There is a considerable amount of uncertainty associated with the design and operation of water distribution systems. These include: long-term projections of the growth in demand; the spatial distribution of the nodal demands coupled with diurnal and seasonal consumption patterns; variations in electricity/energy tariffs; bursts and component failures; possible changes in pipe diameters and roughness with age. Following a pipe failure/removal or large localised increase in demand, flows in pipe networks are rerouted in complex ways which are generally difficult to predict prior to a full simulation of the network concerned. This can cause difficulties and further uncertainty at the design stage, because of the general inability to identify the critical elements of the network.