Remote sensing and in situ-based assessment of rapidly growing South Lhonak glacial lake in eastern Himalaya, India

Melting glaciers are mostly associated with formation of proglacial lakes and the expansion of existing glacial lakes in Himalayan region. These expanding glacial lakes can induce the risk of glacial outburst floods that pose a great potential threat to natural resources and human lives. In Sikkim Himalaya, South Lhonak lake (SLL) (5200 masl) is rapidly expanding over the few decades due to the ongoing glacier melting. We recorded that the lake size increased from 0.20 ± 0.020 to 1.31 ± 0.001 km2 with the length change of 1.6 km during the period from 1976 to 2016. The average rate of expansion was recorded of 0.027 km2 per year; however, it increased drastically since 2000. The in situ-based bathymetric study of SLL showed that the storage volume was 65.81 ± 2.5 million m3 and maximum and average depths were 131 ± 2.5 and 67.05 ± 2.5 m, respectively. We observed that the substantial calving of ice bodies during the melting seasons and partly from the melting of North Lhonak glacier and flow of the Lhonak lake have contributed in expansion of SLL. We have also proposed an empirical equation of volume–area relationship to calculate the storage capacity of similar moraine-dammed glacial lakes in the Himalaya. In addition, we have suggested effective precautionary and mitigation measures to minimize the risk of GLOFs in future. The present study provides vital inputs for hydrodynamic modelling for flood simulation of potentially vulnerable lakes and to formulate the effective strategies in disaster risk reduction and mitigation plan in minimizing the threat of GLOFs.