Reconstruction of the Dynamic Process of the Holocene Gelongbu Landslide-Blocking-Flood Geological Disaster Chain Based on Numerical Simulation

This paper takes the Holocene Gelongbu ancient landslide blocking the river outburst flood geological disaster chain as an example, using field surveys, PFC3D landslide dynamics numerical simulation and HEC-RAS outburst flood simulation to reproduce the whole process of the landslide slip-blocking-break disaster chain. It is found in field investigations that the total volume of Gelongbu landslide is about 7.92×10⁷ m³, the main sliding direction is NW335°, the maximum sliding distance is 2.3 km, and the maximum accumulation thickness is about 150 m. The materials at the front edge of the landslide are looser than those at the back edge, and the degree of fragmentation is higher. The numerical simulation shows that the sliding process of Gelongbu landslide lasts for 103 s, and the maximum velocity can reach 57 m/s. In the sliding process, the kinematic characteristics of the sliding process showed regional differences in the degree of fragmentation. Most of the particles maintained their original positions during the motion, and the accumulation material consisted of individual particles and block clusters with relatively weak fragmentation. The landslide blocked the Yellow River and formed a barrier dam as high as 143 m at the saddle point and elevations of 2 030 m on the left bank and 2 063 m on the right bank. The area of the landslide accumulation body is approximately 1.8×10⁶ m², forming an upstream reservoir with an area of 128 km² and a storage capacity of 4.87×10⁹ m³. By simulating the flood evolution process under different degrees of dam break (15%, 25%, 50%, and 75%), it was observed that the outflow from the breach rapidly increased to a peak within 30 minutes, followed by a gradual decrease in velocity. The peak outflow from the breach at different levels of dam break were 15 137.9 m³/s, 52 192.9 m³/s, 157 375.5 m³/s, and 326 703.6 m³/s, respectively. The peak flood discharge and water level characteristics at various downstream sections were analyzed. The relationship between flood evolution and the Lajia Site is discussed. It is found that the peak discharge of the burst flood is 57 782.3 m³/s when the dam breaks at 25%, and the water depth at the Lajia Site is 27.1 m, which is 6.1 m above the ancient surface of the site. When 75% of the dam breaks, the maximum flow to Erlitou Site was equivalent to the once-in-100-year flood flow of the Yellow River.