On the Physical Meaning of Dam Safety Risk Estimates

摘要

Dam Safety risk analysis has been practiced for over twenty years by the Bureau of Reclamation (Reclamation) and other owners and operators of high-hazard dams. The key steps of the process include a thorough review of all relevant background information, the identification of potential failure modes, the simplification of each credible potential failure mode into a series of component events, the estimation of the conditional probabilities of the component events, and an analysis of the consequences of failure. Once these steps have been completed, normalized measures of failure and life loss risk can be calculated using a pair of relatively simple formulas. Application of the risk analysis process on a variety of levels (screening, comprehensive, issue evaluation) has allowed dams in need of corrective action to be systematically identified and funding within the Reclamation inventory to be prioritized in a manner consistent with the risk. However, while the process used by Reclamation is well known to those who practice it, the fundamental mathematical basis for it may not be. Much of the publicly available training literature prepared by Reclamation and others is focused on the practical aspects of the risk analysis process, originally a result of the difficulty in simply getting deterministically trained engineers to look beyond an allowable safety factor. Over time, however, this disconnect has resulted in published criticism of the Reclamation process, and in proposals for alternative analysis procedures intended to address its presumed flaws. The complexity of many of these alternative procedures belies the remarkable simplicity of the Reclamation process, which is grounded in elementary statistical concepts that are well within the grasp of most engineers. This paper outlines the current Reclamation risk estimation process and explains the physical meaning of the input and output estimates that are generated by it. It also addresses some recent criticisms and provides a Reclamation counterpoint for each.