Changes And Risk Assessment Of Glacial Lakes In Nagqu,Tibet

Under the influence of climate change,the glaciers in the Qinghai-Tibet Plateau have retreated significantly,providing favorable conditions for the formation and expansion of glacial lakes.The dam of glacial lakes is not stable enough,and they are prone to collapse due to internal or external factors,followed by flood disasters caused glacial lake outburst flood(GLOF).With the impact of climate change,the frequency of GLOF events has increased significantly in recent years.Regions that have not reported such disasters in the past have also been affected by GLOF disasters recently.Previous research on glacial lakes has mainly focused on the Himalayas,with less in-depth research on other regions of the Third Pole region.This paper takes the Nagqu region of Tibet as the research area,where at least two large-scale GLOF events have occurred in the past decade.Combined with remote sensing,field investigation,and hydraulic model,the change characteristics of glacial lakes in Nagqu in the past 40 years were obtained,and glacial lakes with high dangerous of outburst were identified,and the degree of risk brought to the downstream area was assessed.The applicability of the HEC-RAS model to the GLOF is verified by simulating the flood evolution process of the Jinwengcuo and Ranzeriacuo GLOFs.The following conclusions were obtained:(1)There are 255 glacial lakes in Nagqu region,with a total area of 27.829±4.62km~2.The glacial lakes mainly concentrated in the southeast and southwest of Naqu.It shows noticeable expansion of the glacial lakes from 1990 to 2020.During this period,the total number of glacial lakes increased by 72,and the area increased by 6.14 km~2.The original glacial lakes expanded rapidly in the low altitude region(less than 4700m),and the new glacial lakes emerged largely in the high altitude area(5100—5700m).Temperature and precipitation are critical factors to the expansion of glacial lakes.Due to the spatial and temporal differentiation of temperature and precipitation in Naqu,the expansion of glacial lake was faster in the south,compared with that in the north,and the expansion rate of the whole region accelerated gradually.The location of glacial lakes can also affect the process of expansion:the closer the glacial lake is to the glacier,the more favorable it is to the development.This can also lead to a significant difference in glacier changes that glaciers connected to lakes shows higher rate of melt.However,there is no apparent evidence showing linear relationship between glacier and glacial lake area change.(2)There are 16 extremely dangerous glacial lakes,52 dangerous glacial lakes,147 moderately dangerous glacial lakes and 40 low dangerous glacial lakes in the Nagqu region,and the glacial lakes above the danger level are mainly distributed in Jiali County,Biru County and Baqing County in the southeast of Nagqu.Dangerous glacial lakes generally have the following characteristics:1)low elevation;2)close distance and large slope between glacial lakes and replenishment glaciers;3)steep glacier tongue at the end of replenishment glaciers;and 4)steep moraine dam.These characteristics show that dangerous glacial lakes may be affected by glacier collapse,landslide,extreme weather and other factors.(3)Glacial lakes with high dangerous of outburst could not certainly at high risk to the downstream areas if the glacial lakes are far away from human settlements or cannot form large-scale floods quickly.Conversely,glacial lakes with a relatively low dangerous may be at higher risk because the outburst will cause enough damage downstream or the flooding is formed too quickly to result in a short time.The glacial lakes with high risk are concentrated in the eastern section of Nyainqentanglha Mountain,and 7 of the 10 glacial lakes with the highest risk are located in Jiali County.The glacial lake with the highest risk is located in Suo County.This glacial lake has been expanding rapidly since 1990,and the village of Yooxiong is located very close to the downstream of the lake.It needs to be paid enough attention in the future.(4)Comparing the simulation results of the flood evolution process of Jinwengcuo and Ranzeriacuo by the HEC-RAS two-dimensional unsteady flow model with the flood data obtained from field investigations,it is shown that the HEC-RAS model perform robust in simulating the inundation range,water depth and flow velocity of the outburst flood.It has good applicability and can be used in the future study of GLOF prediction.