Numerical reconstruction and field investigation of a flash flood in a steep multi-tributary mountain river

Based on rainfall data and high-precision topographic data, a hydrological-hydrodynamic model was employed to conduct numerical reconstruction of the flash flood disaster process that occurred in Zanhuang County, Hebei Province, on July 29, 2023. The main river channel in the study area is approximately 30 km long, with a total drop of 1,000 m, slope changing from 12% to 1%, and 21 tributaries. This configuration fully captures the complete variation process in mountainous rivers: from steep to gentle slopes, from small to large peak discharges, from shallow to deep water depths, and from high to low Froude numbers. Combined with post-disaster field investigations, the rapid transition patterns of riverbed scour and deposition in the river reaches were revealed, and the disaster characteristics of flash floods in each reach were summarized. Based on the research findings, the following recommendations are proposed: (1) To improve the reliability of critical rainfall and water level thresholds for flash flood early warning, water level monitoring points in flash flood gullies should be installed near structures and locations with minimal scour-deposition changes, such as check dams, low weirs, and bedrock outcrops in erosional reaches. (2) Due to the difficulty of hydrodynamic modelling in simulating the rapid transition of riverbed scour and deposition, regular sediment removal is recommended for aggradation reaches; meanwhile, scour protection should be strengthened for embankments, bridges, and roadbeds in high-velocity reaches. (3) Under supercritical flow conditions, the backwater effect of downstream reaches on upstream flood routeing is relatively minor; therefore, for flood control planning purposes, high-precision topographic surveys need only focus on areas near disaster-bearing bodies.