Quantifying the impact of river levels on urban drainage capacity

When river flooding and urban waterlogging occur simultaneously, the rising river water level could reduce the drainage capacity of the urban pipe network, thereby exacerbating the flooding risk. This study quantitatively analyzed the impact of river water level fluctuations on the drainage capacity of the urban pipe network. A 1D pipe network model was coupled with a 2D river hydrodynamic model to simulate different combination scenarios, based on which a flow coefficient formula was fitted. The results indicate that the drainage capacity is closely related to the outflow conditions. Under free outflow conditions, the drainage capacity remains largely unaffected by external water levels. However, when the river water level is 1 to 2 times (in term of the diameter of the outlet) higher than the bottom of the outlet, the phenomenon of inundation outflow begins to emerge, progressively weakening the drainage capacity. When the water level exceeds twice the diameter of the outlet, the drainage capacity diminishes rapidly to zero, and backflow may occur. Taking the Beijing urban Sub-center as a case study, the findings reveal that during normal river water levels, the pipe network operates effectively, ensuring proper drainage. However, under high water level scenarios, the drainage capacity becomes limited or even ineffective. During the water level recession phase, the river's influence on drainage capacity diminishes, leading to partial recovery of drainage, although it cannot be fully restored. These results provide important reference for the combined scheduling of urban floods and waterlogging.