A comparative assessment of human entrapment and instability risks in flash floods

The frequency and intensity of flash flood events are gradually increasing due to global warming. Despite extensive studies on human instability in flash floods, the risk of entrapment caused by flood loads on buildings remains insufficiently addressed. In this study, thresholds of human entrapment risk under different flow conditions were determined based on mechanical theory and compared with instability risk thresholds. An extreme flash flood events occurred in Suizhou, Hubei Province, on August 12th ("8.12" flood event) was chosen as a case study. The Xinanjiang(XAJ) model and Hydrologic Engineering Center's River Analysis System(HEC-RAS) were integrated to simulate the flooding inundation depth and water velocity. The spatial distribution and temporal variation in entrapment and instability risks were analyzed and compared. The results showed that the integrated flood model based on the XAJ and HEC-RAS models demonstrated strong applicability in the study area, with an error range of -0.35-0.15 m and a simulation qualification rate of 87.5 %. Compared with extreme instability risk, extreme entrapment risk covers a wider area and develops 10-30 min earlier. The entrapment risk mainly depends on water depth, while the instability risk is significantly influenced by water velocity. This study demonstrated that entrapment risk occurs earlier and is more fatal than instability risk in flash floods, which is essential for mitigating and managing flood-related risks.