ncdc logo title
New ideas on debris flow disaster assessment in Chengdu mountainous area are put forward
On August 8, 2017, Jiuzhaigou, a world natural heritage site, experienced a strike slip strong earthquake with magnitude of MS 7.0, with a focal depth of 20km (33.20 ° n, 103.82 ° E), causing 25 deaths. Thousands of coseismic landslides induced by earthquake provide abundant loose solid source for debris flow and increase the risk of debris flow. Therefore, how to quickly and accurately evaluate the post earthquake secondary disasters without long-term monitoring data is of great significance to the protection of national treasures, while the existing assessment methods rarely consider the change of sediment supply after the earthquake. In order to understand the impact of the change of sediment supply on debris flow after the earthquake, Hu Kaiheng, a researcher from Chengdu Mountain Institute of Chinese Academy of Sciences, led a team to carry out a detailed field investigation in Jiuzhaigou watershed and made important progress.                          . It is found that the sediment increment of Zharugou, Heye gully, Shuzheng gully, zechawa gully and Haizi gully in the next season is large, and the possibility of debris flow after earthquake is the highest. In the later stage, the sediment increment of five debris flow gullies before and after the earthquake was obtained by multi period high-definition remote sensing images and UAV photos. Therefore, a comprehensive debris flow disaster assessment method is developed by combining sediment increment, flow mass model and numerical simulation. The feasibility of the method is verified by the debris flow events in Haizi gully in the lower season after the earthquake. Using the new method, the key characteristic parameters of debris flow such as scale, flushing distance, accumulation range and accumulation depth of the five debris flow gullies were evaluated, and the debris flow risk zoning map was made, which provided reference for land use planning and debris flow secondary disaster prevention and control in Jiuzhaigou scenic area after the earthquake. The study was recently published in engineering geology, a well-known international journal Link to article: 九寨沟地震诱发的典型地质灾害现场
New Models
A large number of reservoir inflow with preserved data, spillway discharge and discharge of breach ditch (unstable flow with sediment transport capacity) formed along erosion are comprehensively applied in the model. It is assumed that the slope ratio at the bottom of the breach is basically the same as that at the downstream of the dam. The development of breach ditch depends on the material properties of dam body (D50 size, unit weight, friction angle, bond strength). The model takes into account the following possible complexities: 1) the material properties of the dam core and the outside of the dam are different; 2) before the real breach caused by overtopping flow is formed, the erosion ditch along the downstream slope surface of the dam is determined; 3) the downstream slope of the dam may be covered with grass or contain materials larger than the external components of the dam; 4) the shear strength and cohesive force are exceeded The results show that more than one sudden structural collapse mechanism is enlarged due to the hydrostatic pressure; 5) the width of the breach obtained from the slope stability theory is enlarged; 6) the free surface breach flow is developed by piping, which causes the collapse; and 7) the non cohesive material (gravel) or cohesive material (clay) may be carried away by erosion. The discharge water level hydrograph is obtained by time step iterative method, which can be completed in only a few seconds on a main computer. The model is not constrained by numerical stability and convergence stability.