小流域山洪, N-S方程, 径流汇流模型, 成因分析

This study employs a distributed rainfall-runoff routing model, using minute-resolution radar quantitative precipitation esti⁃

mation data and 12.5 m-resolution topographic data. Model parameters were calibrated using observed data from the Shuangxi Hydrological Station. A refined simulation was conducted to reproduce the flash flood and debris flow event that occurred in Baishi Township, Beichuan County, Mianyang City, Sichuan Province, on July 16, 2022. The model effectively retraced the runoff generation and confluence processes and analyzed the underlying causes of the disaster. Results show that the localized model performed well, with a high degree of consistency between simulated and observed water depths. The coefficient of determination (R²) reached 0.86, with a maximum water depth error of 0.25 m, a rise magnitude error of 0.59 m, and a runoff onset timing error of 0.1 h, all within acceptable margins and laying a solid foundation for subsequent analyses. From 00:00 Beijing time on July 16, 2022, Beichuan experienced 8.0 hours of continuous rainfall reaching rainstorm levels. The heaviest precipitation occurred in Baishi Township and the upstream basins of the Qing⁃pian and Tiedong rivers, with rainstorms covering 70% of the total basin area and particularly affecting the middle and lower reaches. The maximum hourly rainfall reached 30 mm, the maximum cumulative rainfall was 120 mm, and the average rainfall in the middle and lower reaches was 88 mm. The long duration, high intensity, and wide spatial coverage of rainfall were the primary triggers of the flash flood. In addition, steep terrain gradients and narrow river channels accelerated runoff and increased water depth. Flow velocities at several locations exceeded 3.3 m·s-1. In the lower Qingpian River, water depth continued to rise for 2.0 hours, reaching a flood peak of 15.4 m that lasted for 2.6 hours, contributing significantly to the runoff at the disaster site. At Baishi Township, the peak flood depth reached 21.0 m and remained at a high level for 3.0 hours. Under complex terrain conditions, prolonged heavy rainfall triggered a largescale flash flood in the small watershed, inundating multiple villages along the river to varying extents. The study area lies within the central Longmenshan Fault Zone and was severely impacted by the 2008 Wenchuan Earthquake. There are 23 identified hazard sites for debris flows and landslides in the basin. The intense flash flood scoured and mobilized abundant loose material, generating a catastrophic debris flow that caused devastating damage to Baishi Township.

small watershed flash floods, N-S equation, runoff convergence model, analysis of flash flood causes