In designing water distribution networks (WDN) against emergency situations, efforts must be made to achieve two objectives: maximizing reliability and minimizing cost. For such optimization, it is necessary first to quantify the reliability of WDN using a surrogate measure. Hydraulic-connectivity is selected as a surrogate measure in this study, which indicates a probability that every demand node in a network is connected to at least one supply source with required flow at adequate pressure. An optimization model using hydraulic-connectivity and the genetic algorithm (GA) is formulated. For illustration, the optimization model is applied to the New York City water supply tunnel. This tunnel cannot satisfy the minimum pressure with existing diameters. The new optimal design with required flow at adequate pressure is found to be lower than the existing tunnel cost by US$25.252 million when the hydraulic-connectivity is 0.9778. It is also noted that this approach allows the exploration of the trade-off between cost and reliability directly. This permits designers to design WDN based on more quantitative information regarding cost and reliability.
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