Impact of rainstorm patterns on the urban flood process superimposed by flash floods and urban waterlogging based on a coupled hydrologic–hydraulic model: a case study in a coastal mountainous river basin within southeastern China

Flash floods (FF) and urban waterlogging (UW) hazards pose a serious threat to citizens and property, becoming a global challenge. However, the rainstorm patterns that influence the urban flood process associated with both FF and UW are still not well understood. This paper utilized and verified a coupled hydrologic–hydraulic model to determine the FF hydrographs, inundation indicators, and statistical relationship of surface inundation under different return periods as well as rainstorm patterns in a mountainous city experiencing both FF and UW. The results show that the effects of the rain peak coefficients on FF hydrographs and urban surface inundation indicators are relatively similar. FF volumes and urban inundation severity increase with an increasing rain peak coefficient, while the lag time of FF hydrograph and inundation depth becomes shorter. The effect of rainstorm patterns on surface inundation has been considerably amplified by FF, especially for high return periods and inundation areas with a high-water depth. When FF and UW superimpose on urban inundation, the impact of the rainstorm patterns on the spatial distribution of the inundation lag time shows a distinctive response due to the different district topographies on the FF propagation pathway. Furthermore, FF dominated the total inundation volume and thus significantly increases the surface inundation spatial connection of different locations. The exponential relationships for maximum inundation depth of many flooding vulnerable points were found. The key findings of this study provide a readymade technical tool and thus aid decision-makers in managing urban flood problems on a basin scale.