glaciers down-waste, thawing of permafrost, changes to the boundary between cold-based and polythermal glaciers, and potential buoyancy of previously submerged stagnant ice masses within lakes, are some of the features that might be expected to become evident.

Climate change is affecting the supply of water from down- wasting glaciers with severe consequences in terms of water resource management. When a GLOF occurs it can cause major changes to the course and characteristics of an affect- ed river system, often for many years afterwards. Commu- nities can suffer damage to houses, bridges, paths, access to grazing lands, and valuable food-producing land can be destroyed. Increased geotechnical effects are observed such

Glacier Lake Outburst Floods Cumulative frequency of events

15 20 25 30 35

10 5 0 1930 1940 1950 1960

Observed events in Nepal, Buthan and Tibet

1970

Regression line 1980

1990 2000

Source: reproduced from Richardson and Reynolds, An overview of glacial hazards in the Himalayas. Quaternary International, 65-66, 31-47., 2000.

Figure 10: Cumulative frequency graph of GLOFs in Nepal, Bhutan and Tibet. The best-fit line suggests an increase in GLOF frequency through time, although older events may be under-reported. GLOFs also took place in Patagonia, Chile in 2008 and 2009 (Dussaillant et al., 2010)(From Richardson and Reynolds, 2000).

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as more landslides resulting from greater erosion of the toes of unstable slopes, and loss of agricultural land and forestry. Suspended sediment loads increase in the river water, with effects on the river ecology and resulting in greater abrasion of hydropower turbine blades and greater rates of sedimen- tation. All of these changes have economic as well as physi- cal consequences.

The vulnerability to GLOFs varies from region to region de- pending on terrain, land use patterns and population den- sity as well as the likelihood of the dams breaching. A great number of people are potentially in danger should the lakes classified as dangerous in the Hindu Kush-Himalayas drain. In most cases, there would be little or no warning, with insuf- ficient time for complete evacuation. Much larger numbers of people would be affected through loss of property, farmland and livelihoods. Damage to households, private property and agricultural lands would be in the range of several tens to possibly hundreds of millions USD (Khanal 2010). A major concern should GLOFs increase is the subsequent damage to hydropower stations downstream. Early warning systems are clearly needed, but difficult and costly to implement. Given the vastness and remoteness of the mountain areas and GLOF locations in question, remote sensing techniques appear as the only feasible means to achieve reasonable pre- dictions of potential GLOFs on a larger scale (Qincey et al., 2005; 2007).

Siphons have also played a key role in remediation of lakes in the Cordillera Blanca, Peru, and have helped reduce the volume of a lake that, when it eventually breached, only af- fected the immediate area downstream, with no loss of life or significant damage. The affected lake was later safely remedi- ated by constructing a series of four tunnels separated by 5 m vertically through bedrock into the base of the lake. On 11th April 2010, a rock and ice avalanche cascaded into this lake and produced an avalanche push wave about 28 m high that overtopped the rock dam and flooded downstream affect- ing farm land and destroying the water supply for the local town. However, no-one was killed. The remediation under- taken in 1993 had lowered the lake level by 20 m providing a freeboard of 23 m. Consequently the bulk of the push wave was retained within the rock basin, significantly reducing the flood volume and attenuating the peak flow rate. It is thought