Economic level of reliability for the rehabilitation of hydraulic networks

The problem of water distribution system rehabilitation is formulated here as a multiobjective optimisation problem under uncertainty. The two objectives are to minimise the structural rehabilitation cost and to maximise the hydraulic reliability of the system. In this context, reliability is defined as a probability of simultaneously satisfying minimum pressure head constraints at all nodes in the network. An economic analysis has been performed, taking into account not only the structural costs but also lost revenue owing to the volume of water required by users but not supplied to them because of the structural inadequacy of the network. Because of the uncertainty in water demand, a probabilistic approach is used within the optimisation model. The probabilistic distribution and its parameters were estimated through an experimental study conducted on a real water distribution network. The recently developed robust Non-dominated Sorting Genetic Algorithm II optimisation algorithm is used to solve the optimisation problem. The methodology presented allows the identification of the specific optimal solution of the Pareto front that corresponds to minimal structural cost and minimal lost revenue yet corresponding to a high level of reliability – the economic level of reliability. This value could be assumed as the optimal solution or as a lower threshold, which restricts the gamut of technically feasible solutions that could be adopted in a rehabilitation programme.