Research On Mechanism And Numerical Simulation Of Post-Fire Debris Flow Disaster In Small Catchment Based On Hydrology And Dynamic Process

In the context of global climate change,wildfires are becoming a global natural disaster.A worthwhile concern is that the susceptibility to debris flow hazards in catchments affected by recent wildfires is much higher than in similarly geographically and geologically similar smaller catchments that have not experienced wildfires,which is a very dangerous subset of geological hazards.Especially in recent years,forest fires have occurred frequently in China,causing the lives and properties of residents in the catchments exposed to forest fires face serious threats from geological disasters such as post-fire debris flow.Therefore,it is urgent to carry out research on the disaster mechanism and numerical simulation of post-fire debris flow in mountainous small catchments based on hydrological and dynamic processes.On July 5,2021,a short-term heavy rainfall caused debris flow to erupt simultaneously in multiple tributaries of the Xiangjiao catchment in Muli County,Liangshan Prefecture,Sichuan Province.The catchment experienced a severe forest fire about a year ago.In order to achieve qualitative and quantitative analysis of the hydrological and dynamic processes of post-fire debris flow disasters in small catchments in mountainous areas,and to ascertain its disaster mechanism,a comprehensive numerical model including physical process models was constructed,combined with field survey observations,remote sensing data interpretation,UAV aerial photography modeling and laboratory testwireless to conduct a comprehensive mechanism analysis of post-fire debris flow in Xiangjiao catchment on July 5,2021.The main research content and results of this thesis are as follows:(1)Based on the disaster characteristics of the post-fire debris flow and the surface cover characteristics of the burned catchment,a hydrological and dynamical comprehensive numerical model including vegetation interception,soil infiltration,surface runoff and soil erosion deposition model was constructed.Combining comprehensive numerical model and multi-source data to carry out numerical simulation of post-fire debris flow in Xiangjiao catchment.The simulated inundated area of the downstream of the Xiangjiao debris flow is 59.8 ha,while the inundated area drawn from the high-definition image modeled by UAV aerial photography is about 50.1 ha.The simulated sedimentary area inundated depth and section erosion depth were compared with the observed results,and the results showed that the simulated results were in good agreement with the actual observed results.The total amount of simulated debris flow is1.02×10~6 m~3,and the average solid concentration of debris flow reaches 0.47.In general,the simulation results at the small catchment scale better reveal the physical characteristics of the reduction of vegetation interception capacity,the decrease of soil infiltration rate,and the severe erosion of surface runoff during the formation of post-fire debris flows.It shows that the comprehensive numerical model constructed in this thesis can effectively describe the characteristics of the initial stage and propagation stage of the post-fire debris flow induced by surface runoff,and provide important insights and guidance for the prediction and prevention of post-fire debris flow disasters in the catchment.And the simulation results can provide the data basis for quantitative analysis for the analysis and research of the disaster mechanism of post-fire debris flow.(2)Through on-site investigation,interpretation of remote sensing data,etc.,the detailed disaster mechanism analysis of the Xiangjiao post-fire debris flow after was carried out,and the soil burn severity map in the catchment was established.The results showed that in the catchment,moderately and highly burned areas accounted for 41.8%of the total area of the catchment,the proportion of surface soil fine particles in the burnt area was larger than that in the unburned area,and the soil saturation infiltration rate was significantly lower than that in the unburned area.Combined with the forest fire burnt situation and field investigation data,it is found that the Xiangjiaogou debris flow is a typical post-fire debris flow disaster induced by excessive infiltration-surface runoff-basal erosion.(3)Through the interpretation of satellite remote sensing time series data and the regression equation of soil saturated infiltration rate,the relevant basic simulation data before the fire and one year after the debris flow occurred were obtained,and the simulation was driven by rainfall data,and the impact of forest fires on the debris flow in the study area was analyzed.The spatio-temporal evolution of disaster susceptibility,found that the catchment experienced rainfall events of the same intensity before the fire may not have debris flow disasters,and the risk of subsequent surface runoff-induced debris flow disasters also tended to decrease over time after the fire,and the results provide theoretical guidance for the research on early warning and forecasting of debris flow after fire.In general,the study on the mechanism and numerical simulation of post-fire debris flow disasters in mountainous small catchment based on hydrological and dynamic processes provides technical and theoretical support for qualitative and quantitative comprehensive mechanism analysis of post-fire debris flow disasters in small catchment.Combining the simulation and reappearance of the whole process of debris flow,traditional field investigation and laboratory experiments,the development characteristics,the response of hydrological and dynamic processes to the impact of fires have been ascertained,and the mechanism of post-fire debris flow disasters has been profoundly revealed.It provides a reliable scientific basis and technical support for the monitoring and warning,disaster prevention and reduction of debris flow disaster in the mountain burnt catchment.