Maintaining network loops, and hence attaining acceptable system reliability, has been a challenge in the optimal design of water distribution networks. Aimed at a possible solution to the problem, this paper has two objectives: to introduce a new reliability-based optimal design formulation and a model, and to examine the effect of specifying minimum allowable pipe sizes during least-cost designs on system reliability. System reliability is estimated using the minimum cut-set method, but instead of using the mechanical failure probabilities of pipes, weighted failure probabilities are calculated by considering the ratio of the actual supply to demand. One of the salient features of this study and the new reliability-based design model in particular, is that a pressure-driven network simulation model is used to determine the actual supply at each demand point when a component fails. A simplified two-loop network is used to illustrate the performance of the new model and to study the effect of specifying minimum allowable pipe sizes. The results obtained show that the new model preserves loops and results in a system with better reliability; and, if appropriate, minimum allowable pipe sizes are specified in the least-cost design, a required reliability can be attained with a reasonably low cost.

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