Hydrological and hydraulic modelling of flash flood induced by extreme rainfall in Liulin town, Hubei province, China, 2021

On 12 August 2021, a high-intensity rainstorm hit Sui County, Hubei Province in China, reaching 500 mm in 12 h during the event, causing a catastrophic flash flood in Liulin town and the tragic loss of 24 people. The study aims to reconstruct the flash flood event through post-flood surveys and hydrological-hydraulic modelling, and analyze the flood generation mechanisms from meteorological, geomorphological, and anthropogenic perspectives. Field surveys revealed that the presence of eight bridges and a culvert measuring at least 100 meters in length, located in the center of town, severely constrained the flood capacity in the channel. The flooding extent and high-water marks were recorded with inundation depths varying from 0.5 to 6.0 meters. Using the modular distributed hydrological model (MDHM), the estimated peak flood is 623 m3/s with a steep rise time of approximately two hours. The hydrodynamic simulation results indicated that the flood diverted at bends and propagated directly along the road, while narrow and steep roads aggravated the severity of flooding. Comparing field survey observations with simulated flood extent showed good consistency with a Critical Success Index (CSI) of 88.27%. Impact analysis based on hypothetical scenarios demonstrated that high-intensity rainfall was the primary cause of flash flood occurrence, with the 6-h precipitation reaching a 1,000-year return period according to the official report. Furthermore, Bridges and culverts situated in unsuitable locations with unsuitable dimensions also hurt the progression of flooding. Additionally, inadequate housing structures and a lack of proper awareness of flash flood response during the event were contributing factors to 24 fatalities. This study provides valuable insights into the underlying reasons for the formation and evolution of flood disasters in the study basin, which can inform strategies for flood disaster investigations, flood hazard assessment, and flood risk mitigation.