Reliability indicators for water distribution system design: comparison

Abstract

When designing a water distribution system (WDS), it is imperative that the reliability of the network is taken into consideration. It is possible to directly evaluate the reliability of a WDS, although the calculation processes involved are computationally intensive and thus undesirable for some state-of-the-art, iterative design approaches (such as optimization). Consequently, interest has recently grown in the use of reliability indicators, which are simpler and faster to evaluate than direct reliability methods. In this study, two existing reliability indicators, the Todini resilience index and entropy for WDS, are examined by analyzing their relationships with different subcategories of reliability, namely the mechanical (network tolerance to pipe failure) and hydraulic reliability (network tolerance to demand change). The analysis is performed by generating comparable solutions through multiobjective optimization of cost against the chosen indicators using the well known Anytown WDS benchmark as a case study. It is found that WDS solutions with high entropy have increased mechanical reliability, yet are expensive and have poor hydraulic operation and water quality. In contrast, high resilience index networks are relatively cheaper and present reasonable hydraulic operational performance, yet have limited improvement in mechanical reliability. Both indicators appear to be correlated to hydraulic reliability, but each has its own associated disadvantages. Including minimum surplus head as an additional objective in the optimization of the reliability indicators appeared to improve the performance. When optimized together, a trade-off between the two indicators is identified, implying that significantly increasing both simultaneously is not possible, and thus a new indicator is recommended to account for both the mechanical and hydraulic reliability while ensuring reasonable standards of hydraulic operation.