Effect Of A Thaw Slump On Soil Freeze-thaw Erosion In The Permafrost Region Of The Qinghai–tibet Plateau,China

Permafrost is an important part of the natural ecosystem in the permaf rost region of the Qinghai–Tibet Plateau,China.Because of the influences of climate warming and various engineering projects,the degradation permafrost on the Qinghai-Tibet Plateau leads to thaw slumps which are typical thaw geological disasters in permafrost regions.Thaw slumps intensify soil freeze-thaw erosion which not only causes collapse and retrogression on slopes which put constructions at risk in permafrost regions,but also changes soil hydrological processes which result in the lost of recovery ability in permafrost with the deterioration of regional ecological system.In this paper,a small thaw slump in the init ial development stage is taken as the research object in the Hoh Xil area.Based on field monitor ing,soil properties analyses,temperature and moisture analyses,slope stability analyses,combined with the fractal theory and the infinite slope model,the research of the effect of a thaw slump on soil freeze-thaw erosion is conducted in the permafrost region of the Qinghai–Tibet Plateau,China.The main results are as follows:(1)The slope deformation of the thaw slump monitoring during 2013-2016 shows the thaw slump is not caused by a one-time landslide.Because of annual thermokarst processes,the edge of the soil structure at the top of the slope is destroyed and collapses,and the thaw soil in the unstable active layer becomes a plastic flow state creeping down along the sliding face,which changes the topography of the study area gradually.The borehole test shows that the creep movement of the thaw slump mainly occurs at soil depths between 0 and 40 cm(not >50 cm depth).The degree of soil freeze-thaw erosion is greater in the unstable slope than that in the unaffected area.(2)The soil freeze-thaw erosion intensified by the thaw slump leads to the significant changes in the soil properties.With the development of the thaw slump and the destruction of the surface vegetation,the soil in the active layer is coarsened and homogenized,and the soil bulk density increases along the slope gradually,and the soil fine particles and the organic matter largely first deposits in the 30-40 cm depth active layer at the gentle slope area then loses rapidly at the slope bottom.The thaw water released by the melting of the underground ice causes the signif icant increase of the soil moisture and the significant reduction of shear strength in the active layer.(3)The analyses results based on the infinite slope model of the thaw slump are consistent with the field observations: sliding movement of the headwall is inevitable in thaw seasons,whereas the gentle slope area and slope bottom are in a more stable state with a very slight creep displacement;the whole slope is stable in non-thaw seasons.(4)The thaw slump weakens the heat transfer buffer of the active layer,and accelerates land surface heat exchange,and enlarges the ablation the underlying ice-rich permafrost,which results in the deepening of the active and the increasing of the thaw settlement.The slope monitoring system shows that the headwall is steeper and steeper,and a big hollow is forming in the front part of the failed slope.With the ablat ion enlargement of the underlying ice-rich permafrost,large amounts of thaw water is released from the thawed permafrost beyond the holding and discharging capacity of the affected active layer,which leads to the formation of overland flow at the gentle slope area and the formation of ponding depression at the slope bottom.What is more,the active layer change regulations of the thaw slump,those of the different extent affected areas around a thermokarst lake and those of the lakeshore slope caused by thaw settlement are similar.It predicts that a thermokarst lake will form in the front part of the failed slope,and the study area will become a thermokarst lake adjacent to shoreline thaw slumps,which is a typical thermokarst landform.