Research On Lahar Erosion Mechanism Of Changbai Mountain

Changbaishan Tianchi volcano is one of the most potentially explosive volcanoes in China and even in the world.The existence of Tianchi caldera lake is like a time bomb embedded in the crater,making the damage of Tianchi volcano deadlier.Once the volcanic activity intensifies,the outburst water gradually develops into lahar by continuously scouring the soft rock strata along the way and the volcanic loose deposits left by volcanic historical eruptions.Erosion in the migration process of lahar will significantly change its flow properties,expand its scale,and increase the peak discharge and submergence range,which seriously threatens the life and property safety around Tianchi volcano and Erdaobaihe River.However,the complex erosion of lahar will seriously affect the accuracy of prediction.Therefore,it is of great theoretical significance and practical value to systematically study the lahar erosion mechanism.Based on the project of "Research on Initiation and Migration Mechanism and Disaster Prediction of Changbaishan Lahar" sponsored by National Natural Science Foundation of China,this paper takes lahar induced by the outburst of Tianchi caldera lake as the research object.According to the survey data,we carry out field investigation to analyze the spatial distribution and characteristics of the volcanic deposits left over from the Tianchi volcano historical eruptions.After the particle size characteristics and physical and mechanical properties of soil samples are tested,the lahar erosion mechanism is explored by carrying out physical simulation experiments.The spatial distribution pattern of erosion and the controlling factors of erosion intensity are clarified.Combined with the CFD-DEM numerical model,the erosion process and the motion characteristics of the particles are studied from the microscopic scale,an erosion mode involving volcanic pumice clasts is proposed.Based on the unique role of volcanic pumice clasts,an erosion rate formula is derived by establishing the erosion relationship between lahar and the eroded bed material with different pumice clast content.And the availability of the formula is verified by experiments.The main research conclusions are as follows:1.Through detailed field investigation,the deposit facies types were identified and the distribution map of volcanic deposits was drawn.The grain size and physical and mechanical properties of volcanic deposits were analyzed through laboratory experiments and mathematical methods.It is found that the scaling parameters are closely related to particle size and physical and mechanical properties of volcanic deposits.By fitting the equations between scale parameters and dry density,pore ratio and internal friction angle,the relationships between scale parameters and the physical and mechanical properties of volcanic deposits were established,which provides a theoretical basis for the subsequent derivation of lahar erosion rate formula.Scale parameters provide new research ideas for volcanic deposits.2.Based on the indoor flume experiments,the influence of pumice clast content in eroded bed,slope of flow channel and flow rate on the spatial erosion pattern from fixed bed to eroded bed were studied.The spatial distribution characteristics of lahar erosion and the controlling factors of erosion intensity were investigated.The results show that the spatial erosion pattern of lahar has the same variation trend under different experimental conditions.For similar terrain and flow conditions,the lahar erosion intensity and the relationship between the experimental conditions largely depend on the pumice clast content of the eroded bed.Volcanic pumice clasts improve the lahar erosion intensity by increasing the collision stress between particles.3.A flume experiment was carried out to simulate the erosion behaviors when a certain flow rate was released from high altitude and flowed through the eroded bed.By using colored sands to trace the movement path and final position of soil particles in the flume experiments,the erosion process was analyzed,the lahar erosion mode and the influence of volcanic pumice clast on the erosion mode were explored.Based on the eroded bed morphology and the spatial distribution characteristics of the colored sands,a new erosion mode involving volcanic pumice clasts,upwelling erosion,is proposed.By using the CFD-DEM model to simulate the movement characteristics of particles during the erosion process,it is proved that the upwelling of volcanic pumice clasts can significantly improve the transient erosion intensity temporarily.4.The influence of volcanic pumice clasts on the whole erosion process is quantitatively evaluated from two aspects: the flow characteristics of lahar and the evolution process of erosion bed morphology.Based on the unique role of volcanic pumice clasts in the erosion process,erosion rate formula was derived by establishing the erosion relationship between lahar and the eroded bed material with different pumice clast content.And the availability of the formula was verified by experiments.The new erosion rate formula can better describe the effect of pumice clast content on the lahar erosion process.Furthermore,it can make the quantitative expression of lahar erosion more accurately.