Study Under Different Source Conditions Risk Assessment Of Typical Single Gully Debris Flow

The Wudu-Diebu section of the Bailong River Basin in the northeastern margin of the Qinghai-Tibet Plateau is affected by complex terrain,a large number of sufficient sources and extreme rainfall weather,and is one of the areas with extremely high debris flow disasters in the country.Continuous heavy rainfall from August 15 to August 17,2020 induced large-scale group debris flows in the region and caused serious losses.In order to explore the dynamic process and risk characteristics of debris flow under different source conditions,this paper comprehensively analyzes the types of debris flow source conditions in the area based on detailed field investigation and remote sensing interpretation.The left branch gully,Shuiyu gully,Moluo gully and Macao gully of Niqian gully were selected as typical single gullies with different source conditions for detailed investigation,and the formation conditions and basic characteristics of debris flow were summarized.According to different dynamic model conditions,FLO-2D and RAMMS were used to simulate the risk of four typical single gully debris flows under different rainfall conditions.Combined with the risk zoning results,the more appropriate software simulation results were selected to further evaluate its vulnerability and risk.The main research contents are as follows :(1)By collecting the topography,geological structure,meteorological and hydrological data of Wudu-Diebu section in Bailong River Basin,the regional geological environment conditions are analyzed.Through field investigation and remote sensing analysis,the source initiation conditions and source types of debris flow in the basin are classified.According to different source conditions,four typical debris flow gullies in the study area were selected to analyze the disaster-forming conditions and basic characteristics of debris flow.The four single gullies are the left branch gully of Niqian gully,Shuiyu gully,Moluo gully and Macao gully representing the source of collapse and slope surface.Among them,the left branch gully of Niqionggou is ice-water mixed hydrodynamic initiation type,and Shuiyugou,Moluogou and Macaogou are rainstorm initiation type.(2)Based on different dynamic model conditions,FLO-2D and RAMMS software were selected for numerical simulation,and the risk assessment of single gully debris flow under different rainfall conditions was carried out.According to the precipitation conditions of 20 years,50 years and 100 years,the debris flow velocity and accumulation depth under the two model conditions are compared and analyzed.The results show that the flow rate and accumulation depth of RAMMS simulation are greater than those of FLO-2D,in which the difference of flow rate is 20% and the difference of accumulation depth is about 35%.(3)Based on the simulation results and the actual disaster range,the accuracy verification was carried out to determine the optimality of different software models to simulate the debris flow results under different source conditions.The simulation accuracy of FLO-2D for the left branch gully,Shuiyu gully,Moluo gully and Macao gully of Niqian gully was 80.23%,63.48%,68.72% and 76.88%,respectively.The simulation accuracy of RAMMS for four single gullies is 67.78%,53.89%,81.45% and 86.32%respectively.The results show that FLO-2D is mainly suitable for collapse and slope debris flow,and RAMMS is suitable for landslide debris flow.Among them,Moluo Gully is a collapse and slope-type debris flow,but the simulation accuracy of FLO-2D is lower than that of RAMMS.The main reason is that the slope source in the basin is relatively large,and the gully is started after continuous heavy rainfall.Therefore,the amount of slope source and rainfall intensity and type can be used as the classification criteria for further research.However,the left branch of Niqian gully is ice-water mixed hydrodynamic start-up type.Considering the increase of temperature and ice-snow melt water in the future,the debris flow accumulation area will increase by 34.67%.(4)Based on the optimal debris flow simulation results,the debris flow intensity under different rainfall conditions is analyzed by flow velocity and accumulation depth.Finally,combined with the intensity of debris flow and the risk analysis of different rainfall conditions,four typical debris flow gully risk zoning maps are obtained.The results show that the high risk area accounts for more than 65 % of the total dangerous area,accounting for a large proportion.(5)Based on the principal component analysis method,the vulnerability of the three main disaster-bearing bodies of buildings,roads and green spaces within the optimal risk results is evaluated.Based on the results of risk and vulnerability zoning,the risk zoning maps of four debris flow gullies are obtained.The results show that the high-risk areas of the four debris flow single gullies account for 12.44%,26.27%,20.01% and 80.19% of the left branch gully,Shuiyu gully,Moluo gully and Macao gully,respectively.