Event Tree Analysis of Lock Closure Risks

Abstract

The use of event trees and subjective probabilities in risk analysis is illustrated via a case study: the estimation of the probability of closure of Poe Lock on St. Marys River (Sault Ste. Marie, Mich.) because of vessel accidents and other nonstructural failures. The risk of closure is needed to determine the benefits of building a second Poe-class lock at that location. An event tree structures the risk analysis. Probabilities associated with the branches of the tree were developed from 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 associated with obtaining the necessary probabilities. It was found that a 30-day closure could occur about once every 50 years. The results are most sensitive to assumptions concerning how often ships hit lock gates.