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

The middle reach of the Taohe River in the southern part of Gansu Province is severely threatened by debris flow hazard. The Eryang River is an important branch of the Taohe River. On May 10, 2012, a large amount of debris flows was triggered by extreme heavy rainfall (with 1% frequency), resulting in serious losses of human life and property in Minxian County of Gansu Province. In order to recognize the hazard of debris flow in small watersheds, six key debris flow gullies in the Eryang watershed were selected as the research areas. The numerical simulation software FLO-2D was used to simulate 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, deposits accumulated in the river valley. According to the remote sensing and field investigation, the simulation results were compared with the actual situation. The comparison shows the simulation effect is good, and the deposition area, the discharge process, and the main damage area are well reconstructed by FLO-2D simulation. The same method and parameters are used to simulate the accumulation range, depth and velocity of debris flow under the precipitation of 2.0% and 0.2% frequencies, and the risk zoning map is produced by the simulation data. The potentially threatened houses and properties are also outlined. These provide references to the local government to control the debris risk. This work also provides a practical approach for the debris flow hazard assessment.