Formation Process,Main Control Factors And Critical Conditions Of Dam-Break Debris Flow In Bailong River Basin

The Bailongjiang River Basin is located in the strong earthquake area in China.The geological structure in the basin is complex,and the mountains and deep valleys are densely distributed.Multiple strong earthquakes in history have led to the fragmentation of landforms,weak lithology,fragile geological environment and extremely developed geohazards.In addition,climate change has led to frequent extreme rainstorm events in recent years,which has induced a large number of geological disasters,and often in the form of geological disaster chain.Because the geological disaster chain is composed of multiple disasters and has chain effect,it can often cause greater harm.Among various forms of disaster chains,dam-break debris flow is the most critical part of the geological disaster chain due to its fast occurrence and strong destructive power,which has attracted the attention of scholars at home and abroad.However,at present,the research on the dam-break debris flow mainly focuses on the glacier lake outburst debris flow.There are few studies on the dam-break debris flow of the channel barrier in the strong earthquake area,and the research on the main control factors and critical conditions of the dam-break debris flow is not sufficient.At present,there is a lack of comprehensive research on the formation process,main control factors and critical conditions of the dam-break debris flow in the strong earthquake areas such as the Bailong River Basin,which needs to be further studied.Therefore,this study collected field investigation,remote sensing image monitoring and text data of typical debris flow gully in Bailongjiang River Basin.The simulation experiment combining indoor flume experiment with field in-situ flume experiment is the main research method.Study on the formation process,controlling factors and critical conditions of landslide debris flow in channel dam.The evolution process of the debris flow caused by the collapse of the cofferdam body is analyzed,and the influences of different gully slope,upstream inflow and dam soil mechanical properties on the formation process of the debris flow are analyzed.The main control factors of the formation of the debris flow were determined,and the critical conditions and discriminant formulas of the formation of the debris flow were calculated.The reliability was verified by the actual disaster events in the basin and similar regions in recent years.It provides scientific basis and technical support for the prediction and early warning of dam-break debris flow disaster chain in Bailongjiang River Basin in strong earthquake area.The main conclusions are as follows :1)According to the dam break process and its flow change process,as well as the bulk density change process of fluid mixture after dam break,the formation process of debris flow is divided into three stages for the first time: The first stage is the initial stage of the formation of the collapsed debris flow,in which the dam body collapse is in the process of retrogressive erosion.The outburst flow is small and slowly rising,and the bulk density of the fluid mixture is low and fluctuates slightly.At this stage,the fluid solid material is basically derived from the dam erosion.The second stage is the acceleration stage of debris flow formation,in which the dam break is in the process of erosion acceleration.The outburst flow and bulk density of the mixture show a large and rapid rise and then a rapid decline.The solid material comes from dam erosion and gully bed erosion.This stage is the most prone to debris flow and the most destructive stage of debris flow.The third stage is the weakening stage of the collapse debris flow,which is in the process of erosion weakening.The burst flow decreased slowly and was finally close to the upstream inflow.The bulk density of the fluid mixture showed a slow decreasing trend as a whole.At this stage,the solid material came from gully erosion and the collapse of some residual dams.2)By changing the four parameters of gully bed slope,upstream inflow,dam porosity and initial water content in the experiment,the effects of different gully bed slope,upstream inflow,dam compactness and dam drying degree on the formation process of debris flow were analyzed.Results showed: 1.From the influence of various control factors on the formation time of debris flow,the increase of gully bed slope and upstream inflow,the decrease of dam drying degree and dam compactness will shorten the formation time of debris flow.2.From the influence of various factors on the maximum bulk density of debris flow formed,the increase of gully bed slope,upstream inflow and the decrease of dam drying degree will cause the increase of the maximum bulk density of debris flow formed by dam break,while the change of dam compactness has no obvious influence on the maximum bulk density of debris flow formed by dam break.3.From the perspective of the influence degree of each factor on the maximum bulk density of debris flow,the influence degree of each factor from large to small is as follows : gully bed slope > upstream inflow > dam drying degree > dam compactness.Among them,gully bed slope and upstream inflow are the key control factors.3)By analyzing the influence mechanism of different control factors on the formation process of debris flow,it is found that: The increase of slope of gully bed and upstream inflow can increase the peak bulk density of debris flow burst by improving hydrodynamic conditions,and shorten the formation time of dam burst and debris flow burst.The dry degree of dam body and the density of dam body are reduced by reducing the anti-erosion ability of dam body to shorten the time of dam break and debris flow formation.It is worth noting that the decrease of the density of the dam body accelerates the erosion rate of the dam body,but also reduces the solid quality provided by the unit volume of the dam body,so the decrease of the density of the dam body cannot significantly increase the maximum bulk density of the outburst debris flow.4)Based on the theory of non-uniform fluid motion and the principle of water balance,combined with the results of indoor flume experiment and field experiment,the peak discharge of break flood and upstream inflow were linked together,and the prediction model of peak discharge of break flood suitable for this study area was proposed,and according to the field experimental data fitting suitable parameters in the Bailong River Basin.Combined with empirical formula and mathematical derivation,the critical condition formula for the formation of debris flow controlled by gully bed slope and upstream inflow is obtained: The applicability of the two critical conditions determination formulas is verified by using the field collection data of Sanyanyu Dayanyu gully dam and Jinshajiang Baige dam in Zhouqu County of Bailong River published in the existing literature.The results show that the determination formula can accurately determine the critical conditions for the formation of dam-break debris flow in Bailong River Basin and its similar regions,and can be used for the early prediction and early warning of dam-break debris flow in this region.