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. 1 ]:ntroduct:ion PURPOSE The goal of this report is the enhancement of dam safety. It was prepared by the Committee on the Safety of Existing Dams, National Research Council (NRC), to present in a single volume all essential aspects of dam safety. A major objective of the report is to provide guidance for achieving improvements in the safety of existing dams within financial restraints. Many dam owners are faced with problems of such a nature and extent that they are unable to finance remedial measures. In May 1982 the U.S. Army Corps of Engineers reported that no remedial measures had been instituted at 64% of the unsafe dams found during its 4-year inspection program, principally because of the owners' lack of resources. To these owners, as well as to regulatory agencies and others concerned with the engineering and surveillance of dams, the committee presents its suggestions and guid- ance for assessing and improving the safety of existing dams. The contents of this report are intended to be informational and not to advocate rigid criteria or standards. In no instance does the committee intend to recom- mend the lowering of existing dam safety standards. SCOPE The scope of the committee's study and the conclusions of this report con- cern technical issues pertinent to dam safety. The study includes examina- tions of risk assessment techniques; engineering methodologies for stability 1

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2 SAFETY OF EXISTING DAMS and hydrologic evaluations; and methods and devices to identify, reduce, and/or eliminate deficiencies in existing dams. Included are case histories demonstrating economical solutions to specific problems and also possible nonstructural approaches. The Committee on the Safety of Existing Dams, operating under a June 1983 completion deadline, arranged a relatively brief but intensive study of its assigned task. It conducted a 2-day meeting in Washington, D.C., on June 2-3, 1982, to initiate the effort; a meeting of the panel chairmen in Spokane, Washington, on August 10, 1982, to plan a workshop; and a 3- day workshop meeting in Denver, Colorado, on October 5-7, 1982. A com- mittee meeting was also held on March 7 and 8, 1983, to complete the draft of the report. Participants in the workshop included committee members, members of NRC staff, and other experts with a broad range of experience in dam engi- neering and dam safety. Task assignments for the workshop were divided among five working groups: (1) Risk Assessment, (2) Stability of Embank- ment Dams and Their Foundations, (3) Stability of Masonry Dams and Their Foundations, (4) Hydraulic/Hydrologic Considerations, and (5) In- strumentation. Advance assignments were made to individual participants for specific contributions to the workshop to ensure complete coverage of all issues. During the workshop a separate task group was designated to address the general subjects of geology and seismology. The participants discussed the various technical aspects of enhancing dam safety with a view to reaching a consensus on desirable approaches whenever possible. From these presentations and discussions, the commit- tee reached the conclusions presented in this report. BACKGROUND This study by the Committee on the Safety of Existing Dams is the second phase of a comprehensive study concerning policy and technical issues re- lated to the safety of dams. In October 1981 the Federal Emergency Man- agement Agency (FEMA3 asked the NRC to undertake such a study. For the first phase, FEMA asked the NRC to identify impediments to state-run programs for dam safety, to suggest federal actions to remove or mitigate those impediments, and to define how the U.S. government could help make nonfederal dams safer. In response, the NRC created the Committee on Safety of Nonfederal Dams to review and discuss the issues involved. The efforts of that committee were completed in February 1982 and re- ported in the 1982 publication entitled Safety of Nonfederal Dams, A Re- view of the Federal Role.

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Introduction 3 Unlike the first phase, the second phase concerns the technical consider- ations relating to dam safety and is applicable to all existing dams, federal as well as nonfederal. The NRC created a new Committee on the Safety of Existing Dams in May 1982 to examine the technical issues of dam safety and to develop guidance on how to achieve improvements in the safety of dams, with due recognition of financial constraints. The committee's task as defined by FEMA was as follows: To inventory and assess risk techniques and formulate guidelines on their use to rectify problems faced by dam owners and states with limited financial resources. To review and evaluate methods and devices that can be applied, along with risk assessments, to identify, reduce, and/or eliminate deficien- cies in existing dams (includes development of a glossary of terms; evalua- tion of hydrologies and stability parameters; and formulation of guidance for mitigation of such problems as overtopping, weak foundations, piping, and seismicity). To examine methodologies for assessing the potential impact of ad- verse conditions (e.g., maximum credible earthquake, probable maximum flood) on existing dams and potential modifications in order to set limits of acceptable damage to a dam. The methodologies must support the assump- tion of nonfailure of the structure. Additionally, the methodologies are not to be applied to major structures where failure is catastrophic. Guidance here is intended to be offered on how to achieve improvements in the safety of existing dams within financial constraints. REFERENCES National Research Council, Committee on Safety of Nonfederal Dams (1982) Safety of Nonfe- deral DamsA Review of the Federal Role, National Academy Press, Washington, D.C. U.S. Army Corps of Engineers (1982) National Program for Inspection of Nonfederal Dams Final Report to Congress.