Safety of Dams Flood and Earthquake Criteria (1985) / Chapter Skim
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Appendix A: Design Criteria in Use for Dams Relative to Hazards of Extreme Floods
Appendix A: Design Criteria in Use for Dams Relative to Hazards of Extreme Floods
Pages 115-174
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From page 115... ...
U.S. Nuclear Regulatory Commission PART 2 STATE AGENCIES RESPONSIBLE FOR DAM SAFETY Alaska .
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From page 116... ...
New York. North Carolina North Dakota Ohio Pennsylvania South Carolina.
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From page 117... ...
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 .
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From page 118... ...
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.
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From page 119... ...
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.)
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From page 120... ...
If structural failure would not present a hazard to human life or cause significant property 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 significantly documented. As a result of the publications of Hydrometeorological Reports Nos.
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From page 121... ...
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.
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From page 122... ...
The Regional Director of Engineering must approve exceptions to the recommended spillway design flood. When documentation is not available to support an exception, use the recommended spillway design flood criteria shown in Table A-1.
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From page 123... ...
(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 balancing the likelihood of failure by overtopping against the consequences of dam failure.
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From page 124... ...
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 developingsuch 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.
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From page 125... ...
Class (b) Dams located in predominantly rural or agricultural areas where failure may damage isolated homes, main highways or minor railroads or cause interruption of use or service of relatively important public utilities.
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From page 126... ...
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 determinations are to be based upon combinations of hydrologic factors which are selected to prevent unrealistic combinations of hydrologic conditions.
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From page 128... ...
Currently such floods are based upon TVA maximum probable 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 maximum and TVA maximum probable floods generally call for average or median conditions observed during maximum past floods.
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From page 129... ...
In application of these standards most Corps of Engineers new dams are designed to pass the PMF with full freeboard; the exceptions being run-ofriver 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.
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From page 130... ...
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, limited agricultural land, or township and country roads.
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From page 131... ...
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 relatively 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 public utilities, main highways, or railroads.
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From page 132... ...
(From letter dated June 8, 1984) The following is extracted from material furnished by NRC: Although the Nuclear Regulatory Commission (NRC)
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From page 133... ...
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".
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From page 134... ...
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 criteria for review of plans for dams and inspections and relies heavily on Corps of Engineers criteria.
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From page 135... ...
can demonstrate to the department 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 that just exceeds the routing capacity of the reservoir.
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From page 136... ...
. .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)
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From page 137... ...
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 maximum precipitation determined from Hydrometeorological Report No.
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From page 138... ...
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 spillway design flood.
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From page 139... ...
Illinois (From letter dated May 1984) Hydrologic requirements have been summarized by a state official as follows: Class I (High Hazard Potential)
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From page 140... ...
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 appropriate standards which are discerned as being reasonable by the National Research Council.
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From page 141... ...
1 (reproduced below) outlines the use of variable probable maximum precipitation based upon hazard classification of structure.
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From page 143... ...
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*
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From page 144... ...
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.
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From page 145... ...
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.
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From page 146... ...
That draft proposes the following 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.
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From page 147... ...
With regard to hydrologic 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
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From page 148... ...
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.
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From page 149... ...
3. Cl~s"C" Shall have adequate capacity to discharge a Spillway Design Flood equal to one-half of the probable maximum flood.
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From page 150... ...
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
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From page 151... ...
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.)
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From page 152... ...
The design flood criteria set out in Pennsylvania's regulations are as follows: 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.
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From page 153... ...
State has no dam safety criteria. Texas (From letter dated May 25, 1984)
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From page 154... ...
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.
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From page 155... ...
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.
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From page 156... ...
(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.
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From page 157... ...
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.
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From page 158... ...
Corps of Engineers have revised both the hydrologic and seismic design criteria for these structures 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)
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From page 159... ...
As for our design procedures, criteria and standards for dam safety and inspections, Santee Cooper is licensed by the Federal Energy Regulatory Commission (FERC)
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From page 160... ...
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, compatible with the temporary surcharge volume deriving from the partial
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From page 161... ...
—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.
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From page 162... ...
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.
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From page 163... ...
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 spillway capacity criteria circumstances alter cases.
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From page 164... ...
and applying COE hazard criteria to select the Spillway Design Flood (SDF)
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From page 165... ...
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.
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From page 166... ...
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 significant damage will take place as a result of those outflows even without the occurrence of a dam failure.
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From page 167... ...
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.
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From page 168... ...
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.
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From page 169... ...
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.
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From page 170... ...
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From page 171... ...
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From page 172... ...
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.
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From page 173... ...
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.
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From page 174... ...
174 Appendix A immediately below a dam and the loss of its output would have grave economic 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.
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