Water distribution network reliability: are surrogate measures reliable?

Water distribution networks are known to be costly infrastructures. A few decades ago the research efforts concerning water distribution network design were focused on economic aspects and the goal was to obtain least cost solutions. Beyond economic, these infrastructures must be reliable since they provide an essential service to society. Reliability assessment is a complex task and involves various aspects: mechanical, hydraulic, water quality, water safety, among others. This paper focus is on the hydraulic reliability. As hydraulic reliability is computationally hard to measure directly, researchers came up with surrogate measures, like the resilience index or the flow entropy. But these surrogate measures had some flaws and researchers quickly started suggesting new ones trying to avoid those known flaws, like the modified resilience index or the diameter-sensitive flow entropy. But are these surrogate measures reliable to be used in the design of water distribution networks? This paper presents a thorough analysis of these reliability surrogate measures, supported by illustrative examples, highlighting their pros and cons to help in deciding which one to use for design purposes. A new reliability index is proposed and used to design an example network, its advantages are highlighted, and the above question answered.