Risks of Damage from Flooding Rivers: Correlation of Weakened Dyke Structures?

Summary

This chapter shows how to incorporate uncertainty on whether two dykes, considered as likely to collapse during high water in a river after weeks of torrential rain, are correlated or independent. Prior to actual collapse of at least one dyke there is no way to be sure of the state of the two dykes in relation to each other. We show that this uncertainty automatically adds an uncertainty to the mean assessed cost estimate should there be a collapse, and that this uncertainty can dominate over uncertainties brought about by imperfect knowledge of the failure probabilities and potential costs of each of the individual dykes, as well as the distributions chosen to represent such uncertainties, and the nominal values of the parameters and their skewness of dynamic range around the nominal values. The main aim is to show which factors dominate in assessments of uncertainty of potential costs should there be a dyke collapse. In this way one can focus on narrowing the relevant parameter uncertainties causing the largest contributions without the need to spend time and effort in addressing other parameter uncertainties that would help but little, if at all, without the main focus on the important contributors to uncertainty of the mean cost assessment.