities. Class (c) High Hazard dams located in areas where failure may cause extensive loss of life, serious damage to homes, industrial and commercial facilities, important public utilities, main highways or railroads. Louisiana (From letter dated May 23, 1984) State has not yet adopted regulations for dam safety but furnished the following"Excerpt of Proposed Rules and Regulations of Dam Safety Law." The minimum performance standards for impoundment are as follows: Minimum Spillway Minimum Hazard Size Design Flood Freeboard Low Small 50 to 100 yr O foot Intermediate 100 yr to ~/e PMF 1 foot Large ~/o PMF to PMF 3 feet Significant Small 100 yr to ~/z PMF O feet Intermediate l/2 PMF to PMF 1 foot Large PMF* 3 feet High Small l/2 PMF to PMF O foot Intermediate PMF* 1 foot Large PMF* 3 feet 100 yr = 100 year exceedance frequency. The flood magnitude expected to be exceeded, on the average, once in 100 years. It may also be expressed as an exceedance frequency with a one-percent chance of being exceeded in any given year. PMF = Probable maximum flood. The flood that may be expected from * The primary spillway may be sized to accommodate not less than one-half (~/~) of the total PMF, with the remainder of the total PMF accommodated by an emergency spillway.

144 Appendix A the most severe combination of critical meteorologic and hydrologic condi- tions that are reasonably possible in the region. The PMF is derived from probable precipitation, which information is generally available from the following National Weather Service publications: (1) NOAA Hydrometeorological Report No. 51 (Schreiner and Riedel, 1978) "Probable Maximum Precipitation Estimates U.S. East of the 105th Meridian". (2) NOAA Hydrometeorological Report No. 52 (Hansen et al., 1982) "Application of Probable Maximum Precipitation Estimates U.S. East of the 105th Meridian". Maine (From letter dated May 10, 1984) Regulations are to be developed in near future. Michigan State has not issued criteria for dams but follows what is considered good engineering practice. The following is quoted from a statement by the Chief, Dam Safety and Lake Engineering, in the Water Management Division of the State: The geology of Michigan does not generally allow the construction of large dams as clefined by USCOLD. The general criteria for spillway capac- ity is the 1% frequency flood in rural and undeveloped] areas. In urban areas the 0.5 % or 200-year frequency flood is a requirement. In addition to this spillway capacity, a freeboard of 11/2 foot is required above the design flood elevation for earthen embankment dams. The normal side slopes are 3 to 1 horizontal to vertical for upstream and 2~/2 to 1 on the downstream. In addition, an appropriate crest width is required. If the side slopes are steeper, a slope analysis by the consulting engineer must be provided to this office for review and acceptance. The foregoing are general criteria for earthen embankment dams which are the majority of the cases in this state. For the larger dams with a higher hazard potential, the guidelines of the National Dam Safety Inspection Program would apply. Mississippi (From letter dated May 23, 1984) The State Dam Safety Coordinator has stated: "We basically follow the criteria and standards of the Soil Conservation Service as published in their

Appendix A Technical Release No. 60 Earth Dams and Reservoirs. For new dams clas- sified as high hazard we require that they pass the full PMF. For existing high hazard dams we require them to pass 50 % PMF." 145 Missouri (From letter dated May 10, 1984) Missouri is in the process of revising its regulations for dam safety. The proposed requirements for spillway design floods are shown tin Table A-83. This proposed revision specifies use of Probable Maximum Precipitation (PMP) where present regulations refer to Probable Maximum Flood (PMF) . The "Environmental Class" listings in this table refer to developments in the area downstream from the dam that would be affected by inundations in the event of dam failure. The classes are defined as: Class I Contains 10 or more permanent dwellings or one or more public buildings. Class II Contains 1 to 9 permanent dwellings; or one or more camp- TABLE A-8 Proposed Required Spillway Design Flood Precipitation Values (Missouri) Stage of Dam Type Construction Special Conditions I Environmental Class II III Conventional Completed Two or more 0.75 pMpa o.5 pMpa o.5 pMpa or industrial dams in a series Storage x height 0.75 pMpa o.5 pMpa 0.4 pMpa greater Industrial Starter After starter dam is finished and before final dam is com- pleted than 30,000 Storage x height less 0.75 pMpa o.5 pMpa 0.3 pMpa than 30,000b Any o.5 pMpa 0.2 pMpa 0.1 pMpa Any 0.75 pMpa o.5 pMpa 0 2PMPa aPMP is probable maximum precipitation. bStorage in acre-feet measured at emergency spillway crest elevation and height in feet.

146 Appendix A grounds with permanent utility service; or one or more roads with average daily traffic volume of 300 or more; or one or more industrial buildings. Class III Everything else. Nebraska (From letter dated May 30, 1984) The following is extracted from letter of Chief, Engineering Branch, Department of Water Resources: Nebraska Department of Water Resources is principally a regulatory agency of the water resources in the State of Nebraska. The hydraulic and earthquake criteria acceptable during reviewing plans and specifications of dams are relatively the same as those used in this region by the Federal Agencies, particularly the Soil Conservation Service, Corps of Engineers and Bureau of Reclamation. Occasionally, some deviations of criteria are necessary based on existing site conditions and these are resolved on a site by site basis. * New Jersey (From letter dated May 25, 1984) Since January 1978, the state has been using criteria established for U.S. Army Corps of Engineers National Dam Inspection Program, but has now drafted proposed state dam safety regulations. That draft proposes the fol- lowing criteria for spillway design: Hazard Classification of Dam I-High hazard potential II-Significant hazard potential III-Low hazard potential IV-Smal1 damsa Minimum Spillway Design Flood (SDF)b PMP 1/2 PMP 24-hour, 100-year frequency, Type II storm 24-hour, 100-year frequency, Type II storm plus 50 % aLess than 10 feet high, impounding less than 15 acre/feet, having drainage area of 100 acres or less and not in hazard classification I or II. bReference to Type II storm is not explained in draft. New Mexico (From letter dated June 4, 1984) The following is extracted from a summary of current practices furn by the State Engineer: fished

Appendix A The State Engineer has not developed a manual of rules and regulations pertaining to the design and construction of clams because each dam is unique and as such must be designed using current good engineering design practices. Each design is submitted to and reviewed by the State Engineer's staff prior to acceptance by the State Engineer. Following the State Engineer's endorsement of approval on the applica- tion and prior to commencement of construction, the owner nominates an engineer registered in New Mexico to supervise construction of the dam. The State Engineer reviews the qualifications of the engineer and if acceptable he issues a letter approving the engineer and setting forth conditions under which he will supervise construction. For the design of the spillway, we require that it be sized in accordance with the minimum emergency spillway hydrologic criteria set forth in U. S. Department of Agriculture Technical Release No. 60, June 1976 (revised August 1981~. The State Engineer has taken the position that criteria now deemed ap- propriate for new structures may not be appropriate for existing structures. As a practical matter it may be necessary to entertain a greater risk with existing structures than would be acceptable in the design of new structures. We have undertaken to consult with engineers in federal and other state governments and in private practice to assess what best represents good engineering judgment. The staff of the State Engineer has undertaken breach analysis studies for selected high hazard dams in New Mexico. The results of these studies are used to better assess the requirements for emer- gency spillway design of existing and new clams in New Mexico. 147 New York (From letter dated May 30, 1984) The following is extracted from material supplied by the Chief, Dam Safety Section, New York State Department of Environmental Conserva- tion, who has described the state's spillway criteria as follows: Dams in New York are classified according to downstream hazard cIassifi- cation. Class "A" is the lowest hazard class and class "C" is the highest. Failure of a class "C" dam could result in loss of life. With regard to hydro- logic design criteria we follow the following standards: New Earth Dams Class "A" 1. Small Dam Spillway shall have sufficient capacity to discharge a Spillway Design

148 Appendix A Flood equal to a 100 year storm and also maintain a minimum freeboard of one foot between design high water and the top of dam. 2. Large Dam Spillway shall have sufficient capacity to discharge a Spillway Design Flood equal to lSO % of the 100 year storm and also maintain two feet of freeboard between design high water and the top of dam. Class "B" 1. Small Dam Spillway shall have sufficient capacity to discharge a Spillway Design Flood equal to 225 % of the 100 year storm ant] also maintain a minimum freeboard of one foot. 2. Large Dam Spillway shall have sufficient capacity to discharge a Spillway Design Flood equal to 40 % of the probable maximum flood and also maintain two feet of freeboard. Class ccC,, 1. Small Dam Spillway shall have sufficient capacity to discharge a Spillway Design Flood equal to one-half of the probable maximum flood and also maintain one foot of freeboard. 2. Large Dam Spillway shall have sufficient capacity to discharge a Spillway Design Flood equal to the probable maximum flood and also maintain two feet of freeboard. Small Dam Definition a. Height of Dam equal to or less than 40 feet b. Storage at normal water surface equal to or less than 1000 acre feet Large Dam Definition a. Height of Dam greater than 40 feet b. Storage at normal water surface equal to or less than 1000 acre feet Ned Concrete or Masonry Dams Dams will be designed for the same Spillway Design Floods as indicated for new earth dams. However, for concrete or masonry gravity dams over- topping will be acceptable providecl that the spillway and nonoverflow cation will he Ho to meet the .ctr'~c~t'~ral stability requirements with regard v__ ~^ ~ ~ ~ ~ ~ ~ ~ ~ to sliding and overturning. Existing Dams 1. CIass"A" Shall have adequate capacity to discharge a Spillway Design Flood equal to a 100 year storm.

Appendix A 149 2. CIass"B" Shall have adequate capacity to discharge a Spillway Design Flood equal to 150 % of the 100 year storm. 3. Cl~s"C" Shall have adequate capacity to discharge a Spillway Design Flood equal to one-half of the probable maximum flood. North Carolina (From letter dated May 18, 1984) The State's Dam Safety Regulations include Tables A-9 and A- 10. In addition, two other criteria for sizing spillways are set out as follows: (1) Within 15 days following passage of the design storm peak the spill- way system shall be capable of removing from the reservoir at least 80 % of the water temporarily detained in the reservoir above the elevation of the primary spillways. (2) Rational selection of a safe spillway design flood for specific site condi- tions based on a quantitative analysis is acceptable. The spillway should be sized so that the increased downstream damage resulting from overtopping failure of the dam would not be significant as compared with damage by the same flood in the absence of overtopping failure. A design storm more fre- quent than once in 100 years will not be acceptable for any class C dam. (The state normally requires that the assumed time of breach development in such analysis be in the range of 15 to 30 minutes.) TABLE A-9 Criteria for Spillway Design Stormsa Size Classification Size Total Storage (ac-ft~a Height (ft~a Small Less than 750 Less than 35 Medium Equal to or greater than Equal to or greater than 750 and less than 35 and less than 50 7,500 Large Equal to or greater than Equal to or greater than 7,500 and less than 50 and less than 100 50,000 Ver,v large Equal to or greater than Equal to or greater than 50,000 100 aThe factor determining the largest size shall govern.

150 Appendix A TABLE A-10 Minimum Spillway Design Storms Spillway Design Hazard Size Flood (SDF) Low (Class A) Small 50 yr Medium 100 yr Large i/3 PMP Very large i/2 PMP Intermediate (Class B) Small 100 yr Medium i/3 PMP Large i/2 PMP Very large 3/4 PMP High (Class C) Small i/3 PMP Medium i/2 PMP Large 3/4 PMP Very large PMP North Dakota (From letter dated May 29, 1984) A state official reports that North Dakota is in process of developing safety criteria for dam design. It is anticipated that the criteria will give consider- ation to the size of dams, downstream hazard categories, probable effects of dam failures, and other pertinent national or man-made conditions. Also, it is anticipated that the hazard classifications and hydraulic analysis guide- lines will follow the criteria established for the U.S. Army Corps of Engi- neers National Dam Inspection Program. Proposed classifications of dams are as follows: Category Storage Capacity (ac-ft) Height (ft) 1. Large 50,000 ac-ft and larger 100 and higher 2. Intermediate 1,000 through 49,999 40 through 99 3. Small 50 through 999 25 through 39 4. Very Small 12~/2 through49 8through24 5. Pond ~/2 through 12.4 2 through 7 Ohio (From letter dated June 5, 1984) The Ohio Administrative Rules relating to dam safety provide for four classes of dams and corresponding spillway design criteria as follows: 1. When failure of the dam would result in probable loss of human life or serious hazard to health, serious damage to homes, high-value industrial or

Appendix A 151 commercial properties, or major public utilities, the dam shall be placed in class I. Dams having a storage volume greater than five thousand acre-feet or a height of greater than sixty feet shall be placed in class I. 2. When failure of the dam would result in a possible health hazard or probable loss of high-value property or damage to major highways, rail- roads, or other public utilities, but loss of human life is not envisioned, the dam shall be placed in class II. Dams having a storage volume greater than five hundred acre-feet or a height of greater than forty feet shall be placed in class II. 3. When failure of the dam would result in property losses restricted mainly to rural lands and buildings and local roads, and no loss of human life or hazard to health is envisioned, the dam shall be placed in class III. Dams having a height of greater than twenty-five feet, or a storage volume of greater than fifty acre-feet, shall be placed in class III. 4. When failure of the dam would result in property losses restricted mainly to the dam and rural lands, and no loss of human life or hazard to health is envisioned, the dam may be placed in class IV. Dams which are twenty-five feet or less in height and have a storage volume of fifty acre-feet or less, or dams, regardless of height, which have a storage volume of fifteen acre-feet or less, may be placed in class IV. No proposed dam shall be placed in class IV unless the applicant has submitted the preliminary design report required by rule 1501:21-5-02 of the Administrative Code. The magnitude of the design flood shall be determined from actual streamflow and flood frequency records or from synthetic hydrologic crite- ria based on current publications prepared by the Ohio Division of Water, the United States Army Corps of Engineers, the United States Geological Survey, the National Oceanographic and Atmospheric Administration, or others acceptable to the Chief of the Division of Water. The minimum design flood will be: 1. For class I dams, the probable maximum flood; 2. For class II dams, fifty per cent of the probable maximum flood; and 3. For class III dams, twenty-five per cent of the probable maximum flood. (The Administrative Rules give no minimum design flood for Class IV dams.) Pennsylvania (E; rom letter dated June 5, 1984) Hydraulic criteria for dam safety are shown by the excerpts reproduced by the following extracts (Tables A-ll and A-12) from Pennsylvania's Rules and Regulations.

152 Appendix A TABLE A-ll Size Classification (Pennsylvania) Class Impoundment Storage (ac-ft) Dam Height (ft) B Equal to or greater than 50,000 Less than 50,000 but greater than 1,000 Equal to or less than 1,000 Equal to or greater than 100 Less than 100 but greater than 40 Equal to or less than 40 NOTE: Size classification may be determined by either storage or height of structure, whichever gives the higher category. TABLE A-12 Hazard Potential Classification (Pennsylvania) Category Loss of Life Economic Loss Substantial 2 Few (no rural communities or urban developments and no more than a small number of habitable structures) None expected (no permanent structure for human habitation) Excessive (extensive residential, commercial, agricultural, and substantial public inconvenience) Appreciable (damage to private or public property and short duration public inconvenience) Minimal (undeveloped or occasional structures with no significant effect on public convenience) The design flood criteria set out in Pennsylvania's regulations are as fol- lows: Size and Hazard Potential Classification A-l, A-2, B-1 A-3, B-2, C-1 B-3, C-2 C-3 Design Flood PMF 1/2 PMF to PMF 100 year to 1/2 PMF 50 year to 100 year freq.

Appendix A 153 The Department may, in its discretion, require consideration of a mini- mum design flood for any class of dams or reservoirs in excess of that set forth above where it can be demonstrated that such a design flood requirement is necessary and appropriate to provide for the integrity of the dam or reservoir and to protect life and property with an adequate margin of safety. The Department may, in its discretion, consider a reduced design flood for any class of dams or reservoirs where it can be demonstrated that such design flood provides for the integrity of the dam or reservoir ant] protects life and property with an adequate margin of safety. The regulations provide, also, that the Probable Maximum Flood (PMF) is to be derived from Probable Maximum Precipitation (PMP) estimates obtained from the National Weather Service of the National Oceanographic and Atmospheric Adminis- tration (NOAA). The Chief, Division of Dam Safety in Pennsylvania, reports that changes in technique in application of PMP estimates advocated in recent NOAA reports have caused problems with dam owners. South Carolina (From letter dated May 17, 1984) Requirements for spillway capacities in the South Carolina dam safety regulations are as shown in Table A-13. South Dakota (From letter ciated May 16, 1984) State does not have a dam safety program but did inspect high hazard dams under the Corps of Engineers national dam inspection program. State has no dam safety criteria. Texas (From letter dated May 25, 1984) Published rules of the Texas Water Development Board have not specified hydraulic criteria for dams. Criteria of such authorities as SCS and U.S. Army Corps of Engineers have been used. However, changes in the rules to include hydraulic criteria for dams are being developed. Utah (From letter dated May 8, 1984) The following material on spillway hydrology is quoted from "Rules and Regulations Governing Dam Safety in Utah," dated January 1982:

154 Appendix A TABLE A-13 Spillway Design Flood Criteria Hazard Size Spillway Design F food (SDF) High Very small 100 yr to 1/2 PMF Small i/2 PMF to PMF Intermediate PMF Large PMF Significant Very small 50 to 100 ye frequency Small 100 yr to i/2 PMF Intermediate i/2 PMF to PMF Large PMF Low Small 50 to 100 yr frequency Intermediate 100 yr to 1/2 PMF Large 1/2 PMF to PMF NOTE: When appropriate, the spillway design flood may be reduced to the spillway discharge at which dam failure will not significantly increase the downstream hazard which exists just prior to dam failure. Unless specifically exempted by the State Engineer, the spillway design calculations shall follow the list below. The spillway shall be sized such that the appropriate floor] can pass through the structure without overtopping. High hazard Moderate hazard Low hazard |/2 PMF PMF 100-year frequency l/2 PMF 100-year frequency Virginia (From letter dated dune 14, 1984) The following material (Table A-14) is extracted from material furnished by the Dam Safety Section, State Water Control Board: Table A-14 defines the appropriate spillway design flood. This is essen- tially the same as the Guidelines of the Corps of Engineers. Presently an amendment is proposer] to permit engineering judgment on the appropriate spillway design flood since the PMP is constantly changing. There are many factors involved in the Spillway Design Flood in addition to the capacity and height. The watershed area ant] the slope should also be considered. It may be well to consider more steps as l/4 PMF, i/2 PMF and 3/4 PMF.

Appendix A TABLE A-14 155 Spillway Class Hazard Potential Size Classification Design of If Impounding Maximum Capacity Flood Dam Structure Fails (ac-ft)a Height (ft)a (SDF)b I Probable loss of Large > 50,000 > 100 PMF life; Medium > 1,000 and > 40 and PMF excessive < 50,000 ~ 100 economic loss Small > 50 and > 25 and \/2 PMF ~ 1,000 ~ 40 to PMF II Probable loss of Large > 50,000 > 100 PMF life; Medium > 1,000 and > 40 and ~/~ PMF appreciable < 50,000 ~ 100 to PMF economic loss Small > 50 and > 25 and 100-yr 1 ,000 < 40 to l/2 PMF III No loss of life Large > 50,000 > 100 PMF expected; Medium > 1,000 and >40 and ~/o PMF minimal < 50,000 < 100 to PMF economic lossC Small > 50 and > 25 and 50 yr < 1,000 <40 to 100 yr aThe factor determining the largest size shall govern. bThe recommended design floods in this column represent the magnitude of the spillway design flood (SDF), which is intended to represent the largest flood that need be considered in the evaluation of a given project, regardless of whether a spillway is provided; i.e., a given project should be capable of safely passing the appropriate SDF. Where a range of SDF is indicated, the magnitude that most closely relates to the involved risk should be selected. CClass III impounding structures for agricultural purposes less than or equal to 100 acre-feet in capacity and less than or equal to 25 feet in height are exempt from regulation per Section 2.01-d-iii upon certification by the owner per Section 2.01q. PMF: Probable Maximum Flood. The flood that may be expected from the most severe combination of critical meteorologic and hydrologic conditions that are reasonably possible in the region. The PMF is derived from the current probable maximum precipitation (PMP), which information is generally available from the National Weather Service, NOAA. Most federal agencies apply reduction factors to the PMP when appropriate. Reductions may be applied because rainfall isohyetals are unlikely to conform to the exact shape of the drainage basin. In some cases local topography will cause changes from the generalized PMP values; therefore, it may be advisable to contact federal construction agencies to obtain the prevailing . a. practice In specific cases. 100-Year: 100-Year Exceedance Interval. The flood magnitude expected to be exceeded, on the average, once in 100 years. It may also be expressed as an exceedance frequency with a one percent chance of being exceeded in any given year.

156 Appendix A Washington (From letter dated June 6, 1984) The Supervisor, Dam Safety Section, reports that his section has been engaged for the past three years in regional frequency analysis of precipita- tion data for the purpose of selecting design storms to be used in the computa- tion of inflow design floods for spillway design. The decision to use this probabilistic approach was based on the perception that basing designs on PMP or percentages of PMP leads to drastically different levels of safety throughout Washington State. It is envisioned that the adopted spillway design guidelines will include two components: (1) a downstream hazard assessment weighting, and (2) a regional precipitation analysis to provide probabilistic information on the magnitude, frequency and temporal distribution of rainfalls within extreme events. West Virginia (From letter dated May 29, 1984) The following are extracts from "Dam Control Regulations," effective February 1, 1982, issued by the West Virginia Department of Natural Re- sources: Hazard Classification The hazard potential shall be determined by the applicant based on the potential loss that would result due to a failure and the classification deter- mined as listed below: (a) Class A Dams located in rural or agricultural areas where failure may damage farm buildings, agricultural land, or secondary highways. Failure of the structure would cause only loss of the structure and loss of property use such as related roads, but with little additional damage to adjacent property. Any impoundment exceeding 25 feet in height or 200 acre-feet storage volume or having a watershed exceeding 500 acres shall not be a Class A structure. (b) Class B Dams located in predominantly rural agricultural areas where failure may damage isolated homes, primary highways or minor railroads or cause interruption of relatively important public utilities. Fail- ure of the structure may cause great damage to property and project opera- tions. (c) Class C Dams located where failure may cause loss of human life, serious damage to homes, industrial and commercial buildings, important

Appendix A 157 public utilities, primary highways, or main railroads. This classification must be used if failure would cause possible loss of human life. Design Requirements Design Storm All dams shall be designed to meet the following mini- mum hydrologic criteria based on hazard classification: (1) Class A dams shall be designed for a minimum of PIOO + 0. 12(PMP-P~oo) inches of rainfall in six (6) hours plus three (3) feet of freeboard. If the storage x effective height is less than 3,000 (acre-feet x feet) then Soil Conservation Pond Standard 378 may be substituted. (2) Class B dams shall be designed for a minimum of P~oo+o.4o~pMp-p~oo3 inches of rainfall in six (6) hours plus three (3) feet of freeboard. (3) Class C dams shall be designed for the probable maximum precipita- tion, or for 80 percent of the probable maximum precipitation plus three (3) feet of freeboard provided the watershed is less than ten (10) square miles in area. PART 3 OTHER GOVERNMENTAL AGENCIES City of Los Angeles, California, Department of Water and Power (From letter dated July 3, i984) The following is adapted from a list of design procedures and criteria relating to extreme floods furnished by the Department of Water and Power: 1. Develop the PMP storm using the procedures in "Hydrometeorological Report No. 36 Interim Report- Probable Maximum Precipitation in Cali- fornia" (U.S. Weather Bureau, 1961a) or "Hydrometeorological Report No. 49 Probable Maximum Precipitation Estimates, Colorado River and Great Basin Drainages" (Hansen et al., 1977~. 2. Calculate runoff generated by the Probable Maximum Precipitation (PMP) storm using the latest version of the HEC-1 Flood Hydrograph Pack- age computer program developed by the U.S. Army Corps of Engineers. 3. Provide adequate spillway capacity to accommodate the PMP storm. 4. Provide adequate freeboard to accommodate the runoff generated by the PMP storm. 5. Provide sufficient storm water bypass facilities for off-stream reser- vo~rs. 6. Provide sufficient blow off capability.

158 Appendix A 7. Meet the requirements of the California Department of Water Re- sources, Division of Safety of Dams. East Bay Municipal Utility District, California (From letter dated May 25, 1984) The East Bay Municipal Utility District (EBMUD) owns a number of dams. EBMUD's manager reports that the District does not have formalized or written criteria for clams but has attempted to apply "state-of-the-art" criteria, standards and procedures in both the design of new facilities and the analysis of existing facilities. Salt River Project, Arizona (From letter dated june 25, 1984) The following two paragraphs are quoted from letter of the General Manager of the Salt River Project (SRP): SRP operates ant! maintains six (6) large high-hazard dams upstream of metropolitan Phoenix. Current studies by the USBR ant] Corps of Engineers have revised both the hydrologic and seismic design criteria for these struc- tures causing them all to be categorized as unsafe due to their inability to safely accommodate the new Inflow Design Floods (IDF) and Maximum Credible Earthquakes (MCE). Recently completed studies also indicate that failure of these dams conic result in the inundation of as many as a quarter million (250,000) Phoenix Valley residents. Although legal title and ultimate Safety of Dams responsibility rests with the USBR, SRP senses a strong obligation to investigate and support all efforts to insure the safety of the dams on the Salt and Verde rivers. Since the Salt River Project does not own the Salt and Verde River dams, it does not establish the hydrologic and seismic criteria but rather operates under the criteria set by the clam owner, i.e. the USBR. SRP has, however, conducted several studies in the past which are directly or closely related to the concerns being investigated by the National Research Council study. Information on the following described studies was furnished by the Gen- eral Manager: 1. Synopsis of Selected USBR Inflow Design FZoodsfor Large Dams, PRC Engineering Consultants, Inc., May 1980. The study was undertaken to compile a synopsis of U.S. Bureau of Reclamation (USBR) hydrologic design criteria to assist the Salt River Project (SRP) in assessing the hydrologic

Appendix A degree of risk associated with the six storage dams that comprise the SRP system. 2. Paleoflood Hydrology Studies on Salt and Verde Rivers, Dr. Victor Baker, University of Arizona. This study is currently in progress and is in- tended to provide additional information on the magnitude and frequency of historic and prehistoric floods on the Salt and Verde rivers. A letter report identifies the cursory results obtained during a reconnaissance investigation and a glimpse of the information hoped to be obtained during the full scale study. This study is scheduled to be completed by October 1, 1984. The report on the first-listed study traces the hydrologic design practices of the U. S. Bureau of Reclamation from the establishment of the Bureau in 1902 till the present and experience of the Bureau in 7,946 dam-years of accumulated exposure at 259 storage dams. The Gibson Dam on the Sun River in Montana was the only Bureau dam overtopped in that period of record. The report noted that estimates of reservoir inflow at the Roosevelt and Horseshoe dams of the Salt River Project for currenfly-used design rainfalls greatly exceed the spillway capacities available at those dams. The letter relating to the second-listed study stated that brief reconnais- sance of pre-historic flood deposits along the Salt and Verde rivers had re- vealed radiocarbon-datable materials. It concluded that an extensive paleoflood record existed that probably could be used to evaluate the se- quence of largest floods over several thousand years. 159 Santee Cooper (South Carolina Public Service Authority) (From letter dated duly 12, 1984) The following is quoted from letter of the President, Santee Cooper: We are very interested] in the establishment of acceptable risk levels for seismicity as well as for spillway adequacy in Federal dams. As for our design procedures, criteria and standards for dam safety and inspections, Santee Cooper is licensed by the Federal Energy Regulatory Commission (FERC) ant] follows FERC guidelines regarding dam safety. PART 4 TECHNICAL SOCIETIES American Society of Civil Engineers (ASCE) On May 9, 1981, the ASCE Board of Directors adopted a policy statement entitled "Responsibility for Dam Safety." As the title indicates, the statement

160 Appendix A was directed primarily at the placement of responsibility for the safety of dams but it did contain references to design criteria. The following are extracts from that statement: In the development of a project involving a dam, there are uncertainties in predicting the natural events to which the dam will be subjected, in estimat- ing future project effects and benefits, and in forecasting the performance of project components. Available design alternatives may involve varying first costs and degrees of risk to the owner. Where only costs to the owner are involved, economic analyses based on historical records of natural events and performance records of similar components at other projects provide bases for selection among design alternatives. However, if possible loss of human life, loss of vital community services or extensive damages to others may be involved, the adopted design should seek to minimize the potential for such losses or damages. Hydrologic relationships used in operations studies to evaluate project effects and to design project components should be based on thorough analy- ses of local and regional hydrologic records. The theoretical probable maxi- mum flood (PMF) potential of the basin should be considered in designing any dam where there would be significant hazard to lives and property in downstream areas. International Commission on Large Dams (ICOLD) The following is extracted from a draft dated February 1984 of a proposed "Guidelines on Dam Safety" prepared by the ICOLD Committee on Dam Safety: As a general rule, the design of the dam and reservoir shall be based on the probable maximum flood. The latter shall derive from the combination of maximum runoff volumes with most unfavorable runoff conditions and is to be used to produce the design flood hydrograph. The capacity of gated spillways shall be sufficient to discharge the full design flood without taking into account the dampening effect resulting from flood routing through the reservoir. A reduction of the design flood as derived from the probable maximum flood, or the consideration of the effect of flood routing when determining the spillway capacity, should be permitted under especially favorable conditions. Such conditions may be: The permanent availability of reserve storage capacity of the reservoir, between the normal top water level and the maximum reservoir level, com- patible with the temporary surcharge volume deriving from the partial

Appendix A 161 retention of the inflowing flood. The availability of the mentioned reserve storage capacity must be combined with highly reliable operating proce- clures that assure the opening of the spillway gates in accordance with the predetermined flood routing program. —The existence of an additional fuse plug type spillway the rupture of which would not increase the downstream flood beyond the acceptable risks. A permanently warranted low downstream risk level that should at no time include any risk to human life. —Other favorable circumstances that permit the exemption from the above mentioned requirements, in accordance with criteria and regulations established by the Government Agency. In any case, however, the determination of design flood and spillway capacity of all dams within the same drainage area must be based on uniform criteria and procedures. U.S. Committee on Large Dams (USCOLD) This United States component of ICOLD has not attempted to promul- gate design standards for dams, but in the late 1960s an USCOLD working committee undertook a survey of design practices in the United States for sizing spillways. The results of that survey, which was accomplished by use of a questionnaire, were presented in a 1970 USCOLD report "Criteria and Practices Utilized in Determining the Required Capacity of Spillways." The following material is extracted from that report: All respondents stated that current policies of their agencies or firms were consistent with the following general statement: "When a high dam, capable of impounding large quantities of water, is constructed above a populated community, a distinct hazard to that commu- nity from possible failure of the dam is created unless due care is exercised in every phase of the engineering design, construction, and operation of the project to assure complete safety. The policy of deliberately accepting a recognizable major risk in the design of a high dam simply to reduce the cost of the structure has been generally discredited from the ethical and public welfare standpoint, if the results of a failure would imperil the lives and lifesavings of the populace of the downstream flood plain. Legal and finan- cial capabilities to compensate for economic losses associated with major dam failures are generally considered as inadequate justifications for accept- ing such risks, particularly when severe hazards to life are involved. Accord-

162 Appendix A ingly, it is the policy of this agency that high dams impounding large volumes of water be designed to conform with Security Standard 1." 1. Standard 1 a. High dam impounding large volumes of water, sudden release of which would create major hazards to life or property downstream. b. Dams of such economic importance that prevention of overtopping during extreme floods including the probable maximum flood is of such importance as to justify the expenditures required, notwithstanding the low probability of occurrence of overtopping. 2. Standard 2 It is recognized that some low head dams can be overtopped without failing or if they fail a hazardous flood wave will not result downstream. In such cases the design capacity of the spillway and related features may be based largely on economic consideration. Typical applications include: a. Dams specifically designed so that overtopping will not cause either failure or serious damage downstream. b. Run-of-river hydroelectric power or navigation dams, diversion dams, and similar structures where relatively small difference between headwater and tailwater elevations will prevail during overtopping floods and where the cost of preventing overtopping is high in comparison with economic losses otherwise probable. c. Sub-impoundment dams adjacent to larger reservoirs, where possible release from breaching can be absorbed by the larger reservoir without major hazard. 3. Standard 3 a. Dams impounding a few thousand acre-feet or less, so designed as to assure a relatively slow rate of failure if overtopped and located where hazard to life and property in the event of dam failure would clearly be within acceptable limits. Under certain circumstances, with special precau- tions larger dams may fall under this standard. b. Sub-impoundment dams of the nature described under Standard 2c. 4. Standard 4 a. Dams forming small recreational lakes or water supply ponds located where the probability of serious property damage would be acceptably small. b. Dams forming relatively small farm ponds where failure would not constitute a serious hazard downstream.

Appendix A TABLE A-15 Number of Projects Designed or Constructed by Reporting Agencies (Approximate Estimates) From 1970 USCOLD Report 163 Reporting Agency Standard Standard Standard or Firm 1 2 3 Total Federal agencies Corps of Engineers, USA 320 70 10 400 Bureau of Reclamation 195 134 0 329 Tennessee Valley Authority 22 8 8 38 Soil Conservation Service 560 — 4,780 5,340 Subtotals (Federal) 1,097 212 4,798 6,107 State agencies and private engineering firms 295 64 18 377 Totals 1,392 276 4,816 6,484 The primary objective of the survey was to compile information concern- ing practices and criteria actually used in the design of existing dams and those scheduled for construction in the near future. Accordingly, each recipi- ent of the questionnaire was requested to indicate the number of projects covered in his reply, identified according to Security Standards 1, 2 and 3 defined above in orcler that the scope of application of various procedures and criteria Night be evaluated. Table A-15 is a breakdown of projects reported in the various classifications. PART 5—FIRMS IN UNITED STATES Acres American, Inc., Buffalo, New York (From letter datecl July 7, 1984) The following extract from a paper prepared for a seminar summarizes the practices of the Acres American organization in determining spillway · . capacities: Like most organizations, Acres has not adopted a rigid position on spill- way capacity criteria circumstances alter cases. General statement for large reservoirs is: Check carefully for largest flood types (spring snowmelt, hurricane, other storm rainfall);

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

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

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-

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

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

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.

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

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.