Tsho Rolpa, looking west

Tsho Rolpa, looking west toward Na;
note the moraine crest that forms the lake basin,
and the trough in which runs a stream, the headwaters of Rolwaling River

On GLOFs in General











The terms glacial lake outburst flood (GLOF) and jokulhlaup are applied to floods caused by a variety of processes.  In many cases, lakes are formed at the snout of receding glaciers, where they are held in place by morainal deposits and sometimes ice. These deposits tend to be relatively unconsolidated, and are subject to destabilization due to a number of factors, which can occur individually or in combinations.







In these processes, the lake banks may simply give way as they are weakened or subjected to increased pressure; or the waters may overtop the banks, leading to rapid erosion and breach.


Other ice-dammed glacial lakes may be trapped alongside glaciers, or in the axis between a glacier and an ice-free tributary valley. At a certain point, the lake may become heavy enough to force its way under the glacier, to burst forth suddenly.  In Iceland, where large glaciers overlie volcanic hotspots (for instance, around Vatnajokull), "lakes" form underneath the ice; when these become large enough, the glacier belches them out. This is the jokulhlaup, literally "glacier leap."



A related process that is worth mentioning in this context is the avalanche-dammed lake outburst flood. In many places, streams or rivers have been blocked by landslides; generally these give way at some point, leading to an event similar to a GLOF. As Ives (1986) notes, such blockages are fairly common in the Himalayas due to the highly seasonal precipitation, and the outburst floods regularly cause substantial loss of lives and property.



Yet another scenario is the GLOF cascade: a highland GLOF delivers a surge to an artificial dam downstream, causing it to give way.



The key element of the GLOF hazard is the near-instantaneous release of large volumes of water, producing sudden peak flows that are sometimes several times greater that the seasonal peaks associated with monsoon rains; the effect is even more pronounced when the GLOF flow is piggy-backed on top of the high volume of monsoon events which are frequently involved in the initiation of the outburst. Eye-witnesses have described roaring walls of water 10 to 20 meters high.




The environmental impact of a GLOF flood may be much greater than the immediate loss of property. The explosive release of lake waters carries a large load of sediment and often debris, which may be deposited over previously productive farmland. (The accelerated melting of Himalayan glaciers over the past decades has resulted in comparatively large accumulations of exposed erosional material; dirtier GLOFs are to be expected.) The stream channel may be reconfigured, river banks and even tall talus slopes and alluvial cones undercut to the point that they collapse -- and continue to collapse for years. 



Efforts to mitigate GLOF impact involve first of all the identification of potential sources. In many cases, the water accumulates under glaciers and is not even visible until the outburst. Ice-dammed lakes, however, tend to dump repeatedly (sometimes regularly) over time.  With morainally dammed lakes, the difficulty of prediction arises from the large number of variables, including meteorolical and seismic processes, as well as the relative obscurity of processes within the moraine walls.




People have been living -- and dying -- with GLOFs a long time.  There are reasons to give more consideration to the possibility of GLOFs now, beyond the simple reason that we now have some capacity do both mitigate and aggravate the danger.   First of all, the upland population tends to be concentrated along rivers, where they and their possessions (including agricultural fields) are extremely vulnerable to flood impact.  As populations increase, and become economically more important,  especially due to tourism opportunities,  the risk increases.  And of course the increasing numbers of tourists, who often outnumber local residents, are also at risk.


Secondly, as development projects proliferate, there is a need to protect both traditional access routes (on which the projects continue to rely) as well as the new infrastructure. Some of the infrastructure becomes (such as hydroelectric plants) becomes so important locally and also to lowland population centers that disruption is unacceptable.


Third, lowland populations and infrastructure also cluster around rivers. GLOFs can cause major damage tens of kilometers away. There is little doubt, for instance, that a sudden breach of Tsho Rolpa would not only destroy the village of Na and a large part of Beding, it would take out most of the villages downstream of Simigaon for a hundred kilometers, including the new hydroelectric plant at Khimti.



Fourth, the high-lowland linkages have evolved in many areas to the point that it is no longer  politically feasible to ignore disasters that at one time would have aroused little sympathy. 


At the very least, we must be aware of the scale of a GLOF hazard in order to take precautions in development projects.  Even though moraine-dammed and supra-glacial lakes (as opposed to englacial and subglacial lakes) may be observed both from the air and often by close inspection, the availability of an approximate assessment of stability does not assure an appropriate response. As Ives (1986) notes:






The very fact that the 4 August 1985, Khumbu catastrophe originated with the outburst of a moraine-dammed lake would serve to justify the mounting of a small research programme aimed at mapping and monitoring such lakes. "Moreover, word-of-mouth reports to the effect that Namche Small Hydel Project engineers were aware that Dig Tsho Lake was overtopping its moraine dam in 1984" (Dr. V. Galay 1986 pers. comm.) begs the question of why steps were not taken to meet the portending catastrophe.  Wider public and official awareness of the jokulhlaup phenomenon might be able to minimize, or even prevent, future disasters.