A computer program for design of sanitary sewer systems

Abstract

A program. is developed for the design of sanitary sewer networks when the sewer lines (links) connecting the manholes (nodes), the nodal elevations, the sewer lengths and a final outlet are specified. The program consists of two algorithms developed for hydraulic design and layout generation. The hydraulic design algorithm is intended for design of networks with specified main or full layouts. The lift stations required may be sited either by the user or may be located automatically by the algorithm at nodes with depths exceeding a specified limit. Even concurrent assignment of both types of lift stations is within the capabilities of the algorithm. The laybut generation algorithm generates a main layout and transfers the data to the hydraulic design algorithm. Unlike model available in the literature, no initial layout is reqtiired for this algorithm. To generate the layout a standard Shortest Path algorithm is used to seek the path with the "shortest length" to the final outlet from every node in the netw6rk. When superposed, these paths generate the routes from all initial manholes to the final outlet. The criteria tested to minimize the total excavation volume, which is found to be the most significant factor determining overall sewen system cost, included: horizontal and surface lengths of sewer links, natural slope and a hypothetical excavation volume (computed by assuming the upper cover depth an;d sewer slope to be minimum) for each sewer line. The relatively high computer storage requirement in generating a layout for large networks is overcomed by subzoning the network and then superposing the layouts generated for each subzone. The layout generation and hydraulic design algorithms are applied together to sewer networks of different sizes, including PINARKENT (a resort town of 13,000), to test the various minimization criteria and hydraulic design parameters. The "hypothetical excavation volume" criterion is found to be quite efficient, especially in view of the fact that true optimality (or minimization) in excavation volume can only be achieved by the simultaneous solution of both the layout and the hydraulic design problems. As such, the program can allow for the easy evaluation of many alternative designs as. well as the effect of various hydraulic design parameters. Hence, the objective of minimizing the dependence on engineering judgement in the overall design process has been achieved to a considerable degree.