7.1 INTRODUCTION

There is a need for the reliability analysis of water supply/water distribution systems. The ideas of using event/fault tree analysis for this purpose were discussed in Mays (1989). Unfortunately, these methods have not been implemented by water utilities for reliability assessment. The purpose of this section is to reintroduce some very valuable methodologies that can be applied in the reliability computations for vulnerability assessment.

A formal quantitative reliability analysis for an engineering system involves a number of procedures as illustrated in Fig. 7.1. First, the system domain is defined, the type of system is identified, and the conditions involved in the problem are defined. Second, the kind of failure is identified and defined. Third, factors that contribute to the working and failure of the system are identified. Fourth, uncertainty analysis is performed for each of the contributing component factors or subsystems. Fifth, based on the characteristics of the system and the nature of the failure, a logic tree is selected to relate the failure modes and paths involving different components or subsystems. Fault tree, event tree, and decision tree are the logic trees often used. Sixth, appropriate method or methods that can combine the components or subsystems, following the logic of the tree to facilitate computation of system reliability, are identified and selected. Seventh, computation following the methods selected in the sixth step is performed...