Assessment of the joint impact of rainfall and river water level on urban flooding in Wuhan City, China

Flood severity in cities along the Yangtze River is vulnerable to multiple influencing factors, such as intense rainfall, higher river water level, and pump operation. This study evaluates the joint impact of rainfall and river water level on urban flooding in Wuhan city. Firstly, the variation characteristics of rainfall in Wuhan and water level at Hankou of the Middle Yangtze River were analyzed by the Mann-Kendall test. High water levels and heavy rainfall mostly occurred in June to July, and the rainstorm and lowest water levels presented increasing trends during this period. Meanwhile, a strong positive dependence was found between rainfall and water level, implying that the probability of flooding cannot be correctly estimated by only considering the influence of a single factor. Secondly, a joint risk probability was estimated using the optimal copula. The joint risk probability was always larger than the exceedance probability of water level or rainfall, which highlighted that ignoring the interaction between different flood drivers can lead to the underestimation of flood hazards. A comparison of the joint risk probabilities during different periods of 2005-2020 and 1990-2004 shows that extreme events of heavy rainstorm and higher water level have occurred frequently in recent years, with the joint risk of urban flood increasing significantly. Finally, flood inundation processes under different combinations of rainfall and water level were simulated using the calibrated model of SWMM, with and without working pumps being considered. The flood volume and flood duration would increase by about 1.7%-3.7% and 0.8%-1.3% respectively with the water level increasing from 21.7 m to 28.0 m, and would reduce by approximately 71.3%-71.9% and 58.2%- 58.5% with the rainfall return period decreasing from 100-yr to 5-yr, therefore urban inundation was mainly caused by heavy rainfall. Moreover, the efficient effect of pumping on the urban drainage decreased gradually with the increasing rainfall. Therefore, the results can be used to provide the support for the design of water resource projects and the disaster mitigation measures of urban flooding.