Reliability analyses of water distribution networks conducted on geometric-, hydraulic- and economic grounds all reflect the complexity of network resilience that is still difficult to generalize or classify, as well as it is impossible to assess without full integration of all three aspects. Practical assessments of hydraulic reliability usually consider the demand loss from consecutive failure of single pipe ending in a form of network resilience index with the value between 0 and 1, which is representing a kind of averaged consequences. This concept can further be expanded by analyzing the effects of increased investment and/or operational costs on the reliability improvement. It is mostly favourable due to its simplicity and having powerful computational tools able to quickly calculate the index of even very large and complex networks. Nevertheless, the average index is potentially hiding the weak points in the network as well as the condition of the network expressed by mechanical reliability i.e. the failure probability may inadequately be captured.

The presentation discusses two commonly applied indices and the improvement suggested by the visual representation of the network reliability, through so called Hydraulic Reliability Diagram. The concussion point the improvements but also spell the need for more rigorous (mathematical) verification of the established hypotheses.