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federal and nonfederal dams (presuming the availability of data from state inventories) . Study the need for continuing a national inventory of nonfederal dams before the end of the phase-out period. 5.2 RISK CLASSIFICATION 5.2.1 Problem Description The analysis of risk involves consideration of two basic elements: these are the probability and the consequences of failure (hazard potential or potential losses in the event of failure), Combining these two elements in a rational manner produces a measure of risk. While the consequences of failure can be realistically estimated, the probability of an individual structure's failing is difficult to assess.' Although the basic principles for making such assessments exist, the methodology has not been applied in practice to dams except in isolated instances and in very simplistic forms. A risk assessment for dam safety would have to consider such information as historical data on dam failuresj the uncertainties about loads (including earthquakes), structural resistances, and geological factors; overtopping probabilities and their structural consequences; and the eXisting conditions of structures. Estimates of risk and the reliability of ri sk predictions change with t.ime since the amounts and quality of information about site conditions and structural properties usually increase with time. By applying risk assessment to dam safety, engineers and safety officials would at least obtain a priority list for inspections, maintenance, and remedial work. Together with pertinent cost data, risk assessment would facilitate the better allocation of financial and personnel resources for mitigating dam failure hazards. In the process, the trade-off between cost and risk would have to be considered. All structures, no matter how~ell engineered, embody some risk. Risks can be lowered by upgraded design, more intensive inspection, rehabilitation, or effective emergency warning systems and evacuation plans; and the -26-