U.S. BUREAU OF RECLAMATION'S USE OF RISK ANALYSIS AND RISK ASSESSMENT IN DAM SAFETY DECISION MAKING

摘要

Reclamation distinguishes between Risk Analysis and Risk Assessment. Risk Analysis determine the risk at a particular site in qualitative and quantitative terms. Risk Assessment is the process that blends Risk Analysis results with other factors to make a decision on the course of action to take at the site. Risk analyses are undertaken to determine the baseline risk represented by the structure as it now stands and is now operated and monitored and risk reduction analyses may be undertaken to determine impacts of proposed alternatives. Reclamation's current methodology for the project team risk analysis begins with formulation of a project team that includes experts in pertinent technical areas. Once formed, the team begins its analysis with input on load probabilities for hydrologic and seismic events along with input on possible consequences. The team then considers all the possible failure modes for the structure, eliminates the ones considered to be trivial, and moves forward to construction of event trees for those remaining. Event trees are constructed to decompose the failure modes into a logical series of events for further study by the team. Included in the event trees are a breakdown of the loading information into appropriate ranges, events to capture the important steps of the structure's response to the load, events to capture the likelihood of intervention, and finally the consequences from the potential dam failure. Consequences in terms of life loss are always considered along with consideration of other types of consequences as-needed. Once the event trees are constructed, the teams consider all pertinent information on each node of the event tree, a form of expert elicitation is used to estimate the likelihood of each event taking place. Once each node of the event tree has been estimated, the product of the events is taken to represent the risk of this failure mode. The analysis team then considers the overall result to determine if it accurately represents the team's overall perception of the risk.