| Glacial lake outburst flood is one of the typical geological disasters in Tibet.Since industrialization,climate warming has been intensifying,and the rate of glacier melting has been accelerating,leading to an increase in the number and volume of ice lakes in Tibet,greatly increasing the risk of ice lake collapse.Glacial lake outburst floods are often complex disasters that can lead to a series of secondary disasters such as landslides,mudslides,ground subsidence,and barrier lakes.Once these secondary disasters form,they can easily cause serious impacts on the residents and ecological environment around the glacial lake.Therefore,conducting monitoring and forecasting research on glacial lake disasters in Tibet has become an urgent task.In this study,the Boiqu River Basin in Tibet Autonomous Region is selected as the study area,which is located in the collision zone between Himalaya and Indian Plate.The mountainous and canyon landforms,glacier landforms are widely distributed in the region,and glacier activities are frequent,which are very likely to cause glacial lake outburst flood disasters,not only affecting Chinese residents at the downstream of the glacial lake,but also bringing harm to neighboring countries.In response to the current situation of low resolution and insufficient timeliness in glacial lake outburst monitoring and prediction based on meteorological and remote sensing methods,this article utilizes infrasound methods with wide monitoring range and strong timeliness.Considering that ice avalanche is one of the important factors inducing glacial lake outburst in Tibet(about70%),this article explores the characteristics of ice avalanche infrasound and conducts research on infrasound based glacial lake outburst prediction technology,Provide important theoretical support for the prevention and emergency dispatch of glacial lake disasters in Tibet.This study firstly compares the progress of research on glacial lake outburst floods from the perspectives of glacial lake outburst mechanism,flood simulation and monitoring and forecasting.Next,we design an indoor ice avalanche infrasound experiment to simulate the infrasound signal generated by natural ice avalanche,and carry out the preprocessing of ice avalanche infrasound signal based on the methods of de-DC drift,low-pass filtering and infrasound event picking,and obtain the time-frequency characteristic parameters of ice avalanche signal by wavelet transform and wavelet packet transform method based on the comprehensive comparison of the advantages and disadvantages of various time-frequency analysis methods.On this basis,the experiments are divided into different stages according to the mechanical characteristics,and the characteristic responses of ice avalanche infrasound in different stages are analyzed to summarize the effects of different natural factors such as ice avalanche speed,freezing time,scale and temperature on the characteristics of infrasound signals.Finally,the research object is to analyze the glacial lake outburst disaster-causing process by taking the Jiarongcuo end moraine-dammed lake in Boiqu River basin as the research object,and compare the classical surge wave climbing height calculation method based on the derivation of the glacial lake outburst criterion with the glacial avalanche height as the main factor,and introduce the glacial avalanche infrasound characteristics and the array infrasound time difference factor to build the forecast model of glacial lake outburst.The main conclusions are as follows:(1)The laboratory ice block loading experiment shows that there is a significant acoustic emission phenomenon during the process of ice block damage under load,which proves the feasibility of monitoring ice avalanche disasters through infrasound from a laboratory perspective.According to mechanical characteristics,the experimental process is divided into five stages: compaction closure,elastic deformation,crack propagation,strain softening,and residual strength.Each stage is accompanied by acoustic emission phenomena,but the characteristics of each stage are different.In the three stages of compaction closure,strain softening,and residual strength,there is only a small amount of acoustic emission phenomenon;In the stages of elastic deformation and crack propagation,the acoustic emission phenomenon is relatively dense,and in these two stages,ice avalanche infrasound events with sound pressure greater than 0.1 Pa account for over 90% of the entire process.In practical monitoring and prediction applications,the maximum through fracture during the crack propagation stage should be selected as the discriminant condition for the occurrence of ice avalanche.(2)The influence of natural factors such as ice avalanche speed,freezing time,size,and temperature on the characteristic parameters of infrasound signals varies.Among them,the ice avalanche speed and freezing time conditions have a significant impact on the infrasound characteristic parameters,reflecting that the faster the ice avalanche movement speed and the longer the glacier accumulation time,the more concentrated the frequency of the ice avalanche infrasound signal is towards high frequencies;The two factors of size and temperature are not sensitive to the characteristic parameters of ice avalanche infrasound.(3)The sound pressure amplitude of indoor ice avalanche infrasound is 0.05-1.80 Pa,and the spectral energy distribution is mainly between 3.13-10 Hz,with a duration of 1-3seconds.The actual monitoring of ice avalanche events in the field mainly involves sound pressure amplitudes ranging from 0.10 to 2.83 Pa,spectral energy distribution ranging from 1.50 to 9.50 Hz,and duration ranging from 3 to 5 seconds.Based on the actual field monitoring data of our research group in the research area,an infrasound based ice avalanche judgment criterion is established as follows: the sound pressure amplitude is greater than 0.05 Pa,the frequency is between 1.50 and 10 Hz,and the duration is greater than 1 second.(4)A set of infrasound based flood prediction methods for glacial lake outburst has been preliminarily formed.Taking the end moraine-dammed lake in Jialongcuo,Boiqu River Basin as the research object,based on the comparison of different calculation methods for surge climbing height,the experience method of the Chinese Academy of Water Sciences was selected to derive the glacial lake collapse judgment criteria based on ice avalanche elevation.The positioning formula and coordinate conversion formula based on array infrasound time difference in the area were derived,and the ice avalanche elevation calculation method based on infrasound was obtained.The judgment criteria for ice avalanche infrasound characteristic parameters were combined,We have jointly established an infrasound based prediction model for the collapse of Jialongcuo end moraine-dammed lake.The results of this study provide important theoretical support for the prevention and reduction of glacial lake disasters in Tibet,and are of great significance for ensuring the safety of local residents and the stable development of the environment. |
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