Abstract:
A model for the optimal design of simple conjunctive - source, looped, gravity - fed water
distribution systems is developed and applied to the redesign of the water distribution system for a
small urban estate. The model proposed is a modified version of that proposed by Hans Jorgen
Rasmusen (1976) and involves two major steps; The Network Solver and The Pipe Modification
Algorithm. The Network Solver utilises the Hardy Cross method for balancing flows and pressures.
The Pipe Modification Algorithm then optimises the entire network of di5trtl1utioo piJXI5 wing II
heuristic procedure on discrete pipe diameters until an optimal solution is arrived at. The variation of
the combination of the diameter of the distribution pipes and the height of the overhead tank
(pressure head) supplying the water is the decision variable. The model allows the optimisation of each
pipe diameter to revolve around one of three choices; reducing to the next smaller available pipe
diameter, leaving the diameter unchanged, or increasing to the next larger available pipe diameter.
While the model is helpful for recommending optimal pipe sizes, the experience of the designer is
most useful in the judicious selection of one of the alternatives proposed, which will best suit the
required conditions on site.
A distribution system of four networks of 74 pipes is proposed for Ijapo estate, Akure city -
Nigeria to replace the existing distribution system of three networks of 64 pipes. Population is
expected to stabilize within Ijapo estate in twenty-five years. Hence, a design period of twenty-five
years is used. Reliability of water flow to every node within the distribution networks despite breakage,
blockage, or leekage in pipes is made possible by the introduction of redundant pipes. From the initial
proposed system of four distribution networks, an efficiency of between 2 - 15% was achieved,
reducing the total initial pipe and energy costs by about #200,000:00.
A conjunctive water supply system comprising of two imported sources of water (the Owena
water supply scheme and, the College of Agriculture mini-water supply scheme) and four boreholes is
recommended to supply the quantity of water required by the residents of Ijapo estate throughout the
design period.
