A framework for amplification flood risk assessment and threshold determination of combined rainfall and river level in an inland city

Urban flooding is becoming frequent in the context of climate change and rapid urban expansion. Under extreme rainfall, urban flooding is not only driven by rainfall, but also the surging water level of neighboring rivers can hinder the drainage of rainwater and aggravate flooding. This study presents a framework for amplification flood risk assessment and threshold determination (FARTD) of extreme rainfall and river levels. Combined scenarios are stochastically simulated using copula functions and Monte Carlo sampling under the consideration of the correlation structure of rainfall and river levels. A comprehensive index (ARI) was proposed for the amplification flood risk assessment by integrating a hydrodynamic-based urban inundation model and entropy weighting method. The amplification flood risk caused by combined rainfall and river levels was determined by ARI, and the combined thresholds of compound flooding were generated by the binary return period of rainfall and river levels. Taking the region of Zhengzhou, China as a case study, the FARTD was used to assess the risk of amplification flooding from rainfall and river levels. The results show that disregarding river levels and relying solely on rainfall as the sole causal factor may lead to an underestimation of urban flooding. The maximum inundation volume, area and depth of compound flooding increased by an average of 6.01 %, 10.03 % and 6.12 %, respectively compared to the only rainfall-driven flooding. The randomly generated combination of rainfall and river level scenarios were identified as high, medium and low risk classes, and the thresholds of binary return period were determined for different risk classes. The study area will be exposed to more than medium amplification flood risk when the binary return period is higher than 20 years. The proposed framework can serve as a valuable reference for flooding warning and reduction in inland cities.