Dam Safety: A Comparison of Flood Hazard Criteria with Acceptable Risk Criteria. David S. Bowles, Utah Water Research Laboratory, Utah State University, Logan, UT, and RAC Engineers and Economists, Logan, UT; Loren R. Anderson, Department of Civil and Environmental Engineering, Utah State University, Logan, UT, and RAC Engineers and Economists, Logan, UT; and Terry F. Glover, Department of Economics, Utah State University, Logan, UT, and RAC Engineers and Economists, Logan, UT
Hazard classification criteria are commonly used as a basis for determining the design flood requirements for dams. Threat-to-life and economic damages are used to assign a low, significant, or high hazard rating with the high rating commonly leading to the requirement for passing the probable maximum flood. By definition hazard is the potential for loss of life and property damage in the event of dam failure. It is not a measure of the adequacy of the dam. Many states, government agencies, and professional bodies have developed their own version of hazard classification criteria which contain different definitions of low, significant or high hazard ratings, and sometimes more than three ratings are used. When these different definitions are carefully examined they can lead to differences in application that result in potential inconsistencies in design flood requirements. These inconsistencies are described in this paper together with a comparison of several different hazard criteria, which are in current use, against internationally-used acceptable risk criteria for life safety. This comparison indicates that, under certain circumstances, hazard classification can lead to dam safety requirements that may exceed those at nuclear power plants, or for the siting of hazardous industrial facilities. The comparison will include a summary of the results of risk assessments of twenty existing dams to show the effects.