The recent increase in extreme climatic phenomena has gradually attracted the attention of researchers regarding urban flooding. This paper used a hydrological model combined with the Computational Fluid Dynamics software to assess the drainage efficiency, predict the hydrological processes under different precipitation intensities and establish the relationship between rainfall intensity and inundation depth under extreme rainstorm events. The study proposed several thresholds such as specific rainfall intensities (that is, RI 95% and RI 50%), drainage efficiency, and the limited flow rate of pipes. Based on 2021 July 20, 0:00 to 24:00 precipitation monitoring data from the “7.20 Zhengzhou Rainstorm Extreme Event”, the relationship between rainfall intensity and road-pipe overflow patterns was determined by analyzing these thresholds for different drainpipe diameters and spacings of catchment wells. The results demonstrated the evaluation parameters varied with rainfall intensities and pipe characteristics and revealed the main limitation of drainage efficiency and flow rate of drainpipes. The simulation helped the drainage systems design for different precipitations and proposed several relative suggestions for drainage-system improvements, wherein the diameter of the branch pipes plays a dominant role in coping with extreme rainstorm events.

The secondary flow is deflected under pressure and superimposed on the main flows. This research investigated its characteristics, including velocity gradient, vorticity, shear stress, and Reynolds stress in unpressurized circular pipes, through physical experiments and Computational Fluid Dynamics numerical simulations. Combining numerical simulations with the physical experiments under three flow rates (30 m 3/h, 35 m 3/h, and 40 m 3/h) and width–perimeter ratios ( Wr = 0.43, 0.4, and 0.35), the experimental data demonstrate the secondary-flow propagation in unpressurized circular pipes. The secondary flows manifest as deviations of velocities, substitutions of secondary vortices, and flips of shear stresses, which present decay tendencies and are negligible at 52 times the pipe diameter. The secondary flows are driven by the velocity gradient or Reynolds stress, the dominance of which shifts with the increase of diffusion distance. The secondary flow turbulence is reduced and smoothing when the width–perimeter ratio reaches a threshold (approximately Wr = 0.40) because of Dean vortices collisional depletion.