Storm drainage design aims to provide safe travel for vehicles and pedestrians as well as to protect properties from damage during storms. The design process must begin in the early phases of urban planning in order to achieve an effective, efficient and economical system. The main problem is that during 100 y rainfall events, urban drainage systems can become flooded. This is due to the fact that unlike main collector systems, which are designed to accommodate such a flow, urban drainage systems are designed to convey runoff from less frequent storms, around 2 y to 10 y. This paper adopts a 1D/2D PCSWMM model to illustrate the dual drainage concept and to investigate how to achieve a more realistic estimate of the coupled flow discharging into a main collector, as well as simulating different scenarios to reduce the damage that could result from extreme events. The results were examined and compared, providing guidance for engineers to reach an optimal storm drainage design. Furthermore, the paper presents different ways to utilize 1D/2D models in simulating multiple solutions for urban flooding.