Societal Risk Criteria: History, Current Risks, Risk Aversion, and Scale Effect
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In August 2005, Hurricane Katrina caused massive flooding in the city of New Orleans. Eighty-percent of the city was left submerged causing 1833 fatalities and immense economic loss. In the wake of this catastrophic event, the need for concise, consistent, and robust societal risk criteria for dam and levee risk management was felt necessary to guide the assessment of risks obtained from risk analysis. In the United States, the U.S. Army Corps of Engineers (USACE) and U.S. Bureau of Reclamation (USBR) have developed a dam safety risk management framework for their portfolios of dams. In this dissertation, new opportunities are suggested to improve the performance of these guidelines, and a method is proposed to extend the guidelines to risk management of levee systems. This dissertation compiles the long history of societal risk criteria and their evolution. It attempts to unravel twisted and obscure themes, clarify implied assumptions, investigate the impacts of these assumptions on resulting guidance, and illuminate paths that lead to current practice. This dissertation addresses technical issues such as risk aversion, scale effect, and marginal vs. cumulative criteria. The data on historical levels of natural and man-made catastrophic risk have been gathered and analyzed, and corresponding F:N curves have been presented as an update to what was originally introduced in Reactor Safety Study (WASH-1400,1975). It is demonstrated that for dam safety, as well as certain other catastrophic hazards, an F:N slope of -½ rather than -1 is more historically supported. The relationship between F:N criterion, f:N limit line, and utility theory to risk aversion has been explicated and mathematically investigated. Finally, the dissertation investigates the relationship between acceptable risk and benefits and makes recommendations on how F:N and f:N criteria should be applied to risk management of dams and levee systems in the future.