Nepal’s Glacier Lakes Are Hazards and Opportunities

DHANANJAY REGMI AND JEFFREY KARGEL

Recent articles in the different national newspapers have discussed Imja Lake lowering and disaster mitigation initiatives undertaken by the Nepal Army and Hydrology and Meteorology Department (DHM). Though informative, the articles missed information about glacier lake formation, why they are dangerous, and whether lowering the lake water levels means that downstream communities are safe forever.  Using knowledge gained from years of studies by different universities, Nepal government agencies and NGOs, The Mountain Institute, and our work with the NASA/USAID/SERVIR and U.N. Development programs, we add this missing information and address how modest lake lowering reduces the danger of Glacier Lake Outburst Floods (GLOFs) and could aid the tourism and energy sectors.

Because of climate change Nepal’s glaciers are speeding their retreat. Consequently, where glaciers have low slopes and develop surface depressions due to melting, glacier lake formation is prominent. Satellite image mapping reveals more than two thousand glacier lakes in Nepal’s Himalaya; these developed mostly since the 1960s. Lakes start with formation of small ponds on the glacier surfaces. The ponds gradually widen and merge. The coalesced lakes then melt underlying ice.  The end of the glacier is normally heavily insulated by debris and melts more slowly, so the lake’s growth cuts off the end of the glacier, which acts as an impermeable ice-cored, debris-covered natural dam. That ice, however, also may slowly melt, forming caverns and natural pipes.  Sometimes the dam can fail suddenly, and lake water can burst to make a GLOF. In minutes to hours, a large fraction of the lake volume can drain.

Because of climate change Nepal’s glaciers are speeding their retreat. Consequently, where glaciers have low slopes and develop surface depressions due to melting, glacier lake formation is prominent. Satellite image mapping reveals more than two thousand glacier lakes in Nepal’s Himalaya; these developed mostly since the 1960s. Lakes start with formation of small ponds on the glacier surfaces. The ponds gradually widen and merge. The coalesced lakes then melt underlying ice.  The end of the glacier is normally heavily insulated by debris and melts more slowly, so the lake’s growth cuts off the end of the glacier, which acts as an impermeable ice-cored, debris-covered natural dam. That ice, however, also may slowly melt, forming caverns and natural pipes.  Sometimes the dam can fail suddenly, and lake water can burst to make a GLOF. In minutes to hours, a large fraction of the lake volume can drain.

Several processes can initiate GLOFs, such as melting, weakening, and seep-induced widening of natural pipes and sudden breakdown of the ice core within the terminal moraine; formation of massive, erosive tsunami-like waves due to sudden calving from the glacier, or landslides or rock and ice avalanches into the lake, or detachment of large blocks of submerged ice; or earthquakes. GLOF mitigation through lake lowering can use outlet control structures, tunneling through the moraine barrier, controlled breaching using explosives and excavation, or pumping or siphoning of lake water.

Peru has controlled more than 40 glacier lakes, and Bhutan and Kazakhstan have also controlled some lakes. Nepal completed its first lake lowering in 2000 at Tso-Rolpa. In October 2016 Nepal successfully reduced Imja lake, which, at 5010 meters a.s.l., is the world’s highest elevation lake lowering. Tso-Rolpa was reduced by 3 m, whereas Imja was reduced by 3.4 m. These lowerings are small compared to the lakes’ maximum depths (over 150 m in the case of Imja Lake).  However, small lowerings can increase the dam height above water level (“freeboard”), making it harder for giant waves or eroding rivers to cause a GLOF, and reducing the flood volume if a GLOF nonetheless occurs.

The lowering of these lakes helped to reassure local people. The modest lowering reduces the risk, but the hazards are not totally eliminated, and the lakes must be closely monitored for hazardous changes.

Nepal’s glacier lakes are located in uninhabited or lightly inhabited areas, so monitoring for hazardous changes is challenging. If we can develop these lakes as safe tourism destinations for trekking and boating, promote these areas as open natural laboratories for educational tours of schools and colleges, and develop the lakes as natural reservoirs, then opportunities exist for better monitoring of the lakes, and the continuing potential dangers can be identified early and those risks can be reduced. Economic incentives may encourage tourism service operators and stewards of the lakes to live nearby year-round, who then can maintain and protect meteorological and hydrological stations and report hazardous changes.

Secondly Nepal is facing an energy crisis, especially during the dry season and peak daytime loads. We suggest using these glacier lakes as natural reservoirs and to release required amounts of water by opening and closing the gates to fulfill peak-hour energy demands. An outlet control structure at these high altitudes costs around $3 million (U.S.). This is a small cost if we consider the energy output and benefit of hydropower and the enhanced safety of downstream residents and infrastructure.

For example, Nepal already has more than 200 MW of running projects and over 1000 MW of projects under construction on the Marshyangdi river corridor. Many glacier lakes exist above these projects. Among them, Thulagi (Dona) lake is already among Nepal’s most dangerous lakes. Our team’s latest study shows that this lake has over 40 million m3 of water. If the lake bursts it may damage or destroy 200 MW of running projects, bridges, agricultural land, and settlements, such as Tal, located along the river banks. We should control the glacier lakes before they burst. We can use these lakes as reservoirs by building outlet control structures and reserving the water within safe levels. By investing $3 million (U.S.), we can make more than this by releasing water during the peak loads and generating more hydroelectric power.

Outlet controls built on selected lakes in major catchments can reduce GLOF hazards, ease Nepal’s energy crisis and load-shedding problem, and provide more water for downstream uses during droughts.

Any high mountain development and GLOF mitigation will have environmental impacts, especially on wild nature. However, through careful planning these impacts can be reduced and offset by compensating investments in expanded parklands at local to national levels, and investments in park protections and wildlife conservation.

A strategy is needed to reassess the hazards and vulnerabilities of glacial lakes; to determine the economic opportunities and cost/benefit of GLOF mitigation, hydropower development, and water resource management; and to make these investments also a net benefit to Nepal’s wildlife and wild nature.   Proper study and management can transform some of the present GLOF risk to opportunities for the tourism, energy, and agriculture sectors.

Dr. Dhananjay Regmi is a Kathmandu-based mountain-geomorphologist and a businessman involved in tourism and hydropower development.  Dr. Jeffrey Kargel is a University of Arizona (Tucson) glaciologist and expert in high mountain hazards. 

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