Glacial Lakes Outburst Floods (GLOFs) Modelling of Thulagi and Lower Barun Glacial Lakes of Nepalese Himalaya

Melting of glaciers is one of the major effects of recent atmospheric warming in the Himalayas which results in the development of supraglacial lakes that coalesce to form proglacial lakes. Large volumes of water in these lakes are considered dangerous due to its unstable surrounding geomorphology. Most of the proglacial lakes are moraine-dammed and breaching of such moraine due to some triggering events may give rise to rapid release of water and sediment, often termed as Glacial Lake Outburst Floods (GLOFs). GLOFs represent catastrophic phenomenon of the Himalaya and pose a risk to downstream communities and infrastructure. Several GLOF events have been recorded in Nepal in the past few years. Since 1990s, the lake area of Thulagi has increased from 0.72 to 0.94 km² while that of Lower Barun has increased from 0.64 to 2 km² and both glacial lakes rank No. I in the Potentially Dangerous Glacial Lakes (PDGLs) in Nepal. In this study, NWS-BREACH and HEC-RAS model are used to access the impact of GLOF on downstream areas. Different dam breach scenarios of 5, 10, 15 and 20 m is evaluated by NWS-BREACH model to estimate the breach hydrograph. The peak flow at the time of breach ranges from 1400 to 5400 m³/s for Thulagi and 2600 to 7900 m³/s for Lower Barun in low and high breach scenarios respectively. The peak flow estimated from NWS-BREACH is used for the flood routing in HEC-RAS to simulate peak discharge and flood height at different distances in the downstream from the lake outlet. The impact on downstream regions of the two glacial lakes are then evaluated by flood inundation map to access the land exposed to GLOF. Nepal consists of large number of glacial lakes and only a few have been studied. Furthermore, detail field survey of the lakes and downstream areas need to be incorporated for better prediction of GLOF events.