What Is Significant About Significant Hazard Dams? A Closer Look at Dam Hazard Classification

Hazard Potential Classification Systems are used by regulatory dam safety agencies and owners to help manage and prioritize their portfolios of dams. In the USA, most hazard classification systems have three levels: High, Significant, and Low hazard. The current standard classification system is provided in FEMA 333, Federal Guidelines for Dam Safety - Hazard Potential Classification System for Dams (2004). While the definitions of High and Low Hazard potential classification for dams in this document seem fairly clear, the same cannot be said for the Significant Hazard definition. FEMA 333 defines Significant Hazard dams as “those dams where failure or mis-operation results in no probable loss of human life but can cause economic loss, environmental damage, disruption of lifeline facilities, or can impact other concerns”. This definition appears to be vague and subjective, as there is no indication of the degree of damages that qualify as being “significant”. Would a single occupied structure being flooded by a foot of floodwater that causes first floor damage count? Or would it take dozens of such homes getting damaged? Also, what if the loss of human life is not probable, as in high hazard, but is still possible? Would that scenario fall under the Significant Hazard Classification? This paper discusses the background of the Hazard Potential Classification System and will discuss the variety in criteria used by the states and Federal agencies to define it. The authors will recommend potential changes to the FEMA 333 definition for Significant Hazard to better define what is “significant” and make it less subjective.