Probabilistic Hydrologic Risk Assessment: Input For the Decision Maker

In recent years dam owners have faced the prospect of major, expensive modifications to existing dams due to the results of the reevaluation of the design basis flood, Experience indicates that a reassessment of the flood design basis, using data and methods of analysis not available at the time the dam was built, produces a design flood that is greater than the current capacity of the project. Coupled with this ratcheting upward of the design basis is the fact that dam owners must balance the potentially expensive costs to modify an existing project with other competing demands on limited financial resources. This problem applies universally to public and private, large and small dam owners alike. As a result, difficult questions are asked as to the benefits (e.g., improved safety) that are derived from an increase in the design basis and at what expense. In a number of cases dam owners have performed a probabilistic hydrologic risk assessment (PHRA) in an effort to gain some insight to the trade-offs in terms of safety, cost and incremental-risk reduction. For the most part the experience in performing PHRAs is limited (compared to PRA experience in other engineering fields). As a result, the risk assessments that have been performed often fail to provide the owner or regulator with the kind of information/insight that can facilitate the decision-making process. This paper discusses some of the questions that should be asked by a manager/regulator (decision maker), the products that should be provided and the format of the results, prior to starting a PHRA. The answers to these types of questions gives some assurance that valuable input to the issue at hand will be provided. The paper also lists some opportunities for developing acceptable risk standards for dams.