On 15 July 2021, the massive floods that occurred in many German states were seen as a national tragedy by Chancellor Angela Merkel. On 20 July, in Zhengzhou, China, a time-interval rainfall of 201.9 mm between 16:00 and 17:00 broke the global land record. Millions of people were affected and the economic loss amounted to 53.2 billion Chinese yuan. On 1 September, record rainfall caused floods in New York that largely paralyzed the city. Flooding has become a major constraint on urban public security and social and economic development (Alexander et al., 2019;Guo et al., 2021;Lee et al., 2020). Quantifying and evaluating the damage of flooded urban areas not only allows for timely mitigation measures, but also provides a basis for post-disaster recovery and reconstruction (Kreibich et al., 2014;Merz et al., 2010;Schröter et al., 2014). Accurate flood damage assessment results are also essential for the natural disaster insurance industry in post-disaster claims settlement (Diakakis et al., 2020;Wagenaar et al., 2017). Hence, scientific and reasonable flood damage quantification plays an important role in urban flood risk control and rescue decision making.The flood damage assessment process is characterized by multidimensionality, dynamic changes and uncertainties due to multiple factors such as regional rainfall characteristics, geographical conditions and socio-economic levels (Gall, 2017;Schoppa et al., 2020). It is a widely used flood damage assessment method to achieve damage results by combining flood damage ratios with inundation depth, land use and economic factors (Chen et al., 2019;Jongman et al., 2012;Scorzini & Frank, 2017). The damage ratio serves as the link between flood inundation

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 analysing 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.

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