Blog 10 - Generating Alternatives

Article Review

Pan TC, Kao JJ (2009) GA-QP Model to Optimize Sewer System Design, JOURNAL OF ENVIRONMENTAL ENGINEERING, 135(1) 17-24

Pan and Kao implemented a Genetic-Algorithm (GA) and Quadratic Programming (QP) to generate alternative to the problem of designing sanitary sewer systems. This approach is proposed in order to reduce excessive computational effort presented by other optimization techniques. An alternative method is discrete differential dynamic programming (DDDP). DDDP, however, restricts the search space decreasing the chances of locating global optimum. Also, DDDP must be manually implemented, difficult to generalize to different systems. Another approach would be a GA-LP (linear programming) model, but linearization of nonlinear functions is complex.

The nice contribution of this work is the recognition that some important design factors and unquantifiable issues (public preference, for example) are not included within the optimization model. So, “the solution obtained from existing sewer design models may not be the best one or even feasible when unmodeled factors or issues are simultaneously evaluated”. Therefore, a model to generate alternatives (MGA) has been developed to evaluate the differences among the designs generated by the GA-QP and DDDP models.

Discussion

This article presents a nice application of generating alternatives, proposed in the previous article by Brill. Specifically to the design of the sewer system, the decision variables considered were slopes and buried depths. However, in real projects, there are a lot of “more important” aspects that may be subject to change by the decision-makers. For example, the network topology. Also, geology is a very important aspect in economic viability.