Many of these networks are tree-like in structure, (having no loops), and most are such that the cost of a link in the network is dependent on both the length of the link and the flow along it. Furthermore, the dependence of the cost on the flow is almost always a non-linear relationship exhibiting some degree of economy of scale. This relationship is often discontinuous, due, for example, to the availability of sewer pipes only in discrete sizes.

Traditionally, the designer of the network must first choose which out of a very large number of possible layouts he considers most suitable. He may then decide on the pressures (or elevations or potentials depending on the application) he requires at the pipe junctions. Finally he must design the diameter of each link in the network to cope with the expected flows. Any attempt at achieving a least cost scheme is usually limited to the examination of a handful of alternatives.

A method for the simultaneous selection of the layout and pipe diameters for the least cost network that connects a large number of sources to a single sink is described in this paper. The pressure (or elevation or potential) at each pipe junction is treated as a variable, and the method successfully handles any form of cost function. This global optimum solution is achieved by the use of Dynamic Programming.

A computer program is an essential part of the design process, and a description of one that the authors have written in Pascal is included together with examples of its use in the solution of civil engineering network problems.

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