Glacial Lake Outburst Flood/Debris Flow Disaster Mechanism And Hazards Assessment In Bhote Koshi Basin

The glacial lake outburst flood/debris flow(GLOFs)is one of the most catastrophic disasters of the cryosphere.The Himalayas is currently the most severely damaged region in the world suffering from GLOFs.With global warming,the glaciers retreat in most parts of the Himalayas has led to the expansion of the glacial lakes,arousing the concerns of local government,disaster management departments,and scholars.Whether global warming is increasing the frequency of GLOFs has become a debate among scholars.In addition,the Himalayas is one of the regions experiencing the most frequent seismic activity.However,few earthquake-induced GLOFs events are reported,and the mechanism of the effect of the earthquake on GLOFs is still yet unclear.The cascading process of GLOFs in the Himalayas influenced by multiple factors,including climate,geomorphology,earthquake,and human activity.However,most of the studies have focussed mainly on mechanisms that cause primary GLOF.This study uses remote sensing interpretation,field investigation,and hydrological model calculation methods based on the GLOFs events in the Bhote Koshi basin,the central Himalayas focusing on the analysis of the Gombongtongsaco GLOFs disaster mechanism after Gorkha earthquake.Based on this,the GLOFs hazard assessment after the earthquake was performed in conjunction with glacial lake inventory.The main results achieved are as follows:The latest glacial lake inventory of the Bhote Koshi basin was established,and the distribution and changes characteristics of glacial from 1976 to 2018 were revealed.The Bhote Koshi was highly developed glacial lakes,where glacial lakes were rapidly expanded driven by significantly rising regional temperatures.A total of 140 glacial lakes larger than 0.01 km2 with an area of 20.59 km2 were identified in 2018.The glacial lakes mainly consisted of small moraine dam lakes,which distributed at elevations ranging from 4100-5750 m a.s.l..The glacial lakes increased by 18% and 77% in numbers and areas respectively from 1976 to 2018.The average annual expansion rate was 0.21 km2/a,where the fastest expansion period was between 2004 and 2010.Significantly increase in temperature at the rate of 0.23°C/10 a,and decrease in precipitation and evaporation in the study area led to rapid expansion of glacial-fed-lakes toward the direction of retreated glacier.The GLOFs in Bhote Koshi was characterized by high frequency,recurrence,cascade disaster chains,and huge hazards.The small outburst-induced major disaster of Gombatongshaco GLOF was driven by climate change,earthquake,and human activities separately.Significant rise in regional long-term temperature led to glacier retreat,leaving large amounts of moraines and debris at the back slope of the glacial lake.The preceding extreme temperature and heavy rainfall on the disaster day triggered debris flow and caused the moraine dam failure.A large amount of loose landslides deposits included by the earthquake were eroded and evolved into the debris flow,which amplified the discharge of debris flow.The Gorkha earthquake exacerbated the social and ecological vulnerability in the downstream region,which resulted in significant post-disaster socio-economic and environmental effects.It showed that GLOFs hazard is a chain process controlled by multiple spheres and factors.A rough but more comprehensive method for evaluating GLOFs hazard was proposed,considering the probability of a flood developing into a downstream debris flow where large numbers of earthquake-triggered landslides are being distributed.There were 10 extremely high hazard,22 high hazard,42 medium hazard and 66 low hazard glacial lakes were identified,and the GLOFs hazards in Chongduipu basin where Nylam County located was found extremely high.However,the GLOF hazards of 25.7% galcial lakes would be underestimated without considering earthquake-induced landslides,and in which most of them were small glacial lakes.Therefore,regional GLOF hazard assessment should not be overlooked for landslide deposits entrainment along a flood route would increase the peak discharge,especially in the earthquake-affected area where large numbers of landslides were triggered.GLOF hazard alleviation coordination mechanism is required in vulnerable transboundary watersheds,including carrying out joint construction and sharing of monitoring and early warning systems for "extremely high" and "high" hazard glacial lakes.In addition,cooperation and assistance from government departments,scientific research institutions,and international organizations,and the establishment of a coordination mechanism for resources and major rescue operations are highly required.