Glacial lake outburst flood in Tista basin on 4 October 2023 - a preliminary investigation

Cloudbursts and Glacial Lake Outburst Floods (GLOFs) are major triggers of flash floods in the Himalayas, leading to sudden destructive surges of water. Breaching of South Lhonak Lake (3 October 2023, around 22:12 IST) in combination of high-intensity rainfall in downstream areas made a torrential water intrusion in the Tista basin of eastern Himalayas on 4 October 2023. It has found that during the last few decades, the South Lhonak Lake was expanding in size, that is possible caused by sudden temperature rise in north Sikkim. On 3 October 2023, a large mass of lateral moraine was collapse into the lake from the north west bank. The debris deposition into the lake elevated the hydrostatic pressure of water, which ultimately triggered tsunami that further breached the entire moraine embankment. The released lake water thereby followed the Goma Chhu (Lhonak valley) and Zema Chhu to reach the river Tista. The flow was very rapid and takes around two hours to strike away Chungthang, one of the nearest towns. The occurrence of GLOF along with heavy rainfall in the mid-region of Tista basin, resulted in a raise of water level up to 3 to 5 m than normal. Significant damage on both the population and infrastructure was experienced across the districts of Mangan, Gangtok, Pakyong and Namchi districts of Sikkim; Darjeeling and Kalimpong in West Bengal. Nearly 41 people died due to this enormously devastating event. Therefore, the present study is an attempt to identify the causes and consequences of recent GLOF event through preliminary investigation and thereby providing a scope for a comprehensive follow up study that would minimise the risk in future.