Shape optimization of egg-shaped sewer pipes based on the nondominated sorting genetic algorithm (NSGA-II)

Overflow pollution is an undesired issue that commonly occurs in combined sewers under wet weather conditions. There is a lack of existing studies on the structural optimization of sewers to prevent siltation, and no previous study on egg-shaped sewers with this purpose has confirmed satisfactory anti-sedimentation performance. To achieve reduced sedimentation and lower energy loss under low- and high-flow conditions, respectively, the nondominated sorting genetic algorithm (NSGA-II) was adopted in this study based on a constant full filling discharge capacity equal to that of a 300 mm (diameter) circular sewer. The results showed that egg-shaped sewers with bottom and top arc radii of 58.3 and 116.6 mm, respectively, and a height of 408.1 mm performed significantly better than circular sewers (d = 300 mm). Notably, at a low flow ratio below 0.2, the shear stress of the optimized egg-shaped sewer was 5.2%–20.6% higher than that of the circular sewer. At a flow ratio of 0.2–0.6, both the egg-shaped and circular sewers were capable of maintaining a balanced amount of sediment between deposition and erosion. As the flow ratio increased to 0.6–1, both types of sewers completely scoured sediments: in this situation, the shear stress of the egg-shaped sewer was 5.5%–10.1% lower than that of the circular sewer, thus exhibiting reduced energy loss. This study indicates that egg-shaped sewers have an attractive future in replacing circular sewers for sedimentation prevention and cost control. •The NSGA-Ⅱ was used to optimize the shape parameters of the egg-shaped sewer (ESS).•Combined with the shear stress characteristics of sediments, the optimal ESS was obtained.•The shear stress of the ESS is 5.2 ∼ 20.6% higher than that of the circular sewer (CS) at low flow.•The ESS, with a 5.5 ∼ 10.1% lower shear stress than CS at high flow, has lower energy loss.