The impacts of global warming trends can be particularly conspicuous in glaciated landscapes, notably in the form of rapidly receding glaciers and the formation of glacial lakes.
Growing evidence suggests that similar changes are also occurring at above 6,000m elevations with the thawing permafrost that can impact the mechanical strength and hydraulic permeability of high elevation rock faces.
Rockfalls previously constrained by year-round frozen temperatures can set in motion a cascade of catastrophic processes below, including more rockfalls, debris flows, explosive impact with glaciers below in addition to triggering glacier-related floods downstream.
On 20 April 2017, a flood on the Barun Valley in the Makalu-Barun National Park in eastern Nepal blocked the confluence of the Barun with the Arun River, resulting in a potentially dangerous 200m long lake.
Our follow-up study during May-June 2017 revealed that the source of the flood was the <0.1 km2 Langmale Glacial Lake. In spite of its small size, the estimated 1.3 × 106m3volume flood caused extensive downstream damage to forests, pastureland, and some infrastructure down to the flood’s attenuation point in Yangle Kharka.
Read also: Forests replace glaciers in the Himalaya, Tufan Neupane
However, instead of the flood trigger being a snow or ice avalanche into the lake, the cause turned out to be a massive, high altitude breakage of solid rock from the east face of Saldim Peak (6,388 m). The debris landslide plummeted 1,200m down to the Langmale Glacier, creating a massive explosion upon impact characterised by a dust cloud and hurricane-force winds.
A 1.1 million m3 debris flow of ice, debris, and sediment then cascaded directly into the Saldim Glacial Lake below, triggering a hyper-concentrated slurry outburst flood that grew progressively larger and more destructive as it continued down valley.
Similar processes are thought to have triggered the 2012 Seti flood near Pokhara that killed 70 people, and 2021 Chamoli flood in Uttarakhand that left 300 dead and destroyed several under construction hydropower projects.
Swiss permafrost specialist Wilfried Haeberli, using recent examples from the Swiss Alps, Cordillera Blanca in Peru, and Mt Everest region of Nepal, believes that the probability of similar flood events will continue to increase with the continued formation of new glacial lakes below high mountains.