A GLOF is a type of outburst flood following the failure of the natural dam containing the glacial lake. It can be very fast and generate a very high discharge: up to 15 000 m3/s, almost 9 times the Rhone’s mean discharge. Because these lakes can contain millions of cubic meters of water, the consequences can be disastrous. In a few hours, almost all of the water contained in the lake, the rocks and the mud flow downstream in the form of mud and debris flows. Torrents transform themselves in a few moments and destroy villages, crops, roads and infrastructures along their path. The damages may reach tens of kilometers downstream of the lake.
Glacial lakes are formed on the outskirts of the glaciers. There are different types:
- Proglacial (1),
- Supra glacial (3),
- Of confluence (4),
- Periglacial (5).
The natural dam is formed of moraines and/or of ice. Global warming is at the origin of the increasing number of glacial lakes.
The moraines are piles of eroded rocks that were carried down and deposited by the glaciers. Different types of moraines exist. They are classified according to their type of formation: lateral, median, frontal… These different types of moraines can be at the origin of the formation of glacial lakes and their weaknesses as well as the cause of failure. Hence, they are closely linked to GLOFs.
It is a flow with a mixture of water, soils and rocky materials. It is composed of mud known as the matrix; its high density allows it to carry large rocks (up to a few meters). In addition, mudflows have the ability to carry organic materials such as trees at a very high speed. These organic materials can create temporary dams during the flood.
A glacial surge is the rapid advance of the glacier. It can last a few hours to a few weeks and can make the glacier advance from tens to hundreds of meters per day (against tens of centimeters in normal times).
The erosion by overflowing or the erosion created by a spill occurs when the water level reaches the tope of the dam. In the case of glacial lakes*, an avalanche or the fall of a sérac could trigger an important wave that, if it reaches the outlet, submerges the natural dam and damages it. A breach forms thereby weakening the dam. From the breach to the total failure of the dam, there is only one step. This type of erosion is the principal cause of the rupture of natural dams.
The erosion by boil is linked to the flow of water within the dam. The tube or « boil » increases up to a particular value of the diameter which causes the layer of soil above it to collapse. This creates a breach, which will then increase in size.