NEPAL – Glacial Lakes Outburst Floods (GLOFs) – A humanitarian perspective of additional risks following the recent earthquakes in Nepal

I am sure we all remember the images of the 2004 Indian Ocean tsunami or the 2011 tsunami in Japan. The destructive force of these tsunamis, however, is nothing compared with the destructive power and likely impact of a glacial lake outburst flood (GLOF). Nepal has seen its fill of these events – the last one hit Pokhara in 2012, causing considerable destruction of houses and infrastructure and resulted in a high death toll when the Seti River carried a massive GLOF downstream. Other sizeable GLOFs occurred in Nepal in 1981 (Sun Koshi Hydroelectric Project), triggered by a GLOF originating in Tibet Autonomous Region, in 1985 (Dig Tsho glacial lake in Khumbu), and in 1994 (Lugge Tsho) in neighbouring Bhutan.

GLOFs are not new phenomena, yet these once rare events are now compounded by climate changes—the resulting warming of the glaciers and increased glacial melt in the Hindu Kush and the Himalayas. GLOFs are typically caused when large parts of a glacier break off or when a sizeable landslide or avalanche crashes into a glacial lake, threatening the stability of the moraines that contain these lakes. Glacial lakes are impounded on the downstream side by end moraines, which consist of unconsolidated composite material, sand, clay and gravel, and the instability of this mix is often compounded by regular freezing and thawing of the ground. The impact of flood waves caused by landslides entering the glacial lake can easily erode unstable end moraines, breaking these barriers and causing outburst floods with enormously destructive force. A GLOF is basically a tsunami-like flood with flood waters squeezed into the narrow gauges below the lake, with surge waves of 10-15 meters and higher that can leave a wake of destruction 150 km or more downstream[1].

Earthquakes are an additional factor that can contribute to the rupture of an unstable natural end moraine. Earthquake tremors can further destabilize the already unconsolidated material, possibly causing fissures or even fractures in the moraine. Nepal just suffered two major earthquakes – the first on April 25th was of 7.9 magnitude, followed by hundreds of strong aftershocks coming in waves and a second quake of 7.3 strength on May 12th. In the past several weeks, there have been 20+ earthquakes of 4.0 – 5.7, all around 10 km in depth. Most of these quakes have occurred to the north and northeast of Kathmandu, indicating volatile tectonic activity beneath areas where some of the most endangered glacial lakes (classified as dangerous or very dangerous) in the Himalayas are located.

Complicating matters, the annual monsoon will commence in Nepal within approximately three weeks, adding multiple hazards to the system related to slope saturation and higher water levels—compounding the already heightened risk of GLOFs.

A 2001 study of Himalayan glacial lakes classified over 200 of the total 8,800 glacial lakes in the Himalayan bioregion as potentially dangerous[2]; and a follow-up study in 2009 using remote sensing technology revealed a total of 1,466 glacial lakes in Nepal alone, of which six were classified as potentially dangerous and in need for further field studies.[3] More detailed studies were undertaken on three highly dangerous lakes (Tsho Rolpa, Imja and Thulagi glacial lakes). In the case of Tsho Rolpa, significant efforts have been made since May 16 to analyse the structure of the end moraine, to reduce water levels in the lake, and to create early–warning systems for downstream communities that are immediately at-risk.[4]

Although the destructive powers of GLOFs are enormous, proper awareness and precautionary measures can help reduce loss of human life, livestock, crops, risk to infrastructure, and other sizeable economic losses. These measures include precautionary artificial lowering of water levels through appropriate drainage (siphoning) and the installation of early warning mechanisms. Furthermore, water storage and more appropriate use of hydroelectric power could limit the danger from GLOFs, linked with a clearer strategy of settlements and possibly even relocation of endangered villages to safer ground.

In the current moment, I am struck by the fact that despite the recent earthquakes, the occurrence of over 3,000(!) dry landslides between ongoing aftershocks, general awareness of the risks of the pending monsoon (both perennial and magnified by seismic activity), and the considerable research and knowledge about GLOFs that exists… very few people apart from specialized scientists seem particularly worried about this real and acute danger. ICIMOD scientists have announced plans to undertake more detailed comparative studies of the most endangered lakes after the monsoon (in October) however, conventional wisdom would urge inclusion of the GLOF risks within this years monsoon planning given the increased vulnerability resulting from the earthquakes. The reality is that the rainy season will likely render most mountain areas inaccessible for months and with precarious communication lines to remote areas, the realities in these areas may remain largely unknown until it is too late.

For centuries, the people of Nepal have trusted in the protection by their deities and undoubtedly this protection will continue one way or the other. However, seismic and geomorphological risks are creating new layers or precarity and exposure in communities living against a backdrop of climactic change. It is hoped that this coming monsoon season passes relatively uneventfully, despite the earthquake—Nepal has seen enough suffering. If the rain comes and the moraines hold, the international community and the respective Nepalese authorities, institutions and NGOs currently focused on the reconstruction of Nepal might recognize this a blessing, and they might be well advised to invest heavily in GLOF preparedness and mitigation efforts to prevent even greater destruction and suffering in years to come.

Gert Venghaus is a humanitarian disaster manager with extensive experience who works in Nepal with Global Emergency Group.



[1] Hurricane Katrina triggered a similar effect when the hurricane, powerful as it was, generated even greater devastation as it contributed to the breaching of the levees holding back Lake Pontchartrain and flooding of New Orleans with additional billions of gallons of water.

[2] Shrestha, Arun Bhakta. Resource Manual on Flash Flood Risk Management, Module 2. Kathmandu: International Centre for Integrated Mountain Development, 2008. Web.

[3] Ives, Jack D., Rajendra B. Shrestha, and Pradeep Mool. Formation of Glacial Lakes in the Hindu Kush-Himalayas and GLOF Risk Assessment. Kathmandu: International Centre for Integrated Mountain Development, 2010. Web.

[4] “In the Aftermath of the Earthquake, UNDP Expedites Installation of Early Warning System in At-risk Glacial Lakes.” UNDP in Nepal. United Nations Development Programme, 20 May 2015. Web.