EVENT TREE ANALYSIS OF LOCK CLOSURE RISKS

The use of event trees and subjective probabilities in risk analysis i s illustrated via a case study: the estimation of the probability of c losure of Poe Lock on St. Marys River (Sault Ste. Made, Mich.) because of vessel accidents and other nonstructural failures. The risk of clo sure is needed to determine the benefits of building a second Poe-clas s lock at that location. An event tree structures the risk analysis. P robabilities associated with the branches of the tree were developed f rom a combination of historical data and subjective probabilities. The latter were obtained in workshops with navigation experts. This paper summarizes the risk analysis, and discusses the difficulties associat ed with obtaining the necessary probabilities. It was found that a 30- day closure could occur about once every 50 years. The results are mos t sensitive to assumptions concerning how often ships hit lock gates.