In sewer layout design, most developed models are designed to determine pipe diameter, slope and overall layout to minimize the least cost for the design rainfall. However, these models cannot consider the superposition effect of runoff hydrographs entering each junction. The suggested Optimal Sewer Layout Model (OSLM) is designed to control flows and distribute the node inflows while taking into consideration the superposition effect for reducing the inundation risk from the sewer pipes. The suggested model used the genetic algorithm to determine the optimal layout, which was connected to the SWMM (Storm Water Management Model) for the hydraulic analysis. The suggested model was applied to an actual urban watershed in Seoul, Korea. By using the suggested model, the peak outflows at the outlet were decreased by approximately 20% than those of the current sewer layout for the design rainfall during 30 minutes.