A flood of evidenceAs glacial lakes are increasing in size, the risk of outburst floods is also rising
Galongco Lake sits at the foot of an unnamed glacier within the Himalayan mountain range in China, close to its border with Nepal, one of more than 1,000 glacial lakes that have formed in the Hindu Kush Himalaya (HKH) in the past three decades. Galongco feeds the Poiqu River in China, which flows into Nepal as the Bhote Koshi and later the Sun Koshi further downstream.
The formation of glacial lakes in the HKH is an impact of melting glaciers. As glaciers retreat, meltwater fills the space between the frontal moraine, and the retreating glacier. In this way, new lakes form, existing ones expand and sometimes merge with each other.
Climate change is exacerbating this slow, natural process. Since the late 1970s, when scientists first started mapping Galongco, the lake has more than tripled in size, from 1.50 sq km in 1977 to 5.07 sq km in 2010. This is a cause for concern as Galongco is the largest of all the glacier-fed lakes in the transboundary Koshi Basin that spans China, India, and Nepal.
Studies by the International Centre for Integrated Mountain Development (ICIMOD) and its partners have identified 42 rapidly growing lakes in the Koshi Basin that have grown by more than 35 percent in the past three decades. Like Galongco, most glacial lakes in the region are dammed by moraines—ridges formed by debris deposited along the sides of glaciers that hold the meltwater. As glacial retreat increases with rising temperatures, the chances that these lakes will burst their moraines also increases, heightening the potential risk of glacial lake outburst floods (GLOFs). The general seismicity of the HKH also contributes to this risk. Luckily, the most recently experienced earthquake in Nepal, in 2015, did not trigger any large GLOFs this time.
ICIMOD and its partners in China and Nepal have been using earth observation tools to map decadal changes in glaciated areas in the region. Landsat imagery data from 1977, 1990, 2000, and 2010 show that glacial lakes in the Koshi Basin have increased consistently in size and number. The research shows that the Himalaya is losing ice mass and snow cover, and witnessing an increase in natural hazards due to the degradation of moraines and permafrost.
A sudden discharge of high volume water from glacial lakes in the mountain valley can have catastrophic effects downstream, as evidenced by five GLOF events that have been recorded along the Koshi River in Nepal since 1935. When Zhangzhangbo Lake breached in 1981, in one hour it released 19 million cubic metres of water, equivalent to 7,600 Olympic-sized swimming pools. The resulting flood claimed five lives and injured hundreds in Nepal. The Sun Koshi Hydropower Dam was destroyed and stretches of the Araniko Highway were severely damaged, halting traffic for over a month. Trade and transportation services were disrupted for almost three years and physical infrastructure and vulnerable landforms were left severely damaged. The total economic loss incurred was estimated to be around $4 million.
Due to increasing population along the corridor, on-going research and several models developed around the Koshi River show that thousands of lives and property worth millions are at currently risk, both in China and in Nepal. The projected losses are higher for Nepal as markets and infrastructure have been set up along the Sun Koshi River in recent years.
Understanding how glaciers and glacial lakes respond to climate change helps understand the long-term impact of climate change on water resources. This provides evidence for government agencies such as Nepal’s Department of Hydrology and Meteorology (DHM) to build timely disaster risk reduction plans and a basis for developing monitoring and early warning systems, and planning and prioritising disaster mitiagtion efforts. In November 2016, as part of the Community Based Flood and Glacial Lake Outburst Risk Reduction Project, the DHM and the United Nations Development Programme (UNDP) were successful in draining water from Imja Lake in the Everest Region to safe levels. According to UNDP, the draining has successfully minimised the threat posed by a potential GLOF to an estimated 96,000 people living downstream along the Dudh Koshi Valley.
While the success at Imja Lake is certainly a significant positive and a wonderful illustration of how evidence generated by scientific knowledge and understanding can effectively influence government plans and action, it is important to note that there are other glacial lakes for which timely disaster risk reduction plans and monitoring and early warning systems still need to be developed. Defining the risks of potential GLOF events continues to be a challenge for researchers as the exact circumstances that lead to risk are difficult to predict.
However, the fact is that glacial lakes need our attention because they are growing dangerously quickly and cannot be ignored. Mitigation intervention at any level is only possible through a long-term collaboration of science and decision making. The glacial lakes of the Koshi Basin demand our attention. There is conclusive scientific evidence that can help government agencies start building timely disaster risk reduction plans and mitigate the effects of possible future diasters.
- Shrestha is a Research Associate, Geographic Information Systems (GIS) and Seldon is a Communications Officer at ICIMOD