Quantitative Assessment Of Retrogressive Thaw Slump Induced Soil Erosion In Permafrost Regions On The Qinghai-Tibet Plateau

The Qinghai-Tibet Plateau(QTP)is the main area for ice-rich permafrost distribution in China.Due to global climate change,the ice-rich permafrost on the plateau is becoming increasingly vulnerable to the impact of terrain-changing thermokarst landforms,and the most rapid and dramatic among them are retrogressive thaw slumps(RTSs).RTSs are typical kind of thermokarst landslides that primarily occur in gently sloping areas underlain by massive ground ice in Arctic regions and the QTP.They are initiated when ice-rich permafrost or massive ground ice is exposed and thaws,leading to active layer detachment failure and the formation of a steep,ice-rich frozen back scarp at the trailing edge of the slope that gradually retreats during the thawing season.This process can cause soil erosion within a few years and release organic carbon frozen in the permafrost.With the intensification of climate warming in the past decade,the number and scale of RTSs in the QTP have increased,resulting in severe impacts on the region’s infrastructure and ecological environment.There have been studies that qualitatively describe the development and evolution process of RTSs from the perspective of cryo-geomorphology,identifying characteristics such as the development area,scale,frequency,and spatial distribution of RTSs.However,the quantification and assessment of the amount of ground ice ablation and soil erosion during the development of RTSs remains elusive,making it difficult to quantify the ecological and environmental impact caused by these RTSs.In this paper,we selected a typical RTS situated on the northeast-facing flank of Gu hill(Gerlama,34°50’N,92°54’E)on the Beiluhe River Basin of the QTP based on remote sensing interpretation results.An extensive field investigation was conducted to obtain a thorough understanding of the hydrogeology,topography,landform,and permafrost conditions(ice content)of the study area.Furthermore,we used a multidisciplinary approach to address the scientific issue of quantifying ground ice ablation,headwall regression,and soil erosion of RTS development in the QTP.This study aimed to explore the hydrothermal conditions,transport laws of fine-grained sediments,as well as volumetric changes and deformation laws of RTS development during the ice-rich permafrost thawing.In addition,we integrated field survey and numerical simulations to develop a semi-empirical and semi-theoretical mathematical model for estimating the soil erosion of RTS development.Starting with an analysis of the development patterns of RTSs,this study employs a comprehensive interpretation approach that incorporates remote sensing imagery of topography,landforms,vegetation,and hydrological characteristics.Through this approach,we assess the soil erosion induced by the entire RTS phenomenon across the plateau.Several significant findings and innovative achievements of the thesis are as follows:(1)Based on the interpretation of remote sensing data of RTSs in the QTP,a field investigation and geophysical exploration were conducted on typical RTS areas to determine the stratigraphic structure,distribution characteristics of ground ice,and volumetric changes.The results showed that the distribution of massive ground ice is the main material condition of RTS development.Furthermore,the erosive process of RTSs will continue to develop in areas where ice-rich permafrost still exists in the headwall.Using nuclear magnetic resonance survey,the study examined the unfrozen water content and permeability coefficient in the upper 3 m of the permafrost table.Results indicated a trend of increasing and then decreasing water content and permeability coefficient.At the base of the active layer,the permeability coefficient was approximately 0.2-0.01 m/d.Additionally,a joint investigation utilizing terrestrial laser scanning and unmanned aerial vehicle revealed that the studied site—K1129W RTS area experienced approximately 1412 m~3 of soil erosion,while approximately 13139 m~3 occurred in the Gushan RTS between September 2021 and August 2022.(2)A three-dimensional coupled water-heat-mass transfer model was constructed to quantify ground ice ablation and the impact on the hydrological and ecological system during RTS development.The development and evolution of RTS has changed the hydrothermal conditions of the soil in the active layer and the permafrost table.The rate of downward thaw in the permafrost table in the development area is significantly higher than that of the natural area.The polycyclic development of RTS is caused by the melting of ground ice during the thawing season each year,with the maximum amount of ground ice melting(492 m~3)occurring in 2016.Furthermore,the melting of ground ice is positively correlated with the high air temperature and precipitation of each year.In addition,the melting of ice-rich permafrost has caused a large amount of fine-grained mass transport,which has affected the hydrological environment near the RTS development area.During the thawing season in 2021,when the daily temperature and precipitation reached their peak for that period,the turbidity of the surface water in the front edge area of the RTS 49 in the subsequent time.(3)Through laboratory experiments on the melting process of high ice-content permafrost under natural conditions,the process of cementation weakening during the melting of high ice-content permafrost under natural conditions was proposed,and the melting and drainage laws of permafrost samples with different ice contents were studied.Under conditions where the room temperature was approximately 18℃,the cumulative thaw settlement of amples with an ice content of 50%under self-weight was approximately 30.26 mm,with a final drainage volume of approximately 6.2 ml and TSS of approximately 866 ppm.A comparison of the melting process of samples with different ice contents showed that the ability of water migration and the suspended particle content of meltwater both increased with increasing ice content.(4)A systematic numerical simulation method was developed for the dynamic evolution process of three-dimensional RTS,which was verified through numerical calculations of the melting process of ice-rich frozen soil and in-situ direct shear tests in the basal zone of active layer.The simulation results showed that the soil erosion during a single thawing season of the K1129W RTS was 1335 m~3,and the headwall regression during this process was approximately2.2～3.5 m.Combining the numerical simulation and field investigation results,We developed a semi-empirical and semi-theoretical mathematical model for quantitatively estimate the soil erosion of RTS.(5)By integrating remote sensing data and mathematical model,we estimated the volume of soil erosion caused by RTSs in the QTP.Starting from the developmental patterns of RTSs,this study combines terrain,landform,vegetation,and hydrological characteristics derived from remote sensing imagery to conduct in-depth interpretation.This leads to the construction of a dataset comprising 1,192 active RTSs with corresponding measurements of the magnitude of headwall retrogression and the area of mass wasting zone up to 2020 or 2021.Using the mathematical model for estimating the soil erosion volume of RTS,the total volume of 2,625active layer detachment induced RTSs in the QTP was estimated during the summer between2008/2013 and 2021,mobilized a combined volume of pproximately 19.4×10~6 m~3/a.This study presents a novel approach by integrating remote sensing and quantitative research methods to investigate the freeze-thaw erosion of soil erosion in the QTP induced by RTSs.