Optimal Design of Water Distribution Networks for Secondary Cities in Imo State Using Pressure-Driven Analysis

Water supply coverage in secondary cities of Imo State, South East Nigeria, including Orlu, Okigwe, and Owerri-North, falls below 35 percent of households receiving treated piped water, reflecting decades of investment underperformance and infrastructure deterioration. This study applies pressure-driven hydraulic analysis combined with mixed-integer nonlinear programming to optimise the design of new and rehabilitated water distribution networks for these three secondary cities. Pressure-driven demand models are used in place of demand-driven models to more accurately represent the partial-pressure supply conditions typical of Nigerian distribution networks. The optimisation model minimises total pipe installation and pumping cost over a 20-year planning horizon, selecting pipe diameters for each link from a discrete set of commercially available sizes subject to minimum pressure head constraints at all demand nodes. Network topology is determined from GIS-mapped road corridors and demand node locations derived from building density analysis of satellite imagery. The nonlinear optimisation is solved using a hybrid genetic algorithm-gradient method. Optimal network designs for Orlu achieve full pressure-standard coverage at 62 percent lower cost than the current state agency's preliminary engineering estimate based on conventional design. Sensitivity analysis confirms robustness of the optimal design to plus or minus 20 percent demand growth uncertainty. Keywords: water distribution network, pressure-driven analysis, optimal design, Imo State, mixed-integer nonlinear programming.