Glacier lake outburst floods
Glacier floods, also known with the Icelandic term, jökulhlaup, are sudden outbursts of enormous quantities of water from within the glacier. These are impressive phenomena because of the volume of water involved and because of the violence with which it floods the valley downstream causing enormous damage and drastic modifications in the landscape. Some glaciers are subject to cyclic outburst activity. As far back as the Little Ice Age, the Rutor Glacier, in Valle d’Aosta, for example, was known to discharge periodically, due to the emptying of the S. Margherita Lake, more than 4-5 million m3 of water in less than 6-7 hours – the effects of the flood could be felt beyond the confluence of the Dora di Verney and the Dora Baltea rivers, about 30 km away.
Most glacial floods are due to the formation of lakes, both on the surface and on the sides of the glacier and the subsequent overflow or sudden emptying caused by the collapse of the glacier walls supporting them – the phenomenon is also known with the acronym GLOF, Glacier Lake Outburst Flood. The latter causes the discharge of millions, at times tens of millions of cubic metres of water downstream which, on their path, pick up and drag along debris of any dimension, further increasing the destructive power of the GLOF. The destruction of moraine ridges, debris layers and alluvial terraces increases the solid load transported by the water, which often is transformed into a muddy flow that buries everything in its path. The violence of the current is such that much damage is caused by the air displacement caused by the mass of moving water: ‘walls’ of water up to 10-15 m high have been observed. The flood can also reach areas very far away, tens of km away: in 1985, in Nepal, the GLOF of Dig Tho, with a capacity of 2,000 m3/s, spread for over 90 km fortunately in a sparsely populated area. Some glacier outburst floods apparently cannot be explained, and for this reason are much more dangerous because unpredictable and without ‘premonitory signs’: they are probably due to the formation and subsequent sudden drainage of lakes within the glacier: the study of glacier caves can be very important in these cases, because it allows us to understand how water behaves within a glacier and to predict, to a certain extent, the possibility that a glacier could face a jökulhlaup.