Debris flow hazard assessment of the Eryang River watershed based on numerical simulation

Abstract

The middle reach of Taohe River, in the south part of Gansu Province, China is severely threatened by debris flow hazard. Eryang River is an important branch of Taohe River. On May 10, 2012, many debris flows were triggered by extreme heavy rainfall (1% frequency), resulting in serious losses of human lives and properties. In order to recognize the hazard of debris flows in small watersheds, six key debris flow gullies in Eryang watershed, Lalonggou, Zhalonggou, Yizigou, Jiehagou, Lalugou, Paizuigou, were selected as the research area. The numerical simulation software FLO-2D was used to analyze the debris flow movement and accumulation characteristics of each debris flow gully under the actual ‘5.10’ rainfall conditions, so as to reconstruct the ‘5.10’ debris flow disaster scenario. Numerical simulation results show that the flow speed increased to the maximum after 15-30 minutes since the outburst. The flow lasted for about 3 hours. The speed in the moving section was very high, and decreased sharply at the gully-mouth, then deposits accumulated in the river valley. According to the satellite images and field investigations, the simulation results were compared with the actual situations. The comparison shows the simulation effect is good, the deposition area, the discharge process, and the main damage area were well-reconstructed by FLO-2D simulation. Then, the same method and parameters are used to simulate the accumulation range, depth and velocity of debris flows under the precipitation of 2 % and 0.2 % frequency (fifty and five hundred years rainfall, respectively), and then the risk zoning map is produced using the simulation data. The potentially threatened houses and properties were outlined. The threatened area is 37900m2 and 60500m2 under 2% and 0.2% rainfall frequencies. There are 22 houses in the highest hazard level. This provides direct reference to the local government to control the debris risk. This work also provides a practical approach for the debris flow hazard assessment.