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Introduction In May 1984, subsequent to a request from the Assistant Secretary of the Interior for Water and Science and the Assistant Secretary of the Army for Civil Works, the National Research Council initiated a study of criteria for the evaluation of the adequacy of spillways and earthquake resistance of dams. An enabling cooperative agreement provided that the Council would establish a Committee on Safety Criteria for Dams; prepare an inventory of existing safety criteria in use by federal, state, and nongovernmental entities in the design of dams, particularly with respect to spillway capacities and earthquake hazard, to serve as an information base for consideration by the committee; and identify and evaluate alternative criteria for establishing minimum levels of safety for federally designed, constructed, or operated dams. The committee was to consider both deterministic and frequency- based criteria, as applied to extreme flood and earthquake events. In identi- fying alternative criteria it was considered essential to reflect the influences of various natural and man-made conditions, size of dam, and probable effects of dam failure. Further, it was agreed that new dams and existing dams should be considered separately. A comparison of the alternative crite- ria, including the impacts, costs, and other implications of their use, was agreed to be an important and integral part of the evaluation process. A Committee on Safety Criteria for Dams was appointed to function under the auspices of the Water Science and Technology Board of National Research Council. The committee included experts in hydrology, hydrau- lics, general dam engineering, seismology, geology, meteorology, earth- quake engineering, economics, law, and risk assessment. The Committee on s

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6 SAFETY OF DAMS Safety Criteria for Dams, with a December 1984 completion deadline, ar- ranged a brief but intensive study of its assigned task. Simultaneous with the appointment of the committee, input to an inven- tory of existing safety-related criteria for dams was sought from essentially every type of organization that might have an interest in dam safety. This inventory was created in an effort to assure consideration by the committee of existing approaches to dam safety standards. In May 1984, two meetings of several committee members, the government technical representatives, and the Water Science and Technology Board members and staff were held to organize the study, sort out assignments, and prepare for working meet- ings of the full committee. The committee met with experts from several of the federal water resources agencies in Washington, D. C., on June 25-26 and August 13-14, 1984, to discuss, review, debate, and develop draft docu- ments. A draft report was produced from the activities of these meetings, and the committee refined its report at a subsequent meeting in Pasadena, California, on September 20-21, 1984. Throughout the study process, the committee was acutely aware of the importance of its report to those responsible for safety of the nation's several thousands of dams. Although the focus of the committee's considerations has been on the dams for which the federal government is primarily responsible, the potential effect of the committee's report on the dam safety programs of state governments has been kept in mind. It is recognized that the level of effort appropriate to evaluating the safety of each of the major federal dams may not be suitable to the differing circumstances encountered in state dam safety programs and in other nonfederal dam safety activities. Such aspects as limitations in available financial and technical resources, differences in size of dams involved, and differences in the general levels and extent of potential hazards may justify use of simplified procedures and criteria, par- ticularly for preliminary screening of large numbers of dams in the interest of public safety. The request by the Departments of Army and the Interior for the National Research Council to undertake this study is one more indication of the high level of interest in dam safety that has been evident throughout the world in the past few years. This interest has been brought about by such develop- ments as the following: The occurrence of several disastrous incidents involving uncontrolled releases of impounded waters in the last two decades (e.g., Vaiont in Italy, Malpasset in France, and Machha II in India, and in the United States such dams as Buffalo Creek, Bear Wallow, Teton, Toccoa Falls, and Laurel Run) and several near failures (severe damage) from earthquakes (e.g., Koyna

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Introduction Dam in India and the Upper and Lower Van Norman (San Fernando) Dams in California). The results of the National Dam Inspection Program conducted by the U.S. Army Corps of Engineers, which found that one-third of the approxi- mately 9,000 dams inspected (all in high-hazard situations) were tentatively classified as unsafe. The realization that improvement of dams in the United States to meet current safety standards would have very high costs and that finding funds to improve many clams would be difficult. The increaser] activity in many states in regulating privately owned dams in the interest of public safety. All of these developments contributed to the general perception that dam safety criteria are important to many interests. However, three other devel- opments may be regarded as proximate causes for the request for this study: 1. The finding, based on investigation of existing projects by the Bureau of Reclamation and the Corps of Engineers in light of current concepts of the effects of earthquakes on dams, that some of those projects need alteration to assure acceptable safety during earthquakes. 2. Findings that many existing dams fail to meet current spillway capac- ity criteria for new dams. 3. Tightened budget requirements at the federal level and need for in- creased justification for requests for funds to improve existing federal dams in the interest of public safety. The committee is gratified that, through its work, a considerable range of professional viewpoints and backgrounds has been brought to bear on the problems involved in dam safety criteria relating to extreme floods and earthquakes. However, establishing such criteria necessarily involves bal- ancing risks among various interests. Establishing acceptable levels of risk to humans and properties is a matter of public policy. Technical experts can only help to determine what level of risks are acceptable. Guidelines for dam safety should not be applied in a mechanical fashion by specialists in a narrow field of technical activity. A review in the broad public interest of dam safety evaluations and plans to improve dam safety is desirable for each project.