Nepal’s mountains are melting
The development of glacial lakes from receding glaciers, contained by either terminal moraines or bedrock, is commonly linked with global warming since the end of the Little Ice Age (LIA), and are thought to be increasing in number and size.
Such lakes are prone to sudden and catastrophic drainage of their water which is known as glacial lake outburst floods (GLOFs). Dam breakage can result from rapid lake area expansion rates, dead-ice melting in moraines, destabilising seepage processes, sudden lake water level change, and surge waves created by rockfall or ice calving.
The stored lake water that is released has caused enormous death and devastation downstream. However, there are a range of other cryospheric processes and hazards that are only beginning to receive attention. They, too, can result in landslides, debris flows, and/or other major glacier-related flood events.
We focus on three of these lesser-known and poorly documented phenomena — englacial conduit floods, permafrost-linked rockfall, and debris flows. All three took place in the Nepal Himalaya during the past decade.
Englacial Conduit Floods
Debris-covered glaciers with gradients steeper than 2° are prone to meltwater drainage as opposed to the pooling and development of large glacial lakes. Consequently, they often develop a network of englacial conduits, or cave-like features, within the glacier itself.
The conduits may be interconnected and linked to surficial meltwater ponds characteristic of stagnating and ablating debris-covered glaciers. These conduits are usually water filled during the summer months, contained within the glacier by an ice lens or dam located below the exterior debris cover. They are water free during the winter months.
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Flood triggers can include the rapid drainage of a surficial meltwater pond directly into water-filled conduits, overland floods from high precipitation events, where floodwaters enter sinkholes that connect directly to the conduits, or the sudden discharge of water from one internal conduit to another.
Retaining ice lenses can then fracture from the increased hydrostatic pressure, with flood waters emerging directly out of the glacier itself.
The discharged water from multiple conduit outlets can then merge to create significant downstream flood activity, with peak discharges in the range of several hundred cubic meters per second.
These floods, while considerably smaller than most known and recorded GLOFs, can nevertheless result in damage downstream.
Permafrost-linked, Rockfall-induced GLOFs and Debris Flows
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.
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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.
Earthquake-induced GLOFs
Seismic activity is a potential trigger of GLOFs. The fragile and unconsolidated nature of most terminal moraines means they can be easily damaged during earthquakes, releasing water downstream.
Following the 25 April 2015 earthquake, a team of international remote sensing specialists from the University of Arizona mapped 4,312 landslides to identify potential hazards. They surveyed 491 glacier lakes for earthquake damage, finding nine landslide-impacted lakes but no visible satellite evidence of GLOFs. The lack of GLOFs was attributed to the impacted region’s extreme topography, which may have played a role in buffering the potentially destructive impact of the shock waves.
An earthquake-triggered avalanche of ice and rock was dislodged from the Langmoche Glacier above Dig Tso glacial lake in 2015 in the Khumbu. The avalanche created a 4.2–8.2m seiche wave and small outburst flood within the Bhote Kosi downstream. Although the flood was largely contained within the river channel, several bridges were destroyed.
Dig Tso first experienced major GLOF activity on 4 August 1985, as a result of a similar ice avalanche from the Langmoche Glacier. A resulting avalanche created a surge wave that breached the terminal moraine, unleashing an estimated 5 million m3 of water that killed five people and caused extensive damage downstream.
The 2015 repeat event is important because it demonstrates that even though a glacial lake has already experienced an outburst flood, it can still be dangerous, subject to additional flooding activity in the future, and in need of regular monitoring.
Each of these phenomena need more detailed study to develop the most effective prevention, mitigation, and adaptation approaches possible. Such studies will most likely be strengthened if they include a on-site or remote sensing reconnaissance of the event as soon after its occurrence as possible, along with the participation, insights, and experience of local people.
Large hydropower projects are particularly vulnerable to the impacts of glacier floods, as has happened repeatedly in recent years. Such projects need to plan for such catastrophic events, which have thus far been largely ignored within the pre-construction feasibility and environmental impact study process.
Transboundary research and cooperation will also be essential to minimising the impacts of future GLOFs, particularly along the Nepal-China border.
If scientists can share the results of their research with decision-makers, more timely mitigation programs will be possible.
Alton C Byers, PhD is a Senior Research Associate and Faculty at the Institute for Arctic and Alpine Research (INSTAAR), University of Colorado at Boulder. A scientific version of this article, recently published in Mountain Research and Development, can be downloaded for free here. Byers is currently researching alpine ecosystems in the Kanchenjunga Conservation Area.