<p>In recent years, urban flooding caused by heavy rainfall events has become increasingly common, severely impacting urban operations and residents' daily lives. Most urban stormwater is discharged into rivers through drainage networks or pumped into rivers by pumping stations. However, when river water levels are excessively high, they can backflow into drainage networks, impeding discharge and worsening flooding. Thus, understanding the outflow capacity of drainage networks and surface inundation under the combined effects of rainfall and high river water levels is crucial for mitigating urban flooding. This study is focused on the Wachong River drainage area in Zhoukou city, and the MIKE FLOOD platform is employed to develop a coupled model of rivers, drainage networks, and surface flooding. The results indicate that the drainage capacity of the pipeline system exhibits a lag effect: the higher the rainfall intensity and the later the rainfall peak (r = 0.75) are, the shorter the lag time. For rainfall events with the same return period, an earlier rainfall peak (r = 0.25) leads to a greater total volume of surface water under flooded pipeline conditions and a greater risk of inundation. The findings of this research provide a scientific basis and guidance for improving the emergency response capacity of urban drainage systems and reducing urban flood risk.</p>