Effect of intense rainfall and high riverine water level on compound flood hazards in a river-valley city: A case study of Yingde, China

River-valley cities are susceptible to compound floods induced by intense rainfall and high riverine water level, especially where design standard of levees is low and overtopping-induced fluvial flood are very likely to run into urban area. However, the holistic impact of high riverine water level and intense rainfall on a river-valley city still needs to be assessed within a quantitative analyzing framework. In this study, a typical river-valley city named Yingde in Guangdong Province of China is selected as the study area. A framework combining multivariate statistical analysis and numerical hydrodynamic modeling is proposed to quantitatively assess the compound flood hazard in river valley cities. To be specific, a compound flood numerical model that can integrate rainfall and overtopping flow processes is first developed and validated based on historical observations. Then, the joint probability distribution of rainfall and riverine water level is established based on the copula function. After that, the compound flood scenarios corresponding to different joint return periods are simulated and compared. Considering only one factor may underestimate the probability of flooding occurrence. The compound flood severity in terms of both inundation area and depth for the study area is majorly dominated by intense rainfall, but the extra contribution from upstream water level cannot be ignored. An inundation amplification factor can be used to assess the amplification effect of high upstream water level on the flood hazard of the riparian area. For a site that is quite near the river, the inundation amplification factor can be drastically decreased as the levee crest elevation is heightened. Without the contribution from the fluvial floods, the rainfall-induced inundation could be several times smaller than the compound floods.