In-situ bathymetry and discharge-based modelling for potential GLOF assessment of Gepang Gath Lake, Lahaul Himalaya, India

The present study covers a vulnerable glacial lake known as Gepang Gath Lake in the Lahaul area of Himachal Pradesh, India. The field investigation was carried out to measure depth and discharge rate for analysing the probability of lake outburst hazard. The lake depth ranges between a minimum of 2m to a maximum of 48 m with an average depth of 19 m with a standard deviation of 13 as per the field surveys. The stream discharge of 10 m/s was measured near Sissu village valley lying downslope from the Gepang Gath Lake on 21 June 2021 in the afternoon. Snowmelt discharge from the temperature index model was compared with the stream discharge on a clear weather day plus the rainfall data of monsoon months (when the possibility for discharge is higher) was analysed and modelled in HEC-HMS software for peak precipitation discharge based on rainfall data of July and August. 1D flood modelling using glacial lake volume in HEC-RAS software was performed and finally, the resultant discharge (a combination of all three discharges from snowmelt, precipitation and lake volume) is 3,696 m3/s. This resultant discharge is considered huge enough to affect the downstream and valley area with a high-intensity flood that can reach up to the Tandi village area.