Comparison of procedures for assessing water demand shortfalls caused by segment isolation

In this paper, two procedures for assessing water demand shortfalls following segment isolation are compared. The first (topological) procedure is based on simple network topological analysis, and identifies the water demand shortfall as the water demand (under normal operational conditions) relative to the directly and/or indirectly isolated segment(s). The second (hydraulic) procedure is based on pressure-driven hydraulic simulation of the network after segment isolation. The difference between the two procedures lies in the fact that the hydraulic procedure enables evaluation of the demand shortfall rate due to pressure drop in the parts of the network that remain connected to the source points after segment isolation, in addition to the rate related to the water demand of the isolated segment(s). Two case studies were used to test the application of the two procedures in the analysis and redesign of their isolation valve systems, considering the economic burden (expressed in terms of number Nval or cost Cval of installed valves) and reliability (expressed in terms of maximum Dmax and weighted average D̄ water demand shortfall) of the systems. As a whole, the results show that the differences between the two procedures depend on which of the global variables (Dmax or D̄) is chosen to characterize the demand shortfalls in network segments. The rationale behind the choice between the two procedures, with a view to balancing the necessity for an accurate demand shortfall characterization with the need for limited computation times, particularly in the phase of multiobjective design, is discussed.